Effect of Bentonite Clay on the Mechanical, Thermal, and

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Effect of Bentonite Clay on the Mechanical, Thermal, and Pervaporation Performance of the Poly(vinyl alcohol) Nanocomposite Membranes Thomasukutty Jose,†,‡ Soney C. George,*,†,‡ Maya M. G,† Hanna J. Maria,§ Runcy Wilson,§ and Sabu Thomas∥ †

Centre For Nano Science and Technology, Department of Basic Sciences, Amal Jyothi College of Engineering, Kanjirapally, Kerala, India-686518 ‡ Research and Development Centre, Bharathiar University, Coimbatore, India-641 046 § School of Chemical Sciences, ∥International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, Kerala, India-686 560 ABSTRACT: Bentonite clay/poly(vinyl alcohol) (PVA) inorganic/organic hybrid nanocomposite membranes were prepared via solvent casting method. A series of PVA nanocomposite membranes have been prepared by varying the concentration of the bentonite clay. Nanoscale dispersion of bentonite clay affected the properties of PVA. PVA with 5 wt % clay showed better enhancement in the properties. Both X-ray diffration and transmission electron microscopy showed uniform dispersion and exfoliated structure of the bentonite in the PVA membranes. The effective separation of azeotropic composition of isopropanol (IPA) and water mixtures was also carried out by using these membranes. The flux and separation factor of the membranes were increased with 1 wt % clay loading and followed by a decrease. The intrinsic properties of the membranes have been calculated using membrane permeance and selectivity. The 1 wt % clay loaded membrane showed enhanced membrane permeance with a water permeance of 6500 gpu and a selectivity value of 46. The effective membrane area for transport has been analyzed from atomic force microscopy analysis. Finally, it is important to mention that the minimum nanoscale filler loading gave rise to the maximum separation efficiency for the azeotropic composition of IPA and water mixtures.

1. INTRODUCTION The introduction of particles in nanometer scale into the polymeric materials leads to the enhancement of their properties. One of the factors for this enhancement is the high aspect ratio of the nanomaterials incorporated in the polymeric matrix. Recently, the clay reinforced polymer nanocomposites were found to have better mechanical as well as thermal properties.1−6 It is due to the nanodispersion of clay as well as the exfoliation of clay particles in polymer matrix and the development of nanolayer clusters in polymeric system. Therefore, now a days, great attention is given to the incorporation of layered nanoparticles into polymeric matrix to develop a new nanocomposite material. There are lots of works on polymer−clay nanocomposites based on different polymers such as polypropylene,7,8 epoxy resin,9 polystyrene,10 styrene butadiene rubber,11 polyethylene,12 and so on. The interaction of filler and polymer, nanoscale structure effect, and easily tailored pore size of nanoclay particles attract the attention of modern researchers to the field polymer/clay nanocomposites. Polymers such as nylon-6 showed 103% increase in its tensile modulus with low filler loading, because individual clay layers acted as the reinforcing components in the matrix.13 The thermal stability of the polymers also was improved by the addition of clay particles, and this was first reported in 1965.14 This is because the layered silicate structures act as an excellent insulator and mass transport barrier, slowing the escape of volatile polymer products on decomposition.15,16 Polyimide hybrid with montmorillonite © XXXX American Chemical Society

showed excellent gas barrier properties and a low thermal expansion coefficient.17 Poly(vinyl alcohol) (PVA) is a water-soluble synthetic polymer that exhibits excellent film forming, emulsifying, and adhesive properties. The nanoclay-filled PVA matrix manifested tremendous increase in its properties. The PVA reinforced with montmorillonite clay showed better thermal, mechanical, and water vapor transmission properties because of the nanoscale dispersion of the filler in the polymeric matrix.18−21 The αzirconium phosphate reinforced PVA matrix resulted in the increase in its storage modulus, tensile strength, and elongation at break with increase in the aspect ratio of the filler.22 Because of the economic and environmental factors, the use of bentonite clay is an attractive option in polymeric matrices. It is also very useful as reinforcement for polymeric materials because of its good mechanical and chemical resistance.23 So, the bentonite nanoclay was used as a reinforcing agent in different polymers. The mechanical, thermal, and sorption properties of these polymeric nanocomposites are remarkable.24−26 Pervaporation is an energy efficient separation method, especially for the separation of azeotropic, isomeric, and close boiling point systems. Selection of proper polymers and Received: July 2, 2014 Revised: September 27, 2014 Accepted: October 2, 2014

