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Ind. Eng. Chem. Res. 2008, 47, 4898–4904
Gas Transport Through Nano Poly(ethylene-co-vinyl acetate) Composite Membranes S. Anil Kumar,*,† He Yuelong,‡ Ding Yumei,‡ Yang Le,§ M.G. Kumaran,| and Sabu Thomas⊥ Department of Chemistry, N. S. S. College, Ottapalam, Palakkad, Kerala, India, College of Mechanical and Electrical Engineering and College of Materials Science and Engineering, Beijing UniVersity of Chemical Technology, Beijing, 100029, China, Rubber Research Institute of India, Kottayam, 686009, Kerala, India, and School of Chemical Sciences, Mahatma Gandhi UniVersity, Priyadarshini Hills Po, Kottayam 686560, Kerala, India
Poly(ethylene-co-vinyl acetate)-clay nanocomposites containing different filler loading were prepared. Transport of gases through the composite membranes was investigated, and the results were compared with the unfilled one. Transport studies revealed that gas transport was considerably reduced by the incorporation of clay filler into the polymer matrix especially at a loading of 3 wt % filler. It was also found that permeability increased for composites containing more than 3 wt %. This is presumably due to aggregation of clay filler at higher loading. The morphology of nanocomposites was analyzed by X-ray diffraction and transmission electron microscopy. Dielectric loss data were also used to understand the uniformity of dispersion of nanoparticles. The effect of free volume on transport behavior was investigated by positron lifetime spectroscopic analysis. The cross-link density values were estimated to correlate with the gas barrier properties. The oxygen/nitrogen selectivity of these membranes was investigated. Introduction Polymer membranes play an important role in many applications such as liquid and gas separations and barriers for packaging.1–4 The transport of gases through polymeric membranes depends on various factors like permeant size and shape, polymer molecular weight, functional groups, density and polymer structure, cross-linking, crystallinity, etc.5 The wide application of membranes for gas separation has attracted polymer technologists to synthesize new polymeric membranes of good permeability and selectivity.6–8 Paul and co-workers9–12 examined the relationship between gas transport and polymer structure. The introduction of functional groups in the polymer chain can alter permeability and selectivity due to the variation of the existing free volume within the polymer. Van Amerogen13,14 extensively studied the permeability of various gases through different elastomers. Chen et al.15 reported the sorption and transport of gases in polycarbonate membranes. Ruaan et al.16 reported the oxygen/nitrogen separation by polycarbonate/cobalt complex membranes and found that both oxygen permeability and O2/N2 selectivity increased when 3 wt % of cobalt was added. Thomas and co-workers17,18 studied the gas transport properties of various rubber blends and found that the rate of diffusion was related to the size of the gas molecule. Polymer-layered silicate nanocomposites are new hybrid materials with layered silicates in the form of sheets of one to several nanometers thick and hundreds of nanometers long. These materials have been studied extensively by many researchers during the past decade.19,20 Polymer clay hybrid nanocomposites have received greater attention due to the * To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: 09446045664. Fax: 91 481 561 190/ 800. † N. S. S. College. ‡ College of Mechanical and Electrical Engineering, Beijing University of Chemical Technology. § College of Materials Science and Engineering, Beijing University of Chemical Technology. | Rubber Research Institute of India, Kottayam. ⊥ Mahatma Gandhi University.
