polyvinyl alcohol nanocomposites

polyvinyl alcohol nanocomposites

Journal of Physics and Chemistry of Solids 115 (2018) 238–247 Contents lists available at ScienceDirect Journal of Physics and Chemistry of Solids j...

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Journal of Physics and Chemistry of Solids 115 (2018) 238–247

Contents lists available at ScienceDirect

Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs

Spectroscopic, thermal, and electrical properties of MgO/ polyvinyl pyrrolidone/ polyvinyl alcohol nanocomposites Gh. Mohammed a, c, *, Adel M. El Sayed b, c, W.M. Morsi d, e a

Physics Department, Faculty of Education, Ain Shams University, Roxy, 11757, Cairo, Egypt Physics Department, Faculty of Science, Fayoum University, Fayoum 63514, Egypt Physics Department, Faculty of Science, Northern Border University, Arar 91431, KSA d Building Physics Institute, Housing and Building National Research Center (HBRC), Dokki, Giza, 11511, Egypt e Al Imam Mohammad Ibn Saud Islamic University (IMSIU), Faculty of Science, Riyadh, Saudi Arabia b c

A R T I C L E I N F O

A B S T R A C T

Keywords: Band gap tuning DC conductivity FTIR spectra MgO nanoribbon Optical constant PVP/PVA nanocomposite

In this study, we aimed to control the optical and electrical properties of polyvinyl alcohol (PVA) in order to broaden its industrial and technological applications, which we achieved by blending PVA with polyvinyl pyrrolidone (PVP) and adding sol–gel prepared MgO nanopowder. The blended film and nanocomposite films were prepared using the solution casting technique. X-ray diffraction analyses showed that the crystallite size was ~18.4 nm for MgO and the highest degree of crystallinity (XC) in the films was about 24.34% at 1.0 wt% MgO. High resolution transmission electron microscopy determined the nanoribbon morphology of MgO. Scanning electron microscopy (SEM) indicated the uniform distribution of the MgO nanoribbons on the surfaces of the PVA/PVP films. SEM and Fourier transform infrared spectroscopy also confirmed the interaction between the blend and MgO fillers. The effects of the additives on the glass transition (Tg) and melting (Tm) temperatures were evaluated by differential thermal analysis and differential scanning calorimetry. The appearance of one melting point confirmed the miscibility of the two polymers. According to ultraviolet–visible–near infrared spectroscopy measurements, the optical properties and optical constants of PVA could be adjusted by the addition of PVP and MgO, where the optical band gap (Eg) determined for PVA increased with the PVP content, whereas it decreased to 4.8 eV as the MgO content increased. The DC conductivity ðσ dc Þ of the films increased whereas the activation energy (Ea) decreased after the addition of MgO, possibly because the nanoribbon shape fixed the preferred conducting pathways. In addition, MgO could break the H-bond in –OH groups of the blends to allow the free movement of the molecular chains.

1. Introduction The blending of different polymers in solution is a simple and practical method for obtaining new materials with various properties [1,2]. The rapid growth of nano-sized materials also allows polymers and polymer blends to be used as matrices for incorporating nano-sized particles. These particles could act as an interconnecting network to broaden the uses of these blends [3] because combining the attractive functions of inorganic nanostructured materials and organic polymers is expected to obtain synergistically enhanced properties [4,5]. Polyvinyl alcohol (PVA) is one of the most widely used synthetic polymers because of its excellent characteristics such as water solubility, high transparency, and very high flexibility. Polyvinyl pyrrolidone (PVP) is also interesting from a biological viewpoint because it has similar structural features to

proteins [6]. PVP is an amorphous colorless polymer that allows faster ionic mobility compared with other semi-crystalline polymers. Due to the – O) in the PVP side chains, it produces a presence of a carbonyl group (C– variety of complexes with different inorganic dopants [7]. In addition, it possesses a high glass transition (Tg) temperature because of the presence of the rigid pyrrolidone group [1]. The effective blending of PVP and PVA is attributed to hydrogen bonding, which may occur between the proton-accepting carbonyl moiety in the pyrrolidone rings and the hydroxyl side group of PVA [8]. PVP/PVA hydrogel exhibits nontoxic, noncarcinogenic, and bio-adhesive properties, so it has been used widely in biomedical and pharmaceutical applications [9], e.g., as a wound dressing material [10], while it also useful for synthesizing new biomaterials [8]. Abdelrazek et al. [1] studied the structural and thermal properties of a PVA/PVP

* Corresponding author. Physics Department, Faculty of Education, Ain Shams University, Roxy, 11757, Cairo, Egypt. E-mail address: [email protected] (Gh. Mohammed). https://doi.org/10.1016/j.jpcs.2017.12.050 Received 29 April 2017; Received in revised form 22 December 2017; Accepted 23 December 2017 Available online 27 December 2017 0022-3697/© 2017 Elsevier Ltd. All rights reserved.

