Solid-state poly(methyl methacrylate) (PMMA) nanofoams. Part II: Low-temperature solid-state process space using CO2 and the resulting morphologies

•PMMA nanofoams with cell size in 30–40 nm range are created for the first time.•The critical amount of CO2 needed for making PMMA nanofoams via the solid-state process is discovered.•We establish the processing conditions for making interconnected nanoporous structures in PMMA.

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 1014 cells/cm3 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 °C to 40 °C. A range of equilibrium mass% of CO2 are achieved via the different saturation temperatures, from 11.4% at 40 °C to 39.3% at −30 °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.

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