Applications of layered double hydroxide biopolymer nanocomposites

Applications of layered double hydroxide biopolymer nanocomposites

Applications of layered double hydroxide biopolymer nanocomposites 15 Shadpour Mallakpour1,2,3 and Leila khodadadzadeh3 1 Organic Polymer Chemistry ...

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Applications of layered double hydroxide biopolymer nanocomposites

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Shadpour Mallakpour1,2,3 and Leila khodadadzadeh3 1 Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan, Islamic Republic of Iran, 2Research Institute for Nanotechnology and Advanced Materials, Isfahan University of Technology, Isfahan, Islamic Republic of Iran, 3Chemistry Group, Pardis College, Isfahan University of Technology, Isfahan, Islamic Republic of Iran

15.1

Introduction

The term “nanotechnology” is widespread nowadays not only in scientific fields, but also in many public sectors. According to the definition by the Royal Society and the Royal Academy of Engineering, “nanotechnologies are the plan, characterization, production and application of structures, devices and systems by controlling the size at the nanometer (nm) scale” (UNESCO, 2006), despite the fact that the US National Nanotechnology Initiative has defined that nanotechnology research and technology development are at the atomic, molecular, or macromolecular levels, in the length scale of approximately 1100 nm range. For the answer of “what is so special about 100 nm?,” it has been said that in this scale (below 100 nm), engineers consider the properties of materials that ordinary engineers ignore, which in particular contain “quantum mechanical effects” and “surface science effects” (Gasman, 2006). Nanotechnology has a great influence on the economy and society in the early 21st century (Lau et al., 2013) compared to information technology, cellular and molecular biology, and semiconductor technology. According to many books and reviews, nanotechnology has had a significant impact on the oil and gas industry (Bera and Belhaj, 2016; Fakoya and Shah, 2017; Negin et al., 2016), petroleum exploration (He et al., 2016), energy research and solar cells (Abdin et al., 2013; Hussein, 2015, 2016), water treatment applications (Kunduru et al., 2017; Olvera et al., 2017; Qu et al., 2013), food safety and food packaging (Dimitrijevic et al., 2015; Duncan, 2011), drug and gene delivery (McDonald et al., 2015; Wen et al., 2016; Wong et al., 2017), cancer therapy (Beik et al., 2016; Xie et al., 2016), tissue engineering (Kingsley et al., 2013; Walmsley et al., 2015), and many other important areas. Composite materials are defined as solid materials with multiple phases. The economic importance of these materials is observed everywhere. Generally, Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00015-X © 2020 Elsevier Ltd. All rights reserved.

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composite materials are engineered materials, made up of more than one component with significantly different physical or chemical properties. The phases not only have intimate contact (at an atomic or molecular level) with each other but also remain individual on a macroscopic level within the finished structure. Matrix and reinforcement (filler) are two main phases in composite materials. Matrix is usually a continual and softer phase that can be a polymer (weak stiffness), metal (medium stiffness but high ductility), or ceramic (strong stiffness but brittle), while the reinforcing or dispersed phase is a strong and stiff part embedded in the matrix which enhances the physical and mechanical properties of matrix. For example, wood is composed of fibrous cellulose as reinforcement in a matrix of lignin, while bone and teeth are made up of hard inorganic crystals (hydroxyapatite or osteones) in a matrix of collagen. There are many routes for the combination of the phases and the most common, especially in polymer and material science, are filling, blending, compounding, mixing, melting, and assembling. It is noteworthy that the significance of composite materials is not only for their bettered strengths but also for their other useful applications such as electrical, biological, thermal, etc. (Durand, 2008; Hull and Clyne, 1996; Youssef, 2013; Zafar et al., 2016) The advance in nanotechnology has given new intuitions into applications of popular materials due to surprising properties as a result of nanoscale technology. As an example, polymer nanocomposites (NCs) are a comparatively new type of composite material, and are regarded as a new alternative to conventional polymers. In polymer NCs, the matrix is an organic polymer and nanoscopic inorganic or organic fillers (with at least one dimension less than 100 nm) are dispersed in it. Nanofillers (NFs) have very high aspect ratio which leads to their effective dispersion in matrix. Such a uniform dispersion creates an ultra large interfacial area between two phases and so the final NCs have predominant properties compared to conventional microcomposites. Furthermore, due to the unique properties of nanosized fillers, reinforcing with only a low loading of these fillers (, 5 wt.%) not only endows NCs with a similar performance to that of conventional composites (containing 4050 wt.% of common fillers) but also resulting in a material with lower weight (Mousa et al., 2016). In comparison with pure polymers or conventional composites, NCs have remarkably ameliorated properties which in particular contain catalytic, gas barrier, thermal, mechanical, flame retardancy, etc. This aspect of nanotechnology has potential in applications such as biomedical applications, water treatment, engineered plastics, rubbers, adhesives, and coatings (Thakur and Thakur, 2015). It is crucial to diminish the environmental effect of materials manufacturing by reducing the environmental impression at all steps of their life cycle. Using nonbiodegradable petro-based synthetic polymers has caused many environmental problems, which in particular include high annual consumption and limitations of landfill sites for their waste accumulation, the production of carbon dioxide and hazardous emissions during incineration, uneconomical waste recycling, etc. (Mousa et al., 2016). Therefore, developing bio-based materials is one of the outstanding subjects for many researchers. As a consequence, fabrication of NCs in which both the matrix and NF are based on renewable resources is the order of the

Applications of layered double hydroxide biopolymer nanocomposites

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day. Therefore, the main focus of this chapter is on polymer NCs containing biopolymer matrix. Biopolymers are polymers that are biodegradable (Grossman et al., 2013) and according to ASTM standard D-5488-94d and European norm EN 13432, “biodegradable” means “capable of undergoing decomposition into water, carbon dioxide, inorganic compounds, methane, and biomass.” The principal mechanism for degradation is the enzymatic action of microorganisms (Ave´rous and Pollet, 2012). Different classifications of various biodegradable polymers are possible. It can be based on the synthesis procedure, processing technique, chemical constitution, application, economic significance, etc. and since each of them supplies different information, it is not simple to choose the classification. In this chapter we choose a classification of biopolymers based on their “origin” which classifies them into three main groups (Smith, 2005): 1. Natural polymers (renewable resource polymers), macromolecules obtained from natural origins. These polymers are subdivided into six classes: a. Polysaccharides; b. Proteins; c. Lipids; d. Polyesters obtained from plants and microorganisms (polyhydroxyalcanoates); e. Polyesters synthesized from bio-derived monomers [polylactic acid (PLA)]; f. A final group of various polymers such as composites and natural rubbers. Note: subdivisions (ac) are called agro-polymers. 2. Synthetic polymers (a nonrenewable resource), macromolecules synthesized from mineral origins (crude oil). These polymers are subdivided into four classes: a. Aliphatic polyesters [polyglycolic acid, polybutylene succinate (PBS), polycaprolactone (PCL)]; b. Aromatic polyesters or blends of aliphatic and aromatic kinds (polybutylene succinate terephthalate); c. Poly(vinyl alcohols) (PVA); d. Modified polyolefin (polyethylene or polypropylene containing sensitive sectors for temperature or light). 3. Blends of polymers from miscellaneous origins. In spite of the appealing properties of biopolymers, such as vast accessibility of raw materials and improved biodegradability, some vital properties like barrier, mechanical, and thermal properties are limited and not comparable with conventional polymers. Production of biopolymer NCs by incorporation of NFs into biopolymers is an outstanding solution for the described problems (Averous and Boquillon, 2004), which makes them a suitable choice for a wide range of applications such as food packaging, water remediation, electrochemical and electroanalytical, medical and tissue engineering, coatings, etc. (Aranda et al., 2006; Costa et al., 2013; DeGruson, 2014; Grossman et al., 2013; Jorfi et al., 2013; Mallakpour et al., 2014; Mousa et al., 2016; Okamoto and John, 2013; Pilla, 2011; Reddy et al., 2013; Rhim et al., 2013; Tan et al., 2010; Zhou et al., 2011). One of the most important groups of NFs is inorganic types. Indeed, inorganic nanomaterials (NMs) are considered as basic building blocks in nanotechnology. Inorganic NMs may be classified according to their geometric shape and have at

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least one dimension in the range of 1100 nm. According to this, they can be spherical (e.g., metal and metal oxide nanoparticles [NPs]), fibril-like (e.g., carbon nanotubes and metal wires), and platelet-shape (e.g., natural smectite clays, layered double hydroxide [LDH], graphite, and graphene sheets), producing particulate, elongated, and layered NCs, respectively (Zafar et al., 2016) (Fig. 15.1). Other geometric shapes and morphologies that have been reported include inorganic nanowires, nanotubes, nanoboxes, nanocubes, nanospheres, and nanorods (Chiu and Lin, 2012). A broad range of inorganic NMs have been used as NF for reinforcement of polymeric matrix but LDHs, a family of lamellar inorganic materials, has been much investigated for this purpose. This is due to their many outstanding properties which in particular include nontoxic, low price, eco-friendly, large surface area, great thermal and mechanical properties, and tendency to interchange their interlayer anions with other anions such as larger organic and inorganic ones (Mallakpour and Hatami, 2017). LDHs are also recognized as anionic clays due to the existence of exchangeable anions in the interlayer area. The structure of LDH composes of brucite-like sheets and the thickness of each sheet is about 0.5 nm. The positive charge of LDH sheets is a result of incomplete substitution of divalent metal cations with trivalent ones which are finally balanced with anions located inside the interlayer area. In general, LDH has the formula of (M211x M31x (OH)2)(Anx/n  mH2O), where M21 and M31 are divalent and trivalent metallic ions which inhabit the center of octahedral units in the hydroxide layers, and An is an interlayer anion. The metal (M) cations located in the octahedral units of hydroxide layers are coordinated by six hydroxyl groups, hence forming M(OH)2 brucite˚ ) and usually like sheets. The radii of these metal ions are similar to Mg21 (0.65 A contain Ni21, Co21, Mg21, Cu21, Zn21, or Cd21 (divalent), and Al31, Mn31, Fe31, Ga31, or Cr31 (trivalent). Many of the common metal ions used to synthesize LDH

Figure 15.1 Nanomaterial classification, together with examples, according to which the physical dimension is in the 1100 nm range. Source: Adapted from Chiu, C.-W., Lin, J.-J., 2012. Self-assembly behavior of polymerassisted clays. Prog. Polym. Sci., 37(3), 406444, with kind permission of Elsevier.

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Figure 15.2 Part of binary LDHs reported in patents and literature. Source: Adapted from Qu, J., Zhang, Q., Li, X., He, X., Song, S., 2016. Mechanochemical approaches to synthesize layered double hydroxides: a review. Appl. Clay Sci., 119, 185192, with kind permission of Elsevier.

which are reported in many patterns and literatures are illustrated in Fig. 15.2. On the other hand, the usual interlayer anions are inorganic one such as CO322, Cl2, NO32, and SO422. It is noteworthy that the most commonly applied LDH is hydrotalcite, containing Mg21 and Al31 as metal cations and CO322 as interlayer anion [general formula: (Mg211x Al31x (OH)2)(CO322x/n  mH2O) (0.2 # x # 0.33)] (Gao, 2012). The most common synthetic routes which have been used for the preparation of LDHs are coprecipitation, reconstruction, ion exchange, and hydrothermal method (Mallakpour and Hatami, 2017). It is noteworthy that recently, mechanochemical methods have received more attention from researchers to resolve the problems that remain with conventional solution methods such as high energy consumption, treatment of aqueous waste, complex operation, etc. (Qu et al., 2016) LDH is a favorable material for a broad range of practical applications such as catalysis, adsorption, pharmaceutics, photochemistry, electrochemistry, and other fields. This is due to its high versatility, easily tailored properties and low cost, which make it possible to produce materials designed to fulfill specific requirements (Li and Duan, 2006). In order for successful preparation of polymer NCs reinforced with lamellar NMs and to obtain the high efficiency of these NMs, rupture of the initial structure of lamellar NMs (exfoliation) or intercalation of polymer chains between their

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layers (intercalation), as well as homogeneous distribution of them in the polymer matrix and good compatibility between two phases, are vital factors. On the other hand, it is not always possible to obtain complete exfoliated morphology and a microcomposite morphology can be observed too, in which the lamellar NM remains in its initial structure while mixed with the polymer matrix (Gao, 2012; Youssef, 2013). Several methods have been used to fabricate polymer/LDH NCs in which the most commonly applied are in situ polymerization, melt-mixing, and solution blending (Gao, 2012). This chapter focuses on the preparation, characterization, and application of NCs from most of the biopolymers reinforced with LDH.

15.2

Biopolymer/layered double hydroxide nanocomposites

15.2.1 Polysaccharide/layered double hydroxide nanocomposites Polysaccharides are renewable resource biopolymers and are a class of carbohydrate polymers consisting of multiple monosaccharide units which are linked together by glycoside linkage (Zafar et al., 2016). Due to possessing unique features such as more stability and usually not being irreversibly denatured on heating, polysaccharides are different from other agro-polymers. The diverse structures of polysaccharides, including linear or branched forms, a wide range of molecular weights from low to high, diverse polydispersities, existing in the forms of both monofunctional (OH) or multifunctional (OH, COOH, NH2), as well as specific properties such as water-soluble or insoluble properties, high level of chirality, environmentally safe, low toxicity, and nonimmunogenic, have attracted a great deal of attention for utilizing them in the production of NMs (as an organic NF) and NCs (as a matrix) (Zheng et al., 2015).

15.2.1.1 Cellulose/layered double hydroxide nanocomposites Cellulose, the most abundant and sustainable structure in nature, is obtained from cotton and wood pulp. It is considered as a renewable source polymer and believed to be an outstanding replacement for petroleum-based compounds. It is a β-1,4linked linear polymer of glucose units and is insoluble in water, dilute acidic solutions, and dilute alkaline solutions at normal temperatures (Chen, 2014). It has attracted considerable attention due to its unique properties such as biodegradability, biocompatibility, and chemical stability (Zafar et al., 2016). There are some reports for the incorporation of various types of LDH into both cellulose and modified cellulose (e.g., cellulose acetate, carboxymethylcellulose, oxidized cellulose, etc.), which have shown potential for applications such as adsorption and separation from solution and wastewater (biosorption for diclofenac and bovine serum albumin [BSA]- pollutant adsorption for fluoride, boron, and

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selenium separation of rare earth elements) (Beyki et al., 2017; Iftekhar et al., 2017; Latorre et al., 2013; Mandal and Mayadevi, 2008; Yan and Yi, 2013; Zhang et al., 2014), drug delivery (Barkhordari and Yadollahi, 2016), sensors and analytical applications (Lai et al., 2016), catalyst (for CC bond formation) (Mahdi et al., 2015), as well as important features for the packaging industry such as mechanical properties (Yadollahi and Farhoudian, 2015; Yadollahi et al., 2014), water vapor (Yadollahi et al., 2014) and gas barrier (Dou et al., 2014), swelling behavior (Yadollahi and Farhoudian, 2015; Yadollahi and Namazi, 2013; Yadollahi et al., 2015), and fire resistance (Ton-That et al., 2015). One of the most common and important effects of the incorporation of NFs into polymer matrix is improving the mechanical properties of the polymer. Yadollahi et al. (2014) investigated the mechanical properties, as well as water vapor permeability, of carboxymethyl cellulose/LDH NC films with an LDH concentration up to 8 wt.%. They synthesized MgAl-LDH by the coprecipitation method. Fabrication of MgAl-LDH was confirmed by observation of diffraction peak at 2θ value of 10.26 degrees in XRD spectrum, corresponding to a (003) spacing of pristine LDH (Fig. 15.3A). Carboxymethyl cellulose/LDH NCs (1, 3, 5, and 8 wt.%) were prepared by a casting/evaporation method. XRD patterns (Fig. 15.3A) of carboxymethyl cellulose/LDH NCs showed characteristic peaks of carboxymethyl cellulose (weak peak at 2θ B 11 degrees and a strong peak at 2θ B 20 degrees). When the LDH loading was 5 and 8 wt.%, the morphology of films was exfoliated/ intercalated due to a new broad peak appearing at 2θ values of 5 and 3.5 degrees. On the other hand, when the amount of LDH was below 5 wt.%, no diffraction peaks were observed around about 2θ 5 2 2 10 degrees, demonstrating the fully exfoliated structure. TEM images (Fig. 15.4) of carboxymethyl cellulose/LDH NCs revealed that the LDH sheets were well distributed in the carboxymethyl cellulose at LDH loadings below 8 wt.% and, when it was increased to 8 wt.%, the LDH tended to aggregate. According to the morphological analysis, the prepared carboxymethyl cellulose/LDH NCs were composed of highly exfoliated LDH layers, including the intercalated regions as well as agglomerated regions. The researchers investigated the water vapor permeability of carboxymethyl cellulose after incorporation with LDH. It was observed that the presence of LDH could decrease the water vapor permeability of carboxymethyl cellulose/LDH NC films and a reduction in the values of these parameters was observed with an increase of the LDH amount. It was decreased by 13%37% depending on the LDH concentration. This observation is attributed to the concept of tortuous paths. In the presence of ordered and dispersed impermeable LDH layers the permeating water molecules are forced to follow longer and more tortuous pathways to diffuse through the carboxymethyl cellulose/LDH NC. They also observed the influence of incorporation of LDH into the carboxymethyl cellulose matrix on the mechanical properties of the final NCs. By incorporation of 3 wt.% LDH into the matrix, both tensile strength and tensile modulus were increased compared to the pure carboxymethyl cellulose, attributed to the strong interaction between LDH plates and polymer chains. However, by increasing the LDH content to 5 and 8 wt.%, both were decreased, due to the aggregation of LDH in the higher content. The elongation at break of the films was