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blend filled with chitosan up to 40 wt%, where they found that the incorporation of chitosan improved the thermal stability but decreased the degree of crystallinity. Ragab [11] reported the effect of NiCl2 salt on the dielectric properties of PVA/PVP films and showed that the AC conductivity increased with the frequency, which corresponded to the greater stability of the films. However, the dielectric permittivity decreased as the NiCl2 concentration increased. Elashmawi and Abdel Baieth [6] reported the effects of hydroxyapatite on the thermal properties of PVP/PVA blends, where they demonstrated that the degradation temperature of the polymer decreased and the thermal stability was reduced due to chemical reactions between the polymer and hydroxyapatite. Bdewi et al. [7] synthesized MgO nanoparticles (NPs) using a wet chemical method with magnesium chloride and sodium hydroxide, and they studied the effects of the MgO NPs on the structural and optical properties of PVA. Recently, nano-sized MgO has received much attention because of its versatile applications due to its high melting point, wear resistance, low erosion rate, non-toxicity, and good biocompatibility [12–14]. In addition, its optical band gap is 4.5 eV compared with 7.8 eV for the regular structure of bulk MgO [13,15], where this enhances its ultraviolet (UV) blocking ability and photocatalytic activity [12]. The nano-sized MgO exhibits greater efficiency in wastewater treatment due to its high surface area [16]. It is applied widely as a protective layer in ceramics, aviation, foundry, optoelectronic materials, transparent fillers, antibacterial agents, fire retardants, and electrochemical biosensors [14, 17]. Various methods have been used to prepare MgO nanostructures with different morphologies. MgO NPs measuring 42 nm in size were synthesized by heat treating a magnesium carbonate/poly(methyl methacrylate) composite [12], and by flame spray pyrolysis [18]. MgO thin films with a honeycomb-like structure were also prepared by a sol–gel route [14]. One-dimensional nanostructures have attracted considerable attention because of their unique properties as well as their potential technological applications. In particular, MgO nano-belts were fabricated by DC arc plasma jet chemical vapor deposition [19] and MgO nano-fibers were synthesized using a hydrothermal method [20]. Das et al. [21] prepared MgO films with nanoribbon structures by heat treating Mg (OH)2 films at 450  C in the air. The sol–gel method is one of the most successful for the synthesis of high purity bulk and nano-structured MgO with uniform and engineered morphologies [22]. Previous studies have not considered the effects of adding PVP and one-dimensional MgO nanostructures on the spectroscopic, thermal, and DC conductivity properties of PVA. Thus, in the present study, MgO nanoribbons and PVA/PVP/MgO nanocomposite films were prepared using a free-template sol–gel method and solution casting technique, respectively. The structural properties of the samples were studied using characterization techniques comprising X-ray diffraction (XRD), high resolution transmission electron microscopy (HR-TEM), scanning electron microscopy (SEM), and Fourier transform infrared (FTIR) spectroscopy. The thermal, optical, and electrical properties as well as the optical constants of the prepared nanocomposite films were determined.

