Probing the mechanical and thermal properties of polysulfone membranes modified with synthetic and natural polymer additives

Probing the mechanical and thermal properties of polysulfone membranes modified with synthetic and natural polymer additives

Polymer Testing 34 (2014) 202–210 Contents lists available at ScienceDirect Polymer Testing journal homepage: www.elsevier.com/locate/polytest Mate...

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Polymer Testing 34 (2014) 202–210

Contents lists available at ScienceDirect

Polymer Testing journal homepage: www.elsevier.com/locate/polytest

Material properties

Probing the mechanical and thermal properties of polysulfone membranes modified with synthetic and natural polymer additives Gcina Doctor Vilakati a, Eric M.V. Hoek a, b, c, Bhekie Brilliance Mamba a, * a

University of Johannesburg, Department of Applied Chemistry, P.O. Box 17011, Doornfontein 2028, Johannesburg, South Africa University of California, Los Angeles, Department of Civil and Environmental Engineering, 5732 Boelter Hall, P.O. Box 951597, Los Angeles, CA 90095-1593, USA c University of California, Los Angeles, California NanoSystems Institute, Los Angeles, CA 90095, USA b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 November 2013 Accepted 31 January 2014

This paper reports on the effects of lignin, polyvinyl pyrrolidone and polyethylene glycol as additives to polysulfone ultrafiltration membranes prepared by nonsolvent induced phase separation. The focus is on the mechanical and thermal properties of the resultant membranes. Differential scanning calorimetry (DSC) and thermogravimetric analysis were used to probe the thermal properties, while an Instron tensile tester was used to characterise the mechanical properties. Morphological studies indicate that the porosity of the bottom sub-layer increased with the use of each additive, suggesting that coagulation in the sub-layer differed from that of the top layer. Membranes fabricated using lignin were thermally stable as the residue at 800  C increased from 13% to 44%, suggesting interaction of lignin with the polymer. The increase in free fractional volume was confirmed by DSC thermograms as the glass transition temperature decreased considerably after incorporating the additives. Generally, the modulus and tensile strength decreased after the introduction of the additives. These results offer new insight into the use of an emerging, cheap and readily available natural additive (lignin) compared to traditional synthetic additives in membrane formation. Ó 2014 Elsevier Ltd. All rights reserved.

Keywords: Glass transition Membranes Mechanical properties Morphology Thermal properties

1. Introduction Materials such as polyethersulfone (PES), polysulfone (PSf) and polyvinylidene fluoride (PVDF) have been used extensively in ultrafiltration membrane preparations owing to their excellent physical, mechanical and chemical characteristics [1,2]. In order to improve their separation properties, organic and inorganic additives are incorporated into the polymer dope solution prior to the phase-inversion

* Corresponding author. E-mail addresses: [email protected] (G.D. Vilakati), [email protected] ucla.edu (E.M.V. Hoek), [email protected], [email protected] (B. B. Mamba). http://dx.doi.org/10.1016/j.polymertesting.2014.01.014 0142-9418/Ó 2014 Elsevier Ltd. All rights reserved.

process. Some examples of well-known additives include glycerol, maleic acid, polyvinyl pyrrolidone (PVP) and polyethylene glycol (PEG) [3–5]. Some additives are used as pore-forming agents while others are used as macro-void suppressors in order to improve interconnectivity of the pores. In either case, the aim is to improve the membrane porosity of both the top layer and sub-layer and, ultimately, the membrane performance [6]. Non-solvent additives are known to suppress macrovoid pore formation, but improve the interconnectivity of pores. This is because their incorporation into the polymer solution reduces its stability, resulting in the solvent rapidly diffusing out of the polymer film into the coagulation bath during phase inversion [7]. An increase in the non-solvent additive can bring the polymer solution nearer

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to the precipitation point since the coagulant tolerance is reduced [8]. However, Wang et al. proposed that the size of pores formed depends on the molecular weight of the additive; low-molecular-weight PVP resulted in small pores while high-molecular-weight PVP formed large pores, although most of it could not be leached out of the membrane and may block the macro-void interconnection path [9–11]. The assumption is that, during the phase-inversion process, the hydrophilic additive, namely PVP or PEG, is removed by dissolution in the non-solvent, and the sites where the PVP exists become the pores [7]. More emphasis has been on the improvement of pure water flux and solute rejection, neglecting the effect on the mechanical properties brought about by the introduction of additives during membrane preparation. The fact is that hydrophilic polymeric additives such as PVP and PEG will remain in the membrane after coagulation and will compromise the mechanical strength of the resultant membrane at elevated temperatures [12]. Adams et al. recently reported that an increase in b-cyclodextrin polyurethane additive in membranes decreased the ultimate

