Molecular Transport Characteristics of Poly(ethyleneco-vinyl Acetate) in Presence of Aliphatic Chlorinated Hydrocarbons

Molecular Transport Characteristics of Poly(ethylene-co-vinyl Acetate) in Presence of Aliphatic Chlorinated Hydrocarbons Molecular Transport Characteristics of Poly(ethyleneco-vinyl Acetate) in Presence of Aliphatic Chlorinated Hydrocarbons S. Anil Kumar 1 and Sabu Thomas 2 * 1 Department of Chemistry, NSS College Nenmara, Palakkad, Kerala, India 2 School of Chemical Sciences, M.G. University, Priyadarshini Hills P.O, Kottayam, Kerala- 686560, India Received: 16 August 2005 Accepted: 11 January 2006 ABSTRACT The molecular transport characteristics of poly(ethylene-co-vinyl acetate) (EVA) with aliphatic chlorinated hydrocarbons as probe molecules have been investigated by simple sorption gravimetric analysis. Uncrosslinked and crosslinked EVA membranes were prepared. The cross links were introduced by peroxide technique using dicumyl peroxide (DCP). Samples with different crosslinking densities were also prepared. The infl uence of crosslinking, different crosslinking density, temperature, size and polarity of penetrants on transport were analyzed. It has been observed that a critical concentration of crosslinker is necessary for maximum sorption. D 1 sample showed a higher solvent uptake than the uncrosslinked sample. But when the extent of crosslinking increases, the equilibrium uptake decreases. The influence of temperature on the sorption and the activation energies of sorption have been calculated. Thermodynamic parameters were also estimated. The mechanism of transport showed anomalous behavior in all solvents. Sorption (S), desorpion (D), resorption (RS), redesorption (RD) experiments have been performed for studying the transport phenomenon. After attaining equilibrium, EVA sample was allowed to desorbe fully. The desorbed sample is then exposed to solvents for resorption followed by redesorption. Transport parameters namely diffusion coeffi cient, sorption coeffi cient and permeation coeffi cient have been calculated. The values of polymer-solvent interaction parameter and molar mass between crosslinks were calculated using Flory-Rehner theory. First order kinetics model was used to investigate the transport kinetics. *To whom correspondence should be addressed. Tel: +31-481-2730-003, Fax: +91-481-561-190/800, E-mail: sabut552001@yahoo.com Rapra Technology, 2006 Progress in Rubber, Plastics and Recycling Technology, Vol. 22, No. 4, 2006 285

S. Anil Kumar and Sabu Thomas INTRODUCTION One of the most important attractions of polymers is their ability to dissolve molecules of gas, vapor, and liquid and to allow the transport of these materials through its solid phase. The knowledge of permeation property through polymers is essential for the practical applications, such as separation process pervaporation (1), reverse osmosis (2), cable coating, food packing and encapsulation of electronic circuits (3). It is also essential to investigate the polymer morphology and molecular interaction that controls transport. These applications have created considerable interest in the study of molecular transport of liquids through polymers. Previous research efforts in this field results in the accumulation of transport of organic liquids through a variety of polymeric materials (4-8). In these studies, liquid sorption was found to be a function of structure of both the polymer and the penetrant molecule. Molecular transport through polymers can be explained by Fick s law of diffusion. Aminabhavi and Phade (9) have investigated the molecular transport of alkanes through santoprene rubber. For all liquids, equilibrium uptake was influenced by factors such as penetrant size, shape and temperature. Transport behavior of aromatic hydrocarbons through crosslinked nitrile rubber membranes was influenced by the type of crosslinking (10). The peroxide system showed the least and the conventional system the highest. The transport of small organic liquids through a variety of polymers and their blends was investigated by our research group (11-14). These studies showed that liquid transport was found to be a function of polymer morphology, size and shape of diffusant, types of cross linking, cross linking density, temperature etc. Poly(ethylene-co-vinyl acetate) is a random copolymer of ethylene and vinyl acetate. The polymer is extensively used in many engineering and industrial areas because of its toughness, chemical resistance and excellent processability. Successful applications of the polymer in these areas depend very much on its response to the liquids and vapors. The objective of the present study is to investigate the transport properties of both uncrosslinked and crosslinked poly(ethylene-co-vinyl acetate) membranes as a function of temperature, solvent size, polarity and crosslinking density. After attaining equilibrium the samples were desorbed fully. The desorbed samples were again exposed to solvents for resorption followed by desorption. Thus a sorption (S), desorption (DS), resorption (RD), redesorption (RD) experiments is an effective tool for studying the transport phenomenon. The sorption, diffusion and permeation coefficients were calculated. Activation energies for diffusion and permeation were also calculated. 286 Progress in Rubber, Plastics and Recycling Technology, Vol. 22, No. 4, 2006

