EVA/Clay Nanocomposites Transport Features

Transcription

1 Chapter 9 EVA/Clay Nanocomposites Transport Features Summary

2 228 Chapter Introduction Polymer-clay nanocomposites are hybrid composite materials consisting of a polymer matrix with dispersed clay nano particles. Nano clays have been widely used as an inorganic reinforcement for polymer matrices with nano scale dispersion of the inorganic phase within the polymer matrix [1-4]. The typical feature size of each filler platelet is approximately 1nm in thickness, and nm in length. Mechanical properties are improved due to the reinforcing effect of the particles [5-7], whereas the thermal stability is increased [8-9] and the thermal expansion coefficient is reduced [10]. The large surface to volume ratio of the nano fillers suggests that the particles may affect the segmental mobility of the polymer phases provided that the polymer molecules are efficiently attracted to the filler particles in a way similar to that of amorphous chain segments to crystals in a semi crystalline polymer. Polymer-clay nanocomposites have attracted the attention of many researchers [11-13]. Drozdav et al. [14] showed that the diffusion process becomes anomalous with higher clay content. Hedengvist et al. [15] studied the diffusion of methanol through spray dried cheese whey proteinmontmorillonite nanocomposites. Musto et al. [16] examined the diffusion of water and ammonia through polyimide-silica nanocomposites. Merkel et al. [17] reported an increase in permeability by adding nano structural fumed silica to several glassy high free volume polymers. Valsaveld et al. [18] prepared polyamide- silicate nanocomposites and analysed the influence

3 EVA/Clay Nanocomposites Transport Features 229 of silicate concentration on the diffusion characteristics and the mechanical properties. Shantaii et al. [19] prepared polyimide-based nanocomposites and their properties were characterised by kinetics of water uptake. Aminabhavi and co-workers [20] used poly (vinyl alcohol)-iron oxide nanocomposite membranes for pervaporation. High permeability, good selectivity and stability are the important factors in choosing suitable pervaporation membranes. The substantial decrease in moisture permeability was reported for nano clay-polyamide composites by Okada et al. [21]. The permeability performance of nano composite normally depends on the clay content, aspect ratio and the degree of dispersion of silicate layers [22]. The reduction in vapour permeability was attributed to the extremely high aspect ratio of clay platelets, which increased the tortuosity of the path of gas or vapour molecules as it diffuses into the nanocomposite. The wide application of membranes for gas separation has attracted many polymer technologists to synthesis new polymeric membranes with good permeability and selectivity [23]. Polymer layered nanocomposites have attracted many scientists due to the dramatic improvements in the gas barrier properties of polymers [24, 25]. In this study EVA/clay nanocomposites containing different filler loading have been prepared. The nano clay used was closite Na+ which has no organic modifier. Transport of aromatic hydrocarbons, pervaporation of chloroform-acetone mixtures, permeation of chlorinated hydrocarbon

4 230 Chapter 9 vapours and gases like O 2 and N 2 were investigated. Morphology of nanocomposites were analysed by XRD and TEM. Positron annihilation lifetime spectroscopic analysis (PALS) has been used to estimate the free volume of nanocomposites Results and Discussion A) Transport of aromatic hydrocarbons through EVA/clay nanocomposites Morphology X-ray diffraction analysis (XRD) XRD is widely used for the characterization of the structure of layered silicate and polymer nanocomposites. The change in the d-spacing of the polymer nanocomposite is observed from the position of the peaks in the XRD patterns in accordance with the well known Bragg s equation. n = 2d Sin (9.1) where n is an integer which gives the order of reflection, is the wave length, d is the d-spacing and is the angle of diffraction. X-ray diffraction method has been used to characterise the formation and structure of polymer-silicate hybrids by monitoring the position, shape and intensity of the basal reflection from the silicate layers. When insertion of polymer chains in the silicate layers occurs, an increase of silicate interlayer volume and corresponding layer spacing could be obtained which in turn gives rise to the shifting of diffraction peaks to lower angles. Diffraction peak cannot be seen in the case of exfoliated structures where silicate

