Chapter 7. Gas Transport through Nano and Micro Composite Membranes. Summary: This chapter deals with the gas permeability of nano and micro filled

Transcription

1 Chapter 7 Gas Transport through Nano and Micro Composite Membranes Summary: This chapter deals with the gas permeability of nano and micro filled composites of natural rubber (NR), carboxylated styrene butadiene rubber (XSBR) and their blend membranes. The gas transport behaviour has been investigated with special reference to type of filler, gases, filler loading and pressure. The effect of free volume on the gas barrier properties has been investigated by positron annihilation lifetime spectroscopy (PALS). It is found that the relative fractional free volume of latex membranes deceased in the presence of layered silicates. It is observed that due to the platelet like morphology and high aspect ratio of layered silicates, the gas barrier properties of nano filled latex membranes are very high. - The results of this cho~ter hare hrrn rrrhmiitrdlo Po!vmer

2 190 Chapter Introduction Membranes of polymers have played an important role in inany applications such as gas and liquid separations and barriers for packing'-5. The transport of gases through a membrane depends on various factors like permeant size and shape, permeant phase, polymer molecular weight, functional groups, density and polymer structure, crosslinking, crystallinity, orientation etc6. The wide application of membranes for gas separation has attracted polymer technologists to synthesise new polymeric membranes with good permeability and Paul and co- worker^'"'^ have investigated the relationship between gas transport and polymer structure. The introduction of functional groups in the polymer chain can alter the permeability and selectivity due to the variation in the existing free volume within the polymer15-'6. The selective transport of gases through polymeric membranes has been reviewed by Aminabhavi and co-workers". ~rnero~en'~.'~ has extensively studied the permeability of various gases through different elastomers. Thomas and co-workers Investigated the gas transport properties of various rubber blends. They concluded that permeation is a process in which gas molecules dissolve in the elastomer on one side of a membrane, diffuse through it to the other side and then evaporate. The rate of diffusion in a given polymer is found to be related chiefly to the size of the gas molecule. It is observed that the presence of polar groups or methyl groups in the polymer molecule reduces the pemleabilitj of ;I given gas. In the case of permeant gases such as nitrogen, oxygen, and hydrogen, the permeability depends on the nature of gas, the membrane material, temperature and pressure. Matayabas and co-workers 22 studied the enhancement in gas barrier properties of poly (ethylene terephthalate) (PET) upon the addition of nano clay. van

3 Gas transport This chapter deals with the gas transport properties of micro and nanocomposites of rubber membranes. The gas permeability of these membranes has been correlated with the free volume. 7.2 Results and discussion Free volume measurements An understanding of the free volume of polymers is crucial for determining the permeability and ionic conductivity of polymers. The experimental quantification of free volume can be carried out by positron annihilation lifetime spectroscopic analysis (PALS) 23-24, This method helps one to estimate the hole size at a nanoscale and its fraction directly. It has been fonnd that the estimated hole size and fraction significantly relate to the free volume propem of polymer material^^'^^^. Ito examined the relationship between the oxygen permeability and the free volume for ethylene-vinyl alcohol copolymer as the ethylene content varies, which significantly followed the free volume theory. The results obtained by them suggest that the molecular mechanism of gas permeation could be considered on the basis of the local motion of the polymer segments and the free volume size. Nagel selective polymer membranes. correlated the free voluine and transport properties of highly The impact of nano and conventional micro fillers on the free volume and gas barrier properties of SBR ha5 been carried out by War~g et rrl". The authors concluded that the gas permeability of SBlZ nanocomposite is less than carbon black filled SBR, it's revealed that the permeability of nanocomposite is mainly influenced by fractional free \,olume and tortuous diffusion path effects explained to the clay platelet morpholog\. The diffusion of permeant through polymeric membranes can be described by two theories, viz, ~nolecular and fr-er volunle theories. According to free volume theory

