Surface Characterization, Modification Chemistry, and Separation

Particularly, the separation performance of CO2/CH4 gas pair can be improved ... in the tradeoff curve for gas permeability and selectivity.16,17 Mean...
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Surface Characterization, Modification Chemistry, and Separation Performance of Polyimide and Polyamidoamine Dendrimer Composite Films Youchang Xiao, Tai-Shung Chung,* and Mei Lin Chng Department of Chemical & Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 Received April 14, 2004. In Final Form: June 21, 2004 6FDA-polyimide films modified by polyamidoamine (PAMAM) dendrimers with generations of 0, 1, and 2 are reported in this article. The actual molecular conformation and bulk size of these three generation dendrimers immobilized on polyimide surface were characterized by atomic force microscopy. After comparing with the results of dynamic simulation, we believe that the disk-shape cluster structure of dendrimers has been developed on the polymer surfaces. The amidation and cross-linking reaction between dendrimers and polyimide were examined and quantified by X-ray photoelectron spectroscopy, attenuated total reflection Fourier transform infrared spectroscopy, and gel content measurements. Modification time and the generations of PAMAM dendrimer have been verified as two important factors in determining the properties of modified polyimide films. These modified polyimide films exhibit excellent gas separation performance. The ideal selectivity of He/N2 increases tremendously to about 200% as compared to that of the original polyimide film. Particularly, the separation performance of CO2/CH4 gas pair can be improved beyond the upper bond limit possibly due to the strong interactions of dendrimer molecules with CO2, which was verified by sorption tests.

1. Introduction For polymeric membrane materials utilized in gas separation processes, high permeability and high selectivity are essential in order to produce high-purity products with minimal operating costs. In addition, good physical and chemical resistances of the materials are necessary to prolong the lifetime of membrane systems.1-4 Among many polymeric materials researched in the past few decades, aromatic polyimides have been found to possess both good physical and gas separation properties due to their rigid chain structure and unique chemistry.5-10 Extensive works have been carried out to tailor the chemical structure of polyimides in order to search for materials with better gas separation performance.11-15 However, these attempts seem to be approaching a limit * Corresponding author. E-mail: [email protected]. Fax: (65)67791936. (1) Stern, S. A. Polymers for gas separations: the next decade. J. Membr. Sci. 1994, 94, 1. (2) Paul, D. R.; Yampol’skii, Y. P. Polymeric gas separation membranes; CRC Press: Boca Raton, FL, 1994. (3) Koros, W. J.; Mahajan, R. Pushing the limits on possibilities for large scale gas separation: which strategies? J. Membr. Sci. 2000, 175, 181. (4) Matsuura, T. Synthetic membranes and membrane separation processes; CRC Press: Boca Raton, FL, 1994. (5) Kim, T. H.; Koros, W. J.; Husk, G. R. Advanced gas separation membrane materials-rigid armotic polyimides. Sep. Sci. Technol. 1988, 23, 1611. (6) Ho, W. S. W.; Sirkar, K. K. Membrane Handbook; Van Nostrand Reinhold: New York, 1992. (7) Chung, T. S.; Kafchinski, R. E. Development of asymmetric hollow fibers from polyimides for air separation. J. Membr. Sci. 1992, 75, 181. (8) Coleman, M. R.; Koros, W. J. Conditioning of fluorine-containing polyimides. 2. effect of conditioning protocol at 8% volume dilation on gas-transport properties. Macromolecules 1999, 32, 3106. (9) Chung, T. S.; Kafchinski, R. E. The effects of spinning conditions on asymmetric 6FDA/6FDAM polyimide hollow fibers for air separation. J. Appl. Polym. Sci. 1997, 65, 1555. (10) Chung, T. S.; Lin, W. H.; Vora, R. The effect of shear rates on gas separation performance of 6FDA-durene polyimide hollow fibers. J. Membr. Sci. 2000, 167, 55.