5 EVA/Clay Nanocomposites Transport Features 231 layers are completely and uniformly dispersed in a continuous polymer matrix [26] The X-ray diffraction patterns of the nano clay and polymer nanocomposites are shown in Figure 9.1. Closite Na+ clay exhibits a single peak at an angle 2θ of 7 o corresponding to a d-spacing of 11.7 A o. For EVA/clay nanocomposites, the characteristic diffraction peak moves to a lower angle with respect to that of nano clay. For the composite samples containing 3 (F 3 ), 5 (F 5 ) and 7 (F 7 ) wt% of clay, the d-spacing were found to be 16.3, 16 and 14.6 A o corresponding to 2θ 5.4, 5.5 and 6.04 o respectively. This shows that EVA chains have intercalated into the interlayers of closite Na +. It is found that in all systems the interlayer spacing increases due to the intercalation of polymer into the layers of nanoclay. Enhanced interlayer distance indicates that the layered structure is retained. With the increase of clay content, the left shift magnitude of diffraction peak decreases, that is, the enlargement extent of the interlayer distance of the clay decreases. This indicates that lower the loading of nanoclay the more favourable it is for the intercalation of EVA chains into the silicate layers.

6 232 Chapter 9 Closite Na + Intensity 2 θ Intensity F 7 F 5 F 3 2 θ Figure 9.1 : XRD of nanoclay and EVA clay nanocomposites Transmission electron microscopic analysis (TEM) The transmission electron micrographs of various EVA-clay nanocomposites are presented in Figure 9.2. The dark lines in the transmission electron micrographs show the dispersion of silicates in the polymer matrix. It can be seen that in F 3 sample, the clay is well dispersed in the matrix and is having a more ordered exfoliated structure. When the percentage of clay increases, dispersion decreases and clay exists as large aggregates and is unable to undergo exfoliation. The above observation is consistent with the data observed from the XRD patterns given in Figure 9.1.

7 EVA/Clay Nanocomposites Transport Features 233 a) EVA+DCP+3%F (F 3 ) b) EVA+DCP+5%F (F 5 ) c) EVA+DCP+7%F (F 7 ) Figure 9.2 : TEM images of nanocomposites Positron annihilation lifetime spectroscopic analysis (PALS) Free volume present in nanocomposite systems play a major role in determining the overall performance of the membranes. PALS is an efficient technique used for analysis of free volume. The diffusion of permeant through polymeric membranes can be described by two theories, viz. molecular and free volume theories. According to free volume theory the diffusion is not a thermally activated process as in molecular model but it is assumed to be the result of random redistributions of free volume voids within a polymer matrix. Cohen and Turnbull [27] developed the free volume models that describe diffusion process when a molecule moves into void larger than a critical size; V c. Voids are formed during the statistical redistribution of free volume within the polymer. The effect of layered silicates on o-ps lifetime (τ 3 ), o-ps

8 234 Chapter 9 intensity (I 3 %) and relative fractional free volume % which are presented in Table 9.1. It can be deduced from the table that relative fractional free volume % is lowest for F 3 system. It is found that the relative fractional free volume of unfilled polymer decreases upon the addition of layered silicates. The decrease is attributed to the interaction between layered silicates and polymer due to the platelet structure and high aspect ratio of layered silicates. The decrease is explained to the restricted mobility of the chain segments in the presence of layered silicates. This results in reduced free volume concentration or relative fractional free volume. The contact surface area between the filler and the matrix is higher in nanocomposites owing to its high aspect ratio, which in turn reduces the free volume concentration. It is also found that the relative fractional free volume % increases with clay loading. The increase in the values of fractional free volume values, can be attributed to the aggregation of fillers and the consequent additional void formation. The impact of nano particles on the free volume and the barrier properties has been studied by Wang et al. [3] and R. Stephen et al. [2]. They concluded that the permeability of nanocomposite is mainly influenced by fractional free volume effects.

9 EVA/Clay Nanocomposites Transport Features 235 Table 9.1 : PALS measurement data of nanocomposites Sample o. P s lifetime, τ 3 + n s o. P s intensity I % Relative fractional free volume % F F F F Influence of nano particles on diffusion The influence of nano clay on the sorption behaviour of EVA is presented in Figure 9.3. The experiment was conducted at 28 o C and the solvent used was benzene. Unfilled (F o ) sample showed the maximum and filled sample with 3wt% of clay (F 3 ) showed the least solvent uptake values. A similar trend was also observed with other solvents. The increase in the barrier properties of EVA membranes reinforced with layered silicates are due to the exfoliation of silicates in the polymer matrix. The molecular level interaction of polymer/clay results in reduced availability of free volume which in turn reduces the diffusion through the membrane. The reduced sorption and diffusion of filled membranes is owing to its platelet like morphology and high aspect ratio of clay particles. Similar results were reported previously [2,15]. The influence of weight % of clay on the equilibrium uptake of solvents is given in Figure 9.4. It is observed that, as the amount of clay increases, the equilibrium uptake decreases. This is attributed to the difference in the dispersion of clay particles in the matrix. TEM images shown in Figure 9.2