Molecular Transport Characteristics of Poly(ethylene-co-vinyl Acetate) in Presence of Aliphatic Chlorinated Hydrocarbons EXPERIMENTAL Reagents and materials Poly(ethylene-co-vinyl acetate), EVA was obtained from Poly olefin Industries Limited, Chennai, India. The crosslinking agent used was dicumyl peroxide (DCP). The solvents carbon tetrachloride, chloroform, dichloromethane and dichloroethane were of reagent grade. Sample preparation Crosslinked and uncrosslinked poly(ethylene co-vinyl acetate) membranes were prepared. To prepare uncrosslinked samples, the EVA granules were sheeted out in a two roll mixing mill having a friction ratio 1:1.4. The sheeted out stock was compression moulded in a hydraulic press at 170 C and under a load of 25 tonnes. The sample is represented as D 0. EVA was crosslinked by peroxide technique using dicumyl peroxide. Samples with different loading of DCP were also prepared. The crosslinked samples are represented as D 1, D 2, D 4, D 6 and D 8 according to the DCP content where the subscript numbers represent the grams of DCP used per 100 grams of the polymer. The corresponding number of moles is 0.002, 0.003, 0.006, 0.009 and 0.012 moles respectively. The mixing was done in a two roll mixing mill as before. Knowing their cure characteristics the samples were cured to their optimum cure times at 170 C in a hydraulic press under a load of 25 tonnes. Diffusion experiments Diffusion experiments were monitored by the weight gain analysis of the polymer samples. Disc shaped samples (diameter 1.96 cm) were cut from the dried EVA samples by means of a sharp-edged steel disc. The thickness of the samples were measured at several points and average value was taken as thickness. These samples were soaked in 20 ml of the solvent taken in test bottles. They were weighed at regular intervals after wiping off the adhering solvent. The process of weighing is repeated until equilibrium swelling is attained. After attaining equilibrium, the EVA sample was desorbed fully. The desorbed sample was exposed to solvent for resorption followed by redesorption. Thus a sorption (S), desorption (D), resorption (RS), redesorption (RD) experiments has been used for studying the transport mechanism. Diffusion experiments were also conducted at 28, 45 and 55 C for all solvents except dichloromethane. For dichloromethane, experiments were carried out at Progress in Rubber, Plastics and Recycling Technology, Vol. 22, No. 4, 2006 287

S. Anil Kumar and Sabu Thomas 28 C because of its low boiling point. For uncrosslinked samples experiments were conducted at 28 C only. The results of sorption experiments are expressed as moles of liquid Q t, sorbed by 100 grams of polymer. The Q t values were calculated as mass of solvent sorbed/molar mass of solvent Q= t x100 initial weight of the polymer (1) The Q t values obtained were plotted as function of the square root of time of immersion of the polymer in liquids. RESULTS AND DISCUSSION The sorption curves, expressed as mol% uptake, Q t of the liquid by 100 grams of the polymer versus square root of time, t 1/2 were plotted for uncrosslinked and cross linked EVA samples. Figure 1 represents the liquid sorption behavior of uncrosslinked and crosslinked samples. The solvent used was chloroform Figure 1. Mol%chloroform uptake of EVA samples at 28 C 288 Progress in Rubber, Plastics and Recycling Technology, Vol. 22, No. 4, 2006