which is shown in the tradeoff curve for gas permeability and selectivity.16,17 Meanwhile, other progress on the crosslinking modification of polyimides has received worldwide attention because neat polyimides tend to be attacked by the highly soluble impurities in the feed stream. Experimental results suggest that the cross-linking modification imparts polyimides with antiplasticization, chemical resistance, and sometimes better gas separation properties when compared to the normal tradeoff line. Cross-linking modifications of polyimides can be induced by several methods, such as ultraviolet (UV) light, ion beam radiation, thermal treatment, and chemical modification.18-37 However, the difficulties of implementing irradiation (11) Lin, W. H.; Vora, R. H.; Chung, T. S. Gas transport properties of 6FDA-durene/pPDA copolyimides. J. Polym. Sci., Part B: Polym. Phys. 2000, 38, 2703. (12) Kim, T. H.; Koros, W. J.; Husk, G. R.; O’Brien, K. C. Relationship between gas separation properties and chemical structure in a series of aromatic polyimides. J. Membr. Sci. 1988, 37, 45. (13) Hirayama, Y.; Yoshinaga, T.; Kusuki, Y.; Ninomiya, K.; Sakakibara, T.; Tamari, T. Relation of gas permeability with structure of aromatic polyimides I. J. Membr. Sci. 1996, 111, 169. (14) Akira, S.; Tsukasa, M.; Masatoshi, M.; Kenichi, I. Relationships between the chemical structures and the solubility, diffusivity, and permselectivity of propylene and propane in 6FDA-based polyimides. J. Polym. Sci., B: Phys. 2000, 38, 2525. (15) Ayala, D.; Lozano, A. E.; Abajo, J. D.; Perez, C. G.; Campa, J. G.; Peinemann, K. V.; Freeman, B. D.; Prabhakar, R. Gas separation properties of aromatic polyimides. J. Membr. Sci. 2003, 215, 61. (16) Robeson, L. M. Correlation of separation factor versus permeability for polymeric membranes. J. Membr. Sci. 1991, 62, 165. (17) Freeman, B. D. Basis of permeability/selectivity tradeoff relations in polymeric gas separion membranes. Maromolecules 1999, 32, 375. (18) Hayes, R. A. polyimide gas separation membranes, US Patent no. 4,717,393, 1988. (19) Hayes, R. A. Amine-modified polyimide membranes, US Patent no. 4,981,497, 1991. (20) Kita, H.; Inada, T.; Tanaka, K.; Okamoto, K. Effect of photocross-linking on permeability and permselectivity of gases through benzophenone-containing polyimide. J. Membr. Sci. 1994, 87, 139. (21) Matsui, S.; Ishiguro, T.; Higuchi, A.; Nakagawa, T. Effect of ultraviolet light irradiation on gas permeability in polyimide membranes. 1. irradiation with low-pressure mercury lamp on photosensitive and nonphotosensitive membranes. J. Polym. Sci. 1997, 35, 2259.

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uniformly on the hollow fiber and deteriorations of the subtle asymmetric structures by high-temperature treatments may limit these two applications. To overcome the above drawbacks, different chemical modification methods have been proposed. Hayes18,19 at DuPont developed novel chemical cross-linking modifications for polyimides by immersing the polyimide membranes in amino compound solutions followed by thermal treatment. Staudt-Bickel et al.30 cast films from the solution of diaminobenzoic acid (DABA) based copolyimide and ethylene glycol, followed by cross-linking under solid-state conditions. Wind et al.31,32 not only extended the work of Staudt-Bickel et al. but also evaluated other types of glycols to investigate their antiplasticization characteristics. Liu et al.33 reported p-xylenediamine could induce chemical cross-linking reactions between its free primary amine groups and the imide groups of polyimides at room temperature. This approach has been successfully applied to other polyimide films and hollow fiber membranes with enhanced antiplasticization properties by our group.34-37 It was proven that the cross-linking modification tends to increase chain packing and inhibits the intrasegmental and intersegmental mobility. Therefore, an increase in the degree of cross-linking may result in higher gas selectivity, but lower gas permeability for most cross-linked polymeric membranes. A brand new cross-linking reagent, polyamidoamine (PAMAM) dendrimer, generation 0, was recently reported (22) Coleman, M. R.; Xu, X.; Ilconich, J.; Hu, L. Effective hydrogen separation using ion beam modified polymeric membranes. Polym. Prepr. 2004, 45, 5. (23) Bos, A.; Punt, I. G. M.; Wessling, M.; Strathmann, H. Suppression of CO2-plasticization by semi-interpenetrating polymer network formation. J. Polym. Sci., Part B: Polym. Phys. 1998, 36, 1547. (24) Liu, Y.; Pan, C.; Ding, M. X.; Xu, J. P. Gas permeability and permselectivity of photochemically cross-linked copolyimides. J. Appl. Polym. Sci. 1999, 73, 521. (25) Bos, A.; Punt, I. G. M.; Wessling, M.; Strathmann, H. Plasticization-resistant glassy polyimide membranes for CO2/CH4 separations. Sep. Purif. Technol. 1998, 14, 27. (26) Krol, J. J.; Boerrigter, M.; Koops, G. H. Polyimide hollow fiber gas separation membranes: Preparation and the suppression of plasticization on propane/propylene environments. J. Membr. Sci. 2001, 184, 275. (27) Kang, J. S.; Won, J.; Park, H. C.; Kim, U. Y.; Kang, Y. S.; Lee, Y. M. Morphology control of asymmetric membranes by UV irradiation on polyimide dope solution. J. Membr. Sci. 2000, 169, 229. (28) Won, J.; Kim, M. H.; Kang, Y. S.; Park, H. C.; Kim, U. Y.; Choi, S. C.; Koh, S. K. Surface modification of polyimide and polysulfone membranes by ion beam for gas separation. J. Appl. Polym. Sci. 2000, 75, 1554. (29) Rezac, M. E.; Sorensen, E. T.; Beckham, H. W. Transport properties of cross-linkable polyimide blends. J. Membr. Sci. 1997, 136, 249. (30) Staudt-Bickel, C.; Koros, W. J. Improvement of CO2/CH4 separation characteristic of polyimides by chemical cross-linking. J. Membr. Sci. 1999, 155, 145. (31) Wind, J. D.; Staudt-Bickel, C.; Paul, D. R.; Koros, W. J. The effect of cross-linking chemistry on CO2 plasticization of polyimide gas separation membranes. Ind. Eng. Chem. Res. 2002, 41, 6139. (32) Wind, J. D.; Staudt-Bickel, C.; Paul, D. R.; Koros, W. J. Solidsate covalent cross-linking of polyimide membranes for carbon dioxide plasticization reduction. Macromolecules 2003, 36, 1882. (33) Liu, Y.; Wang, R.; Chung, T. S. Chemical cross-linking modification of polyimide membranes for gas separation. J. Membr. Sci. 2001, 189, 231. (34) Cao, C.; Chung, T. S.; Liu, Y.; Wang, R.; Pramoda, K. P. Chemical cross-linking modification of 6FDA-2,6-DAT hollow fiber membranes for natural gas separation. J. Membr. Sci. 2003, 216, 257. (35) Liu, Y.; Chung, T. S.; Wang, R.; Li, D. F.; Chng, M. L. Chemical cross-linking modification of polyimide/poly(ether sulfone) dual-layer hollow-fiber membranes for gas separation. Ind. Eng. Chem. Res. 2003, 42, 1190. (36) Tin, P. S.; Chung, T. S.; Wang, R.; Liu, Y.; Liu, S. L.; Pramoda, K. P. Effects of cross-linking modification on gas separation performance of Matrimid membranes. J. Membr. Sci. 2003, 225, 77. (37) Zhou, C.; Chung, T. S.; Liu, Y.; Wang, R.; Goh, S. H. The accelerated CO2 plasticization of ultrathin polyimide films and the effect of surface chemical cross-linking on plasticization and physical aging. J. Membr. Sci. 2003, 225, 125.

