Thermal decomposition of poly(.BETA.-hydroxybutyrate) and its copolymer.

Abstract

Thermal decomposition processes of poly (β-hydroxybutyrate) PHB and poly (β-hydroxybutyrate-co-β-hydroxyvalerate) P (HB-co-HV) were investigated by means of thermogravimetry and differential thermal analysis (TG/DTA), gel permeation chromatography (GPC) and mass spectrometry (MS). The main decomposition of PHB occurred at ca. 250°C and that of the copolymer P (HB-co-28mol% HV) occurred at a temperature ca. 10°C lower than that of PHB. The activation energies estimated by the TG method were 109±13 kJ/mol for PHB and 126±13 kJ/mol for P (HB-co-28% HV). The GPC curves of PHB (and the copolymer) decomposed at 245°C showed a series of peaks which were assigned to decomposition products, i.e., crotonic acid (and 2-pentenoic acid) and dehydrated oligomers of PHB (and the copolymer). In the MS study, the mass spectrum of each of PHB and the copolymer showed a series of triplet or doublet peaks assigned to the decomposition products originated from each repeating unit and the oligomers (ranging up to pentamer). Moreover, the mass spectrum of thermal decomposition products of P (HB-co-28mol% HV) showed the intense peaks assigned to products of the mixed oligomers of both units, suggesting that it is an almost statistically random copolymer.