A Facile and Efficient Method for Preparing Chain Extended Poly(lactic acid) Foams with High Volume Expansion Ratio

In recent years, poly(lactic acid) (PLA) foams have drawn considerable attention worldwide due to their environmental friendliness and excellent biocompatibility. However, it was a big challenge to prepare PLA foams with high volume expansion ratio owing to their poor crystallization properties and melt strength. In this work, the random copolymer of ethylene and glycidyl methacrylate as an efficient chain extender (CE) was chosen and employed to improve the crystallization behaviors and foamability of PLA. The chain extension reaction between PLA and CE was proved by torque curves, Fourier transform infrared spectra, gel fraction, and intrinsic viscosity measurements. Differential scanning calorimetry results and polarized optical microscope images showed that the crystallinity and spherocrystal number of various PLA samples increased notably as well as their spherocrystal size decreased with the CE content increasing. The viscoelasticity of various PLA samples was improved by the increment in the CE content. Various PLA samples were foamed by a batch foaming method at different foaming temperature with supercritical CO2 to research the influence of crystallization behaviors and/or rheological properties on the foaming behaviors of PLA. At the high foaming temperature, the cell density and volume expansion ratio (VER) of various PLA foams were affected by rheological properties significantly and their highest VER could reach to 42.41 +/- 0.01 times. At the low foaming temperature, the cellular morphology of various PLA foams was improved notably, due to the increased melt strength induced by the cooling and the generation of crystallization region.