Molecular Motions of Poly(γ-monosubstituted-benzyl D-glutamate) in the Solid State

Abstract

Dielectric measurements were performed at frequencies ranging from 30 Hz to 300 KHz and at temperatures between −150 and 120°C for the following Poly(γ-monosubstituted-benzyl D-glutamate)s; Poly(γ-o-chlorobenzyl D-glutamate) (o-ClPBDG), Poly(γ-m-chlorobenzyl D-glutamate) (m-ClPBMDG), Poly(γ-p-chlorobenzyl D-glutamate) (p-ClPBDG), Poly(γ-p-nitrobenzyl D-glutamate) (p-NPBDG), Poly(γ-p-methylbenzyl D-glutamate) (pMePBDG) and a racemic mixture of p-ClPBDG and p-ClPBLG (p-ClPBDG+p-ClBLG). Each sample exhibited a dielectric loss maximum in the temperature range of 30—90°C. These loss maxima were attributed to the onset of large scale motion of side chains. The temperatures of maximum loss (the dispersion temperature) are shifted by the introduction of substituents. The dispersion temperature is highest for p-ClPBDG among three monochlorosubstituted PBDGs. For p-monosubstituted-PBDGs, p-NPBDG, which has the side chain with the largest dipole moment among three p-substituents studied, showed the highest dispersion temperature. The results are explained in terms of restriction caused by dipole–dipole interaction among dipoles in the side chain. The enormous relaxation strength of o-ClPBDG was considered to reflect a certain conformation of the side chain originating from steric hindrance between the ester and the o-substituent of a side chain. Racemic mixtures of p-ClPBDG and p-ClPBLG showed no evidence of the first-order solid–solid transition observed for racemic mixtures of PBDG and PBLG.