4 192 Chapter 7 the diffusion is not a thermally activated process as in molecular model but it is assumed to result from random redistributions of free volu~ne voids within a polymer matrix. Cohen and ~urnbu11~~~~' developed the free volume models that describe diffusion process when a molecule moves into void larger than a critical size; V,. Voids are formed during the statistical redistribution of free volume within the polymer. Free volume in polymer is represented as Vfand is defined as Vr V-V,, where V is the specific volume and V, is the specific molecular volume due to steric size and thermal vibrations. The theoty of Cohen- Turnbull is given by, where y is an overlap factor (y=l, for most polymers). This theory is not applicable for polymers far below their 7, and at high temperature because the chain motions are arrested below T, but at higher temperature an activation energy term is needed. ~ujita" relates the thermodynamic diffusion coefficient, DT and fractional free volume of the polymer, Vr by, ;I D, = RTA,, [ exp -- where Ad is related to the size and shape of the permeant by, where M is the molecular we~ght of the permeant, o IS the Lennard- Jones size *, parameter and Bd is a parameter describing the amount of tree volume needed. Ft~jita's theory is based on the assumption that a diffusing ~nolecule can only move from one place to another when the local free volume around that molecule exceeds a certain critical value.

5 Gas transport The effect of layered silicates such as sodium bentonite and fluorohectorite and the conventional micro fillers on o-ps life time (r3), o-ps intensity (I3(%)) and relative fractional free volume are presented in Table 7.1. The free volume of nanocomposites decreases. This indicates that the positronium atoms are formed and annihilate only in the pre-existing holes of virgin polymers. It is found that the relative fractional free volume of virgin poly~ners decreases upon the addition of layered silicates. The decrease is explained to the interaction between layered silicates and polymer due to the platelet str~~cture and high aspect ratio of layered silicates. However, the effect of micro fillers on free volume is rather complex. It shows increased relative fractional free volume values, which may be due to the aggregation of fillers and the consequent additional void formation. Layered silicates filled NR and XSBR have reduced o-ps lifetime because the Ps atonis formed can annihilate only through the free volume holes present in the virgin polymer. The decrease can be explained in terms of the restricted mobility of the chain segments in the presence of layered ~ilicates. In the case of blend system, due to the uneven distribution of silicates it shows enhanced Ps lifetime. The intensity of Ps decreases with the addition of layered silicates and this can be explained in terms of the restricted mobility of chain segments on incorporation of fillers resulting in reduced free volume concentration or relative fractional free volume.

6 1 94 Chapter 7 Table 7.1 PALS measurements data 1 / Relative fractional %, o-ps intensity, free volume b1' Permeation of gases The gas transport properties of micro and nano tillers reinforced latex membranes have been analysed using oxygen and nitrogen gabes. Nitrogen and oxygen gas permeability coefficient of micro and nano filled me~nbranes are shown in Figures

7 It is found that the transport of gases th~.ough layered silicates tilled latex membranes is lower than conventional micro filled samples. The enhancement in gas barrier properties of nano clay reinforced latex membranes indicate strong polymerlfiller interaction resulting in more tortuous path for the permeant molecules to travel through the membranes. 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 ~imha" reported on the reduced free volume available in the polymer matrix after the incorporation of clay platelets. According to them in exfoliated polymer nano composite the accessible clay surface area is proportional to organo clay loading. They observed that the addition of 4 wt% of organo clay (Cloisite 15) could reduce the matrix hole fraction twice as large as that observed for polymer nanocomposite with 2 wt% organo clay. The incorporation of 1.1 and 2.42 wt'% of montmorrillonite (MMT) can reduce the matrix free volume to 4.7 and 8.0% respectively. Upon the addition of nano clay sometimes small change can be observed due to the intercalation of the polymer into the clay layers I bar 1 Figure 7.1 Nitrogrn permeability of nano and micro composites of NR (2.5 phr)

8 Figure 7.2 Oxygen permeability of nano and micro composites of NR (2.5 phr) Figure 7.3 Nitrogen permeability of nano and micro composites of XSBR (2.5 phr)

9 Gas transport Figure 7.4 Oxygen permeability of nano and micro composites of XSBR (2.5 phr) Figure 7.5 Nitrogen permeability of nano and micro composites of N~~(2.5 phr) In most of the cases, micro fillers containing systems exhibit increase in permeation of gases due to the aggregation of particles resulting in the formation of voids as shown in Figures The increase ~II gas barrier properties of latex membranes reinforced with layered silicates are due to the exfoliation of silicates in the poly~ner matrix leading to the nanometric level dispersion ofthe organic and inorganic phases. The presence of aqueous phase in latex can easily disperse the silicate particles into individual layers. The moleculal- level of polyme~-/filler