10 236 Chapter 9 clearly reveals that at higher filler loading, aggregation of filler particles occurs due to its poor dispersion in the matrix. Thus a microphase separation was formed between the polymer and clay particles, resulting in an increased uptake of solvents. Similar results were reported in the literature [26]. 2.5 F 0 F Q t (mol%) Time 1/2 (min) 1/2 Figure 9.3 : Influence of nano particles on diffusion Q Filler loading (wt%) Figure 9.4 : Influence of the weight percentage of filler on diffusion

11 EVA/Clay Nanocomposites Transport Features Sorption behaviour The sorption behaviour for the system under investigation has been followed by the equation 3.1 (Chapter 3). The values of n and k are determined by power regression analysis of the linear portion of plots Q t versus square root of time. To ensure linearity, values upto 50% of the equilibrium uptake were only used. The values of n and k are placed in Table 9.2. The values of n for nanocomposites lie in between 0.5 and 1, thus the sorption behaviour was found to be anomalous. Table 9.2 : Analysis of Sorption data at 28 o C Solvent n K x 10-2 F 0 F 3 F 5 F 7 F 0 F 3 F 5 F 7 Benzene Toluene Xylene Diffusion coefficient The diffusion coefficient (D) was calculated using the equation 4.3 (Chapter 4). The calculated values of diffusion coefficients are given in Table 9.2. It is found that EVA/clay nanocomposite membranes show reduced diffusion coefficient values. The result shows that the barrier properties of the polymer nanocomposite membrane are remarkable. The values of

12 238 Chapter 9 diffusion coefficient show that the impermeable clay layers produce a tortuous pathway for a penetrant to transverse the nanocomposite. This not only enhances the barrier characteristics but also reduces the solvent uptake. Chen et al. [28] showed the reason for the reduction of the solvent diffusion coefficient of nanocomposites. They explained that the reduction is due to the hindered diffusion pathways caused by the dispersion of the individual nano sheets of the layered silicates in the nanocomposite. The decrease in the diffusion rate of the polymer membranes modified with nano clay is due to the nano metric level dispersion of the organic and inorganic phases. Hence the available free volume decreases and this results in the reduction of diffusion. Due to the platelet like morphology of silicates the nano filled matrix exhibits reduced diffusivity owing to the increase in tortuosity of the path. The ordered dispersion of clay is maximum for F 3 sample, which is evident from TEM picture. Hence F 3 sample showed the least diffusivity but the diffusivity increases as a function of filler loading. This can be explained in terms of aggregation of fillers at higher filler loading which leads to an increase in free volume of the samples. Table 9.3 : Values of diffusion coefficient (Dx10 11 m 2 s -1 ) at 28 o c. Solvent F 0 F 3 F 5 F 7 Benzene Toluene Xylene

13 EVA/Clay Nanocomposites Transport Features Temperature effects and activation parameters The temperature dependence of diffusion through nano clay filled EVA was followed by conducting the experiments at 50 and 70 o C in addition to those at 28 o C. In Figure 9.5, Q t mol% uptake is plotted as a function of time at various temperatures for the F 3 system. The solvent used was benzene. It has been observed that maximum solvent uptake increases with increase in temperature. All other systems showed the same trend. It is also found that the slope of the linear portion increases with temperature showing that the transport process is temperature activated Q t (mol%) Time 1/2 (min) 1/ C 50 0 C 70 0 C Figure 9.5 : Influence of temperature on the sorption behaviour of nanocomposite The energy of activation for the diffusion and permeation process is calculated from the Arrhenius relationship.

