Transport of Aromatic Hydrocarbons

Transcription

1 Chapter 7 Transport of Aromatic Hydrocarbons.. Abstract The transport behaviour of HDE/EVA blends in toluene, xylene and mesitylene was analysed at different temperatures. The effects of blend ratio, compatibilisation, crosslinking, penetrant size and temperature on the sorption behaviour have been studied. HDE shows the minimum and EVA shows the maximum uptake of toluene. The blends show an intermediate behaviour, which increases with increase in EVA content in the blend. The observed sorption characteristics are correlated to the blend morphology. The mechanism of transport has been analysed and found that the mode of transport is close to Fickian. The variation of diffusion, sorption and permeation coefficients with blend ratio, compatibilisation and crosslinking has been analysed. The solvent uptake decreases with increase in size of the penetrant molecule. The effects of compatibilisation and crosslinking on the sorption behaviour have been studied and found that the uptake of 70/30 HDE/EVA blends decreased by compatibilisation and crosslinking. In the case of neat components and blends the uptake of toluene is increased by an increase in temperature. The activation energy for permeation is calculated using Arrhenius theory and found that as the concentration of EVA in the blend increases the activation energy values decreases. The first order kinetic model has been used to follow the kinetics of diffusion of solvents through HDE/EVA blends. Various theoretical models have been used to study the permeability of blends... * The results of this chapter have been submitted for publication in Journal of hysical Chemistry B (ACS).

2 00 Chapter Introduction The transport of various organic solvents through polymer blends is of great importance due to the fact that these materials are widely used in various barrier applications. In the case of polymer blends, in addition to the factors like physical and chemical structure of polymers, crosslink density, shape and size of the penetrant molecule and temperature, the transport phenomena depend on the miscibility of the component polymers and blend morphology -. Hence, study of transport phenomena in blends gives an idea about the miscibility and morphology of the system. The transport behaviour of organic liquids through polymers and polymer blends has been studied extensively by several researchers -0. Fick s law of diffusion can be used to explain the diffusion of solvents through rubbery polymers. Because of their time dependent response glassy polymers exhibit a non-fickian or anomalous diffusion kinetics. The effect of blend ratio, vulcanising systems, and different fillers on the diffusion of aromatic hydrocarbons through polypropylene/nitrile rubber (/NBR) blends was reported by George et. al 7. The uptake of solvents and the diffusion coefficients increases with increase in rubber concentration and decreases with molar volume of solvents. Diffusion and permeation properties of substituted benzenes through blends of NBR and natural rubber (NR) have been investigated by Mathai et al 0. The effects of blend ratio and penetrant size on the transport properties were studied. In the blends, the equilibrium solvent uptake decreases with an increase in concentration of NBR. The relationship between the transport behaviour and the morphology of the system was also examined. Aminabhavi and hayde 3 studied the transport properties of haloalkanes through ethylene propylene copolymer / blends. Sorption, desorption, re-sorption, re-desorption and degree of overshoot were influenced by nature of penetrant and temperature. The effect of crosslink density on diffusion has been studied by several researchers,4,5. For low levels of crosslinking, there is a linear relationship exists between the decrease in diffusion coefficient and crosslink density. At higher levels of crosslinking the rate of decrease levels off. Solvent transport of

3 Transport of Aromatic Hydrocarbons 0 aromatic hydrocarbons through both crosslinked and uncrosslinked ethylene vinyl acetate (EVA) membranes have been reported by Unnikrishnan and Thomas. It was found that as the extent of crosslinking increases, the equilibrium uptake decreases. It was also found that temperature activates the phenomenon of diffusion according to Arrhenius equation. Blends of HDE and EVA are a new class of thermoplastic elastomers, which couple the superior properties of HDE and EVA. These blends will exhibit good mechanical properties, processability, insulating property etc. These blends may find use as barrier layers, cable coatings, food packaging, encapsulation of electronic circuits, controlled drug release, reverse osmosis, pervaporation separation, microelectronics etc. It is quite essential to evaluate the dimensional stability of the polymeric materials in the presence of aggressive liquids. The diffusion process is also important because the permeating molecule can be used as a molecular probe to determine the polymer morphology. However these blends are immiscible and incompatible due to high interfacial tension and poor interfacial adhesion - resulting from unfavourable interfacial interactions, which arise due to the polarity differences between the components. Therefore these blends are compatibilised by suitable interfacial agents or vulcanised under appropriate conditions so as to achieve necessary superior properties suitable for commercial applications. This chapter examines the diffusion of aromatic hydrocarbons (Toluene, Xylene and Mecetylene) through HDE/EVA copolymer blends. The effects of blend ratio, dynamic vulcanisation, compatibilisation, penetrant size and temperature on transport behaviour have been studied. The experimental results were correlated to the blend morphology and compared with various theoretical predictions. 7.. Results and discussion 7... Effect of blend ratio on diffusion The transport behaviour of HDE/EVA blends in toluene was analysed at 8 C. The effect of blend ratio on the sorption behaviour of HDE/EVA blends was studied and the mol% uptake (Q t ) vs. square root time is given in Fig.7.. From the figure it is clear that the uptake of toluene is minimum for HDE and

