We report the successful production of thick, homogeneous nanocellular foams with high pore volume by carbon dioxide foaming of thermoplastic polymers. The addition of nanoscale additives, either silica nanoparticles or POSS, was shown to enhance cell nucleation density for polymethymethacrylate and styrene acrylonitrile copolymers by three orders of magnitude. This approach is especially effective using acrylic copolymers with improved CO2 affinity such as poly(methyl methacrylate-co-ethyl acrylate) or poly(methyl methacrylate-co-ethyl methacrylate), for which nanoscale additive levels below 0.5 wt% contributed to the production of nanofoams with 100 nm average cell size, relative density of 0.15 (85% porosity) and cell densities exceeding 10(16) cells/cm(3). Well-dispersed additives with size of 7 nm or less gave the finest cell morphologies. Designed experiments also showed that foaming at higher pressures (above 30 MPa) and lower temperatures (40 degrees C and below) increased cell density and reduced cell size, and that porosity could be maximized by adjusting post-foaming annealing temperature. (C) 2013 Elsevier Ltd. All rights reserved.