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 CO2. We investigated the effect of saturation temperature on the solid-state processing of PC foams in the range of -30 degrees C to 80 degrees 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 Delta 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 CO2. The dissolution of CO2 in PC dramatically reduces its glass transition temperature in proportion to the amount of CO2 absorbed: incorporation of 20.4% CO2 in PC decreased the T-g from 147 degrees C down to -7.5 degrees C. We have identified a critical CO2 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. (C) 2014 Elsevier Ltd. All rights reserved.