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J. Phys. Chem. C 2007, 111, 652-657
A General Strategy for Adhesion Enhancement in Polymeric Composites by Formation of Nanostructured Particle Surfaces Shu Shu, Shabbir Husain, and William J. Koros* School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst DriVe Northwest, Atlanta, Georgia 30332 ReceiVed: September 2, 2006; In Final Form: October 25, 2006
In this work we present a general methodology to enhance the interfacial adhesion in polymeric composites by creating nanoscale morphologies on the surfaces of particles. The surface morphology, which appears as inorganic whiskers, is achieved by treating silicate particles (zeolites 4A) with thionyl chloride, followed by subsequent reaction with methylmagnesium bromide (Grignard reagent). Poly(vinyl acetate) and Ultem composites containing this type of modified particles exhibit defect-free interfaces. Dynamic mechanical analysis testing reveals that such composites have higher moduli as compared to those embedded with nontreated fillers with the same loadings. Furthermore, gas permeation measurements demonstrate that these materials also show impressive enhancements in gas separation efficiency. The dramatic increase in the topological roughness (physical heterogeneity) on the sieve surfaces is proposed to provide improved interaction at the interface via thermodynamically induced adsorption and physical interlocking of polymer chains in the nanoscopic inorganic whisker structure. The presented strategy need not be tailored to a specific polymer/ filler pair and therefore has potential to be extended to many other polymeric composites for a variety of applications, where tailoring polymer/solid interface compatibility is important.
Introduction The incorporation of inorganic silicate materials into a polymeric matrix has been explored extensively as an efficient way for improving various performances such as thermal stability,1 optical properties,2,3 electrical properties,4,5 catalytic reactivity,6,7 and transport properties8-10 of materials. Nevertheless, due to limited intrinsic compatibility between inorganic and many organic polymers, lack of adhesion at the interfaces poses challenges for applications of such hybrid materials. Better control and tailoring of the nanoscale interfacial regions to optimize the desired properties would enable successful fabrication of composite materials. Prior work to control interfacial compatibility can be divided into three categories: (1) modifying the filler surface chemistry via the use of coupling agents such as silane coupling agents to covalently bond these two phases,11-14 (2) surface-initiated polymerization with preformed particles,15-19 (3) in situ synthesis of particles within prefabricated polymer matrixes.20-22 Use of coupling agents is usually limited to a specific polymer/ filler pair because the selection of coupling agents depends on the detailed chemical structure of the polymeric matrix. The latter two methods have offered possibilities to improve interfacial contact; however, a generally applicable approach involving the blending of premade particles into a presynthesized polymer is most attractive. This potential approach provides full synthetic control over both the filler and the matrix and has the ability to generate a wider variety of composite materials.23 While chemical bonding via silanes and other reactive units can be applied in many cases, a more general method to engineer the particle surface without being limited to a specific material pair is highly desired. * To whom correspondence should be addressed. E-mail: wjk@chbe. gatech.edu. Phone: (404) 385-2845. Fax: (404) 385-2683.
In this paper, we report, for the first time, such a general approach to promote the interfacial adhesion in polymer/silicate composites by creating a nanostructured morphology, which appears as inorganic whiskers, on the outer surfaces of the filler particles. For the case of zeolite 4A, this inorganic nanoscale “whisker” structure can be obtained by treating the silicate material with thionyl chloride followed by subsequent reaction with methylmagnesium bromide (Grignard reagent). In the present study, composite membranes for gas separation were primarily used as the model system. Since transport is very sensitive to subnanometer defects due to the angstrom-sized gas molecules, it is ideally suited to probe the interfacial integrity between the inorganic and polymer phases. The unmodified and modified 4A sieves were incorporated into either poly(vinyl acetate) (PVAc) or Ultem to generate composite membranes. Scanning electron microscopy (SEM) is used to examine the surface morphologies of the particles, as well as the interfacial morphology in the resultant composite films. Dynamic mechanical analysis (DMA) is utilized to test the elastic moduli of these films. Pure gas permeation measurements were conducted to examine the gas transport properties of the films. Numerous variations on the specific case illustrated here can be envisioned, and a few additional examples will also be suggested. Experimental Section Modification of Particles. Zeolite 4A, which is an aluminosilicate (Advanced Specialty Gas Equipment) with a characteristic 5 µm cubic form, was selected as the model dispersed phase. Samples of 5-7 g of 4A particles were placed in a threeneck reaction flask. All the particles and the glassware used in the treatment were first dried in a vacuum oven at 150 °C for 24 h, followed by flame-drying of the glassware with a propane torch prior to the treatment. An 80 mL portion of anhydrous toluene (99.8%, Aldrich) and 20 mL of thionyl chloride (99.5%,
10.1021/jp065711j CCC: $37.00 © 2007 American Chemical Society Published on Web 12/09/2006
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Figure 1. Representative SEM micrographs: (a) unmodified 4A surface, (b) modified 4A surface with the inorganic whisker morphology, (c) Ultem composite film containing unmodified 4A particles, (d) Ultem composite film containing modified 4A particles, (e) high-magnification image of the surface whisker structure as shown in (b). In all the composite films, the zeolite particles are evenly distributed throughout the polymer matrixes without apparent aggregation.