4 0 Chapter 7 maximum for EVA. The blends show an intermediate behaviour, which increases with increase in EVA content in the blend. The lowest mol% uptake of HDE can be correlated to its high degree of crystallinity. HDE is a semicrystalline polymer and in a semicrystalline polymer, there will be some amorphous region along with the crystalline regions and only these amorphous regions will contribute to the uptake of solvents. In the case of blends, the crystalline HDE phase makes a tortuous path to the transport of solvent through the amorphous region in the blend. As the EVA content in the blend increases, the crystallinity of the blends decreases. As the crystallinity decreases, the hindrance for the transport of toluene decreases and hence the uptake increases..4. Q t moi % H 00 H 70 H 50 H 30 H (t min) / Figure 7.: Variation of mol % uptake (Q t ) of toluene with square root of time at 8 C The variation of Q (equilibrium uptake) with weight % of EVA is given in Fig.7.. From the figure, it is clear that, as the wt % of EVA increases the Q value increases linearly up to about 70 wt % of EVA in the blend and then a change in slope of the sorption curve at higher concentration. The variation of Q can be correlated to the morphology of the blend. The scanning electron micrographs (SEM) of H 70, H 50 and H 30 are given in Fig.7.3 (a-c). It is seen that in H 70 and H 50, EVA phase gets dispersed as spherical domains in continuous HDE matrix. In

5 Transport of Aromatic Hydrocarbons 03 H 30 both HDE and EVA exhibit a co-continuous morphology. In the case of pure HDE, only the amorphous phase is responsible for sorption and Q is very low due to its high crystallinity Q mol % Weight % of EVA Figure 7.: Variation of equilibrium uptake (Q ) of toluene with weight % of EVA at 8 C In H 70 the EVA phase gets dispersed as spherical domains in the continuous HDE matrix and these low crystalline EVA phase increases the sorption. The continuous HDE phase makes a tortuous path to the transport of solvent and hence the increase is not much high. When a polymer interacts with solvents, the surface of the polymer sample swells immediately but the lateral expansion (due to swelling) is prevented by the underlying unswollen material. As a result a stress is developed and it is dissipated either by further swelling or rearrangement of segments. Thus the continuous crystalline HDE phase hinders the transport of solvents and restricts the swelling. So the solvent uptake is negligible for H 70 and it is schematically represented in Fig.7.4 (a). In H 50 dispersed as well as continuous EVA phase can be seen. As the EVA content in the blend increases from 30-50wt %, the average size of the dispersed EVA phase increases and the inter-particle distance decreases considerably (Table 7.). More over there is some continuous EVA phase. All these factors contribute towards an increase in uptake of H 50 than H 70 [Fig.7.4 (b)].

6 04 Chapter 7 (a) (b) (c) Figure 7.3 (a-c): SEM Micrograph of H 70, H 50 and H 30 Table 7.: Morphological parameters of HDE/EVA blends from SEM analysis HDE/EVA Blends Domain diameter (µm) D n D w Critical inter-particle distance (d c ) (µm) H H H H H 30 Co- Continuous morphology

7 Transport of Aromatic Hydrocarbons 05 In H 30 both HDE and EVA phases exhibit a co-continuous morphology. Due to the fully continuous nature of the EVA phase, one may expect a higher value of Q for H 30. But actually the uptake of H 30 is slightly higher than H 50. This is because the continuous and dispersed EVA phase of H 50 becomes fully continuous in H 30. This co-continuous interpenetrating structure, which obstructs the transport of the penetrant molecules 8 due to the presence of HDE crystallites and restricts the swelling process [Fig.7.4 (c)]. Further increase in concentration of EVA leads to a change in morphology ie. a phase inversion, which causes the change in slope at higher EVA concentration. In other words, when the concentration of EVA is higher than 70 wt %, the EVA phase becomes the continuous phase and the HDE becomes the dispersed phase. Since EVA is the continuous phase, the sorption is very high. Figure 7.4(a-c): Schematic representation of the tortuous path exhibited by HDE phase to the transport of solvent Further increase in concentration of EVA leads to a change in morphology ie. a phase inversion, which causes the change in slope at higher EVA concentration.

