Lyotropic Phase Behavior of Concentrated Solutions of Diblock Copolymers in an Ionic Liquid

Concentrated solutions of three different poly((1,2-butadiene)-$b$-ethylene oxide) (PB-PEO) diblock copolymers in the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF$_{6}$] were prepared, and the lyotropic phase behavior of the ordered copolymer nanostructures was investigated via small angle x-ray scattering (SAXS). The three copolymers contained PB blocks of the same length (9 kDa), and different length PEO blocks (3, 4, and 6 kDa). At moderately high concentrations (50-80 wt{\%} polymer) all three of the block copolymers adopted a lamellar morphology, while at very high concentrations (80-95 wt{\%} polymer) the two block copolymers with lower PEO weight fractions formed hexagonally packed cylinders. For lower concentrations ($<$40-50 wt{\%} polymer), a broadening of the SAXS peaks indicated a loss of long-range order in the copolymer nanostructure.