Transition from microcellular to nanocellular chain extended poly(lactic acid)/hydroxyl-functionalized graphene foams by supercritical CO2

Abstract

Currently, preparing nanocellular semi-crystalline polymer foams by supercritical CO2 is a big and newly developing challenge. In this paper, chain extender (CE) and hydroxyl-functionalized graphene (HG) were introduced into poly(lactic acid) (PLA) through melt blending method to improve the crystallization behaviors, rheological properties and foaming behaviors of PLA. Differential scanning calorimetry results showed that the cold crystallization temperature of chain extended PLA (CPLA)/HG was higher 8.2 °C than that of CPLA, due to the introduction of HG and the strong interaction between CPLA and HG. The viscoelasticity of PLA was improved by the addition of CE and HG, due to the formation of branching structure and the interaction between CPLA and HG. Compared with that in PLA/HG, HG aggregation in CPLA/HG became many but small, indicating that the aggregation of HG in the matrix released. A facile batch foaming method with constant foaming temperature slightly lower than melting temperature was employed to fabricate nanocellular PLA foams in the presence of supercritical CO2. The transition temperature from microcells to nanocells in various PLA foams was confirmed. The effect of chain extension, foaming temperature and the introduction of HG on cell size, cell density, cell size distribution and volume expansion ratio (VER) was studied systematically. For the CPLA/HG foam prepared at 130 °C, its cell size could reach 350 ± 247 nm as well as its cell density and VER were 1.76 × 1013 cells/cm3 and 3.71 ± 0.16 times, respectively. Finally, the foaming mechanism for the nanocell formation was proposed and explained by schematic diagram.