Quantitative DSC Assessment of the Polymorph-Specific Crystallinity of Poly(Lactic Acid) and the Impact of a Self-Assembling Nucleating Agent and PEG Plasticizer

This study examines the temperature-resolved, polymorph-specific crystallinity of poly(lactic acid), PLA, during cooling and heating at 10 °C/min, with a focus on the effects of N, N-bis(benzoyl) hexanedioic acid dihydrazide (BHAD, commercially known as TMC306) as nucleating agent and PEG 1000 as plasticizer. A semicrystalline (PLA-1) and amorphous (PLA-2) PLA grade were investigated. The study emphasizes the importance of using temperature-dependent, polymorph-specific transition enthalpies to accurately calculate crystallinities from Differential Scanning Calorimetry (DSC). Polymorphism is independently confirmed using Wide Angle X-ray Diffraction (WAXD). Pure PLA-1 reached an α′ crystallinity of 2% during cooling, which increased to 38% through cold crystallization upon heating. At BHAD concentrations of at least 0.4%, α crystallites formed instead of α′, reaching a maximum crystallinity of 38% during cooling. The addition of 10 wt% PEG to PLA-1 facilitated primary α crystallization during cooling, followed by secondary intraspherulitic α′ crystallization upon heating, resulting ultimately in a crystallinity of 34%. Adding 1 wt% BHAD into PLA-1 with 10 wt% PEG shifted the crystallization temperature upward by 40 °C and enhanced the α crystallinity to 44%, highlighting the synergistic effect of PEG and BHAD on crystallization.