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Figure 15.3 (A) X-ray diffraction patterns of CMC film, MgAl-LDH, and CMCLDH NC films and effect of LDH content (wt.%) on (B) tensile strength (C) tensile modulus, and (D) elongation at break of CMC-based films. CMC, carboxymethyl cellulose. Source: Adapted from Yadollahi, M., Namazi, H., Barkhordari, S., 2014. Preparation and properties of carboxymethyl cellulose/layered double hydroxide bionanocomposite films. Carbohyd. Polym., 108, 8390, with kind permission of Elsevier.

decreased by incorporation and increasing the amount of LDH, which led to increasing the brittleness of the films (Fig. 15.3BD). One of the most important factors for packaging applications, especially in the fields of food and drug packaging, is possessing high barrier properties for degradative gases (e.g., O2) and water. Although there are many benefits to polysaccharides being a matrix in polymer NCs for packaging applications, their performance is often limited by high gas permeability. To solve this problem, incorporation of LDH and other inorganic NFs into the polysaccharides has been proved to be an effective way of improving the barrier behavior through the extensive diffusion path for permeating molecules. One example in this area is the fabrication of cellulose acetate/LDHNC transparent and flexible films with a tremendous oxygen barrier, as reported by Dou et al. in 2014 (Dou et al., 2014). The films were prepared

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Figure 15.4 TEM images of the CMCLDH NC film with 3 wt.% LDH (A and B) and 8 wt.% LDH (C and D) at low and high magnifications, respectively. CMC, carboxymethyl cellulose. Source: Adapted from Yadollahi, M., Namazi, H., Barkhordari, S., 2014. Preparation and properties of carboxymethyl cellulose/layered double hydroxide bionanocomposite films. Carbohyd. Polym., 108, 8390, with kind permission of Elsevier.

by spin-coating technique of cellulose acetate and LDH followed by thermal annealing. Compared to other oxygen barrier compounds, two factors including highly oriented nanoplatelets in the polymer matrix and hydrogen bonding network could cause coextensive diffusion length and strong resistance and finally lead to suppressing the oxygen permeability. Good dispersion of LDHs in cellulose acetate was successfully confirmed by XRD, SEM, and EDX. The absence of any nonbasal reflections (h, l 6¼ 0) compared with pure LDH demonstrated a preferred orientation of LDH platelets with the ab plane parallel to the substrate, which not only confirmed the uniform distribution of LDHs but also account for this high level of transparency. Furthermore, the SEM images of films showed the uniform layered architecture consisting of densely packed LDH nanoplatelets with good corientation. According to the EDX, the Mg, Al, and C were uniformly dispersed throughout the 2D-organized film. It is noteworthy that the ratio between metallic ions (divalent and trivalent) in the LDH can affect the reinforcement behavior of LDH for cellulose matrix and the thermal stability of the final NC. As an example, Mekdad et al. (2014) studied the effect of Mg/Al ratio and the rates of reinforcement for cellulose/hydrotalcite NCs. Hydrotalcite belongs to the family of LDHs. The authors synthesized hydrotalcite with two different ratio of Mg/Al, 2 and 3 (HT2 and HT3). Cellulose was extracted from yucca leaves and the NCs were prepared with 2, 5 and 10 wt.% of each NF. According to the XRD spectrum, the average sizes of the synthesized hydrotalcite particles confirmed the nanometric size of NFs (18.71 nm for HT2 and 19.70 nm

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for HT3, calculated by DebyeScherer equation). The XRD spectra of cellulose showed peaks at 2θ 5 16, 22, and 34 degrees, where the first represented amorphous cellulose while the latter two were attributed to the crystalline shape of the cellulose. In terms of NCs, an extra wide stripe overlapped with the stripe (006) of hydrotalcite (2θ 5 20 degrees) in the XRD pattern of both NCs, corresponded to the most intense peak of microcrystalline cellulose. Furthermore, the disappearance of the peak corresponding to the amorphous shape of cellulose (2θ 5 16 degrees), indicated that the cellulose was in an ordered form in the NCs. A low intense peak at 2θ 5 29 degrees was observed for the NC of hydrotalcite with the ratio of Mg/Al 5 2 containing 5 and 10 wt.% hydrotalcite, while this peak was absent for the NC of hydrotalcite with the ratio of Mg/Al 5 3 (5 and 10 wt.%). This peak can be attributed to the formation of another phase when the load rate increased for the NC of HT2. This can be related to the load of the layer of the hydrotalcite, which was weaker in the case of HT3 due to the influence of the ratio of Mg/Al on the synthesis of composite materials. In addition to XRD, TEM observations (Fig. 15.5) were used for investigation of the morphology of the obtained NCs. Hydrotalcite NPs were well dispersed in the matrix, and the size of the majority of the clay particles was smaller than 100 nm. In addition, in the case of NCs of HT2, 2 wt.%, the particles were homogeneously dispersed and had a size of approximately 4060 nm (Fig. 15.5C). The influences of the rate of reinforcement showed that the optima was 2% for the hydrotalcite of ratio of Mg/Al 5 2 while it was 5% for hydrotalcite of ratio of Mg/Al 5 3. Moreover, the hydrotalcite of ratio of Mg/Al 5 2

Figure 15.5 TEM micrographs of (A) MCC, (B) HT3, (C) MCC-HT2.2%, and (D) MCCHT3.5% NCs. MMC, microcrystalline cellulose; HT2 and HT3, hydrotalcite with ratios of Mg/Al 5 2 and 3.

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[comparison between NC of HT2 (5%) and NC of HT3 (5%)] gave a more thermally stable NC. Natural cellulose is hydrophilic and therefore it absorbs moisture which can be indicated by its low water contact angle (2030 degrees). This is despite the fact that hydrophobic cellulose materials are always in high demand in applications such as water-proof packaging, thin films, paper, sorbents, sanitation, fabrics, etc. There are several reports of modifying cellulose (e.g., paper) surfaces while there is little research on hydrophobic materials derived from cellulose fibers. Interestingly, Sobhana et al. (2017) used LDH as first-of-its-kind material in order to meet the challenges in cellulose-based super-hydrophobic materials. They hydrophobized cellulose by environmentally benign stearic acid with the aid of inorganic linker/ interface/sandwich material, LDH, which had layers linked with hydrophobic stearic acid and hydrophilic cellulose simultaneously (Fig. 15.6). Indeed, there was no direct attachment between hydrophobizer and cellulose but LDH created this conjugation. Cellulose conjugated with LDH through its hydrophilic layers, while the negatively charged polar head of stearic acid was attracted toward the positively charged brucite layers of LDH through an electrostatic interaction.

Figure 15.6 Schematic illustrations for the preparation of the super-hydrophobic SA/LDH/ cellulose NC. SA, stearic acid. Source: Adapted from Sobhana, S.L., Zhang, X., Kesavan, L., Liias, P., Fardim, P., 2017. Layered double hydroxide interfaced stearic acid 2 cellulose fibres: a new class of superhydrophobic hybrid materials. Colloids Surfaces A: Physicochem. Eng. Aspects, with kind permission of Elsevier.

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Fig. 15.7 shows SEM images of pristine cellulose fibers, cellulose fibers with stearic acid, and stearic acid/LDH/cellulose hybrid fibers with different metal precursors and LDH concentrations. The pristine cellulose fibers showed a wellpacked structure due to the compactness of fibers (Fig. 15.7A), while stearic acid/ LDH/cellulose hybrid fibers were much looser than they should have been

Figure 15.7 SEM images of the fibers: (A) pristine cellulose fibers, (B) cellulose fibers with stearic acid (0.02 M); SA-LDH/cellulose hybrid fibers at different metal precursor and stearic acid concentrations under static conditions (C) 500 mM M21:M31/0.02 M SA, (D) 100 mM M21:M31/0.02 M SA, (E) 100 mM M21:M31/0.002 M SA, (F) 100 mM M21:M31/0.001 M SA, (G) 50 mM M21:M31/0.002 M SA, (H) SA/LDH/cellulose fibers prepared under shaking conditions. SA, stearic acid. Source: Adapted from Sobhana, S.L., Zhang, X., Kesavan, L., Liias, P., Fardim, P., 2017. Layered double hydroxide interfaced stearic acid 2 cellulose fibres: a new class of superhydrophobic hybrid materials. Colloids Surfaces A: Physicochem. Eng. Aspects, with kind permission of Elsevier.

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(Fig. 15.7C). Furthermore, when the metal salts used for making the LDH were highly concentrated, the quantitative making of LDH particles was very high, which richly covered the fiber surface and consequently resulted in loosening the fiber to fiber interaction. In all these cases, the dimensions of the LDH platelets remained uniform. The utilizing of LDH as an interface/linker/sandwich material in these hybrids was caused by increasing the hydrophobicity of cellulose, even in its wet and maiden condition, as well as decreasing the processing time of about 5 days in making hydrophobic cellulose material. It was also made possible using even a minimal amount of stearic acid, which could lead to great hydrophobicity. The hybrid of capsulated LDH in carboxymethyl cellulose was used for cephalexin (as a drug model) oral delivery in gastrointestinal tract conditions (Barkhordari and Yadollahi, 2016). For this purpose, cephalexin was intercalated between LDH layers through the coprecipitation route and the as-obtained LDHcephalexin nanohybrid was capsulated in pH-sensitive carboxymethyl cellulose hydrogel beads. The results showed that carboxymethyl cellulose/LDH-cephalexin NC beads had a pH-sensitive swelling behavior, which increased when the pH was increased. Furthermore, an in vitro release study of cephalexin in conditions that simulate the passage through the gastrointestinal tract, showed better protection against drug release for carboxymethyl cellulose/LDH-cephalexin beads compared to LDH-cephalexin at the stomach pH and a controlled release in the intestinal tract conditions due to the pH-sensitive swelling behavior of carboxymethyl cellulose. It was observed that drug release was decreased at acidic pH, attributed to the shrinkage of carboxymethyl cellulose at acidic pH, so the drug was protected against digestion within the stomach. The schematic illustration of cephalexin release from carboxymethyl cellulose/LDH-cephalexin beads is depicted in Fig. 15.8. The shapes of carboxymethyl cellulose/LDH-cephalexin beads and their SEM images are shown in Fig. 15.9. The large size of the wet beads confirmed their ability for great swelling and water retention. On the other hand, the appearance of LDH-cephalexin nanohybrids on the surface of beads proved a good interaction between LDH-cephalexin and carboxymethyl cellulose, resulting to good dispersion of them in the matrix.

Figure 15.8 Cephalexin release from CMC/LDH-CPX NC bead. CMC, carboxymethyl cellulose; CPX, cephalexin. Source: Adapted from Barkhordari, S., Yadollahi, M., 2016. Carboxymethyl cellulose capsulated layered double hydroxides/drug nanohybrids for Cephalexin oral delivery. Appl. Clay Sci., 121, 7785, with kind permission of Elsevier.

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Figure 15.9 Digitals photo of CMC/LDH-CPX NC bead at (A) wet and (B) dry state; SEM micrographs of CMC/LDH-CPX NC beads at (A) low magnification ( 3 10,000) and (B) high magnification ( 3 70,000). CMC, carboxymethyl cellulose; CPX, cephalexin. Source: Adapted from Barkhordari, S., Yadollahi, M., 2016. Carboxymethyl cellulose capsulated layered double hydroxides/drug nanohybrids for Cephalexin oral delivery. Appl. Clay Sci., 121, 7785, with kind permission of Elsevier.

15.2.1.2 Starch/layered double hydroxide nanocomposites Starch is one of the most widely available and inexpensive biopolymers. It is arranged in individual particles called granules which are partly crystalline regions consisting mostly of two homopolymers of glucopyranose: amylose, a linear polymer with α-(1 ! 4)-linked glucose units, while amylopectin is a highly branched polymer consisting of α-(1 ! 4)-linked glucose units with branches made by α-(1 ! 6) linkages at the branch points (Bertolini, 2009). Starch needs to be plasticized to obtain a thermoplastic material. Different plasticizers can be used, including sorbitol, maltose, xylitol, glucose, ethylene-bis-formamide, and glycerol (Mikus et al., 2014). There are many reports on the preparation of starch NCs incorporated with inorganic NFs, such as clay (Perotti et al., 2017), carbon nanotubes (Cheng et al., 2013), metal oxides (Ma et al., 2016), etc. Also, starch has been successfully reinforced with LDH (Chung and Lai, 2010; Privas et al., 2013; Wu et al., 2011). Starch/LDH NCs can be prepared by various methods. For example, Chung and Lai (2010) synthesized LDH in acid-modified corn starch (AMS) dispersion in a direct manner. This technique involves a rapid LDH nuclei precipitation followed by a

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Figure 15.10 (A, B) TEM images of starch/LDH NCs and (CE) schematic representations of LDH dispersion in starch matrix. (A, C) NCS4 and (B, D) AMS4. NCS, native corn starch; AMS, acid-modified starch. Source: Adapted from Chung, Y.-L., Lai, H.-M., 2010. Preparation and properties of biodegradable starch-layered double hydroxide nanocomposites. Carbohyd. Polym., 80(2), 525532, with kind permission of Elsevier.

hydrothermal treatment that simultaneously leaches starches from the granules and grows the LDH nuclei. They compared the influence of using AMS and native corn starch (NCS) as matrices on the mechanical properties, opacity, and moisture adsorption of the final NCs. TEM images (Fig. 15.10) of NCs revealed good distribution of LDH in AMS while aggregation was observed in terms of NCS. This is due to the lower viscosity of modified starch which not only facilitated the distribution of LDH in the matrix but also improved the modulus of NCs without sacrificing their transparency and moisture sensitivity. Investigation of the mechanical properties revealed that when the LDH was absent, the Young’s modulus and tensile strength of the NCS were larger compared to the AMS, while the elongation at break was reduced. This was due to the lower molecular weight of starch as well as increasing the linear short chains after acid hydrolysis which leads to generating a softer material. As can be seen in Table 15.1, the tensile strength and elongation at break of NCS-based NCs decreased by increasing the amount of LDH (NCS1 . NCS2 . NCS3 . NCS4). This was due to the phase separation of NCs with a high LDH content which might destroy the integrity of the NCS-based NC structure. For AMS-based NCs, the Young’s modulus increased by increasing the amount of LDH (AMS4 . AMS3 . AMS2 . AMS1) (Table 15.1). This was due

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Table 15.1 Mechanical properties of NCSLDH and AMSLDH NCs Sample

Young’s modulus (MPa)

NCS1 NCS2 NCS3 NCS4 AMS1 AMS2 AMS3 AMS4

2805 2519 2734 2841 2424 2701 2734 3316

6 305a 6 258b 6 267a 6 177a 6 1 68a 6 152b 6 259b 6 363c

Tensile strength (MPa) 35.73 31.13 30.43 27.51 31.12 33.89 32.51 31.85

6 6 6 6 6 6 6 6

3.92a 2.25b 2.74b 2.84c 2.48a 2.32a 3.50a 3.30a

Elongation at break (%) 2.77 3.14 2.58 1.80 3.55 3.52 3.53 1.81

6 6 6 6 6 6 6 6

0.73a 1.38a 0.95a 0.66b 0.82a 0.99a 1.10a 0.54b

The samples were conditioned at 53% RH before measurement NCS, native corn starch; AMS, acid-modified starch. a,b,c Values followed by the different letters are significantly different at p , 0.05 within NCSLDH and AMSLDH nanocomposites Source: Adapted from Chung, Y.-L., Lai, H.-M., 2010. Preparation and properties of biodegradable starch-layered double hydroxide nanocomposites. Carbohyd. Polym., 80(2), 525532, with kind permission of Elsevier.

to increasing the stiffness as well as decreasing the phase separation of AMS matrix by incorporation of LDH. This is despite the fact that the tensile strength of AMSbased NCs was not enhanced by increasing the amount of LDH. This indicated that the incorporation of LDH, which increased the stiffness of the matrix at a low strain, did not influence its stiffness at a high strain. Dropping the elongation at break of AMS-based NCs at higher LDH amount (AMS4) was attributed to the formation of a higher amount of aggregated LDH. Investigation of the opacity of NCs revealed that the opacity of NCS and AMS samples without LDH was similar. By increasing the loading of LDH, the opacity of NCS-based NCs increased significantly. However, the AMS-based NCs were uniformly translucent between 0% (AMS1) and 10% (AMS4) of LDH loadings. This suggested that the dispersion and miscibility of the nanophases in AMS were much better than those in the NCS matrix. The moisture content of samples slowly increased with an increase in the equilibrium relative humidity up to 70%. Above that a steep rise in moisture content was observed. The moisture content of NCs did not significantly change when LDH was added to both the NCS and AMS systems. This revealed that the water adsorption of starch/LDH NCs during storage is dominated by the starch matrix because of the relatively strong hydrophilicity of starch molecules. Wu et al. (2011) modified the LDH with modified polysaccharide, carboxymethyl cellulose via encapsulation of LDH in polysaccharide for stabilization of LDH in water. The prepared LDH-carboxymethyl cellulose NFs (with 37.3 wt.% of carboxymethyl cellulose) were then incorporated into the matrix of glycerol-plasticized starch for the fabrication of NCs. As can be seen in the TEM image (Fig. 15.11) of LDHcarboxymethyl cellulose, LDH platelets were encapsulated into carboxymethyl cellulose with smaller size (layer number) of each LDH stack, which helped the uniform distribution of LDH-carboxymethyl cellulose NFs in glycerol-plasticized starch matrix. SEM images of LDH-carboxymethyl cellulose/glycerol-plasticized starch 2 wt.% NCs (Fig. 15.12A,B) indicated the good distribution of prepared NFs in the matrix

Applications of layered double hydroxide biopolymer nanocomposites

Figure 15.11 TEM image of LDHCMC. The thickness of platelets is shown with white circles. CMC, Carboxymethyl cellulose. Source: Adapted from Wu, D., Chang, P.R., Ma, X., 2011. Preparation and properties of layered double hydroxidecarboxymethylcellulose sodium/glycerol plasticized starch nanocomposites. Carbohyd. Polym., 86(2), 877882, with kind permission of Elsevier.

