Organic−Inorganic Hybrid Membranes with Simultaneously Enhanced

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SEPARATIONS Organic-Inorganic Hybrid Membranes with Simultaneously Enhanced Flux and Selectivity Fubing Peng, Lianyu Lu, Honglei Sun, Fusheng Pan, and Zhongyi Jiang* Key Laboratory for Green Chemical Technology, Ministry of Education of China, School of Chemical Engineering and Technology, Tianjin UniVersity, Tianjin 300072, China

Tradeoff phenomenon is a sticky problem, which has been perplexing researchers in the polymeric membrane area for quite a long time. Since the inherent limitation of polymeric membrane and inorganic membrane, organic-inorganic hybrid membrane seems a feasible solution because it effectively combines the rigidity of inorganic moiety and the flexibility of polymer moiety. This study demonstrated such kind of hybrid organicinorganic membrane composed of poly(vinyl alcohol) (PVA) and crystalline flake graphite and found that simultaneous increase of permeability and selectivity of the hybrid membranes in pervaporation separation of benzene and cyclohexane mixtures was achieved. More specifically, the hybrid membranes showed about 4-fold permeation flux and 6-fold separation factor increase in comparison to pure PVA membrane. In addition, it was found that pervaporation properties of the hybrid membranes were quite sensitive to both graphite particle size and content. Introduction Organic-inorganic hybrid membranes in which inorganic filler with specific physicochemical characteristics is spatially dispersed into the bulk of polymeric membranes seems a facile and feasible solution to cross the tradeoff hurdle by combining the high selectivity with desirable mechanical properties of inorganic materials and tunable flexibility with excellent processability of polymeric materials.1,2 Many studies have demonstrated that the dramatically improved separation properties of organic-inorganic hybrid membranes could be realized by incorporating porous or nonporous inorganic absorbents, including zeolite, silica, carbon molecular sieve, and activated carbon into the matrixes of glassy polymers and rubbery polymers. There are also some studies on incorporating mineral flakes into polymeric membranes to selectively permeable membrane3,4 or impermeable barriers.5 Much effort has been devoted to obtain organic-inorganic hybrid membranes with both high permeability and high selectivity. The permeability of organicinorganic hybrid membranes depends on not only the intrinsic properties of the inorganic material and polymer and the interaction between the two phases but also the inorganic content and particle size. Maxwell,6 te Hennepe et al.,7,8 and Koros and co-workers9 have subsequently proposed and commonly employed to theoretically describe the transport behavior of hybrid organic-inorganic membranes, where inorganic particle content is the sole key variable. Recently, based on the experimental and theoretical analysis in membrane-based gas separation process, Moore and Koros proposed that the organic-inorganic interface morphologies would strongly affect the permeability and selectivity of organic-inorganic hybrid membranes.10 They pointed out that the dispersed phase might cause an undesirable void at the interface or create varying degrees of rigidification * To whom correspondence should be addressed. Tel.: +86-2227892143. Fax: +86-22-27892143. E-mail: [email protected].

in the surrounding polymer. Until now, no similar report specifically dealing with the pervaporation process can be found. Therefore, in this study we focus on a detailed investigation and elucidation about the organic-inorganic hybrid pervaporation membranes with simultaneously enhanced permeability and selectivity. Previous studies showed that permeability of hybrid membranes was often sensitive to inorganic particle size.11 However, the changing tendency was quite diversified. For example, the permeability of PDMS-zeolite hybrid membranes increased as zeolite particle size increased,12 but the permeability of PTMSP-silica nanocomposite membranes decreased linearly with the increase of primary particle size. These results may deserve further experimental and theoretical exploration.13 The objective of this study is to find a strategy achieving simultaneous enhancement of permeability and selectivity, and thus establishing a set of methodologies for hybrid pervaporation membrane preparation and process development. Poly(vinyl alcohol) (PVA)-graphite hybrid membranes were prepared and characterized by PALS, FTIR, SEM, and XRD techniques. The effects of graphite particle content and size on pervaporation properties of the hybrid membranes for separation of benzene/ cyclohexane mixtures were systematically investigated. The reversal tradeoff effect was actually found and tentatively elucidated. Experimental Section Selection of Membrane Materials. In this study, poly(vinyl alcohol) (PVA) was chosen as the bulk polymer and crystalline flake graphite was chosen as the inorganic component. PVA has been proven to be an appropriate material for pervaporative separation of benzene/cyclohexane mixtures because benzene, with π-electron and 1 order of magnitude of solubility in water larger than that of cyclohexane, has stronger interaction with PVA than cyclohexane. But due to the tight polymer chain

