Time and Temperature Evolution of the Rigid Amorphous Fraction and Differently Constrained Amorphous Fractions in PLLA

In semi-crystalline polymers, amorphous segments located in different regions and distance from the crystalline domains can exhibit different dynamics. In poly(L-lactic acid) (PLLA), the existence of (i) two distinct mobile amorphous fractions (a completely mobile amorphous fraction and a slightly constrained mobile amorphous fraction), which vitrify/devitrify in the Tg region, and (ii) a rigid amorphous fraction, which vitrifies/devitrifies at temperatures higher than Tg, has been reported in the literature. The percentage of the three different amorphous fractions, characterized by different segmental mobility, is generally calculated at Tg. In the present study, for the first time, the time and temperature evolution of all the amorphous fractions of PLLA are measured in parallel with the crystalline phase (i) during crystallization at two different temperatures, and (ii) upon the successive cooling step down to room temperature. The kinetics of vitrification/devitrification of the different amorphous fractions in diverse conditions of crystallization and solidification has been determined during the entire thermal history of the material. Rigid amorphous fraction is found to develop not only during crystallization but also upon cooling, after completion of crystallization.