Viscosity of Biodegradable Polymers Saturated with Carbon Dioxide

The viscosities of biodegradable polymers saturated with high pressure carbon dioxide were measured experimentally, at temperatures of 40 – 100 degrees C and pressures of 0 – 20 MPa (gage). The viscosities of low molecular weight polycaprolactone, poly(ethylene glycol) and poly(lactide-co-glycolide) were determined using a high pressure rheometer with a rotating, partially submerged cylindrical bob that had been calibrated with Newtonian fluids. Up to an order of magnitude decrease in polymer viscosity occurred due to the presence of carbon dioxide. Data from different temperatures and saturation pressures were correlated as a function of the reduced density predicted by the Sanchez-Lacombe equation of state, suggesting that free volume theory could be used to describe the transport properties. Although each studied polymer followed the Doolittle free volume equation for the zero shear rate viscosity, the universal constants of the WLF equation did not apply. The viscosity behavior is believed to be related to the propensity of each polymer to form drug delivery particles after depressurization from the saturated state.