DSC Characterization of Polylactic Acid Synthesized by Non-Catalyzed Direct Polycondensation under Vacuum

Polylactic acid (PLA) was synthesized from l-lactic acid by direct polycondensation under vacuum without the use of catalysts (NC-DP). It was studied in order to adapt it to on-site cell production where safety treatment, compact capacity, and simple operations are required to reduce production costs. Experiments were conducted at polymerization temperatures (Tp) between 150 and 250°C and under reduced pressure. The resulting PLA was characterized using gel permeation chromatography (GPC) to determine the molecular weight as well as by differential scanning calorimetry (DSC) to examine the thermal properties. The maximum weight average molecular weight of PLA obtained was 90 kDa after 89 h at 200°C. Above 200°C, PLA was thermally degraded, and racemization occurred as PLA became atactic. Indeed, PLA only exhibited a glass transition temperature (Tg) and exhibited neither a crystallization temperature nor a melting point. The relations between Tg and the molecular weight were in agreement with the Flory–Fox equation.