14 240 Chapter 9 The values of activation energy for diffusion, E D and the activation energy for permeation, E P were estimated. From the difference between E P and E D, the heat of sorption, H S was estimated. The values of E P, E D, and H S in benzene are complied in Table 9.4. It is found that activation energy for diffusion and permeation for the unfilled sample is lower than that of nano clay filled polymer membranes. The permeant molecules require greater activation energy to travel through the layered silicates. The large aspect ratio of the clay platelets, effectively increased the diffusion path, which was responsible for the increased activation energy. Table 9.4 : Activation parameters of diffusion Sample E D kj mol -1 E P kj mol -1 H S kj mol -1 F F F F The value of H S gives additional information about the molecular transport through the polymer matrix. H S is a composite parameter involving contribution from Henry s law and Langmuir type sorption. All values are positive suggesting that, sorption is mainly dominated by Henry s law, i.e. the formation of sites and the filling of these sites by penetrant molecules.

15 EVA/Clay Nanocomposites Transport Features Polymer-Solvent interaction parameter The polymer-solvent interaction parameter (χ) has been calculated from the equation 4.11 (Chapter 4). The polymer-solvent interaction parameter has been utilized to explain the interaction between the solvents and the EVA samples. A low value of χ indicates stronger interaction with solvents. The calculated values are placed in Table 9.5. The χ values of the nano clay modified EVA samples are higher than that of the unfilled sample. This shows that the interaction of nanocomposites with the solvents is minimum. All the samples showed maximum interaction with benzene. Table 9.5 : Values of interaction parameter Solvent F 0 F 3 F 5 F 7 Benzene Toluene Xylene Network structure analysis The molecular mass between crosslinks was estimated using the Flory- Rehner equation 4.13 (Chapter 4). The calculated M C values are given in the Table 9.6. The decrease in Mc values of filled samples compared to unfilled one is due to the reinforcement of clay in the polymer and hence the stiffness of the material

16 242 Chapter 9 increases. Lower values of M c indicate that the network is more restrained and this result in lower swelling of these samples. Table 9.6 : Values of molecular weight between crosslinks (g/cc) M c Solvent F 0 F 3 F 5 F 7 Benzene Mc Toluene Xylene The molecular weight between crosslinks (M c ) for the affine limit of the model [M c (aff)] was calculated using the equation 4.14 (Chapter 4). The molecular weight between crosslinks for the phantom limit of the model [M c (ph)] was calculated using the equation 4.15 (Chapter 4). The values are given in Table 9.7. Table 9.7 : Values of molecular weight between crosslinks (g/cc) M c Solvent F 3 F 5 F 7 Benzene M c (aff) Toluene Xylene Benzene M c (ph) Toluene Xylene

17 EVA/Clay Nanocomposites Transport Features 243 It is found that M c values of EVA/clay nanocomposites are close to M c (aff). This shows that in the solvent swollen state, the network deforms affinely Comparison with theory The theoretical sorption curves were generated using the equation 3.6 (Chapter 3). Experimentally obtained values of diffusion coefficients are substituted in the equation and the resulting curve is shown in Figure 9.6. The theoretical and experimental results were not in good agreement. The experimental curve deviates from the theoretical curve which is fully a Fickian mode of diffusion Q t /Q Theoretical Experimental Time 1/2 (min) 1/2 Figure 9.6 : Comparison between experimental and theoretical sorption curves of F 3 at 28 o C B) Pervaporation characteristics of EVA/clay nanocomposites Swelling ratio Figure 9.7 shows the swelling ratio values of unfilled, and nano clay filled EVA films. The unfilled membranes (F 0 ) showed maximum swelling ratio

18 244 Chapter 9 values for all the feed concentrations. Modified EVA films with 3 wt% of nano clay (F 3 ) showed the minimum and the value increases with increase in wt% of the filler. The two main factors that influence the swelling of the films; the fraction of the amorphous phase in the polymer and the chemical compatibility between the polymer chain and the solvent mixture. When the crystalline fraction of the polymer decreases there is an increase of both the volume of amorphous fraction and chain lengths that connect the crystalline domains. A higher material volume accessible for the liquid sorption and a higher flexibility of the network allow an increased solvent uptake. Swelling ratio F 0 F 3 F 5 F Chloroform in feed (wt%) Figure 9.7 : Swelling ratio of unfilled and EVA nanocomposite films The decreased swelling ratio values of filled EVA films (F 3 ) is explained as follows. The impermeable clay layers dictate a tortuous path way

