Ultra-High-Expansion-Ratio Nanocellular Semicrystalline Polymer Foams: Preparation of Chain-Extender-Mediated PLA/PPC...

The inherent brittleness, retarded crystallization kinetics and insufficient melt strength of polylactic acid (PLA) severely limit its applications in foaming. This study proposed a reactive chain extender and compatibilizer strategy, which involved introducing the multifunctional epoxy-based chain extender KL-E4370 (0.6 wt%) into a PLA/polypropylene carbonate (PPC) blend system to simultaneously enhance melt strength and interfacial compatibility, with the aim of preparing high-expansion-ratio nano-cellular foams by means of the physical foaming assisted by supercritical CO₂. The impact of PPC loading and foaming temperature on the crystallization behavior, rheological properties, cell morphology, and compressive properties of the blends were systematically investigated. As a result, chemical interactions occurred between the chain extender and terminal carboxyl and hydroxyl moieties of PLA and PPC during ring-opening reactions, which produced a PLA-chain extender-PPC copolymer in situ. This significantly improved melt elasticity, reduced interfacial tension, and promoted the even distribution of the PPC phase. At the same time, the addition of PPC enhanced the CO₂ adsorption capacity of PLA. The PLA/PPC15-CE0.6 formulation exhibited the highest crystallinity (5.9%) and a refined spherulite morphology, providing abundant interfaces between crystalline and amorphous regions that serve as auxiliary nucleation sites. At the optimal foaming temperature of 115 °C, this formulation achieved an excellent foaming volume expansion ratio of 7.3 times, an average cell size of 660 nm, and a maximum cell density of 1.1 × 10¹³ cells/cm³. With further elevation of the foaming temperature to 117 °C and beyond, excessive softening of the matrix triggered cell coalescence, causing the cell sizes to increase sharply beyond the nanoscale range and the cell density to drop significantly. Samples foamed at 115 °C retained a compressive strength of 2.7 MPa under 70% strain, demonstrating a good balance between lightweight properties and mechanical integrity. This study provides a systematic mechanism understanding and operational directions for the sustainable fabrication of high-performance bio-based biodegradable polymer nanoporous materials.