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 heatrncapacity of the solid (crystalline and rigid amorphous) and the liquid (mobile amorphous)rnfraction. Under certain conditions, which will be discussed in detail, TMDSC allows for therndirect measurement of baseline heat capacity. But the occurrence of an excess heat capacity,rnwhich is often observed for semicrystalline polymers, makes it impossible to determine fractionsrnof different molecular mobility by means of the measured heat capacity. The question arises ifrnthere is some coupling between the rigid amorphous fraction and the occurrence of reversiblernmelting (excess heat capacity). Baseline and excess heat capacity are distinguished by means ofrnfrequency dependence of measured heat capacity. If base line heat capacity is measured nornfrequency dependence is observed while excess heat capacity due to reversing melting alwaysrnshows frequency dependence. Fractions of different mobility are additionally determined byrnmeans of solid state NMR techniques.rnWe present temperature and frequency dependent heat capacity data for poly(3-rnhydroxybutyrate) (PHB) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV)rncopolymers. For pure PHB an excess heat capacity is observed above the isothermalrncrystallization temperature (80℃) while for the copolymer with 12% PHBV frequencyrndependent excess heat capacity is observed already at lower temperatures. For PHB there is goodrnagreement between the fractions of different mobility determined by calorimetry and NMR. Forrnthe copolymers reliable fractions of different mobility (mobile and rigid amorphous) arerntherefore available from calorimetry in a limited temperature range only. While for PHBrnreversing melting is observed at temperatures where the rigid amorphous fraction is devitrifiedrnsuch strong coupling does not exist for the copolymers.