Morphological evolution of PLA foam from microcellular to nanocellular induced by cold crystallization assisted by supercritical CO2

[Display omitted] •Inducing crystals as bubble nucleation sites in range of cold crystallization temperature.•Micro/nano transformation of cells induced by crystallinity and transformation from α to α'.•Preparation of nano foams in a semi-crystalline polymer using a physical blowing agent. The use of physical foaming agents to form nanofoams in semicrystalline polymers is still a tremendous challenge. The change from microcellular to nanocellular bubbles (the micro/nano transition) in poly(lactic acid) (PLA) foam was investigated by adjusting the type of crystals induced in PLA during cold crystallization. The results showed that at saturation temperatures below the micro/nano transition temperature, α′ crystals formed; their coarser surfaces and higher crystallinity resulted in a higher cell nucleation efficiency, and thus a microcellular foam was converted to a high-cell-density nanofoam. After chain extension, the micro/nano transition temperature decreased, and the properties of the foam were improved at temperatures above the micro/nano transition temperature. A nanocellular foam with a cell density of 1015 cells/cm3 and a cell size of approximately 30 nm was obtained.