dramatic improvement in mechanical and electrical properties, heat resistance, radiation resistance, and gas barrier properties.21–24 Polymer clay nanocomposites based on organic polymer and inorganic clay minerals consisting of silicate layers are the most promising nanocomposite system.25,26 The gas permeability of rubber-clay hybrids was reduced by ∼30% with 4 vol % of exfoliated clay.27 An 80% decrease in water absorption was also reported for poly(-caprolactone) nanoclay composites.28 The permeability performance of nanocomposite normally depends on the clay content, aspect ratio, and degree of dispersion of silicate layers.29 The permeability reduction was attributed to the extremely high aspect ratio of clay platelets, which increased the tortuosity of the path of gas as it diffuses into the nanocomposite. The concept originated from the tortuous path model where permeability was a function of volume fraction and aspect ratio of the platelets. Matayabas and co-workers30 studied the enhancement in gas barrier properties of poly(ethylene terphthalate) upon the addition of nanoclay. The aim of the present study is to investigate the gas transport properties of poly(ethylene-co-vinyl acetate) nanocomposite membranes. The morphology of nanocomposites was analyzed by X-ray diffraction (XRD) and transmission electron microscopic (TEM) analysis. Positron annihilation lifetime spectroscopic analysis was used to estimate the free volume of nanocomposites. Experimental Section Materials. The nanoclay, cloisite Na+, was obtained from Southern Clay Products. It is a layered silicate. There are silicate layers of 1-mm thickness of 200-300 mm in lateral dimension. The cation exchange capacity of cloisite Na+ is 92.6 mequiv/ 100 g and its specific gravity is 2.86 g/mL. It contains no organic modifier. Poly(ethylene-co-vinyl acetate) (EVA) was supplied by Polyolefin Industries Ltd. (Chennai, India). It contains ∼18% vinyl acetate. The cross-linking agent used was dicumyl peroxide (DCP). Sample Preparation. Unfilled and nanoclay-modified composites of EVA were prepared. EVA was vulcanized by a
10.1021/ie071624h CCC: $40.75 2008 American Chemical Society Published on Web 06/21/2008
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peroxide technique using DCP. The mixing was done on a tworoll mixing mill (150 × 300 mm) at a nip gap of 1-3 mm and a friction ratio of 1:1.4. The nip gap, mill speed ratio, time of mixing, and temperature of the rolls were kept the same for all mixes. The amount of DCP used was 1 g/100 g of the polymer. The vulcanization behavior of the mixes was followed by using a Monsanto rheometer at a rotational frequency of 100 cycles/ min. Blank cuts from the uncured sample were marked with the direction of the mill grain and were vulcanized at 160 °C in a hydraulic press under a load of 24.5 × 10-4. The samples were designated as F0, F3, F5, and F7, where the subscript numbers represent the number of grams of the nanoclay per 100 g of the polymer. Thickness of the membranes were ∼100 µm Membrane Characterization: X-ray Diffraction Analysis. X-ray diffraction patterns were taken by using Ni-filtered Cu K∞ radiation (λ ) 0.154nm) by X’pert diffractometer, Philips at 40 keV and 30 mA. The samples were scanned in step mode by 1.5°/min scan rate. The operating voltage and the current of the tube were kept the same throughout the investigation. Transmission Electron Microscopic Analysis. TEM is the most effective method for the analysis of dispersion of layered silicates in polymer nanocomposites. The dispersion of layered silicates in EVA matrix was investigated by TEM. Transmission electron micrographs of the nanocomposites were taken in a Leo 912 Omega transmission electromicroscope with an acceleration voltage of 120 keV. The specimens were prepared using an ultracut E cryomicrotone. Thin sections of about ∼100 nm were cut with a diamond knife at -120 °C. Dielectric Measurements. The dielectric loss of the composites was recorded by an Agilent 4294A Precision impedence analyzer at low-frequency range (up to 1 MHz). The 8 mm × 8 mm rectangular samples were dried in a desiccator at reduced pressure and made into a parallel plate capacitor by placing between two electrode plates for analysis. All the measurements were made at room temperature. Thermogravimetry. Thermogravimetric analyses of the composites were performed using a TG/DSC analyzer (SDT Q600, TA Instruments) to study the systematic weight loss and thus the thermal stability of the specimens. The measurements were carried out between ambient temperature and 800 °C at a heating rate of 10 °C/min under nitrogen with a flow rate of 100 mL/min. Positron Annihilation Lifetime Spectroscopic Analysis. Positron annihilation lifetime spectra (PALS) are used to examine the free volume present in unfilled and nanofilled EVA samples. The positron lifetime spectrometer consists of a fast-fast coincidence system with BaF2 scintillators coupled to photomultiplier tubes type XP2020/Q with quartz window as detectors. The detectors were conical shaped to achieve better time resolution. A17 mCi 22Na positron source, deposited on a pure Kapton foil of 0.0127-mm thickness was placed between two identical pieces of the sample under investigation. This sample-source sandwich was positioned between the two detectors of PALS to acquire lifetime spectrum. The spectrometer measures 180 ps as the resolution function with 60Co source. However, for better count rate, the spectrometer was operated at 220-ps time resolution.31 All lifetime measurements were performed at room temperature, and two to three positron lifetime spectra with more than 1 million counts under each spectrum were recorded. In PALS analysis, there are only two measured parameters, namely, o-Ps lifetime (τ3) and o-Ps intensity I3. The o-Ps lifetime τ3 measures the size of the free
volume holes (Vf) and I3 is a relative measure of the number of free volume sites in the polymer matrix. Permeability Measurements. The measurements were done using an ATS FAAR gas permeability tester in nanometric method in accordance with the ASTM standard D1434. The permeability of O2 and N2 gases through EVA composites were tested at various pressures. Cross-Link Density Measurements. Cross-link density values of the samples were determined by an equilibrium swelling method using chloroform. Circular samples were punched from the sheet, the weighed samples were kept in chloroform until they reached equilibrium swelling, and then the weights were taken for the calculation. Results and Discussion Morphology. X-ray Diffraction Aanalysis. XRD is widely used for the characterization of the structure of layered silicate and polymer nanocomposites. The change in the d-spacing of the polymer nanocomposite is observed from the position of the peaks in the XRD patterns in accordance with the well-known Bragg equation. nλ ) 2d Sinθ
(1)
where n is an integer that gives the order of reflection, λ is the wavelength, d is the d-spacing, and θ is the angle of diffraction. The X-ray diffraction method has been used to characterize the formation and structure of polymer-silicate hybrids by monitoring the position, shape, and intensity of the basal reflection from the silicate layers. When insertion of polymer chains in the silicate layers occurs, an increase of silicate interlayer volume and corresponding layer spacing could be obtained, which in turn gives rise to the shifting of diffraction peaks to lower angles. Diffraction peaks cannot be seen in the case of exfoliated structures where silicate layers are completely and uniformly dispersed in a continuous polymer matrix.32 The X-ray diffraction patterns of unmodified EVA and nanoclay and polymer nanocomposites are shown in Figure 1a and b, respectively. Cloisite Na+ clay exhibits a single peak at an angle 2θ of 7° corresponding to a d-spacing of 11.7 Å. For EVA/clay nanocomposites, the characteristic diffraction peak moved to a lower angle with respect to that of nanoclay. For the composite samples containing 3 (F3), 5 (F5), and 7 (F7) wt % clay, the d-spacing were found to be 16.3, 16, and 14.6 Å corresponding to 2θ 5.4, 5.5, and 6.04°, respectively. This shows that EVA chains have intercalated into the interlayers of cloisite Na+. It was found that in all systems the interlayer spacing increased due to the intercalation of polymer into the layers of nanoclay. Enhanced interlayer distance indicated that the layered structure was retained. With the increase of clay content, the left shift magnitude of diffraction peak decreased; that is, the enlargement extent of the interlayer distance of the clay decreased. This indicated that the lower the loading of nanoclay the more favorable it was for the intercalation of EVA chains into the silicate layers. Transmission Electron Microscopic Analysis. The transmission electron micrographs of various EVA-clay nanocomposites are presented in Figure 2a-c, respectively. The dark lines in the transmission electron micrographs show the dispersion of silicates in the polymer matrix. It can be seen that, in F3 sample, the clay is well dispersed in the matrix and is having a more ordered exfoliated structure. When the percentage of clay increases, dispersion decreases and clay exists as large aggregates and is unable to undergo exfoliation. The above
4900 Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008
Figure 1. (a) X-ray diffraction patterns of unmodified EVA. (b) X-ray diffraction patterns of nanoclay and EVA clay nanocomposites.