low ion solution, Aldrich) were added to the reaction flask which contained the 4A powders. Both liquids were transferred carefully through a dry transfer line to maintain a moisturefree environment. This dispersion was kept in a sonication bath (model 1510, Branson) for 12 h, after which toluene and extra thionyl chloride were evaporated by a constant nitrogen sweep through the system. Then 80 mL of anhydrous toluene and 15 mL of methylmagnesium bromide (3.0 M solution in diethyl ether, Aldrich) were added to the reaction flask in the same manner as described above. The dispersion was again kept in a sonication bath for 12 h. After the reaction finished, extra methylmagnesium bromide was quenched by anhydrous 2-propanol (99.5%, Aldrich), and the particles were collected and rinsed with 2-propanol followed by deionized water several times. Formation of Polymeric Composites Films. In this study PVAc (Aldrich, Mw ) 500000) and a poly(ether imide), Ultem 1000 (GE Plastics, Mw ) 56000), were used as the polymer matrixes. The polymers were dissolved by an appropriate solvent (toluene or dichloromethane, Aldrich) to form a viscous solution (∼20 wt %). The proper amounts of 4A particles (to form 15, 30, and 40 wt % sieve loading films) were first dried in a vacuum oven at 250 °C and then dispersed in the corresponding solvent via a sonicator (model VC 50, Sonics & Materials Inc.) for 1-2 min. The two solutions were then mixed together to form a final homogeneous suspension. The composite film was prepared by drawing this solution with a doctor’s knife on a smooth glass substrate, followed by annealing in a vacuum oven above the glass transition temperatures (Tg) of the polymers. For PVAc-based films, the annealing temperature was 100 °C, while Ultem-based films were annealed at 230 °C. The thickness of the resultant films was measured using a dial micrometer (AMES, model 56212). For each film, 10 measurements were performed and an average thickness was obtained. All the films used in this work had a thickness of around 62.5 µm (the standard deviation was less than 2.5 µm). The particles were uniformly distributed in the films shown by SEM (Figure 1).
Characterization. SEM images were obtained with an SEM JEOL 1503, equipped with a thermally assisted field emission gun operating at 10 keV. DMA tests were performed on a dynamic mechanical analyzer (model 2980, Texas Instrument) at room temperature. Three films were examined for each type of pure or composite films. Additionally, statistical ANOVA analysis was performed to evaluate the significance of the variance among the data. The thickness for all the tested films is around 62.5 µm. Pure gas permeation measurements were conducted in a permeation system following a procedure described in detail elsewhere.24 The testing temperature was 35 °C, and the upstream pressure was 4.5 atm. Results and Discussion Morphology Characterization of Zeolites and Composite Films. Figure 1a shows the surface morphology of the asreceived 4A particles, while Figure 1b shows the modified counterparts. These SEM images clearly show that the surfaces of the sieves are changed significantly after the treatment. A new morphology that resembles inorganic whiskers or asperities was formed on the outer surface of the particle, the length of which is estimated to be around 50 nm according to the SEM images (Figure 1b,e). The width of these asperities appeared to be 1-2 orders of magnitude smaller than the length (Figure 1e); thus, high aspect ratio structures exist on the surfaces. The significance of this scaling will be discussed later. We anticipate that the detailed dimensions of these features are adjustable via control of the treatment conditions, and this issue is currently under investigation. On the basis of the surface characterizations of the modified particles, such as X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), magnesium hydroxide was detected as the component of this inorganic whisker structure. The characterization results can be found in the Supporting Information. When this type of modified particles were incorporated into a commercially available polymeric matrix (Ultem) to form
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Figure 2. Structures of Ultem and PVAc.
