Coil Dimensions of Poly(ethylene oxide) in an Ionic Liquid by Small-Angle Neutron Scattering

The infinite dilution radii of gyration (R_g,0) for five different molecular weights (10-250 kg/mol) of perdeuterated poly(ethylene oxide) (d-PEO) have been determined at 80 °C in the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF₄]) using small-angle neutron scattering (SANS). The results establish the dependence of R_g,0 on polymer molecular weight as R_g,0 ∼ M_w^0.55. An excluded volume exponent of ν ≈ 0.55 indicates that [BMIM][BF₄] is a moderately good solvent and that PEO remains a flexible random coil in this ionic liquid. These results clarify the uncertainty surrounding PEO coil dimensions in this ionic liquid, as computer simulations at various levels of complexity have led to conflicting results, and prior experimental results also do not present a completely consistent picture.