Rheology of polydimethylsiloxane swollen with supercritical carbon dioxide

Viscosity curves were measured for polydimethyl siloxane (PDMS) melts swollen with dissolved carbon dioxide at 50 and 80 degrees C for shear rates ranging from 40 to 2300 s(-1), and for carbon dioxide contents ranging from 0 to 21 wt %. The measurements were performed with a capillary extrusion rheometer modified for sealed, high-pressure operation to prevent degassing of the melt during extrusion. The concentration-dependent viscosity curves for these systems are self-similar in shape, exhibiting low-shear rate Newtonian plateau regions followed by shear-thinning ''power-law'' regions. Considerable reduction of viscosity is observed as the carbon dioxide content is increased. Classical viscoelastic scaling methods, employing a composition-dependent shift factor to scale both viscosity and shear rate, were used to reduce the viscosity data to a master curve at each temperature. The dependence of the shift factors on polymer chain density and free volume were investigated by comparing the shift factors for PDMS-CO2 systems to those obtained by iso-free volume dilutions of high molecular weight PDMS. This comparison suggests that the free volume added to PDMS upon swelling with dissolved carbon dioxide is the predominant mechanism for viscosity reduction in those systems. (C) 1997 John Wiley & Sons, Inc.