Cellular polymers with closed cells about 10 μm in size and a cell density of 108 cells/cm3 and higher are called microcellular foams (Martini-Vvedensky, 1984) and those with cells less than 1 μm in size and cell density greater than 1012 cells/cm3 are called ultramicrocellular foams. The latter have also been termed supermicrocellular foams (Baldwin et al., 1994). Microcellular foams have been made by saturating the glassy polymer with a blowing agent, usually CO2, and then inducing cell nucleation by a rapid temperature soak (Kumar, 1993) or a pressure quench (Goel and Beckman, 1994). The cells are allowed to grow for a short duration before the cellular morphology is locked-in by rapidly cooling the system. Ultramicrocellular foams have been made the same way except that saturation with the blowing agent is done at a much higher pressure (Baldwin et al., 1994; Goel and Beckman, 1994).

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