Molecular Transport Characteristics of Poly(ethylene-co-vinyl Acetate) in Presence of Aliphatic Chlorinated Hydrocarbons and experiments were conducted at 28 C. It is clear from the figure that the mode of diffusion for crosslinked and uncrosslinked samples is same. The uncrosslinked EVA shows a lower solvent uptake where as D 1 sample the mol% uptake is higher. Similar trends were observed with other solvents also. Figure 2 shows the variation of equilibrium mol% uptake (Q ) values for the four different solvents with number of moles of DCP used for crosslinking. For all solvents the transport behavior follows the order D 1 >D o >D 2 >D 4 >D 6 >D 8. When the number of moles of DCP used for crosslinking increases, Q value increases, reaches a maximum and then decreases. Pure poly(ethylene-covinyl acetate) is a semi crystalline polymer. This crystalline nature generates a compact structure and hence a lower porosity. But when moderate amounts of DCP were introduced the crystallinity was reduced by the formation of crosslinks in the networks. This explains the sharp increase in Q value from D 0 to D 1. X-ray diffraction studies of the uncrosslinked and crosslinked samples support the above data and are given in Figure 3. At the same time when the amount of DCP goes up the extent of crosslinking also goes up. This Figure 2. Variation of equilibrium mol% uptake with number of moles of DCP used for crosslinking Progress in Rubber, Plastics and Recycling Technology, Vol. 22, No. 4, 2006 289

S. Anil Kumar and Sabu Thomas Figure 3. X-ray diffraction patterns of unmodified and modified EVA membranes explains the reduction in Q values from D 1 to D 8. When the amount of DCP becomes too large, the polymer chains become very rigid and this prevents the diffusion of solvent molecules. This accounts for the low equilibrium uptake values for D 8 samples. Thus a moderate amount of crosslinks is necessary for maximum sorption. Knowing that diffusion is influenced by the polymer morphology, we have calculated the molecular mass between crosslinks (M c ) using the equation (15) M c 13 / ρpvφ = ln( 1 φ)+ φ+ χφ 2 (2) Where, ρ p is the density of the polymer, V, is the molar volume of solvent; φ, the volume fraction of swollen polymer and χ, is the polymer solvent parameter. 290 Progress in Rubber, Plastics and Recycling Technology, Vol. 22, No. 4, 2006

Molecular Transport Characteristics of Poly(ethylene-co-vinyl Acetate) in Presence of Aliphatic Chlorinated Hydrocarbons The volume fraction of the polymer was calculated using the equation (16) φ = w w1 / ρ1 / ρ + w / ρ (3) 1 1 2 2 Where, w 1 and ρ 1, are the weight and density of the polymer, w 2 and ρ 2, are the weight and density of solvent respectively. The values of φ are placed in Table 1. The value of volume fraction is a measure of the degree of crosslinking. When the amount of DCP used for crosslinking is increased, the degree of crosslinking and the density of the matrix also increased. The highest value of volume fraction for D 8 sample shows that crosslinking is maximum in D 8 sample. Thus φ value also supports the higher solvent uptake of D 1 sample. Values of volume fraction decreased when temperature was increased. This trend was observed for all solvents. The polymer solvent interaction parameter χ is calculated using the equation (5). χ ( ) { = } dφ/ dt φ/ ( 1 φ) + N ln( 1 φ)+ Nφ 2 2 2 φ( dφ/ dt) φ N( dφ/ dt) φ / T (4) Where, φ is the volume fraction of the polymer in the solvent-swollen sample and N is calculated from φ using the equation φ N = φ 23 / 13 / / 3 2/ 3 2φ/ 3 (5) The calculated values of polymer-solvent interaction parameter are placed in the Table 2. It is a measure of polymer-solvent interaction. A high value of χ indicates minimum interaction with the solvent. Hence D 1 sample showed higher interaction with all the four solvents. The D 8 sample showed the least interaction with the solvents. This accounts for its lower Q values. All the samples showed minium χ values in chloroform and maximum for dichloroethane. This shows that polymer chloroform interaction is higher. Molar mass between crosslinks (M c ) for all crosslinked samples in two solvents were calculated and placed in Table 2. The higher equilibrium solvent uptake of D 1 sample compared to D 0 is due to its higher M c values. This can be explained by the higher crosslink density of uncured EVA (D 0 ) due to the physical crosslinks formed by the crystalline regions. Upon crosslinking by DCP the folded chains of EVA rearrange themselves resulting in low crosslink density and higher free volume. The x-ray diffraction patterns in Figure 3 complements the above facts. The molecular mass between crosslinks is least Progress in Rubber, Plastics and Recycling Technology, Vol. 22, No. 4, 2006 291

