Sulfonated copolyimides: influence of structural parameters on gas separation properties

Structure-properties relationships in terms of gas transport have been established for a large series of sulfonated copolyimides. The permeability and diffusion coefficients of different gases (H2, O2, CO2, N2) differing by their size and interaction capacity towards the polymers have been determined. CO2 sorption experiments have also been carried out allowing a direct determination of the solubility coefficients. The analysis of the transport parameters as a function of the copolyimides structure and composition underlines the predominant role of the non-sulfonated sequences and shows that the permeability is mainly governed by the diffusion coefficient. A large range of permeability values can thus be obtained by varying the nature and the content of the non-sulfonated diamine in the copolyimides and for some copolymers, H2 and CO2 permeability values are similar. Furthermore, O2/N2 selectivity coefficients determined for all the membranes are located near the Robeson curve. As a consequence, some systems, notably those based on (CARDO/ODA) diamines mixture, present a good selectivity/permeability balance.

Non-sulfonated diamine Pe H2 Pe CO2 Pe O2 (barrer) (barrer) (barrer) Pe N2 (barrer) CARDO/ODA CARDO BAPHF ODA 1.7 4.5 0.64 0.1 5.2 4.6 0.83 (CARDO: 9,9-bis(4-aminophenyl fluorene); ODA: oxydianiline; BAPHF: bis [4-(4-aminophenoxyphenyl) hexafluoropropane] dioxide and this can be in part explained by the high CO2 solubility coefficients measured for these materials. Concerning now the selectivity factors, they have been calculated for O2/N2 from the permeability values. They are all located near the curve depicted by Robeson [5] (Fig. 3). It can be observed that introducing small amounts of sulfonated sequences in CARDO/ODA based polyimide greatly improve Sulfonated copolyimides represent an interesting structural family for both fundamental understanding of transport mechanism and selective applications. Indeed, the possibility of governing the structure and composition of the copolymers by the choice of the non sulfonated diamine, its content and the length of the blocks in the copolymer allows to improve the comprehension of the transport mechanisms in these hydrophobic/ hydrophilic materials. Furthermore the great potentiality of these materials in the domain of gas separation applications has been demonstrated and copolymers with interesting gas fluxes and selectivity factors have been synthesized. Further work is now under progress in order to improve these properties.