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

We describe the formation of bicontinuous microemulsions (B mu E) 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 B mu E 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 B mu E phase over a wide range of compositions and temperatures, it is remarkable that the B mu E 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 B mu E by modulating ion concentration.