19 EVA/Clay Nanocomposites Transport Features 245 for a permeate to pass through the nanocomposite. The diffusion path is schematically represented in Figure 9.8. (a) (b) Figure 9.8 : Schematic representation of diffusion through (a) composite with conventional filler (b) nanocomposites From the Figure 9.8 (a), it is clear that as in the case of micro composite the penetrant molecules can easily pass through the

20 246 Chapter 9 interphase between filler and the matrix. However, in the case of clay filled nanocomposite (Figure 9.8 (b)) penetrant molecule experiences a difficult pathway due to molecular level dispersion of clay in the matrix Pervaporation of chloroform-acetone mixtures The pervaporation performance of unfilled and EVA nanocomposite membranes were analysed using chloroform-acetone mixtures. Both unfilled and nano clay filled membranes showed chloroform selectivity from chloroform-acetone mixtures. The affinity of EVA membranes towards chloroform is higher than acetone and this creates a remarkable difference in the separation of chloroform from chloroform-acetone mixtures [29]. Table 9.8 shows the permeation rate and the selectivity of unfilled (F 0 ) and modified films with 3wt% of filler (F 3 ) membranes. EVA/ clay nanocomposites showed a higher selectivity but a lower permeation rate than the unfilled ones. The increased selectivity is due to the exfoliation of silicates in the polymer matrix leading to the nanometric level dispersion of the organic and inorganic phases. The molecular level of polymer/filler interaction results in a reduced availability of free volume, as a result the permeation rate decreases and separation factor increases. The enhanced selectivity of nano filled membranes are owing to its platelet like morphology and high aspect ratio of the fillers. Due to the high aspect ratio of layered silicates the contact area between filler and the matrix increases. Hence

21 EVA/Clay Nanocomposites Transport Features 247 there will be more resistance towards molecular diffusion resulting in a reduced permeation rate. The above results have been complemented by PALS analysis. Table 9. 8 : Pervaporation characteristics of EVA films (22 wt% of chloroform-acetone mixture) System Permeation composition (wt%) Selectivity α ij Flux (kg/m 2 h) F F Influence of nano clay loading The influence of the wt% of nano clay on pervaporation performance is given in Figure 9.9. The selectivity factor decreases sharply when the clay content becomes higher than 3 wt%. This can be explained in terms of aggregation of clay particles with increase in concentration of clay, resulting in the weakening of polymer chain. When the clay composition is greater than 3 wt%, the compatibility of filler and the matrix decreases, resulting in microphase separation. PALS analysis also showed that the fractional free volume % decreases at higher clay loading. Hence a drop in selectivity and an increase in permeation rate was observed. Wang et al. [26] investigated the effect of clay content on the pervaporation performance of 90wt% ethanol aqueous solution through the polyamide/clay nanocomposite membrane. They

22 248 Chapter 9 found that selectivity decreases sharply when the clay content becomes higher than 2wt% Selectivity Selectivity Flux Permeation flux (kg/m 2.h) Clay loading (wt%) 0.14 Figure 9. 9 : Influence of filler loading on pervaporation Calculation of membrane selectivity ( mem ) The overall selectivity is a combination of the effects of the membrane selectivity (sorption-diffusion selectivity) and the volatility or evaporative selectivity. If the downstream pressure is negligible, the apparent separation factor or selectivity is given by [30]. ij = mem. evp (9.2) = ( Pi ) ( P ) j ri. pi r. p j j (9.3)

23 EVA/Clay Nanocomposites Transport Features 249 where P refers to the permeability and p and r refers to vapour pressure and activity coefficients of the components i & j. The membrane selectivity ( mem ) can be calculated using the formula. mem = α α ij evp (9.4) The membrane selectivity, mem is calculated for unfilled (F 0 ) and nanocomposite membrane (F 3 ) using the equation (9.4). The values of membrane selectivity will provide an interesting study of how a mass separating agent can overcome the intrinsic volatility differences and enables to permeate the less volatile component in a mixture. Table 9.9 gives the membrane selectivity for unfilled and nanocomposite membrane. It can be seen from the table that the evaporative selectivity is being overcome by the membrane selectivity. The membrane selectivity is much higher for nano clay modified EVA membranes. Table 9.9 : Membrane performance (22 wt% of chloroform mixture) Sample α mem α ev F F