S. Anil Kumar and Sabu Thomas D D D D D Table 1. Volume fraction of swollen polymer (φ) Solvent Temp Uncrosslinked o C D 0 1 2 4 6 8 Carbon 28 0.135 0.124 0.144 0.189 0.209 0.228 tetrachloride 45 0.069 0.105 0.173 0.176 0.198 55 0.068 0.098 0.141 0.156 0.177 Chloroform 28 0.176 0.114 0.129 0.183 0.203 0.218 45 0.068 0.103 0.167 0.173 0.193 55 0.066 0.096 0.159 0.164 0.181 Dichloro 28 0.394 0.393 0.395 0.396 0.398 0.401 methane Dichloroethane 28 0.589 0.587 0.591 0.594 0.597 0.599 45 0.392 0.396 0.403 0.412 0.417 55 0.254 0.278 0.316 0.346 0.353 Table 2 Values of polymer solvent interaction parameter (χ) and molecular mass between crosslinks (M c ) Polymer Solvent interaction parameter (χ) Molecular mass between crosslinks (M c ) Solvents D 1 D 2 D 4 D 6 D 8 Carbon tetrachloride 0.514 0.548 0.555 0.561 0.569 Chloroform 0.491 0.534 0.539 0.551 0.558 Dichloromethane 0.757 0.766 0.779 0.789 0.803 Dichloroethane 0.852 0.867 0.881 0.909 0.915 Carbon tetrachloride 44848 39058 33381 29789 22864 Chloroform 57752 49010 45694 39245 33981 for D 8 sample. This indicates that maximum chemical crosslinks are formed in D 8 sample. As a result the free volume of the matrix decreases and this makes the diffusion process more tortuous. Effect of penetrant size The influence of molecular size of chlorinated hydrocarbons on transport through EVA is presented in Figure 4. There is no systematic trend in the sorption behavior. 292 Progress in Rubber, Plastics and Recycling Technology, Vol. 22, No. 4, 2006

Molecular Transport Characteristics of Poly(ethylene-co-vinyl Acetate) in Presence of Aliphatic Chlorinated Hydrocarbons Figure 4. Mol% uptake of solvents by D 1 samples at 28 C Among the solvents used in this work dichloroethane showed the lowest and chloroform showed the maximum uptake followed by carbon tetrachloride and then dichloromethane. This abnormal trend can be explained by considering the interaction parameter, χ between the solvent and the polymer. The χ values calculated are given in the Table 2. The χ value is least for chloroform and maximum for dichloroethane. High value of χ shows that interaction of polymer with the solvent is minimum. This abnormal trend can be explained also on the basis of solubility parameter (δ) of the solvents and polymers. The closer the solubility parameters, greater will be the solubility of the polymer. The solubility parameter of EVA is 18.8 (MPa) 1/2. This value is close to the solubility parameter of chloroform (19 (MPa) 1/2 ). The closeness in their solubility parameters account for maximum chloroform uptake. Mechanism of transport The results of the sorption experiments were analyzed by using the empirical relation (8,17). Progress in Rubber, Plastics and Recycling Technology, Vol. 22, No. 4, 2006 293

S. Anil Kumar and Sabu Thomas log ( Q / Q )= log k+ nlog t t (6) where, Q t is the mol% sorption at time t and Q is the mol% sorption at equilibrium. The value of k depends on the structural features of polymer and in addition to its interaction with the solvent. The value of n indicates the mechanism of sorption. If n=0.5 the mechanism of sorption is Fickian and this occurs when the rate of diffusion of permeant molecules is much less than polymer segmental mobility. If n=1 the mechanism of sorption is termed as case II. This arises when the rate of diffusion of permeant molecules is much greater than polymer segmental mobility. If n lies between 1 and 0.5 the mechanism of diffusion follows an anomalous trend. Then the permeant mobility and polymer segment relaxation rates are similar. The estimated values of n and k are placed in the Table 3. It is seen that the n values are in between 0.5 and 1 for all systems under investigation, suggesting an anomalous mode of transport. No systematic trend was observed for the values of k. Diffusion coefficient The kinetic parameter, the diffusion coefficient D, was calculated using the equation (18). D π hθ/4q = ( ) 2 Where, θ is the slope of the linear portion of the sorption curves, h, is the initial sample thickness and Q the mol% solvent uptake at equilibrium. Since EVA samples undergo significant amount of swelling, intrinsic diffusion coefficient D* is calculated (19) D (7) D = φ 13 (8) * / The calculated values of D * are given in the Table 4. For the crosslinked samples the value of D * decreases from D 1 to D 8 in all solvents. When the amount of DCP used for crosslinking was increased, rate of diffusion decreases. This is due to the reduction in free volume within the polymer matrix due to large number of crosslinks. For the uncrosslinked samples D* value is in between D 1 and D 2. These observations are parallel to their solvent uptake nature. The intrinsic diffusion coefficient D * is higher for chloroform, indicating a higher chloroform polymer interaction. 294 Progress in Rubber, Plastics and Recycling Technology, Vol. 22, No. 4, 2006

