Mechanistic Study of the Influence of Salt Species on the Lower Disorder-to-Order Transition Behavior of Poly(ethylene oxide)-b-Poly(ionic liquid)/Salt Hybrids

Recently, lower disorder-to-order transition (LDOT) phase behavior was observed in the poly(ionic liquid) (PIL)-containing block copolymers, poly(ethylene oxide)-b-poly(1-((2-acryloyloxy) ethyl)-3-methyl imidazolium) with counterions of X (PEO-b-P(AOEMIm-X)), where the competitive hydrogen (H)-bonding and Coulombic interaction are present. In this work, the influence of different trifluoromethanesulfonate salts, such as LiTFO, NaTFO, KTFO, and Mg(TFO)(2), on the LDOT phase behavior of PEO-b-P(AOEMIm-TFO) and the related mechanism were explored. It is found that the disorder-to-order transition temperature (T-DOT) of PEO-b-P(AOEMIm-TFO) can be efficiently tuned by changing the salt cation. The salt affects the H-bonding and Coulombic interaction in different ways. The salts increase the miscibility and the T-DOT of PEO-b-P(AOEMIm-TFO) via diminishing the dissimilarity between PEO and P(AOEMIm-TFO) microdomains from the viewpoint of Coulombic interaction, which depends on the number of dissociated free ions. On the other hand, cations also interfere with the H-bonding between PEO and P(AOEMIm-TFO) through competitive complexation with PEO. These two factors have opposite effects on microphase separation. The former weakens microphase separation, while the latter enhances the immiscibility.