2.1.2. Preparation of samples MgO nanoribbons were prepared using a template-free sol–gel method as follows. First, a 0.5 M solution was prepared by dissolving 10.72 g of the high purity Mg (CH3⋅COO) 2.4H2O and 9.46 g of OAD in 100 mL DD water under magnetic stirring for 2.0 h. The sol obtained was kept in an oven at 100  C for 8 h and then cooled to 60  C, before stirring for another 45 min to obtain the gel. The gel was aged for 15 h at room temperature (RT). Finally, the gel was calcined at 400  C and then at 550  C for 2.0 h to obtain MgO nanoribbons. PVP/PVA films with a composition of 70% PVA and 30% PVP were prepared using the solution cast method, where 1.4 g PVA was dissolved in 70 mL DD water at 90  C with continuous stirring until a clear and homogeneous solution was obtained. Next, 0.6 g PVP was dissolved in 30 mL DD water at RT and mixed with the PVA solution. During the preparation process, we observed that a PVP:PVA ratio of 30%:70% yielded more flexible and highly homogenous films. The mixed PVP þ PVA solution was then stirred for another 30 min at RT. The required masses (0.0, 0.5, 1.0, 1.5, and 2.0 wt%) of the sol–gel synthesized MgO nanoribbons were added to the PVP/PVA solution under continuous stirring for 1.0 h at RT to prevent any agglomeration. The aqueous solutions of the mixtures were then poured into glass Petri dishes, which were placed in an oven at 50  C in the air and peeled off after 24 h. Care was taken to obtain films with uniform thickness to record the optical absorption spectra and DC conductivities. 2.2. Measurements The XRD patterns of the MgO nanoribbons (calcined at 400  C and 550  C), pure PVA, PVP/PVA (blend), and the blends loaded with 0.5, 1.0, 1.5, and 2.0 wt% MgO nanoribbons were obtained using a PANalytical's X'Pert PRO diffractometer. The samples were scanned directly at 2θ angles between 10 and 60 with a Cu Kα source at a wavelength of 1.5418 Å and at RT. HR-TEM (JEM 2100, Jeol, Japan) was used to determine the crystalline size and morphology for the as-synthesized MgO nanoribbons. SEM (Inspect S, FEI, The Netherlands) images were acquired for the pure PVP/ PVA blend and the nanocomposite films in order to study the distribution and dispersion of MgO inside he PVP/PVA. FTIR spectroscopy was performed for the polymeric films at RT (JASCO, FT/IR-6200) in the wavenumber range of 4000–400 cm1. Thermal measurements were performed for the different films by differential thermal analysis (DTA; Shimadzu DTA-50) in the temperature range of RT–400  C at a heating rate of 10  C/ min in a nitrogen atmosphere. In addition, differential scanning calorimetry (DSC; DSC 131 model Setaram calorimeter) was performed in the temperature range of RT–150  C with the same heating rate. Optical characterization was conducted at RT using a Shimadzu UV3600 ultraviolet–visible–near infrared (UV-VIS-NIR) spectrophotometer in the wavelength range of 200–1200 nm at an accuracy of 0.2 nm. The thickness of the films was evaluated using a digital micrometer at an accuracy of 0.001 mm. Finally, the DC conductivity ðσ dc Þ of the films was d determined based on the well-known relationship: σ dc ¼ RA ; where d is the film thickness in m, A is its cross-sectional area in m2, and R is the film resistance in Ω. The film resistance was measured directly with an electrometer (Keithley 616) in the temperature range of 30–105  C. Samples in the form of discs with a radius of 5 mm and thickness of ~0.5 mm were used, which were coated with conductive silver paste on both sides to guarantee good ohmic contacts. They were then placed in an electrical furnace in a holder to undergo the required heating process. A copper/ constantan thermocouple was used to monitor the temperature precisely.

2. Experimental 2.1. Materials preparation 2.1.1. Chemicals Magnesium acetate tetrahydrate (Mg(CH3⋅COO)2⋅4H2O, MW ¼ 214.45; Nova Oleochem Limited, India) and oxalic acid dehydrate (OAD) (H2C2O4⋅2H2O, MW ¼ 126.07; LOBA Chemie, India) were used as the source material and chelating agent, respectively. PVA (Avondale Laboratories, Banbury, Oxon, UK; average MW ¼ 1.7  103) and PVP (Sigma, k 30; MW ¼ 4  104) were used to prepare the PVA and PVP/PVA films. Double distilled (DD) water was used as the solvent for preparing MgO and the nanocomposite films.

3. Results and discussion 3.1. Powder characterization The structural properties of the powders prepared using the sol–gel method were assessed by XRD, as shown in Fig. 1(a). The XRD pattern 239

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Fig. 1. (a) XRD patterns obtained for the MgO nanoribbons annealed at 400  C and 550  C prepared by a sol–gel route. (b) HR-TEM images of MgO nanoribbons annealed at 550  C. (c) Selected area electron diffraction pattern.

obtained for the powder calcined at 400  C indicated that the powder comprised cubic MgO and hexagonal Mg (OH)2 (or brucite, according to JCPDS Card No. 07-0239). The XRD pattern obtained for the synthesized MgO after calcination at 550  C had 2θ values at 36.88 , 42.83 , 62.23 , 74.5 , and 78.3 , which corresponded to the (111), (200), (220), (311), and (222) planes, respectively, and space group F m3m (JCPDS Card No. 45-0946). These sharp diffraction peaks indicated the presence of cubic MgO (periclase) alone, which possessed good crystallinity. Bdewi et al. [7] detected MgCO3 based on the XRD pattern obtained for MgO NPs measuring 20.62 nm, which were prepared using a microwave-assisted sol–gel method. In our study, the proposed reaction is:

a ¼ 4.20 Å by using a modified organic gel combustion route with urea-formaldehyde as the fuel. The HR-TEM image in Fig. 1(b) was obtained to analyze the morphology of the MgO nano-powder calcined at 550  C. The sample had a ribbon-like morphology with variable widths and lengths in the order of 0.6–1.5 μm. The difference in the sizes measured by XRD and HR-TEM is explained by the fact that XRD measures the crystallite size whereas TEM measures the particle or grain size, which usually comprises more than one crystallite. Similar results were reported by Justus et al. [25] for an α-Fe2O3 nanopowder prepared using a chemical solution method. This indicates that under the conditions employed, our sol–gel method is a suitable process for obtaining MgO with a nanoribbon structure. As shown in Fig. 1(c), the selected area electron diffraction pattern obtained for an isolated particle exhibited bright rings, which corresponded to the lattice planes of MgO, where (200) was the most intense plane. This was consistent with the XRD results, where the (200) plane also exhibited the highest intensity (Fig. 1(a)). These results suggest that the preferred direction for the growth of nanoribbons was perpendicular to the (200) plane.

