Comparison of glass formation kinetics and segmental relaxation in polymers

Abstract

Comparisons were made of the fragility ( T g-normalized temperature dependence of segmental relaxation times) derived from mechanical and dielectric measurements to the same quantity determined from the activation energy for enthalpy relaxation as measured by differential scanning calorimetry (DSC). For linear polystyrenes of varying molecular weight, as well as for several other polymers, there was near quantitative agreement between the two measures of fragility. However, for poly(vinylethylene) (PVE) networks, whose normalized temperature dependence by either method increased with increasing crosslink density, the relationship was not quantitative. In contrast to the assessment of fragility, the shape (breadth) of the relaxation function cannot be reliably obtained from DSC measurements. The analysis yields results which depend on thermal history, contradicting the method’s assumption of thermorheological simplicity.