Confinement-induced vitrification in polyethylene terephthalate

Abstract

Dynamic mechanical thermal analysis performed on cold-drawn polyethylene terephthalate (PET), cold crystallized (annealed) in the temperature interval $100--140\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, 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 $\ensuremath{\delta}\ensuremath{\mu}$ 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 $\ensuremath{\delta}\ensuremath{\mu}$ is calculated by means of the Gaussian chain model (this implicitly assumes that $\ensuremath{\delta}\ensuremath{\mu}$ is related to the density fluctuations). The resulting expression is then used to discuss the structural differences between cold-drawn and unoriented PET samples reported in the literature.