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tensile strength. The decrease was attributed to the presence of macro-void pores in the membrane sub-layer [13]. Ma et al. reported that the mechanical strength of polysulfone membranes decreased when the dosage content of PEG was increased [14]. However, the elongation at break increased to 35% at 6% PEG dosage and then decreased when 10% PEG dosage was used in a separate study. Ma et al. also reported that an increase in clay dosage weakened the mechanical properties of polysulfone membranes [15]. Zafar et al. reported the results of a contrasting study when a 600 Da PEG additive was incorporated into cellulose acetate membranes. It was reported that the tensile strength and the modulus improved with the PEG content. The explanation was based on the interaction of the polymer and additive molecules which increased the polymer chain mobility, and hence the polymer toughness improved [16]. An additive to a polymer solution during membrane fabrication normally improves the thermal properties of the membrane. The incorporation of PEG 600 into cellulose acetate membranes has resulted in the improvement

Fig. 1. A hypothetical structure of lignin from wheat straw. (Reprinted with permission from Elsevier).

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Fig. 2. SEM cross-section images of polysulfone (PSf) and PSf modified with 0.125 wt% PEG, PVP and lignin additives.

of the thermal stability since the degradation temperature shifted to higher temperatures [16,17]. The glass transition temperature (Tg) can be used to probe the membrane morphology. A lower Tg normally indicates a higher free volume fraction in the membrane, and hence a looser structure [17,18]. From the literature review presented in this study, PEG and PVP additives compromise the tensile strength and modulus of polysulfone membranes while improving the thermal stability of most polymers. Membrane toughness is the most crucial property in membrane applications that require high pressure, because the membrane should be resistant to compaction which can destroy the sub-layer pore structure if it is unable to withstand high pressure. The modulus will decrease due to the fact that additives induce macrovoids in the membrane structure; however, there should be a balance between the two. Natural plantbased polymers such as lignin have been used to improve the toughness of polymer composites [19,20]. In membranes, lignin derivatives have been used as additives. For example, Zhang et al. used lignosulfonate in polysulfone membranes to impart electrolyte transference and Nevarez

et al. used propionated lignin to synthesize cellulose triacetate membranes [21,22]. Based on the literature cited, it is envisaged that the use of an inexpensive plant-based polymer, lignin, can improve the thermal and mechanical properties of membranes compared to relatively expensive synthetic PEG and PVP additives, which have been widely studied. This study reports the effect of incorporating lignin, PVP or PEG on the mechanical and thermal properties of the resultant membranes. Although permeation studies are important, it is necessary to compare the results with commercial membranes (normally cast on a fabric) and that makes it difficult to monitor the mechanical and thermal properties of the membranes (cast on a glass plate), and as such they are not included in this study The membrane fabrication process described in this paper not only introduces a novel additive, but could also lower the cost of membrane manufacture because lignin is readily available and relatively cheap. The exact structure of lignin is unknown, but a schematic diagram of a proposed structure is shown in Fig. 1 [23]. Although PEG and PVP have been used as experimental controls because their behaviour is known when used as

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Fig. 3. SEM cross-sectional images showing the effect of varying amounts (0.125 wt% and 0.5 wt%) of added lignin.

additives, their extensive characterisation is necessary because their loading is smaller here than that used in most past studies (2–10%) to enable direct comparison with the lignin loadings [14]. Since the lignin used had a molecular weight of 28 kDa, we selected PVP and PEG controls of comparable size, that is 29 and 10 kDa. Note that polysulfone dope solutions cast with 35 kDa PEG were unstable (precipitated before casting). 2. Experimental 2.1. Materials Polymer solutions were prepared from polysulfone beads (22 KDa), lignin alkali (28 KDa), polyvinylpyrrolidone

