Comparison of glass formation kinetics and segmental relaxation in polymers

Comparisons were made of the fragility (T-g-normalized temperature dependen ce of segmental relaxation times) derived from mechanical and dielectric me asurements to the same quantity determined from the activation energy for e nthalpy 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 meas ures of fragility. However, for poly(vinylethylene) (PVE) networks, whose n ormalized temperature dependence by either method increased with increasing crosslink density, the relationship was not quantitative. In contrast to t he assessment of fragility, the shape (breadth) of the relaxation function cannot be reliably obtained from DSC measurements. The analysis yields resu lts which depend on thermal history, contradicting the method's assumption of thermorheological simplicity. (C) 2000 Elsevier Science B.V. All rights reserved.