Solute diffusion into polymer swollen by supercritical co2 by high-pressure electron paramagnetic resonance spectroscopy and chromatography

Abstract

High-pressure electron paramagnetic resonance (EPR) was used to measure translational diffusion coefficients (Dtr ) of a TEMPONE spin probe in poly(D,L-lactide) (PDLLA) and swollen in supercritical CO2 . Dtr 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 × 10−12 m2/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 (τcorr = 6 × 10−11 s) than expected, based on its Dtr . 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.