Thermal degradation behaviour of poly(lactic acid) stereocomplex

Abstract

The thermal degradation of poly(lactic acid) stereocomplex (scPLA) was investigated to clarify the pyrolysis mechanism. Three scPLA samples with different chain end structures were prepared, namely, as-polymerised scPLA-ap, precipitated-with-methanol scPLA-pr, and purified metal-free scPLA–H. From the analyses of thermal degradation kinetics and pyrolysates of the scPLA samples, typical degradation mechanisms of these scPLAs were proposed as follows. The pyrolysis of scPLA-ap proceeds through main unzipping depolymerisation caused by Sn–alkoxide chain ends with apparent E a =80–100 kJ mol −1 , showing zero-order weight loss behaviour. The pyrolysis of scPLA-pr also proceeds via a zero-order weight loss process consisting of main Sn-catalyzed selective lactide elimination with apparent E a =100–120 kJ mol −1 caused by Sn–carboxylate chain ends. The pyrolysates from scPLA-ap and scPLA-pr were predominantly l, l-/ d, d-lactides. In the case of scPLA–H, random degradation is a main process, producing a large amount of meso-lactide and cyclic oligomers. These degradation mechanisms were nearly the same as those of the corresponding PLLAs, except that the scPLA-ap pyrolysis started at higher temperature due to the higher melting point of scPLA.