8 06 Chapter 7 In other words, when the concentration of EVA is higher than 70 wt %, the EVA phase becomes the continuous phase and the HDE becomes the dispersed phase. Since EVA is the continuous phase, the sorption is very high Mechanism of transport The results of diffusion experiments were expressed as moles of solvent uptake by 00g of polymer sample, Q t mol% Q t mol % = ( Mass of solvent sorbed ) Molar mass of solvent Mass of polymer X00 (7.) The mechanism of transport in HDE/EVA blends was analysed using the relationship (7.-7.3), log( / Q ) = log k + n logt Q t (7.) where, Q t and Q are the mol % solvent uptake at time t and at equilibrium respectively, k is a constant which depends on the structural characteristics of the polymer and gives information about the interaction between the solvent and polymer and n indicates the mechanism of sorption. When the value of n=0.5, the mechanism of transport is termed as Fickian and this occurs when the rate of diffusion of the penetrant molecule is much less than the relaxation rate of the polymer chains. When n=, the mechanism of transport is termed as non-fickian (case II-relaxation controlled) which arises when the rate of diffusion of the penetrant molecule is much greater than the relaxation process. However, the value of n between 0.5 and indicates anomalous transport behaviour and it is due to the fact that the rate of diffusion of the penetrant molecule and the relaxation rate of the polymer are similar. The values of n and k for HDE/EVA blends were obtained by regression analysis of the plot of log (Q t /Q ) vs. log t and the results are given in Table 7.. The correlation coefficient value is found to be Since, the value of n lies in between 0.5 to 0.59, the mode of transport is close to Fickian. Similar Fickian mode transport of several semicrystalline and elastomeric polymers has already

9 Transport of Aromatic Hydrocarbons 07 been reported 6,7. As the concentration of EVA in the blend increases, there is a slight decrease in the value of n and the sorption behaviour approaches the Fickian mode. In polymeric systems the transport of small molecules occurs via. a solution diffusion mechanism ie. first the penetrant molecules are sorbed by the polymer and then diffused through it. The kinetic parameter, the diffusion coefficient (D) or diffusivity can be calculated using the expression 8-30, hθ 4 D = π Q (7.3) where, h is the initial sample thickness, θ is the slope of the initial linear portion of the sorption curve ie. before the attainment of 50% of equilibrium uptake and Q has the same meaning as in equation 7.. Table 7.: n and k (in g.g - min n ) values of sorption of aromatic hydrocarbons by HDE/EVA blends Blend Toluene (8 C) Xylene (8 C) Mesitylene (8 C) n k x0 n k x0 n k x0 H H H H H The permeation of a penetrant to a polymer membrane depends on the diffusivity as well as the sorption of the penetrant into the membrane. Hence the sorption coefficient (S), which is the maximum saturation value, has been calculated using the equation 9,

10 08 Chapter 7 S M = s M p (7.4) where, M s is the mass of the penetrant at equilibrium swelling and M p is the mass of the polymer sample. As mentioned earlier the permeation is a combined process of diffusion and sorption 3, the permeability () is given by the expression, = D x S (7.5) The variation of the D and with wt % of EVA in the blend is given in Fig.7.5.HDE has the lowest and EVA registered the highest D and values. The lowest D and value of HDE is due to its lowest swelling behaviour, which is attributed to its high degree of crystallinity Diffusion coefficient(d x 0 8 ) D Wt % of EVA ermeation coefficient ( x 0 9 ) Figure 7.5: Variation of D and with wt % of EVA As the wt % of EVA in the blend increases the considerable extent of swelling takes place and the value of D and increases. The increase in diffusion and permeation coefficients with increase in EVA content may be due to the fact that the tortuosity exhibited by HDE to the penetration of solvent molecules decreases with increase in EVA content. The D and value of H 70 is not much higher than H 00 because, in H 70 the EVA phase gets dispersed in the continuous

11 Transport of Aromatic Hydrocarbons 09 crystalline HDE - which hinders uptake of solvents. That is, the increase in D and values are relatively small up to 30 wt % of EVA and in the % composition range, it is moderate. However, when the concentration of EVA is greater than 70% the values increase sharply, which may be attributed to the phase inversion of EVA from the dispersed to the continuous phase. Table 7.3: Diffusion (in cm s - ), Sorption (in g.g - ) and ermeation (in cm s - ) coefficients of HDE/EVA blends Blend Toluene (8 C) Xylene (8 C) Dx0 8 S x0 x0 9 D x0 8 S x0 x0 9 H H H H H Blend Mesitylene (8 C) D x0 8 S x0 x0 9 H H H H H The values of D, S and of the pure components and blends are given in Table 7.3. As the amount of EVA in the blend increases the values of diffusion, sorption and permeation coefficient increase. The maximum sorptivity of EVA reveals the fact that the absorbed solvent molecules are better accommodated in it. The experimental diffusion results were compared with theoretical predictions using the relation 3, Q t 8 = Q π n = /( n + ) 0 exp[ D ( n + ) π t / h ] (7.6)

