Phase Transitions of Anionic Block Copolymers with Modulated Charge Density

Abstract Ionic block copolymers have great potential for diverse applications. Their phase behaviors have been intensively studied at limited charge densities and remain to be explored toward the high-density limit. Here, we establish a synthetic method to modulate the charge density of sulfonate groups in a broad range and study its effect on phase behaviors systematically. Incorporation of a low charge density is found to improve the long-range order of microphase separation significantly. In comparison, high charge density hinders the ordering process and traps the microphase separation in less ordered state, due to associated ionic clusters and crystallization of sulfonate groups. The crystallinity of sulfonates increases with charge density, while the dissociation of charge pairs decreases. The strong electrostatic cohesion of dense charges maintains the phase structures above the melting temperature and improves the thermal stability of block copolymers. These findings are validated in sulfonated block copolymers with sodium and lithium as counterions. This work provides an efficient chemical approach to modulate the charge density of block polymers and sheds insights into the physical mechanism of phase behaviors, which are very useful for exploring high-density ionic materials.