Interferometric observation of the process of CO₂ dissolving into PMMA under high pressure

In situ interferometric observation of the process of CO₂ dissolving into PMMA (polymethyl methacrylate) was carried out at high pressure and room temperature, in view of the interest to polymer foam technology. By analysing interferograms, the phase shift of a laser beam transmitted though the sample (the phase is related to the dissolved gas content and polymer thickness) was obtained as a function of time and pressure. At constant pressure, the phase shift initially increased over time and reached a maximum value, and then decreased asymptotically to a constant value as equilibrium was approached. In addition, the equilibrium value of the phase shift increased with increase in pressure for pressures lower than the glass transition pressure P_g, and decreased at pressures higher than P_g. Thus, the optical properties exhibited during the process of the gas dissolving were drastically different for the glassy state and the rubbery state of the polymer.