24 250 Chapter Comparison of pervaporation results with vapour-liquid equilibrium (VLE) data Chloroform and acetone form an azeotrope at 80 wt% of chloroform. Separation of azeotropes by simple distillation is possible only by adding a third component, i.e., an entrainer such as benzene, which is known as deadly carcinogen. In the membrane based pervaporation separation, membrane acts as a third phase to break the azeotrope. Thus pervaporation is more effective in separating azeotropes than conventional distillation. Figure 9.10 shows that pervaporation curve is higher than that of the vapour-liquid equilibrium (VLE) curve throughout the composition range of the feed mixture. 1.0 Chloroform fraction(w/w vapour) B D Chloroform fraction(w/w feed) B:Vapor composition of the permeate D:Liquid-vapour equlibrium curve Figure 9. 10: Comparison of vapour-liquid equilibrium curve with pervaporation data for nanocomposite films.

25 EVA/Clay Nanocomposites Transport Features 251 C) Transport of organic vapours through EVA / clay nanocomposites Mechanical properties The tensile properties of EVA-clay nanocomposites are shown in Table The tensile strength of the EVA-clay composites are higher than the unfilled ones. The tensile strength of EVA/clay nanocomposite with 3 wt% clay is 25% higher than that for the EVA membrane. This may be due to the reinforcing effect of the silicate sheets through the random dispersion in the polymer matrix. However, the tensile strength decreased on further increase in clay content and it is reduced to 6.8 MP a for samples containing 7wt% of clay content. The decrease in tensile strength at higher filler loading is due to the uneven distribution of stress by the aggregated clay present in the matrix. Table : Effect of clay content on tensile properties Clay Content (Wt%) Tensile Strength (MP a ) Vapour transport The vapour permeation coefficient of unfilled and nano clay modified composites are shown in Table The vapour permeation characteristics of EVA/clay nanocomposites were analysed using

26 252 Chapter 9 chloroform. The value of permeation coefficient is maximum for unfilled membranes. Among the filled samples, the permeation coefficient increases with increase in weight percentage of clay. The reduced vapour permeability of membranes reinforced with layered silicates are due to the molecular level of polymer / clay interaction resulting in a reduced free volume. The large aspect ratio of the clay platelets effectively increase the penetration path, which was responsible for the reduced permeability. However, it was found that when the filler weight percentage was greater than 3%, there is a gradual increase in permeability due to the decrease in the dispersion of nano particles. Table : Permeability coefficient [Px10 10 (mol Pa)] System Permeability F F F F D) Gas transport through EVA/clay nanocomposites EVA/Clay nanocomposite membranes for gas separation The gas transport properties of nano clay reinforced polymer membranes have been analysed using oxygen and nitrogen gases. The results were compared with that of unfilled ones (F 0 ). Oxygen and nitrogen gas permeability coefficients are shown in Figures 9.11 and 9.12 respectively. It is found

27 EVA/Clay Nanocomposites Transport Features 253 that the transport of gases through layered silicate filled membranes is lower than that of unfilled ones. The enhancement in gas barrier properties of nano clay reinforced membranes indicate strong polymer/filler interaction. Since the chain segments get immobilized in the presence of layered silicates, the free volume decreases and as a result the gas permeability coefficient reduces. Utracki and co-workers [31] studied the reduced free volume available in the polymer matrix after the incorporation of clay platelets. According to them, in exfoliated polymer nanocomposite the accessible clay surface area is proportional to organo clay loading. They observed that the addition of 4 wt% of organo clay (closite 15) can reduce the matrix hole fraction twice as large as that observed for polymer nanocomposite with 2wt %. The incorporation of 1.1 and 2.42 wt% of montmorillonite (MMT) can reduce the matrix free volume to 4.7 and 8.0% respectively. 5 1bar O 2 gas P x (mol/mspa) F0 F3 F5 F7 Figure : Variation in oxygen permeability

28 254 Chapter bar N 2 gas P x (mol/mspa) F0 F3 F5 F7 Figure : Variation in N 2 permeability The addition of fillers reduce gas permeability of polymers according to a tortuous path model developed by Neilson [32]. P c = P p 1- Q f 1+ αq f /2 (9.5) where P c and P p are the permeability of composite and polymer, Q f is the volume fraction of filler and α is the aspect ratio of platelets. The calculated value of α is The high aspect ratio of the clay platelets effectively increased the gas penetration path, which is responsible for the reduced permeability. Schematic representation of the tortuous path model is given in Figure From the figure, it is clear that the gas molecules have to travel through a tortuous path in the presence of layered silicate. The reduced gas permeability of nanocomposites is influenced by two factors, viz. geometry of the filler and the molecular level interaction of the