Molecular Transport Characteristics of Poly(ethylene-co-vinyl Acetate) in Presence of Aliphatic Chlorinated Hydrocarbons Table 3 Analysis of sorption data of chlorinated hydrocarbons in EVA at different temperature Solvents Temp o C n K x 10 2 (gg -1 min n ) D 0 D 1 D 2 D 4 D 6 D 8 D 0 D 1 D 2 D 4 D 6 D 8 Carbon tetrachloride 28 0.72 0.68 0.70 0.65 0.63 0.62 2.47 0.59 0.70 1.03 1.17 1.37 45 0.89 0.89 0.81 0.79 0.80 0.29 0.45 0.85 0.81 1.04 55 0.91 0.94 0.86 0.89 0.90 0.42 0.46 0.82 0.62 0.68 Chloroform 28 0.79 0.73 0.83 0.71 0.72 0.70 1.06 0.99 1.01 1.63 1.77 1.99 45 0.95 0.87 0.80 0.78 0.79 0.42 0.80 1.52 1.49 1.82 55 0.97 0.96 0.86 0.85 0.93 0.47 0.62 1.37 1.25 1.07 Dichloro methane 28 0.68 0.65 0.66 0.63 0.61 0.61 3.98 3.71 3.61 3.70 3.80 4.06 Dichloro ethane 28 0.59 0.56 0.56 0.58 0.56 0.57 3.89 4.17 4.52 4.75 3.76 3.59 45 0.73 0.72 0.74 0.70 0.72 1.64 1.81 1.68 1.95 1.76 55 0.84 0.87 0.83 0.84 0.87 0.88 0.92 1.19 1.18 1.06 Progress in Rubber, Plastics and Recycling Technology, Vol. 22, No. 4, 2006 295

S. Anil Kumar and Sabu Thomas Table 4. Values of intrinsic diffusion coefficient D * x 10 3 (cm 2 s -1 ) at different temperatures Solvent Temp C D 0 D 1 D 2 D 4 D 6 D 8 Carbon tetra 28 24.98 39.07 19.91 4.22 2.41 1.59 chloride 45 5201.05 570.75 32.74 24.05 15.62 55 8748.64 1142.95 185.46 69.34 33.64 Chloroform 28 192.29 263.76 186.37 22.63 13.17 9.15 45 19545.20 1670.02 94.72 74.89 40.84 55 25122.3 2673.41 114.52 111.55 92.99 Dichloro 28 0.297 0.316 0.276 0.207 0.195 0.190 methane Dichlor ethane 28.0094 0.0099 0.009 0.0085 0.008 0.007 45 0.146 0.1391 0.130 0.124 0.113 55 2.391 1.6768 0.835 0.582 0.485 Sorption constant (s) and permeability coefficient (P*) The permeation of a penetrant into a polymer matrix depends on diffusivity as well as the solubility or sorptivity of the penetrant. Hence the sorption coefficient, S, which is the maximum sorption value was calculated by the equation (20). S= M /M 0 (9) Where, M, the mass of the solvent at equilibrium and M 0, is the initial mass of the polymer. The estimated values are given in Table 5. It is observed that S is maximum for D 1 and minimum for D 8 sample. The maximum value for the D 1 sample is an indication of better accommodation of the solvent molecules in the highly flexible polymer networks. The lowest value for D 8 system shows the least capability to accommodate the solvent molecules in the less flexible c-c network. The sorption coefficients are highest for chloroform. The permeability coefficient, P * is determined from the following equation (21) P * = D * x S (10) Where, D *, is the diffusivity and S, is the sorption coefficient. The calculated values are given in the Table 5. The permeability coefficient shows the net 296 Progress in Rubber, Plastics and Recycling Technology, Vol. 22, No. 4, 2006

