Rheological and Dielectric Behavior of Polyisoprene under Pressurized Carbon Dioxide

Abstract

For a well-entangled polyisoprene (PI; molecular weight = 260k) equilibrated under pressurized carbon dioxide (CO₂) at 25 °C, linear viscoelastic and dielectric data, respectively, were measured with a stress-controlled rheometer and a dielectric bridge being equipped with respective high-pressure cells. The viscoelastic and dielectric data shifted to higher frequencies with increasing CO₂ pressure, indicating that the pressurized CO₂ dissolved into PI thereby accelerating the global motion of PI. For those data at various CO₂ pressure, time-CO₂ pressure superposition held well and a single master curve was obtained, and the horizontal/vertical shift factors were consistent for the viscoelastic and dielectric data. These results indicated that the dissolved CO₂ behaved just as an ordinal solvent to accelerate the global motion of PI as in ordinary solutions. In fact, the dynamic tube dilation (DTD) relationship between the viscoelastic and dielectric data, known to be valid for ordinary solutions/bulk of linear PI, was found to work also the PI/CO₂ system, which confirmed the simple solvent role of the pressurized CO₂ for the global motion of PI. Thus, the knowledge for ordinary polymer solutions would work for processing of polymer/CO₂ systems.