A new experimental system for combinatorial exploration of foaming of polymers in carbon dioxide: The gradient foaming of PMMA

Abstract

A unique foaming system has been developed to explore the consequences of different pressures and temperatures employed in foaming of polymers in supercritical carbon dioxide. The cell employs a temperature gradient field extending over 25cm. It thereby generates a continuum of temperatures to which a long strip of polymer is exposed at a given CO2 pressure in a single experiment. It allows a powerful combinatorial approach to explore foaming. This manuscript describes the experimental system and demonstrates its utility in gradient foaming of polymers using long strips (25cm) of poly(methyl methacrylate) in thick sheet (2.1mm) and thin film (100μm) forms at nominal pressures of 14, 21, 28, 35MPa under an imposed temperature gradient extending over 25cm with a temperature difference from 40°C to 125°C. By generating a rich array of sections foamed at different temperatures at a given pressure, the experimental approach provides new insights not only on the pore formation and growth as a function of pressure and temperature, but also on the importance of other factors such as the time allowed or needed for CO2 sorption and escape, and the changes in the softening and vitrification state of the polymer that are important in cell nucleation, growth, or collapse. Progress of the diffusion front, which is readily identified as the boundary between the plasticized and glassy domains, is documented which provides an assessment of the diffusion coefficient as a function of temperature at a given pressure, and as a function of pressure at a given temperature.