12 0 Chapter 7 where, Q t and Q are the mass of mole percent solvent uptake at time t and at equilibrium and h is the initial thickness of the polymer sample. This equation represents a Fickian mode of transport. The experimentally determined values of D of H 70, H 50 and H 30 were substituted in the equation and the curves obtained are given in Fig.7.6 (a-c). The total agreement is fairly good. The slight deviation is due to the anomalous diffusion behaviour Q t /Q Experimental Theoretical Q t /Q Experimental Theoretical t / (min) (a) t / (min) (b)..0 Q t /Q Experimental Theoretical t / (min) Figure 7.6(a-c): Experimental and theoretical diffusion curves of H 70, H 50 and H 30 (in Toluene) (c)

13 Transport of Aromatic Hydrocarbons Effect of penetrant size The sorption behaviour of polymer blends is affected by the size, shape and polarity of the penetrant molecules. Homologous series of aromatic hydrocarbons such as, toluene, p-xylene and mesitylene were selected to study the effect of penetrant size. Fig.7. 7 shows the variation of mol % uptake (Q t ) with square root of time of H 50 blend for the diffusion of toluene, xylene and mesitylene at 8 C. From the figure it is clear that the solvent uptake decreases with increase in size of the penetrant molecule. In other words the low molecular weight solvent shows the highest uptake and the high molecular weight solvent shows the lowest uptake. The decrease in uptake with increase in penetrant size might be due to the grater activation energy needed for the activation of the diffusion process 4. The variation of Q with molar mass of the solvent for pure components and blends are given in Fig For pure components and blends the Q values decrease linearly with molar mass of the penetrant. In the case HDE and HDE rich blends, the sorption is low and the values do not change appreciably with change in molar mass of the solvent. The rate of decrease of Q with molar mass increases with increase in EVA content in the blend Q t mol % Toluene Xylene Mesitylene (t min) / Figure 7. 7: Variation of mol % uptake (Q t ) with square root of time of H 50 blends for toluene, xylene and mesitylene at 8 C

14 Chapter 7 The influence of penetrant size on the mechanism of transport, diffusion, sorption and permeation coefficients are given in Table 7. & 7.3. From Table 7. it is clear that the value of n decreases with increase in molar mass of the penetrant. Moreover the value of n is in between 0.48 and 0.53 i.e. the mode of transport is close to Fickian. In the case of all the solvents the D, S and values of the pure components and blends decrease with increase in molar mass of the penetrant indicating a decreased sorption of the solvents with increase in molar mass...0 H 00 H 70 H 50 H 30 H Q (mol %) Molecular mass of the Solvent Figure 7.8: The variation of Q with molar mass of the solvent in HDE/EVA blends Effect of compatibilisation Most of the polymer blends are immiscible and incompatible and their properties can be improved by the addition of a compatibiliser. During compatibilisation of an immiscible blend, the compatibiliser will generally locate at the interface between the dispersed and continuous phase. This will lead to an increase in the interfacial thickness.

15 Transport of Aromatic Hydrocarbons 3 The effect of compatibilisation on the Q t mol % uptake of compatibilised and uncompatibilised H 70 blend with square root of time is given in Fig From the figure it is clear that the uptake of H 70 blends decreased by compatibilisation. As the wt % of the compatibiliser increases the uptake decreases. The n, k, D, S and values of the sample are given in Table 7.4. From the table it is clear that the value of n decreases on compatibilisation. In all the cases the value of n is in between 0.49 and 0.58 ie, the sorption behaviour follows the Fickian mode of transport. For compatibilised blends, the values of D, S and are lower than those of the uncompatibilised blends (Table 7.4). Moreover the values of D, S and decrease with increase in concentration of the compatibiliser. The decrease in value of diffusion parameters upon compatibilisation can be explained on the basis of the morphology change occurred during compatibilisation. 0.0 Q t (mol%) H 70 H 70 M 0.5 H 70 M H (t min) / Figure 7.9: Variation of Q t mol % uptake of compatibilised and uncompatibilised H 70 blend with square root of time for toluene (8 C)

16 4 Chapter 7 Table 7.4: n, k(in g.g - min n ), Diffusion (in cm s - ), Sorption (in g.g - ) and ermeation (in cm s - ) coefficients of compatibilised HDE/EVA blends Blend Toluene (8 C) n k x0 Dx0 8 S x0 x0 9 H H 70 M H 70 M H H H 50 M H H 30 M Figure 7.0: SEM micrograph of H 70 M In the case of compatibilised blends, the MA-E and h-e compatibilisers locate at the interface of HDE and EVA. The non-polar part of the compatibiliser is wetted by the HDE phase and the polar part is wetted by the EVA phase because of the dipolar interactions between the MA groups of MA- E and EVA and phenolic groups of h-e and EVA 3. As a result the interfacial thickness increases and this leads to the effective stress transfer between the dispersed phase and the continuous phase and an increase in interfacial adhesion. This dipolar interaction causes a reduction in the domain

