Gelation and Flow Behavior of Amphiphilic Block Copolymers in Selective Solvents

Self-assembly of surfactants in the presence of selective solvent(s) can result in mesomorphic structures in the range of 2-50 nm, which are called lyotropic liquid crystals (LLCs). LLCs can be used as a template to produce nanoporous membranes and hydrogels. Water has been conventionally used as a solvent for self-assembly of Pluronic surfactants, which are triblock copolymer of poly(ethylene oxide) and poly(propylene oxide). Ionic liquids (IL) can also result in self-assembled structures with higher dynamic moduli than that of water due to their higher viscosity, which enhances the control and retention of the structure for templating applications. The self-assembly of Pluronic F127 in ethylammonium nitrate (EAN), a hydrophilic IL, has been previously investigated in detail by other researchers. Both F127/EAN and F127/water systems show sol-gel transition (i.e., disorder-to-ordered cubic transition), which decreases to lower temperatures with increasing surfactant concentrations, mainly driven by changes in the extent of hydrogen bonding. In this work, we studied the effect of solvent properties (EAN and water mixture) on the flow behavior of Pluronic F127 mesophases. All samples show Newtonian behavior at sol state and Type III nonlinear behavior at gel state accompanied by yielding. Studying the sol-gel transition of this system by both rheological and calorimetric methods shows that the sol-gel transition temperature has a negative deviation from the mixing rule when water and EAN mixtures are used. This phenomenon could be attributed to the strength of interactions between the solvent mixture and polyethylene oxide shell of the spherical micelles in the face-centered cubic structures.