29 EVA/Clay Nanocomposites Transport Features 255 matrix and the clay. Also the extent of exfoliation is found to be maximum in F 3 samples (Figure 9.2 (a)). Therefore, these samples exhibit reduced gas permeability. Figure 9.13: Schematic representation of the tortuous path model developed by Nielsen for the transport of gases through filled membranes Selectivity of membranes The polymeric membranes used for gas separation processes have certain significance such as high permeability to the desired gas, high selectivity and the ability to form useful membrane configurations. The requirement of an ideal membrane is high permeability along with high permselectivity. The permselectivity of membrane is given by, α (O 2, N 2 ) = P(O 2 ) P(N ) 2 (9.6) where, α is the permselectivity of a membrane towards O 2 and N 2 gas, P(O 2 ) and P(N 2 ) are the permeability coefficients of O 2 and N 2 gases, respectively.

30 256 Chapter 9 The permselectivity values of the membranes are given in Table The nanocomposites possess higher selectivity than the unfilled one. These might be ascribed to the interaction between the polymer and the filler. Table 9. 12: Oxygen to Nitrogen selectivity values of unfilled and filled EVA membranes [P(O 2 )/P(N 2 )] Sample Permselectivity F F F F Effect of pressure The permeation mechanism can be obtained by examining the variation in permeability coefficient as a function of pressure. The effect of pressure on nitrogen gas permeability of F 0 and F 3 membranes is given in Figure The unfilled (F 0 ) system exhibit increase in permeability with pressure. This is due to the higher solubility of the permeant molecules in the polymer chain as a function of pressure. The effect of pressure on the filled system is negligible due to the close packing of fillers in the polymeric matrix.

31 EVA/Clay Nanocomposites Transport Features F F 3 Px10 10 (mol/m.spa)) Pressure(bar) Figure : The effect of pressure on the permeability of nitrogen gas through filled (F 3 ) and unfilled membranes 9.3. Conclusion EVA/clay nanocomposites containing different filler loading have been prepared and the transport features of the membranes were investigated. Morphology of the composite membranes were analysed by XRD and TEM. It has been found that the diffraction peaks were shifted to lower angles with an increase in d-spacing. Samples with 3wt% of filler showed maximum increase in d-spacing. TEM images showed that sample with 3 wt% of clay showed excellent dispersion of clay particles resulting in an exfoliated structure. The dispersion of nano particles decrease with an increase in the clay loading. The fractional free volume % was determined using positron annihilation lifetime spectroscopic analysis. Sample with 3 wt% clay showed the least free volume.

32 258 Chapter 9 The liquid transport characteristics of EVA/clay nanocomposite membranes were investigated using aromatic hydrocarbons as probe molecules. Due to enhanced polymer/filler interaction, sample with 3wt% of clay exhibited lower solvent uptake. However, the solvent uptake tendency increased at higher clay loading. This is ascribed to the poor physical interaction between the matrix and filler, leading to aggregation of fillers. The diffusion coefficient values also showed the same trend. The mechanism of transport was found to be anomalous. The pervaporation performance was analysed using chloroform-acetone mixtures. These membranes exhibited a far superior selectivity to chloroform molecules than the unfilled one. The vapour permeability was examined using chloroform vapours and nanocomposite membranes exhibited very low vapour permeability compared to unfilled one. The gas transport properties of nano clay filled and unfilled (F 0 ) membranes were investigated using permeant gases such as O 2 and N 2. Due to enhanced polymer/filler interaction, nanocomposite membranes exhibited lower permeability to oxygen and nitrogen gases. Increase in effective penetration path due to the very large aspect ratio of the silicate layers was responsible for the reduced gas permeability. As the filler loading increased the permeability of the polymer increased due to the aggregation of the filler particles. From the plots of pressure against permeability it can seen that pressure has little influence on the permeation of gases through nanofilled composites. Finally it is

33 EVA/Clay Nanocomposites Transport Features 259 important to mention that by the incorporation of nanofillers into EVA matrix, new gas barrier membranes could be developed.