17 Transport of Aromatic Hydrocarbons 5 size of the dispersed EVA particles, which is evident from Fig.7.0. This reduction in particle size with the addition of compatibilisers is due to the reduction in interfacial tension between the dispersed EVA and the continuous HDE matrix and also due to the suppression of coalescence. So we can say that the decrease in the value of diffusion parameters on compatibilisation is due to the increased adhesion of the two phases upon compatibilisation-which increases the tortuosity of the system Effect of dynamic vulcanisation The dynamic vulcanisation of the rubbery phase during mixing has been used as a way to improve the physical properties of several thermoplastic elastomers based on rubber/ plastic blends. During the process of dynamic vulcanisation the viscosity of the rubber phase increases due to crosslinking and the rubber domains can no longer be sufficiently deformed by the local shear stress and eventually broken down in to small droplets Q t (mol %) H 70 H 70 D 0.5 H 70 D H 70 D (t min) / Figure 7.: Variation of Q t mol % uptake of crosslinked and uncrosslinked H 70 blend with square root of time for toluene (8 C) Fig. 7. gives the variation of Q t mol % uptake of crosslinked and uncrosslinked H 70 blend with square root of time. From the figure it is clear that the uptake of toluene is decreased with increase in concentration of the vulcanising agent,

18 6 Chapter 7 dicumyl peroxide (DC). The n, k, D, S and values of the sample are given in Table 7.5. It can be noted that the value of n increases slightly on crosslinking. In the case of low EVA crosslinked blends (H 70 D 0. 5, H 70 D, H 70 D.5 and H 50 D ) the value of n exceeds 0.57 ie, the Fickian mode of transport changes to anomalous mode which arises due to similarity in the rate of diffusion of the penetrant molecule and the relaxation rate of the polymer. In the case of all crosslinked blends, the value of diffusion, sorption and permeation coefficients are lower than those of the uncrosslinked sample. Moreover the values of diffusion, sorption and permeation coefficients decrease with increase in DC content. The decrease in the value of diffusion parameters depends on the extent of crosslinking i.e. the crosslink density. Whenever a polymer is immersed in an organic liquid, its molecules will diffuse into the solid polymer film to produce a swollen gel. Dissolution is prevented if the attraction between neighbouring polymer molecules is sufficiently great, perhaps due to cross-linking. Swelling equilibrium is approached when the chemical potential of the solvent inside the swollen polymer becomes equal to that of the outside phase. The investigation of swelling equilibrium helps to elucidate the structure of the polymer network formed upon vulcanisation. Diffusion into solid polymers depends on the availability of appropriate molecular size holes in the network, in addition to the attractive forces between the penetrant molecules and the polymer. From the structure and morphology of the polymer, the presence of holes is determined in terms of chain entanglement densities and its dependence on swelling. In order to get a clear idea about the sorption process in relation to the morphological characteristics of the polymer, the molecular weight between cross-links (M c ) has been estimated by using the equation ρ VS ( φ φ / ) M = (7.7) c ln( φ) + φ + χφ

19 Transport of Aromatic Hydrocarbons 7 where, ρ p is the density of the polymer, Vs the molar volume of the solvent, φ the volume fraction of the polymer in the fully swollen state and χ is the polymer penetrant interaction parameter which is calculated from the equation 35, VS χ = β + ( δ ) S δ (7.8) RT Where, δ s and δ p are the solubility parameters of the solvent and the polymer, β the lattice constant whose value is taken as 0.34, R is the universal gas constant and T is the absolute temperature. Table 7.5: n, k(in g.g - min n ), Diffusion (in cm s - ), Sorption (in g.g - ) and ermeation (in cm s - ) coefficients of vulcanised HDE/EVA blends Blend Toluene (8 C) n k Dx0 8 S x0 x0 9 H H 70 D H 70 D H 70 D H H 50 D H H 30 D The volume fraction of the swollen rubber is estimated by considering HDE as filler in EVA (since the uptake of HDE is low) using the expression, ( d fw) ρ φ = ( d fw) ρ + A ρ p S S (7.9) where, d is the swollen weight of the polymer, f is the volume fraction of the filler, w is the initial weight of the sample, ρ p is the density of the polymer, ρ s is the density of the solvent and A s is the weight of the solvent in the swollen sample. The values of M c and crosslink density are given in Table 7.6.

