About the transformation of low Tm into high Tm poly(l-lactide)s by annealing under the influence of transesterification catalysts

Abstract

Cyclic polylactides were prepared in bulk at 170 °C, crystallized at 120 °C and then annealed at temperatures between 130 and 170 °C with variation of catalyst, catalyst concentration and annealing time. The transformation of the initially formed low melting (LT_m) crystallites, having melting temperatures (T_m) < 180 °C into high melting (HT_m) crystallites having T_m values > 189 °C was monitored by means of DSC measurements and characterized in selected cases by SAXS measurements. It was confirmed that the formation of HT_m crystallites involves a significant growth of the thickness of the lamellar crystallites along with smoothing of their surface. Annealing at 170 °C for 1 d or longer causes thermal degradation with lowering of the molecular weights, a gradual transition of cyclic into linear chains and a moderate decrease of lamellar thickness. An unexpected result revealed by MALDI TOF mass spectrometry is a partial reorganization of the molecular weight distribution driven by a gain of crystallization enthalpy.