Figure 15.12 SEM micrograph of the fragile fractured surface of NCs with different LDHCMC contents (A, B) 2 wt.% and (C, D) 8 wt.%. CMC, carboxymethyl cellulose. Source: Adapted from Wu, D., Chang, P.R., Ma, X., 2011. Preparation and properties of layered double hydroxidecarboxymethylcellulose sodium/glycerol plasticized starch nanocomposites. Carbohyd. Polym., 86(2), 877882, with kind permission of Elsevier.

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due to the improved hydrophilic property of LDH after stabilization, decreasing the size of LDH platelets after cellulose introduction and strong interaction between two polysaccharides, starch and carboxymethyl cellulose. Furthermore, low loading of NFs (6 wt.%) led to improvement in mechanical properties (an increase in the tensile strength and elongation) and water resistance (decrease in the water vapor permeability) compared to the pure glycerol-plasticized starch. There are two reasons for this improvement: (1) because polysaccharides could form complexes with metal ions due to the high number of coordinating functional groups (hydroxyl and glucoside groups), strong associations between the metal ions and the carboxymethyl cellulose occurred for the nucleation and initial crystal growth of the LDH, and thus the LDH was successfully encapsulated by the carboxymethyl cellulose and (2) the hydrophilic carboxymethyl cellulose component and the smaller size of each LDH stack allowed the LDH to be well dispersed in the starch matrix and good interactions between the NF and the matrix were formed because of the carboxymethyl cellulose component (Ramawat and Me´rillon, 2015). However, the bio-NCs displayed a decrease in the thermal decomposition temperature because the weak thermal stability of the carboxymethyl cellulose could weaken the interactions between the LDH filler and the starch matrix and facilitate the decomposition of the starch. Furthermore, a high LDH content (8 wt.%) could result in the agglomeration of the NF in the matrix and thus reduce the mechanical properties and water vapor permeability (Figs. 15.12C,D and 15.13). Privas et al. (2013) modified LDH with organic compound, lignosulfonate, and employed this filler (LDH-LS) with a concentration of 1 up to 4 wt.% for incorporation into thermoplastic corn starch (TCS) as matrix. Incorporation of LDH/LS in starch was done using LDH/LS slurry instead of powder in order to avoid secondary particle aggregation. This reinforced starch was used for preparing a

Figure 15.13 (Left) The effect of LDHCMC contents on tensile yield strength and elongation at break of the composites. (Right) The effect of LDHCMC contents on water vapor permeability of the composites. CMC, carboxymethyl cellulose; WVP, water vapor permeability. Source: Adapted from Wu, D., Chang, P.R., Ma, X., 2011. Preparation and properties of layered double hydroxidecarboxymethylcellulose sodium/glycerol plasticized starch nanocomposites. Carbohyd. Polym., 86(2), 877882, with kind permission of Elsevier.

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Figure 15.14 Transmission electron microscopy images of thermoplastic corn starch mixtures containing 1 wt.% of LDH/LS at different magnifications. LS, lignosulfonate. Source: Adapted from Privas, E., Leroux, F., Navard, P., 2013. Preparation and properties of blends composed of lignosulfonated layered double hydroxide/plasticized starch and thermoplastics. Carbohyd. Polym., 96(1), 91100, with kind permission of Elsevier. Table 15.2 Tensile properties of TCS and LDH/LSTCS NC Sample

Young’s modulus (MPa)

Stress at break (MPa)

Elongation at break (%)

TCS LDH/LS 1%TCS LDH/LS 2%TCS LDH/LS 4%TCS

7.3 (1.0) 10.2 (2.3) 2.3 (0.4) 1.5 (0.1)

2.7 (0.2) 3.2 (0.1) 1.7 (0.1) 1.4 (0.1)

61 (5) 100 (7) 101 (2) 104 (6)

TCS, thermoplastic corn starch; LS, lignosulfonate. Source: Adapted from Privas, E., Leroux, F., Navard, P., 2013. Preparation and properties of blends composed of lignosulfonated layered double hydroxide/plasticized starch and thermoplastics. Carbohyd. Polym., 96(1), 91100, with kind permission of Elsevier.

starchpolyethylene-based polymer blend composite. For this purpose, LDH/ LSstarch NCs were mixed with Lotader 3210 [a random terpolymer of ethylene, butyl acrylate (6%) and maleic anhydride (3%)] at concentrations of 20 and 40 wt. %. Finally, the researchers evaluated the interest of composites, mainly considering gas barrier and mechanical properties. To characterize the dispersion of LDH/LS in TCS, XRD and TEM (Fig. 15.14) analysis were employed. TEM images showed a uniform distribution of LDH/LS NFs into the TCS for all NF loading. The dimensions of the LDH/LS particles (70 nm length and B3 nm) proved an exfoliated morphology. Table 15.2 shows the mechanical properties of TCS and LDH-LS/TCS NCs. Mechanical properties of LDH-LS/TCS are shown in Table 15.2. As can be

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seen, starch with 1 wt.% LDH/LS showed an interesting property improvement, while higher LDH-LS concentrations exhibited degraded properties. One reason could be that the addition of hydrophilic NF enabled the building of strong polar interactions with water which may help to retain water molecules, leading to a more plasticized material. Lignosulfonate is already known to plasticize starch materials. After melt processing, thermoplastic starch undergoes retrogradation, which provides crystallization and reduces the elongation at break. Introduction of NF may stop or reduce this recrystallization process, a phenomenon already observed with starchclay NC. Similar to the mechanical properties, when thermoplastic starch was filled with a small amount of LDH/LS, as low as 1 wt.%, an increase of oxygen and water barrier properties was observed. Lotader/(LDH-LS/ TCS) with two composition ratios 80/20 and 60/40 by weight was prepared and the mechanical properties of these materials, as well as pure Lotader and unfilled TCS, were examined (Table 15.3). As can be seen, the most interesting blends were with 20 wt.% of starch-based content. Tensile strength is around 76.9 MPa for Lotader/ (LDH/LS 1%TCS), much higher than for Lotader/TCS (38.7 MPa). Tensile strength and elongation at break for Lotader80%/(LDH/LSTCS) are similar to

Table 15.3 Tensile properties of Lotader, TCS, and Lotader/TCS with 0, 1, 2, and 4 wt.% LDH/LS materials Sample

Young’s modulus (MPa)

Tensile strength (MPa)

Elongation at break (%)

41.3 (1.8) 7.3 (1.0)

80.0 (2.8) 2.7 (0.2)

229 (3) 61 (5)

27.5 (0.5) 31.5 (0.8) 32.1 (1.4) 32.3 (1.3)

38.7 (4.8) 76.9 (7.6) 77.6 (9.3) 74.3 (8.2)

183 (11) 221 (7) 214 (8) 202 (8)

45.5 (2.7) 38.9 (1.2) 39.3 (1.6) 42.1 (3.4)

16.8 (0.9) 20.6 (2.8) 21.2 (8.1) 24.4 (4.8)

91 (8) 106 (13) 104 (32) 112 (14)

References Lotader 3210 TCS

80/20 Lotader/TCS Lotader/(LDH/LS 1%TCS) Lotader/(LDH/LS 2%TCS) Lotader/(LDH/LS 4%TCS)

60/40 Lotader/TCS Lotader/(LDH/LS 1%TCS) Lotader/(LDH/LS 2%TCS) Lotader/(LDH/LS 4%TCS)

TCS, thermoplastic corn starch; LS, lignosulfonate. Source: Adapted from Privas, E., Leroux, F., Navard, P., 2013. Preparation and properties of blends composed of lignosulfonated layered double hydroxide/plasticized starch and thermoplastics. Carbohyd. Polym., 96(1), 91100, with kind permission of Elsevier.

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pure Lotader. The improvement in tensile strength can be attributed to the formation of a three-dimensional network reinforcing the blend through a percolation mechanism Wilhelm et al. (2003) investigated the influence of various types of natural [kaolinite (neutral mineral clay) and hectorite (cationic exchanger mineral clay)] and synthetic [LDH (anionic exchanger) and brucite (neutral)] layered NFs on the properties of various types of starch (native starch, oxidized starch, glycerol-plasticized starch, and oxidized plasticized starch). The starch/hectorite proportions were 100/ 0, 95/05, 90/10, 85/15, 80/20, and 70/30, relative to starch mass, while the composite films of starch with other layered compounds were prepared only in 90/10 proportions. The composite films were prepared from the respective aqueous suspensions by casting. The effects of the filler type and the plasticizer were analyzed by XRD, dynamic mechanical analysis, and thermogravimetric analysis (TGA). The XRD results of plasticized starch composites revealed that only hectorite showed an increase in the amount of interplanar basal distance, which was attributed to the intercalation of means that kaolinite, LDH, and brucite were not influenced when the starch matrix was present. Substitution of plasticized starch matrix by a plasticized oxidized starch or native/oxidized starch blend gave rise to composites with higher interplanar basal distances, indicating that both short oxidized starch chains and glycerol molecules could be intercalated between clay layers. In the absence of glycerol, oxidized starch was preferentially intercalated in relation to native starch chains due its lower chain size and probable higher diffusion rate.

15.2.1.3 Chitosan/layered double hydroxide nanocomposites Chitosan is a cationic, biodegradable, biocompatible, eco-friendly, and inexpensive polysaccharide which is obtained by partial deacetylation of chitin (Zafar et al., 2016). It is composed of β-(1-4)-linked glucosamine units together with some N-acetyl-D-glucosamine units (Chen et al., 2017; Kim, 2010). Chitosan has been successfully utilized as a matrix for the preparation of organicinorganic hybrid materials and there are some reports for the preparation of chitosan/LDH NCs where the matrix is neat chitosan (Darder et al., 2008; Depan and Singh, 2010), modified chitosan (Wang and Zhang, 2014), or a blend of chitosan with other biopolymers such as alginate and PVA (Ribeiro et al., 2014). Due to having multifunctional groups, chitosan can be used as a biosorbent with high adsorption capacity and sorbent for the removal of pollutants from wastewater. Elanchezhiyan and Meenakshi (2017) produced a chitosan/LDH NC by coprecipitation technique and used it as an adsorbent for the removal of oil particles from an oil in water emulsion. These NCs showed improved adsorption efficiency for oil adsorption at acidic PH compared to raw LDH or chitosan. This observation was a result of a high amount of LDH in chitosan, which facilitated the immobilization of oily particles. As can be seen in the SEM of NCs and oil-adsorbed NCs in Fig. 15.15, a rough coral reef heterogeneous structure of NCs improved the diffusion of oil particles onto the surface of the adsorbent and consequently enhanced adsorption efficiency.

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Figure 15.15 SEM micrograph of (A) CS-LDHCs with respective mapping images and (B) oil sorbed CS-LDHCs with respective mapping images. CS, chitosan; Cs, composite. Source: Adapted from Elanchezhiyan, S.S., Meenakshi, S., 2017. Synthesis and characterization of chitosan/Mg-Al layered double hydroxide composite for the removal of oil particles from oilin-water emulsion. Int. J. Biol. Macromol., with kind permission of Elsevier.

On the other hand, due to the occupation of oil molecules on the adsorbent surface, the coral reef structure was destroyed and a smooth heterogeneous structure was observed after oil adsorption. The mechanism for the adsorption of oil on NCs was predominantly controlled by a hydrophobichydrophobic interaction as well as physical forces. According to the thermodynamic results, the nature of adsorption was spontaneous and endothermic. Due to its biocompatibility and biodegradability, chitosan has displayed an outstanding potential in a variety of biomedical applications such as drug delivery and tissue engineering (Ahmed and Ikram, 2016; Gohil et al., 2016; Jayasuriya, 2016; Moura et al., 2016; Pathania et al., 2016; Rijal et al., 2016; Ryu et al., 2015; SaberSamandari and Saber-Samandari, 2017; Sionkowska, 2016; Vunain et al., 2016; Zare and Shabani, 2016; Zhang et al., 2017a). It is noteworthy that most of the applications of chitosan/LDH NCs are around biomedical applications too (Chen et al., 2017; Chi et al., 2017; Qin et al., 2015; Rezvani and Shahbaei, 2015; Ribeiro et al., 2014; Wei et al., 2012; Wei et al., 2015; Zhao et al., 2015). In 2017, Chen et al. (2017) reported the self-assembly preparation of the pifithrin-α-LDH/chitosan nanohybrid composites as drug-delivery systems for stem cell osteogenic differentiation. As can be seen in SEM and TEM images of NCs (Fig. 15.16), the LDH nanoplates self-assembled into a flower-like shape and the chitosan was covered uniformly by the nanoplates which increased the drug loading efficiency and drug release property as compared with the pure LDH (Fig. 15.17).

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Figure 15.16 (A) SEM image, (B)TEM image, (C) EDS spectrum, and (D) ED pattern of PFTα-LDH/CS nanohybrid composites. CS, chitosan; PFTα, pifithrin-α. Source: Adapted from Chen, Y.-X., Zhu, R., Xu, Z.-L., Ke, Q.-F., Zhang, C.-Q., Guo, Y.-P., 2017. Self-assembly of pifithrin-α-loaded layered double hydroxide/chitosan nanohybrid composites as a drug delivery system for bone repair materials. J. Mater. Chem. B, 5(12), 22452253, with kind permission of the Royal Society of Chemistry.

Figure 15.17 Schematic illustration of sustained release of PFTα from PFTα-LDH/CS. CS, chitosan; PFTα, pifithrin-α. Source: Adapted from Chen, Y.-X., Zhu, R., Xu, Z.-l., Ke, Q.-F., Zhang, C.-Q., & Guo, Y.-P. (2017). Self-assembly of pifithrin-α-loaded layered double hydroxide/chitosan nanohybrid composites as a drug delivery system for bone repair materials. Journal of Materials Chemistry B, 5(12), 22452253, with kind permission of the Royal Society of Chemistry.

The mesopores throughout the LDH nanoplates acted as channels for loading pifithrin-α. The results revealed that the combination design of the pifithrinα/LDH/chitosan nanohybrid composites created a reserving method for bone tissue regeneration. It is noteworthy that chitosan/LDH NCs have been employed in other applications such as preparation of biosensors (Ai et al., 2008; Ding et al., 2011; Han et al., 2007), food packaging due to possessing the remarkable oxygen barrier

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properties (Pan et al., 2015), and catalyst (Adwani et al., 2015). Adwani et al. (2015) synthesized bio-NCs of chitosan and LDH (hydrotalcite) by the coprecipitation method. The prepared NCs showed efficient catalytic activity for selective synthesis of jasminaldehyde by aldol condensation of benzaldehyde and 1-heptanal (Fig. 15.18). The characterization of the catalyst was carried out by N2 sorption, FT-IR, TGA, XRD, SEM, and TEM. The BET surface area of NCs was found to be 3.77 m2/g. Fig. 15.19 shows the XRD pattern of hydrotalcite, chitosan, and the final NC. The sharp, intense, and symmetric peaks at lower diffraction angles (2θ 5 1025 degrees) and a broad asymmetric reflection at higher diffraction angles (2θ 5 3050 degrees) in the XRD pattern of hydrotalcite, are characteristic of a

Figure 15.18 Schematic illustration of synthesis of jasminaldehyde. CMA, chitosan/Mg/Al hydrotalcite bio-NC. Source: Adapted from Adwani, J.H., Noor-ul, H.K., Shukla, R.S., 2015. An elegant synthesis of chitosan grafted hydrotalcite nano-bio composite material and its effective catalysis for solvent-free synthesis of jasminaldehyde. RSC Adv., 5(115), 9456294570, with kind permission of the Royal Society of Chemistry.

Figure 15.19 XRD pattern of HT, CT, and CMA. HT, hydrotalcite; CT, chitosan; CMA, chitosan/Mg/Al hydrotalcite bio-NC. Source: Adapted from Adwani, J.H., Noor-ul, H.K., Shukla, R.S., 2015. An elegant synthesis of chitosan grafted hydrotalcite nano-bio composite material and its effective catalysis for solvent-free synthesis of jasminaldehyde. RSC Adv., 5(115), 9456294570, with kind permission of the Royal Society of Chemistry.

