Microcellular polymer foams afford a wide variety of attributes relative to their dense analogues, and efforts remain underway to establish viable routes to generate foams with substantially reduced pore cell size and increased pore cell density. Barrier constraints are applied in the present work to achieve diffusion-controlled isothermal foaming of thin polymer films in the presence of supercritical carbon dioxide (scCO(2)). Poly(methyl methacrylate) (PMMA) films measuring ca. 95-100 mum in thickness are physically constrained between two impenetrable plates so that scCO(2) exit diffusion is restricted to the film edges. Results obtained here demonstrate that the pore size can be systematically reduced to less than 100 nm in such systems by applying high saturation scCO(2) pressures, relatively low foaming temperatures (near the glass transition temperature of the scCO(2)-plasticized polymer), and a rapid pressure quench. Classical nucleation theory (CNT) modified to account for the compressible nature of scCO(2) is used to describe pore cell growth as a function of foaming temperature and scCO(2) saturation pressure. Incorporation of a gradient model based on the Sanchez-Lacombe equation of state to account for PMMA-CO2 interfacial tension in conjunction with the CNT yields accurate predictions of foam cell densities as a function of relevant system parameters.