Charged polymer nanostructures

Diblock copolymers—made up of a chain of monomer A bound to a chain of monomer B—are of theoretical and practical interest. When the A and B monomers are sufficiently immiscible, they can segregate into self-assembled periodic nanostructures such as alternating layers or hexagonally ordered rods. The morphology of those two-phase structures depends on the phase immiscibility, the relative lengths of the A and B chains, and other factors. Choosing the A and B phases with complementary properties enables a variety of applications. For example, to make a solid-state electrolyte for a battery or fuel cell, one can optimize the A phase for ionic conductivity (with negative charges bound to the polymer chains balanced by unbound positive ions) and the B phase for mechanical stability. Numerous experiments, however, have shown that the theoretically derived phase diagram that predicts the nanophase morphology no longer applies when one of the phases is charged. Now Monica Olvera de la Cruz and colleagues at Northwestern University have developed a new theory that accurately describes the charged A phase by accounting for correlations among the charged monomers and ions. Tuning the strength of those correlations (which depends, most importantly, on the dielectric constant of the A phase) can induce qualitative changes in the A phase’s morphology, from disconnected cylindrical rods (seen in cross section in the top row of the figure) to a continuously connected “percolating” phase (bottom row) that allows long-range ion transport in any direction. (C. E. Sing, J. W. Zwanikken, M. Olvera de la Cruz, Nat. Mater. 13, 694, 2014 .)