Solute Diffusion into Polymer Swollen by Supercritical CO by High-Pressure Electron Paramagnetic Resonance Spectroscopy and Chromatography.

High-pressure electron paramagnetic resonance (EPR) was used to measure translational diffusion coefficients (D) of a TEMPONE spin probe in poly(D,L-lactide) (PDLLA) and swollen in supercritical CO. D was measured on two scales: macroscopic scale (>1 μm), by measuring spin probe uptake by the sample; and microscopic scale (<10 nm), by using concentration-dependent spectrum broadening. Both methods yield similar translational diffusion coefficients (in the range 5-10 × 10 m/s at 40-60 °C and 8-10 MPa). Swollen PDLLA was found to be homogeneous on the nanometer scale, although the TEMPONE spin probe in the polymer exhibited higher rotational mobility (τ = 6 × 10 s) than expected, based on its D. To measure distribution coefficients of the solute between the swollen polymer and the supercritical medium, supercritical chromatography with sampling directly from the high-pressure vessel was used. A distinct difference between powder and bulk polymer samples was only observed at the start of the impregnation process.