Analysis of stress relaxation data for linear aromatic polyesters

The stress relaxation of linear aromatic polyesters was measured at various temperatures for 1 h at a strain of less than about 0,7%. The polyesters of the terephthalate series had the highest relaxation moduli, E(t), when compared with the isophthalate and copolyester series of the same methylene sequence lengths at a fixed temperature and time. Within the individual series, the E(t) decreased with increasing methylene sequence length. Time‐temperature superposition was found to be applicable to the viscoelastic behaviours of only some of the semicrystalline polyesters under certain conditions. Subsequently, master curves for these polyesters were constructed. The data were studied by means of a generalized Maxwell model which fitted the corresponding master curves exactly. By the use of Andrews' second approximation, the relaxation time spectrum of these polyesters was obtained over a very wide time scale range. The resultant spectrum was fairly high and flat for the terephthalate and isophthalate series while that of the copolyester series resembled that of a typical amorphous polymer.