Effect of Ionic Liquid Components on the Coil Dimensions of PEO

Abstract

Small-angle neutron scattering is used to measure the infinite dilution radius of gyration (Rg,0) for a range of molecular weights (10–250 kg/mol) of perdeuterated poly(ethylene oxide) (d-PEO) in various imidazolium-based ILs at 80 °C. The Flory exponents (ν) evaluated in the ILs studied (1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM][TFSI]), 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]), and 1-hexyl-3-methylimidazolium tetrafluoroborate ([HMIM][BF4])) lie between 0.58 and 0.60, indicating good solvent behavior. The coil dimensions of d-PEO increase on increasing the alkyl chain length of the cation and on decreasing the anion basicity. A more pronounced effect on coil dimensions is observed upon altering the anion, compared to the dependence on cation alkyl chain length. The temperature dependence of Rg,0 is found to be moderate in 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]), where the coil size decreases upon increasing the temperature from 80 to 135 °C. Addition of up to 10 wt % of a salt, lithium bis(trifluoromethylsulfonyl)imide, to mixtures of d-PEO and [BMIM][TFSI] has no significant effect on Rg,0. These results are discussed in terms of the relative interactions among PEO, the cation, and the anion.