Microphase Separation of Ionic Liquid-Containing Diblock Copolymers: Effects of Dielectric Inhomogeneity and Asymmetry in the Molecular Volumes and Interactions between the Cation and Anion

We study the phase behavior of a block copolymer and ionic liquid mixture using the Landau theory of Leibler and field-theoretic techniques in polymer physics. Our weak-segregation theory of microphase separation accounts for electrostatic screening, the excluded volume effect, and the dielectric contrast between the species. The results of the phase diagrams for ionic liquid-containing poly(styrene-block-2-vinylpyridine) and poly(ethylene oxide-block-styrene) qualitatively agree with the observed transitions between the ordered microstructures corresponding to lamellae, hexagonally packed cylinders, and body-centered cubic lattices. We show that moderate changes in the model parameters for molecular volumes and nonbonded interactions can qualitatively alter the trend of the phase boundaries. Specifically, our theory highlights the following features: (a) The effect of the dielectric contrast between the constituent blocks may cause a driving force for microphase separation, whereas the effect of electrostatic screening is likely minimal. (b) Reentrance into a lamellar phase, hexagonally packed cylinder phase, or disordered phase may occur when the volume fraction of an ionic liquid is increased. (c) Asymmetry in the molecular volumes and interactions between the cation and anion in an ionic liquid may cause substantial changes in the phase boundaries. The present results also suggest that despite its simplicity, the Born solvation energy of the ions can be used to consider the dissolution of an ionic liquid in block copolymers when the dielectric contrast is large.