(553c) Self-Assembly of Pluronic Micelles In a Ethylammonium Nitrate

Nanoscale self-assembly in ionic liquids (IL) is of great interest as these fluids have special thermal and chemical stability, negligible vapor pressure, high ionic conductivity and wide electrochemical window.  In this work we study self-assembly and the structure-rheological properties relationships of amphiphilic Pluronic block copolymers (triblock copolymers made of a central polypropylene oxide (PO) and two end polyethylene oxide (EO) blocks) dissolved in a protic ionic liquid (ethylammonium nitrate, EAN). Using a combination of experimental methods including rheology, cross-polarized light microscopy, and small angle neutron scattering, we characterized the phase diagram of the Pluronic/IL model systems: F127 (EO₁₀₆PO₇₀EO₁₀₆)/EAN and L121(EO₅PO₇₀EO₅)/EAN. The length of the EO block has a significant effect on the shear viscosity and elastic modulus of the micellar aggregates. Spherical micelles are formed for higher Mw EO blocks, which stabilizes the highly curved spherical interface. For lower EO Mw steric effects are less significant and the interface curvature is lowered, forming cylindrical (wormlike) micellar aggregates. Spherical micellar solutions behave as non-birefringent Newtonian liquids whereas wormlike micelles show a considerable viscoelasticity and strong flow birefringence.  A comparison with pluronic self-assembly in water is interpreted with molecular thermodynamics, providing guidance for the self-assembly of non-ionic surfactants and block copolymers in ionic liquids.