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PAMAM Dendrimer-Induced Cross-Linking Modification of Polyimide Membranes Tai-Shung Chung,*,† Mei Lin Chng,† K. P. Pramoda,‡ and Youchang Xiao† Department of Chemical & Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 and Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602 Received April 9, 2003. In Final Form: January 7, 2004
Introduction Gas separation by glassy polymer membranes is known to have excellent technical and commercial potential. However, for large-scale gas separation, not only a good separation property but also the ability to work in a harsh and complex environment is required.1 To overcome the challenges mentioned earlier, lots of works2-9 were performed to cross-link polyimide structure. Staudt-Bickel and Koros found that co-polyimides containing strong polar carboxylic acid can be cross-linked by ethylene glycol with thermal treatment.2 Wind et al. also used ethylene glycol and aluminum acetylacetonate to cross-link the above copolyimides in order to suppress the undesirable plasticization effect in CO2/CH4 separation.3 Kita et al. applied UV light to induce photochemical cross-linking reactions in benzophenone-containing polyimides.4 Bos et al. successfully stabilized Matrimid films by forming a semiinterpenetrating network with oligopolyimide containing acetylene end groups at an elevated temperature5 and by cross-linking with heat treatment at 300 °C.6 Hayes invented a new way of cross-linking polyimide by immersing the polymer in an amino compound solution followed by thermal treatment.7-8 Liu et al. used pxylenediamine as a cross-linking agent to modify polyimide membrane at ambient temperature.9 It has been proven that the cross-linked membrane will not plasticize easily since cross-linking structure will prevent the material from swelling in the presence of plasticizing agents as well as promote chemical and thermal stability.1 With * To whom correspondence should be addressed. Fax: (65)67791936. E-mail:
[email protected]. † National University of Singapore. ‡ Institute of Materials Research and Engineering. (1) Koros, W. J.; Mahajan, R. Pushing the limits on possibilities for large scale gas separation: which strategies? J. Membr. Sci. 2000, 175, 181. (2) Staudt-Bickel, C.; Koros, W. J. Improvement of CO2/CH4 separation characteristic of polyimides by chemical crosslinking. J. Membr. Sci. 1999, 155, 145. (3) 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. (4) Kita, H.; Inada, T.; Tanaka, K.; Okamoto, K. Effect of photocrosslinking on permeability and permselectivity of gases through benzophenone-containing polyimide. J. Membr. Sci. 1994, 87, 139. (5) 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. (6) 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. (7) Hayes, R. A. Polyimide gas separation membranes. U.S. Patent 4,717,393, 1988. (8) Hayes, R. A. Amine-modified polyimide membranes. U.S. Patent 4,981,497, 1991. (9) Liu, Y.; Wang, R.; Chung, T. S. Chemical cross-linking modification of polyimide membranes for gas separation. J. Membr. Sci. 2001, 189, 231.
increasing degree of cross-linking, higher gas selectivity can be achieved because of reduced polymer swelling and chain mobility. However, the gas permeability of those cross-linked polymer membranes is also decreased as a result of lower free volume. Starburst dendrimer discovered by Tomalia10,11 resembles a Cayley tree, which is a regularly branched polymer with a starlike cascade topology. Their high density of terminal amine group provides a large number of reactive sites for many potential applications such as acting as a vehicle for controlled-release systems and as molecular scaffolds for chemical catalysts.12-13 Several researchers add PAMAM dendrimers on polymer surfaces or into polymer blocks in order to achieve novel functional materials.14-18 Recently, Kovvali et al. used dendrimers as CO2-facilitated transporter in polymer films to improve the CO2 selectivity.19-21 In our study, PAMAM (polyamidoamine) dendrimer, generation 0, was employed as the novel cross-linking agent at ambient temperature. The modified polyimide membranes not only have good chemical and physical stability but also excellent permeability and permselectivity. The effects of modification on solubility and gasseparation properties of the modified polyimide were investigated. The cross-linking process was monitored by XPS, FTIR-ATR, and XRD to explore the possible mechanisms of cross-linking reaction. Experimental Section Materials and Dense Film Preparation. The chemical structure of 6FDA-durene polyimide is shown in Figure 1. It was synthesized in our lab.9 A 2% (w/w) polymer solution was prepared by dissolving the polyimide in dicholoromethane. The polymer solution was then filtered through a Whatman filter (1µm) and cast onto a silicon wafer at ambient temperature. After slow solvent evaporation, the film was dried in a vacuum oven at 250 °C for 48 h to remove the residual solvent. After the drying (10) Tomalia, D. A.; Baker, H.; Dewald, J.; Hall, M.; Kallos, G.; Martin, S.; Roeck, J.; Ryder, J.; Smith, P. Polym. J. (Tokyo) 1985, 17, 117. (11) 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. (12) Bosman, A. W.; Janssen, H. M.; Meijer, E. W. About dendrimers: structure, physical properties, and applications. Chem. Rev. 1999, 99, 1665. (13) 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. (14) Sui, G.; Micic, M.; Huo, Q.; Leblanc, R. M. Synthesis and surface chemistry study of a new amphiphilic PAMAM dendrimer. Langmuir 2000, 16, 7847. (15) 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. (16) Zhao, M. Q.; Liu, Y. L.; Crooks, R. M.; Bergbreiter, D. E. Preparation of highly impermeable hyperbranched polymer thin-film coating using dendrimers first as building blocks and then as in situ thermosetting agents. J. Am. Chem. Soc. 1999, 121, 923. (17) Mackay, M. E.; Hong, Y.; Jeong, M.; Hong, S.; Russell, T. P.; Hawker, C. J.; Vestberg, R.; Douglas, J. F. Influence of dendrimer additives on the dewetting of thin polystyrene films. Langmuir 2002, 18, 1877. (18) Cha, B. J.; Kang, Y. S.; Won, J. Preparation and characterization of dendrimer layers on poly(dimethylsiloxane) films. Macromolecules 2001, 34, 6631. (19) Kovvali, A. S.; Chen, H.; Sirkar, K. K. Dendrimer membranes: A CO2-selective molecular gate. J. Am. Chem. Soc. 2000, 122, 7549. (20) Kovvali, A. S.; Sirkar, K. K. Dendrimer Liquid Membranes: CO2 separation from gas mixtures. Ind. Eng. Chem. Res. 2001, 40, 2502. (21) Kovvali, A. S.; Sirkar, K. K. Carbon dioxides separation with novel solvents as liquid membranes. Ind. Eng. Chem. Res. 2002, 41, 2287.
