The cell nucleation and real-time cell growth with increasing cell growth time in microcellular foaming were investigated by means of temperature quenching in a supercritical CO2 pressure-quench process. Samples of uniform size and shape were saturated in a vessel under conditions of 100 - 180degreesC and 30 MPa, and then depressurized to the atmosphere in 10 s. After depressurization, these samples were removed from the vessel at prescribed intervals, and immediately immersed in an ice-water slurry to obtain foamed samples with various cell growth times. It was found that the nucleation density is closely correlated to the gas absorption capacity of the polymer matrix, so that the final cell density should not be adopted as the nucleation density, as done commonly. The change of cell structure and mass density with increasing cell growth time was dominated by gas diffusion behavior, which was strongly influenced by the temperature. The final cell structure was mainly determined by the cell growth step, where gas diffusion played a key role. The final cell density was in direct proportion to the gas remaining in the substrate, which ranged from 6.0 x 10(9) to 4.7 x 10(6) cells/cm(3). (C) 2004 Wiley Periodicals, Inc.