10 198 Chapter 7 interaction results in reduced availability of free volume and as a result the permeability of latex membranes decreases. The poor gas barrier properties of micro filled systems as compared to those with layered silicates can be explained in terms of the poor physical and chemical interaction between the organic and inorganic components leading to the aggregation of fillers. The enhanced gas barrier property of nano filled latex membranes are owing to its platelet like morphology and high aspect ratio of fillers. The effect of penetrant size on the diffusion of gas molecules through these membranes can also be understood from the Figures When compared to nitrogen, oxygen has more permeability than nitrogen due to the lower covalent radii of oxygen. This can be explained by using Stoke- Einstein equation; according to them the diffusion of gas molecules is inversely related to the friction exerted. The equation is given by, where kb is the Boltzmann constant, T is the absolute temperature and f is the friction factor. As the radius of the gas molecule increases the friction factor also increases by the relation, and thereby the permeability decreases. In this equation p is the viscosity of the solvent and Ro is the radius of the diffusing gas molecule. The schematic representation of increase in tortuosity on the addition of fillers is shown in Figure 7.6. Tortuosity factor is the ratio of the actual distance that a penetrant must travel in the presence of layered silicate to the shortest distance that it would travel in the ahsence of layered silicates.

11 Gas transport *&- k Rubbe? (rs, Rubber +Layered Pilirrte Pu(leulrtr WPf)..I * Rubber +PF Figure 7.6 Schematic illustration of diffusion of gas molecules through nano filled membranes micro and I'he activation energy needed for the diffusion of trapped ~nolecules from one cavity to another one is related to cohesive energy density by the equation developed by Meares "and is given by, where o' is the cross section of the penetrant molecule, d is the jump length and NA is Avogadros' number. Due to the polarity of XSBR latex its cohesive energy density is high and hence the ~enneability of gases through these membranes is very less. The addition of tillers further enhances its gas barrier properties. 1,iterature shows that increase in pressure will enhance the segmental #nobility and, will result in the occupation of gas molecules in the voids". The effect of pressure on the nitrogen gas permeability of NR latex with nan<) and conventio~lirl tillers

12 200 Chapter 7 are given in Figure 7.7 Sodium bentonite tilled and particulate clay filled systems show marginal effect of pressure on the gas transport properties while rest of the systems are almost independent of the pressure exerted due to the close packing of tillers in the polymeric materials, especially for fluorohectorite tilled system 4.,., I6 Pressure (bar) Figure 7.7 Effect of pressure on the nitrogen permeability composites of NR nano and micro -d I? I I 16 Pressure (bar) Figure 7.8 Effect of pressure on the nitrogen permeability of nano and micro composites of XSBR

13 A plot of permeability change with increase in pressure of XSBR membrane containing fillers is given in Figure 7.8. The unfilled XSBR systems show slight increase in permeability with pressure due to the increase in segmental mobility. The permeability of filled XSBR system increases slightly with pressure similar to blend system. From the results we can conclude that the effect of pressure on the permeability of all systems is negligible. The addition of fillers reduces gas permeability of polytners according to a tortuous path model, developed hy ~eilson)~, where PC and P, are the permeability of composite and polymer, #,,.is the volume fraction of filler and a is the aspect ratio of platelets. Schematic representation of the tortuous path model is given in Figure 7.9. From this it is clear that the gas molecules have to travel through a tortuous path in the presence of layered silicate. T~OIS pth Figure 7.9 Schematic representation of tortuous path model

14 202 Chapter 7 The effect of filler loading on the permeation of gases through NR latex membrane containing layered silicates and conventional clay is shown in Figure The gas permeability of silicates filled palymer system decreases constantly with filler loading due to the tortuous path the gas molecule has to have1 in the presence of layered silicates. In contrast, the gas permeability of micron sized filled system increases as a function of filler Loading. This can be explained in terms of aggregation of fillers with increase in weight percentage of filler resulting in the weakening of polymer chain. The high gum strength of NR latex and the lack of active centers will enhance the aggregation of conventional filler. The stnrctural peculiarity of layered silicates is the reason for the enhancement in gas barrier properties of NR latex membrane with the addition of filler. The aqueous medium of NR latex can easily disperse the platelets of silicates into individual layers. Figures 7.11& 7.12 are the plots of variation in gas permeability of filled XSBR and NTO latex membranes. It is interesting to note that in these two systems the gas barrier properties enhanced with the increment in the weight percentage of filler. The decrease in gas permeability is sharp for nano systems due to greater polymerlfiller interaction. Gas permeability coefficient of micro filled XSBR and N70 decreases as a function of filler loading hut the change is not so sharp as seen in layered silicates reinforced systems., n 1 2, 4 k *T7-7- Wc~aht%oCfiller (phr) Figure 7.10 Permeability of nano and mlero composites of NR a,ith filler loading

