Aliphatic polyesters as models for relaxation processes in crystalline polymers: 2. Dielectric relaxation in copolymers of adipic acid with 1,6 and 2,5-hexanediols

Abstract

Linear aliphatic polyesters are interesting model systems for studying relaxation processes in crystalline polymers because they show both γ and β relaxations in a prominent way whereas in many highly crystalline linear polymers (LPE, POM etc.) the β process is suppressed and, unlike branched polyethylene which also has a prominent β process, the polyesters have no chemical structural complications due to branches. To study the hypothesis that variations in the appearance of the β (glass-rubber) relaxation are due to varying degrees of immobilization of long-range segmental motion in the amorphous fraction by the presence of the crystal phase measurements are made of dielectric relaxation in a series of the title copolymers that span the crystallinity range of zero (homopolymer with 2,5-hexanediol) to ≈60% (homopolymer with 1,6-hexanediol). The data are analysed in terms of the effect of degree of crystallinity on both kinetic parameters (activation energies and relaxation width) and equilibrium parameters (dipole correlation factor deduced from relaxation strength). In the latter case special care is taken to properly assess composite mixture effects on the relaxation strength. The shape of the β process is extremely sensitive to the degree of crystallinity, broadening greatly with the onset of the presence of crystallinity and increasing in broadness with increasing degree of crystallinity. Although less sensitive than the width, the β relaxation dipole correlation factor depends on crystallinity also, the availability of chain configurations within the amorphous fraction being noticeably reduced in the crystalline specimens. The γ process kinetic parameters are relatively insensitive to the presence of the crystal phase, a behaviour consistent with a localized motion mechanism. The γ relaxation strength correlates with the process having an amorphous phase origin if it is assumed that the 2,5 diol units contribute less to relaxation strength than the 1,6 diol units; a result also consistent with the localized motion concept.