composite films, the resultant films exhibited better interaction at the interface. The film composed of Ultem and unmodified 4A had apparent voids between the polymer and particles (Figure 1c), indicating poor adhesion between these two phases. These undesirable voids were eliminated in the films embedded with the modified particles having the whisker surface morphology (Figure 1d), implying enhanced interaction at the interfacial regions. This improvement suggests that this nanoscale whisker morphology stabilizes the polymer chains at the interface and promotes compatibility between the polymer and filler particles. The structures of Ultem and another polymer used in this work PVAc are provided in Figure 2. Since the resolution of SEM is not adequate to observe angstrom-sized defects, more characterization is needed to demonstrate that the interface is intact at such small dimensions. DMA and gas permeation tests will yield more information with regard to this issue and will be discussed in detail in the subsequent sections. Mechanical Properties of the Composite Films. Besides Ultem, PVAc composites were also explored to demonstrate the versatility of this method with different kinds of polymers. Impressive changes are observed in the mechanical strength of both PVAc and Ultem composites containing the modified 4A fillers. DMA tests for the PVAc-based composite films containing either unmodified or modified 4A particles (both at 15 wt % sieve loading) revealed an increased elastic modulus over pure PVAc, as displayed in Figure 3a. This trend suggests that the incorporation of 4A fillers into PVAc formed good mixed matrix materials with improvement in the mechanical property. The fact that the PVAc/fresh 4A film has a higher modulus than the neat polymer film is expected since PVAc is a very flexible polymer with a glass transition temperature (Tg) of 35 °C, and it is known to have affinity for silicate surfaces.25 Thermodynamically, it is likely that the acetate groups on PVAc may interact favorably with the natural silanols on the unmodified 4A surfaces.26 Thus, it is very easy for PVAc chains to adhere well to the sieve surface and form a desirable interface without any modification. Even accounting for this fact, it is clear that the moduli of the PVAc/modified 4A film are even higher than those of PVAc/fresh 4A films, suggesting better adhesion provided by the modified sieves with nanostructured surfaces. In Ultem-based films the situation is completely different as illustrated in Figure 3b. The Ultem composite films loaded with unmodified and modified 4A fillers (both at 15 wt % sieve loading) exhibited markedly different properties. The Ultem/ unmodified 4A films had an elastic modulus even less than that of the neat polymer, reflecting the apparent voids between Ultem and the sieves (Figure 2) due to poor interfacial adhesion between the two phases. On the other hand, Ultem/modified 4A films showed a higher elastic modulus than the neat polymer, obviously pointing to the presence of strong interaction between Ultem and the sieves with a whisker structure on the outer surface. This difference in the adhesion behavior of Ultem for the two types of particles is believed to reflect the intrinsic nature of the rigid polyimide backbone of Ultem, which has a Tg of 205 °C. Low chain flexibility impedes the adsorption ability onto a particle surface to form a defect-free interface. Moreover,
Figure 3. DMA test results of two sets of films: (a) neat PVAc, PVAc containing pristine 4A (15 wt % sieve loading), and PVAc containing modified 4A particles (15 wt % sieve loading), (b) neat Ultem, Ultem containing pristine 4A (15 wt % sieve loading), and Ultem containing modified 4A particles (15 wt % sieve loading). Statistical ANOVA analysis was performed to evaluate the significance of the variance. For PVAc-based films, the three groups showed an F of 240.6 and a p value of 0.0043, and for Ultem-based films, the three groups showed an F of 62.14 and a p value of 0.0159. Since both the p values were less than 0.05, the null hypothesis could be rejected.27
the lower affinity of the Ultem backbone for the silanol-rich native 4A surfaces predicts poor adhesion. Thus, the voidcontaining interfacial morphology of Ultem and unmodified 4A (Figure 1c) is believed to be attributed to the intrinsically less favorable interaction between the comparatively smooth particle surface and rigid polymer chain. The nanoscale inorganic whiskers on the modified particle surface greatly enlarge the contact area and serve as interlock points to stabilize the polymer chains, so the void morphology is eliminated even in the absence of highly favorable energetic interaction. This issue will be explored in more detail below. In any case, the DMA test results are consistent with the SEM observations in Figure 1 and further confirm that the interfacial adhesion is promoted by the modified fillers whose surfaces consist of inorganic whiskers. Gas Separation Performance of Composite Films. Gas transport measurements are used in this study as another probe for the interfacial morphologies in composite membranes. Since transport is very sensitive to subnanometer defects due to the angstrom-sized gas molecules, it is ideally suited to probe the interfacial integrity between the inorganic and polymer phases. In addition to the improvement of mechanical properties, remarkable changes have been observed in gas transport properties of composite films using the modified particles with nanostructured surface morphology. Zeolite 4A is a well-known