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highly crystalline layered structure. In the XRD pattern of NC, characteristic peaks of semicrystalline chitosan (broad peaks at 2θ 5 10 and 21 degrees) were present and no significant change in the characteristic planes of hydrotalcite was observed, indicating retention of the structure of hydrotalcite. However, after the formation of the NC, a slight shift towards lower 2θ values for peaks corresponding to (003) and (006) planes was observed, which was due to a concurrent decrease in the positive charge of the layers of hydrotalcite. The XRD pattern was also indicative of a slight intercalation of the biopolymer into the LDH layer as observed by a marginal shift of (001) peak relative to parent LDH along with the retention of both phases (chitosan and hydrotalcite). The TEM of the NC showed that the polymer was grafted over LDH material (Fig. 15.20A). Fig. 15.20B reveals the presence of an interplanar distance of approximately 1.4 nm, indicating some exfoliation of the LDH material. The inset in Fig. 15.20B shows presence of single LDH layers in the composite sample. Fig. 15.20C shows the presence of the small particles of the layered structure whilst Fig. 15.20D shows the lattice fringes of the LDH material. The inset in Fig. 15.20D shows the electron diffraction pattern of the composite. It confirms the presence of pure composite material. The investigations were performed in detail as a function of amount of the catalyst, temperature, and molar ratio of 1-heptanal to

Figure 15.20 TEM images of CMA. CMA, chitosan/Mg/Al hydrotalcite bio-NC. Source: Adapted from Adwani, J.H., Noor-ul, H.K., Shukla, R.S., 2015. An elegant synthesis of chitosan grafted hydrotalcite nano-bio composite material and its effective catalysis for solvent-free synthesis of jasminaldehyde. RSC Adv., 5(115), 9456294570, with kind permission of the Royal Society of Chemistry.

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benzaldehyde to observe the effect of these reaction parameters on the conversion, selectivity, and rate of the formation of the condensation products. All these parameters were found to influence the performance of the catalyst. On increasing the catalyst amount, the rate of formation of jasminaldehyde was effectively increased, while that of 2-pentyl-2-nonenal, increased first and then showed a decreasing trend. On increasing the temperature, the rate of formation of jasminaldehyde increased while the rate of formation of 2-pentyl-2-nonenal tended to decrease. The initial rates of the formation of jasminaldehyde were found to be more, while the rate of formation of 2-pentyl-2-nonenal was less with NC as compared to the individual catalysts, chitosan and hydrotalcite. The catalyst was operative under solvent-free conditions, giving the highest selectivity to jasminaldehyde 89%, with .99% conversion with 100 mg catalyst at 160 C under optimized reaction conditions. Furthermore, the catalyst could be elegantly separated and found to be effective for recycling six times without any substantial loss in its activity.

15.2.1.4 Alginate/layered double hydroxide nanocomposites Alginate is a natural anionic polysaccharide usually extracted from brown seaweed and also some special bacteria can synthesize it. It is a water-soluble, linear copolymer of guluronic acid and mannuronic acid which are presented irregularly (Darder et al., 2008). The ratio and pattern of these blocks is mainly governed by the origin of alginate. Biocompatibility, biodegradability, low toxicity, low cost, and mild and easy gelation are promising properties which are made possible with the employing of alginate and also its nanohybrid composites in a wide range of applications such as biotechnology (immobilization of biocatalysts) (Cheryl-Low et al., 2015; Dekamin et al., 2016; Saha et al., 2009), biomedical (wound healing, tissue engineering, drug delivery, and controlled release of encapsulated substrates) (Guarino et al., 2015; Lee and Mooney, 2012; Sood et al., 2016), food (thickeners, viscosifiers, emulsifiers, stabilizers and gel-formers, film-formers, or water-binding agents) (Bierhalz et al., 2012; Brownlee et al., 2009), and environmental engineering (biosorbent for removal of heavy metals or other pollutants) (Bertagnolli et al., 2015; YuLin et al., 2010). There are some reports for the preparation (Landman and Focke, 2006) of alginate/LDH NCs and application of them for catalytic performance (YanChun and Chen, 2014; Zhao et al., 2011), sensing (Darder et al., 2005; Lopez et al., 2010; Shou-Nian et al., 2009), drug delivery (Alcantara et al., 2010; Mahkam et al., 2013; Rezvani and Shahbaei, 2015), and removal of water pollutants such as phosphates, dyes, and heavy metal ions (Kim Phuong, 2014; Lee et al., 2012; Lee et al., 2013; Phuong, 2014; Sebastian et al., 2014) due to the ability of both alginate and LDH in these areas. Furthermore, Wang et al. (2010) reported alginate assistance for LDH assembling. The catalytic activity of alginate/LDH NCs was reported by YanChun and Chen (2014) for the oxidation of α-pinene (Fig. 15.21). Microwavecrystallization and low saturated state of coprecipitation were employed for the intercalation of sodium alginate into a series of binary Mg-Al, Cu-Al, and Zn-Al

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Figure 15.21 The reaction of α-pinene oxidation.

Figure 15.22 SEM images of Zn-FLDH/SA (A) and Zn-CFLDH/SA (B). FLDH, flaky layered double hydroxides; CFLDH, calcined flaky layered double hydroxides.

flaky LDH. Then the NCs were calcined and employed for the catalytic oxidation of α-pinene using 30 wt.% hydrogen peroxide as oxidant. SEM images of both Znflaky LDH/sodium alginate NC and the calcined one showed petal-like structures with many cavities (Fig. 15.22). The observation of interleaved flexible curled nanosheets was attributed to the extra expansion of LDH as a result of large ions of inserted sodium alginate between its sheets. Therefore, by such a huge increase in the interlayer spacing of LDH, the layers completely disappeared and the LDHs appeared as monolayers, which then created Zn-flaky LDH/sodium alginate nanoflakes. The calicined type of Zn-flaky LDH/sodium alginate NC maintained the flake-like morphology but more cavities were observed which were attributed to peeling out the water during the calcination procedure. According to the results, the calcined Zn-flaky LDH/sodium alginate NC had better catalytic performance for the oxidation of α-pinene compared with the other method. By using this catalyst, α-pinene conversion and the selectivity of α-pinene epoxide, verbenol and verbenone could reach 69.6% and 29.1%, 39.6% and 12.0%, respectively. Due to the order of catalytic activity (calcined Zn-flaky LDH/sodium alginate . Zn-flaky LDH/sodium alginate . calcined Zn-flaky LDH . Zn-flaky LDH) it is concluded that the existence of sodium alginate has the main role in α-pinene oxidation. Use as an adsorbent is another example of the application of alginate/LDH NC. Sebastian et al. (2014) prepared alginate/Mg-Al LDH NCs (with 3, 5.9, 11, and 20 wt.% alginate concentrations) by in situ coprecipitation method and used them for the removal of anionic dyes (Acid Green 25 and Acid Green 27) from water. XRD (Fig. 15.23) and SEM (Fig. 15.24) were used to characterize the alginate/MgAl LDH NCs. As can be seen, the XRD pattern of MgAl-Alg (20%) NC showed a

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Figure 15.23 X-ray diffraction patterns of MgAl LDH and MgAl-Alg composites with varying amount of sodium alginate. Alg, sodium alginate.

Figure 15.24 SEM images of MgAl, sodium alginate and their composites. (A) MgAl LDH; (B) sodium alginate, (C) MgAl-Alg (3%), (D) MgAl-Alg (5.9%), (E) MgAl-Alg (11%), and (F) MgAl-Alg (20%). Alg, sodium alginate.

shift in 003 and 006 diffraction peaks, from higher to lower 2θ values, indicating an increase in the interlayer space of the layered clay after composite formation. This revealed that alginate ions, when present in high concentration, get intercalated into the interlayer space of the layered clay structure. According to the SEM images, pristine MgAl crystals were flake-like with sharp edges, stacked in the form of layers (Fig. 15.24A), whereas sodium alginate (Fig. 15.24B) had a smooth surface. Small alginate particles can be seen spread over the clay surface and between layers of the clay, at low alginate concentrations (Fig. 15.24C,D). At higher concentrations (Fig. 15.24E,F), alginate started forming a smooth continuous/semicontinuous layer

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Figure 15.25 Influence of sodium alginate concentration on the adsorption capacity of MgAl-Alg composites for the dyes, AG 25 and AG 27 (wt. of adsorbent: 0.1 g; vol. of adsorbate: 50 cm3; adsorption time: 3 h; adsorption temp.: 25 C). Alg, sodium alginate; AG, Acid Green.

on the clay. The results showed that the adsorption behavior of the NCs was superior to that of the pristine clay, however, it varied with the alginate concentration in the NC. The maximum adsorption capacity of the composite was enhanced by 51% for Acid Green 25 and 160% for Acid Green 27, compared to the pristine layered clay sample. The maximum adsorption rate increased by 48% and 147% for the adsorbates AG 25 and AG 27, respectively. Fig. 15.25 shows that the adsorption capacity for both dyes was varied with the sodium alginate concentration and passed through a maximum at an alginate concentration of 5.9%. The increase in adsorption capacity may be directly correlated with the increase in specific surface area of the NCs up to an alginate concentration of 5.9%.

15.2.1.5 Other polysaccharides There are some reports for the preparation of LDH NCs with other polysaccharides. Carrageenan was incorporated with LDH and characterized by Gwak et al. (2016). Darder et al. (2005) synthesized bio-NCs by intercalation of anionic polysaccharides including alginic acid, pectin, κ-carrageenan, ι-carrageenan, and xanthan gum in [Zn2Al(OH)6]Cl,nH2O LDH. The “coprecipitation” versus the “reconstruction” method was confirmed as the best method for the intercalation of such highmolecular-weight biopolysaccharides within the LDH. The “reconstruction” procedure from the calcined LDH in the presence of the anionic polysaccharides only resulted in a partial intercalation of the organic guest. In agreement with the fact that most of the studied biopolymers interact strongly with calcium ions producing homogeneous gels, the prepared biopolymer/LDH NCs were operative as active

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Figure 15.26 (A) Potentiometric responses of the CPEs prepared with the [Zn2Al]Cl material (K) and the ι-carrageenan-LDH NC (’) in CaCl2 solutions of increasing activity. (B) Potentiometric responses of the PVC membrane-based electrodes prepared with the alginateLDH (K), ι-carrageenan-LDH (’), pectin-LDH (▼), and κ-carrageenan-LDH (▲) NCs in CaCl2 solutions of increasing activity. Source: Adapted from Darder, M., Lo´pez-Blanco, M., Aranda, P., Leroux, F., Ruiz-Hitzky, E., 2005. Bio-nanocomposites based on layered double hydroxides. Chem. Mater., 17(8), 19691977, with kind permission of the American Chemical Society.

phases of sensors for the recognition of calcium ions. Hence, the biopolymer/LDH NCs were incorporated in carbon paste or PVC matrices for the development of potentiometric sensors. These devices were applied to calcium determination by direct potentiometry and the best responses were obtained for the sensors based on alginate/LDH and ι-carrageenan-LDH NCs (Fig. 15.26). One of the other applications for carrageenan/LDH NCs is catalytic performance for CC bond formation, which was reported by Mahdi et al. (2015). Huang et al. (2013) synthesized a dextran-magnetic LDH-fluorouracil NC for drug delivery. Pectin is another polysaccharide which has been incorporated with LDH for drug delivery (Darder et al., 2005; Gwak et al., 2016). Gorrasi et al. (2012) prepared LDH with intercalated active molecules: benzoate, 2,4-dichlorobenzoate, parahydroxybenzoate, and ortho-hydroxybenzoate, incorporated into apple pectin. Incorporation of these active molecules gave antimicrobial properties to the NC films, indicating the potential application of prepared NCs in the packaging field and opened new perspectives in using pectin-antimicrobials as coating agents for a wide number of packaging polymers. The prepared NCs were characterized with XRD analysis (Fig. 15.27), which showed the absence of the peak corresponding to the basal spacing of the LDH hybrids in the composite samples, suggesting the exfoliation of the filler in all cases. Thermal, mechanical, and barrier properties of the prepared NCs were investigated. TGA showed a better thermal resistance of pectin in the presence of fillers, especially para-hydroxybenzoate and ortho-hydroxybenzoate. Such an improvement could be due either to the LDH layers that created a more tortuous path, lowering the diffusion of oxygen, or with a protecting

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Intensity (a.u)

(A) d c b

5

10

15

20

25

30

35

a 40

θ 2θ Intensity (a.u)

(B)

a b c d e 5

10

15

20

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35

40



Figure 15.27 X-ray diffraction patterns of (A): (a) LDH-Bz, (b) LDH-DCBz, (c) LDH-oOHBz, (d) LDH-p-OHBz; and (B): (a) pectin, (b) pectin/LDH-Bz, (c) pectin/LDH DCBz, (d) pectin/LDH-o-OHBz, (e) pectin/LDH-p-OHBz. Bz, benzoate; DCBz, 2,4-dichlorobenzoate; P-OHBz, p-hydroxybenzoate; o-OHBz: o-hydroxybenzoate. Source: Adapted from Gorrasi, G., Bugatti, V., Vittoria, V., 2012. Pectins filled with LDHantimicrobial molecules: Preparation, characterization and physical properties. Carbohyd. Polym., 89(1), 132137, with kind permission of Elsevier.

effect of LDHs increasing the thermal stability of the biopolymer. Such increasing of thermal stability was confirmation of a strong interaction between the pectin and the inorganic phase. Mechanical properties showed an improvement of elastic modulus in particular for the LDH-para-hydroxybenzoate nanohybrid, probably for stronger interactions between pectin matrix and nanohybrid layers via formation of hydrogen bonds, better favored by the para-hydroxybenzoate molecule. Barrier properties (sorption and diffusion) to water vapor showed an improvement in the dependence on the intercalated active molecule, the best improvement was achieved for NCs containing para-hydroxybenzoate molecules, suggesting that the interaction between the filler phase and the polymer plays an important role in sorption and diffusion phenomena. Antimicrobial activity of the NC films was examined by storing them at room temperature and humidity conditions, along with the control pectin films (Fig. 15.28). As can be seen, mold formation was noticed in the pectin films after 2 weeks of storage, but there was no such indication in the NC films even after 12 months. These results clearly suggested the potential of utilizing pectin films enriched with LDH intercalated antimicrobial compounds as novel packaging materials. Other polysaccharides such as xanthan gum and microbial polysaccharides such as welan gum, scleroglucan, and EPS I (a novel polysaccharide with structural

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Figure 15.28 Pictures from cast film of pectin and composites with nanohybrids after storage for 12 months at ambient temperature. Source: Adapted from Gorrasi, G., Bugatti, V., Vittoria, V., 2012. Pectins filled with LDHantimicrobial molecules: Preparation, characterization and physical properties. Carbohyd. Polym., 89(1), 132137, with kind permission of Elsevier.

similarities to xanthan gum) have been used in the preparation of LDH NCs (Darder et al., 2005; Gwak et al., 2016; Plank et al., 2012). Plank et al. (2012) investigated the intercalation ability of three microbial polysaccharides, welan gum, scleroglucan, and EPS I into the Zn-Al-LDH (Fig. 15.29). The NCs were synthesized by direct coprecipitation of Zn(NO3)2 and Al(NO3)3 in the polysaccharide solutions at pH B 8.5. Results from XRD and TEM (Fig. 15.30), proved that welan gum was successfully intercalated into the ZnAlLDH structure, while neutral scleroglucan failed to be intercalated. Instead, this biopolymer was only surface-adsorbed on inorganic CaAlOHLDH platelets, as was evidenced by dewashing experiments. As can be seen in Fig. 15.30, well-ordered layered structures with an average d-spacing of B 0.9 nm could be observed for pure LDH, whereas more disordered layers at much higher interlayer distances were found for the sample incorporating welan gum. This despite the fact that, in the case of scleroglucan, only layered structures characteristic of LDH could be observed, confirming that no intercalation into the LDH framework occurred with this biopolymer. In contrast to regular xanthan gum, EPS I was intercalated into the LDH structure to give a sharp X-ray reflection representing a

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Figure 15.29 Chemical structure of the microbial polysaccharides, welan gum and scleroglucan.

