The development of energy conversion and storage devices is at the forefront of research geared toward a sustainable future. However, materials limitations serve as bottlenecks in this progress. Typical liquid electrolytes used for the transport of ions between an anode and cathode are highly flammable and pose a safety hazard. Research efforts have focused on exploring solid electrolytes that do not have these limitations. Block copolymers are an alternative as solid electrolytes because they can self-assemble into nanostructures, which enables ion transport and maintains structural integrity. Furthermore, the ion conductivity can be controlled according to the nanostructure geometry.

Researchers Charles E. Sing, Jos W. Zwanikken, and Monica Olivera de la Cruz from Northwestern University have used a theoretical model to demonstrate that the distribution of charge on the blocks comprising block copolymer systems can be used to manipulate the copolymer through inducement of an array of nanostructures. As reported in the July issue of Nature Materials (DOI: 10.1038/NMAT4001; p. 694), the researchers varied the charge density along the block copolymer backbone through straightforward design of parameters. Charges along one of the blocks as well as the corresponding counterions dictate the equilibrium properties of the block copolymers through a combination of entropy, ion solubility, and electrostatic cohesion, which taken together drastically manipulate the phase behavior. The accompanying figure depicts the effect of charge cohesion on nanostructure phase behavior, demonstrating that highly asymmetric charge cohesion effects can induce the formation of nanostructures that are inaccessible to conventional uncharged block copolymers, including percolated phases desired for ion transport.

This work presents an orthogonal route toward tuning nanostructures that will have a significant impact on the design of block copolymer materials for use in numerous applications, such as in lithographic templates for small electronics, sensors, drug delivery, vehicle filtration membranes, and materials with novel and tunable mechanical properties.

Effect of charge cohesion on nanostructure phase behavior showing electrostatic cohesion between the charged A-blocks and the counterions. When the interactions are strong, the components are electrostatically correlated in a liquid-like ordered structure, which has a significant effect on the phase diagram. Reproduced with permission from Nat. Mater. 13 (2014), DOI: 10.1038/nmat4001. © 2014 Macmillan Publishers Ltd.