Characterization of poly(lactic acid) enhancement via poly(_L-lactic acid)/ poly(_D-lactic acid) stereocomplexation and its influence on material crystallinity and morphology

Poly(lactic acid) (PLA) is a biobased, biodegradable material that has shown great potential as an alternative to fossil-based polymers, already used in various fields due to its biocompatibility and advantageous physico-chemical properties. However, PLA lacks the thermal stability and impact resistance needed for engineering applications. To this end, the current study proposes a solvent-free methodology for the physical preparation of a PLA stereocomplex (PLA-SC) between enantiomeric poly(_D-lactic acid) and poly(_L-lactic acid), and its use in small amounts as a self-nucleating agent. The strength of the internal hydrogen-bonding (−CH₃… O=C) within the stereocomplex crystallites was altered by applying three threshold extrusion temperatures, and its influence on crystallinity and morphology was investigated. The crystallinity of PDLLA showed a sixfold increase with the addition of PLA-SC, while the crystallinity of PLLA doubled, as indicated by differential scanning calorimetry (DSC) and corroborated by Raman spectroscopy. The self-nucleation effect of the stereocomplex was observed under polarized optical light microscopy (POM). Atomic force microscopy (AFM) revealed two distinct morphologies correlated to the crystallinity trends recorded, namely spherulites and shish-kebabs, the latter being known as flow-induced, oriented semi-crystalline conformations. The intrinsic crystallization kinetics of each matrix promoted lamellar or fibrillar packing, respectively. The heat deflection temperatures (HDT) also increased with an increase in PLA-SC content for both PDLLA and PLLA matrices. Incorporating very low amounts of PLA-SC into PLA matrices of different optical purities under industrial processing conditions enhanced material properties, crucial for widening the applicability of this bioplastic.