10.1021/ie061622h CCC: $37.00 © 2007 American Chemical Society Published on Web 03/21/2007

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packing, PVA membrane often exhibits low permeability toward organics. To solve this problem, crystalline flake graphite can be incorporated based on the following considerations. (1) Graphite is the most commonly used friction-reducing and diffusion-enhancing additive for polymers owing to its weak interaction between adjacent graphite layers.14 (2) Graphite with conjugated π-bond structure may be one of the preferential candidate materials to show affinity toward benzene.15,16 (3) Compared with zeolite and carbon molecular sieve, graphite has much higher flexibility, and the considerable interaction between PVA polymer chains and graphite sheets may lead to defect-free voids at the PVA-graphite interface, loosing the chain packing of PVA and increasing the free volume of membrane accordingly. Membrane Preparation. PVA-graphite hybrid membranes were prepared through solution casting of PVA and graphite blends. PVA (degree of polymerization was 1750 ( 50, Tianjin Yuanli Chemical Co. Ltd., China) was dissolved in distilled water at 363 K and the resulting 10 wt % PVA homogeneous solution was filtered. To the filtrate, a certain amount crystalline flake graphite (particle size was 1, 2, 5, 10, and 15 µm, Qingdao Graphite Co. Ltd., China), 2 mL of cross-linker glutaradehyde (25 wt % aqueous solution), and 1 mL of concentrated HCl catalyst were added to initiate the cross-linking reaction. The solution was gently stirred for about 4 h at room temperature and the resulting homogeneous solution was cast onto a glass plate with the casting knife. The wet membranes were air-dried for 24 h at room temperature and then annealed at 393 K for 1-3 h. Pure PVA membrane was prepared in the same procedure as PVA-graphite hybrid membranes, but without crystalline flake graphite. The final membrane thickness was 80 ( 2 µm. FTIR, SEM, and XRD were employed to probe the composition and structural characteristics of graphite and PVA-graphite hybrid membranes. Details about Free Volume Characterization. The positron annihilation lifetime spectroscopy (PALS) was carried out at an EG&G ORTEC fast-fast system with a resolution of 264 ps (under 22Na window setting). The positron source (22Na) material was sandwiched between two pieces of PVA-graphite hybrid membranes (sample-source-sample). A million counts were collected for each spectrum. In these membranes, opositronium (o-Ps) could pick off an electron of opposite spin from the surrounding medium (the so-called pick-off process), and the lifetime was shortened to 1-5 ns depending on the average electron density surrounding the o-Ps. The pick-off annihilation lifetime of o-Ps could be directly correlated to free volume cavity size by a semiempirical equation proposed by Nakanishi and Jean17 according to the quantum-mechanical model of Tao18 later developed by Eldrup er al.19 It has been shown that o-Ps is formed and annihilates in regions of low electron density. In a polymer, the low electron density regions are the free volume sites of the polymer, and the lifetime of o-Ps is therefore related to free volume cavity size. Assuming that the positronium is localized in a spherical potential well surrounded by an electron layer of thickness ∆r equal to 0.1656 nm, and the following expression was used to correlate o-Ps pick-off lifetime, τ3 and τ4, and average radius of free volume cavities, r.

τ)

1 2πr 1 r + 1sin 2 r + ∆r 2π r + ∆r

[

( ) (

)]

-1

(1)

The volume of the equivalent sphere can be calculated by eq 2.

