Modification of biomedical polymers in dense fluids. Miscibility and foaming of poly(p-dioxanone) in carbon dioxide + acetone fluid mixtures

This paper reports on the miscibility and foaming of a bioabsorbable polymer, poly(p-dioxanone) in CO2 + acetone mixtures at high pressures. Liquid–liquid miscibility pressures have been determined in acetone for polymer concentrations in the range from 1 to 15 wt% over a temperature range from 70 to 170 °C. Phase boundaries were also determined for 5, 7.5 and 10 wt% solutions in CO2 (11 wt%) + acetone (89 wt%); and for 7.5 wt% polymer solutions in CO2 (25 wt%) + acetone (75 wt%) fluid mixture. The solutions in both acetone and in acetone + CO2 mixtures were found to display UCST type behavior. In solvent mixtures, miscibility pressures show a marked increase with increased carbon dioxide content. At 150 °C, for the 7.5 wt% polymer solution, the miscibility pressures increased from about 25 MPa in pure acetone to 40 MPa in 10 wt% CO2 mixture and to about 65 MPa in 25 wt% CO2 containing mixture. Foaming experiments were carried out in a specially designed mold with porous plates in pure carbon dioxide and also in mixtures containing acetone at low addition levels (<1 wt%) at temperatures between 80 and 100 °C and at pressures between 20 and 45 MPa. Foams with relatively large and distinctly interconnected pores with pore diameters in the range from 20 to 100 μm were generated if pressures were greater than 20 MPa and the temperatures were less than 92 °C. Larger pores and interconnectivity were favored when foaming was carried out from mixtures containing acetone.