Figure 15.30 TEM images of the pure [Zn2Al]NO3LDH and of coprecipitates from welan gum and scleroglucan.

d-spacing of 2.77 nm. This behavior proved that slight modifications of the polysaccharide could greatly improve its intercalation ability. It was found that the intercalation ability of these biopolymers depends on two main factors; the charge and the steric position of the anionic functions in the biomolecule. Successful incorporation

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was only possible when the biopolymer possessed an anionic charge, which was sufficient to compensate a significant portion of the positive charge in the inorganic frame. Additionally, intercalation was more favored when the anionic charges were present on the backbone of the biopolymer, instead of its side chains. Therefore, when LDHs were utilized for chemical encapsulation of such biopolymers, for example, for medical applications to provide a time-controlled release, these two factors should be taken into account. Agarose and hyaluronic acid are other polysaccharides which have used for the preparation of bioNC with LDH for drug delivery (Gwak et al., 2016)

15.2.2 Protein/layered double hydroxide nanocomposites Proteins are a great choice between natural polymers for the preparation of composite materials. They exist in either globular or fibrous structures. The globular proteins are spherical in shape and coupled by an ordering of hydrogen, ionic, hydrophobic, and covalent (disulfide) bonds, while the fibrous ones are completely extended and arranged in parallel form, commonly via a hydrogen bond. Gluten, milk protein, and soy protein are employed in the production of edible films, despite the fact that keratin is employed in the fabrication of nanofiber, film, and composites for material fields. It seems that protein-based biopolymers can be employed in the areas of biomaterials, packaging materials, and in coatings industries in the future (Gupta and Nayak, 2015). Similar to polysaccharides, proteins have some disadvantages, such as poor mechanical, thermal, barrier, water resistance properties, etc., which can be improved by the incorporation of low loading of NFs into them. For example John and Thomas (2012) reviewed soy protein NCs which were reinforced with inorganic NFs such as clay, carbon nanotubes, etc. Also, there are some reports on the preparation of protein/LDH NCs (Yasutake et al., 2008) for bone tissue engineering (Fayyazbakhsh et al., 2011, 2012, 2017), drug delivery (Gwak et al., 2016), and fluorescent biosensors (Zhang et al., 2017b). As an interesting example we can point to the preparation of the gelatin/LDH and gelatin/LDH-hydroxyapatite NC for bone tissue engineering scaffolds using coprecipitation and solvent-casting techniques, as reported by Fayyazbakhsh et al. (2017). They investigated both in vitro and in vivo studies. As can be seen in Fig. 15.31, SEM images of NCs showed the likeness between the microstructures of both scaffolds with natural spongy bone and interconnected macropores. The porosities of 90% and 92%, as well as Young’s modulus of 19.8 and 12.5 GPa, were obtained for gelatin/LDH and gelatin/LDH-hydroxyapatite scaffolds, respectively. Moreover, the SEM images revealed that between two scaffolds, the gelatin/ LDH-hydroxyapatite with needle-like secondary hydroxyapatite crystals showed better bioactivity. Alkaline phosphatase activity and alizarin red staining results indicated that gelatin/LDH-hydroxyapatite scaffolds improved bone-specific activities compared to gelatin/LDH scaffolds, as well as a control sample. Finally, all the scaffolds suffered implantation (Fig. 15.32) on animals and by in vivo results indicated that both implanted groups (acellular and cell-seeded groups) did not show a serious inflammatory response and provided new bone

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Figure 15.31 Micropraphs of (A) pure LDH/GEL scaffold, (B) LDH-HA/GEL scaffold, (C) natural bone microstructure for comparison with synthetic scaffolds (high structural similarity can be seen between bone and LDH-HA/GEL interconnected scaffold) (magnification: 100 3 ). The SEM images of scaffolds after immersing in SBF. (D, E, F) Pure LDH/GEL scaffold, (G, H, I) LDH-HA/GEL scaffold with different magnification. (D, G) The nucleation of secondary HA through the pores after day 3, (E, H) the growth of secondary HA crystals and scaffold degradation after day 14, (F, I) needle-like morphology of secondary HA on the surface of scaffolds after day 21. GEL, gelatin; HA, hydroxyapatite. Source: Adapted from Fayyazbakhsh, F., Solati-Hashjin, M., Keshtkar, A., Shokrgozar, M.A., Dehghan, M.M., Larijani, B., 2017. Novel layered double hydroxides-hydroxyapatite/gelatin bone tissue engineering scaffolds: Fabrication, characterization, and in vivo study. Mater. Sci. Eng. C., with kind permission of Elsevier.

formation. Consequently, gelatin/LDH-hidroxyapatite scaffold satisfied the crucial demands of BTE and had the potential to be employed in orthopedic and reconstructive surgery.

15.2.3 PHA/layered double hydroxide nanocomposites PHAs are bacterial biopolyesters that generally consist of (R)-3-hydroxy fatty acids, with various side chains. The pendant group (R) varies from C1 to C13 and is saturated or unsaturated, or contains a substituent (Roy and Visakh, 2014). Similar to

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Layered Double Hydroxide Polymer Nanocomposites

Figure 15.32 The general implantation process: (A) fat tissue sampling from intrascapular area, (B) OM image of cultured ASCs (magnification 40 3 ), (C) measuring and marking the defect site on radius bone, (D) cutting the radius to create the defect by surgical saws, (E) with/without ASC scaffolds were soaked in culture media and then swelled, the swollen scaffold vs. dissected bone. The size differences can be seen but the swollen scaffold showed a flexible behavior, therefore it was placed in the defect site, (F) the scaffold placed in the defect site (i.e., implanted), (G) muscle sutures after implanting, (H) dermal sutures, (I) radiography position and the aluminum graded phantom to evaluate the density of newly formed bone. Source: Adapted from Fayyazbakhsh, F., Solati-Hashjin, M., Keshtkar, A., Shokrgozar, M.A., Dehghan, M.M., Larijani, B., 2017. Novel layered double hydroxides-hydroxyapatite/gelatin bone tissue engineering scaffolds: Fabrication, characterization, and in vivo study. Mater. Sci. Eng. C., with kind permission of Elsevier.

other biopolymers, to overcome the disadvantages of PHAs, there are reports for the preparation of hybrid NCs with PHA, a blend or a copolymer of PHA (Roy and Visakh, 2014), especially with LDH and the authors have usually investigated the mechanical and thermal properties of prepared NCs (Bunea et al., 2016; Ciou et al., 2014; Dagnon et al., 2009a; DeGruson, 2014; Liau et al., 2014; Pak et al., 2013; Zhang et al., 2012). For example, Ciou et al. (2014) prepared bio-NCs based on poly(3-hydroxybutyrate) and a copolymer of poly(3-hydroxybutyrate), poly(3hydroxybutyrate-co-3-hydroxyvalerate) with poly(3-hydroxyvalerate) content of

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5 and 12 wt.%, as matrices and an oleate-modified LDH as NF. They also investigated the degradation behavior of bio-NCs by using Caldimonas manganoxidans as a microbial catalyst. The oleate intercalated LDH was synthesized by a novel one-step coprecipitation forming structure of bilayer and monolayer in which the molecules were expected to lay on or tilt at a fixed angle to enlarge the interlayer ˚ ). XRD and TEM images of both NCs 5 wt.% (Fig. 15.33) showed spacing (34.0 A that most of the hydroxide layers of LDH were exfoliated and randomly distributed in both matrices.

Figure 15.33 TEM micrographs of 5 wt.% (A) PHB/m-LDH and (B) PHBV12/m-LDH NCs. TEM micrograph obtained from the embedded and cut 5 wt.% PHB/m-LDH NC is shown in (C). PHB, poly(3-hydroxybutyrate); PHBV12, poly(3-hydroxybutyrate-co-3-hydroxyvalerate) with 12% poly(3-hydroxyvalerate); m-LDH, modified Mg-Al LDH. Source: Adapted from Ciou, C.-Y., Li, S.-Y., Wu, T.-M., 2014. Morphology and degradation behavior of poly (3-hydroxybutyrate-co-3-hydroxyvalerate)/layered double hydroxides composites. Eur. Polym J., 59, 136143, with kind permission of Elsevier.

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Table 15.4 Dynamic storage modulus and crystallinity (Xc) of PHB/m-LDH and PHBV/ m-LDH NCs Sample code PHB 1 wt.% PHB/m-LDH 3 wt.% PHB/m-LDH 5 wt.% PHB/m-LDH PHBV5 1 wt.% PHBV5/m-LDH 3 wt.% PHBV5/m-LDH 5 wt.% PHBV5/m-LDH PHBV12 1 wt.% PHBV12/m-LDH 3 wt.% PHBV12/m-LDH 5 wt.% PHBV12/m-LDH

E0 at 250oC (MPa) % 1680 2280 2710 3540 1000 1520 1930 2060 670 960 1220 1610

Xc (%) 66 65.4 64.7 65.1 51.5 53.2 52.4 52.0 36.5 36.1 37.1 37.2

PHB, poly(3-hydroxybutyrate); PHBV: poly(3-hydroxybutyrate-co-3-hydroxyvalerate); m-LDH, modified Mg-Al LDH. Source: Adapted from Ciou, C.-Y., Li, S.-Y., Wu, T.-M., 2014. Morphology and degradation behavior of poly (3hydroxybutyrate-co-3-hydroxyvalerate)/layered double hydroxides composites. Eur. Polym J., 59, 136143, with kind permission of Elsevier.

It was shown that the storage modulus of both poly(3-hydroxybutyrate) and poly (3-hydroxybutyrate-co-3-hydroxyvalerate) and their NCs depended on the temperature and the storage modulus of both, and were highest at 250 C and decreased with increasing temperature. As can be seen in Table 15.4, at 250 C, the storage modulus of all NCs was increased by introducing and increasing the amount of modified LDH, which was attributed to the reinforcement effect of the presence of the rigid LDH layers as well as the superior interaction between the polymers and modified LDH, leading to a prominent improvement in the stiffness of the polymer matrix. In terms of biodegradability, it was observed that although the introduction and increasing of modified LDH in all poly(3-hydroxybutyrate)-based film improved their mechanical properties, but the addition of modified LDH up to 5% showed an insignificant difference for biodegradability. Microbial degradation offered that the poly(3-hydroxybutyrate) depolymerase of C. manganoxidans was governed by the exo-type hydrolysis activity, where the degradation of poly(3hydroxybutyrate) polymer was started from both ends of polymer chains. Pak et al. (2013) used a blend of poly-3 hydroxybutyrate/poly(butyleneadipateco-terephthalate) (PHB/PBAT) as matrix for the preparation of NCs with 1, 2, 3, 4, and 5 wt.% of LDH. The Zn3Al LDH was synthesized using a coprecipitation method and modified with stearate anion surfactant via an ion exchange reaction. The stearate anions were successfully intercalated into pristine LDH confirmed by the observation of the alkyl group in the FT-IR spectrum. According to the XRD pattern of pristine LDH and modified LDH, the basal spacing of the LDH was ˚ after modification with stearate. The remaining enhanced from 8.77 to 24.94 A peaks of PHB/PBAT blend in the XRD patterns of NCs (Fig. 15.34C) suggesting

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Figure 15.34 (A and B) TEM micrographs of PHB/PBAT/2.0 wt.% stearate-Zn3Al LDH. (C) XRD pattern for (a) pure PBAT, (b) pure PHB, (c) PHB/PBAT blend, and NCs of 1.0, 2.0, 3.0, 4.0, and 5 wt.% of stearate-Zn3Al LDH (dh). PHB, poly-3 hydroxybutyrate; PBAT, poly(butyleneadipate-co-terephthalate).

that the polymer blend crystalline lattice was not modified appreciably in the existence of LDH. TEM images (Fig. 15.34A,B) of NCs indicated that the NCs formed an intercalated structure as the modified NF percentage increased, thus providing better compatibility between the blend matrix and the galleries of LDH. It was observed that the tensile strength of PHB/PBAT was decreased by increasing the percentage of PBAT from 10% to 50% compared to the neat PHB, while the tensile modulus and elongation at break of this blend were increased by the addition of 10% PBAT and then decreased by increasing up to 50%. Therefore, the PHB/ PBAT blend with 90/10 ratio was chosen as the best ratio among others for investigation of the mechanical properties of NCs. As can be seen in Fig. 15.35, adding 2.0 wt.% modified LDH into the blend matrix enhanced the elongation at break (from 35.03% up to 54.58%), with an improvement of 56% compared to that of the unfilled blend. This was due to the presence of the long-chain hydrocarbon parts of stearate anions in the modified LDH that acted as a plasticizer. However, after more addition of 3.05.0 wt.% of LDH, decreasing elongation at break was observed, which may be due to the presence of large agglomerates which resulted in more brittle NCs. Zhang et al. (2012) prepared the exfoliated bio-NCs based on poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P(3,4)HB) and Co-Al LDH (SSLDH) (1, 3, 5 wt.%)

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Figure 15.35 Tensile strength, tensile modulus, and elongation at break of PHB/PBAT blends with different stearate-Zn3Al LDH content. PHB, poly-3 hydroxybutyrate; PBAT, poly(butyleneadipate-co-terephthalate).

via melt intercalation. Moreover, mechanical, thermal, and flame-retardant properties for these bio-NCs were systematically investigated. The TEM (Fig. 15.36) images clearly reveal that the LDH sheets were mainly exfoliated and disorderly dispersed in the P(3,4)HB matrix and confirmed that exfoliated P(3,4)HB/SS-LDH bio-NCs were successfully prepared by the melt blending process. DMA (dynamic mechanical analysis) testing, which is one of the techniques commonly used to characterize the time, frequency, and temperature dependency of the viscoelastic nature of polymers, showed that a small amount of LDH could significantly enhance the storage modulus and thermomechanical properties of bio-NCs. TGA results indicated that the thermal stability of the bio-NCs decreased with the increasing loading of LDH. The reduction of thermal stability was probably related to the degradation of the SS-LDH, which influenced the decomposition of P(3,4) HB. However, the mass loss rates for the bio-NCs were reduced as the weight percentage of LDH increased, which was possibly attributed to the barrier effect of the nanosheets of LDH. Thermal combustion properties of P(3,4)HB/SS-LDH bio-NCs were evaluated by a microscale combustion calorimeter (MCC), which is a rapid, and small-scale flammability testing instrument to research polymer combustion properties. From MCC data (Table 15.5), it was found that the flame retardancy of P(3,4)HB/SS-LDH bio-NCs was enhanced with the addition of LDH content. As can be seen, compared to neat P(3,4)HB, the PHRR (peak of heat release rate), HRC (heat release capacity), which is an important parameter of the fire hazard,

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Figure 15.36 TEM images of P(3,4)HB/5 wt.% SS-LDH bio-NC (A) at low magnification and (B) at high magnification. P(3,4)HB, poly(3-hydroxybutyrate-co-4-hydroxybutyrate); SSLDH: Co-Al LDH. Source: Adapted from Zhang, R., Huang, H., Yang, W., Xiao, X., Hu, Y., 2012. Preparation and characterization of bio-nanocomposites based on poly (3-hydroxybutyrate-co-4hydroxybutyrate) and CoAl layered double hydroxide using melt intercalation. Compos. Part A: Appl. Sci. Manuf., 43(4), 547552, with kind permission of Elsevier.

Table 15.5 MCC results for neat P(3,4)HB and its bio-NCs Samples

PHRR (W/g)

THR (kJ/g)

HRC (J/g K)

TPHRR ( C)

P(3,4)HB P(3,4)HB/1 wt.%SS-LDH P(3,4)HB/3 wt.%SS-LDH P(3,4)HB/5 wt.%SS-LDH

761.9 518.6 468.7 435.7

13.6 13.3 12.6 11.8

743 525 472 451

298.5 300.7 289.2 282.8

HRC, heat release capacity, 6 5 J/g K; PHRR, peak of heat release rate, 6 5 W/g; THR, total heat release, 6 0.1 kJ/ g; TPHRR, temperature at PHRR, 6 2 C. Source: Adapted from Zhang, R., Huang, H., Yang, W., Xiao, X., Hu, Y., 2012. Preparation and characterization of bio-nanocomposites based on poly (3-hydroxybutyrate-co-4-hydroxybutyrate) and CoAl layered double hydroxide using melt intercalation. Compos. Part A: Appl. Sci. Manuf., 43(4), 547552, with kind permission of Elsevier.

and THR (total heat release) values for P(3,4)HB/LDH bio-NCs were remarkably reduced, resulting from the barrier effect of exfoliated LDH layers. Bunea et al. (2016) synthesized novel biocomposites based on poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBHV) and modified LDH for tissue engineering. For this purpose, LDH was modified with SDS by a coprecipitation method. The

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Figure 15.37 SEM microphotographs reveal the morphology of: PHBHV-film (A), PHBHV/ LDH-SDS 1% (B), and PHBHV/LDH-SDS 3% (C). PHBHV, poly(3-hydroxybutyrate-co-3hydroxyvalerate).

prepared NF (LDH-SDS) (1, 2, 3 wt.%) was incorporated into PHBHV and the biocomposites were obtained by a solvent casting technique. To investigate the morphology of NCs, SEM was employed for PHBHV film and PHBHV/LDH-SDS NCs (Fig. 15.37). As can be seen, PHBHV microparticles were dispersed in a compact structure in the PHBHV film. A compact surface could be observed in the composites where the LDH-SDS particles were embedded into the polyester film. TGA curves of PHBHV/LDH-SDS (1% and 2%) revealed that the presence of the clay did not influence the thermal properties of the composite films. The composites started to decompose at lower temperatures compared to PHBHV. This may be due to the presence of dodecyl sulfate. The residue of these samples increased compared to the pure polymer. The biocompatibility of these novel biocomposites was studied in relation to human adipose-derived stem cells. Live/dead fluorescence microscopy assay [based on the simultaneous staining of live (green-labeled) and dead (redlabeled) cells] was performed to evaluate hASC viability in direct contact with PHBHV film (B) and PHBHV_LDH-SDS composites with 1% (B1), 2% (B2), and

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Figure 15.38 Confocal fluorescence microscopy micrographs revealing live and dead cells on PHBHV film (B) and PHBHV/LDH-SDS composites with 1% (B1), 2% (B2), and 3% (B3) LDH-SDS, after 24 h of culture. PHBHV, poly(3-hydroxybutyrate-co-3hydroxyvalerate).

