Residual strain recovery, molecular mobility, and plasticity of solid aromatic copolyesters of p-hydroxybenzoic acid with poly(ethylene terephthalate)

The processes of thermally stimulated recovery of internal energy stored by a specimen during deformation and of residual strain ɛres in weakly oriented liquid-crystalline 80: 20 and 60: 40 (by mole) copolyesters of p-hydroxybenzoic acid with PET, as well as in nonoriented (amorphous and crystalline) PET, were studied. All the materials were deformed via uniaxial compression at room temperature. It was shown that ɛres is built up in all phases that coexist in the materials: the glassy, crystalline, and intermediate phases, of which the last presumably contains imperfect crystals of a small size. Excess energy of strain is stored in the glassy phase with a built-in LC order and, probably, in copolyester crystallites. The main deformation processes in the glassy component of the materials are the nucleation and development of small-scale shear transformations in exactly the same manner as in the earlier studied glassy polymers that do not form an LC structure. Consequently, enhanced rigidity of the polyesters chains and their LC order have no qualitative effect on the mechanism of plastic deformation of the copolyester glasses. However, the LC structure leads to a decrease in the yield stress σy and the compression and shear moduli of the copolyesters in comparison with those of conventional glassy polymers i.e., reduces the resistance of the materials toward plastic deformation. With an increase in strain, various forms of Brownian motion of chains, beginning from the rotation of p-hydroxybenzoic acid fragments, become successively involved in the process of their deformation at room temperature. Correspondingly, the thermally stimulated recovery of ɛres exhibits peaks of small-scale motions of aromatic chain fragments in the glassy phase of copolyesters, a phenomenon that is not observed in PET and other aliphatic polymer glasses. The intensity of these peaks depends on the value of ɛres built up by the specimen. Even small strains (≈5−40%) in the specimens irreversibly destroy the initial orientation of chains in copolyesters. To explain this effect, the concept of domain disorientation of their structure is proposed.