Fabrication of functionally graded porous polymer via supercritical CO2 foaming

A fabrication process to produce functionally graded porous polymer via supercritical carbon dioxide (ScCO2) foaming is reported in this paper. It utilizes a partial gas saturation technique to obtain non-equilibrium gas concentration profiles in thermoplastic polymer. Once foamed the polymer material obtains a graded foam–solid-foam structure with varying pore size distributions. This functionally graded material fabrication method was studied with polymethyl methacrylate (PMMA) under a ScCO2 saturation condition. A diffusion model was developed to estimate the gas diffusion coefficient and to predict the gas concentration profiles inside the polymer samples. Scanning electron microscopy images were used to analyze the effects of partial saturation on the graded porous structure. Mechanical properties of the foamed samples were characterized using a three-point bending test. It was found that the gas concentration profiles resulted from partial saturation corresponded well to the graded structure after foaming. The proportion of the foam and solid regions inside the polymer sample can be manipulated by controlling the partial saturation profile. The test results also suggested that the graded foam structure could be optimized to achieve desirable mechanical properties.