VF )

4π 3 r 3

(2)

Further, the apparent fractional free volume fapp may be estimated from eq 3.

fapp ) VF3 I3 + VF4 I4

(3)

where VF and I are free volume of the sphere and intensity of o-Ps, respectively. Pervaporation Experiment. The pervaporation properties of the hybrid membranes to benzene/cyclohexane mixtures were investigated on a P-28 membrane module (CM-Celfa AG Company, Switzerland). The effective surface area of the membrane in contact with feed was 28.0 cm2. The vacuum in the downstream side was maintained at 1 kPa; feed concentration was 50 wt % benzene; feed temperature was 323 K; feed flowing rate was 60 L/h. The permeate was collected in liquid nitrogen cold traps. The compositions of the feed and permeate were measured using Agilent 6820 gas chromatography equipped with a FID detector and a PEG20M column. The pervaporation properties of organic-inorganic hybrid membranes are evaluated by two parameters, flux J, which is defined as J ) W/At, and separation factor R, which is defined as R ) (yB/yC)/(xB/xC), where W is the mass of permeate collected in time t, A is the effective membrane area, and x and y represent the weight fractions of benzene (B) and cyclohexane (C) in the feed and permeate, respectively. The membranes were tested three times to obtain the averaged permeation flux and separation factor. Results and Discussion Membrane Characterization. The FTIR results of PVA and PVA-graphite hybrid membranes are shown in Figure 1a. There were plenty of hydroxyl groups and carboxyl groups on the graphite surface, which was generated during the forming process of natural graphite, and it was confirmed by the FTIR spectrum shown in Figure 1b. PVA membrane showed a broad band at 3100-3500 cm-1 that was attributed to -OH stretching vibrations. Compared with the spectrum of PVA membrane, the bands of PVA-graphite hybrid membranes at 3100-3500 cm-1 obviously broadened and shifted to lower wavenumber; such results may be caused by the hydrogen bonding of the hydroxyl group of PVA and the hydroxyl and carboxyl groups on the graphite surface. Through this weak nonspecific interaction, the defect-free voids could be possibly constructed at the PVA-graphite interface. Figures 2a and 2b show the cross section of PVA-graphite hybrid membrane. The particle size of graphite flake and the graphite content in the membrane were 2 µm and 6 wt %, respectively. As shown in Figures 2a and 2b, the graphite flakes were homogeneously dispersed throughout the hybrid membranes, and quite interestingly, the graphite flakes were all in the direction parallel to the membrane surface due to the gravity effect, which is quite favorable for disturbing the alignment of polymer chains, and hence for increasing the free volume of the membranes. The XRD patterns of graphite, pure PVA membrane, and PVA-graphite hybrid membranes are shown in Figure 3. Due to the semicrystalline character of PVA, the pure PVA membranes exhibited a typical peak at 2θ ) 19.6° and some other sharp diffraction peaks. In comparison, the peak intensity or crystalline degree of PVA-graphite hybrid membrane was less than that of pure PVA membrane. Moreover, those sharp diffraction peaks all disappeared except for the characteristic

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Figure 1. FTIR spectrum of (a) PVA and PVA-graphite hybrid membrane and (b) crystalline flake graphite for confirming the groups on the graphite surface.

peak of graphite at 26.5°, which may have resulted from the smaller crystal particles of PVA that were destroyed by the incorporation of graphite. The lifetimes and intensities of the different positron annihilation processes in PVA and PVA-graphite hybrid membranes are shown in Table 1and Table 2. The shortest observed time, τ1 ∼ 0.13-0.15 ns, is close to the intrinsic p-Ps lifetime; the second shortest lifetime, τ2 ∼ 0.40-0.50 ns, is usually associated with the annihilation of positrons with valence or core electrons of the constituent atoms and the long lifetimes τ3 and τ4 are due to o-Ps pick-off annihilation. In this study, only the long lifetime τ3 and τ4 are taken into account. Based on the mechanism developed by Matsuura and Sourirajan for reverse osmosis process,20 the polymer-based pervaporation membranes consisted of two types of pores, network pore (r3) and aggregate pore (r4), respectively. The network pores are the small cavities between polymer chains constituting the polymer chain aggregates, whereas the aggregate pores are the large cavities surrounding the polymer chain aggregates and organic-inorganic interfacial voids. Effect of Graphite Content on Pervaporation Properties. Figure 4 illustrates permeation flux of benzene through PVAgraphite hybrid membranes as a function of graphite content (graphite particle size was about 2 µm). Within the range of graphite content from 0 to 10 wt %, permeation flux of benzene in PVA-graphite hybrid membranes is significantly larger than that in pure PVA membrane. When the graphite content was 6 wt %, PVA-graphite hybrid membranes exhibited the highest permeation flux of benzene, approximately 4-fold increase

