Effect of Ion Concentration on the Formation of Bicontinuous Microemulsions in Partially Charged Ternary Polymer Blends

We describe the formation of bicontinuous microemulsions (BμE) from a series of three model charged ternary polymer blends comprising an AB diblock copolymer and the constituent A and B homopolymers, where A is a partially charged poly[(oligo(ethylene glycol) methyl ether methacrylate-co-oligo(ethylene glycol) propyl sodium sulfonate methacrylate)] (POEGMA) block with systematically tuned ion concentration and B is a neutral polystyrene (PS) block. The mole fraction of charged groups in the POEGMA block and the corresponding POEGMA homopolymer was 7, 23, or 36%. The phase diagrams of ternary blends with variable ion concentrations have been mapped out along the volumetrically symmetric isopleth by using a combination of rheology, small-angle neutron and X-ray scattering, and cloud point measurements. A well-defined BμE channel is found over an unusually wide range of homopolymer compositions (2-7%), in the proximity of the Lifshitz multicritical point anticipated by mean-field theory. While the ion concentration significantly impacts both the morphological length scale and the extent of the BμE phase over a wide range of compositions and temperatures, it is remarkable that the BμE persists over a significant range of segregation strength. This study provides new insight into the rational design of charged ternary polymer blends to tune the structural characteristics of the BμE by modulating ion concentration.