observation is consistent with the data observed from the XRD patterns given in Figure 1. Dielectric Measurements. The formation of aggregates in higher filler loadings was also evident from the dielectric loss data given in Figure 3. The dielectric loss for the F3 sample, where the clay is uniformly dispersed, was found to be minimum where as the loss factor was more for samples with higher filler loadings. The nonuniform distribution of the clay particles and the aggregates in the matrix results in more dielectric loss. Thermal Behavior. The thermogravimetric analysis (TGA) of the samples was done to understand the thermal behavior, and the TGA curves are given in Figure 4. The weight loss in pure EVA starts at ∼250 °C. In the presence of clay, the onset of weight loss in the composites occurs at a slightly higher temperature. That is, the heat stability of the polymer is improved by the incorporation of clay as the filler. The sudden
weight loss observed between 400 and 500 °C due to the thermal degradation of the polymer is also shifted to higher temperatures in clay-filled EVA when compared to that of pure polymer. Thus, it is clearly depicted that the presence of clay particles enhances the thermal stability of the polymer. Positron Annihilation Lifetime Spectroscopic Analysis. Free volume present in nanocomposite systems plays a major role in determining the overall performance of the membranes. PALS is an efficient technique used for analysis of free volume. The diffusion of permeant through polymeric membranes can be described by two theories, viz. molecular and free volume theories. According to free volume theory the diffusion is not a thermally activated process as in a molecular model but it is assumed to be the result of random redistributions of free volume voids within a polymer matrix. Cohen and Turnbull33 developed the free volume models that describe diffusion process when a molecule moves into void larger than a critical size, Vc. Voids are formed during the statistical redistribution of free volume within the polymer. The effect of layered silicates on o-Ps lifetime (τ3), o-Ps intensity (I3percentage) and relative fractional free volume percentage are presented in Table 1. It can be deduced from the table that relative fractional free volume percentage is lowest for F3 system. It was found that the relative fractional free volume of unfilled polymer decreased upon the addition of layered silicates. The decrease is attributed to the interaction between layered silicates and polymer due to the platelet structure and high aspect ratio of layered silicates. The decrease is explained to the restricted mobility of the chain segments in the presence of layered silicates. This results in reduced free volume concentration or relative fractional free volume. The contact surface area between the filler and the matrix is higher in nanocomposites owing to its high aspect ratio, which in turn reduces the free volume concentration. It is also found that the relative fractional free volume percentage increased with clay loading. The increase in the fractional free volume values can be attributed to the aggregation of fillers and the consequent additional void formation. Wang et al.32 and Stephen et al.34 studied the impact of nanoparticles on the free volume and the barrier properties and came to the conclusion that the permeability of nanocomposite was mainly influenced by fractional free volume effects. EVA/Clay Nanocomposite Membranes for Gas Separation. The gas transport properties of nanoclay-reinforced polymer membranes have been analyzed using oxygen and nitrogen gases, and the results were compared with that of the unfilled one (F0). Nitrogen and oxygen gas permeability coefficients of unfilled and nanoclay-filled membranes are shown in Figure 5a and b, respectively. It is found that the transport of gases through nanoclay-filled membranes is lower than that of unfilled one. The enhancement in gas barrier properties of nanoclay-reinforced membranes indicate strong polymer/filler interaction. The increases in gas barrier properties of the membranes reinforced with layered silicates are due to the nanometric level dispersion of the organic and inorganic phases. The molecular-level dispersion of polymer/filler interaction results in the reduced availability of free volume, and as a result, the permeability of filled membrane decreases. The enhanced gas barrier property of nanofilled membranes is due to the platelet-like morphology and high aspect ratio of fillers. Due to the high aspect ratio of layered silicates, the contact area between the filler and the matrix increases. Utracki and co-workers35 studied the reduced free volume available in the polymer matrix after the incorporation of clay platelets. According to them, in exfoliated polymer nanocomposite, the
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Figure 2. Transmission electron micrographs of nanocomposites, (a) F3, (b) F5 (c) F7.
Figure 3. Dielectric loss data for EVA-clay nanocomposites. Figure 4. TGA curves for EVA-clay nanocomposites.