Increasing calcination temperature from 400 C to 550 C

⇒ MgOðnanoribbonsÞ: MgðOHÞ2 The mean crystallite size (D) for the MgO nano-powder was estimated using Scherer's formula: D ¼ 0.9λ/(β cos (θ)), where λ is the X-ray wavelength (0.154 nm), β is the peak width at half-maximum (in radians), and θ is the angle of the corresponding peak. The calculation yielded a D value of 18.4 nm. The lattice parameter (a) was calculated using the relationship: dhkl ¼ a=ðh2 þ k2 þ l2 Þ1=2 , and the value obtained was a ¼ 4.219 Å. The specific surface area (A) of MgO is related to D by the formula: A ¼ 6=ρD [23], where ρ is the density of MgO (3.58 g cm3) [24], and Aav. ¼ 91.06 m2/g according to this equation. In a previous study [14], MgO nanowires were prepared by a solvothermal method where the lattice constant a ¼ 4.21 Å, average D ¼ 17 nm, and Aav. ¼ 98.6 m2/g. In addition, Halder and Bandyopadhyay [13] prepared a nanocrystalline MgO powder that measured 18 nm in size where

3.2. Structural properties of the polymeric and nanocomposite films 3.2.1. XRD and SEM analysis The XRD patterns obtained for the pure PVA, PVP (30%)/PVA (70%) blend, and blends loaded with 0.5, 1.0, 1.5, and 2.0 wt% MgO 240

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nanoribbons are depicted in Fig. 2. In the PVA film, the main peak occurred at 2θ ¼ 19.6 , which corresponded to (101) reflection. The dspacing obtained by XRD was 4.59 Å. Another weak peak was observed at ~40.9 , which confirmed the semi-crystalline structure of PVA. Karthikeyan et al. [24] also reported that PVA film exhibited two peaks around 2θ ¼ 19.20 and 40.55 . The degree of crystallinity (XC) determines the macroscopic properties of the polymer. Thus, as the content of the crystalline phase increases in PVA, the mechanical properties improve, the water resistance increases, and the permeability to gases decreases [26]. The relative XRD intensity ratios based on the peak area were used under crystalline peaks to calculate XC according to [27]: XC % ¼ Area Total area under all peaks  100.

The values obtained are shown in Table 1. Clearly, the XC value for pure PVA was 20.34 but it decreased to 14.35 after blending with PVP. Thus, adding PVP to PVA reduced the crystallinity of the PVA film by decreasing the area under the halo [28]. This is because PVP is an amorphous colorless polymer [2] and it is logical to expect that the amorphous regions in the blended films would be augmented. XC changed as the MgO content increased in a non-monotonic manner. An XC value of about 17.8 was reported for PVA by Tretinnikov and Zagorskaya [26], where this value increased to 60.2% after annealing the film at 150  C. The H and OH in the alcohol tend to interact with O in MgO, where this interaction occurs between MgO nanoribbons and the blend may cause the breakage of the H-bond in the –OH groups of the blend, so the molecular chains are free to rotate [6]. All of the characteristic peaks for MgO were absent from the composite but a higher value of XC was obtained at the 1.0 wt% MgO loading, thereby illustrating the interaction between MgO and the blend, and the enhancement of the ordering characteristic of the PVP/PVA blend at this ratio. The film surface near the fracture cross section and the distribution of the MgO nanoribbons inside the PVP/PVA blend are shown in Fig. 3. The surfaces of both the PVA and PVP/PVA appeared smooth (Fig. 3(a–b)),

Fig. 2. XRD patterns obtained for pure PVA, PVP/PVA (blend), and the blends loaded with 0.5, 1.0, 1.5, and 2.0 wt% MgO nanoribbons.