(29 KDa), polyethylene glycol (10 KDa) and NN-dimethyl formamide (DMF), obtained from Sigma Aldrich and Nmethyl pyrrolidone (NMP) bought from Merck. 2.2. Membrane fabrication The membranes were prepared using the Loeb-Sourirajan wet phase inversion method with a few modifications [24]. Polymer solutions were prepared by first dissolving the additives in a 3:1 (NMP:DMF) solvent ratio under magnetic stirring while heating in a water bath at 50  C for a period of 30 min. The 3:1 ratio was adapted from Yip et al. [25]. In order to establish the impact of the various additives when incorporated into polysulfone membranes, the amount of each additive incorporated was increased from 0 wt% to, 0.125 wt%

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Table 1 Bulk porosity measurements estimated using the wetting method. The glass transition temperature of the membranes is also shown. Membrane

Bulk porosity (%)

Glass transition, Tg ( C)

PSf 0.125%_Lig_PSf 0.5%_Lig_PSf 0.125%_PEG_PSf 0.5%_PEG_PSf 0.125%_PVP_PSf 0.5%_PVP_PSf

20.08 46.23 53.99 24.80 30.07 35.30 63.26

187 180 – 179 166 187 185

(6.79) (5.12) (10.65) (0.35) (0.85) (8.10) (1.04)

NB: The standard deviation is shown in parentheses.

and 0.5 wt%. A sufficient quantity of polysulfone beads was dissolved in a cooled solution of solvent (83 wt%) and additive for a period of 8 h, and the solution was allowed to settle overnight. The polymer dope solutions were then hand-cast on a glass plate using a doctor knife with the blade adjusted to a casting height of 150 mm. Thereafter, the glass plate was immediately immersed in a coagulation bath consisting of deionised (DI) water at 23  C in order to initiate coagulation. The formed membranes were then rinsed three times with DI water for 30 min each time and stored in a refrigerator before characterisation. 3. Characterisation 3.1. Scanning electron microscopy (SEM) The membranes were analysed using a FEI-SIRION Quanta SEM. The membranes were first freeze-fractured in liquid nitrogen to expose a cross-section and then gold-coated to impart electrical conductivity. 3.2. Bulk porosity The unsupported membrane bulk porosity was estimated using Equation (1). This was done by submerging the membranes in deionised water at room temperature for a period of 6 h, after which they were dried in an oven at 80  C overnight. Replicates were performed from four similar membranes cast on different days using a similar method.

ε ¼

mwet  mdry V



rw

 100%

(1)

where: mwet is the wet mass of the membrane mdry is the dry mass of the membrane rw is the density of the wetting solvent (water) V is the wet volume of the membrane 3.3. Thermogravimetric analysis (TGA) The thermal experiments were carried out on a Perkin Elmer TGA 4000 (sample weight of about 20 mg) in the temperature range of 30  C to 800  C at a heating rate of 10  C/min under nitrogen atmosphere with a flow rate of 20 m[/min. Before analysis, the membrane samples were first dried overnight at 80  C to remove moisture.

Fig. 4. TGA graphs of neat additives.

3.4. Differential scanning calorimetry (DSC) Differential scanning calorimetry measurements were performed on a DSC Q2000 Differential Scanning Calorimeter (DSC supplied by TA Instruments – Waters LLC, New Castle, Delaware, USA). These analyses were carried out at temperatures of between 30  C and 300  C at a heating and cooling rate of 10  C/min. The experiments were performed under nitrogen atmosphere at a flow rate of 20 m[/min. The glass transition temperature values of the polymers were taken from the second heating scan. 3.5. Mechanical tests An InstronÒ machine was used to test the tensile strength of the membranes. Samples of about 25 mm, by 0.11 mm by 10 mm with a gauge width of 3.2 mm were used for the analysis. The samples were analysed at a cross-head speed of 5 mm/min and seven replicates were analysed with the average values reported. 4. Results and discussion 4.1. Morphology Figs. 2 and 3 show cross-sectional images before and after the addition of pore-forming agents. In general, all the images show an asymmetrical structure which consists of the top skin layer, the middle sub-layer and the bottom sub-layer. However, the bottom sub-layer thickens and becomes spongy after the introduction of the additives. This observation can be explained considering the processes that take place during coagulation. A porous membrane forms as a result of instantaneous coagulation and a spongy membrane forms when there is delayed demixing [26]. The addition of the hydrophilic additives increased the water tolerance of the casting solution, hence the outflux rate of the solvent decreased on the glass-contacting (bottom) surface [28]. Fig. 3 shows the effects of varying amounts of added lignin (0.125 wt% and 0.5 wt%) on the resultant membrane

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Fig. 5. TGA graphs of PEG, PEG and lignin-modified membranes showing the amount of residue at 800  C.

morphology. The thickness of the bottom part of the sublayer increases with the lignin content, which clearly shows the effect of further delayed de-mixing [27].

been added to cause a change in the viscosity of the dope solution.