Figure 2. SEM graphs of full scale (a) and partial scale (b) of cross section of PVA-graphite hybrid membrane and crystalline flake graphite (c).

compared with that in pure PVA membrane. This phenomenon is tentatively explained as follows: when the graphite content was less than 6 wt %, the organic-inorganic interface number in hybrid membrane was relatively small, and the PVA polymer chain packing was only partly interfered. Therefore, as shown in Table 1, the fractional free volume and the number of network pores and aggregate pores of PVA-graphite hybrid membranes increased with graphite content increasing, which resulted in the higher benzene permeation flux with graphite content increasing. When graphite content was greater than 6 wt %, the organic-inorganic interface number increased, and the disturbing effect of graphite flakes on PVA polymer chain packing became more pronounced. However, the existence of excess graphite flakes would result in the decrease of network pore and aggregate pore number and thus the decrease of fractional free volume in PVA-graphite hybrid membranes, and subsequently the lower benzene permeation flux in the hybrid membranes. In addition, the permeation flux of cyclohexane in

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Figure 3. X-ray diffraction patterns of pure PVA membrane, PVA-graphite hybrid membrane, and crystalline flake graphite (CG). Table 1. Free Volume Parameters of Pure PVA and PVA-Graphite Hybrid Membranes with Different Graphite Contentsa system

τ3 (ns)

τ4 (ns)

I3 (%)

I4 (%)

r3 (nm)

r4 (nm)

fapp

PVA-graphite(4%) PVA-graphite(6%) PVA-graphite(8%)

1.80 1.80 1.83

5.56 4.45 4.56

6.38 7.55 5.68

0.68 0.60 0.56

0.27 0.27 0.27

0.45 0.45 0.45

0.79 0.85 0.68

a

Figure 5. Comparison of the permeation flux and selectivity of PVAgraphite hybrid membranes with the upper bound curve proposed by Lue and Peng.

Graphite particle size: 2 µm.

Table 2. Free Volume Parameters of Pure PVA and PVA-Graphite Hybrid Membranes with Different Graphite Particle Sizesa system

τ3 (ns)

τ4 (ns)

I3 (%)

I4 (%)

r3 (nm)

r4 (nm)

PVA PVA-graphite (1 µm) PVA-graphite (2 µm) PVA-graphite (5 µm) PVA-graphite (10 µm) PVA-graphite (15 µm)

1.35 1.89 1.80 1.75 1.74 1.66

2.30 4.48 4.45 4.15 4.10 3.29

4.10 7.83 7.55 6.93 6.69 6.42

2.86 0.62 0.60 0.64 0.61 0.81

0.22 0.27 0.27 0.26 0.26 0.25

0.31 0.45 0.45 0.43 0.43 0.38

a

Graphite content: 6 wt %.

Figure 4. Effect of graphite content on the permeation flux for PVAgraphite hybrid membranes.

the hybrid membranes slightly decreased with the increase of graphite content. The effect of network pore and aggregate pore number on diffusion of larger cyclohexane molecules was not obvious due to the stronger affinity interaction of graphite toward benzene and the stronger competitive adsorption ability of benzene in PVA-graphite hybrid membranes than that of cyclohexane.21 As we all know, the intrinsic tradeoff between permeability and selectivity exists in common polymeric membranes. As a

Figure 6. Effect of graphite particle size on the permeation flux and separation factor of PVA-graphite hybrid membranes.

pioneering work, Robeson1 plotted the “upper bound tradeoff curve” for O2/N2, CO2/CH4, etc. gas separation polymeric membranes, which spurred the intense research and development of polymeric membrane with high permeability and high selectivity. Lue and Peng2 had summarized the pervaporation results of benzene/cyclohexane separation using various membrane materials and deduced the relationship between benzene/ cyclohexane selectivity and benzene permeation flux, and a distinct upper bound curve was presented. As shown in Figure 5, the pervaporation outcome of pure PVA membrane is all below this upper bound curve. In sharp contrast, the pervaporation outcome of PVA-graphite hybrid membranes are all above the upper bound curve. Effect of Graphite Particle Size on Pervaporation Properties. Figure 6 presents permeation flux and separation factor of PVA-graphite hybrid membranes with 6 wt % graphite as a function of graphite particle size. With graphite particle size increasing, the permeation flux values of PVA-graphite hybrid membranes decreased linearly but were still higher than that of pure PVA membrane. It should be mentioned that when the particle size is less than 1 µm, serious aggregation of graphite particles will occur during the membrane preparation, and it is almost impossible to acquire a sheet of intact membrane (that is, graphite particles cannot be homogeneously distributed in the membrane). When the graphite content in the hybrid membranes was fixed, the smaller particle sizes certainly meant the larger number of graphite particle. Accordingly, the probable number of organic-inorganic interfaces increased. Therefore,

