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 degrees C, melting and crystallization temperatures are depressed by up to 18 degrees 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.