Viscosity studies of poly(DL -lactic acid) in supercritical CO ₂

The effect of supercritical CO ₂ on the viscosity and activation energy to viscous flow of P _DLLA is investigated, using a high pressure parallel plate rheometer, over a range of temperatures (50-140 °C) and pressures (5-12 MPa). The Cross model is fitted to the data to enable calculation of the zero shear viscosity and critical shear rate. A significant decrease in the viscosity is observed on increasing both variables; however, at high temperatures, the pressure effect becomes negligible. An increase in the critical shear rate is also observed on raising the pressure, indicative of a reduction in the relaxation time of the polymer. Manipulation of the Arrhenius equation shows a reduction in the activation energy to viscous flow as the pressure is increased. Together, these results show that the melt processing temperature of P _DLLA can be reduced in the presence of supercritical and high pressure CO ₂.