A simple mechanism regulating polymer mobility is demonstrated to determine initial and final growth states of solid-state microcellular foams. This mechanism, governed by the extent of plasticization of the polymer by the dissolved gases, is examined with a mass balance model and results from foam growth experiments. Polycarbonate was exposed to CO2, which acted as both a plasticizing gas and a physical blowing agent driving foam growth. The polycarbonate specimens were saturated to the equilibrium gas concentration at 25 degreesC for CO2 pressures of 1-6 MPa in 1-MPa increments. Equilibrated specimens were heated in a glycerin bath until thermal equilibrium was reached, and a steady foam structure was attained. Glycerin bath temperatures of 30-150 degreesC in 10 degreesC increments were examined. Using knowledge of gas solubility, the equation of state for CO2, the effective glass-transition temperature as a function of gas concentration, and a model for mass balance within a solid-state foam, we demonstrate that foam growth terminates when sufficient gas is driven from the polycarbonate matrix into the foam cells. The foam cell walls freeze at the elevated bath temperature because of gas transport from the polycarbonate matrix and the associated rise in the polymer glass-transition temperature to that of the heated bath. (C) 2001 John Wiley & Sons, Inc.