A

dx.doi.org/10.1021/ie502632p | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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bentonite and PVA were poured on a clean glass plate and dried in a hot air oven at 40 °C for 48 h. Dried membranes were peeled off carefully. Similarly 3, 5, and 10 wt % clay reinforced PVA membranes were prepared. The samples were denoted as PVA0, PVA1, PVA3, PVA5, and PVA10 for 0, 1, 3, 5, and 10 wt % clay, respectively. 2.3. Characterization of the PVA/Bentonite Clay Nanocomposite Membranes. To characterize the nanocomposite membranes, X-ray diffraction (XRD), Fourier transform infrared (FT-IR), transmission electron microscopy (TEM), and atomic force microscopy (AFM) have been used. The thermal stability was obtained from differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) techniques. TEM images of the sample were obtained from Jeol JEM 2100, Japan at 200 kV. AFM studies were performed on few samples using Digital Instrument’s Multimode Scanning Probe Microscope with Nanoscope controller; X-ray diffraction patterns were studied using Bruker Kappa Apex II analyzer with Cu X-ray source and wavelength 1.5406 A0. The FT-IR analysis was performed with PerkinElmer analyzer at the spectrum range 400−4000 cm−1. Thermogravimetric analysis was performed with a PerkinElmer, Diamond TG/DTA analyzer at a heating rate of 10 °C/min. Differential scanning calometry was performed using a Mettler Toledo DSC 822e. The water contact angle was obtained by the sessile drop method using a contact angle meter SEO Phoenix 300 contact angle analyzer. 2.4. Mechanical Properties of the Nanocomposite Membranes. The variation of tensile properties with clay loading was determined on a Tinius Olsen Universal Testing Machine H60KT with cross sectional speed 50 mm−60 kN with sample of dimension 10 cm × 1 cm. The tensile properties such as tensile strength, modulus at different elongation, Young’s modulus, and elongation at break were evaluated. To explain the reinforcing effect of filler on polymeric matrix, Guth and Gold39 introduced a model to explain the reinforcement effect of colloidal filler on polymers. In our present study, the moduli of composite materials are explained by choosing Guth and Halpin−Tsai equations.

preparation of a suitable membrane with high performance are the main factors in pervaporation. Many researchers developed different polymeric membranes for the pervaporation separation of different organic−water mixtures.27−29 However, the performance of PVA/bentonite clay membranes was not well explored in this field. Isopropyl alcohol (IPA) is a widely used solvent in pharmaceutical and chemical industries. IPA forms an azeotrope with water at 87.5 wt %, and most of its application needs high purity. In the industrial point of view, the dehydration of the azeotropic composition of IPA is an important one. The separation of the azeotropic composition of the isopropanol (IPA)−water system is too difficult to carry out by using conventional methods such as distillation. The pervaporation separation of an azeotropic composition of IPA and water using the pervaporation technique would be more efficient than other conventional techniques.30 The dehydration of isopropanol (IPA) using pervaporation process was studied by many researchers.30−33 The majority of researchers concentrate on the wide range of water−isopropanol composition.34−38 However, there is no systematic pervaporation study focused on the separation of an azeotropic composition of IPA and water. So, considering the environmental and economic aspects, in our present study, we choose PVA as the polymeric matrix and bentonite nanoclay as the reinforcing material. The effect of concentration of bentonite clay on PVA nanocomposite was investigated. The nanocomposites membranes were well characterized, and their mechanical and thermal properties and pervaporation performance were investigated. The relevance of the present study is that the PVA membranes are used for the separation of azeotropic composition of isopropanol−water mixture. However, to the best our knowledge, the role of bentonite clay on the pervaporation performance has not been studied yet.

2. EXPERIMENTAL MATERIALS AND METHODS 2.1. Materials. The polymer matrix poly(vinyl alcohol) (PVA), with a number-average molecular weight 1, 25 000 and degree of hydroxylation 86−89%, was supplied by SD Fine chemicals, Mumbai. The Bentonite clay was purchased from Sigma-Aldrich, U.S.A. All other chemicals were laboratory reagent grade and procured from Merck Limited, Mumbai, India. 2.2. Preparation of PVA/Bentonite Clay Nanocomposite Membranes. Solution casting is one of the simplest methods for preparing thin membranes with a thickness of around 100−150 μm. The bentonite clay reinforced PVA nanocomposite membranes were prepared by this method. The 5 wt % PVA membrane was prepared by mixing 5 g PVA in 95 mL water and was stirred up for 1 h using a magnetic stirrer. The magnetically stirred solution was kept for 30 min to remove the air bubbles. Then, the homogeneous mixture was poured on a clean glass plate with the aid of a casting knife in a dust-free atmosphere at room temperature and dried in a hot air oven at 40 °C for 48 h. Dried membranes were peeled off carefully. The bentonite clay reinforced PVA nanocomposite membranes were prepared by varying the concentration of the clay. Bentonite clay (1 wt %) reinforced membranes were prepared as follows. The homogeneous solution of 5 wt % PVA was mixed with 1 wt % bentonite clay, which was sonicated at 40 °C with sufficient amount of distilled water, and magnetically stirred at 40 °C for 2 h. The well dispersed solution of

