Gas/supercritical fluid (SCF) and polymer mixtures are applied broadly in industry, such as rigid and light foam products for aircraft use, foamed cushion in chairs or shoes, and gas separation membranes for capturing methane from landfill gas. To understand the gas transport properties and polymer thermal properties of the mixture, we detected the dielectric properties of the mixture at two conditions and developed a novel self-designed system for high temperature and high pressure measurement.

The transport properties of physical blowing agents in polymers is viewed as a critical parameter controlling the final foam product’s foam density, cell density, and cell size. We calculated theoretical CO₂ diffusivity coefficients in amorphous polymers using dielectric constant changes during CO₂ desorption. Such values were demonstrated to match experimental diffusivity coefficients measured with a gravimetric method. Three amorphous polymer films, polystyrene (PS), polycarbonate (PC), and cyclic olefin polymer (COP), were saturated with supercritical CO₂ in a pressure chamber. The CO₂ infused films were removed from the chamber for gas desorption experiments. The capacitance of the samples and the CO₂ weight percentages (CO₂ wt%) were obtained during gas desorption. It was found that the trend of dielectric constant changes was approximately that of the CO₂ wt% changes. This approximation was used to build a mathematical model for the prediction of CO₂ wt% during gas desorption using the measured dielectric constants. The two diffusivity coefficients obtained from experiments and this prediction model were compared with each other which successfully proved the reliability of our prediction model.

Gas/SCF also changes the melting and crystallization temperature (Tm and Tc) of polymers. These changes result from gas/SCF’s plasticization and hydrostatic pressure effects. In order to determine Tm and Tc and study the plasticization and hydrostatic effect of gas in the polymer, dielectric constant, ε′, and damping factor, tan(δ), of the polymer/gas mixture were measured and plotted as a function of temperature. A new system with control, measuring, and data acquisition elements, was designed to evaluate how the dielectric properties of high density polyethylene (HDPE) change with temperature. CO₂ and helium (He) were used separately as the pressurizing mediums in the experiments. At the same pressure, CO₂ and He had the same hydrostatic pressure effect on the polymer behavior. However, the plasticization effects of the two gases on HDPE were significantly different because of their different solubilities. There is a competing relationship between hydrostatic pressure and plasticization. The effect of plasticization is strong, when CO₂ is used, which leads to a decrease in Tm and Tc of HDPE with CO₂ pressure increase. Alternatively, hydrostatic pressure dominates when He is used, which results in an increase in Tm and Tc in HDPE as He pressure increase.