Table 1 The variation the degree of crystallinity (XC) with the film's composition. Film composition

XC

Pure PVA PVP/PVA blend 0.5% MgO/blend 1.0% MgO/blend 1.5% MgO/blend 2.0% MgO/blend

20.34 14.35 16.29 24.34 13.84 16.1

Fig. 3. SEM images obtained for: (a) pure PVA, (b) PVP/PVA (blend), (c) 0.5 wt% MgO/blend, and (d) 2 wt% MgO/blend. 241

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most appropriate techniques for studying these phenomena [32]. In addition, DTA can be employed to investigate the miscibility of the polymers forming a blend [1], where this miscibility can actually predict the final properties of blends. The DTA curves obtained for the pure PVA, PVP/PVA blend, and MgO/PVP/PVA nanocomposite films in the temperature range of RT–400  C are depicted in Fig. 5(a). The exothermic peak determined in all the films at about 40–55  C was attributed to the volatilization of moisture present in the films, which will be present unless they are carefully vacuum dried. The glass transition temperature (Tg) is usually identified as a “step” in the DTA curves because it is a second order endothermic phase transition. At Tg, the substance transformed from an amorphous and glassy state to a rubbery state because of the increase in polymer chain segmental motion. The Tg value determined for pure PVA was about 80  C, which agreed with the previously reported value [33], and it changed slightly after PVP and MgO loading. The broad exothermic peak in the range of 105–180  C was assigned to the relaxation associated with the crystalline regions in the films [34], where the film containing 70% PVA was a semi-crystalline polymer. The endothermic peak located at ~217  C for pure PVA indicated its melting temperature (Tm), which was in good agreement with the previously reported value [35]. PVA segments readily undergo dehydration followed by chain scission during the heating process [36]. The Tm value determined for PVP/PVA decreased to ~200  C due to the interaction between PVP and PVA, which caused a decrease in the crystallinity of the PVA segments [28], and this agreed with the XRD measurements. The appearance of a single melting peak demonstrated that PVA and PVP exhibited good miscibility [37] due to the formation of hydrogen bonds between the hydroxyl groups of PVA and the carbonyl groups of PVP. The addition of MgO nanoribbons again increased Tm to about 203  C, 212  C, and 214  C for the 0.5%, 1.5%, and 2% MgO/blends respectively. Thus, MgO slightly enhanced the crystallinity of the blends because Tm was shifted to higher temperatures and the melting peak also sharpened, especially for the 2% MgO/blend. Finally, a broad exothermic peak appeared related to the decomposition (degradation) of the films at 317  C and 325  C for the pure PVA and PVP/PVA blend, respectively, whereas it decreased to about 270  C with the addition of MgO. This confirmed the interaction between the film components comprising PVP and MgO with the PVA. During the decomposition process via the chain-stripping mechanism [38], the expected gases released comprised water vapor, carbon monoxide, and carbon dioxide [39]. To confirm the

whereas the fracture cross sections were different and this confirmed the interaction between PVP and PVA. Fig. 3(c–d) shows the uniform distribution of the MgO nanoribbons. Loading MgO into the blend resulted in the production of small holes. In addition, increasing the MgO content to 2.0 wt% increased the surface roughness of the film. 3.2.2. FTIR spectroscopy The FTIR transmittance spectra obtained for the films at RT are shown in Fig. 4(a–f). The broad band observed at 3640 cm1 as well as the bands between 628 and 632 cm1 were attributed to the O–H stretching frequency, thereby confirming the presence of OH groups [1,29]. The bands observed at 2873 cm1 and 2963 cm1 characterized the symmetric and asymmetric stretching modes of the CH2 group, respectively [7]. The first band at 2873 cm1 was enhanced slightly after MgO loading and the second band at 2963 cm1 shifted to 2990 cm1 after mixing PVP and MgO. An absorption peak at 1695 cm1 was present in the spectra for the – O stretching PVP/PVA blend (Fig. 4(b–f)), which was due to the C– vibration, thereby confirming the intermolecular interaction between the OH groups in PVA and carbonyl groups in PVP [30,31]. The bands observed at 1375 and 1331 cm1 were attributed to the combination frequencies of (CH þ OH) [29]. The intensity of the absorption band at 1331 cm1 in pure PVA was shifted to 1285 cm1 and it was enhanced after blending with PVP, whereas it decreased as the MgO content increased, while the small band at 1376 cm1 was also reduced gradually as the MgO content increased. The sharp absorption peak at 1157 cm1 was an ether stretching band (C–O) and it was attributed to the crystallinity of PVA, which is used for assessing the PVA structure. The band observed at 853 cm1 was attributed to CH2 in the stretching mode (CH2 bending) [29]. Finally, shifts occurred at 3640, 2990, and 1285 cm1 only when the MgO content 1.5%, while the decreased intensity of some bands indicated that the amorphous regions in the composite polymer films were augmented [6], which were consistent with the XRD measurements. These findings also confirmed the interaction between MgO and the functional groups of the PVP/PVA blend chains. 3.3. Thermal properties of the nanocomposite films When heating polymeric materials, different phenomena may occur such as a glass transition, melting, and decomposition. DTA is one of the

Fig. 4. FTIR spectra obtained for: pure PVA, PVP/PVA (blend), and blend films loaded with 0.5, 1.0, 1.5, and 2.0 wt% MgO. 242

