Supercritical carbon dioxide is known to swell and plasticize poly(methyl methacrylate), PMMA. dramatically. We have employed a pressure quench in a CO2-swollen PMMA sample to generate a microcellular core structure encased by a nonporous skin. Further. we have demonstrated that classical nucleation theory can be used to model the effects of saturation pressure. temperature, and time on the cell density of the porous materials, provided that the effects of the CO2-diluent on the surface tension of PMMA are adequately taken into account. This is because our system is in a homogeneous liquid state at our operating conditions because of the plasticization. Both model predictions and data indicate that cell density uses sharply at a saturation pressure of approximately 14 MPa (at 40-degrees-C), leveling out above 27 MPa. By contrast, the effect of temperature on cell density in the range 40-degrees-C to 80-degrees-C is minimal.