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by our group for the modification of polyimides.38 Starlike PAMAM dendrimers with regular and highly branched three-dimensional architecture were discovered by Tomalia et al.39,40 The molecular functionalities such as the number of end groups, branch points, molecular weight, and size can be perfectly controlled by using different generations of synthesis. The high-density functional groups at the surface or in the cavities of PAMAM dendrimers, offer us several potential applications based on their chemical, physical, optical, multiredox, and catalytic properties.41,42 Use of dendrimers as a membrane material was first reported by Sirkar and co-workers.43-45 In their research, high-performance CO2 selective liquid membranes were developed using dendrimers as a CO2selective molecular gate. Their discovery opens new dimensions of research and potential applications of dendrimers in the modifications of membrane materials. Recently, polymer films chemically grafted by PAMAM on the surface have been reported for gas separation by Fail et al.46 and Cha et al.47 Fail et al. immobilized the PAMAM dendrimers onto the surface of a polypropylene film by means of plasma followed by heat treatment, inducing a highly cross-linked structure layer which improved the gas barrier. Using the same method, Cha et al. prepared poly(dimethylsiloxane) dendrimer composite films which formed a complex with AgBF4, and thus exhibited excellent gas separation performance. In our previous note,37 it was proven that surface cross-linked fluoropolyimide membranes could be prepared by immersing polyimide membranes into PAMAM methanol solutions at room temperature, followed by drying at room temperature. Compared with plasma-induced covalent attachment of dendrimers,46,47 our approach is more convenient and feasible in the modification of hollow fiber membranes. This report expands our previous research scope and incorporates other generations into the study for a more in-depth comparison. PAMAM dendrimers with generations up to 2 are utilized, and their molecular sizes and shapes are theoretically simulated by Cerius2 software. AFM (atom force microscopy) characterizes the actual morphology of PAMAM dendrimers immobilized onto the polyimide surfaces. The possible interactions between dendrimers and polyimide are investigated by XPS (Xray photoelectron spectroscopy), attenuated total reflection Fourier transform infrared (FTIR-ATR) spectroscopy, and gel content measurements. These experiments in addition (38) Chung, T. S.; Chng, M. L.; Pramoda, K. P.; Xiao, Y. C. PAMAM Dendrimer Induced Cross-linking Modification of Polyimide Membranes. Langmuir 2004, 20, 2966. (39) Tomalia, D. A.; Baker, H.; Dewald, J.; Hall, M.; Kallos, G.; Martin, S.; Roeck, J.; Ryder, J.; Smith, P. Dendritic macromolecules: Synthesis of starburst dendrimers. Macromolecules 1986, 19, 2466. (40) Tomalia, D. A.; Frechet, M. J. Discovery of dendrimers and dendritic polymers: a brief historical perspective. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 2719. (41) Bosman, A. W.; Janssen, H. M.; Meijer, E. W. About Dendrimers: Structure, Physical Properties, and Applications. Chem. Rev. 1999, 99, 1665. (42) Zhuo, R. X.; Du, B.; Lu, Z. R. In vitro release of 5-fluorouacil with cyclic core dendritic polymer. J. Controlled Release 1999, 57, 249. (43) Kovvali, A. S.; Chen, H.; Sirkar, K. K. Dendrimer membranes: A CO2-selective molecular gate. J. Am. Chem. Soc. 2000, 122, 7549. (44) Kovvali, A. S.; Sirkar, K. K. Dendrimer liquid membranes: CO2 separation from gas mixtures. Ind. Eng. Chem. Res. 2001, 40, 2502. (45) Kovvali, A. S.; Sirkar, K. K. Carbon dioxide separation with novel solvents as liquid membranes. Ind. Eng. Chem. Res. 2002, 41, 2287. (46) Fail, C. A.; Evenson, S. A.; Ward, L. J.; Schofield, W. C. E.; Badyal, J. P. S.; Controlled attachment of PAMAM dendrimers to solid surfaces. Langmuir 2002, 18, 264. (47) Cha, B. J.; Kang, Y. S.; Won, J.; Preparation and characterization of dendrimer layers on poly(dimethylsiloxane) films. Macromolecules 2001, 34, 6631.

