Perturbation of the Experimental Phase Diagram of a Diblock Copolymer by Blending with an Ionic Liquid

Abstract

Understanding the phase behavior of block copolymer/ionic liquid mixtures is an important step toward their implementation in commercial devices. Here we report a high throughput and systematic small-angle X-ray scattering study of the lyotropic phase behavior of a series of polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) block copolymers in the ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide (EMIM Tf2N). The ionic liquid induces disorder-order transitions for a number of low molecular weight systems, and the onset points of these transitions are used to calculate the dependence of the effective Flory-Huggins interaction parameter (χeff) on the ionic liquid concentration. This enabled construction of an experimental phase diagram, which reveals that after taking volumetric swelling into account, at higher ionic liquid concentrations, the experimental phase boundaries shift significantly when compared to theoretical calculations for block copolymer melts. It is also demonstrated that the scaling of the domain spacing with ionic liquid concentration is dependent on the molecular weight for low degrees of polymerization. Finally, it is demonstrated that the addition of the ionic liquid is able to induce phase separation in normally disordered block copolymers to achieve individual lamellar domains as narrow as 7.2 nm, which is significantly narrower than those for neat PS-b-PMMA. These findings should be an important tool in future investigations that target specific self-assembled morphologies to suit a desired application.