CONCENTRATION-DEPENDENT VISCOELASTIC SCALING MODELS FOR POLYDIMETHYSILOXANE MELTS WITH DISSOLVED CARBON-DIOXIDE

Recent measurements of the viscosity of polydimethylsiloxane (PDMS) me lts swollen with dissolved carbon dioxide at 50 and 80 degrees C by Ge rhardt et al. have shown that classical viscoelastic scaling factors c an be employed to superpose the viscosity curves for CO2-swollen melts onto the viscosity curve for pure PDMS at the same temperature and pr essure. Here a free volume expression for the viscosity of a diluted p olymer melt is combined with equation-of-state theories for the volume tric properties of PDMS-CO2 mixtures to develop models for the CO2 con centration-dependent viscoelastic scaling factors, Both the Sanchez-La combe and Panayiotou-Vera equations-of-state are investigated for this purpose. The predicted viscoelastic scaling factors are found to be i n very good agreement with the data of Gerhardt et al. The model predi ctions show that mixing of gaseous carbon dioxide into the swollen PDM S phase is highly non-ideal, and that significant free volume is added to the melt upon carbon dioxide dissolution. The free volume added to the melt by dissolved carbon dioxide is the principal mechanism for v iscosity reduction, relative to pure PDMS, in this system. (C) 1998 Jo hn Wiley & Sons, Inc.