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285 nm and a hump at 335 nm. The absorption bands at 285 and 335 nm were assigned to the electronic transitions comprising n→π* (R-band) and π→π* (k-band), respectively [41]. The absorption intensity of these bands decreased significantly in the PVA film after blending with PVP, but they increased gradually as the MgO content increased in the blend. The changes in the absorption edge determined for the nanocomposite samples suggest that there was an interaction between the polymer blend chains and the MgO nanoribbons. Thus, UV transmission through the nanocomposites was significantly reduced as the filler content increased due to the UV-shielding ability of the MgO nanoribbons. Similar results were reported in a previous study where UV protection in chitosan was enhanced by incorporating MgO spherical NPs [12]. Fig. 6(b) shows the reflectance (R %) as a function of the wavelength λ for the films investigated in the present study. Similar to the behavior of k, R % decreased for PVA after blending with PVP whereas R % increased gradually for the PVP/PVA blend as the MgO content increased. The refraction is high when the incident light interacts with a material containing many particles, [42]. In the high absorption region, the optical band gap Egopt was calculated r

using the relationship: hν ¼ Aðhν  Egopt Þ , where A is a constant and r is a parameter that determines the optical transition type. Davis and Shilliday [43] reported that near the fundamental band edge, the direct transition can be determined by plotting ðαhνÞ2 as a function of photon energy ðhνÞ and then plotting the linear portions of the curves to zero absorption. These plots are shown in Fig. 7(a) and the values obtained are given in Table 2. According to Fig. 7(a) and Table 2, Eg direct was 5.1 eV for the PVA film but it increased to 5.2 eV after blending with PVP, whereas it decreased to 4.8 eV as the MgO content increased to 2.0 wt%. This decrease in Eg may be attributed to the creation of new energy levels in the optical band gap of the blend due to increased disorder, which facilitated the crossing of electrons from the valence band to the local levels and to the conduction band, and thus the band gap decreased as the MgO content increased. In the absorption spectra, the extending tail for lower photon energies hν

below the band edge can be described by α ¼ αo e =EU , where αo is a constant and EU is the Urbach energy corresponding to the width of the band tails for the localized states in the band gap. The EU values were calculated as the reciprocal gradient of the linear portions of the plot of ln(α) versus hυ, as shown in Fig. 7(b). The EU values increased from 0.523 eV to 0.595 eV as the MgO nanoribbon contents increased. This result supports the XRD measurements, where EU can be attributed to the disorder in the material that leads to the tail in the valence and conduction bands. Moreover, the k and R spectra as well as the Eg values of PVA films can easily be tuned by adding PVP and mixing with MgO. The refractive index n is a very important physical parameter related to microscopic atomic interactions and it is very important for the design of optoelectronic devices. The refractive index, n, is given by Ref. [44]:

Fig. 5. (a) DTA curves obtained for: pure PVA, PVP/PVA (blend), and blend films loaded with 0.5, 1.5, and 2.0 wt% MgO. (b) DSC curves obtained for different films in the temperature range of RT–150  C.

Tg values for the different films, DSC measurements were obtained in the temperature range of RT–150  C, as shown in Fig. 5(b). Clearly, the Tg values were in the range of 82–86  C, where a slight change was observed with PVP and MgO loading. Finally, the Tg value determined for the PVP/PVA blend agreed with that reported by Nishio et al. [40].



1þR þ 1R

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 4R  k2 ; ð1  RÞ2

(1)

where R is the recorded reflectance and k is the extinction coefficient. Fig. 8(a) shows the dependence of the refractive index (n) on the wavelength λ. In most of the visible regions of the spectra, the n values decreased slightly as λ increased. For the pure PVA film, n was between 1.4 and 1.66, whereas the range was 1.24–1.5 for PVP (30%)/PVA (70%). The n value increased as the MgO nanoribbon contents increased in the PVP/PVA matrix, where the maximum n value was 1.82 for 2.0% MgO/PVP/PVA film at λ ¼ 406 nm. Similar results were reported previously for a Poly(ethylene glycol)/PVA blend doped with hematite (α-Fe2O3) nano-rods [45]. The changes in the physical properties of a material are highly dependent on its internal structure, e.g., the packing density and molecular weight distributions. Doping with MgO nanoribbons increased the degree of disorder and it may have increased the mobility of the free radicals, which are chemically active and led to

3.4. Optical properties and optical constants of the nanocomposite films Studying the fundamental absorption edge in the UV region is useful for elucidating the optical transitions and electronic band structures of crystalline/non-crystalline materials. PVP/PVA is used as an insulator in various applications. In this study, we investigated the effect of 30% PVP and loading MgO up to 2.0 wt% on the absorption spectra and optical band gap of PVA. Fig. 6(a) shows the dependence of the extinction coefficient k on the wavelength λ, where k ¼ αλ/4π and α ¼ the recorded absorption/film thickness (d). The spectra obtained for all the films contained two absorption bands, with a shoulder-like band around 243

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Fig. 6. (a) Extinction coefficient (k) and (b) reflectance spectra (R%) as a function of wavelength for pure PVA, PVP/PVA (blend), and blends loaded with 0.5, 1.0, 1.5, and 2.0 wt% MgO.