Adhesion Enhancement in Polymeric Composites molecular sieve with a pore diameter of 3.8 Å and a 3-dimensional pore network of interconnected cages that is ideally suited to the separation of O2/N2, and it has been estimated to have a very attractive oxygen permeability of 0.75 barrer and an ideal O2/N2 selectivity of roughly 40, which are much higher than the performance properties for polymers. Similarly attractive performance is expected for the CO2/CH4 pair on the basis of their relative sizes (3.3 Å minimum dimension for CO2 and 3.8 Å for CH4). Unfortunately, pure defect-free zeolite membranes are extremely difficult and expensive to fabricate into largearea applications. Ultem is a polymer widely used in industry for gas separation due to its good mechanical properties and desirable separation efficiency. By combination of these two materials, a new composite is generated which can potentially provide improved gas separation efficiency along with desirable productivity. PVAc has been shown to form a favorable interface with 4A particles due to its intrinsic flexibility,25 but this polymer is not suitable for industrial gas separation because it is hard to process into hollow fibers and it can be easily plasticized by CO2. Ultem has great advantages over PVAc in its processability and resistance to plasticization; however, it presents challenges due to difficulty in forming defect-free interfaces.28,29 Our work reported here, therefore, focuses on this polymer. Pure gas permeation experiments were performed to measure the gas transport properties in Ultem-based films, and the results are shown in Figure 4. Figure 4a demonstrates the separation performance for oxygen/nitrogen. The bold line is called the “Robeson upper bond tradeoff curve”, which sets the upper limit for the separation performance of the available solutionprocessable polymeric materials.30 The goal of creating the composite films is to push the performance beyond this curve. The transport property of neat Ultem lies below this curve. The dots represent the theoretical transport properties of Ultem/4A films predicted by the Maxwell model. This model has been widely used to predict the electrical conductivity31 and gas transport properties32 for composite systems. The squares and triangles are the experimental data obtained for the composite films containing modified 4A particles and pristine 4A, respectively. The numbers designate the percentage of filler loadings in the polymer matrix by mass. It is apparent that the Ultem/fresh 4A films demonstrate no improvement in separation efficiency with the O2/N2 selectivity equal to that of the neat polymer. This could be easily explained by the fact that the voids existing between the polymer and fillers (Figure 1c) provide a free bypass for the gas molecules. They allow both oxygen and nitrogen molecules to transport directly through the voids without entering the selective micropores in 4A zeolites. In other words, 4A zeolites did not participate in selecting O2 or N2; therefore, no selectivity enhancement was observed. On the other hand, significant selectivity enhancement is achieved by incorporating the modified 4A particles with the special surface morphology into the Ultem matrix. As indicated on the plot, at sieve loadings of 15 and 30 wt %, the selectivity values obtained by experiment are almost equal to the Maxwell model predictions and significantly exceed those of the neat polymer. This improvement clearly further supports the absence of defects at the interface and thus better adhesion in the composites containing modified sieves. The permeabilities of these films are relatively lower than the model prediction, which is believed to be caused by immobilization of polymer chains due to adsorption onto the particle surfaces.33,34 At 40 wt % loading, the selectivity is a bit lower than the model prediction but still impressively higher
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Figure 4. Gas transport properties of Ultem composite films: (a) separation performance for O2/N2 at different filler loadings, (b) separation performance for CO2/CH4 at different filler loadings (pure gas tests, 35 °C, upstream pressure of 4.5 atm).
