Investigations of the equilibrium melting temperature in PET and PC/PBT blends

The melting behavior of poly(butylene terephthalate) and its blends with bisphenol-A polycarbonate was investigated with differential scanning calorimetry. The aim of this work was to determine the equilibrium melting temperature and its dependence on the blend composition using the Hoffman-Weeks plots. It is shown that the critical analysis of various influences on the melting peak is necessary for the reorganization processes and crystallized content of blends. The experimental conditions and the corrections of measured temperatures were derived and discussed. It was found that the use of the extrapolated onset temperature T-m,T-o of the melting peak is more efficient than the maximum temperature T-m for the Hoffman-Weeks plots. The equilibrium values of pure PET are determined to be T-m,o(o) = 501 K and T-m(o) = 506 K. The equilibrium temperatures of the blends do not show a depression with increasing PC content. Using the Nishi-Wang relation, the results can be qualitatively interpreted with a polymer-polymer interaction coefficient chi greater than or equal to 0 between both components. A weak increase in the equilibrium temperature with increasing PC content was observed. A hypothesis to explain this is based on the possibility of a changed population of the different spherulites with various melting temperatures in dependence on PC content. (C) 1996 John Wiley & Sons, Inc.