High-Pressure NMR of Polymers Dissolved in Supercritical Carbon Dioxide

High-pressure, high-resolution proton nuclear magnetic resonance spectroscopy has been utilized to study the solution behavior of poly(1,1-dihydroperfluorooctyl acrylate) (PFOA) and poly(1,1-dihydroperfluorooctyl acrylate-block-styrene) (PFOA-b-PS) in supercritical carbon dioxide. The proton chemical shift was monitored as a function of solvent density and temperature. A change in the solvent quality of CO2 from that of a poor solvent to that of a good solvent is postulated to describe a discontinuous change in the 1H NMR spectra of the polymers with increasing CO2 density and increasing temperature. At a CO2 density of 0.7 g cm-3 at 64.6 °C (0.8 g cm-3 at 41.2 °C) there is an apparent transition between a two-phase systemthe coexistence of two polymer species in different environmentsand a homogeneous solution. The volume fraction of CO2 in the polymer-rich phase is estimated from the proton chemical shift and the contributions from the bulk susceptibilities of PFOA and PFOA-b-PS.