than the property of the neat Ultem. At a higher loading of fillers, more interfaces exist in a composite film, and any difficulty of maintaining a completely defect-free material becomes more pronounced. In any case, the results are quite impressive. The separation performance for carbon dioxide and methane follows the same trend as in the preceding discussions and is shown in Figure 4b. In conclusion, the enhancement in gas separation efficiency of composite films is strong proof that the nanoscale whisker structure on the filler surface has greatly improved the effective compatibility at the interface between the organic and inorganic phases. And due to the size of the gas molecules themselves, this improvement also gives evidence that the defects were eliminated on the angstrom scale. All previous results shown by SEM images, elastic moduli, and gas separation properties show the significant potential to use this new surface tailoring approach to nanoscopically engineer advanced materials. Hypothesis on Adhesion Enhancement Enabled by the Inorganic Whisker Morphology. All the preceding results suggest that formation of nanoscale structures on the surfaces of particles promotes the interfacial adhesion in the resultant polymeric composite films. Here we propose a mechanism to qualitatively explain how this nanostructure enhances the interfacial adhesion on the basis of thermodynamic arguments.
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Shu et al. whisker structure also plays a crucial role in achieving the improvement. The length of such whiskers formed in our experiment is around 50 nm, with an apparent aspect ratio well above 10. This structure provides a dimension that is close to the scaling of a polymer chain in the present study. The “diameter” of a linear polymer molecule is roughly given by its root-mean-square (rms) end to end length, which for many carbon backbone polymers is given approximately by
〈r2〉1/2/nm ) 0.06M1/2 Figure 5. Polymer adsorption onto a flat, ordered surface versus its adsorption onto a heterogeneous surface with random nanostructures. ∆S1 designates the entropy change of a polymer chain to adsorb onto a flat surface, and ∆S2 symbolizes the entropy change of the same chain to adsorb onto a whisker structure.
A schematic vision of the scenario enabled by the inorganic whisker morphology is depicted in Figure 5. For typical polymers, the preferred conformation state is as random coils; however, the surfaces of pristine silicate particles are comparatively smooth with few irregularities (Figure 1a). When a polymer chain adsorbs onto this type of surface, it must significantly deform to adhere via the formation of “loops, trains, and tails”,35 which are obviously much more ordered configurations than the initial random situation. In this case, the entropy penalty of a polymer’s adsorption onto a solid surface, ∆S1, is considerable. However, in the case of a heterogeneous particle surface, a polymer coil need not change its original conformation too much; therefore, the entropy penalty, ∆S2, is substantially lower as compared to the situation of a smooth particle surface.36,37 The inorganic whisker morphology created in our experiment represents a very disordered surface topography where polymer chains can entangle into the whiskers without much change in configuration, and may even show a favorable mixing entropy with the nanoscale entities comprising the morphology. The free energy of adsorption can be represented as
∆G ) ∆H - T∆S If an adequately negative enthalpy of interaction (∆H) offsets a negative entropy change, adsorption can still occur (∆G < 0). This is presumably the situation for PVAc, which has intrinsic affinity toward silicate particles via the formation of hydrogen bonds. On the other hand, for the Ultem system, the less favorable enthalpic interaction (∆H) is presumably unable to overcome the entropy penalty if Ultem chains adsorb onto smooth 4A surfaces. Thus, effective adsorption does not occur in this case (∆G > 0), consistent with the voids present at the interface (Figure 1c). On the other hand, the whisker morphology created via the treatment can significantly minimize any entropy penalty and enable Ultem’s adsorption with such whisker structures (∆G < 0), as shown by the void-free interface (Figure 1d). In conclusion, adsorption onto a heterogeneous particle surface is much more favored by this entropy effect. In addition to the reduction in entropy penalty, the contact areas between the polymer and fillers are greatly increased by the presence of these inorganic whisker structures. Indeed, in cases where there is even a small negative enthalpic interaction between the whiskers and polymers, this will further promote adhesion. In other words, the whiskers serve as effective interlocks between the sieves and polymer to grab tightly and stabilize the contact of polymer chains. Furthermore, it is important to point out that the length scale of the inorganic