20 8 Chapter 7 Table 7.6: M c and crosslink density of dynamically crosslinked HDE / EVA blends Blend M c Toluene Xylene Mesitylene Crosslink Density (x0-3 g mol/cm 3 M c Crosslink Density (x0-3 g mol/cm 3 M c Crosslink Density (x0-3 g mol/cm 3 H 70 D H 70 D H 70 D H 50 D H 30 D From the table it is clear that, the M c values ie, molar mass between the crosslinks, decrease with increase in DC content in the case of crosslinked H 70 blends. More over there is no appreciable change in the value of M c when the concentration of DC increases from to.5 phr. Since M c is inversely related to the crosslink density, we can say that the crosslink density values increase with increase in DC content in the blends. The crosslink density values also do not change appreciably upon increasing the DC concentration from to.5phr. This result is in good agreement with the mechanical properties of the blendswhich do not show a remarkable increase beyond phr of DC. So we can conclude that the decrease in diffusion, sorption and permeation coefficients upon vulcanisation is due to the predominant crosslinking of the EVA phase. The experimental chemical crosslink density values can be correlated to that obtained from affine and phantom network models 36. In the affine model, it is assumed that the components of each chain vector transform linearly with macroscopic deformation and the junction points are assumed to be embedded in the network without fluctuations. Then the molecular weight between crosslinks (Mc) for the affine limit of the model [Mc (aff)] was calculated using the formula 36

21 Transport of Aromatic Hydrocarbons 9 M c ( aff ) = / 3 / 3 µ ρv ( / 3 s ν c ν m ν m ) ν (ln( ν ) m + ν m + χν m (7.0) where, V s is the molar volume of the solvent, µ and ν are called the number of effective chains and junctions, ν m, the polymer volume fraction at swelling equilibrium, ν c, the polymer volume fraction during cross-linking, and ρ is the polymer density. In the case of phantom network model, the chains may move freely through one another. The junction points fluctuate over time around their mean position without being hindered by the presence of the neighbouring chains and are independent of deformation. The molecular weight between cross-links for the phantom limit of the model [Mc (ph)] was calculated by 36 M c ( ph ) = ( ) ρ V sν φ (ln( ν m ) + ν / 3 c m ν / 3 m + χν m (7.) where, φ is the junction functionality. Mc (aff) and Mc (ph) were compared with Mc (expt) and these values are also given in Table 7.7. It is observed that Mc values are close to Mc (aff). This suggests that in the highly swollen state, the chains in the blends and in the component polymers deform affinely, i.e. the chains in the network are freely moving without fluctuating the junction points. Table 7.7: Experimental and theoretical M c values of dynamically crosslinked HDE / EVA blends (Toluene 8 C) Blend M c (expt) M c (aff) M c (ph) H 70 D H 70 D H 70 D H 50 D H 30 D

22 0 Chapter Effect of temperature In order to study the effect of temperature on the transport properties, the sorption experiments were carried out at 8 60 C. In the case of pure components and blends the uptake of toluene is increased by an increase in temperature. The values of diffusion and permeation coefficients at different temperatures are given in Table 7.8. In the case of pure components and blend the values of diffusion and permeation coefficients increase with increase in temperature indicating increased sorption behaviour at higher temperatures. Table7.8: Diffusion (in cm s - ), Sorption (in g.g - ) and ermeation (in cm s - ) coefficients of HDE / EVA blends at different temperatures Diffusion Coefficient (Dx0 8 ) ermeation Coefficient (x0 9 ) 8 C 40 C 50 C 60 C 8 C 40 C 50 C 60 C H H H H H H 70 D H 50 D H 30 D H 70 M H 50 M H 30 M The temperature dependence of transport properties can be used to evaluate the activation energy for the permeation process using the Arrhenius relation, E = exp X (7.) 0 RT

23 Transport of Aromatic Hydrocarbons where, is the permeation coefficient, E X, the activation energy, R the universal gas constant, and T the absolute temperature. Arrhenius plot of log versus /T is given in Fig. 7.. From the Arrhenius plots the value of activation energy of the blends are calculated and are given in Table H 00 H 70 H 50 H 30 H 0 H 70 D H 50 D H 30 D H 70 M H 50 M 5 H 30 M log( X 0 9 )(cm 3 /S) /Tx 0 +3 (K - ) Figure 7.: Arrhenius plot of log versus /T for HDE/EVA blends From Table 7.9, it is seen that HDE shows the highest activation energy. The activation energy values decreases 7 with increase in concentration of EVA in the blend. The solvent uptake decreases with increase in size of the penetrant molecule. The solvent uptake decreases with increase in size of the penetrant molecule. Since the activation energy is inversely related to the sorption behaviour, the sorption behaviour of the blends increases with increase in concentration of EVA in the blend. Crosslinking and compatibilisation increases the activation energy of blends.

