Influence of Charge Fraction on the Phase Behavior of Symmetric Charged-Neutral Diblock Copolymers*

Nanostructured polymer electrolytes have gained significant scientific interest as potential replacements for liquid electrolytes in lithium-ion batteries. In this work, a series of poly[(oligo(ethylene glycol) methyl ether methacrylate–co–oligo(ethylene glycol) propyl sodium sulfonate methacrylate)]-block-polystyrene diblock copolymers were synthesized as a model system to probe the effect of charge fraction on the phase behavior of symmetric charged-neutral diblock copolymers. Small-angle X-ray scattering (SAXS) experiments showed that increasing the charge fraction does not alter the ordered phase morphology (lamellar), but increases the order-disorder transition temperature (T_ODT) significantly. Additionally, the effective Flory-Huggins interaction parameter (χ_eff) was found to increase linearly with the charge fraction, similar to the case of conventional salt-doped diblock copolymers. This indicates that the effect of counterion solvation, attributed to the significant mismatch between the dielectric constants of the two blocks, provides the dominant effect in tuning the phase behavior of this charged diblock copolymer. Electrostatic interaction is therefore a relatively weak contributor, likely due to the high dielectric constant of the charged block.