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Figure 7. Relationship between the permeation flux and the apparent fractional free volume of PVA-graphite hybrid membranes with different graphite particle size.

the interaction between graphite particle and PVA polymer chain was intensified, which would disrupt polymer chain packing more effectively and thus acquire more accessible free volume. As shown in Table 2, the free volume in PVA-graphite hybrid membranes with smaller graphite particle size is larger than that in PVA-graphite hybrid membranes with larger graphite particle size, and even the negligible small change of the accessible free volume in the hybrid membrane would lead to significant increase of permeation flux. Meanwhile, as shown in Figure 7, the changing tendencies of apparent fractional free volume were consistent with the permeation flux of corresponding PVA-graphite hybrid membranes with different graphite particle size. The increased free volume was ascribed to the size and number of network pore and aggregate pore. A small portion of aggregate pore was contributed by the organicinorganic interfacial void, where most of the mass transfer resistances would occur. The free volume and aggregate pore size both increased with graphite particle size decreasing, which might lead to decreased mass transfer resistance at the organicinorganic interfaces. Hence, the increased permeation flux was attributed not only to network pore with radius from 0.25 to 0.27 nm but also to aggregate pore size, which was composed of the large cavities surrounding the polymer chain aggregates and organic-inorganic interfacial voids. Elucidation for Simultaneously Enhanced Permeability and Selectivity. Based on the sorption-diffusion model, the permeability P may be conveniently expressed as

P)S×D

(4)

where S is solubility coefficient and D is concentration-average effective diffusion coefficient. The separation factor toward benzene, RB/C, is defined as the product of the ratios of their solubility selectivity SB/SC and diffusivity selectivity DB/DC.

RB/C )

SB DB × SC DC

(5)

According to our sorption experimental data,21 both solubility selectivity and diffusivity selectivity of PVA-graphite hybrid membrane increased compared with those of pure PVA membrane. The penetrant diffusion coefficients could be calculated by following22

D ) A exp(-γν/VFV)

(6)

where A and γ are positive constants, ν is the minimum volume required for penetrant to execute a diffusion step and hence is a measure of penetrant size, and VFV is polymer free volume. According to eqs 5 and 6, diffusivity selectivity DB/DC is proportional to exp(γ(VC - VB)/VFV). Therefore, an increase in polymer free volume was expected to be accompanied by the increase in diffusion coefficients and the decrease in diffusivity selectivity since the collision radius of benzene and cyclohexane molecules were 0.263 and 0.303 nm, respectively. This was just opposite to our results that diffusivity selectivity DB/DC increased for PVA-graphite hybrid membrane compared with pure PVA membrane. We thus turned to seeking the plausible explanation from the size of the network pore and aggregate pore. In PVA-graphite hybrid membranes, the radius of network pore r3 ranged from 0.25 to 0.27 nm and the radius of aggregate pore r4 ranged from 0.38 to 0.45 nm [see Table 2]. But in pure PVA membrane, the radius of network pore r3 and aggregate pore r4 were 0.22 and 0.31 nm, respectively. The network pore size of PVA-graphite hybrid membranes lies just between the collision radius of benzene and cyclohexane, which allows benzene molecules to permeate freely while rejecting cyclohexane molecules mostly. The greater the concentration of the small free volume cavity, the easier for benzene molecules to permeate; correspondingly, higher diffusion selectivity will be acquired. On the other hand, the aggregate pore size is larger than the size of both benzene and cyclohexane molecules, which allows both benzene and cyclohexane molecules to permeate. In addition, smaller collision radius of benzene and higher affinity of graphite toward benzene are both favorable for benzene molecules taking up more free volume cavity, which effectively prevent cyclohexane molecules from diffusing through. The free volume cavity size r3 and r4 as well as concentration I3 (except for I4) of all PVA-graphite hybrid membranes with 6 wt % graphite content increase as particle size decreases. Based on the above discussions, the diffusivity selectivity of PVA-graphite hybrid membranes increases with graphite size decreasing from 15 to 1 µm, but the solubility selectivity remains nearly constant. Overall, the separation factor of PVA-graphite hybrid membranes increases with the particle size decreasing. Conclusions In this study, hybrid organic-inorganic membrane which incorporated flexible impermeable inorganic particles into rigid polymer was introduced to achieve simultaneous high permeability and selectivity. In particular, a PVA-graphite hybrid membrane containing 6 wt % graphite particles (particle size: 1 µm) exhibited 91.3 g/(m2 h) of permeation flux and 91.6 of separation factor to benzene/cyclohexane mixtures, while the pure PVA membranes only exhibited 23.1 g/(m2 h) of permeation flux and 16.9 of separation factor. These unusual pervaporation properties were reasonably explained by the free volume characteristics of PVA-graphite hybrid membranes with different graphite particle size and loading content. Acknowledgment The authors are grateful for the financial support from the Cross-Century Talent Raising Program of Ministry of Education of China and the Program for Changjiang Scholars and Innovative Research Team in University from the Ministry of Education of China, and the SINOPEC Research Program (NO.

