Bicontinuous Microemulsions in Partially Charged Ternary Polymer Blends

We describe the phase behavior of a partially charged ternary polymer blend model system, comprising a compositionally symmetric poly[(oligo(ethylene glycol) methyl ether methacrylate-co-oligo(ethylene glycol) propyl sodium sulfonate methacrylate)]-b-polystyrene (POEGMA23-PS) diblock polymer and the constituent POEGMA23 and PS homopolymers, along the volumetrically symmetric isopleth, where 23 denotes the percentage of charged monomers in the POEGMA chain. Small-angle neutron and X-ray scattering and dynamic mechanical spectroscopy measurements reveal morphological transitions from a layered superlattice to swollen lamellae to a bicontinuous microemulsion (BμE), followed by macroscopic phase separation, with increasing homopolymer content. The BμE channel occurs between 85 and 90% homopolymer addition, positioned approximately at the isotropic Lifshitz composition predicted by mean-field theory for neutral systems. The resulting BμE morphology exhibits a periodicity of 26 nm, yielding a mesoscopically structured but macroscopically disordered bicontinuous structure. That this structure can be achieved in a charged polymer system is surprising, given the huge asymmetries typically induced by adding charge to either diblock copolymers or binary polymer blends.