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 (T _g) change, and the chains' conformational behavior of PLLA during sub-T _g and supra-T _g 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).