Molecular Transport Characteristics of Poly(ethylene-co-vinyl Acetate) in Presence of Aliphatic Chlorinated Hydrocarbons Table 5. Values of sorption coefficients (S) and permeation coefficient (P X =D X.S) Solvents Temp Sgg -1 P* cm 2 s -1 C D D 0 D 1 D 2 D 4 D 6 D 8 D 0 D 1 D 2 4 D 6 D 8 Carbon 28 10.13 11.95 10.07 7.27 6.43 5.73 0.435 0.467 0.201 0.027 0.015 0.009 tetra chloride 45 22.87 14.48 8.12 7.91 6.85 118.95 8.263 0.266 0.190 0.107 55 23.39 15.64 10.33 9.21 7.89 204.61 17.86 1.915 0.638 0.265 Chloroform 28 10.62 12.32 10.60 7.05 6.22 5.67 2.04 3.24 1.97 0.159 0.081 0.051 45 21.83 13.81 7.84 7.55 6.60 426.66 23.04 0.744 0.566 0.269 55 22.34 14.92 8.34 7.18 7.16 552.68 39.88 0.955 0.801 0.665 Dichloro methane Dichloro ethane 28 2.19 2.19 2.17 2.17 2.14 2.13 0.6x10-3 0.69x10-3 0.6x10-3 0.4510-3 0.410-3 0.410-3 28 0.936 0.943 0.926 0.915 0.903 0.897 0.008x10-3 0.009x10-3 0.008x10-3 0.0077x10-3 0.0073x10-3 0.007 x10-3 45 2.07 2.04 1.98 1.91 1.87 0.304x10-3 0.275x10-3 0.257x10-3 0.238 x10-3 0.21 x10-3 55 3.93 3.43 2.90 2.53 2.45 0.009 0.0057 0.0024 0.0014 0.0011 Progress in Rubber, Plastics and Recycling Technology, Vol. 22, No. 4, 2006 297

S. Anil Kumar and Sabu Thomas effect of sorption and diffusion. It can be seen that the permeability coefficient value decreases with increase in DCP content in the EVA samples. The presence of large amount of crosslinks prevents the movement of solvent molecules through the polymer chains. The permeability coefficient value is very high for chloroform and least for dichloroethane. This again indicates that polymer has a strong affinity towards chloroform. Effect of temperature Sorption experiments were conducted at 45 and 55 C in addition to those at 28 C. For dichloromethane, the experiments were conducted at 28 C only. Figure 5 illustrates the sorption curves of D1 in chloroform at different temperatures. It has been observed that Qt value increases with rise in temperature. The same trend is shown by other systems also. The increase in equilibrium uptake is due to the increase in free volume and greater segmental mobility at higher temperatures. Figure 5. Mol% uptake of D 1 sample at different temperatures 298 Progress in Rubber, Plastics and Recycling Technology, Vol. 22, No. 4, 2006

Molecular Transport Characteristics of Poly(ethylene-co-vinyl Acetate) in Presence of Aliphatic Chlorinated Hydrocarbons The estimated values of the intrinsic diffusion coefficient D *, sorption constant, S, and permeability coefficient P * are given in the Tables 4 and 5 respectively. There is a systematic increase in the values of D *, S and P * with the rise in temperature. At higher temperatures the chains are more flexible and hence the process of diffusion becomes much easier. The diffusion, sorption and permeation process are temperature activated and hence the energy of activation for diffusion, E D and that for permeation, E P, can be calculated using Arrihenius equation (18). X= X O e -Ex / RT (11) Where, X is P, D and X O denotes P o or D o, which is a constant. The values of the activation energy for diffusion and permeation were estimated. The difference between E P and E D gives the heat of sorption. The values of E P, E D and H s for carbon tetrachloride and chloroform are given in the Table 6. The E P and E D values are found to increase from D 1 to D 8. When the amount of DCP is increased the chain becomes more and more rigid, due to large number of crosslinks. Hence more activation energy is required. It can also see that the activation energy for chloroform is less compared to carbon tetrachloride. This again indicates the ease of diffusion of chloroform through EVA membranes. The heat of sorption ( H s ) is a composite parameter involving contribution from Henry s Law and Langmuir type sorption. In Langmuir type sorption mechanism, the solvent molecules fill the holes already existing within the polymer matrix, which give rise to an exothermic process. All the H s values are positive suggesting that sorption is mainly dominated by Henry s law, where the solvent molecules have to make room for themselves in the polymer matrix. Hence the process of sorption is endothermic. Thermodynamic constants The molecular equilibrium sorption constant (K s ) is defined as (22) K s = No of moles of the solvent sorbed at equilibrium Mass of the polymer (12) The values of K s can be used to calculate the entropy change ( S) for crosslinked EVA by using Van t Hoff s equation (18). log K s = S H 2. 303 R 2. 303 RT (13) Progress in Rubber, Plastics and Recycling Technology, Vol. 22, No. 4, 2006 299