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X503029). We also appreciated the help from Prof. Dashu Yu and Mr. Liqun Wang with the PALS experiment. Literature Cited (1) Robeson, L. M. Correlation of separation factor versus permeability for polymeric membranes. J. Membr. Sci. 1991, 62, 165. (2) Lue, S. J.; Peng, S. H. Polyurethane (PU) membrane preparation with and without hydroxypropyl-β-cyclodextrin and their pervaporation characteristics. J. Membr. Sci. 2003, 222, 203. (3) Wang, Y.-C.; Fan, S.-C.; Lee, K.-R.; Li, C.-L.; Huang, S.-H.; Tsai, H.-A.; Lai, J.-Y. Polyamide/SDS-clay hybrid nanocomposite membrane application to water-ethanol mixture pervaporation separation. J. Membr. Sci. 2004, 239, 219. (4) Adoor, S. G.; Sairam, M.; Manjeshwar, L. S.; Raju, K. V. S. N.; Aminabhavi, T. M. Sodium montmorillonite clay loaded novel mixed matrix membranes of poly(vinyl alcohol) for pervaporation dehydration of aqueous mixtures of isopropanol and 1,4-dioxane. J. Membr. Sci. 2006, 285, 182. (5) DeRocher, J. P.; Gettelfinger, B. T.; Wang, J.; Nuxoll, E. E.; Cussler, E. L. Barrier membranes with different sizes of aligned flakes. J. Membr. Sci. 2005, 254, 21. (6) Maxwell, C. Treatise on electricity and magnetism; Oxford University Press: London, 1873; Vol. 1. (7) te Hennepe, H. J. C.; Smolders, C. A.; Bargeman, D.; Mulder, M. H. V. Exclusion and totuosity effects for alcohol/water separation by zeditefilled PDMS membranes. Sep. Sci. Technol. 1991, 26, 585. (8) te Hennepe, H. J. C.; Boswerger, W. B. F.; Bargeman, D.; Mulder, M. V. H.; Smolders, C. A. Zeolite filled silicone rubber membranes: Experimental determination of concentration profiles. J. Membr. Sci. 1994, 89, 185. (9) Zimmerman, C. M.; Singh, A.; Koros, W. J. Tailoring mixed matrix composite membranes for gas separation. J. Membr. Sci. 1997, 137, 145. (10) Moore, T. T.; Koros, W. J. Non-ideal effects in organic-inorganic materials for gas separation membranes. J. Mol. Struct. 2005, 739, 87. (11) Andrady, A. L.; Merkel, T. C.; Toy, L. G. Effect of particle size on gas permeability of filled glassy polymers. Macromolecules 2004, 37, 4329.

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ReceiVed for reView December 17, 2006 ReVised manuscript receiVed February 10, 2007 Accepted February 14, 2007 IE061622H