10.1021/la034610z CCC: $27.50 © 2004 American Chemical Society Published on Web 03/02/2004
Notes
Langmuir, Vol. 20, No. 7, 2004 2967 Table 1. Gel Contents of Chemical Cross-Linked Polyimides immersion time
Figure 1. Chemical structure of 6FDA-Durene polyimide.
percentage of gelation (%)
unmodified 20 min 60 min 24 hours 7 days 0
13.49
53.13
92.93
95.83
Table 2. Dielectric Constants of Chemical Cross-Linked Polyimides immersion time
dielectric constant (1 Hz)
dielectric constant (100 kHz)
unmodified 20 min 60 min 24 h 7 days
2.03 2.37 2.48 3.18 3.47
2.00 2.31 2.39 2.95 2.99
Table 3. Effect of Immersion Time on N and F Elements and N to F Ratio on Polyimide Surfaces
Figure 2. Chemical structure of PAMAM dendrimer.
immersion time
F atom concentration %
N atom concentration %
N to F ratio
procedure, all membranes were cut into circles of 38 mm diameter and their thicknesses were measured by a micrometer. Only the membranes with about 50 µm thickness were used in the following studies. Membrane Modification. The chemical structure of generation 0 PAMAM dendrimer containing four free amine groups is shown in Figure 2. The modification was carried out by immersing polymer films into a 5% (w/v) dendrimer methanol solution for certain period of time, after which the films were taken out followed by washing with fresh methanol and drying at ambient temperature. Characterization. The gel contents of the cross-linked polyimides were measured by extracting the films in dichloromethane for 24 h. The insoluble fractions were dried at 150 °C in a vacuum oven for 24 h to constant weight. The gel contents were calculated by gel % ) W1/W0 , where W0 and W1 are the original weight and the insoluble fraction weight of the polyimide films, respectively. Their dielectric properties were also investigated using the DEA 2970 Di-electric Analyzer. An X-ray photoelectron spectrometer (Kratos XPS System-AXIS His-165 Ultra) was used to measure the surface elements of the polymer film. ATR measurements were carried out using a Perkin-Elmer FTIR microscope. X-ray diffraction (XRD) spectra were recorded using a Bruker X-ray diffractometer (equipped with a 2D detector) in reflection mode with 2θ scanned between 2° and 30° using nickel-filtered Cu KR1 radiation (λ ) 0.15418 nm) under a voltage of 40 kV and a current of 40 mA. Gas Permeation Experiments. The pure gas permeabilities for O2, N2, CH4, and CO2 were obtained by a constant volume method at 35 °C and 10 atm. A detailed description of the permeation cell design and testing conditions can be found elsewhere.22 The ideal permselectivity is defined as RA/B ) PA/PB, where PA and PB are the permeabilities of gas A and B. The apparent diffusion coefficient Dapp is obtained from the timelag (θ) as Dapp ) L2/6θ, where θ is the diffusion time-lag. L is the thickness of films. The apparent solubility coefficient Sapp is evaluated by the following expression: Sapp ) P/Dapp.
unmodified 20 min 60 min 24 h 7 days
9.39 6.68 13.47 7.53 4.89
2.48 3.68 9.02 10.08 6.1
0.26 0.55 0.67 1.34 1.25
Results and Discussion Characterization of the Cross-Linked Polyimides. The gel contents of the modified films were measured in dicholoromethane, which is the solvent used to cast 6FDAdurene dense films. Table 1 summarizes that the gel content increases with an increase in the immersion time, indicating that there is a reaction between the polymer and dendrimer, and this reaction decreases the solubility (22) 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.