Em = E0[1 + 2.5ϕ + 14.1ϕ2]

(1)

where Em and E0 are the Young’s modulus of the filled polymers and matrix, respectively, and ϕ is the volume fraction of the filler. However, eq 1 is applicable for spherical fillers in polymeric composites. Later, Guth modified the above equation for rodlike filler particles by introducing a shape factor α (length/ width).40 Em = E0[1 + 0.67αϕ + 1.62(αϕ)2 ]

(2)

Halpin−Tsai equations are also widely used to predict the reinforcement effect of filler in nanocomposites.41 The Halpin− Tsai equations are used to study the reinforcement of filler in the composites.

Em =

E0(1 + ξηϕ) 1 − ηϕ

(3)

where ξ=2× B

length of dispersed phase thickness of dispersed phase dx.doi.org/10.1021/ie502632p | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX


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Figure 1. Schematic representation of the pervaporation experiment.

η=

Ef Em Ef Em

−1

J=

−ξ

(5)

J is the molar flux of the membrane in kg m−2 h−1, Q is the quantity of permeate in kilograms, A is the effective area of the membrane in m2 used for separation, t is time in hours Component flux 45 for the separation of azeotropic composition of IPA and water mixture were calculated by using eq 6

2.5. Pervaporation Experiments. The separation of azeotropic composition of isopropanol and water (87.5−12.5 wt %) was carried out by pervaporation technique using the nanocomposite membranes. The feed liquid mixture (azeotropic composition of IPA and water) is in direct contact with the one side of the membrane, and permeate is removed as vapor by applying low downstream pressure. The generally accepted mechanism for pervaporation is the solution-diffusion model.42,43 A schematic diagram of the system used to carry out the pervaporation experiments is shown in Figure 1. It consisted of a detachable glass cell where the feed was maintained at room temperature for each run. The upper part of the cell holds the feed solution at atmospheric pressure, and the lower part holds the PVA membranes. The permeate was condensed and collected in a liquid nitrogen trap. The flux was calculated by weighing permeate on a digital microbalance Mettler Toledo (JB1603-C/FACT) having an accuracy ±0.0001 g. The feed and permeate compositions were analyzed by gas−liquid chromatography. All experiments were carried out at room temperature. The experiments were repeated three times, and the results were averaged. The permeate pressure was kept at 2 mm/Hg with a rotary vacuum pump. The pervaporation performance of the PVA membranes were evaluated by the separation factor (α),44 and the separation flux (J) by the following expressions

⎛ XB ⎞ ⎜Y ⎟ α = ⎜ XB ⎟ A ⎝ YA ⎠

Q At

JH O = JX H2O 2

JIPA = JYIPA

and

(6)

JH2O and JIPA are the component fluxes, J is the flux, XH2O and XIPA are the permeate composition of the mixtures. Permeability, Permeance, and Selectivity of the Nanocomposite Membranes. The membrane properties in terms of flux and separation factor lag the intrinsic, driving force normalized properties behind the separation. Permeability (Pi), permeance (Pi/l) and selectivity, proposed by Baker et al., might overcome this and lead to understanding the real membrane intrinsic properties.46,47 Membrane permeability is a component flux normalized for membrane thickness and driving force, and membrane permeance is a component flux normalized for driving force. These are given from the expressions Permeability (PiG) = Ji

Permeance (PiG/l) =

l Pi0 − Pil

(7)

Ji Pi0 − Pil

(8)

Permeance is most commonly reported as gas permeation unit (gpu) 1 gpu = 3.349 × 10−10 mol/m 2· s· Pa

(4)

XA and XB are the composition of water in the feed and permeate, respectively. YA and YB are the composition of isopropanol in the feed and permeate, respectively.