Fig. 7. (a) Allowed direct and (b) Urbach plots obtained for pure PVA, PVP/PVA (blend), and blends loaded with 0.5, 1.0, 1.5, and 2.0 wt% MgO.

Table 2 Optical properties of MgO/PVP/PVA nanocomposite films; (the direct optical band gap Eg direct, Urbach energy EU, the ratio of carrier concentration to electron effective mass (e2/π C2)(N/ m*), the high frequency refractive index(n∞), the average oscillator wavelength (λo), the average oscillator strength S0 ¼ ðn2∞  1Þ=λ2o , the dispersion energy Ed and the single oscillator energy Eo. Film composition

Eg

pure PVA PVP/PVA blend 0.5% MgO/blend 1.0% MgO/blend 1.5% MgO/blend 2.0% MgO/blend

5.1 5.2 5.05 5.0 4.9 4.8

direct

(eV)

EU (eV)

(e2/πC2)(N/m*) 107 (nm)2

n∞

λo (nm)

So  1012 (m2)

Ed (eV)

Eo (eV)

0.459 0.523 0.576 0.614 0.563 0.595

2.85 2.49 3.82 2.62 3.13 4.73

1.397 1.235 1.257 1.433 1.527 1.760

264.4 293.7 315.3 227.3 207.2 133.4

13.6 6.08 5.83 20.4 31.02 117.8

4.92 2.12 2.40 4.71 6.38 13.55

5.16 4.14 4.26 4.61 4.91 6.64

244

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Fig. 8. Dependences of the refractive index n on λ and n2 on λ2 for pure PVA, PVP/PVA (blend), and blends loaded with 0.5, 1.0, 1.5, and 2.0 wt% MgO.

increases in the excited states, thereby increasing the refractive index [45]. The ratio of the carrier concentration relative to the electron effective mass (e2/πc2) (N/m*) was calculated for our films by considering the dependence of n2 on λ2 (Fig. 8(b)) according to the following dispersion relationship [46]:  n2 ¼ εl 

e2 π c2



 N 2 λ; * m

n2 ¼ 1 þ

Ed Eo Eo2  ðhνÞ2

;

(3)

where Eo is the oscillator energy and Ed is the dispersion energy or oscillator strength, which is a measure of the strength of interband optical transitions. The Eo and Ed values were obtained by plotting (n2 – 1)1 versus (hυ) 2 from the resulting straight lines, as shown in Fig. 9(a), and their values are given in Table 2. Moreover, in order to determine the average interband oscillator wavelength (λo) for the films, the refractive index n was also analyzed further by using the following single Sellmeier oscillator at low energies [46]:

(2)

where εl is the lattice dielectric constant. The (e2/π c2) (N/m*) values are listed in Table 2. The value decreased from 2.5  107 (nm)2 for pure PVA to 2.49  107 (nm)2 for the PVP/PVA blend, but the value increased to 4.73  107 (nm)2 as the loading with MgO nanoribbons increased. This is a reasonable result because the (e2/π c2) (N/m*) values decreased/increased as the Eg values increased/decreased, respectively. The Wemple–DiDomenico single-oscillator model was used to analyze the dispersion of the refractive index of the films, where the dispersion parameters were calculated to obtain more information about the degree of disorder. According to this model:

 2 n2∞  1 λo ¼1 ; 2 n 1 λ

(4)

where n∞ is the long wavelength refractive index. The parameters n∞ and λo values were obtained from the slope and intercept of the (n2–1)1 versus λ2 curves, as shown in Fig. 9(b). The values of n∞ and λo as well as the average oscillator strength S0 ¼ ðn2∞  1Þ=λ2o are summarized in

Fig. 9. (a) (n2  1)1 vs (hν)2 plots and (b) (n2  1)1 vs λ2 plots obtained for pure PVA, PVP/PVA (blend), and blends loaded with 0.5, 1.0, 1.5, and 2.0 wt% MgO. 245

