Plasticization of Poly(ethylene oxide) in Fluid CO₂ Measured by in-Situ NMR

While use of CO₂ as a solvent for polymer synthesis has been well established, applications to polymer processing have followed more slowly. Motivated by success using supercritical CO₂ as a mediating solvent in forming composites of montmorillonite “nanoclay” with plasticized polymer melts, this study makes a detailed quantification of the CO₂ plasticization of solid poly(ethylene oxide) (PEO) using NMR spectroscopy and relaxation experiments. The study of solid polymers using NMR presents challenges but can yield detailed information about polymer morphology, dynamics, and chemistry. Immersing bulk polymers under fluid CO₂ in an NMR spectrometer allows direct access to plasticization dynamics relevant to polymer processing. Using a simple high-pressure apparatus and temperatures <90 °C, melting and crystallization temperatures are depressed by up to 18 °C when solid PEO is immersed in CO₂ at modest pressures (≤82 bar). A detailed analysis of NMR spectral components also quantifies the crystalline/amorphous fractions in PEO as a function of temperature and CO₂ pressure. Thus, this system shows a distinct plasticization effect even at modest CO₂ pressures, emphasizing the utility of CO₂ as a polymer processing aid.