of the polymer solvent. Compared with ref 9, the gel content of these cross-linked films increases more slowly, as the big molecular size of PAMAM dendrimer needs more time to penetrate into the polymer matrix. A dielectric analyzer was also used to observe the changes of polymer chains during the modification process. Table 2 shows that the dielectric constant increases with prolonged immersion time. The increase of the dielectric constant may be due to the decrease of the polymer chains’ mobility and free volume after modification. For the unmodified 6FDA-durene, its dielectric constant is very low because the polymer chains, which have many polar and bulky side groups, can rotate freely. However, the modification reduces the free volume and restricts chain mobility, thus making the polymer more polar and increasing the dielectric constant in the process. Table 3 lists XPS measurements of the ratio of elements N to F. During the modification process, the content of F elements in polyimides remains constant. Thus, the increase of the ratio of N to F indicates that the N element content is increased during the cross-linking reaction. After an immersion time of 24 h, the N content does not increase further, indicating saturation of the modification reaction on the surface. Figure 3 illustrates the XPS peak of N 1s and shows that the bonding energy of N 1s moves toward the lowenergy direction during the modification process. We thus conclude that the chemical environment of N is changed in the modification process. The chemical structure changes during the cross-linking process were also monitored by ATR and are shown in Figure 4. The intensities of the characteristic peaks of imide group at 1780, 1728, and 1380 cm-1 decrease with increasing immersion time. Meanwhile, the characteristic peaks of amide groups at 1656, 1550, and 3300 cm-1 (attributed to N-H bond) appear and become stronger with immersion time. Figure 5 illustrates the cross-linking structure drawn by Chem3D, where amine groups of PAMAM react with some imide groups of 6FDA-durene to form amide bonds. Since PAMAM contains four free amine groups, the resulting cross-linked structure is more complicated than those induced by two functional end groups.2-3,7-9
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Figure 3. Effect of immersion time on N 1s’ XPS of polyimides.
Figure 4. Comparison of FTIR spectra of 6FDA-durene.
Dielectric constant measurements already proved that dendrimer molecules will fill in the free volume between the polymer chains and cross-linking the polymer chains to reduce the free volume and restricts chain mobility of polyimide. With this space-filling effect, due to the bulk of dendrimer molecules, D-spacing should be changed during modification. D-spacing changes of modified polyimides were monitored by XRD. From the first peak listed in Table 4 we can conclude that cross-linking reagent opened the polymer’s chains when the immersion time is short and then chains became much tighter due to a higher degree of cross-linking when prolonging the immersion time. However, we cannot draw any conclusion from the second peak data. Gas Separation Properties of Cross-Linked Polyimides. The gas permeabilities of the cross-linked 6FDA-durene dense films are summarized in Table 5. During the immersion process, due to the slow diffusion of dendrimer, different degrees of cross-linking throughout the films may create asymmetric structure films. We only can get the apparent permeabilities of cross-linked films to show the effects of cross-linking modification on gasseparation properties. Since an increase in the immersion time leads to a higher degree of cross-linking, the gas permeability of the modified polyimides decreases with the immersion time like other cross-linked polymer films.2-9 However, the order of decrease is CH4 > N2 > O2 > CO2, which is different from other references.2-9 Except CO2, this decreasing order is of the same order as the kinetic diameter of penetrant gases. Table 6 tabulates the calculated diffusion coefficients and solubility coefficients from the time-lag method as a function of immersion time and indicates that the decrease in
Notes
permeability mainly results from the decrease in the diffusion coefficients because of cross-linking. Interestingly, the changes of solubility coefficients are not very obvious. Since the free volume, free volume distribution, and chain mobility affect the gas diffusion rate in a polymer matrix, an increase in the degree of cross-linking results in a reduction in the interstitial space among chains, chain mobility, and free volume. As a consequence, the diffusion coefficients decrease significantly following the order CH4 > CO2 > N2 > O2. However, the decreasing rate of diffusion coefficients of polymer cross-linked by PAMAM is significantly slower than using other crosslinking agents containing amino groups.9 This phenomenon may be due to the big molecular size of PAMAM dendrimer. Before reaction with the polymer chain, dendrimer molecules penetrate into the methanol-induced swollen polymer matrix.9 Although its large size opens the polymer chains as shown in Table 4, the cross-linking reaction restricts the mobility of polymer chains. As a result, the diffusion coefficients decrease very slowly if the immersion time is short. However, the increased intersegmental interaction among the newly formed amide groups with the aid of hydrogen bonds and the reduced free volume due to the space-filling effect of the PAMAM dendrimer become dominant factors if the immersion time increases, causing significant reductions in both the diffusion coefficient and permeability. A more interesting phenomenon is that the CO2 permeability increases at the beginning of modification and then decreases with an increase in immersion time. Table 6 shows that the CO2 solubility of the polyimides increases significantly after modification but decreases when the cross-linking density is very high. This may arise from the fact that PAMAM dendrimer provides all kinds of amine groups such as -NH2, -NH-, or -N
