Is devitrification of the rigid amorphous fraction of semicrystalline polymers a prerequisite for reversing melting?

摘要

Baseline heat capacity of semicrystalline polymers is the superposition of the heat capacity of the solid (crystalline and rigid amorphous) and the liquid (mobile amorphous) fraction. Under certain conditions, which will be discussed in detail, TMDSC allows for the direct measurement of baseline heat capacity. But the occurrence of an excess heat capacity, which is often observed for semicrystalline polymers, makes it impossible to determine fractions of different molecular mobility by means of the measured heat capacity. The question arises if there is some coupling between the rigid amorphous fraction and the occurrence of reversible melting (excess heat capacity). Baseline and excess heat capacity are distinguished by means of frequency dependence of measured heat capacity. If base line heat capacity is measured no frequency dependence is observed while excess heat capacity due to reversing melting always shows frequency dependence. Fractions of different mobility are additionally determined by means of solid state NMR techniques. We present temperature and frequency dependent heat capacity data for poly(3- hydroxybutyrate) (PHB) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) copolymers. For pure PHB an excess heat capacity is observed above the isothermal crystallization temperature (80°C) while for the copolymer with 12% PHBV frequency dependent excess heat capacity is observed already at lower temperatures. For PHB there is good agreement between the fractions of different mobility determined by calorimetry and NMR. For the copolymers reliable fractions of different mobility (mobile and rigid amorphous) are therefore available from calorimetry in a limited temperature range only. While for PHB reversing melting is observed at temperatures where the rigid amorphous fraction is devitrified such strong coupling does not exist for the copolymers.