Multistep Relaxation Pathway from Glass to Crystal Development in Poly(L-lactide)

The thermal transitions of amorphous poly(l-lactide) (PLLA) were investigated by combining standard calorimetry (DSC), fast scanning calorimetry (FSC), and infrared spectroscopy. By following the enthalpic evolution, the glass transition temperature (Tg) change, and the chains’ conformational behavior of PLLA during sub-Tg and supra-Tg annealing isotherms, a transition was identified in the supercooled liquid (SCL) phase─the so-called SCL transformation─occurring before crystallization. SCL transformation kinetics was assessed and introduced into the relaxation map of amorphous PLLA, revealing a hierarchical pathway. The SCL transformation allows the unstable PLLA-quenched liquid to relax toward a metastable SCL with a lower free energy. This occurs through conformational rearrangement of a small fraction of gg conformers into the more stable gt form. Ultimately, SCL kinetics was compared to PLLA liquid dynamics, providing a suggestive scenario where the SCL transformation is mediated by both α-relaxation and non-α relaxation mechanisms─the Slow Arrhenius Process (SAP).