24 Chapter 7 Table 7.9: Values of the activation energy and thermodynamic parameters of HDE/EVA blends Blend Activation Thermodynamic parameters Energy (KJ/mol) H (KJ/mol) S (J/mol/deg) - G (KJ/mol) H H H H H H 70 D H 50 D H 30 D H 70 M H 50 M H 30 M Thermodynamic parameters The thermodynamic parameters for diffusion, H and S can be calculated using van t Hoffs relation 7, 0 0 K S H log = S.303R. 303 RT (7.3) where, K s is the equilibrium sorption constant, which is given by, K S No.of molesof solvent sorbedat equilibrium = Massof thepolymer (7.4) The values of H and S are obtained by the regression analysis of the plots of log K s Vs /T. The values of H and S are given in Table 7.9. From the table it is clear that the pure components and the blends have a positive value of H, indicating endothermic sorption behaviour. The H is a composite parameter

25 Transport of Aromatic Hydrocarbons 3 having the contributions from (i) Henry s law needed for the formation of a site and the dissolution of the species into that site, the formation of the site involves an endothermic contribution and (ii) Langmuir s (hole filling) type sorption mechanism, in which case the site already exists in the polymer matrix and sorption by hole filling gives exothermic heat of sorption. Thus, the positive H values of the blends suggest a Henry s type sorption behaviour for the blend. HDE has the highest value of H. As the concentration of EVA in the blend increases, the H values decreases. Compatibilisation and crosslinking increases also the H values. HDE has the lowest S value. As the concentration of EVA in the blend increases, the S values increases indicating an increased sorption (uptake) of the solvents. Compatibilisation and crosslinking decreases the S values. Using the H and S values we can calculate the Gibbs free energy ( G ) of the blends using the Gibbs Helmholtz equation. The G values of the blends are given in Table 7.9. The pure components and blends have a negative value of G indicating the fact that the sorption process is spontaneous. As the concentration of EVA in the blend increases, the G values decreases indicating an increased degree of spontaneity. Compatibilisation and crosslinking increases the G values of the blends Kinetics of diffusion The first order kinetic model has been used to follow the kinetics of diffusion of solvents through HDE/EVA blends. In order to apply this kinetic model it is assumed that during the sorption of solvents, structural changes may occur in polymer chains, which require a rearrangement of the polymer segments that can dominate the kinetic behaviour. According to the first order kinetic equation, dc dt = K ( C C t ) where, K is the first order rate constant, C t and C are the concentrations at time t and at equilibrium respectively. Equation 7.5 on integration gives (7.5) log( C C ) logc Kt t =.303 (7.6)

26 4 Chapter 7 The plot of log (C - C t ) Vs t gives a straight line (Fig.7.3) with slope equal to K /.303. Since the plot is a straight line we can find that sorption of aromatic hydrocarbons through these blends follows first order kinetics H 00 H 70 H 50 H 30 H 0 H 70 D H 70 M log (C -C t ) t (min) Figure 7.3: Variation of log (C-Ct) versus time (min) of HDE/EVA blends Table 7.0: The rate constant values of the transport of toluene in HDE/EVA blends Blend Rate Constant (K x 0 3 min - ) H H H H H H 70 D.73 H 50 D.8 H 30 D 3.88 H 70 M.85 H 50 M 5.54 H 30 M 3.9

27 Transport of Aromatic Hydrocarbons 5 From the slope the value of rate constant is determined and is given in Table 7.0. From the table it is clear HDE has the lowest value of rate constant. As the concentration of EVA in the blend increases the rate constant values increases. The rate constant values are a quantitative measure of the speed (ease) with which polymer uptake the solvent. So as the weight percentage of EVA increases the extent of sorption increases. Compatibilisation and crosslinking decreases the value of rate constant Comparison with theoretical predictions The permeability of heterogeneous polymer blends can be interpreted in terms of various theoretical models. The two commonly used models are Robeson s limiting models, namely series and parallel 7,37. According to arallel model, φ φ C + = According to Series model, ( ) C φ φ + = where, C, and are the permeation coefficients of the blend components and respectively and φ and φ are their corresponding volume fractions. Further for conducting spherical filler, the overall permeation coefficient is given by Maxwell s equation as 7, = ) ( ) ( d m d m d d m d m d m C φ φ where, the subscripts d and m correspond to the dispersed phase and the matrix respectively. Robeson 37 extended the Maxwell s analysis to include the continuous and discontinuous nature of both phases at the intermediate compositions and expressed the equations as, = ) ( ) ( ) ( ) ( X X b a φ φ φ φ where, X a and X b are the contributions of the continuous phase so that X a + X b =. (7.7) (7.8) (7.9) (7.0)

