Melting and reorganization of the crystalline fraction and relaxation of the rigid amorphous fraction of isotactic polystyrene on fast heating (30,000 K/min)

Abstract

For polymers the origin of the multiple melting peaks observed in DSC curves is still controversially discussed. This is due to the difficulty to investigate the melting of the originally formed crystals exclusively. Recrystallization is a fast process and most experimental techniques applied so far do not allow fast heating in order to prevent recrystallization totally. Developments in thin-film (chip) calorimetry allow scanning rates as high as several thousand Kelvin per second. We utilized a chip calorimeter based on a commercially available vacuum gauge, which is operated under non-adiabatic conditions. The calorimeter was used to study the melting of isothermally crystallized isotactic polystyrene (iPS). Our results on melting at rates up to 30,000K/min (500K/s) give evidence for the validity of a melting–recrystallization–remelting process for iPS at low scanning rates (DSC). At isothermal conditions iPS forms crystals, which all melt within a few dozens of K slightly above the isothermal crystallization temperature. There is no evidence for the formation of multimodal populations of crystals with significantly different stability (melting temperatures). Furthermore, relaxation (devitrification) of the rigid amorphous fraction occurs in parallel to melting. Superheating of the crystals is of the order of 25K at 30,000K/min.