Sorption of Supercritical Carbon Dioxide in Polymers: Experimental Determination and Modelling

Recent studies on supercritical fluid technology have focused on polymer processing with supercritical fluids (SCFs). Among others, an interesting field of research is the swelling and sorption of an amorphous polymer with a dense gas. The swelling of the O-ring seals when contacted with a SCF, and the effect of a quick release in pressure causing the rapid desorption of the SCF, can both damage the O-rings. With a good swelling database it should be possible to choose the proper material for each application. An interesting and useful property of a swelled polymer is the increment of solute diffusion coefficients; this enhancing effect can be as large as two order of magnitude, This can be exploited to extract impurities from a polymer matrix: the enhanced diffusion coefficients in the polymer makes it easier for the SCF solvent to reach the impurities trapped into the matrix, to solubilize them and to carry them away. Of course, the process can be conducted at low-to-moderate temperatures, so polymer degradation is avoided. At the same time also the impregnation of a polymer matrix with additives can be performed: for example biocompatible polymers can be charged with drugs to obtain controlled release pharmaceuticals. The principle is to swell the polymer with the dense gas, thus allowing the substance to migrate into the matrix. Then pressure is suddenly released, the gas is removed from the polymer, and the heavy drug is trapped into the now unswollen matrix. Another way to solubilize a drug in a polymer matrix is to dissolve it in a supercritical fluid, and to flow the supercritical solution through a column containing the matrix: the drug splits between the fluid and the stationary phase, following a distribution coefficient which is a function of process conditions (temperature, pressure, mole fraction of the solute in the fluid phase). To gain a better insight into these processes, a large database of swelling and sorption data should be useful. In practice the database available is quite small. The purpose of this work is to measure the sorption of CO2 in biocompatible polymers without the direct knowledge of the swelling data, which are difficult to measure for polymers in powder form. The experimental sorption data are correlated using the dual sorption model