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About the transformation of low Tm into high Tm poly(l-lactide)s by annealing under the influence of transesterification catalysts
Summary
Researchers studied how polylactic acid (PLA) crystals transform into higher-melting forms during heating with catalysts. Understanding the crystallization behavior of biodegradable PLA plastics is important for designing materials that degrade more effectively in the environment.
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.
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