Kahle, S. et al.:
Confirmation of a calorimetric peculiarity in the crossover region of glass
transition in poly(n-hexyl methacrylate) by differential scanning calorimetry
Kahle, S.; Hempel, E.; Beiner., M; Unger, R.; Schröter, K.; Donth, E.:
Confirmation of a calorimetric peculiarity in the crossover region of glass
transition in poly(n-hexyl methacrylate) by differential scanning calorimetry,
Journal of Molecular Structure 479 (2–3) (1999) 149–162,
DOI: 10.1016/S0022-2860(98)00866-7.
The Narayanaswamy model for vitrification is modified to deliver local information about the underlying equilibrium trace of calorimetric response in an Arrhenius plot. The analysis of a simple differential scanning calorimetry (DSC) thermogram for polystyrene reproduces the calorimetric equilibrium trace from heat capacity spectroscopy with an accuracy of few tenths of a frequency decade. Further, contour maps for linear dielecric, heat capacity, and shear response in the crossover region of dynamic glass transition in poly(n-hexyl methacrylate) are presented. The calorimetric map has a saddle between the death of the high-temperature process (a) and the onset of the low-temperature process (α). The sigmoid bend in the equilibrium trace near this peculiarity can independently be confirmed by the modified Narayanaswamy model evaluation of a simple DSC thermogram. A speculative, free-volume physical picture of the crossover region is described.
| Keywords: | Glass transition; Calorimetry; Vitrification; Free volume; Cooperativity; Strucrural relaxation; Secondary relaxation; Characteristic length; Enthalpy relaxation; Dielectric Relaxation; Cooperativity onset; Heat capacity |