The effects of supercritical fluids on amorphous polymers

The changes in the glassy state of amorphous polymers due to the sorption of pressurized gases were studied, and used to delineate the dominant effects governing the extraction of monomers and low molecular weight oligomers from polystyrene.;A Tian-Calvet type heat flow calorimeter was modified to take measurements at high pressures. The apparatus was used to quantify the effects of carbon dioxide, ethylene, and methane on the glass transition temperature of polystyrene and poly(methyl methacrylate). Both carbon dioxide and ethylene were found to be strong plasticizers of polystyrene. Carbon dioxide proved to be an even better plasticizer of poly(methyl methacrylate), while ethylene was equally efficient for both. Both polymers were plasticized to a liquid state at ambient temperatures with pressures below 100 atm of carbon dioxide and ethylene. Methane was only a weak plasticizer for both polymers. It was shown that changes in the glass transition temperature of the polymer-gas mixture depends only on the concentration of the gas, and is not directly affected by polymer-gas interactions.;The solubilities of carbon dioxide, ethylene, and methane in both polystyrene and poly(methyl methacrylate) were determined from pressure decay measurements. The trends in solubility with pressure were identical to the trends in the glass transition temperature with pressure. The changes in the glass transition temperature with pressure are thus a reflection of the relationship between the solubility of the gas in the polymer and pressure. The rate of sorption of these gases was found to be a strong function of temperature and gas concentration even in the glassy state of the polymer-gas mixture.;Extractions of styrene from polystyrene were performed using carbon dioxide and ethylene. The effects on the rate of extraction were tested using two separate sets of temperature and pressure conditions, and using static conditioning. Extractions using carbon dioxide were found to be limited by the thermodynamics of partitioning of styrene from polystyrene into supercritical carbon dioxide. This was a result of the strong interactions between styrene and polystyrene, and the weak interactions between carbon dioxide and styrene. Ethylene was found to be a better extractant than carbon dioxide due to its superior ability to plasticize the polymer matrix at these conditions, and due to a higher affinity of styrene for ethylene over that for carbon dioxide as reflected by the values of the partition coefficients for styrene. Extractions using ethylene were found to be limited mostly by kinetic factors, i.e. the rate of desorption of styrene from the polymer surface, and the rate of styrene diffusion through the polymer matrix.