(139b) Viscosity and Density of Poly (E-Caprolactone) Solutions in Acetone + Carbon Dioxide Binary Fluid Mixtures

Viscosity is an important transport property that plays a key role in polymer formation, modification and processing. It influences flow dynamics and heat and mass transfer characteristics. Viscosity of the medium is an important factor in polymerization reactions in that it influences chain termination reactions and thus reaction kinetics. Perhaps a lesser-recognized but a very important aspect is the influence of viscosity on the physical rate processes such as phase separation processes in polymer solutions. Reducing the viscosity of polymer melts with addition of supercritical fluids like carbon dioxide, or increasing the viscosity of dense fluids with polymeric additives in deep well injections fluids for tertiary oil recovery are among other important application areas where viscosity plays a key role. In addition to its value in such practical applications, viscosity data on polymer solutions are used to gain fundamental insights on polymer-solvent interactions and the goodness of a given solvent and also on crossover concentrations, which influence chain flexibility and conformational states, and the morphology of the end products.

This presentation is on dissolution, volumetric properties and viscosity of a biodegradable polymer , poly(e-caprolactone), -[(CH2)5-COO]n -,in a new solvent mixture ?acetone + carbon dioxide? for these polymers. Poly(e-caprolactone) is of biomedical signioficance, specifically in controlled release-drugs and tissue engineering applications. The interest in processing or modification of biodegradable polymers with carbon dioxide or carbon dioxide containing fluid mixtures is of interest for the well known reasons of (a) tunability of the properties supercritical carbon dioxide, (b) the possibility of replacing or significantly reducing the use of toxic organic solvents, and (c) for the possibilities of easier processing and separation.

Experiments were conducted with a polymer sample with Mw = 14,300; Mw/Mn = 2.34 with 5 wt % solutions in acetone and in acetone + CO2 mixtures over a temperature range from 323 to 398 K at pressures in the range from 7 MPa to 35 MPa. The effect of adding CO2 on viscosity was investigated for the 5 wt % solutions at CO2 concentrations of 1, 2, 4, and 40 wt %. The effect of phase separation on viscosity was studied for the 5 wt % polymer solution in the binary fluid mixture with high (40 wt %) carbon dioxide content. The solution viscosities ranged from 0.2 to 0.6 mPa?s with flow activation energies in the range 7 - 9 kJ/mol and flow activation volumes in the range from 12 - 29 cm3/mol. Close-packed volumes were estimated to be in the range 0.5 - 0.6 cm3/g. In ternary solutions containing carbon dioxide, solution densities were higher, reflecting the higher density of compressed CO2 compared to that of acetone, but viscosities were lowered. A viscosity reduction of about 50 % was observed with 40 wt % CO2 addition.