Micelle structure of novel diblock polyethers in water and two protic ionic liquids (EAN and PAN)

Small angle neutron scattering has been used to probe the self-assembled structures formed by novel block copolymers in water and two protic ionic liquids (ILs), ethylammonium nitrate (EAN) and propylammonium nitrate (PAN). The block copolymers consist of solvophilic poly(ethylene oxide) (PEO) tethered to either poly(ethyl glycidyl ether) (PEGE) or poly(glycidyl propyl ether) (PGPrE) solvophobic blocks. Four block copolymers (EGE₁₀₉EO₅₄, EGE₁₁₃EO₁₁₅, EGE₁₀₄EO₁₇₈, and GPrE₉₈EO₂₆₀) have been investigated between 10 and 100 °C, showing how aggregate structure changes with increasing the EO block length, by changing the insoluble block from EGE to the more bulky, hydrophobic GPrE block, and with temperature. EO solubility mainly depends on the hydrogen bond network density, and decreases in the order H₂O, EAN, and then PAN. The solubility of the EGE and GPrE blocks decreases in the order PAN, EAN then water because the large apolar domain of PAN increase the solubility of the solvophobic blocks more effectively than the smaller apolar domains in EAN, and water, which is entirely hydrophilic; GPrE is less soluble than EGE because its larger size hinders solubilization in the IL apolar domains. Large disk-shaped structures were present for EGE₁₀₉EO₅₄ in all three solvents because short EO chains favor flat structures, while GPrE₉₈EO₂₆₀ formed spherical structures because long EO chains lead to curved aggregates. The aggregate structures of EGE₁₁₃EO₁₁₅ and EGE₁₀₄EO₁₇₈, which have intermediate EO chain lengths, varied depending on the solvent and the temperature. Solubilities also explain trends in critical micelle concentrations (cmc) and temperatures (cmt).