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shape allowed the preferred conducting pathways to be fixed. The sudden decrease in σ dc above 368 K may have been due to structural changes in the crystalline phase inside the blend matrix [53]. In general, the behavior of σ dc with T confirms that the films were thermally activated, and similar results were reported for a PVA/Carboxymethyl cellulose blend doped with CuO NPs [54]. The absence of sudden jumps in the σ dc values after an increase of more than one order suggests the amorphous nature of these films [55]. The maximum value of σ dc for the different films was due to the increase in the number of charge carriers introduced into the films. The increase in σ dc as the MgO content increased (see Fig. 10(a)) could be explained by MgO nanoribbons forming three-dimensional conductive pathways via their contacts with each other [54] inside the polymer matrix, which may have facilitated the transport of charge carriers through the polymeric films. The morphology of the MgO nanoribbons may have augmented and fixed these conductive paths. The interaction between MgO and the blend broke the H-bonds in the blend –OH groups, as mentioned earlier, thereby freeing the motion of the molecular chains [7]. This could have increased the matrix heterogeneity and thus the free volume to enhance the conducting pathways. Finally, MgO as a semiconducting material where Eg ¼ 4.5 eV [13,15] could have created new energy levels in the optical band gap of the blend to allow electrons to move from the valence band to the conduction band, which is in accordance with the optical properties of the films. Fig. 10(b) plots Ln σ dc against 1000/T for the films, where the symbols indicate the Ln values of the experimental conductivities and the solid lines indicate the linear fits for the experimental data. The behavior   a of σ dc followed the Arrhenius behavior: σ dc ¼ σ o exp E kT , where σ o is the

Table 2. According to Figs. 8 and 9, and Table 2, the optical constants of PVA film can be tuned easily by blending with PVP and embedding MgO nanoribbons. Quantitatively determining these parameters may help to tailor and model the properties of these films to facilitate their use in optical components and devices. 3.5. DC electrical properties of the nanocomposite films The dependences of σ dc on temperature (T) and the MgO content of the films are shown in Fig. 10(a). We found that σ dc increased as T increased up to 368 K, before decreasing again, except for the PVP/PVA film. The increase can be attributed to enhanced polymer chain segmental motion [47] due to the high energy gained by the chains segments, where this is in accordance with free volume theory [48], and thus the chains could resist the hydrostatic pressure exerted between neighboring molecules [49]. In addition, the chains could create a small space around their own volume where the vibrational motion may occur [50]. The heating process also creates small holes (free volume) that allow free charges to hop from one site to another [51]. Indeed, the kinetic energy and mobility of the charge carriers present in the host matrix increase as T increases [52]. Thus, the MgO nanoribbons could connect with each other to facilitate electrical conduction, where the nanoribbon

pre-exponential factor and Ea is the activation energy, which is the minimum energy required to overcome potential barriers in the system, and it was calculated based on the slope of the straight lines in Fig. 10(b). The values of Ea determined for the different films are presented in Table 3. We found that the Ea values decreased with the MgO loading because the dispersion of MgO in the blend matrix introduced more conducting paths, which led to higher values of σ dc and lower values of Ea, which agreed with the decrease in the degree of crystallinity shown by the XRD measurements. 4. Conclusions In this study, a free-template sol–gel method was used successfully to prepare cubic MgO nanoribbons with an average size of 18.4 nm, length of 0.6–1.5 μm, and average surface of 91.06 m2/g, where (200) was the preferred growth direction. PVP reduced the semi-crystallinity of PVA. MgO nanoribbons were well dispersed on the PVP/PVA surface and they affected the roughness of the film's surface. The interaction between PVP and MgO nanoribbons with the PVA changed the transmittance intensity and the positions of some functional groups in the FTIR spectra. Thermal analyses confirmed that PVA and PVP exhibited good miscibility. In addition, the melting temperature of the PVP/PVA blend decreased compared with that of PVA due to the reduced crystallinity. The absorption and reflection in the UV–Vis region as well as the optical constants of PVA could be tuned easily by loading PVP and MgO. PVP increased the insulation of PVA whereas MgO increased the

Table 3 The activation energy values for the films under investigations.

Fig. 10. (a) Dependence of DC conductivity in PVP/PVA (blend) and blends loaded with 0.5, 1.0, 1.5, and 2.0 wt% MgO on temperature, where the inset shows the same dependence for pure PVA. (b) Ln σdc against 1000/T for the investigated films. 246

Film composition

Ea (eV) Stage 1

Ea (eV) Stage 2

pure PVA PVP/PVA 0.5% MgO/blend 1.0% MgO/blend 1.5% MgO/blend 2.0% MgO/blend

23.00 17.42 11.84 9.26 8.81 5.68

4.60 4.97 6.33 6.87 7.67 6.63

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semiconducting ability of PVA. The addition of PVP and MgO nanoribbons greatly affected the DC conductivity of PVA. The MgO morphology could fix the preferred conducting networks to facilitate electrical conduction through the MgO/PVP/PVA nanocomposites films, which were considered to be thermally activated. According to these results, loading with PVP and MgO can be employed to obtain PVA polymers with the desired structural, optical, and electrical properties, thereby broadening its potential industrial applications, especially in optical components and devices.

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