Solute Diffusion into Polymer Swollen by Supercritical CO₂ by High-Pressure Electron Paramagnetic Resonance Spectroscopy and Chromatography

• Main Authors: Oleg I. Gromov, Mikhail O. Kostenko, Alexander V. Petrunin, Anastasia A. Popova, Olga O. Parenago, Nikita V. Minaev, Elena N. Golubeva, Mikhail Ya. Melnikov
• Format: Article
• Language: English
• Published: MDPI AG 2021-09-01
• Series: Polymers
• Subjects: poly(D,L-lactide); supercritical fluid; carbon dioxide; diffusion; electron paramagnetic resonance
• Online Access: https://www.mdpi.com/2073-4360/13/18/3059

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