Element-specific study of local segmental dynamics of polyethylene terephthalate upon physical aging

Time-dependent relaxation processes upon physical aging below the glass transition temperature have been studied in polyethylene terephthalate by high-precision dilatometry (DLT), differential scanning calorimetry (DSC), and element-specific positron and positronium (Ps) annihilation spectroscopy. The macroscopic volume change observed by DLT can be described by the Kohlrausch-Williams-Watts decay function, whereas changes in the relaxation enthalpies evaluated by DSC and free volumes probed by positron and Ps annihilation spectroscopy are reproduced by two superimposed exponentials. The multi-method approach reveals three kinds of relaxation processes with characteristic relaxation times: (a) fast Arrhenius-type β relaxation involving the instantaneous local segmental densification along with the exclusion of oxygen atoms from free volumes, (b) macroscopically observable non-Arrhenius-type α relaxation originated from a distribution of relaxation times due to the heterogeneous dynamics of solid-state- and liquid-state-like local segments, and (c) extremely slow Arrhenius-type α relaxation as the consequence of a uniform relaxation time solely due to the thermal dependence of nanometer-scale solid-state-like local segments.