Enthalpy relaxation of the glass of poly (L-lactic acid) of different D-isomer content and its effect on mechanical properties

The relaxation of the glass of poly (l-lactic acid) (PLLA) and its random copolymers with 2 and 4 mol% d-isomer co-units has been analyzed by conventional differential scanning calorimetry. It was found that the presence of up to 4% d-isomer co-units in the PLLA chain has no measurable effect on the glass relaxation kinetics in fully amorphous samples. At ambient temperature, after rapid cooling of the melt at a rate of 50 K/min, to avoid crystallization, the glass relaxation begins after about 10 min and is not finished within 5000 min. The relaxation of the glass is accompanied by a major change of mechanical properties as it was revealed by time-resolved measurements of the micro-indentation hardness and Young's modulus. During annealing of amorphous PLLA at ambient temperature, the microhardness and Young's modulus increase by more than 40 and 10%, respectively, within a time period of about one month, and with an equilibrium value not yet achieved even after such a long annealing time. As in case of the enthalpy-relaxation kinetics, both, absolute values and the time evolution of the analyzed mechanical properties are independent on the presence of d-isomer co-units in the PLLA chain within the investigated concentration range.