Membrane selectivity, defined as the ratio of pemeabilities or permeance of components i and j through the membrane, is calculated using eq 9: C


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PiG PGj

matrix has shifted the characteristic peak 2θ toward the left. The angle of diffraction decreased with increase in the dspacing between clay layers, which indicates the expansion of clay layers and thereby intercalation between PVA and clay layers in the nanocomposite. PVA with 1 wt % clay membranes showed a left shift of angle 2θ with increase in its d-spacing values, and this exhibited a well intercalated network of polymer and clay in the nanocomposite structures. The intercalated structures were formed by inserting several polymer chains in the interlayer of the filler.48 The d-spacing value of PVA1 is being expanded from 14.48 to 16.50 Å, but to a limited extent, which supports the intercalation of the nanocomposite membranes. It can be seen from Figure 2 that with increase in the concentration of nanoclay in the polymeric system, the characteristic peak disappears, supporting the fact that the nanoclay could be totally exfoliated in the PVA matrix. At lower concentrations, nanoclays are intercalated in the polymeric matrix and get exfoliated at higher concentrations. A schematic representation for intercalation is shown in Figure 3. Crystallinity of the PVA was altered upon the addition of bentonite nanoclay. The peak ∼20° 2θ from XRD is due to the semicrystalline structure of the PVA. This peak broadens with an increase in the clay content, suggesting an alteration of crystallinity in the polymer by the addition of clay to the polymer matrix. This was in agreement with the findings of Manians et al.49 and Sammon et al.50 The nanoclay would alter the crystalline nature of the semicrystalline polymer. So, the stiffness of the resulting polymer nanocomposite increases upon addition of nanoclay. Fourier Transform-Infra Red Spectroscopy (FT-IR) Analysis. Figure 4 shows the Fourier transform infrared (FT-IR) spectrum of PVA/bentonite clay nanocomposite membranes. The obtained FT-IR spectra of PVA are in good agreement with literature.51−53 Absorption band around 518(w), 1663(m) can be assigned for bending and symmetric vibrations. The hydroxyl group shows a strong absorption band around 3200 cm−1. The bands around 2900 and 2850 cm−1 correspond to the asymmetric and symmetric stretching of −CH2− groups, respectively. The peaks at 1450 and 1360 cm−1 can be attributed to C−H bending and rocking, respectively. The band at 1028 cm−1(m) can be assigned to Si−O in the bentonite clay. The FT-IR spectrum in the hydroxyl region shows a peak at 3640 cm−1 (m, broad) for silanol −OH group.54 The interaction of polymer and clay in the nanocomposite was clearly identified from the four regions of Figure 4. The characteristic peak of Si−O diminishes in the spectra of the nanocomposites (1st region in Figure 4), which implied the better interaction of polymer and clay in the composites. With increase in the concentration of nanoclay, the peak appearing at

(9)

3. RESULTS AND DISCUSSION 3.1. Characterization of the Nanocomposite Membranes. X-ray Diffraction. Generally, the structural features of the nanocomposites were determined by the X-ray diffraction techniques. The XRD also could provide the exfoliation and intercalation in polymer−clay nanocomposites. The X-ray diffraction patterns of the nanoclay and polymer nanocomposite are shown in Figure 2. In the XRD spectra,

Figure 2. XRD spectra of the PVA/bentonite clay nanocomposites membranes.

bentonite clay showed a characteristic peak at 6.096° with dspacing 14.486 Å. The addition of nanoclay to the polymeric

Figure 3. Schematic representation of the clay intercalation in the polymeric matrix. D



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beyond 5 wt %, agglomeration of the nanoclay layers occurred in the structure of nanocomposite membranes (Figure 5e). The TEM images of the nanocomposites support the intercalated network of the PVA/bentonite nanocomposite membranes with an interlayer distance of 2.04 nm (Figure 5f). The intercalated network of clay is obtained at lower concentration, but exfoliates when concentration of the clay increases. The intercalation and exfoliation is clearly seen in the TEM images and it supports the XRD analysis. 3.2. Mechanical Properties. Stress−strain curve for the nanocomposite membranes were presented in Figure 6. The stress−strain values increases linearly with the concentration of the filler and maximum at 5 wt % clay loading. Thus, the nano dispersed clay with high aspect ratio possesses a higher stress bearing capability and efficiency to the virgin polymer.58 Well dispersed nanolayer in polymeric matrix would cause the improvement in the mechanical properties of the nanocomposite membranes. The tensile testing was performed to evaluate the effect of the bentonite clay on the mechanical properties of the nanocomposite membranes. Tensile properties of the composites are given in Table 1. The data revealed a remarkable increment in the tensile properties of 5 wt % clay nanocomposite membranes. The nanocomposite membranes with 5 wt % clay showed 50% increase in their tensile strength as compared to the pure PVA membranes. The bentonite clay exhibited a layered structure, and it was clearly identified from the TEM images of the nanocomposite membranes (Figure 5f). It has been already reported that the addition of layered filler particles can easily enhance the mechanical properties of the polymers.59 Thus, the mechanical properties of the PVA matrix increase with increase in the filler loading. The well dispersed layered silicate in the polymeric matrix enhances the tensile properties of the nanocomposite membranes. It is evident from the TEM images of 5 wt % clay membranes that a uniform dispersion of clay particles happened in the polymer matrix. On the other hand, if the concentration of clay exceeds 5 wt %, the tensile properties decrease. The dispersion of nanoclay becomes more difficult with increases in clay content and the nanosized particle tends to agglomerate. This results in the decreased tensile properties of the nanocomposite with higher filler loading. The percentage of EB decreases with increases in the nanoclay loading. PVA with 5 wt % clay loaded membrane showed 20% decrease in EB, which suggest that the addition of nanoclay increases the strength of the resulting nanocomposite membranes. The Young’s modulus of the nanocomposite membranes also increases with the filler loading. PVA with 5 wt % clay membranes shows 54% increase in its modulus. The overall mechanical strength of the membranes increases with bentonite loading. Theoretical Modeling. In bentonite, the silicate layers exhibit good planar orientation. Halpin−Tsai equations could be directly applied to predict the modulus of layered silicate nanocomposites. The aspect ratio of clay platelets must be needed for the modeling of modulus of PVA-clay nanocomposites. The calculated data of the aspect ratio for the nanocomposites membranes are listed in Table 2. These data were obtained by measuring TEM images of the nanocomposites having clay content 1−10 wt %. The aspect ratio showed small variation with the filler concentration. The experimental data and modulus predicted by the Guth and Halpin−Tsai are presented in Figure 7. As shown in Figure 7, the predicted moduli by two models are higher than the

