Photoreversible Order-Disorder Transitions in Block Copolymer/Ionic Liquid Solutions

External stimuli are of interest in controlling the nanoscale morphology in self-assembling polymer materials. While temperature is commonly used to control the state of microphase separation in block polymer solutions, light is a highly tunable, contactless method that can be spatially selective. Here, we use UV irradiation to induce order-disorder transitions (ODTs) in concentrated solutions of four poly(methyl methacrylate)-b-poly(benzyl methacrylate-s-4-phenylazophenyl methacrylate) block copolymers in three imidazolium-based ionic liquids. Due to a significant change in polarity in the phenylazophenyl methacrylate monomer upon UV irradiation, a bistable temperature window exists between the ODT in the dark and under UV irradiation, allowing for light-triggered ODTs. UV-irradiated small-amplitude oscillatory shear rheology is used to locate the bistable window and then to trigger ODTs reversibly with light. The ordered states formed in the dark are determined by small-angle X-ray scattering. Three tunable factors are explored within the system: the relative lengths of the copolymer blocks, the selectivity of the solvent, and the content of the photoactive monomer. The block composition was found to mainly influence the ordered state formed by the system after microphase separation. The selectivity of the solvent is the most promising method for tuning the ODT temperature, and the amount of photoactive monomer controls the width of the bistable window.