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Figure 1. The chemical structure of 6FDA-Durene polyimide.

to the pure gas permeability tests under 10 atm and 35 °C condition are used to determine the effects of the different generations of PAMAM dendrimer and immersion times on the properties of modified polyimide films. CO2 and CH4 sorption tests were carried by a microbalance to verify the interaction between CO2 and PAMAM dendrimer molecules. 2. Experimental Section 2.1. Materials. The polyimide material studied in this work was synthesized in our lab from 2,2′-bis(3,4′-dicarboxyphenyl)hexafluoropropane diandydride (6FDA) and 2,3,5,6-tetramethyl1,4-phenylenediamine (durene diamine). The chemical structure of 6FDA-Durene polyimide is shown in Figure 1. Its glass transition temperature, which was determined using a differential scanning calorimeter (Perkin-Elmer Pyris 1) at a heating rate of 20 °C/min under nitrogen atmosphere, is 424 °C. Synthesis details of this polyimide had been reported elsewhere.9,10 Solutions (20 wt %) of zero-generation (G0), first-generation (G1), and second-generation (G2) amino-terminated PAMAM dendrimers in methanol were purchased from Aldrich and used after dilution to a 5 wt % methanol solution. The planar schematic structure of these dendrimers is drawn in Figure 2. All solvents employed were of reagent grade or higher. The purities of He, O2, N2, CH4, and CO2 are 99.99%. All the solvents and gases were used without further purification. 2.2. Simulation of Dendrimer Molecular Size and Conformation. All molecular modeling studies were performed on a SGI (Silicon Graphics Inc.) work station. Dendrimers were constructed and visualized using the builder module of Cerius2 4.8 software published by Accelrys Inc. Due to the lack of other supporting information such as detailed NMR (nuclear magnetic resonance) and SAXS (small-angle x-ray scattering) data, energy minimization is used to determine the most likely conformation for the dendrimers. To illustrate a large number of conformation possibilities, molecular dynamics simulations have been applied. Since the structures may appear slightly different depending on

Xiao et al. the selected angle of viewing, the images in the figures are chosen in order to show the maximum detail and balance. 2.3. Dense Membrane Preparation. Before chemical modification, 6FDA-Durene polyimide was first prepared as a dense film. The polymer was dried overnight at 120 °C under vacuum prior to be used. A 2% (w/w) polymer solution was prepared by dissolving polyimide powder in dichloromethane. The polymer solution was then filtered using Whatman filters (1 µm) to remove insoluble materials and dust particles. After which, the solution was cast onto a silicon wafer at ambient temperature. After most of the solvent had evaporated slowly for about 5 days, the dense membrane was formed. Subsequently, the dense membranes were dried in a vacuum oven to 250 °C. All membranes were cut into circles with a diameter of 38 mm. Only the membranes with a thickness of about 50 ( 5 µm were used in the following studies. 2.4. Polyimide Membrane Modification and Methanol Treatment. For surface modification, 5 wt % methanol solutions of different generation PAMAM dendrimers were prepared. The modification was carried out by immersing the polyimide films into dendrimer solutions for a certain period of time. The films were then washed with fresh methanol with intensive oscillation for 5 min immediately after being taken out from the reagent solution. After the residual dendrimers on the surface were washed away, the modified films were dried at ambient temperature for more than 24 h before other measurements. The above modification procedures were repeated by using pure methanol instead of the dendrimer solutions in order to compare and eliminate solvent effects. 2.5. Characterization. An X-ray photoelectron spectrometer was utilized to measure the ratio of elements and to monitor the chemical reaction on the polyimide film surface. The XPS measurements were carried out by an AXIS HSi spectrometer (Kratos Analytical Ltd., England) using the monochromatized Al KR X-ray source (1486.6 eV photons) at a constant dwell time of 100 ms and a pass energy of 40 eV. The anode voltage and anode current were 15 kV and 10 mA, respectively. The pressure in the analysis chamber was maintained at 5.0 × 10-8 Torr or lower during each measurement. All core-level spectra were obtained at a photoelectron takeoff angle of 90° with respect to the sample surface, and the X-ray penetration depth is about 7.5 nm for polymer materials. FTIR-ATR measurements were carried out using a PerkinElmer FTIR microscope at 8 cm-1 resolution over the 500-2200 cm-1 range. Each sample was scanned 20 times. The surface morphology of the membranes was studied using a Nanoscope IIIa atomic force microscope from the Digital

Figure 2. Planar schematic of the basic PAMAM dendrimer functionality.

