Dielectric measurement was conducted for cis-polyisoprene (PIP) in carbon dioxide (CO 2 ) at high pressures. CO 2 is dissolved in PIP more with increasing pressure, and this dissolution is expected to affect the chain dynamics. In our experimental window, the PIP chains having type-A dipoles parallel along their backbone exhibited the dielectric dispersion reflecting their global motion. This dispersion shifted to higher frequencies with increasing CO 2 pressure. Time -CO 2 pressure

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... n was found to hold well, which suggests that the global motion of the chain is accelerated by CO 2 without any heterogeneity in space and along the chain backbone. The vertical shift factor (intensity shift factor) for this superposition gave the CO 2 concentration in PIP. The plasticizing capability of CO 2 evaluated from the horizontal shift factor (frequency shift factor) was found to be a little larger than that of tetradecane in the same range of concentration. No peculiarity was found for plasticizing ability of CO 2 even in the supercritical state (at a pressure of 10 MPa).