Physical ageing of semi-crystalline PLLA: Role of the differently constrained amorphous fractions

The enthalpy relaxation of poly(L-lactic acid) (PLLA) crystallized at 95 °C and 135 °C is compared with the behavior of a completely amorphous sample in the same undercooling conditions. The enthalpy loss (?ha) normalized to the mobile amorphous fraction has allowed to distinguish the PLLA samples crystallized at different temperatures. From the Tg values an almost unconstrained mobile amorphous fraction and a slightly constrained mobile morphous fraction have been identified in PLLA crystallized at 135 °C and 95 °C, respectively. The normalized ?ha data of the semicrystalline samples have been found to deviate from the ?ha values of the totally amorphous sample at high undercooling. This behavior has been rationalized by assuming that also the rigid amorphous fraction contributes to the structural relaxation process in the semi-crystalline samples through sub-segmental local motions. The experimental data suggest that the contribution of small-scale local motions to physical ageing becomes recognizable with reducing Ta, likely in parallel with the concomitant decrease in the cooperative segmental motions, which could be prevalent at low undercooling.