S. Anil Kumar and Sabu Thomas The free energy change ( G) was calculated using the equation G = H - T S (14) The values of G and S for two solvents are given in Table 6. The standard entropy values are positive and the values decreases from D 1 to D 8 system. The free energy change values are negative; showing that sorption of chlorinated hydrocarbons through EVA is spontaneous. The free energy values also decrease from D 1 to D 8. Table 6. Activation parameters of diffusion and thermodynamic parameters Solvent D 1 D 2 D 4 D 6 D 8 Carbon tetra chloride E D KJ/mol 8.65 16.42 17.88 19.27 20.84 E P kj/mol 25.05 26.89 28.13 30.12 32.16 H KJ/mol 16.36 10.47 10.25 10.8 11.36 G (KJ/mol) -4.75-4.29-3.65-3.51-3.31 S (J/mol) 126.90 93.72 59.96 55.76 44.95 Chloroform E D KJ/mol 3.89 4.724 8.45 12.45 14.17 E P kj/mol 13.08 14.02 15.26 16.43 18.64 H KJ/mol 9.19 9.30 6.86 3.98 4.46 G (KJ/mol) -5.46-5.01-4.25-4.11-3.95 S (J/mol) 119.50 86.34 48.45 46.50 37.42 S D RS RD experiments The sorption process was conducted on polymer samples by the usual method. After sorption samples were desorbed fully. The desorbed samples were again exposed to solvent for resorption followed by redesorption. Sorption, desorption, resoprtion and redesorption curves of D 1 sample are displayed in Figure 6. The solvent used was chloroform. The sorption and resorption curves do not exhibit identical patterns. But desorption and redesorption follow identical patterns. The resorption curves showed higher equilibrium uptake values compared to sorption process. In a sorption, desorption cycle the available free volume of the polymer might increase and hence subsequent sorption process is different from that of the original (6,23). 300 Progress in Rubber, Plastics and Recycling Technology, Vol. 22, No. 4, 2006

Molecular Transport Characteristics of Poly(ethylene-co-vinyl Acetate) in Presence of Aliphatic Chlorinated Hydrocarbons Figure 6. Sorption, resorption, desorption and redesorption curves of D 1 sample Transport kinetics The kinetics of transport of chlorinated hydrocarbons was studied by first order kinetics (24). The first order equation is k 1 t= 2.303 log C / (C - C t ) (15) Where, k 1 is the first order rate constant, C t and C represent concentration at a time t and at equilibrium. Plot of log(c - C t ) versus time is shown in Figure 7. Since a straight-line graph is obtained, the transport follows first order kinetics. Progress in Rubber, Plastics and Recycling Technology, Vol. 22, No. 4, 2006 301

S. Anil Kumar and Sabu Thomas Figure 7. First order kinetics plot of D 1 sample in chloroform CONCLUSIONS Transport characteristics of poly(ethylene-co-vinyl acetate) in presence of chlorinated hydrocarbons have been analyzed by the sorption gravimetric technique. It is found that the introduction of crosslinks in EVA has profound influence on the sorption process. The equilibrium solvent uptake decreases in the order D 1 >D 0 >D 2 >D 4 >D 6 >D 8. The lower uptake of the uncrosslinked sample is due to its crystalline nature. When a critical concentration of the crosslinker was introduced, the sorption process is maximum due to the reduction in crystallinity. But when the extent of crosslinking goes up, solvent uptake tendency was reduced due to large number of crosslinks. The values of volume fraction and molar mass also support the solvent uptake behavior of EVA samples. The uptake of chloroform is maximum followed by carbon tetrachloride and dichloromethane and least for dichloroethane. It was found that temperature activates the diffusion process. The transport data were used to estimate the activation energy, the enthalpy and the entropy of sorption. Gibb s free energy for sorption is found to be negative. The solvent uptake 302 Progress in Rubber, Plastics and Recycling Technology, Vol. 22, No. 4, 2006

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