Introduction of a long-chain branching structure by ultraviolet-induced reactive extrusion to improve cell morphology and processing properties of polylactide foam

Abstract

In this paper, long-chain branched polylactide (LCB-PLA) prepared by UV-induced reaction extrusion with trimethylolpropane triacrylate (TMPTA) was foamed by supercritical carbon dioxide (scCO₂), and the effect of the long-chain branching structure on the cell morphologies of PLA foams was investigated. The LCB-PLA displayed higher complex viscosity, melting point and crystal nucleation potential under scCO₂, and these factors could influence the foaming behavior of PLA which was proved by the different cell morphologies of samples foamed after various saturation times. The advantage of LCB-PLA on foaming was remarkable at high temperature and high pressure. LCB-PLA with more than 0.5% TMPTA showed nano-cells while the other samples showed micro-cells at 142 °C under 12 MPa, and the samples displayed elliptic cells with horizontal semimajor axis in linear PLA and circular cells or oval cells with vertical semimajor axis in LCB-PLA with increasing temperature. The improved cell morphology with reduced coalescence, no collapse and uniform cell distribution was also shown in LCB-PLA under higher pressure. All these results were due to the increasing matrix strength and higher crystal nucleation potential of LCB-PLA. The findings indicate that LCB-PLA possesses better foaming behavior at high temperature and high pressure. The wide foaming processing window of LCB-PLA would benefit the high temperature and high pressure foaming of PLA such as bead foaming and continuous extrusion foaming, thus broadening its application.