Glass transition dynamics of poly(L-lactic acid) during isothermal crystallisation monitored by real-time dielectric relaxation spectroscopy measurements

Dielectric relaxation spectroscopy was used to monitor the development of crystallinity in poly(L-lactic acid) (PLLA) at 80°C. The continuous shifting of the main loss peak towards lower frequencies was modelled considering the evolution of three relaxation processes: the α-process of the bulk-like (non-restricted) amorphous phase, the α-process of the amorphous fraction influenced by the crystalline structure and the β-relaxation. It was found that the shape parameters and the position of the loss peaks are essentially constants during crystallisation, just with their dielectric strengths varying. This indicates that (i) the two segmental dynamics process develops independently and (ii) the dynamics features of the (slower) confined amorphous phase do not change during crystallisation. Optical micro-scopy results showed that a correlation exists between the evolution of the spherulitic morphology and the dielectric strength of the α-processes.