Thermal characterization of polylactides

Three important thermal characteristics of polylactides have been measured and discussed. These are glass transition temperature, melting temperature, and degradation behaviour. The glass transition temperatures at infinite molecular weight for l- and d,l-optical isomers were 58 and 57°C, respectively. Melting temperature for the crystalline l-isomer was 184°C at infinite molecular weight. Number-average molecular weight, melt viscosity, differential scanning calorimetry, infra-red analysis and thermogravimetric analysis were used to study the degradation behaviour under different conditions. Polylactides were found to be highly sensitive to heat, especially at temperatures higher than 190°C. Most of the degradative reactions were thought to involve the highly concentrated ester bond on the main chain. These reactions included thermohydrolysis, depolymerization and cyclic oligomerization, intermolecular and intramolecular transesterifications. Low molecular weight compounds associated with the polymer seemed to play an important role in lowering the molecular weight at high temperatures, as well as the hydroxyl end group of the main chain. The compounds include water, monomers, oligomers, and polymerization catalysts. Removal of the non-polymeric contents and blocking the hydroxyl end-group enhanced the thermal stability of the polymers. The increased amount of the polymerization catalyst in the end product contributed to the degradative reactions.