Figure 4. FT-IR spectra of the PVA/bentonite nanocomposite membranes.

1028 cm−1 in FT-IR spectrum is shifted to1085 cm−1 because of the Si−O stretching. The shift in peak position with change in the intensity indicates the interaction between PVA and layers of clay.55 An important absorption band was observed at 1040−1150 cm−1. According to the literature,56,57 this band corresponds to the crystallinity of PVA and is related to the C−O stretching. So, for PVA-like semicrystalline materials, the absorption band at 1142 cm−1 can be used as assessment tool for its structure. The C−O stretching band is altered by the addition of nanoclay to the PVA matrix (region 1 in Figure 4). The addition of nanoclay should affect the crystalline nature of the semicrystalline polymer, PVA. The intensity of −OH peak is decreased upon the addition of nanoclay (region 4 in Figure 4). The decrease in peak intensity of −OH stretching in this region also revealed the better interaction between nanoclay and the polymeric matrix. Transmission Electron Microscopy (TEM). For making good nanocomposite membranes, the well dispersion and distribution of nanofillers in polymeric matrices is one of the important factors. TEM is used to observe the dispersion of the bentonite clay in the PVA matrix. The TEM images of all the nanocomposites prepared are displayed in Figure 5. The TEM images revealed well dispersion of the nanoclay in the polymer matrix. The dispersion of nanoclay increases with increase in the concentration of the nanoclay without aggregation. The most uniform and well dispersed structure was shown by 5 wt % nanoclay nanocomposite membranes (Figure 5c and d). On increase the concentration of the clay E



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Figure 5. TEM images of the PVA/Bentonite clay nanocomposite membranes. (a) PVA1, (b) PVA3, (c and d) PVA5, (e) PVA10, (f) intercalated structure of 1 wt % PVA nanocomposite (20 nm).

Em = E0[1 + 0.67α(MRF)ϕ + 1.62(MRF)2 (αϕ)2 ]

experimental data. This is due to the contribution of twodimensional plate-like clay to the modulus being less than that of one-dimensional fillers.60−62 Therefore, the modulus reduction factor (MRF) is introduced to overcome the morphology difference between plate-like fillers and fiber-like fillers.62 MRF is related to the aspect ratio and morphology of the filler, and eqs 2 and 3 become eqs 10 and 11, respectively.

Em =

E0(1 + ξ(MRF)ηϕ) 1 − ηϕ

(10)

(11)

With the introduction of MRF, as in Figure 7, the moduli values at filler concentration up to 5 wt % are well fitted to the F



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experimental data. So Halpin−Tsai and Guth model with MRF can be effectively used for the theoretical predictions of the modulus of the nanocomposite and it shows good agreement with experimental values up to 5 wt %. At the higher filler loading, the theoretical model cannot be used effectively to explain the modulus of the composite, because of the agglomeration of nanofillers. 3.3. Differential Scanning Calorimetry and Thermogravimetric Analysis. The thermal stability of the nanocomposites was analyzed by DSC and TGA. DSC is the technique used to determine the quantity of heat either absorbed or released when substances undergo physical or chemical changes. In the present study, the glass transition temperature and the effect of clay on the glass transition temperature (Tg) values of the polymer was revealed using DSC analysis. The Tg values are summarized in Table 3. The Tg Table 3. Effect of Bentonite Clay on the Glass Transition Temperature (Tg) of the PVA/Clay Nanocomposite Membranes

Figure 6. Stress−strain curve of PVA/bentonite clay nanocomposite membranes.

