In this article, we report on the process for creating microcellular and nanocellular polysulfone (PSU) foams. Microcellular foams with cell size up to 8 mm and nanocellular foams with cell size in the range of 20-30 nm were created. A range of CO 2 concentration was achieved by varying saturation temperature, from 5% at 60 C to 14.7% at 210 C. The CO 2 concentration has a strong influence on the cellular structure.There exists a critical concentration window, between 10.7% and 12.3%, within which cell nucleation densities increase rapidly and cell sizes drop from micrometer range to below 1 mm into the nanometer range. Nanofoams with cell nucleation densities exceeding 10 15 cells/cm 3 and void fraction of up to 48% are achieved. At the high CO 2 concentration region, the change from closed nanocellular structure to bicontinuous nanoporous structure is observed. Also, nanostructures on the cell wall of microcells are observed and believed to be formed via stressinduced nucleation/spinodal decomposition. The PSU nanofoams produced in this study present an opportunity to produce polymer nanofoams with a relatively high service temperature. The ability to create cells of different length scales provides an opportunity to study the effect of cell size on the foams properties.
In this paper, solid-state poly(methyl methacrylate) (PMMA) nanofoams are fabricated via a lowtemperature CO 2 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 °C to 40 °C. A range of equilibrium mass% of CO 2 are achieved via the different saturation temperatures, from 11.4% at 40 °C to 39.3% at-30 °C. The amount of CO 2 absorbed greatly influences cellular structure of PMMA foams. We identify a critical mass% CO 2 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.
In this paper, we describe fabrication of micro- and nano-structured foams in high temperature polyphenylsulfone (PPSU) with a Tg of 219°C using a solid-state carbon dioxide (CO2) foaming process. We have fabricated microcellular foams with cell size in 1–10 μm range, and for the first time, nanocellular foams with cell size in 20–40 nm range. We discovered a critical CO2 concentration window 10.5%–11.8%, where a rapid 2–3 orders of increase in cell nucleation density is observed and cell size drops to below 100 nm. Two approaches are demonstrated to create PPSU nanofoams with a cell size of 20–40 nm: increasing saturation pressure to 7 MPa or decreasing saturation temperature to 10°C. Both approaches use high pressure liquid CO2 and the CO2 concentration in the polymer are above the critical concentration window, which result in the formation of nanocells. At high CO2 concentration of 11.8% and 13%, as foaming temperature increases, the nanostructures transition from closed structure to an open porous structure.
Porous polymer sheets created by the “solid state microcellular foaming process” possess an impermeable solid skin on the boundary. In order to experimentally determine the internal structure's fluid permeability, the skin is removed by machining a pattern of holes or channels so that fluid flow can be established when applying a pressure difference across the faces. In this work, a fluid flow model for this particular geometry assuming a Newtonian fluid and isotropic internal porous structure is established. This yields formulae used to compute the structure's Darcy permeability from experimental flow data. Experimental verification of the model is provided for a sample of Polyetherimide (PEI) nanoporous polymer sheet whose skin was drilled in various hole patterns, thus establishing the mathematical foundation for future work in investigating this class of porous materials.
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