accessible clay surface area was proportional to organoclay loading. They observed that the addition of 4 wt % organoclay (cloisite 15) could reduce the matrix hole fraction by twice that observed for polymer nanocomposite with 2 wt %. The incorporation of 1.1 and 2.42 wt % montmorillonite can reduce the matrix free volume to 4.7 and 8.0%, respectively. PALS studies showed that the relative fractional free volume of virgin polymer decreases upon the addition of layered silicates. The decrease is due to the platelet structure and high aspect ratio of layered silicates. The intensity of Ps decreases with the addition of layered silicates and is attributed to the restricted mobility of chain segments resulting in reduced free volume concentration. The contact area between the filler and the matrix is higher owing to the high aspect ratio, which in turn reduces the free volume concentration. However, the unfilled system exhibits an increase in intensity of o-Ps. Therefore, the enhanced gas barrier property of the nanofilled membranes are due to the reduced free volume caused by favorable polymer-clay interaction. The influence of aspect ratio on the permeability of gases can be determined from Nielson equation.36
Table 1. PALS Measurement Data of Nanocomposites sample F0 F3 F5 F7
o-Ps lifetime, τ3 + ns
o-Ps intensity I3 + 0.1%
relative fractional free volume, %
2.33 2.38 2.35 2.32
20.48 7.29 14.61 17.85
4.78 1.77 3.46 4.13
Pc)Pp
[
1 - Qf 1 + RQf ⁄ 2
]
(2)
where Pc and Pp are the permeability of composite and polymer, Qf is the volume fraction of filler, and R is the aspect ratio of platelets. The calculated value of R is 112.9. The high aspect ratio of the clay platelets effectively decreases the available free volume, which is responsible for the reduced permeability. The influence of clay loading on the permeability of gases can be seen from Figure 5a and b, respectively. The gas permeability increases as a function of filler loading. This can be explained in terms of aggregation of fillers with increase in concentration of filler, resulting in the weakening of polymer
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Vrf )
(d - fw)Fp d - fw ⁄ Fp + As ⁄ Fs
(5)
where d is the deswollen weight, f is the volume fraction of the filler, w is the initial weight of the sample, Fp is the density of the polymer, Fs is the density of the solvent, and As is the amount of solvent absorbed. Figure 6 is a plot of cross-link density values of EVA samples. EVA nanocomposite sample with 3 wt % filler (F3) showed maximum cross-link density values. The amount of solvent absorbed by the sample decreases, and hence, the value of volume fraction of the polymer in the solvent-swollen condition increases leading to higher cross-link density. The increase in cross-link density of polymers with layered silicates signifies the reinforcement of clay in the polymer and as a result the stiffness of the material increases. Higher cross-link density values mean more restraint on the network and thus result in lower swelling due to the presence of fillers. When the number of cross-links per unit volume increases, it becomes very difficult for the gas molecules to penetrate through the tightly crosslinked system. Hence, the high cross-link density of the F3 sample is the reason for the reduced gas transport of these samples. However, at higher filler concentration, the cross-link density is found to decrease due to the aggregation of fillers. Hence, these samples F5 and F7 showed increased gas permeability. Selectivity of Membranes. The polymeric membranes used for gas separation processes have certain significance such as high permeability to the desired gas, high selectivity, and the ability to form useful membrane configurations. The requirement of an ideal membrane is high permeability along with high permselectivity. The permselectivity of a membrane is given by Figure 5. (a) Variation in permeability of oxygen gas through unmodified and EVA-clay nanocomposites. (b) Variation in permeability of nitrogen gas through unmodified and EVA-clay nanocomposites.