34 260 Chapter 9 Reference [1] S. Takahashi and D. R. Paul, Polymer, 47, 7535, (2006). [2] R. Stephen, S. Varghese, K. Joseph, Z. Oommen and S. Thomas, J. Membr. Sci., 282, 162 (2006). [3] Z. F. Wang, B. Wang, N. Qi, H.F. Zhang and L. Q. Zhung, Polymer, 46, 719 (2005). [4] J. K. Kim, C. Hu. R. S. C. Woo, and M. L. Sham, Comp. Sci. Technol., 65, 805 (2005). [5] Q. Liu and, D. De Lee, J. Non-Newtonian Fluid Mech., 131, 32 (2005). [6] T. Jiang, Yu-huawang, J. Yeh and Zhi-giang Fan, Eur. Polym. J., 4, 459 (2005). [7] S. Wang, C. Long, X. Wang, LiQ, and Qiz, J. Appl. Polym. Sci., 69, 1557 (1998). [8] D. C. Lee and L. W. Jang, J. Appl. Polym. Sci., 68, 1997, (1998). [9] M. W. Noh and D. L. Lee, Polym. Bull., 42, 619, (1999). [10] Y. Yang, Z. K. Zhu, J. Yin, X. Y. Wang and Z-e Qi, Polymer, 40, 4407 (1999) [11] J. J. Luo and I. M. Daniel, Compos. Sci. Technol., 63, 1607 (2003). [12] M. Krook, A.C. Albertsson, U.W. Gedde, M.S. Hedenqvist, Polym. Eng. Sci., 42, 1238, (2002).

35 EVA/Clay Nanocomposites Transport Features 261 [13] R. K. Bhardwaj, Macromolecules, 34, 9189, (2001). [14] A. D. Drozdov, J.D. Christiansen, R.K. Gupta and A.P. Shah, J. Polym. Sci. Part. B : Polym. Phys., 41, 476 (2003). [15] M.S. Hedengvist, A. Backman, M. Gallstedt, R.H. Boyd and U.W. Gedde, Comp. Sci. Technol., 241, 156, (2006). [16] P. Musto, L. Mascia, G. Mensiteri and Rayosta, Polymer, 46, 4492 (2005) [17] T. C. Merkel, Z. He, Pinnaul, B.D. Freeman, P. Meakin and A.J. Hill, Macromolecules, 36, 6844 (2003) [18] D. P. N. Vlasveld, J. Groenewold, H. E. N. Bersee and S. J. Picken, Polymer, 46, (2005). [19] T. A. Shantalii, I. L. Karpova, K. S. Dragan, E. G. Privalko and UV.P. Privalko, Sci. and Tech. of Advanced Materials, 4, 115 (2003). [20] M. Sairam, B. V. K. Naidu, S. K. Natraz, B. Sreedhar and T. M. Aminabhavi, J. Membr. Sci., 283, 65 (2006). [21] A. Okada, M. Kawasumi. M,. Unuki. A, Kojima. Y, Kuranchi. T, Kamigailo. O, Mater. Res. Sac. Prox., 45, 171 (1990). [22] S. Varghese and J. Karger-Kocsis, Polymer, 44, 4921 (2003). [23] B. Barbi, S. S. Funari, R. Gehrke, N. Scharnagl, N. Stribeck, Macromolecules, 36, 749 (2003).

36 262 Chapter 9 [24] X. Fu and S. Qutubuddin, Polymer, 42, 807 (2001). [25] N. Hasegawa, H. Okamoto, M. Kato, A. Usuki, N. Sato, Polymer, 44, 2933 (2003). [26] Y.C. Wang, S.C. Fan, K.R. Lee, C. L. Li, S.H. Huang, H.A. Tsai and J.Y. Lai, J. Membr. Sci., 239, 219 (2004). [27] D. Turnbull and M.H. Cohen, J. Chem. Phys., 34, 120 (1961). [28] C. Chen, M. Khobaih and D. Curliss, Progress in Organic Coatings, 47, 376, (2003). [29] S. Anilkumar, P.H. Gedam, V.S. Kishan Prasad, M.G. Kumaran and Sabu Thomas, J. Appl. Polym. Sci., 60, 735, (1996). [30] Wenyuan Zil, Donald R. Paul, William J. Kores, J. Membr. Sci., 89, 219 (2003). [31] L. A. Utracki and R. Simha, Macromolecules, 37, (2004). [32] L. E. Neilson, J. Macromol Sci. (Chem)., A1 (5), 929 (1967).