Confinement-induced vitrification in polyethylene terephthalate

Abstract

Dynamic mechanical thermal analysis performed on cold-drawn polyethylene terephthalate ͑PET͒, cold crystallized ͑annealed͒ in the temperature interval 100-140°C, reveals the presence of marginally glassy domains above the annealing temperature T a . This suggests that the thermodynamic force driving crystallization causes the structural arrest of some noncrystalline domains. The latter thus need a temperature higher than T a to completely defreeze. Differential scanning calorimetry supports this point of view. Analogous investigations on unoriented PET, cold crystallized in the same conditions, do not show the same peculiarities; thus, chain orientation is relevant to vitrification. This phenomenology is first cast in the language of thermodynamics by introducing an excess chemical potential ␦ describing the presence of structural constraints in the amorphous domains and the effect of chain orientation. For a first test of this picture, the orientation contribution to ␦ is calculated by means of the Gaussian chain model ͑this implicitly assumes that ␦ is related to the density fluctuations͒. The resulting expression is then used to discuss the structural differences between cold-drawn and unoriented PET samples reported in the literature.