Chapter 18
Indan-Containing Polymers for Gas Separation Membranes 1
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Gerhard Maier , Martin Wolf , Miroslav Bleha , and Zbynek Pientka
Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on June 23, 2016 | http://pubs.acs.org Publication Date: September 2, 1999 | doi: 10.1021/bk-1999-0733.ch018
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Technische Universität München, Lichtenbergstraβe 4, D-85747 Garching, Germany Institute of Macromolecular Chemistry, Heyrovsky Sq. 2, Prague 6 CZ-16206, Czech Republic
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The permselectivity properties of two series of poly(ether ketone)s and polyimides containing various bulky indan groups in the main chain are described. Hydrogen, carbon dioxide, oxygen, and nitrogen permeabilities were determined. Despite their bulkiness, the indan groups did not lead to increased permeability coefficients or selectivities. This result is explained by the presence of relatively flexible phenylether linkages between the ri gid indan groups.
Amorphous polymers with relatively high glass transition temperatures such as poly imides, poly(aryl ether)s, or polycarbonates are generally promising candidates for gas separation membranes (1-4). This view is based on a widely accepted model for the transport of gas molecules through a polymer matrix (1-4). According to this model, gas transport occurs by diffusional "jumps" of the penetrant molecules as a result of thermally activated motions of segments of the polymer backbone. These segmental motions open "jump channels", through which the gas molecules can pass. For a binary gas pair, selection based on penetrant size occurs by the opening of channels which are wide enough for passage of only the smaller penetrant, but are too narrow for the larger one. The channel width depends on the polymer backbone flexibility. More flexible chains allow wider openings, resulting in decreased selectivity. From this model, a ge neral principle for simultaneous improvement of permeability coefficients and selecti vity was deduced in order to overcome the undesired common "trade-off" behavior (14) between permeability and selectivity. Increased rigidity of the polymer main chain is expected to increase selectivity. At the same time, the introduction of bulky, rigid groups leads to increased fractional free volume and hence increased permeability co efficients. Many attempts have been made to optimize polymer structures using this strate gy. The results of the structural modifications could be qualitatively understood in most cases on the basis of main chain flexibility, bulkiness of certain structural ele-
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© 1999 American Chemical Society
Freeman and Pinnau; Polymer Membranes for Gas and Vapor Separation ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
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257 ments, packing density, and fractional free volume. However, quantitative predictions of how a certain polymer structure modification will affect its permeability behavior are still very difficult. In a recent paper (5), increments for the permeability coeffi cients of a number of structural elements were deduced. The application of these in crements for the calculation of permeability coefficients of a number of polymers for oxygen, nitrogen and helium was quite successful. However, the authors state that in crements for new groups can not be deduced simply based on the chemical structure. Thus, predictions for new polymer structures are not possible with this approach. In our opinion, a more detailed understanding of the relationship between the chemical structure of a polymer and its permselectivity characteristics can be achieved by identification of the segments which control diffusional jumps. For this purpose, we are studying various series of polymers containing rigid, bulky, substituted indan groups in the main chain, linked by phenylether segments of controlled flexibility. Ide ally, such polymer architectures should lead to a situation where the diffusional jumps are controlled predominantly by the flexible phenylether segments. If this hypothesis is confirmed, modifications of the chemical structure of these segments should allow op timization of the flexibility and hence selectivity for any gas pair, without reducing the permeability coefficients. This paper describes the results of permeability mea surements of two series of polyimides and poly(ether ketone)s with indan groups in the main chain. Polymer Structures The synthesis and characterization of the polymers have been described earlier (6-8) and will only be outlined briefly. First, the indan group 3 is prepared by cyclodimerization of an α-substituted styrene derivative 2 or the corresponding tertiary alcohol 1 under acidic conditions, as shown in Scheme 1 below.
R = methyl, cyclohexyl Scheme 1: Synthesis of indan groups
The 4-fluorobenzoyl groups are introduced by Friedel-Crafts acylation of the 3phenylindan derivative with 4-fluorobenzoic acid (Scheme 2).
Freeman and Pinnau; Polymer Membranes for Gas and Vapor Separation ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
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+2 F
Ρ
A1CU
R = methyl, cyclohexyl Scheme 2: Synthesis of monomers 5 (R = methyl) and 6 (R = cyclohexyl) Nucleophilic replacement of the fluorine atoms in both monomers by 4-hydroxyaniline results in the diamine monomers 7 and 8 (Scheme 3).
+2
H
°^O^NH
2
|K