3% (B3) LDH-SDS biomaterials (Fig. 15.38). The results showed that hASCs survived after 24 h of culture in contact with all tested biomaterials. Moreover, no significant differences were detected in the ratio between live and dead cells on the PHBHV film and PHBHV/LDH-SDS composites. Consequently, due to good biocompatibility, these materials could be promising candidates for in vivo testing for wound dressing applications. Dagnon et al. (2009a) reported that the thermomechanical performance of poly [(3-hydroxybutyrate)-co-(3-hydroxyvalerate)] (PHBV) is associated with its crystallization. They suggested enhanced nucleation using a stearate-functionalized LDH (LDH-SA) as a solution. Stearate anions were intercalated into the LDH via an ion exchange reaction. Then, the various amounts of prepared NF (1, 3, 5, and 7 wt.%) were incorporated into the PHBV matrix and the NC films were obtained via a solution casting technique. According to the XRD results, increasing the basal spacing of the LDH-SA in all hybrids indicated some degree of intercalation of PHBV chains within the LDH-SA interlayer galleries. On the other hand, the existence of sharp Bragg peaks after solution casting indicated that the dispersed LDH-SA still retains an ordered structure and proved that the LDH-SA crystals remain essentially integral within the PHBV matrix and, consequently, exfoliation of LDH-SA did not occur. TEM showed the intercalated structure of the LDH-SA and there was limited dispersion of LDH-SA in the PHBV matrix as the LDH-SA concentration increased (Fig. 15.39). The effect of LDH-SA on the crystallinity of PHBV was also evaluated with WAXD. According to WAXD, PHBV is a semicrystalline polyester (rhombic cell). The reflections of all NCs were observed at the same values as for the neat biopolymer and this indicated that, in the NCs, PHBV crystallized in its typical crystalline form and its unit cell was not changed after being incorporated in LDH-SA. However, lamella size for the (020) and (021) directions increased with increasing LDH-SA indicating that the crystalline lamella size increased in the presence of LDH-SA. The TGA results showed that the NF destabilized the matrix, leading to decreased thermal stability of the NC with increasing LDH-SA loading. This could be due to the release of water from LDH, which hydrolyzed the ester bonds of PHBV. According to Table 15.6, the thermomechanical properties were modified by the presence of LDH-SA. As can be seen from the maxima of tan δ, Tβ of the matrix polymer (c. 2101 C), which is conventionally associated with

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Figure 15.39 TEM micrographs of PHBV/SAnNCs: (A) PHBV/SA1; (B) PHBV/SA3; (C) PHBV/SA5; (D) PHBV/SA7. PHBHV, poly(3-hydroxybutyrate-co-3-hydroxyvalerate); SA, stearate anion. Source: Adapted from Dagnon, K. L., Chen, H. H., Innocentini-Mei, L. H., & D’Souza, N. A. (2009a). Poly [(3-hydroxybutyrate)-co-(3-hydroxyvalerate)]/layered double hydroxide nanocomposites. Polymer International, 58(2), 133141, with kind permission of John Wiley and Sons. Table 15.6 Dynamic thermomechanical properties of PHBV and PHBV/SAn NCs Sample

Tg ( C)

Tβ ( C)

E0 (GPa) at 2125 C

E0 (GPa) at 25 C

PHBV PHBV/SA1 PHBV/SA3 PHBV/SA5 PHBV/SA7

13 12 10 10 10

2 101.8 2 102.5 2 102.4 2 102.6 2 102.0

6.61 ( 6 0.05) 6.92 ( 6 0.07) 7.59 ( 6 0.10) 8.13 ( 6 0.10) 7.24 ( 6 0.09)

2.45 ( 6 0.05) 2.40 ( 6 0.03) 3.21 ( 6 0.10) 3.40 ( 6 0.10) 2.87 ( 6 0.07)

PHBHV, poly(3-hydroxybutyrate-co-3-hydroxyvalerate); SA, stearate anion. Source: Adapted from Dagnon, K. L., Chen, H. H., Innocentini-Mei, L. H., & D‘Souza, N. A. (2009a). Poly [(3hydroxybutyrate)-co-(3-hydroxyvalerate)]/layered double hydroxide nanocomposites. Polymer International, 58(2), 133141, with kind permission of John Wiley and Sons.

local crankshaft motion of the (CH2)n segment, was unaffected by the addition of LDH-SA. The Tg of the hybrids was also unaffected by the addition of LDH-SA. The PHBV-based NCs showed higher storage moduli than pure PHBV. The higher E0 values of the PHBV/SAn NCs reflected the reinforcement potential of LDH-SA in the biopolymer matrix. The increase in rubbery modulus could be ascribed to the reinforcing effect of the nanofiller. Although E0 generally increased with increasing LDH-SA content, PHBV/SA7 showed E0 lower than that of PHBV/ SA3 and PHBV/SA5. This could be attributed to increased aggregate formation, leading to a deterioration in the mechanical properties.

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15.2.4 PLA/layered double hydroxide nanocomposites PLA is a biocompatible and biodegradable polymer included in the class of aliphatic polyesters which is obtained from α-hydroxy acids. D-/L-lactic acid (D-/L-2hydroxy propionic acid) is the fundamental constituent of PLA, which is generated by carbohydrate fermentation or chemical synthesis. The properties of PLA are governed by the ratio between two enantiomers which has caused the generation of a broad range of PLA polymers to fit the performance demands. High-molecularweight PLA polymers (larger than 100,000 Da) can be synthesized by various methods, such as direct polycondensation, azeotropic dehydrative polycondensation, and ring-opening polymerization of lactide. PLA has been applied in different fields, particularly including the commodity field and industry products, such as packaging and service ware, fiber and nonwoven, engineering plastic, biodegradable hot melt adhesive, environmental remediation, paints, cigarette filters, 3D printing, and parts for space exploration, etc., as well as the field of biomedical materials including tissue engineering scaffolds, controllable drug delivery, surgical sutures, ideal fillers for soft-tissue augmentation, and mesh insertion for groin hernia repair (CastroAguirre et al., 2016; Ren, 2011). There are many reports of the synthesis, characterization, and applications of organic/inorganic hybrid materials based on PLA, many of which have utilized LDH as NF. In some of them neat LDH has been used, while in several reports to enhance the compatibility between NF and matrix, LDH has been modified with various molecules such as surfactants, antioxidants, polymer, lignosulfonate (Hennous et al., 2013), phosphinic acid derivative, ionic liquids (Ha and Xanthos, 2010; Livi et al., 2012), drugs (Dagnon et al., 2009b), etc. which are discussed below. On the other hand, copolymers of PLA with other polymers such as poly(lactic-co-glycolic acid) have also used as matrix (Chakraborti et al., 2011; Chakraborti et al., 2012). For example, Chiang et al. used both neat and modified LDH [modified with PLA-COOH by ion exchange process (Chiang et al., 2011; Chiang and Wu, 2010) and with γ-polyglutamate by melt blending process (Chiang and Wu, 2012)] and then incorporated the new NFs into the PLLA (PLA obtained from L-lactic acid). The morphology, water vapor permeability, as well as barrier, mechanical, and thermal properties of obtained NCs were investigated. LDH was modified by polylactide with carboxyl end group (PLACOOH) using an ionexchange process and then incorporated into the PLLA matrix via solution intercalation. XRD and TEM images (Fig. 15.40) of NCs demonstrated that the modified LDH was exfoliated and randomly distributed into the PLLA matrix. As can be seen in Table 15.7, the mechanical properties of the NC with 1.2 wt.% NF showed a significant increase in the storage modulus as compared to unfilled PLLA. However, by introduction of more modified LDH into the PLLA matrix, a decrease in the storage modulus of NCs was observed, which was probably due to the excessive amount of PLACOOH molecules with low mechanical properties. In another work, Chiang and Wu (2012) modified LDH with γ-polyglutamate via anion exchange method using LDHNO3 as a precursor. Then, both unmodified LDH and γ-polyglutamate modified LDH (γ-LDH) (1, 3 and 5 wt.%) were incorporated into the PLLA by a melt blending process. The XRD and TEM

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Figure 15.40 TEM micrographs of: (A) 5 wt.% PLLA/P-LDH and (B) 10 wt.% PLLA/PLDH NCs. P-LDH, PLA-COOH modified LDH. Source: Adapted from Chiang, M.-F., Wu, T.-M., 2010. Synthesis and characterization of biodegradable poly (L-lactide)/layered double hydroxide nanocomposites. Compos. Sci. Technol., 70(1), 110115, with kind permission of Elsevier.

Table 15.7 Dynamic storage modulus, degradation activation energies (Ea) and Mg and Al residues of PLLA and PLLA/P-LDH NCs Sample code

PLLA 0.4% PLLA/P-LDH 1.2% PLLA/P-LDH 2% PLLA/P-LDH 4% PLLA/P-LDH

Storage modulus (MPa) 220 C

80 C

459 750 865 718 651

105 175 231 164 158

Eaa (kJ/mol)

Rb

Mgc (wt.%)

Alc (wt.%)

158.04 140.50 129.52 105.17 102.92

0.9998 0.9993 0.9902 0.9978 0.9999

 0.020 0.137 0.579 1.110

 0.027 0.100 0.320 0.574

P-LDH, PLA-COOH modified LDH. a The degradation activation energies (Ea) were obtained from the Kissinger equation. b R: correlation coefficient. c Mg and Al ratio were obtained from ICP-AES experiment. Source: Adapted from Chiang, M.-F., Wu, T.-M., 2010. Synthesis and characterization of biodegradable poly (Llactide)/layered double hydroxide nanocomposites. Compos. Sci. Technol., 70(1), 110115, with kind permission of Elsevier.

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Figure 15.41 Bright-field TEM micrographs of (A) 5 wt.% PLLA-L and (B) 5 wt.% PLLAγL. The inset in (B) is the high-magnification TEM images of 5 wt.% PLLA-γL. L, LDH; γL, γ-polyglutamate modified LDH. Source: Adapted from Chiang, M.-F., Wu, T.-M., 2010. Synthesis and characterization of biodegradable poly (L-lactide)/layered double hydroxide nanocomposites. Compos. Sci. Technol., 70(1), 110115, with kind permission of Elsevier. Table 15.8 Mechanical properties of PLLA/LDH NCs Sample

Young’s modulus (MPa)

Tensile strength (MPa)

Elongation at break (%)

PLLA 1 wt.% PLLA-L 3 wt.% PLLA-L 5 wt.% PLLA-L 1 wt.% PLLA-γL 1 wt.% PLLA-γL 1 wt.% PLLA-γL

453 ( 6 16) 417 ( 6 46) 412 ( 6 14) 396 ( 6 13) 462 ( 6 18) 478 ( 6 25) 507 ( 6 44)

46.5 ( 6 2.8) 43.4 ( 6 3.3) 37.8 ( 6 3.1) 34.7 ( 6 2.4) 45.1 ( 6 5.6) 42.0 ( 6 1.4) 39.5 ( 6 2.5)

18.0 ( 6 2.9) 16.7 ( 6 1.6) 14.3 ( 6 1.5) 13.9 ( 6 1.0) 14.5 ( 6 0.3) 14.0 ( 6 0.6) 12.5 ( 6 1.1)

L, LDH; γL, γ-polyglutamate modified LDH. Source: Adapted from Chiang, M.-F., Wu, T.-M., 2010. Synthesis and characterization of biodegradable poly (Llactide)/layered double hydroxide nanocomposites. Compos. Sci. Technol., 70(1), 110115 with kind permission of Elsevier.

(Fig. 15.41) results of NCs revealed that the unmodified LDH with a certain amount of aggregates was unevenly distributed throughout the matrix, while γ-LDH allowed the formation of an intercalated NC. As can be seen in tensile properties of NCs in Table 15.8, the Young’s modulus of NCs based on γ-LDH was increased by increasing the content of the γ-LDH to 5 wt.%, while, when unmodified LDH was

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Figure 15.42 TEM images of PLA/stearate-LDH NCs with (A) 3.0, (B) 5.0, (C) 7.0, and (D) 10.0 wt.% stearate-LDH content. (Magnification is 200 3 .)

used, the modulus of NCs was slightly reduced by increasing the loading of unmodified LDH and was almost independent of the LDH contents. These results indicated that the aggregated and unevenly dispersed unmodified LDH in PLLA matrix would not act as a reinforcing filler for the final NCs. On the other hand, it can be seen that either for unmodified LDH or modified LDH, the tensile strength was decreased by increasing the loading of NF, which may be due to some inevitably agglomerated unmodified and modified LDH platelets in the PLLA matrix that acted as sites of stress concentration. Furthermore, elongation at break of two types of NCs was decreased by increasing the amount of NF as compared to pure PLLA. This observation was attributed to high stiffness provided by the LDH sheets and the restraints on the mobility of the molecular chains caused by possible interactions between hydroxide layers and PLLA backbones. Eili et al. (2012) studied the preparation and characterization of PLA/stearatemodified LDH (stearate-LDH) NCs. The bio-NCs were prepared by a solution casting method and from XRD and TEM (Fig. 15.42) analysis; the stearate-LDH lost its ordered stacking structure and was greatly exfoliated in the PLA matrix. The effect of stearate-LDH loading on tensile properties and soil biodegradation was studied. Significant improvement in elongation at break was observed as a result of the addition of 1.03.0 wt.% of stearate-LDH to the PLA. Moreover, results showed an enhancement in the biodegradation of PLA in soil by incorporation of stearate-modified LDH. This effect can be caused by the catalytic role of the stearate groups in the biodegradation mechanism leading to much faster disintegration of NCs than pure PLA.

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Figure 15.43 TEM micrographs of PLA/LDH-Bph at low (left) and high (right) magnification. Bph, 4-biphenyl acetic acid. Source: Adapted from Oyarzabal, A., Mugica, A., Mu¨ller, A.J., Zubitur, M., 2016. Hydrolytic degradation of nanocomposites based on poly (L-lactic acid) and layered double hydroxides modified with a model drug. J. Appl. Polym. Sci., 133(28), with kind permission of John Wiley & Sons.

Figure 15.44 Visual aspect of (A) neat PLA, (B) PLA/Bph, and (C) PLA/LDH-Bph after 0, 14, 22, 28, 35, 42, and 56 days under incubation in PBS (pH 5 7.2) at 37 C. Bph, 4biphenyl acetic acid. Source: Adapted from Oyarzabal, A., Mugica, A., Mu¨ller, A.J., Zubitur, M., 2016. Hydrolytic degradation of nanocomposites based on poly (l-lactic acid) and layered double hydroxides modified with a model drug. J. Appl. Polym. Sci., 133(28), with kind permission of John Wiley & Sons.

Stearate-modified LDH could also cause better flexibility of PLA (Mahboobeh et al., 2010). Drugs also can be used for the modification of LDH. Oyarzabal et al. (2016) incorporated the LDH into the PLA after modification with 4-biphenyl acetic acid (Bph) as a drug model. NCs were prepared by solvent casting with 5 wt. % of drug-modified LDH. The obtained NC had a partially exfoliated morphology as determined by TEM (Fig. 15.43). Moreover the hydrolytic degradation was carried out in a PBS solution at pH 7.2 and 37 C and compared with neat PLA with 5 wt.% Bph (Fig. 15.44). For PLA/Bph, an acid catalytic effect, caused by the drug, accelerated the PLA mass loss. However, for PLA/LDH-Bph, the presence of LDH produced a barrier effect that initially reduced the diffusion of the oligomers produced during hydrolytic degradation. Dagnon et al. (2009b) also functionalized LDH with ibuprofen (Ibu) and prepared PLLA/LDH-Ibu NCs by 1, 3, and 5 wt.% of LDH-Ibu using the solution

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Figure 15.45 TEM micrographs of the PLLA/LDH-Ibu NCs: (A) 1LDH-Ibu, (B) 3LDH Ibu, and (C) 5LDH-Ibu. Ibu: ibuprofen. Source: Adapted from Dagnon, K.L., Chen, H.H., Innocentini-Mei, L.H., D’Souza, N.A., 2009a) Poly [(3-hydroxybutyrate)-co-(3-hydroxyvalerate)]/layered double hydroxide nanocomposites. Polym. Int., 58(2), 133141; Dagnon, K.L., Ambadapadi, S., Shaito, A., Ogbomo, S.M., DeLeon, V., Golden, T.D., et al., 2009b. Poly (L-lactic acid) nanocomposites with layered double hydroxides functionalized with ibuprofen. J. Appl. Polym. Sci., 113(3), 19051915, with kind permission of John Wiley and Sons.

casting route. The TEM micrographs of the PLLA/LDH-Ibu NCs are shown in Fig. 15.45, and intercalated and exfoliated structures of the LDH-Ibu component were observed. It is clear, however, that the localized structure showed an intercalated/exfoliated dispersion and that aggregation and clustering of the intercalated/ exfoliated regions existed. Thus, the uniform dispersion of LDH-Ibu in the PLLA matrix was limited as the NF concentration increased. The authors examined the potential to decrease cell proliferation, while simultaneously increasing mechanical performance through LDH organically modified with Ibu dispersed in PLLA. For this purpose, smooth muscle cells (SMCs) were used for in vitro studies of the NCs. LDH-Ibu incorporated in PLLA inhibited the proliferation of SMCs after 5 days of exposure. By comparing Ibu, PLLA/Ibu, and LDH functionalized with Ibu incorporated into PLLA, it was concluded that the Ibu component was the dominant cause of the decreased cell proliferation. Incorporating Ibu into the LDH resulted in effective drug release, which led to a multibiofunctional NC with a significant mechanical advantage. Leng et al. (2015) studied the structureproperty relationships of PLA/SDBSmodified LDH NCs which were produced by the melt blending procedure. They investigated the various properties of obtained NCs, such as the influence of the modification of LDH with SDBS on the dispersity of NF in the matrix, degradation stability, crystallinity, dielectric relaxation behavior, and thermal properties. Smallangle X-ray scattering showed the increase in the space of layers in LDH after SDBS incorporation indicated the successful modification process. This observation was also observed in terms of NC and from this it was concluded that the NCs had a partly exfoliated morphology with mixed nanostacks. It is noteworthy that size exclusion chromatography (SEC) measurements showed a small increase in the degradation of PLA due to the LDH NPs but it was too small to influence the properties of NCs significantly. This is despite the fact that Gerds et al. (2012) reported the degradation of PLLA during melt processing with LDH. They proposed that the