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Figure 3. Dimension of different generation PAMAM dendrimers simulated by Cerius2. Instruments Inc. In each case, an area of 200 nm × 200 nm was scanned using the tapping mode. The drive frequency was 330 ( 50 kHz, and the voltage was between 3 and 4.0 V. A drive amplitude, set point, and scan rate of 300 mV, 3.34 µV, and 1.0 Hz were used, respectively. The gel contents of the modified films were measured by extracting the films in dichloromethane for 24 h, after which the insoluble fractions were dried to constant weight at 150 °C in a vacuum oven for 24 h. The weights of polymer films before and after extraction were measured. The gel contents were calculated by gel % ) (W1/W0) × 100, where W0 and W1 are the original weight and the insoluble fraction weight of the polyimide films, respectively. The error of gel content test is around 0.5%. 2.6. Gas Permeation and Sorption Measurements. The pure gas permeabilities were obtained by a constant volume method at 35 °C and 10 atm in the sequence of He, O2, N2, CH4, and CO2. A detailed description of the permeation cell design and testing conditions can be found elsewhere.10 The modified film was mounted onto the permeation cell and vacuumed at 35 °C for more than 24 h before the gas permeation test was carried out. The permeability of each gas was obtained from the average value of at least three tests with a difference smaller than 1%, and each test was carried out at an interval of 6-8 h. The ideal selectivity is defined as follows: RA/B ) (PA/PB) where PA and PB are the permeabilities of gases A and B, respectively. CO2 and CH4 sorption tests were conducted for the modified polyimide films using a Cahn D200 microbalance sorption cell. The microbalance was first calibrated with gas as a function of pressure. Then approximately 200 mg of the film materials was placed on the sample pan, following by evacuation for 24 h. Gas at a specific pressure was fed into the system and the sample started to sorb the gas until equilibrium was achieved. From the weight gain, the amount of gas dissolved in the material was calculated after accounting for the buoyancy correction.

3. Results and Discussion 3.1. PAMAM Dendrimers with Generations 0, 1, and 2. It is well-known that the number of end groups, molecular weight, and the size of dendrimers are related to their generations, which will directly affect the modification ability of dendrimers. The molecular sizes of much higher generation PAMAM dendrimers in methanol solutions have been measured by Prosa et al. using smallangle X-ray scattering.48 However, their method is not suitable to resolve the small dendrimers such as generations 0, 1, and 2, since the shapes of small dendrimer cannot be claimed to be spherelike, starlike, or an ellipsoid of revolution. In this study, Cerius2 software is utilized to simulate molecular sizes and conformation of generations 0, 1, and 2 PAMAM without consideration of the solvent effect. As illustrated in Figure 3, the smallest G0 PAMAM dendrimer has four branches and four free primary amine surface groups. With each increasing

generation of PAMAM, the numbers of branches and surface groups are doubled. Due to the interaction between branches, the increase in molecular size is slower as compared to that of the branches and surface groups. Therefore, low generation PAMAM dendrimers show a rather open and incompact structure. When the generation increases, the structure tends to become somewhat denser and more spherical. As a result, the density of surface functional groups and accessibility for chemical reaction might be enhanced by increasing the dendrimer’s generation. To the best of our knowledge, only molecular morphology of higher generation (g4) PAMAM dendrimers on substrates has been studied.49-51 Tsukruk et al.51 reported a self-assembled multilayer structure constructed by the G4 dendrimer on the SiO2 surface. In this study, the AFM micrographs reveal the real morphology of smaller generation (G0, G1, and G2) PAMAM dendrimers attached onto the polyimide surface. As illustrated in Figure 4, the modified surface appears to have a nodule structure with an increased surface roughness. The diameter and the height of the nodular structure are much bigger than the simulation results as displayed in Figure 3, which shows the simulated molecular diameters of G0, G1, and G2 PAMAM dendrimers are 1.6, 3.0, and 4.0 nm, respectively. In addition, the nodule height is smaller than its diameter. This indicates that dendrimer molecules accumulate together to form disk-shaped clusters, grafting on polyimide surface during the immersion for chemical modification. The grafted polyimide surface appears to have different morphologies with different generations of dendrimers. Because of the incompact structure, smaller generation PAMAM dendrimers can easily interpenetrate into one another and construct clusters consisting of a large number of dendrimer molecules. As a result, the size difference between the single molecule and the cluster is significant (i.e., for G0 PAMAM, a cluster size is about five times bigger than a single molecular size, however, for G1 and (48) Prosa, T. J.; Bauer, B. J.; Amis, E. J.; Tomalia, D. A.; Scherrenberg, R. A SAXS study of the internal structure of dendritic polymer systems. J. Polym. Sci., Part B: Polym. Phys. 1997, 35, 2913. (49) Li, J.; Piehler, L. T.; Qin, D.; Baker, J. R.; Tomalia, D. A. Visualization and Characterizatoin of Poly(amidoamine) Dendrimers by Atomic Force Microscopy. Langmuir 2000, 16, 5613. (50) Betley, T. A.; Holl, M. M. B.; Orr, B. G.; Swanson, D. R.; Tomalia, D. A.; Baker, J. R. Tapping mode atomic force microscopy investigation of poly(amidoamine) dendrimers: effects of substrate and pH on dendrimer deformation. Langmuir 2001, 17, 2768. (51) Tsukruk, V. V.; Rinderspacher, F.; Bliznyuk, V. N. SelfAssembled multilayer films from dendrimers. Langmuir 1997, 13, 8.

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Figure 4. AFM of 24 h modified polyimide surfaces.

