Molecular modeling study of CO2 plasticization and sorption onto absorbable polyesters

Abstract

Drug delivery systems are often made of porous polymer matrices. One method used to prepare a foamed polymer matrix is the controlled expansion of saturated polymers process in which amorphous polymer is exposed to CO2 at high pressure with a significant lowering of glass transition temperature. This plasticizing effect allows us to process temperature-sensitive polymers at relatively low temperatures. In the present study, computational tools were applied to estimate plasticizing effect of CO2 and calculate CO2- and H2O-loading capacities for three absorbable polyesters: polycaprolactone and two copolymers of (poly-d,l-lactid-co-glycolid)-co-polyethylenglycol. Plasticization caused by CO2 was estimated by solubility parameter and radial distribution function at several CO2 concentrations and by enlargement of free volume detected by mean square displacement of helium atoms, calculated after dynamic simulation. It was found that the maximal values of the solubility parameter and density can serve as a tool to predict saturation concentration. The loading capacities of the biopolymers that were preloaded with CO2 molecules were significantly higher than those of the nontreated polymers. Similar results were obtained for H2O molecules loading.