Optical anisotropy of structurally modified polycarbonates having cyclohexylidene and methyl substituents using the rotational isomeric state method

Abstract

An extension of the valence optical scheme-rotational isomeric state method (RIS) applicable to calculation of polarizability tensors and optical anisotropy (γ²) of molecular fragments constituting substituted polycarbonate chains is presented. The theoretical approach utilizes experimentally derived anisotropic polarizability tensors of molecular groups in order to be able to account for induction effects and interdependence of backbone conformational states. The presence of a cyclohexylidene group at the bisphenyl C_α carbon significantly lowers γ² due to the low intrinsic polarizability of the cycloaliphatic substituent and due to the conformational states of backbone phenylene rings. Optical anisotropy value of the bisphenyl containing methyl substituents on the phenyl rings is lower than the value for the unsubstituted bisphenyl. Among the structures investigated here, the repeat unit containing cyclohexylidene group at C_α and methyl groups on phenylene rings, leads to the polycarbonate that shows the lowest optical anisotropy. Quantitatively, cyclohexylidene at C_α is more effective in lowering the optical anisotropy than methyl groups on backbone phenylene rings. Calculated <γ²>/x of the repeat units follows a linear behavior with respect to experimentally observed stress-optical coefficient of these polycarbonates in the melt (C_m). Calculated <γ²>/x of these structurally modified polycarbonate chains are all lower than that of BPAPC, and the relative trend in <γ²>/x is similar to that observed for C_m and C_g (glassy state) from experiments in the literature.