Table 1. Mechanical Properties of PVA/Clay nanocomposite Membranes with Different Concentration of Bentonite Nanoclay sample PVA0 PVA1 PVA3 PVA5 PVA10

tensile strength (MPa) 19.83 20.30 20.75 29.50 26.70

± ± ± ± ±

5 2 5 3 2

elongation at break (EB%) 480 470 472 422 435

± ± ± ± ±

2 2 3 3 2

Young’s modulus (MPa) 41 44 63 69 82

± ± ± ± ±

3 2 3 2 4

sample

aspect ratio, α (length/thickness) 20.54 21.92 23.42 20.20

Tg (°C) 34 36 62 66 60

value of the composite is shifted to higher temperature with increase in the nanoclay content. The movement of the macromolecular polymer chain is restricted by the nanoclay (Figure 8), so Tg increases. The uniformly distributed clay layers can distribute the applied heat throughout the polymer chain. Thus, the Tg value of the composite was shifted to higher temperature. The incorporation of nanoclay had an effect on the amorphous region of the polymer matrix, and so, the Tg of the nanocomposite is shifted to higher temperature. On minimum filler loading, the nanoclay has no significant effect on the amorphous region and so the Tg value of 1 wt % clay loaded membrane is close to pristine polymer. The TGA used to determine materials thermal stability and its fraction of volatile components by monitoring the weight loss of the sample in a chosen atmosphere as a function of temperature. The TGA experimental results of the PVA/ bentonite clay nanocomposites are shown in Figure 9. Figure 9 shows that the weight loss of the nanocomposite membranes decreases as the temperature increases. The thermal degradation of the nanocomposites was less compared to the pure sample. The nanoclay protects the polymer against thermal degradation. The DTG results of all samples exhibited three distinct degradation stages (Figure 10). The first weight loss occurred at around 100 °C because the moisture escaped during heating process. The major weight loss occurs around 200−350 °C for all samples, which corresponds to the structural decomposition of PVA. The third stage corresponds to the side chain decomposition of PVA around 430°. The observed three stage degradation is supported by the literature.63 The PVA with 5 wt % clay shows maximum thermal stability, and the 50% degradation occurs around 354 °C, which indicates the addition of nanoclay altered the thermal stability of the polymeric membranes. The TGA results are given in Table 4. The thermal stability is higher for 5 wt % bentonite filled samples and is around 436

Table 2. Aspect Ratio of the Bentonite Clay in PVA/Clay Nanocomposite Membranes PVA1 PVA3 PVA5 PVA10

sample PVA0 PVA1 PVA3 PVA5 PVA10

Figure 7. Theoretical modeling of Young’s moduluscomparison with experimental data. G



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Figure 8. Schematic representation of the immobilization of the polymer chains by the clay.

Table 4. Thermogravimetric Analysis (TGA) of the PVA/ Clay Nanocomposite Membranes sample

T10 (°C)

T50 (°C)

TMAX (°C)

residue at 350 °C (%)

residue at max. sample temp. (%)

PVA0 PVA1 PVA3 PVA5 PVA10

231 258 224 271 247

337 349 327 354 341

430 431 434 436 435

41 41 34 53 45

0.05 0.08 0.09 7.00 0.50

the polymeric matrix, the thermal stability of the polymer enhanced significantly. 3.4. Contact Angle Analysis. The hydrophilicity variation of membranes upon addition of nanoclay was investigated by contact angle measurements. The data obtained from the analysis is shown in Table 5. The water contact angle of the Table 5. Water Contact Angle of the PVA/Clay Nanocomposite Membranes

Figure 9. TGA curve. Weight loss of the PVA/clay nanocomposite membranes against temperature.

sample

contact angle (deg)


63.00 61.16 59.85 58.00 52.00

nanocomposite membranes decreases with increase in the concentration of nanoclay. As expected, the hydrophilic bentonite clay in the polymer increased the hydrophilicity of the nanocomposite membranes. The hydrophilic nature of the membrane surface increases with the presence of the hydrophilic bentonite, so the contact angle decreases. This indicates that the affinity between the water molecules and the PVA/clay membrane, which is higher than that between the isopropanol and the PVA/clay membrane. The membranes are selective toward water than that of organic isopropanol. The hydrophilicity is one of the important factors for the pervaporation separation of water and isopropanol mixture. 3.5. Pervaporation Performance of the Nanocomposite Membranes. The separation of azeotropic composition of isopropanol−water mixtures through PVA/clay nanocomposite membranes was carried out. In pervaporation, the better performance of the membranes is achieved by getting increased flux with higher selectivity. Thus, development of such membranes was highly challenging one. Here, Figure 11 shows the effect of concentration of bentonite on flux and separation factor of the nanocomposite membranes. The

Figure 10. TGA curve. Derivative weight of the PVA/clay nanocomposite membranes against temperature.

