PEG Middle-Block Molecular Weight as a Quantitative Design Variable for Stereocomplex Formation Efficiency in High-Molecular-Weight PLLA–PEG–PLLA/PDLA Blends

Abstract Achieving complete stereocomplex (SC) formation in high-molecular-weight PLLA/PDLA blends by injection molding remains a challenge because the rapid mold-cooling rate suppresses the slow enantiomeric chain-recognition process. Here, we demonstrate that the PEG middle-block molecular weight in PLLA–PEG–PLLA triblock copolymers is the decisive kinetic control variable governing SC formation efficiency (fSC). Copolymers with PEG Mn = 400, 4000, and 35,000 g/mol were synthesized by bulk ring-opening polymerization (ROP) at two PLLA-block Mn targets (100,000 and 200,000 g/mol) and blended 1:1 with PDLA before injection molding. WPLA-corrected differential scanning calorimetry (DSC) showed that fSC increased monotonically from approximately 75–76% (PEG400) to 93–95% (PEG4k) and to 100% (PEG35k), where the value was confirmed independently by X-ray diffraction (XRD). PEG35k-containing SC specimens crystallized completely during mold cooling without any postcooling cold crystallization, establishing Mn,PEG ≥ 35,000 g/mol as the practical threshold for complete in-mold stereocomplex at Mn ≈ 200,000 g/mol without postprocessing annealing. Toughness increased 7.1-fold, from 10.1 MJ m–3 (neat PLLA) to 71.5 MJ m–3 (L100 K-35k-L100 K/PDLA), driven by synergistic PEG-induced ductility and SC crystal reinforcement. All SC–PLLA-PEG-PLLA specimens maintained dimensional stability at 200 °C, whereas non-SC counterparts melted.