Figure 5. The possible chemical mechanism of polyimide modification by G0 PAMAM dendrimer.

G2 PAMAM, the difference decreases to three times). Moreover, because of the interactions between dendrimers, there are some linkages between clusters. These linkages appear to be stronger for the surface modified by smaller generation dendrimers, which result in a more uniform surface structure. When higher generation dendrimers are used, a fewer number of dendrimer molecules can accumulate on the polyimide surface because of their denser molecular structure and larger steric intermolecular hindrance. Consequently, the size difference between the single molecule and the cluster is smaller. The weaker intermolecular interactions result in weak molecular linkages, thus making clusters circular in shape in order to lower the surface energy. 3.2. Characterization of Modified Polyimide Films by PAMAM Dendrimers. Covalent and hydrogen bonds between the dendrimer and polyimide appear to play a crucial role in the formation of composite dendrimerpolyimide membranes. In our previous short note,38 the reaction between polyimide and G0 PAMAM dendrimer was proven by FTIR-ATR spectra which implied the possible chemical mechanism as illustrated in Figure 5. In this report, detailed XPS experiments are used to monitor the modification process. Figure 6 shows the respective C 1s core-level spectra of the original polyimide surface (a) and the modified surface by the G0 PAMAM dendrimer for durations of 20 min, 1 h, 24 h, and 1 week (b, c, d, and e, respectively). The C 1s core-level spectrum of the original polyimide membrane can be curve-fitted to four peak components, having bond energies at 284.6 eV

Figure 6. XPS C 1s core-level spectra of G0 PAMAM dendrimer modified 6FDA-polyimde films.

for the C-H species, at 285.8 eV for the C-N, at 288.4 eV for the N(CdO)2 (imide) species, and at 292.8 eV for the CF3 species.52 After modification, the intensity of the peak for imide groups (288.4 eV) decreases, indicating that the imide groups react with primary amine groups of PAMAM dendrimers to form amide groups. As a result, modified membranes exhibit a new peak at 287.9 eV for -NH(CdO)- (amide) species, which may be contributed by the amidation reaction and PAMAM dendrimer. The peak at 292.8 eV for the CF3 species was selected as an internal reference, since this group is not involved in the modification reaction. Therefore, the degree of (52) Beamson, G.; Briggs, D. High-Resolution XPS of Organic Polymers: the Scienta ES CA300 Database; John Wiley: New York, 1992

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Table 1. XPS Analysis of PAMAM Dendrimer (G0) Modified 6FDA-Polyimide Films

immersion time

N/F ratio ((0.05)

average no. of polyimide units per PAMAM molecule loading

original 20 min 60 min 24 h 7 days

0.3 0.6 0.7 1.3 1.3

6.3 4.5 1.7 1.7

reaction can be estimated from the area ratio of imide group at 288.4 eV to the internal reference peak at 292.8 eV. As shown in Table 1, there is a decrease in area ratio with an increase in immersion (modification) time, indicating an increase in the degree of reaction. Table 1 also lists the quantitative ratio of elements N to F on modified membrane surfaces as a function of immersion time. As the content of F elements in polyimides remains constant during the modification process, the increase in the N/F ratio can be used to determine the exact loading quantity of PAMAM dendrimer upon the outer polyimide skin. For the original 6FDA-polyimide membrane, the measured N/F ratio is 0.3 and Aimide/ACF3 is 2.0, which are consistent with the theoretical values calculated from the molecular structure shown in Figure 1, where a single 6FDA-polyimide unit contains of two N atoms and six F atoms, as well as four imide C atoms and two trifluoride C atoms. For the 20 min immersion sample, due to the loading of each G0 PAMAM dendrimer molecule containing of 10 N atoms, the N/F ratio is increased to 0.6. This indicates that each polyimide unit reacts with an average of 0.16 PAMAM dendrimer molecules (for the reader’s purpose, the following equation shows the formulation of calculation: (0.6 × 6(F/unit) - 2(N/unit))/10(N/PAMAM) ) 0.16(PAMAM)). In other words, one PAMAM dendrimer molecule reacts with an average of 6.3 polyimide units (i.e., 1/1.6 ≈ 6.3), with the total of 25.2 (6.3 × 4) imide carbon atoms. In addition, Aimide/ACF3 drops from 2 to 1.4, indicating that approximately 30% of the imide groups located near the outer skin have converted to amide groups. Therefore, we may conclude that a PAMAM dendrimer would react with a total of 7.6 (25.2 × 30%) imide C atoms (from 6.3 polyimide units) during modification. Since a free primary amine group will react with an imide group from PAMAM dendrimers to convert two imide C atoms to amide C atoms, the above calculation also suggests that an average of 3.8 (7.6/2) free primary amine groups of a PAMAM dendrimer molecule would react with 6.3 polyimide units during the modification. The calculated results for other modified membranes are also listed in Table 1. The tabulated data reveal that the N/F ratio increases, while the Aimide/ACF3 decreases with immersion time, indicating both PAMAM dendrimer loading and the average degree of conversion of imide groups increase with immersion time. However, the number of primary amine groups involving in amidation reaction decreases with immersion time. Moreover, after a lengthy immersion time of more than 24 h, the PAMAM dendrimer loading and chemical modification at the outer skin of polyimide membranes have apparently reached equilibrium, probably resulting from a decrease in the available sites for polyimides to be cross-linked. As a consequence, the subsequent loading of PAMAM dendrimer can only physically adsorb on the top of the surface to form clusters without chemically reacting with polyimides, which is in agreement with the AFM observation as illustrated in Figure 4.