28 6 Chapter 7 Fig.7.4 shows the variation of experimental and theoretical values of the as a function of volume fraction of EVA in the blend. The experimental value lies between the two limiting models, upper bound parallel model and lower bound series model. The experimental data are close to Maxwell model with continuous HDE phase up to H 70. Similarly the experimental data is close to Maxwell model with continuous EVA phase up to H 30. The Robeson model gives permeability values equally consistent with the experimental values at all blend ratios. 8 ermeability ( x 0 7 cm /s) Experimental arallel Series Maxwell(HDE,cont.) Maxwell(EVA,cont.) Robeson Volume fraction of EVA Figure 7.4: Experimental and theoretical permeation coefficients as a function of volume fraction of EVA 7.3. Conclusion The effect of blend ratio on the sorption behaviour of HDE/EVA blends in toluene was analysed at 8 C. The crystalline HDE phase makes a tortuous path to the transport of solvent. As the EVA content in the blend increases, the crystallinity of the blends decreases-the hindrance for the transport of toluene decreases - and hence the uptake increases. As the wt % of EVA increases the Q value increases linearly up to about 70 wt % of EVA in the blend and then a change in slope of the sorption curve at higher concentration. The variation of Q is correlated to the morphology of the blend.

29 Transport of Aromatic Hydrocarbons 7 In H 70 and H 50, the EVA phase gets dispersed as spherical domains in the continuous HDE matrix and these low crystalline EVA phase increases the sorption. In H 30 both HDE and EVA phases exhibits a co-continuous morphology and this co-continuous interpenetrating structure obstructs the transport of the penetrant. Further increase in concentration of EVA leads to an increase in sorption.. The slope of the plot of log (Q t /Q ) vs. log t is given by the n values, which indicate the mechanism of sorption which lies in between 0.5 to 0.59, indicating that the mode of transport is close to Fickian. As the amount of EVA in the blend increases the values of diffusion, sorption and permeation coefficient increases. The increase in diffusion and permeation coefficients with increase in EVA content may be due to the fact that the tortuosity exhibited by HDE to the penetration of solvent molecules decreases with increase in EVA content. The experimental permeability values of HDE/EVA blends are correlated with various theoretical models. The experimental value lies between the two limiting models, upper bound parallel model and lower bound series model. The experimental data are close to Maxwell model with continuous HDE phase up to H 70. Similarly the experimental data is close to Maxwell model with continuous EVA phase up to H 30. The Robeson model gives permeability values equally consistent with the experimental values at all blend ratios. The solvent uptake decreases with increase in size of the penetrant molecule. The decrease in uptake with increase in penetrant size might be due to the greater activation energy needed for the activation of the diffusion process. The effect of compatibilisation on the sorption behaviour has been studied and found that the uptake of H 70 blends decreased by compatibilisation. As the weight percentage of the compatibiliser increases the uptake decreases. The value of n decreases on compatibilisation. The effect of crosslinking on the transport behaviour has also been studied and found that the uptake of H 70 blends decreased by crosslinking. The uptake of toluene is decreased with increase in concentration of DC. The value of diffusion, sorption and permeation coefficients decrease during compatibilisation and crosslinking. The decrease in

30 8 Chapter 7 the value of diffusion parameters depends on the extent of crosslinking i.e. the crosslink density. The decrease in diffusion, sorption and permeation coefficients upon vulcanisation is due to the predominant crosslinking of the EVA phase. In the case of pure components and blends the uptake of toluene is increased by an increase in temperature. The value of diffusion and permeation coefficients increase with increase in temperature indicating increased sorption behaviour at higher temperatures. The activation energy for permeation is calculated and found that as the concentration of EVA in the blend increases the activation energy values decreases. Crosslinking and compatibilisation increases the activation energy of blends. The thermodynamic parameters such as H, S and G are calculated. The pure components and the blends have a positive value of H, indicating a Henry s type sorption behaviour of the blend. As the concentration of EVA in the blend increases the H values decreases. Compatibilisation and crosslinking increases the H values. The pure components and blends have a negative value of G indicating the fact that the sorption process is spontaneous. As the concentration of EVA in the blend increases the G values decreases. The first order kinetic model has been used to follow the kinetics of diffusion of solvents through HDE/EVA blends. As the weight percentage of EVA increases the rate constant increases. Compatibilisation and crosslinking decreases the value of rate constant References. Unnikrishnan G., Thomas S., olymer, 35,5,5504,994.. Mathai A.E., Thomas S., J Macromol.Sci.hy.,B35(),9-53, Anilkumar S., Thomas S., Kumaran M. G., olymer,38,8,469, George S.C.,Thomas S.,Ninan K. N., olymer,37,6,5839, George S. C., Knörgen M., Thomas S., J. Membrane Sci.,63,,999.

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