(535d) Rheology and Microstructure of Thermoreversible Micellar Crystals in Ionic Liquid

The structural and rheological properties of a model series of binary Pluronic block copolymer mixtures dissolved in a protic ionic liquid are studied as a strategy to modulate and control the soft solid behavior of amphiphilic block copolymers in ionic liquids. The softness of the soft solids are controlled via tuning the mixture composition of Pluronic block copolymers P123 and F127 self-assembled in deuterated ethylammonium nitrate (dEAN).^1,2 Equilibrium microstructures are studied by linear viscoelasticity and small angle neutron scattering (SANS) measurements, while the shear induced microstructures are probed by state of the art in situ rheo-SANS in the radial direction (1-3 plane of flow) under steady shear flow.^3,4 It is found that the mixture systems could be divided into two categories: hard-sphere-like and star-polymer-like. The equilibrium microstructures of hard-sphere-like mixture systems arrange into body centered cubic (BCC) lattice, star-polymer-like systems arrange into face centered cubic (FCC) lattice. And the measurement of the microstructure of the crystal show that the cubic phase lattice is formed by quiescently heating the mixture systems from the liquid region of the phase diagram, i.e., inverse melting. Under steady shear flow, increasing shear rate induces three structural transitions: polydisperse FCC lattice at low shear rates, formation of HCP layers at intermediate shear rates, shear-melting at high shear rates. This study points out a strategy for modification of the structural, and hence rheological, properties of Pluronic block copolymers in ionic liquid solvent, providing insights useful for comparing the properties of self-assembled amphiphilic block copolymers in molecular versus ionic liquid solvents under rest as well as steady shear flows.

References:

(1) López-Barrón, C. R.; Wagner, N. J.; Porcar, L. J. Rheol. (N. Y. N. Y). 2015, 59 (3), 793–820.

(2) López-Barrón, C. R.; Li, D.; Wagner, N. J.; Caplan, J. L. Macromolecules 2014, 47 (21), 7484–7495.

(3) López-Barrón, C. R.; Porcar, L.; Eberle, A. P. R.; Wagner, N. J. Phys. Rev. Lett. 2012, 108 (25), 258301.

(4) Eberle, A. P. R.; Porcar, L. Curr. Opin. Colloid Interface Sci. 2012, 17 (1), 33–43.