Aimide/ACF3 ((0.05)

average no. of imide group disappearances per PAMAM molecule loading

possible no. of reacted free primary amine groups

2.0 1.4 1.2 0.7 0.7

7.6 7.2 4.4 4.4

3.8 3.6 2.2 2.2

Figure 7. A comparison of ATR-FTIR spectra of 6FDApolyimide films immersed in (G0, G1, and G2) solutions for 24 h.

Figure 7 demonstrates the ATR-FTIR spectra of modified polyimides induced by different generation PAMAM dendrimers in a 24 h immersion time. On comparison with the original polyimide, the polyimide film modified by G0 PAMAM dendrimer shows a stronger intensity of the characteristic peaks of amide group at 1656 and 1550 cm-1 but a weaker intensity of the characteristic peaks of imide group at 1780 and 1380 cm-1. The degrees of modification induced by higher generation dendrimers appear to be smaller. The possible cause for this difference may arise from the fact that G0 PAMAM dendrimer can penetrate into the polymer matrix deeply and react with polyimide chains, thus yielding higher dendrimer loading and amidation reaction, while the higher generation PAMAM dendrimers could only penetrate slightly because of bigger molecular sizes and thus they might mainly adsorb and react on the outer surface region. Although the larger molecular size of high-generation PAMAM dendrimers may result in a lower loading, a thin cross-linking structure may still be formed on the polyimide surface due to its multifunctional groups. Tg measurements were conducted to verify if the cross-linking modification takes place since cross-linking may block the intersegmental motion of polymeric chains. Unfortunately, the decomposition of PAMAM under 300 °C makes this attempt a failure. Therefore, the gel contents of the modified polyimide films were measured to testify to the hypothesis. Table 2 illustrates that the gel content increases with immersion time, indicating that the crosslinking reaction between the polyimide chains and dendrimer takes place which results in a decrease in the polyimide solubility in dichloromethane. It also gives evidence that high generation PAMAM dendrimers can

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Figure 8. Immersion time effects on performance of modified polyimide films. Table 2. Gel Contents of Different Generation Dendrimer Modified 6FDA-Polyimide Films gel content (%)

generation 0 generation 1 generation 2

unmodified

20 min immersion time

60 min immersion time

24 h immersion time

7 days immersion time

0 0 0

14 4.2 5.3

53 5.6 9.7

93 39 38

96 99 97

effectively modify the polyimide and enhance their chemical resistance to dichloromethane by forming a surface cross-linking structure if longer immersion times are provided to offset the larger steric hindrance. Interestingly, Table 2 suggests that G2 shows stronger ability in modifying polymer films than G1 if the immersion time is short. The simulated conformations tell us that the higher generation dendrimer has a higher density of surface functional groups and accessibility for chemical reaction. Hence, G2 dendrimer may induce more crosslinking structure on the surface than G1 dendrimer in the initial stage of chemical modifications. 3.3. Pure Gas Separation Transport Properties. During the immersion process, dendrimers penetrated into the polymer matrix and cross-linked the polyimide films to change the gas transport properties. The slow penetration rate will make an asymmetric structure of chemical modification through the films. Therefore, only the apparent permeability was obtained to demonstrate the effects of PAMAM modification on gas separation properties. The relative gas permeabilities of He, O2, N2, CH4, and CO2 through modified polyimide films are depicted in Figure 8. The relative gas permeability is defined as the permeability ratio of the modified to unmodified ones. Since G0 dendrimer has the smallest molecular size, it can penetrate into polymer deeply, thus it affects gas permeabilies more significantly than G1 and G2 dendrimers. The gas permeabilities are found to attain their maximum values after the 20 min modification, subsequently followed a decrease with an increase in immersion time. The initial permeability increase may be attributed to the effects of methanol swelling.36 To eliminate the solvent swelling effects and to obtain the real PAMAM modification effects on gas permeation, a series of contrast

experiments were carried out by using pure methanol instead of PAMAM dendrimer solutions. As shown in the last graph of Figure 8, solvent-induced structure swelling significantly increases the gas permeability because of the increased free volume and chain mobility. The percentage of gas permeability increase is in the order of CO2 (4.0 Å) > CH4 (3.8 Å) > N2 (3.64 Å) > O2 (3.46 Å) > He (2.6 Å), which is consistent with the order of the Lennard-Jones diameter (collision diameter) of gas molecules. In other words, solvent-induced swelling has a strong effect on the gas permeability of a big collision diameter gas (usually a slow gas except CO2) than a small one (usually a fast gas). For excluding the solvents effects, the relative permeability was obtained by the equation: Pr ) P/Ps, where Pr is the relative permeability, P is the permeability of PAMAM modified polyimide films, and Ps is the permeability of pure methanol immersed polyimide films. After the solvent effects are excluded, Figure 9 illuminates the true relationship of gas permeability vs immersion time. The decrease of gas permeabilities are mainly attributed to (1) the cross-linking structure, (2) the increased intersegmental interaction among the amide groups with the aid of hydrogen bonds, and (3) the reduced free volume due to the space filling effect by dendrimers, which retard gas diffusion through the outer skin of modified films. As verified by XPS in Table 1, the modification induced by G0 dendrimer reaches equilibrium at the outer skin after 24 h of immersion time. However, the gas permeability shown in Figure 9 still goes down when the modification time exceeds 24 h. This may have resulted from the fact that the G0 dendrimer will continuously diffuse into the polymer matrix deeper than the response depth of XPS and increase the gas barrier. Bigger size PAMAM den-

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Figure 9. Immersion time effects on performance of modified polyimide films excluding solvent swelling effects.

