We use here molecular dynamics (MD) simulations to study the structure and mobility of bulk melt PEO at the molecular level in response to a volume expansion such as might occur at the interphase of compatibilized polymer blends. PEO is simulated under constant-pressure conditions and compared to lower density constant-volume systems in the range similar to 90-100% of the ambient pressure density at 343 K. Analyses include energies and pressure, conformational data, intra- and intermolecular structural features, and the evolution of chain mobility and mechanical properties. The decrease in density induces negative pressures and an increase in molecular mobility. Moreover, a decrease of 10% in density is found to correspond to a temperature shift of approximately 60 K for the polymer backbone torsional interconversion relaxation times around C-O bonds.