°C. All other samples show better thermal stability than the pure polymer. The increase in the thermal stability was due to the presence of nanoclay layers, which minimize the permeability of volatile degradation product to the material and maximize the heat insulation. On addition of nanofillers to H



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flux.64 So, the incorporation of minimum amount of bentonite clay to the PVA matrix showed 400% increase in its pervaporation flux. Figure 11 also presents the behavior of the separation factor of the nanocomposite membranes with the concentration of bentonite clay. The separation factor increases upon the addition of 1 wt % clay membranes and then decreased with a further increase in the concentration of bentonite clay. This kind of behavior of separation factor might be due to the hydrophilic attraction of PVA/bentonite membranes with water. The separation factor increased 60% for 1 wt % clay loading, as compared to pure polymer. Thus, the minimum amount of clay provided the best PV performance. With this minimum concentration of bentonite clay (i.e., 1 wt %), we developed a membrane with increased flux and separation factor. Figure 13 displays the variation of component flux with the concentration of the bentonite. From the figure, it is evident

Figure 11. Variation of flux and separation factor with concentration of nanoclay for the separation of azeotropic composition of IPA−water mixture.

affinity of membranes toward water is a major fact for the separation of water and IPA azeotropic mixtures. It can be observed from Figure 11 that the greatest increase in the flux was showed by PVA with 1 wt % clay; that is, PVA1 nanocomposite membrane is more suited for solution-diffusion mechanism. According to this mechanism, there are three consecutive steps in pervaporation: (a) sorption of permeate from the feed liquid to the membrane, (b) diffusion of permeate in the membrane, and (c) desorption of the permeate to the vapor phase (Figure 12). The preferential affinity of

Figure 13. Variation of component flux with concentration of bentonite clay.

Figure 12. Schematic representation of the mechanism of pervaporation separation of liquid mixtures through intercalated network structure.

that the component flux increases by the addition of 1 wt % clay. The majority of total flux was for water than IPA, so PVA/ bentonite clay membrane is an effective membrane for the dehydration of IPA from its azeotropic composition. The hydrophilic nature of the PVA and bentonite was more favorable to water components for their transport through the membrane, but comparatively larger molecules such as IPA hindered the diffusion through the membrane, and thus, the component flux decreases. The values of enrichment factor were higher for PVA1 compared to other PVA membranes (Table 6). Thus, on increasing the concentration of hydrophilic clay, the separation factor decreases. The bentonite clay induces hydrophilicity to the polymeric membranes, so the organics (IPA) are retained on the feed side, and the membrane is more selective toward the water at minimum filler loading. When the concentration of clay increases, the hydrophilic nanocomposite restricts the permeation of particles through it; then, the flux and the separation factor of the membrane drop. Thus, the PVA/ bentonite clay membranes are effective membranes for the

membranes toward water would increases the permeation rate. This occurs because the swelling of the membrane in the water mixture increases with the addition of clay. Thus, PVA with 1 wt % clay membranes showed a maximum enhancement for flux compared to other nanocomposite membranes. However, flux values decrease upon the addition of more bentonite clay into the polymer matrix. This occurs because the addition of clay decreases the free volume for the movement of solvent molecule through it. Thus, the flux reduces. The combined effect of intercalation and exfoliation of the clay layers in the matrix and increment in the glass transition temperature of the composite plays key role in the overall reduction in the flux. Thus, the membrane becomes stiff, and the permeation rate decreases for higher filler loading and correspondingly the flux, also. The improvement in the hydrophilic character of the polymeric membrane would increase the extent of permeation I