4.3. Thermogravimetric analysis (TGA) 4.2. Bulk porosity In order to elucidate the effects of incorporating the different additives into the membrane structure, bulk porosity measurements were performed. In Table 1, the results show that the bulk porosity of lignin-modified membranes increased from 20% to 53%. The increase in the bulk porosity implies an increase in macrovoids. The SEM images of the cross-sectional orientation show no difference in the membrane micro-structure after the addition of PEG or PVP. This suggests that the addition of PEG or PVP to polysulfone does not change the thermodynamic stability of the polymer solution. It is known that the addition of PVP reduces the demixing gap and, in most cases, the thin skin becomes porous and thick due to instantaneous liquid-liquid demixing [28]. The SEM images (Figs. 2 and 3) do not show any increase in the selective skin thickness because of the lower content of the additive used. It should, however, be noted that the porosity of PEGmodified membranes was lower than those modified by PVP and lignin. This can be attributed to the lower molecular weight of the PEG (10 KDa) compared to its counterparts. Membranes with low molecular weight additives have been found to have lower porosity compared to high molecular weight PEG additive [14]. This is because high molecular weight additives have low mobility and, during coagulation, they become entrapped in the membrane matrix and with time leach out. One other explanation for the difference in the porosity is the degree to which each additive affects the thermodynamic and the kinetic stability of the polymer solution [29]. When added, the additive causes a reduction in the miscibility of the casting solution and nonsolvent, which favours thermodynamic enhancement during phase separation. The kinetic hindrance has a negligible effect because very low amounts of additive have

Fig. 4 shows TGA graphs of additives before incorporation into membranes. The first degradation step of each additive defines the extent of its water absorption. The graph shows the weight loss for lignin and PVP, at 96 0C but not for PEG. When considering the amount of lost moisture at 430  C, lignin is more thermally stable than PVP and PEG. The most important observation is the amount of lignin residue at 800  C which stands at 13%. This amount changes significantly when lignin is incorporated into the membranes (Fig. 4). Membranes modified with additives showed no difference from pure polysulfone in the onset of thermal degradation, as shown by the insert in Fig. 5. The amount of 0.5%_Lig_PSf residue remaining at 800  C is the only major

Fig. 6. Effect of increasing the lignin content on the thermal stability.

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Fig. 7. DSC thermograms showing the glass transition temperatures of the different membranes. The effect of using different additives and increasing the additive is shown.

difference observed. When the amounts of alkali-lignin residues (Fig. 4) and PSf (Fig. 5) are compared, they do not add up to the 44% residue of 0.5%_Lig_PSf. This can only be attributed to the interaction of lignin with polysulfone causing a change in its structural formation, since it is not observed with the other additives. The lignin structure has numerous free hydroxyl groups which can bind to the oxygen atom bonded to the sulphur group within the polysulfone polymer skeleton to form hydrogen bonds. These new bonds increase the thermal stability of the entire membrane. The same phenomenon was observed when the content of lignin was increased; the residue also increased as depicted in Fig. 6. Therefore, the amount of hydrogen bonds formed was proportional to the amount of lignin added, and hence the residue remaining. 4.4. Differential scanning calorimetry (DSC) Differential scanning calorimetry can be used to study the membrane structure. In the case of amorphous polymers such as polysulfone, the glass transition temperature, Tg is normally employed to interpret the membrane structure. A lower glass transition temperature generally indicates more free volume in the polymer which tends to loosen its molecular structure [30,31]. It can be seen in Table 1 and Fig. 7 that, without any additives, the glass transition temperature was 187  C and it decreased to 179  C and 180  C after the addition of PEG and lignin, respectively. This can be due to the plasticising effect of PEG. Good agreement was found between these results and those recently reported by Laboulfie et al. when PEG of different molecular size was incorporated into hydroxypropyl methylcellulose (HPMC) [32]. With an increase in PEG additive to 0.5 Wt%, the membrane’s Tg step on the thermogram becomes indistinct, broadens and almost disappears. This is because the pore volume of the membranes increases (corresponding to the bulk porosity) and