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state of LDH dispersion in the PLA matrix, which influenced the accessibility of PLA to the hydroxide layer, catalytically active Mg sites and release of water by LDH during melt compounding were causative factors in the PLA degradation, while the latter had the minor role in the degradation process. In addition to solution casting, melt processing (Katiyar et al., 2011) and other methods for the preparation of PLA/LDH NCs, in situ polymerization and in situ bulk polymerization of lactide monomer have also used (Katiyar et al., 2010; Nogueira et al., 2016). Although by using this method, because of chain termination via LDH hydroxyl groups and/or metal-catalyzed degradation, the significant reduction in the molecular weight of PLA was observed, this method was effective for the preparation of NC due to ensuring good distribution of LDH in the matrix (Katiyar et al., 2010). Neat and modified LDH (LDH-CO3 and laurate-modified) (Katiyar et al., 2010), salicylate intercalated and sebacate intercalated were used (Nogueira et al., 2016). PLA/LDH NCs can be used as a packaging material (Bugatti et al., 2013). For example, Demirkaya et al. (2015) fabricated PLA/LDH NC films by utilizing two different Mg:Al ratios of LDH (2:1 and 3:1) via the solution casting method. To improve the compatibility between NF and matrix, they also modified the LDH by sodium dodecyl sulfate (SDS) surfactant via intercalation between the layers of LDH and the properties of NCs with both neat and modified LDH were compared. As can be seen in the AFM images of NCs in Fig. 15.46, the intercalation of SDS in LDH caused better dispersion and exfoliation of NF into the matrix. Furthermore, using modified NF resulted in better mechanical properties and Tg. The highest oxygen and water vapor permeability values were observed for the NCs, prepared with LDH-SDS, which was suitable for packaging materials. Compared to pure PLA (497 cm3/m2.bar.day) and NCs with 3, 5, and 10 wt.% of LDH, the films containing 5% of modified NF at Mg:Al ratio of 3:1 (380 cm3/m2. bar.day) and 5% of modified NF at an Mg:Al ratio of 2:1 showed the best oxygen barrier. The oxygen barrier property for SDS-modified LDH containing NCs was found to increase by 23%, while unmodified LDH containing NCs remained almost unchanged. This was due to the high degree of dispersion and uniformity between the SDS-modified Mg-Al LDH particles and PLA matrix. The water vapor permeability of NC films decreased about 80% for PLA/SDS-Mg-A-LDH (2:1) 5% compared to neat PLA film. This is mainly attributed to the tortuous path for water vapor diffusion due to the impermeable clay layers distributed in the polymer matrix consequently increasing the effective diffusion path length. Due to the application of LDH and PLA for drug delivery and other biomedical applications, there are some reports for the application of PLA/LDH NCs for drug delivery using PLA or a copolymer of PLA. Drugs were usually used for the modification of LDH. For example, San Roma´n et al. (2013) intercalated the drugs (diclofenac, chloramphenicol, and ketoprofen) into LDH and then supported and dispersed these NFs into the PLA for drug delivery. One of the other applications of PLA/LDH NCs is the preparation of fibers by electrospinning technique, as reported by Zhao et al. (2008). Miao et al. (2012) also prepared electrospun fibers of PLA/LDH NCs for drug delivery. For this purpose, they intercalated ibuprofen

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Figure 15.46 AFM images of (A) PLA/Mg-Al LDH (2:1) 3%, (B) PLA/SDS- Mg-Al LDH (2:1) 3 %, (C) PLA/ Mg-Al LDH (3:1) 3%, (D) PLA/SDS- Mg-Al LDH (3:1) 3%, NCs and (E) pure PLA. SDS, sodium dodecyl sulfate. Source: Adapted from Demirkaya, Z.D., Sengul, B., Eroglu, M.S., Dilsiz, N., 2015. Comprehensive characterization of polylactide-layered double hydroxides nanocomposites as packaging materials. J. Polym. Res., 22(7), 124, with kind permission of Springer.

into LDH via coprecipitation and the LDH-ibuprofen NF was incorporated into the PLA. Poly(oxyethylene-b-oxypropylene-b-oxyethylene) (Pluronic) was also added to the fibers for increasing the hydrophilicity and modulation of release. As can be seen in the TEM images of PLA/LDH-ibuprofen fibers (Fig. 15.47), NFs were homogeneously distributed throughout the NC fibers. Interestingly, it was observed that the initial ibuprofen release from PLA/LDH-ibuprofen and PLA/Pluronic/LDHibuprofen fibers was faster than those fibers without LDH, which was a result of the strong interaction between alkyl groups in ibuprofen and methyl substituent groups in PLA, as well as the hydrophilicity of LDH-ibuprofen NPs leading to an easier diffusion of water with a faster release of ibuprofen. Amaro et al. (2016) reported the thermo-oxidative stabilization of PLA after incorporation with antioxidant-modified LDH. The utilized antioxidants are 3-(3,5di-tert-butyl-4-hydroxyphenyl)propionic acid (IrganoxCOOH) and 6-hydroxy2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox, a water-soluble analog of vitamin E), which have phenolic moieties. Interestingly, after modification of LDH with antioxidants, antioxidant power remained in terms of Trolox, and was even

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Figure 15.47 SEM images of electrospun fibers of (A) neat PLA, (B) 2 wt.% IBU/PLA and (C) 5 wt.% PLA/LDH-IBU and low- (D) and high-magnification (E) TEM images of 5 wt.% PLA/LDH-IBU composite fibers. IBU, ibuprofen. Source: Adapted from Miao, Y.-E., Zhu, H., Chen, D., Wang, R., Tjiu, W.W., Liu, T., 2012. Electrospun fibers of layered double hydroxide/biopolymer nanocomposites as effective drug delivery systems. Mater. Chem. Phys., 134(2), 623630, with kind permission of Elsevier.

amplified in the case of IrganoxCOOH, so after a controlled period of aging, both LDH-antioxidants prevented the occurrence of chain scission in PLA (SEC measurements). Furthermore, antioxidants showed a low tendency to migrate away from the LDH-antioxidant incorporated in the matrix (from preliminary migration test) so retaining the antioxidant protected inside the NF layers and leading to it remaining active for a longer time. There is also a report for the preparation of PLA/LDH NCs with fire-retardant behavior and good transparency (Ding et al., 2015). For this purpose, 2-carboxyethyl-phenyl-phosphinic acid (CEPPA) was used for the modification of LDH which not only increased the flame retardancy of PLA but also improved the compatibility between PLA and LDH. The SEM and TEM images of LDH before and after modification with CEPPA are shown in Fig. 15.48, which indicates the exfoliated structure after modification. All the NC films showed good transparency even with a high content of LDH-CEPPA (up to 10 wt.%). Furthermore, the films absorbed the ultraviolet light, which alleviated the embrittlement of PLA films in the procedure. The uniform dispersion of LDH-CEPPA in PLA, coupled with the low visible light absorption of LDH-CEPPA and PLA, lead to good transparency of the PLA/LDH-CEPPA films. The flame-retardant properties of the PLA/LDH-CEPPA films were evaluated using MCC analysis and the corresponding data are summarized in Table 15.9. As can be seen, the peak heat release rate (PHR) value increased with the incorporation of LDH-CEPPA,

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Figure 15.48 Particle morphology and size of LDH and LC samples. (A) SEM and (C) TEM of LDH; (B) SEM and, (D) TEM images of LC. CEPPA:2-carboxylethyl-phenyl-phosphinic acid, LC: LDH-CEPPA. Source: Adapted from Ding, P., Kang, B., Zhang, J., Yang, J., Song, N., Tang, S., et al., 2015. Phosphorus-containing flame retardant modified layered double hydroxides and their applications on polylactide film with good transparency. J. Colloid. Interface. Sci., 440, 4652, with kind permission of Elsevier. Table 15.9 The MCC data of the LCP films with different LC loadings Sample

LC content (wt.%)

PHR (W/g)

THR (kJ/g)

PLA LCP-1 LCP-5 LCP-8 LCP-10

0 1 5 8 10

294.9 373.8 450.9 458.5 404.2

12.0 11.9 11.2 10.7 9.7

LCP, PLA/LDH-CEPPA; LC:LDH-CEPPA. Source: Adapted from Ding, P., Kang, B., Zhang, J., Yang, J., Song, N., Tang, S., et al., 2015. Phosphoruscontaining flame retardant modified layered double hydroxides and their applications on polylactide film with good transparency. J. Colloid. Interface. Sci., 440, 4652, with kind permission of Elsevier.

indicating that the modified NFs promoted the degradation rate of the PLA/LDHCEPPA films. However, since endothermic decomposition of the LDH during combustion is negative for the heat release rate, especially at the higher LDH loadings, thus, these two opposite effects led to a maximum PHR value when LDH-CEPPA loadings were lower than 10 wt.%. Moreover, the total heat release (THR) value was decreased by increasing the LDH-CEPPA loadings. Improved flame retardancy property of PLA/LDH-CEPPA may due to the following factors. The first is attributed to the surface property of LDH and LDH after modification with CEPPA. The zeta potential of LDH was positive, indicating the hydrophilic property of the LDH

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surface, while after modification by CEPPA, the zeta potential of LC LDH-CEPPA changed to a negative value. This change from a hydrophilic to a hydrophobic property improved the compatibility between inorganic LDH NFs and organic PLA matrix. Second, such an exfoliated CEPPA-grafted-LDH structure into PLA improved the dispersion stability of CEPPA with low melting point and enhanced the synergistic effect of flame retardancy. Furthermore, LDH layers can protect the underlying material from further burning and reduce its heat release as a result of the physical barrier property of LDH that can decelerate heat and mass transfer between the gas and the condensed phases. On the other hand, CEPPA is an effective class of reactive flame retardants for polyesters, realized by the esterification reactions between PLA (or its degradative products) and CEPPA. The esterification reactions can be catalyzed by acid point on LDH layers or their metal oxides (degraded during combustion). This favored the improvement of PLA thermal stability.

15.2.5 PVA/layered double hydroxide nanocomposites Unique properties of PVA including chemical stability, biodegradability, biocompatibility, good mechanical and optical properties, etc., make possible its broad range of applications, such as packaging, medicine, drug delivery, bioplastics, membranes and coatings, water treatment, etc. (Goodship and Jacobs, 2009; Wang et al., 2017). Reinforcement of PVA or a blend of PVA with other polymers such as chitosan (Bercea et al., 2015a; Bercea et al., 2015b) or alginate (Kim Phuong, 2014; Lee et al., 2012; Phuong, 2014) with LDH has been reported recently. The prepared NCs have properties such as PH-sensitivity (Bercea et al., 2015a; Bercea et al., 2015b), catalytic (Shu et al., 2015), photostability (Gaume et al., 2013) (necessary for coating applications), improved mechanical and thermal properties (Dinari and Nabiyan, 2016; Huang et al., 2011; Li et al., 2003; Mallakpour and Dinari, 2014a,b, 2016; Mallakpour et al., 2015a,b; Ramaraj and Jaisankar, 2008; Ramaraj et al., 2010; Shu et al., 2014; Tian et al., 2014; Wang et al., 2017; Zhou et al., 2017), electrical conductivity (Chen et al., 2010), flame retardancy (Zhou et al., 2016, 2017), etc. and they have shown the potential for a wide range of applications such as packaging (Du et al., 2014), fibers (Qin et al., 2012; Zhao et al., 2010), water remediation by using a blend of alginate/PVA as matrix (Kim Phuong, 2014; Lee et al., 2012; Phuong, 2014), luminescent ultrathin films (Liang et al., 2014; Zhang et al., 2016), electrospinning (Lv et al., 2016), and fuel cells (Zeng et al., 2012). Researchers have used either pristine LDH (Ramaraj and Jaisankar, 2008; Ramaraj et al., 2010; Zhou et al., 2017) or a modified type as NFs. PVA hydrogels have attracted much attention due to their potential applications as biomedical materials, drug delivery, bioseparation in biotechnology, and carriers for cell immobilization. There are several methods for the preparation of PVA hydrogels. Huang et al. (2012) prepared PVA/LDH NC hydrogels using physical crosslinking by a freezing and thawing technique and investigated the mechanical properties and water swelling behavior of NCs. The freezing and thawing technique has been indicated as having great potential for various applications due to its

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simplicity, lack of toxicity, and low temperature. However, a critical barrier to their applications as load-bearing tissue replacements is a lack of sufficient mechanical properties. In this work, for the preparation of PVA/LDH NC hydrogels, PVA/LDH (0.2 and 0.5 wt.%) solutions were poured into a poly(ethylene terephthalate) (PET) mold and hydrogels were obtained by subjecting the tightly sealed aqueous solution to freezing/thawing cycles: freezing at 220 C for 20 h and thawing at 25 C for 4 h (tightly sealed PET mold guarantees that water does not evaporate and the diameter of the samples does not change with freezethaw cycles). Different cycle numbers (7, 9, and11) were chosen to prepare neat PVA and PVA/LDH NC hydrogels. TEM results (Fig. 15.49) showed that the LDH were well dispersed in PVA matrix, and thus increased the physical crosslinking density by inducing PVA crystallization and the formation of more uniform polymer networks. As a result, Young’s modulus (for PVA/LDH NC 5 wt.%: 25%, 22%, and 149% after 7, 9, and 11 freezing/ thawing cycles, respectively), tensile strength (for PVA/LDH NC 5 wt.%: 100%, 100%, and 300% after 7, 9, and 11 freezing/thawing cycles, respectively), and elongation at break of PVA hydrogels were greatly improved, even at very low LDH loading levels. On the other hand, the water swelling ability of PVA/LDH NC hydrogels decreased due to high crosslinking density caused by nonswollen LDH. Mallakpour et al. modified LDH with diacids (Mallakpour and Dinari, 2014a,b, 2016; Mallakpour et al., 2015a,b) via ion exchange reaction and coprecipitation. By

Figure 15.49 TEM images at low (A) and high (B) magnifications, showing a fine dispersion of LDH throughout PVA NC hydrogel containing 0.2 wt.% LDH. The dark platelets or disks represent LDH, and the light region is the polymer matrix. (C) Schematic of the formation of PVA/LDH NC hydrogels. Source: Adapted from Huang, S., Yang, Z., Zhu, H., Ren, L., Tjiu, W.W., Liu, T., 2012. Poly (vinly alcohol)/nano-sized layered double hydroxides nanocomposite hydrogels prepared by cyclic freezing and thawing. Macromol. Res., 20(6), 568-577, with kind permission of Springer.

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Figure 15.50 XRD patterns of different samples. MLDH, modified LDH. Source: Adapted from Mallakpour, S., Dinari, M., 2014a. Manufacture and characterization of biodegradable nanocomposites based on nanoscale MgAl-layered double hydroxide modified with N, N0 -(pyromellitoyl)-bis-L-isoleucine diacid and poly (vinyl alcohol). Polym. Plast. Technol. Eng., 53(9), 880-889; Mallakpour, S., Dinari, M., 2014b. Novel bionanocomposites of poly (vinyl alcohol) and modified chiral layered double hydroxides: Synthesis, properties and a morphological study. Prog. Org. Coat., 77(3), 583589, with kind permission of Elsevier.

using N,N0 -(pyromellitoyl)-bis-l-phenylalanine diacid as modifier (Mallakpour and Dinari, 2014b), an expansion in interlayer distance was observed in the XRD pattern of this modified chiral LDH compared to the unmodified type. On the other hand, the complete disappearance of LDH peaks in the XRD patterns of NCs (4, 6, 8 wt.%) could be due to the complete exfoliated structure (Fig. 15.50). Furthermore, extensive TEM observations of NC 8 wt.% revealed the coexistence of organo-nano-LDH layers in the intercalated and partially exfoliated morphology (Fig. 15.51). In another work, Mallakpour et al. (2015a) synthesized a diacid from the reaction of tetra-bromophthalic anhydrides and L-aspartic acid and used this diacid for modifying LDH. It was observed that by incorporation of modified LDH into the PVA matrix, the mechanical behavior of NCs was improved significantly compared to pure PVA (Table 15.10). This was due to the strong interaction between modified LDH and PVA via hydrogen bonding which could cause good dispersion of LDH in the matrix. On the other hand, the increase in the stiffness of NCs by increasing the amount of LDH caused a decrease in the elongation at break of NCs compared to pure PVA. Dinari and Nabian (2016) used citric acid (CA) as a modifier for the preparation of bio-NCs based on PVA. The CA-modified ZnAl-LDH (CA-LDH) was synthesized by coprecipitation method in aqueous media and different amounts of this NF

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Figure 15.51 TEM micrographs of MLDH (A, B) and PVA hybrids containing 8 wt.% of MLDH (C, D). MLDH, modified LDH. Source: Adapted from Mallakpour, S., Dinari, M., 2014a. Manufacture and characterization of biodegradable nanocomposites based on nanoscale MgAl-layered double hydroxide modified with N, N0 -(pyromellitoyl)-bis-L-isoleucine diacid and poly (vinyl alcohol). Polym. Plast. Technol. Eng., 53(9), 880-889; Mallakpour, S., Dinari, M., 2014b. Novel bionanocomposites of poly (vinyl alcohol) and modified chiral layered double hydroxides: Synthesis, properties and a morphological study. Prog. Org. Coat., 77(3), 583589, with kind permission of Elsevier.

