Triblock Copolymer Self-Assembly in Ionic Liquids: Effect of PEO Block Length on the Self-Assembly of PEO-PPO-PEO in Ethylammonium Nitrate

The microstructure and rheological properties of micellar solutions of Pluronic block copolymers (PEOxPPOyPEOx) in deuterated ethylammonium nitrate (dEAN, a protic ionic liquid) are studied by small- and ultrasmall-angle neutron scattering (SANS and USANS), fluorescence microscopy, and shear rheology. A homologous series of three Pluronics with similar PPO block length (F127 (PEO106PPO70PEO106), P123 (PEO20PPO70PEO20), and L121 (PEO5PPO70PEO5)) are examined to explore the effect of the relative balance of solvophilic to solvophobic block size on the self-assembled microstructure. Both F127 and P123 form spherical micelles in dEAN that assemble into face-centered and body-centered cubic supramolecular crystals, respectively, upon increasing concentration or temperature. The transition from disordered to crystalline (ordered) structures manifests as a solgel rheological transition. A richer phase diagram is observed in the L121/dEAN system: due to its low solvophilicity in dEAN, L121 self-assembles into structures with low curvatures, namely, vesicles in the dilute regime, elongated (wormlike) micelles at intermediate concentrations and low temperatures, a nematic phase in concentrated solutions, and multilamellar phases at high temperatures. Highly viscoelastic solutions are created by the self-assembly of entangled L121 wormlike micelles. Structural length scales of the three Pluronic/dEAN systems in the various mesophases are determined from SANS and USANS.