the structure easily collapses with heating when compared to membranes without any additive. The addition of PVP into the polysulfone solution also caused a shift in the glass transition temperature to lower temperatures, but by a small margin (4  C) compared to PEG, which shifted the Tg by 20  C. The decrease in the Tg also confirms that the inexpensive and readily available lignin is a comparable additive to the well-known PEG and PVP. 4.5. Mechanical tests The incorporation of additives during membrane preparation usually alters the micro-structure of the membranes, and hence their transport and mechanical properties [15,16]. The modulus decreased considerably when either of the additives was incorporated (Table 2). This is due to the formation of pores within the thin skin and the sub-layer [33]. When 0.125 wt% of lignin was used, the tensile modulus decreased from 194 MPa to 79 MPa compared to the reduction to 53 MPa and 46 MPa for PEG and PVP, respectively. This indicates the level of interaction between polysulfone and the different additives. Although lignin interacts poorly with some commercial

Table 2 Effect of lignin, PEG and PVP additives on the mechanical properties of polysulfone ultrafiltration membranes. Membrane

Tensile stress at break (MPa)

Modulus (segment 1–3 MPa) (Mpa)

Elongation at break (%)

PSf 0.125%_Lig_PSf 0.5%_Lig_PSf 0.125%_PEG_PSf 0.5%_PEG_PSf 0.125%_PVP_PSf 0.5%_PVP_PSf

6.04 4.05 3.49 3.93 4.22 4.22 4.24

194.9 79.52 47.6 53.7 47.0 46.4 46.3

20.0 17.3 12.4 21.9 29.8 32.4 43.0

(0.37) (0.33) (0.25) (0.90) (0.40) (0.50) (0.70)

(16.9) (11.4) (11.8) (7.0) (6.5) (8.9) (6.0)

(1.5) (4.1) (2.6) (3.8) (3.0) (1.9) (15.9)

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polymers, it is known to improve polymer toughness [34]. This could be due to the fact that lignin has numerous functional groups when compared to PEG and PVP. Several researchers have reported that the tensile strength decreases after the incorporation of polymeric additives into polysulfone membranes [12–15]. This is due to the increase in the porosity of the membranes; a similar trend was observed in this study. An increase in the content of lignin further decreases the tensile strength because the membrane becomes more porous. Increasing the PEG and PVP content to 0.5 wt% does not result in a major increase in the tensile strength because the overall amount of the additive is far lower than the amounts reported in the literature; hence fewer traces of the additive remain in the membrane, which is responsible for the reduction in tensile strength [12]. The elongation at break showed contrasting results when lignin and the synthetic polymeric additives were used. An appreciable decrease in the membrane strain was observed when the lignin content was increased. The increase in porosity resulted in numerous breaking points within the membrane, hence a reduced elongation at break. PVP and PEG additives are known to form thick spongy membrane skin layers which may increase the flexibility and mobility of the polymer chains compared to lignin. 5. Conclusions Based on the results obtained from porosity measurements, it can be concluded that the introduction of lignin alters the thermodynamic composition of the polymer solution since it resulted in increased macrovoids. The numerous hydroxyl groups in the lignin polymer necessitated the interaction of polysulfone and lignin through hydrogen bonding, resulting in the formation of thermally stable membranes relative to membranes modified with PEG and PVP. The notable increase in bulk porosity after lignin incorporation renders lignin a preferred additive due to low cost and availability. Although lignin is a good additive, its interaction with the polymer through hydrogen bonding makes it a reinforcing agent. Treating the membranes with an alkali could disrupt the bulk lignin structure and can release it from the polymer in order to open more pores on the membrane. Future work will explore the impacts of chemical post-treatments on mechanical and transport properties of lignin-polysulfone composite membranes. Acknowledgement The authors would like to acknowledge the University of Johannesburg, ESKOM and the DST/Mintek Nanotechnology Innovation Centre for funding of this work. References [1] W. Zhao, Y. Su, C. Li, Q. Shi, X. Ning, Z. Jiang, Fabrication of antifouling polyethersulfone ultrafiltration membranes using pluronic F127 as both surface modifier and pore-forming agent, J. Membr. Sci. 318 (2008) 405–412.

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