Table 15.10 Mechanical properties from tensile testing of PVA and PVA/mLDH NCs Sample

Tensile strength (MPa)

Young’s modulus (MPa)

Elongation at break (%)

Pure PVA PVA/mLDH NC 2 wt.% PVA/mLDH NC 4 wt.% PVA/mLDH NC 8 wt.%

40.00 40.66 51.74 116.64

1.101 1.328 1.965 5.106

8.00 5.31 5.51 4.86

mLDH, modified LDH. Source: Adapted from Mallakpour, S., Dinari, M., Hatami, M., 2015a. Novel nanocomposites of poly (vinyl alcohol) and MgAl layered double hydroxide intercalated with diacid N-tetrabromophthaloyl-aspartic. J. Therm. Anal. Calor., 120(2), 12931302; Mallakpour, S., Dinari, M., Talebi, M., 2015b. Exfoliation and dispersion of LDH modified with N-tetrabromophthaloyl-glutamic in poly (vinyl alcohol): morphological and thermal studies. J. Chem. Sci., 127(3), 519525, with kind permission of Elsevier.

(2, 4, and 6 wt.%) were added to the PVA solution for the preparation of PVA/CALDH NC materials under ultrasonic irradiation by solution film-casting techniques. TEM images (Fig. 15.52) of NCs revealed the good dispersion of the LDH platelets into the PVA matrix in addition to the well-organized intercalated regions. The

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Figure 15.52 TEM micrographs of the (A, B) CA-LDH and (C, D) NCs of PVA with 4% of CA-LDH. CA, citric acid. Source: Adapted from Dinari, M., Nabiyan, A., 2016. Citric acid-modified layered double hydroxides as a green reinforcing agent for improving thermal and mechanical properties of poly (vinyl alcohol)-based nanocomposite films. Polym. Compos., with kind permission of John Wiley and Sons. Table 15.11 Mechanics properties of pure PVA and different NCs of PVA and CA-LDH Sample

Tensile strength (MPa)

Ultimate strain (%)

Pure PVA NC2% NC4% NC6%

57.6 68.1 80.3 70.2

106.8 89.4 74.6 62.7

CA, citric acid. Source: Adapted from Dinari, M., Nabiyan, A., 2016. Citric acid-modified layered double hydroxides as a green reinforcing agent for improving thermal and mechanical properties of poly (vinyl alcohol)-based nanocomposite films. Polym. Compos., with kind permission of John Wiley and Sons.

effective incorporation of modified LDH into the matrix led to an increase in thermal decomposition temperature and improved mechanical properties. Tensile stress increased by increasing the CA-LDH loading (when LDH loading is less than 4 wt. %), and then decreased with incorporation of more LDH such as 6 wt.% (Table 15.11). The main explanation for the improvement in tensile modulus in PVA NCs was the good interaction between matrix and LDH layers via formation of hydrogen bonds of the clay edges. The improvement in thermal properties was attributed to the homogeneous dispersion of CA-LDH in matrix and the strong hydrogen bonding between OH groups of PVA and the oxygen atoms of LDH

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Figure 15.53 The schematic representation of the preparation procedure of the PC-LDH hybrid. PC, phosphorylated cellulose. Source: Adapted from Wang, W., Kan, Y., Pan, H., Pan, Y., Li, B., Liew, K., et al., 2017. Phosphorylated cellulose applied for the exfoliation of LDH: An advanced reinforcement for polyvinyl alcohol. Compos. Part A: Appl. Sci. Manuf., 94, 170177, with kind permission of Elsevier.

layers or the carbonyl group of intercalated citrate anion. In addition, the UV-Vis transmission spectra exhibited that the samples retained high optical clarity, even at high clay loadings (6 wt.%). Therefore, these materials may have potential applications in the packaging industry. Some works report the flame-retardant properties of PVA/LDH NCs. Wang et al. (2017) used phosphorylated cellulose (PC) as a modifier agent for LDH and different amounts of this filler (PC-LDH) were used for the reinforcement of PVA matrix (Fig. 15.53). The results showed that incorporation of PVA with PC-LDH could cause improved mechanical and thermal properties of PVA NCs. Improvement in mechanical properties of PVA after incorporation of PC-LDH (Table 15.12) was due to the excellent compatibility between PVA and PC, which existed due to hydrogen bonds caused by the OH groups in PVA and the ring oxygen in PC. The flame retardancy of the pure PVA and its NCs was investigated by MCC test and the corresponding data are collected in Table 15.12. After the incorporation of PC-LDH, PVA-PC-LDH showed significantly improved flame retardancy, and the HRR value was also decreased with the increase in the PC-LDH content. With the same loading, PC-LDH-based PVA performed better than LDHfilled PVA, which showed lower HRR and THR values. The THR value was reduced after the addition of PC-LDH. These results were attributed to the synergistic effect, which contained a physical barrier effect of LDH layers and the catalytic charring effect of phosphorus element.

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Table 15.12 MCC and tension tests results of pure PVA and its composites Sample

HRR (W/g)

PVA0 PVA-LDH2 PVA-P-LDH0.5 PVA-P-LDH1 PVA-P-LDH2

201.5 237.3 198.4 155.0 116.9

6 6 6 6 6

16.2 13.3 9.5 11.3 7.9

THR (kJ/g)

24.0 21.7 22.8 20.1 61.4

6 6 6 6 6

1.2 0.9 0.7 0.9 0.6

Tensile strength (MPa) 38.7 36.4 45.4 52.4 61.4

6 6 6 6 6

3.9 2.8 2.7 3.3 4.2

Elongation at break (%) 178 6 19 86 6 10 151 6 16 135 6 17 98 6 8

Source: Adapted from Wang, W., Kan, Y., Pan, H., Pan, Y., Li, B., Liew, K., et al., 2017. Phosphorylated cellulose applied for the exfoliation of LDH: An advanced reinforcement for polyvinyl alcohol. Compos. Part A: Appl. Sci. Manuf., 94, 170177, with kind permission of Elsevier.

Figure 15.54 Illustration for the preparation of MoS2-LDH nanohybrids by self-assembly method. Source: Adapted from Zhou, K., Hu, Y., Liu, J., Gui, Z., Jiang, S., Tang, G., 2016. Facile preparation of layered double hydroxide/MoS 2/poly (vinyl alcohol) composites. Mater. Chem. Phys., 178, 15, with kind permission of Elsevier.

Zhou et al. (2016) prepared LDH/MoS2 hybrid as a promising NF for flame retardancy by self-assembly of exfoliated MoS2 nanosheets and LDH nanoplates via electrostatic interaction (Fig. 15.54). Subsequently, the prepared hybrids (loading of 0.5, 1, and 3 wt.%) were incorporated into PVA to serve as reinforcements by a solution blending method. According to TGA thermograms of PVA and its NCs (Fig. 15.55A), the presence of LDH/MoS2 hybrids in the composites resulted in poor thermal stability. However, PVA composites with LDH/MoS2 hybrids resulted in a higher amount of residues than neat PVA, owning to the catalytic carbonization effect of the LDH/MoS2 hybrids. The combustion behavior of PVA and its NCs was evaluated by MCC and their heat release rate (HRR) curves are shown in Fig. 15.55B. It was observed that incorporation of a small amount of

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Layered Double Hydroxide Polymer Nanocomposites

Figure 15.55 TG and HRR curves of PVA and PVA composites. Source: Adapted from Zhou, K., Hu, Y., Liu, J., Gui, Z., Jiang, S., Tang, G., 2016. Facile preparation of layered double hydroxide/MoS 2/poly (vinyl alcohol) composites. Mater. Chem. Phys., 178, 15, with kind permission of Elsevier.

LDH/MoS2 hybrids significantly reduced the fire risk of the PVA NC. PVA is a flammable polymer with high peak heat release rate (PHRR). The PHRRs of all the PVA composites were lower than pure PVA and decreased gradually as the loading of the LDH/MoS2 hybrids increased. When the loading of LDH/MoS2 hybrids reached 3 wt.%, the PHRR value was decreased from 288 W/g for neat PVA to 151 W/g with a reduction of 48%, indicating that the LDH/MoS2 hybrids could significantly reduce the fire risk of the PVA. Moreover, the microstructure of the char residues was investigated by SEM, as shown in Fig. 15.56A,B. As can be observed in Fig. 15.56A, neat PVA showed a discontinuous char layer with a large amount of cracks and holes. However, after addition of 3 wt.% LDH/MoS2 hybrids, the char

Applications of layered double hydroxide biopolymer nanocomposites

(A)

661

(B)

(C)

G

(D)

ID/IG = 3.69

G ID/IG = 3.07

D

400

600

800

1000

1200

1400

Raman shift (cm–1)

1600 1800 400

600

800

1000

1200

D

1400

1600 1800

Raman shift (cm–1)

Figure 15.56 SEM images of the char residues for PVA (A) and PVA composites (B), and Raman spectra of the char residues for PVA (C) and PVA composites (D). Source: Adapted from Zhou, K., Hu, Y., Liu, J., Gui, Z., Jiang, S., Tang, G., 2016. Facile preparation of layered double hydroxide/MoS 2/poly (vinyl alcohol) composites. Mater. Chem. Phys., 178, 15, with kind permission of Elsevier.

residues became more compact and dense, which provided a more effective protecting layer during combustion. Fig. 15.56C,D shows the Raman spectra of the residual char of neat PVA and PVA composites with 3 wt.% LDH/MoS2 hybrids. Raman spectroscopy is widely used to prove the existence of carbonaceous char in the residue and to analyze the special component of the char. It can be observed that the ID/IG ratio followed the sequence of PVA composites (3.07) , pure PVA (3.69), indicating a higher graphitization degree and the most thermally stable char structure of the PVA composites with LDH/MoS2 hybrids. The high content of graphitized carbons in the residual char could act as a barrier to mass and heat transfer and decrease the heat release rate during combustion. Qin et al. (2012) prepared PVA/LDH NC nanofibers using an electrostatic fiber spinning. In this work, either inorganic LDH carbonate (LDH-CO3) or L-lactic acid-modified LDH (Lact-LDH) were used as NF. As can be seen in Fig. 15.57, neat PVA, PVA/LDH-CO3, and PVA/Lact-LDH showed different stability behavior in aqueous solution after ultrasonication. The LDH-CO3 nanoparticles deposited at the bottom of the bottle after 1 h because of the agglomeration, whereas the LactLDH could keep stable even after a long storage time. Therefore, the dispersion of Lact-LDH improved the electrospinnability of the mixture well, which resulted in a decrease in the average diameter of fibers. TEM investigations (Fig. 15.58) also indicated that the dispersity of the LDH in PVA matrix was much improved after modification with L-lactic acid. The lactic acid in the interlayer of LDH may

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Layered Double Hydroxide Polymer Nanocomposites

Figure 15.57 Images of the appearances of LDH dispersion in PVA solutions. (A) After ultrasonication, (B) after storage for 1 h. Source: Adapted from Qin, Q., Liu, Y., Chen, S.C., Zhai, F.Y., Jing, X.K., Wang, Y.Z., 2012. Electrospinning fabrication and characterization of poly (vinyl alcohol)/layered double hydroxides composite fibers. J. Appl. Polym. Sci., 126(5), 15561563, with kind permission of John Wiley and Sons.

Figure 15.58 TEM images of electrospun fibers. (A, D) PVA/5% LDH-CO3, (B, E) PVA/ 3% Lact-LDH; (C, F) PVA/5% Lact-LDH. Lact, L-lactic acid. Source: Adapted from Qin, Q., Liu, Y., Chen, S.C., Zhai, F.Y., Jing, X.K., Wang, Y.Z., 2012. Electrospinning fabrication and characterization of poly (vinyl alcohol)/layered double hydroxides composite fibers. J. Appl. Polym. Sci., 126(5), 15561563, with kind permission of John Wiley and Sons.

weaken the strong cohesion between layers of the nanoparticles and interact with PVA chains, and therefore greatly improves the dispersibility of LDH in PVA matrix. The mechanical properties of the PVA/LDH fibers were obviously enhanced compared to those of neat PVA. For example, the tensile stress and elongation at break of the PVA/Lact-LDH electrospun fibrous mat with 5 wt.% Lact-LDH were

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Figure 15.59 Top: Schematic illustration of the synthetic strategy of artificial nacre-like Au NPs 2 LDH 2 PVA hybrid films. Bottom: Cross-sectional SEM images of Au NPs 2 LDH 2 PVA nacre-like hybrid films with different Au NPs densities. (A) WAu:WLDH 5 0.5:1 hybrid film. (B) WAu:WLDH 5 1:1 hybrid film. (c) WAu:WLDH 5 2:1 hybrid film. (D) WAu:WLDH 5 4:1 hybrid film. NPs, nanoparticles. Source: Adapted from Shu, Y., Yin, P., Liang, B., Wang, H., Guo, L., 2015. Artificial nacre-like gold nanoparticleslayered double hydroxidepoly (vinyl alcohol) hybrid film with multifunctional properties. Ind. Eng. Chem. Res., 54(36), 89408946, with kind permission of the American Chemical Society.

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Layered Double Hydroxide Polymer Nanocomposites

31.7 MPa and 36.7%, respectively, which were significantly higher than those of neat PVA, and also higher than those of PVA/LDH-CO3 owing to the better dispersity of Lact-LDH nanoparticles. Shu et al. (2015) synthesized LDH-Au NPs hybrid as NF which caused nacrelike PVA/Au NPs 2 LDH NCs with multifunctional properties including Raman scattering and catalytic properties for a reduction reaction. For this purpose, the nanosheets of LDH were initially modified with (3-aminopropyl) triethoxysilane to increase the interactions between LDH and Au NPs via a coordination reaction with amino groups of silane modifier. The NC films were fabricated via bottom-up assembly of pretreated LDH-Au NPs nanosheets and subsequent spin-coating of PVA (Fig. 15.59). The reduction of 4-nitrophenol (4-NP) by NaBH4 was chosen as a model reaction to study the catalytic properties of multifunctional PVA/Au NPs 2 LDH hybrid films. The reduction was followed by UV-Vis analysis. In the absence of hybrid films, no signs of reduction were observed, even in a period of 3 days. However, an obvious change in the UV-Vis spectra was found upon the addition of a piece of PVA/Au NPs 2 LDH hybrid film (WAu:WLDH 5 4:1). As can be seen in Fig. 15.60, the absorption at 400 nm significantly decreased as the reaction proceeded. Meanwhile, a new peak appeared at 295 nm and gradually increased as the reaction went on, revealing the successful reduction of 4-NP to 4-aminophenol (4-AP). Moreover, observation of an isosbestic point (320 nm) between two absorption bands indicated that the catalytic reduction of 4-NP yielded 4-AP without any byproducts.

Figure 15.60 UV-vis spectra of the reduction of 4-NP in an aqueous solution recorded every 2 min using the Au NPs 2 LDH 2 PVA hybrid film as a catalyst. 4-NP, 4-nitrophenol; 4-Ap, 4-aminophenol; NPs, nanoparticles. Source: Adapted from Shu, Y., Yin, P., Liang, B., Wang, H., Guo, L., 2015. Artificial nacrelike gold nanoparticleslayered double hydroxidepoly (vinyl alcohol) hybrid film with multifunctional properties. Ind. Eng. Chem. Res., 54(36), 89408946, with kind permission of the American Chemical Society.

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15.3

665

Conclusions

The focus of this chapter is the recent advances in biopolymer/LDHs NCs based on most of the biopolymer matrices including polysaccharide, protein, PHA, PLA, and PVA with LDH NFs. Researchers have used both pristine LDH and modified types as NFs, where the modifiers are various molecules such as drugs, surfactants, biomolecules, etc. Several methods such as coprecipitation, ion exchange, in situ polymerization, etc., have been used for the fabrication of biopolymer/LDH NCs. Based on the literature, incorporation of LDH into biopolymers can cause improved mechanical, optical, barrier, and thermal properties, which endows the biopolymer/ LDH NCs with possible utilization in many important applications, such as flame retardants, water treatment, drug delivery, tissue engineering, packaging, and catalysts.

Acknowledgments The authors acknowledge the Research Affairs Division Isfahan University of Technology (IUT), Isfahan, I. R. Iran, for partial financial support. Further financial support from National Elite Foundation (NEF), Tehran, I. R. Iran, Iran Nanotechnology Initiative Council (INIC), Tehran, I. R. Iran and Center of Excellence in Sensors and Green Chemistry Research (IUT)), Isfahan, I. R. Iran, is gratefully acknowledged.

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