GAS PERMEATION IN POLYARYLATES - EFFECT OF BISPHENOL AND ACID SUBSTITUTION SYMMETRY

The effect of bisphenol substitution symmetry on gas permeation as wel l as other relevant structural properties of polyarylates has been inv estigated both experimentally and by molecular modelling. Asymmetric o r di-substitution of methyl groups on the phenyl rings of bisphenol-A or phenolphthalein resulted in polyarylates with similar packing densi ty and permeability and with increased permselectivity compared to the corresponding polymers with the unsubstituted bisphenols. Symmetric o r tetra-substitution of methyl groups on the bisphenol-A phenyl rings led to polyarylates with decreased packing density, increased permeabi lity and similar selectivities as the corresponding polymers with the unsubstituted bisphenol. Molecular modelling studies of the chain conf ormation gave further insight into the mechanism by which substitution symmetry affects the polymer properties. The differences in the minim um energy chain conformation of symmetrically and asymmetrically subst ituted bisphenol-A polyarylate chains help in explaining the variation in packing density and permeation properties. Calculations of relativ e bond flexibility and the energy barrier for bond rotation of specifi c moieties in the minimized energy chain conformation correlate with m olecular mobility as measured by sub-T-g transition temperatures. The diacid used for polyarylate synthesis was also varied in order to inve stigate the effect of acid linkage symmetry. Polyarylates based on the above bisphenols and asymmetrically linked isophthalic acid were comp ared with the corresponding polymers based on symmetrically linked ter ephthalic acid or 2,6-naphthalene dicarboxylic acid. Isophthalic acid- based polyarylates had higher packing density, chain mobility, and per mselectivity and lower permeability than their terephthalic acid-based counterparts. Incorporation of the naphthalene acid along with the te rephthalic acid also results in polyarylates with lower packing densit y and higher chain rigidity than the isophthalic acid-based polymers. (C) 1998 Elsevier Science Ltd. All rights reserved.