Rheology of Polydimethylsiloxane Swollen With Supercritical Carbon Dioxide LINDA J. GERHARDT, CHARLES W. MANKE, ESIN GULARI Department of Chemical Engineering and Material Science, Wayne State University, Detroit, Michigan 48202 Received 3 June 1996; revised 28 August 1996; accepted 29 August 1996 ABSTRACT: Viscosity curves were measured for polydimethyl siloxane ( PDMS ) melts swollen with dissolved carbon dioxide at 50 and 80°C for shear rates ranging from 40 to 2300 s 01 , and for carbon dioxide contents ranging from 0 to 21 wt %. The measure- ments were performed with a capillary extrusion rheometer modified for sealed, high- pressure operation to prevent degassing of the melt during extrusion. The concentra- tion-dependent viscosity curves for these systems are self-similar in shape, exhibiting low-shear rate Newtonian plateau regions followed by shear-thinning ‘‘power-law’’ re- gions. 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-CO 2 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.  1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 523–534, 1997 Keywords: Keywords: polymer solutions; supercritical fluids; non-Newtonian viscos- ity; free volume; polydimethylsiloxane; carbon dioxide; capillary rheometer INTRODUCTION agents. Evaluation of the rheological properties of molten polymers with dissolved supercritical The processing of thermoplastic polymers, foams, gases is essential to many of these applications. and composites by such operations as extrusion While some data are available in the literature and injection molding is carried out at polymer for the rheology of polymer melts with volatile melt temperatures, which typically range from solvents, very few rheological measurements have 150 to 350°C, and at pressures that may reach been performed on molten polymers containing several hundred atmospheres. These processing dissolved gas under high pressures. Here we em- conditions fall within the regime of supercritical ploy an Instron capillary rheometer modified with behavior of various gases, including carbon diox- a pressurized extrudate chamber to measure vis- ide and low molecular weight hydrocarbons. Thus cosity curves for mixtures of high-molecular supercritical fluids ( SCFs ) may provide an alter- weight polydimethylsiloxane ( PDMS ) containing native to conventional organic solvents, blowing dissolved carbon dioxide. The temperatures of the agents, and plasticizers as polymer processing measurements, 50 and 80°C represent conditions well above the glass transition temperature of PDMS ( 0128°C), and near the critical tempera- Correspondence to: C. W. Manke ture of CO 2 (31°C) where CO 2 exists as a dense Contract grant sponsor: Dow Chemical Co.  1997 John Wiley & Sons, Inc. CCC 0887-6266/97/030523-12 gas at high pressure, facilitating dissolution of 523 9606001 / 8Q20$$6001 07-29-97 18:20:29 polpa W: Poly Physics