Impact of Electrostatic Interactions on the Self-Assembly of Charge-Neutral Block Copolyelectrolytes

The self-assembly of diblock copolymers (BCPs) comprising flexible polymer chains, driven by a balance of enthalpic and entropic forces, is well understood. If one of the blocks is a polyelectrolyte, forming a charge-neutral BCP (CN-BCP), Coulombic interactions can play a significant role in the self-assembly. Here, electron microscopy and small-angle X-ray scattering, in combination with free-energy arguments and a scaling model inspired by surfactant self-assembly, are used to investigate the microphase segregation of CN-BCPs having pendent trisaminocyclopropenium (TAC) ions. We find that the TAC polymer electrolytes have an unexpectedly low dielectric constant (similar to 2.5) and that CN-BCPs containing a TAC polymer electrolyte block exhibit highly asymmetric morphology diagrams. These CN-BCP morphology diagrams have an unexpectedly large range of CN-BCP compositions where cylinders form, with the TAC block forming the continuous matrix, and in contrast to conventional BCPs, these cylindrical phases form even when the charged block is a minority constituent. These unusual morphologies observed in CN-BCPs with strong electrostatic interactions may thus provide a foundation for the exploration of new modes of ion transport in BCP self-assemblies.