In this paper, solid-state poly(methyl methacrylate) (PMMA) nanofoams are fabricated via a low-temperature CO2 saturation process. Nanofoams with smallest cell size in 30-40 nm range and cell nucleation densities exceeding 10(14) cells/cm(3) are achieved. We investigated the effect of saturation temperature on the solid-state foaming of PMMA and resulting morphologies of the foams in the range of -30 degrees C to 40 degrees C. A range of equilibrium mass% of CO2 are achieved via the different saturation temperatures, from 11.4% at 40 degrees C to 39.3% at -30 degrees C. The amount of CO2 absorbed greatly influences cellular structure of PMMA foams. We identify a critical mass% CO2 window between 30.1% and 32.6%, within which cell nucleation density rapidly increases and consequently foamed microstructure changes from microcellular to nanocellular. Nanofoams with void fraction as high as 86% are created. A transition from closed nanocellular structure to bicontinuous nanoporous structure, and also novel worm-like nanostructures have been observed. (C) 2015 Elsevier Ltd. All rights reserved.