Thermal degradation behavior of poly(4-hydroxybutyric acid)

摘要

Thermal degradation behavior of poly(4-hydroxybutyric acid) (P(4HB)) was investigated by thermogravimetric and pyrolysis-gas chromatography mass spectrometric analyses under both isothermal and non-isothermal conditions. Based on the thermogravimetric analysis, it was found that two distinct processes occurred at temperatures below and above 350 degrees C during the non-isothermal degradation of P(4HB) samples depending on both the molecular weight and the heating rate. From H-1 NMR analysis of the residual P(4HB) molecules after isothermal degradations at different temperatures, it was confirmed that the omega-hydroxyl chain-end was remained unchanged in the residual P(4HB) molecules at temperatures below 300 degrees C, while the omega-chain-end of P(4HB) molecules was converted to 3-butenoyl units at temperatures above 300 degrees C. In contrast, the majority of the volatile products evolved during thermal degradation of P(4HB) was gamma-butyrolactone regardless of the degradation temperature. From these results, it is concluded that during the thermal degradation of P(4HB), the selective formation of gamma-butyrolactone via unzipping reaction from the w-hydroxyl chain-end predominantly occurs at temperatures below 300 degrees C. At temperatures above 300 degrees C, both the cis-elimination reaction of 4HB unit and the formation of cyclic macromolecules of P(4HB) via intramolecular transesterification take place in addition to unzipping reaction from the w-hydroxyl chain-end. Finally, the primary reaction of thermal degradation of P(4HB) at temperatures above 350 degrees C progresses by the cyclic rupture via intramolecular transesterification of P(4HB) molecules with a release of gamma-butyrolactone as volatile product. Moreover, we carried out the thermal degradation tests for copolymer of 93 mol% of 4HB with 7 mol% of 3-hydroxybutyric acid (3HB) to examine the effect of 3HB units on thermal stability of P(4HB). (c) 2006 Elsevier Ltd. All rights reserved.