Cellular morphology evolution in nanocellular poly (lactic acid)/ thermoplastic polyurethane blending foams in the presence of supercritical N2

Currently, the formation of nanocells and complex cellular structure (CCS) in semi-crystalline polymer using supercritical fluids as blowing agent had become a big and newly developing challenge worldwide. In this paper, a facile methodology of polymer blending and batch foaming was proposed to fabricate CCS with large cells in micro-size and small cells in nano-size in semi-crystalline poly (lactic acid) (PLA) foams. Thermoplastic polyurethane (TPU) was introduced into PLA through melt mixing method to improve the viscoelasticity and foaming behaviors of PLA. Differential scanning calorimetry results showed that with the addition of TPU, the cold crystallization temperature of various PLA samples increased and their crystallinity kept unchanged almost. Compared with those of pure PLA, the complex viscosity, and storage modulus of PLA/TPU blends increased. Interestingly, with the increasing TPU content, the CCS appeared and became distinct in PLA/TPU blending foams as well as the thermal conductivity of various PLA foams decreased, respectively. The formation mechanism of CCS in nanocellular PLA/TPU blending foams was proposed and explained by schematic diagram. With the foaming temperature rising from 135 to 140 degrees C, a transition from nano-cells to micro-cells was also observed in PLA/TPU 5 blending foam, due to the decreased melt viscosity.