Some thermodynamic and kinetic low-temperature properties of the PC-CO2 system and morphological characteristics of solid-state PC nanofoams produced with liquid CO2

Abstract In this paper, we describe a novel process for creating polycarbonate (PC) nanofoams with cells in the range of 20–30 nm that is based on saturating PC in liquid CO 2 . We investigated the effect of saturation temperature on the solid-state processing of PC foams in the range of −30 °C to 80 °C. Saturation temperature significantly affects the solubility and diffusivity, and the Arrhenius equation can describe the increase of solubility and decrease of diffusivity as the temperature decreases. We observed a drop in Δ H s , the heat of sorption, at the transition from vapor phase to liquid phase, leading to a weaker dependence of solubility on temperature in the liquid CO 2 . The dissolution of CO 2 in PC dramatically reduces its glass transition temperature in proportion to the amount of CO 2 absorbed: incorporation of 20.4% CO 2 in PC decreased the T g from 147 °C down to −7.5 °C. We have identified a critical CO 2 concentration window between 15.9% and 18.9%, within which cell nucleation density rapidly increases and consequently foamed microstructure changes from microcellular to nanocellular. Nanofoams with cell nucleation densities exceeding 10 15 cells/cm 3 and void fraction of up to 60% were achieved. The low-temperature route to enhanced cell nucleation opens up new possibilities for creating nanofoams in thermoplastics. The ability to create cells of different length scales in PC provides a unique opportunity to study the effect of cell size on mechanical and other properties of interest over a cell size range that spans several orders of magnitude.