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.
Bibliographical noteFunding Information:
This work was supported by the Office of Basic Energy Sciences (BES) of the U.S. Department of Energy (DoE) Division of Materials Sciences and Engineering, under Contract DE-FOA-0001664. The SAXS experiments were performed at the DuPont–Northwestern–Dow Collaborative Access Team (DND-CAT) 5-ID at the Advanced Photon Source, Argonne National Laboratory. The SANS experiments were performed on the NG7 30 m SANS instrument at the National Institute of Standards and Technology (NIST), U.S. Department of Commerce.
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