where M ) molecular weight.38,39 This approximation can be used to roughly calculate the dimensions of PVAc and Ultem chains in this work. For PVAc, Mw ) 500000, the approximate r ) 42.2 nm; for Ultem, Mw ) 56000, the approximate r ) 15.0 nm. This similarity in dimensions of the whiskers and the polymer chains makes adsorption even easier, since the chains can relax in an environment close to their own configuration. The detailed relationship between different treatment conditions and the whisker morphology requires more in-depth investigation to be defined clearly. A great advantage of this approach is that it can potentially be generalized and is not confined to a polymer/silicate system. Moreover, unlike the utilization of coupling agents, it need not be tailored to a specific polymer/particle pair since the thermodynamic entropy effect and physical interlocking are universal. This approach appears potentially applicable in many occasions in which similar whisker morphologies can be created on filler surfaces. Once the surface patterns are achieved, the modified fillers can be incorporated into any desired polymer matrix to form void-free composites. Hence, the approach presented here has the potential to be extended to a variety of other materials. A possible avenue in this regard might be deposition of an outer layer of disordered carbon nanotubes on a glassy carbon to form new composite materials. This topic could be interesting and useful to look into. Conclusion In summary, in this work we present a general strategy to enhance the interfacial adhesion in polymeric composites by creating nanoscale morphologies on the surfaces of particles. The surface morphology, which appears as inorganic whiskers, is achieved by treating silicate particles (zeolites 4A) with thionyl chloride, followed by subsequent reaction with methylmagnesium bromide (Grignard reagent). Polymeric composites containing this type of modified particles exhibit defect-free interfaces. DMA testing reveals that such composites have higher moduli than those embedded with nontreated fillers with the same loadings. Furthermore, gas permeation measurements demonstrate that these materials also show impressive enhancements in gas separation efficiency. A thermodynamic argument was proposed to qualitatively explain why such nanoscale structures contribute to improved adhesion at the interfaces. A surface roughening effect has presumably contributed to the enhancement via thermodynamically induced adsorption and physical interlocking in the whisker structure. The entanglement can therefore effectively stabilize contact between the polymer and inorganic fillers. A great advantage of this methodology is that it need not be confined to a specific material pair, since the thermodynamic principle and physical interlocking mechanism provided by the inorganic whiskers are universal and potentially applicable on occasions where similar nanoscopic morphologies can be created on the filler surfaces. Thus, the presented strategy has potential to be extended to many other
Adhesion Enhancement in Polymeric Composites polymeric composites for a variety of applications, where tailoring polymer/solid interface compatibility is important. Acknowledgment. We gratefully acknowledge MEDA L.P for funding support of this work. Supporting Information Available: XPS spectra and X-ray diffraction patterns of the modified particles. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Zhang. J.; Hou, L. Y.; Xu, L.; Xu, Z. L. Chem. Mater. 1999, 11, 3177. (2) Beecroft, L. L.; Ober, C. K. Chem. Mater. 1997, 9, 1302. (3) Molenkamp, W. C.; Watanabe, M.; Miyata, H.; Tolbert, S. H. J. Am. Chem. Soc. 2004, 126, 4476. (4) Quaglia, M.; Lorenzi, E. D.; Sulitzky, C.; Sellergren, B. Electrophoresis 2003, 24, 952. (5) Raghavan, S. R.; Riley, M. W.; Fedkiw, P. S.; Khan, S. A. Chem. Mater. 1998, 10, 244. (6) Clark, J. H.; Macquarrie, D. J. Chem. Commun. 1998, 853. (7) Wright, A. P.; Davis, M. E. Chem. ReV. 2002, 102, 3589. (8) Lu, X.; Manners, I.; Winnik, M. A. Macromolecules 2001, 34, 1917. (9) Teshima, K.; Sugimura, H.; Takai, O. Langmuir 2003, 19, 8331. (10) Jeong, H. K.; Nair, S.; Vogt, T.; Dickinson, C.; Tsapatsis, M. Nat. Mater. 2003, 2, 53. (11) Wall, J. S.; Hu, B.; Siddiqui, J. A.; Ottenbrite, R. M. Langmuir 2001, 17, 6027. (12) Xie, X. L.; Tang, C. Y.; Zhou, X. P. Chem. Mater. 2004, 16, 133. (13) Kulkarni, Gas separation membrane with organosilicon-treated molecular sieve. U.S. Patent 6,508,860, Jan 21, 2003. (14) Mahajan, R.; Koros, W. J. Polym. Eng. Sci. 2002, 42, 1432. (15) Perruchot, C.; Khan, M. A.; Kamitsi, A.; Armes, S. P.; Patten, T. E. Langmuir 2001, 17, 4479. (16) Pyun, J.; Matyjaszewski, K. Chem. Mater. 2001, 13, 3436. (17) Wang, J. Y.; Chen, W.; Liu, A. H.; Lu, G.; Yang, B. J. Am. Chem. Soc. 2002, 124, 13358.
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