Experimental measurement of CO₂ diffusion in PMMA and its effect on microcellular foaming

The CO₂ diffusion coefficient in polymethyl methacrylate (PMMA) was measured to investigate the relationship between the saturation time and cell nucleation in batch foaming. Measurements were conducted using a visualized high-pressure cell at various temperatures from 303 to 343 K and CO₂ pressures of 6, 12, and 18 MPa. The CO₂ diffusion coefficient depended strongly on the temperature but weakly on the pressure. Additionally, the CO₂ diffusion was promoted by the glass transition behavior of PMMA but was suppressed by the hydrostatic pressure at high pressures. Based on the sorption isotherms of CO₂ in PMMA, microcellular PMMA foams were prepared with saturation times from 1800 to 12600 s. Cell nucleation was initiated when the CO₂ concentration, which depended upon the CO₂ diffusion and saturation time, was sufficient to minimize the energy barrier. Furthermore, the critical CO₂ concentration to initiate cell nucleation decreased with an increase in temperature.