Gas permeation in polyarylates: effect of bisphenol and acid substitution symmetry

The effect of bisphenol substitution symmetry on gas permeation as well as other relevant structural properties of polyarylates has been investigated both experimentally and by molecular modelling. Asymmetric or di-substitution of methyl groups on the phenyl rings of bisphenol-A or phenolphthalein resulted in polyarylates with similar packing density and permeability and with increased permselectivity compared to the corresponding polymers with the unsubstituted bisphenols. Symmetric or tetra-substitution of methyl groups on the bisphenol-A phenyl rings led to polyarylates with decreased packing density, increased permeability and similar selectivities as the corresponding polymers with the unsubstituted bisphenol. Molecular modelling studies of the chain conformation gave further insight into the mechanism by which substitution symmetry affects the polymer properties. The differences in the minimum energy chain conformation of symmetrically and asymmetrically substituted bisphenol-A polyarylate chains help in explaining the variation in packing density and permeation properties. Calculations of relative bond flexibility and the energy barrier for bond rotation of specific moieties in the minimized energy chain conformation correlate with molecular mobility as measured by sub-Tg transition temperatures. The diacid used for polyarylate synthesis was also varied in order to investigate the effect of acid linkage symmetry. Polyarylates based on the above bisphenols and asymmetrically linked isophthalic acid were compared with the corresponding polymers based on symmetrically linked terephthalic acid or 2,6-naphthalene dicarboxylic acid. Isophthalic acid-based polyarylates had higher packing density, chain mobility, and permselectivity and lower permeability than their terephthalic acid-based counterparts. Incorporation of the naphthalene acid along with the terephthalic acid also results in polyarylates with lower packing density and higher chain rigidity than the isophthalic acid-based polymers.