Tuning Melt Rheology of PLA through Chiral Hydrogen-Bonding Comonomers and Multi-Arm Topologies

Poly(lactic acid) (PLA) is a promising biodegradable polymer but suffers from poor melt strength, which limits its applicability in advanced processing technologies. In this study, we present a molecular architecture design strategy that combines chiral copolymerization and controlled branching to systematically enhance the melt strength of Poly(l-lactic acid) (PLLA). A series of chiral morpholine-based monomers bearing amide groups were synthesized from amino acids and incorporated into the PLLA backbone via ring-opening copolymerization. These amide groups introduce bifurcated hydrogen bonds that enhance intermolecular interactions and increase chain entanglement. We found that monomer side-chain flexibility, stereochemistry, and comonomer content play key roles in tuning melt rheology, with flexible, opposite-chirality comonomers significantly boosting melt strength through chain expansion and physical entanglement. Additionally, multiarm star architectures were constructed by using polyhydroxyl initiators, further amplifying melt strength via topological constraint. Among the topologies examined, 4-arm PLLA copolymers showed the most balanced performance, improving melt strength by 10,370% while maintaining good thermal and mechanical properties. This synergistic approach not only enhances melt elasticity but also enables improved foaming behavior with uniform cell morphology under supercritical CO2. The strategy demonstrated here offers a scalable and sustainable route for developing high-performance PLLA materials suitable for industrial melt-processing applications.