chains. Therefore, F5 and F7 samples showed aggregated structure because of the poor/filler interaction. This will result in the formation of microcavities at the interfacial region. Therefore, F5 and F7 samples showed increased gas permeability. The above results have been complemented by PALS analysis. It can be seen from Table 1 that the relative fractional free volume percentage is lowest for the F3 system. The decrease is due to the maximum interaction between layered silicate and the polymer matrix. Hence, the F3 membrane showed decreased gas permeability but the relative fractional free volume percentage increase with clay loading. At higher clay loading, aggregation of filler and consequent void formation occurred. Hence, F5 and F7 membranes showed higher free volume and they possessed increased gas permeability. Cross-Link Density Measurements. The cross-link density values can be calculated from swelling method using the equation37 ν)
1 2Mc
(3)
Mc is the molecular weight of polymer between cross-links.34 -FrVsVrf1 ⁄ 3 Mc ) 2 In[1 - Vrf] + Vrf + χVrf
(4)
Fr is the density of polymer, Vs is the molecular weight of solvent, χ is the interaction parameter, and Vrf is the volume fraction of rubber in the solvent-swollen filled sample. Vrf is given by the equation of Ellis and Welding.38
R(O2, N2) )
P(O2) P(N2)
(6)
where R is the permselectivity of a membrane toward O2 and N2 gas and P(O2) and P(N2) are the permeability coefficients of O2 and N2 gases, respectively. The permselectivity values of the membranes are given in Table 2. The nanocomposites possess higher selectivity than the unfilled one. The increase in the selectivity of the polymer membranes modified with nanoclay is due to the nanometric level dispersion of the organic and inorganic phases. Due to the platelet-like morphology of silicates, the nanofilled matrix exhibits reduced diffusivity. The reinforcement of clay in the polymer matrix was supported by the high cross-link density values. Hence, higher cross-link density values are responsible for the improved selectivity of nanocomposite membranes. Effect of Penetrant Size. The effect of penetrant size on the diffusion of gas molecules through these membranes can also be understood from Figure 5a and b,respectively. As compared to nitrogen, oxygen has more permeability than nitrogen. For solution diffusion transport,all polymers are oxygen selective over nitrogen because oxygen diffuses faster and is more soluble than nitrogen. Conclusions EVA/clay nanocomposites containing different filler loading have been prepared, and the transport features of the membranes were investigated. The morphology of the composite membranes was analyzed by XRD and TEM. It has been found that the diffraction peaks were shifted to lower angles with an increase in d-spacing. Samples with 3 wt % filler showed maximum
Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008 4903
Figure 6. Cross-link density variation of unmodified and EVA-clay nanocomposites. Table 2. Oxygen to Nitrogen Selectivity Values of Unfilled and Filled EVA Membranes [P(O2)/P(N2)] sample F0 F3 F5 F7
permselectivity 3.5 5.02 4.5 3.8
increase in d-spacing. TEM images showed that sample with 3 wt % clay showed excellent dispersion of clay particles. The dispersion of nanoparticles decreased with an increase in the clay loading. The formation of aggregates in higher filler loadings was also evident from the dielectric loss measurements. The fractional free volume percentage was determined using positron annihilation lifetime spectroscopic analysis. Sample with 3 wt % clay showed the least free volume. The thermal stability of the membranes was found to be improved on the incorporation of clay filler. Nanocomposites exhibited lower permeability to oxygen and nitrogen gas due to the enhanced polymer/filler interaction. However, the permeability increased at higher filler concentrations. This was explained in terms of the aggregation of filler particles at higher concentrations. The cross-link density of the samples was computed by an equilibrium swelling method. The cross-link density values were found to be higher for layered silicate-reinforced polymer membranes supporting polymer/filler interaction. The size of the penetrant gas molecule affected the permeability coefficient. Finally, it is important to mention that by the incorporation of nanofiller into EVA, highly selective gas barrier membranes could be developed. Literature Cited (1) Koros, W. J.; Mahajan, R. Pushing the limits on possibilities for large scale gas separation: which strategies. J. Membr. Sci. 2000, 175, 181. (2) Koros, W. J.; Fleming, G. K. Membrane-based gas separation. J. Membr. Sci. 1993, 83, 80. (3) Cornelius, C. J.; Marand, E. Hybrid silica-polyimide composite membranes: gas transport properties. J. Membr. Sci. 2002, 202, 97. (4) Moon, E. J.; Yoo, J. E.; Choi, H. W.; Kim, C. K. Gas transport and thermodynamic properties of PMMA/PVME blends containing PS-b-PMMA as a compatibilizer. J. Membr. Sci. 2002, 204–283. (5) NaylorT. Permeation properties. In Handbook of ComprehensiVe Polymer Science, 1st ed.; Pergamon Press: Oxford, U.K., 1989; Vol. 2. (6) Compan, V.; Zanuy, D.; Andrio, A.; Morillo, M.; Aleman, C.; Guerra, S. M. Permeation Properties of the Stereoregular Nylon-3 Analogue, Poly(RL-aspartate). Macromolecules 2002, 35, 4521.
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ReceiVed for reView November 29, 2007 ReVised manuscript receiVed April 5, 2008 Accepted May 7, 2008
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IE071624H
