Field-theoretic study of salt-induced order and disorder in a polarizable diblock copolymer

We study a salt-doped polarizable symmetric diblock copolymer using a recently-developed field theory that self-consistently embeds dielectric response, ion solvation and van der Waals (vdW) attractions via the attachement of classical Drude oscillators and/or fixed dipoles to the constituent fluid elements. This field theory can be directly simulated via the complex Langevin sampling technique, requiring no approximations beyond the phenomenology of the underlying coarse-grained model. We measure the shift in the order-disorder transition with salt-loading in our simulations and observe rich non-monotonic behavior in which ion solvation competes with dilution and charge screening effects to determine whether the ordered or disordered phase is stabilized. At low salt concentrations, the salt behaves as a selective solvent, localizing into the high-dielectric domains and stabilizing the ordered phase. At high salt concentrations, however, the salt localization vanishes due to charge screening effects, and the salt behaves as a nonselective solvent that screens vdW attractions and stabilizes the disordered phase. Our results raise questions regarding the conditions under which it is appropriate to ignore the screening effect of the ion cloud in theories of salt-doped polymers.