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selectivity of 46. Thus, on minimum filler loading, the membrane is more water selective. The selectivity of the membranes shows 85% increase as compared to pure polymer membrane. The water permeability of the nanocomposite increases with filler loading and maximum obtained with 1 wt % clay loading. This fact should be ascribed to a selective interaction of water molecule and the membrane. PVA contains free −OH groups, which can form H-bonds with water molecules.66 The bentonite nanoclay should induce hydrophility to the PVA membranes, so the selectivity and permeance of the membrane increases with minimum filler loading. The minimum filler loading forms an intercalated network of polymeric membrane. Thus, the swelling of the amorphous region of the membrane become more and hence polymeric chain become more flexible. Therefore, minimum energy is required for the diffusive transport through the membrane. After 1 wt % clay loading, the membrane selectivity and flux decrease, the hydrophilic bentonite nanoclay dispersion should cause increased hydrophilicity (contact angle decreases). Thus, the water permeation through the membrane decreases. These phenomena might occur because of increased swelling at higher filler loading and the higher mobility of polymer chains. This accelerates the transport of isopropanol molecule along with water molecules. This diffusive transport of IPA results in a decreased selectivity at higher filler loading.

Table 6. Effect of Bentonite Clay on the Membrane Permeance and Selectivity of PVA/Clay Nanocomposite Membranes sample

enrichment factor

permeance (gpu)

selectivity


5.76 6.56 6.40 6.08 5.92

5799 6576 6364 6330 4826

25.96 46.06 40.39 33.86 33.40

dehydration of azeotropic composition of IPA with minimum filler loading. Atomic force microscopy (AFM) is used to evaluate the surface roughness of the nanocomposite membranes. The values of surface roughness and AFM images are shown in Figure 14. The roughness values for PVA1 and PVA5 are 9.48

4. CONCLUSION The nanodispersion of bentonite nanoclay in the PVA matrix was achieved by simple solution casting method. Clay layers showed good interaction with PVA matrix and formed intercalated structure at lower clay loading and exfoliation at higher filler loading. The TEM images confirmed the well dispersion of nanoclay in the PVA nanocomposites. XRD and TEM supported the intercalation and exfoliation of PVA/clay nanocomposite membranes. The mechanical properties of the PVA/clay nanocomposite membranes increased remarkably by the incorporation of nanoclays. Moreover, a good agreement was obtained between the experimental and the theoretical prediction of modulus by the Halpin− Tsai and Guth hypothesis for layered filler network. The interaction of PVA/ clay is also confirmed from FT-IR technique. The bentonite clay layers reduced the degradation of the polymer chains, and therefore, the thermal stability of PVA/clay nanocomposite membranes showed significant enhancement. The pervaporation properties of the PVA membranes for the separation of azeotropic composition of IPA and water were also found to be affected by the nanoclay in the nanocomposite membranes. Bentonite clay incorporated membranes exhibited a significant improvement in the performance while separating water−isopropyl alcohol azeotropic mixtures. The addition of 1 wt % bentonite clay in the membrane matrix resulted in a simultaneous increase of both the flux and selectivity. This was attributed to a significant enhancement of the hydrophilic character of the nanocomposite membranes. The minimum amount of clay loaded membrane effectively separated the azeotropic composition of IPA and water. The intrinsic property of the membranes in terms of permeance and water selectivity is also calculated. Higher membrane permeance and selectivity are obtained with minimum filler loading. Thus, the developed PVA/clay membranes could be used commercially as selective membranes for the separation of azeotropic

Figure 14. AFM images of PVA1 and PVA5 nanocomposite membranes.

and 3.76 nm, respectively. The surface roughness of the 5 wt % PVA/clay membrane is lower than that of the PVA1 membrane. The effective transport area of the membrane increases with an increasing membrane roughness.65 Thus, the separation factor and selectivity increases with 1 wt % clay loading and decreases sharply when the clay content is higher than 1 wt % in the PVA/clay nanocomposite membranes. Permeability, Permeance, and Selectivity of the Nanocomposite Membranes. The selectivity can be expressed in terms of membrane permeance. The calculated value for membrane permeance and selectivity was shown in Table 6. Pure PVA membrane is water selective with a water permeance of 5799 gpu and isopropanol permeance of 223 gpu. On the addition of nanoclay to the PVA matrix, the water permeance increases and the membrane become more selective toward water. The maximum membrane permeance is shown with 1 wt % clay loading, and the membrane permeance is 6576 gpu with J



Article

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composition of isopropanol and water. The commercial scale up of the work is in progress.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 09447870319. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Kerala State Council for Science, Technology, and Environment (KSCSTE), Thiruvananthapuram, Kerala, India, for the financial support of the project, and Sophisticated Analytical Instrument Facility (SAIF), Sophisticated Test and Instrumentation Centre, Cochin, Kerala, India, for the analysis carried out. We thank Dr. N. K Jayaraj, Department of Physics, Cochin University of Science and Technology (CUSAT) Cochin, Kerala, India, for AFM analysis.

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Effect of Bentonite Clay on the Mechanical, Thermal, and

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