Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) microcellular foams using a melt memory effect as bubble nucleation sites

Abstract

Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) foams were prepared by a foam injection molding (FIM) process using nitrogen (N2) as a physical blowing agent. PHBH is a marine degradable polymer known for its heat treatment weakness. Gel permeation chromatography and rheological measurements showed that the molecular weight was reduced when the polymer was processed at a temperature higher than 160°C; however, it had a prominent melt memory of crystallization at a temperature lower than 170°C. The melt memory effect increased the crystallization temperature and provided crystal nucleation sites, which became bubble nucleation sites in the foaming process. Microcellular foams, whose cell size is smaller than 30 μm and cell density is higher than 3 × 10⁸ cells cm⁻³, were prepared using the melt memory effect while suppressing the thermal decomposition by tuning the injected polymer's temperature (melting temperature) and the core-back timing. The mechanical properties of the resulting foams were evaluated, indicating that a high ductility PHBH microcellular foam was obtained at a melting temperature of 150°C.