Small angle neutron scattering study of the conformation of poly(ethylene oxide) dissolved in deep eutectic solvents

Hypothesis The conformation of poly(ethylene oxide) (PEO) in deep eutectic solvents (DESs) is determined by the polymer–solvent interactions, especially hydrogen bonding interactions. The hypothesis for this work is that the hydrogen bonding environment of a DES can be varied via changing the cation or hydrogen bond donor (HBD), and therefore the solvent quality for PEO; the anion species will also effect hydrogen bonding, but this is not examined here. Experiments Small angle neutron scattering (SANS) is used to probe the concentration dependent conformation of 36 kDa PEO dissolved in DESs formed by mixing ethyl or butyl ammonium bromide with a molecular HBD (glycerol or ethylene glycol) in a 1:2 molar ratio. Findings The radius of gyration (R_g), Flory exponent and crossover concentration (c^*) from the dilute to the semi-dilute regime of PEO in the DESs revealed by SANS and Zimm plot analysis show that these DESs are moderately good solvents for PEO. When the ammonium alkyl chain length is increased, the hydrogen bond density per unit volume decreases, and with it the solvent quality for PEO. The solvent quality is improved when the HBD is changed from glycerol to ethylene glycol due to differences in the hydrogen bonding environment for PEO.