Gas transport properties of polysulphones: 1. Role of symmetry of methyl group placement on bisphenol rings

Abstract

Gas sorption and transport properties are reported for a series of polysulphones having either two or four phenylene hydrogens per repeat unit replaced with methyl groups. The results for tetramethyl bisphenol A polysulphone (TMPSF), dimethyl bisphenol A polysulphone (DMPSF) and dimethyl bisphenol Z polysulphone (DMPSF-Z) are compared to unsubstituted bisphenol A polysulphone (PSF). The effect of the substituents on chain mobility and chain packing has been related to the gas transport properties. Dynamic mechanical thermal analysis and differential scanning calorimetry were used to judge chain mobility, while X-ray diffraction and free-volume calculations give information about chain packing. Permeability measurements were made for He, H 2, O 2, N 2, CH 4 and CO 2 at 35°C over a range of pressures up to 20 atm. Sorption experiments were also done for N 2, CH 4, and CO 2 under the same conditions. The permeability coefficients of these polymers rank in the order: TMPSF ≧ PSF ≧ DMPSF > DMPSF-Z for all of the gases. This order correlates well with free volume as well as with the γ transition temperature. The symmetric methyl substitution of TMPSF yields a relatively open structure with improved separation characteristics over PSF. The asymmetric substitution results in tightly packed structures for the dimethyl materials that are highly selective but less permeable than PSF or TMPSF. The flexible cyclohexyl substituent of DMPSF-Z further reduces the gas permeability and improves the selectivity.