Figure 10. CO2 and CH4 sorption isotherms at 35 °C in original and 1 day modified polyimide films.

drimers also can penetrate slightly into the polymer matrix and, consequently, open the polyimide chains. Thus the gas permeabilities of polyimide films modified by G1 and G2 dendrimers increase slightly when the immersion time exceeds 24 h. Analogous results were reported by Wind et al.31,32 The gas permeability decrease is in the order of CH4 > N2 > CO2 > O2 > He and close to the order of the Lennard-Jones diameter of gas molecules except for CO2. The abnormal behavior of CO2 could be explained by the strong interaction between CO2 and PAMAM dendrimers, reported by Sirkar and co-workers,40-42 or by its molecular linearity.53 It was proven by XPS that the loaded PAMAM dendrimers could provide a high concentration of free primary amine groups, which may react with carbon dioxide according to CO2 + 2RNH2 T RHNCOO- + RNH3+. This interaction may be verified with the aid of sorption tests. Figure 10 depicts CO2 and CH4 sorption isotherms of the original and 1 day modified polyimide films under 35 °C. Because the cross-linking modification reduces free volume, the solubility of CH4 in the modified polyimide decreases because of a much denser film structure. However, the solubility of CO2 in the modified polyimide (53) Shieh, J. J.; Chung, T. S. Gas permeability, diffusivity and solubility of poly(4-vinylpyridine) film. J. Polym. Sci., Part B: Polym. Phys. 1999, 37, 2851.

remains almost the same as it in the original polyimide. This implies the effects of cross-linking and reduced free volume are offset by the interaction between dendrimer and CO2, which increases the solubility of CO2 in modified polyimide films. Therefore, compared with other gases, CO2’s transport might be facilitated by PAMAM dendrimer modification. Figure 11 illustrates the effect of immersion time on ideal gas selectivities for He/N2, O2/N2, CO2/CH4, and CO2/ N2. The improvement of He/N2 selectivity mainly resulted from the reduced interstitial space of polymer chains after the PAMAM dendrimer modification. The ideal He/N2 selectivity of G0 PAMAM dendrimer modified films increases 10 times after a long immersion time, while modifications made from higher generation dendrimers only double this gas pair selectivity. This resulted from stronger ability of G0 PAMAM dendrimer to modify the structure of polyimide films. Nevertheless, the ideal selectivity for O2/N2 does not change significantly because of their closer diameters (3.46 Å/3.64 Å) compared with He/N2 (2.6 Å/3.64 Å). Although the collision diameter of CO2 (4.0 Å) is relatively close to that of CH4 (3.82 Å) and N2 (3.64 Å), the ideal selectivity of PAMAM dendrimer modified polyimide films for both CO2/CH4 and CO2/N2 are improved to almost 200% because PAMAM dendrimers present particular affinity to CO2. Consequently, the CO2/CH4 separation properties of the modified polyimides go above the “upper bound” as illustrated in Figure 12, which signifies great potential in the application of natural gas separation. 4. Conclusion 6FDA-polyimide films were modified by G0, G1, and G2 PAMAM dendrimers using a simple immersion procedure at room temperature and characterized by AFM, XPS, ATR-FTIR spectroscopy, and gas transport measurements. Compared with simulated results, the morphology and conformation of grafted PAMAM dendrimers on polymer surfaces are disk-shaped molecular clusters investigated by AFM. The amidation was proven and quantified by XPS and ATR-FTIR. In addition, gel content

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Figure 11. Immersion time effect on gas selectivities of modified 6FDA-polyimide films.

two important factors in determining the characteristics of modified polyimide films. Longer immersion time yields higher dendrimer loading; equilibrium may be reached at or after 24 h of modification. Due to their bigger molecular size, higher generation dendrimers have less effect on film properties. Modified polyimide films also exhibited better gas separation performance, especially for CO2/CH4, because of the modified structure and the affinity between PAMAM dendrimer and CO2. The residual primary amine groups of dendrimers fixed onto the polyimide surface are available for further chemical reaction to achieve novel functional materials.

Figure 12. The “tradeoff” line for the CO2/CH4 separation.

tests verified the formation of cross-linking structure. Immersion time and different dendrimer generation are

Acknowledgment. The authors thank A*Star and NUS for funding this research under Grant Numbers R-279-000-113-304 and R-279-000-108-112, respectively. Special thanks are due to Dr. Chun Cao, Ms. Pei Shi Tin, and Mrs. Huaimin Guan for their useful assistance. LA049060Z