15 ,. - i i Wetght % of filler (phr) Figure Permeability of nano and micro composites of XSBR with filler loading 04.,.,.,. >. 8., ' > ' ' ' Wetght % of Filler (phr) Figure 7.12 Permeability of nano and micro composites of NTO with filler loading 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 together with high perrnselectivity. The per~nselectivity of a membrane is given by,

16 204 Chapter 7 where a is the permselectivity of a tnetnbranc towards 0 2 and N2 gas, P(02) and P(N2) are the permeability constants of 0 2 and N, gases respectively. The permselectivity values of filled latex membranes are given in Table 7.2. Sodium fluorohectorite filled XSBR system exhibits higher selectivity value. The layered silicates reinforced XSBR and N70 blend systems possesses higher selectivity than the micro tilled and untilled polymers. These might be ascribed to the interaction between the polymer and the tiller. The introduction of silicates into NR latex has varied effects on the selectivity of membranes. Compared to nitrogen gas the NR latex membrane containing micro fillers is highly permeable to oxygen gas and thereby the selectivity of these membranes increases. Table 7.2 Oxygen-to- nitrogen selectivity values of nano and microcomposites Permselectivity Sample 1 (P(OW'(N.)I I The selectivity and permeability of these membranes can be further understood from the diffusivity and solubility values. The permeability of a polymeric material is the product ofdiffusivity and solubilit). It is usually represented as.

17 Gas Iransporl where P is the permeability coefficient, D is the diffusivity and S is the solnbility. The diffusivity D is a kinetic parameter related to the size of the permeant and to the polymer- segment mobility. The solubility factor S is a thermodynamic parameter, which depends on the polymer- permeant interactionh. The diffusion coefficient, D is calculated by the time- lag method using the equation, where x is the thickness of the sample and t' is the time-lag, i.e., the time needed to attain the steady- state condition. Table 7.3 tabulates the NZ and 0 2 gas diffusivity and diffusive selectivity values of filled and unfilled latex membranes. Table 7.3 Diffusion coefficient and selectivity values of nano and micro composites Sample N I ~ S P N 1m~pE2 5 D x 10' (cm21sec) 0, gas N2 gas D(Oz)ID(Nz) 1.OY I.OO

18 206 Chapter 7 The diffusion coefficient of bentonite filled NR is lower than other tilled systems. It can be seen that the diffusivity values of nano filled NR is lower than other systems. This is due to the reduction in the overall segmental mobility in the presence of inorganic fillers, particularly at the interfacial phase surrounding the silica particles. The fluorohectorite filled system shows higher diffusion selectivity values. The gas diffusion coefficient of nano filled membranes exhibit lower values than the conventionally filled and unfilled systems. It is interesting to note that the diffusion selectivity of nano filled latex membranes is higher than other systems. These membranes show a drastic decrease in diffusion coeff~cient values, it is ascribed to the comparatively homogeneous distribution of nano particles in the polymer chain resulting in the compact packing between the polymer chains and the nanoparticles. It is also well known that the crosslinking evidently causes a decrease in the total amount of free volume in the polymer chain. Sulphur prevulcanised latex membranes are used for these experiments. Crosslinked polymeric systems show a remarkable reduction in diffusivity. The crosslinking and the presence of layered silicates decrease the segmental mobility and hence the diffusion process Barrier property The barrier enhancement in latex membranes in the presence of fillers for N2 and O2 gases are given in Table 7.4. Barrier is defined as the ratio of permeability of pure polymer to the permeability of polymer being compared. The barrier enhancement can be seen in the case of layered silicate filled systems than the micro filled system. The layered silicates enhance the gas barrier of polymers according to a tortuous path model, developed by ~eilson'~, in which the layered silicates obstruct the passage of gases and other penneants through polymer matrix as represented in Figure 7.9.

19 Gus transport Table 7.4 Barrier property values of filled latex membranes 7.3 Conclusion.:- Latex membranes reinforced with nano silicates exhibited reduced permeability indicating higher polymer/ filler interaction.:. The relative fractional free volume of nano filled latex membrane decreased. t. The effect of micro fillers on the gas transport property of latex ~ne~nbranr is negligible due to its filler- filler interaction..> Nano filled membrane exhibited good barrier property.

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