Construction of a Highly Oriented Poly(lactic acid)-Based Block Polymer Foam and Its Self-Reinforcing Mechanism

Abstract

Developing poly(lactic acid) (PLA)-based biodegradable polymer foams with high performance is crucial for sustainable energy conservation and the ecological environment. In this work, by applying poly(lactide-co-caprolactone) (PLCL) as a compatibilizer, the block polymer of poly(l-lactic) acid (PLLA)-b-polycaprolactone (PCL) with a long-chain branched structure was prepared by reactive processing. By utilizing the difference between the glass-transition temperature (Tg) and melting temperature (Tm) of PLA and PCL, PLLA-b-PCL-PLCL foams with a PCL chain as the foaming phase and an oriented PLLA chain as the enhancing phase were prepared by establishing a solid-phase hot drawing@supercritical CO2 foaming method. The incorporation of PLCL links significantly improved the interfacial compatibility between PLLA and PCL phases and chain entanglements, exhibiting strain-hardening behavior and leading to an improvement in the stretchability of the block polymer. In the hot drawing process in the range of Tg(PLA)–Tm(PLA), both PLLA and PCL phases were elongated along the drawing direction, and with an increasing draw ratio, the orientation factor and the crystallinity of the two phases increased evidently, accompanied by the formation of a shish–kebab crystalline structure. In the subsequent foaming process in the range of Tg(PCL)–Tm(PCL), the molecular chains of the PLLA phase were frozen and the retention of the orientation factor reached over 90%, while under CO2, crystallinity and crystallite size increased and a more perfect crystalline structure formed for the PCL phase. With an increasing draw ratio, the saturated dissolved amount of CO2 increased and the cell density and cell size of foam reached 5.13 × 109 cells/cm3 and 4.06 μm, respectively. The fibrillar structures of the PLLA phase appeared on the cell wall, while the tensile strength and modulus of foam reached as high as 65.9 MPa and 1.01 GPa, respectively. As a result, a super-strong PLA-based microcellular foam with dense micropores and highly oriented microfibrillar structures was constructed.