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

Abstract

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.

Impact of Photoactive Monomer Location in Photoresponsive Block Copolymer/Ionic Liquid Solutions

“…As demonstrated previously, a spherical particle phase is expected at 35 wt % for a symmetric MBsA in [C 1 mim][TFSI]. 20 Across the three samples, MBsA (Figure 1a) had the lowest T ODT at 130 °C. Above this temperature, Bragg scattering peaks indexed to hexagonally close-packed spheres (HCP), indicated by the (100), (020), and (101) indices situated close together at low q (Figure S22a).…”

“…The difference in T ODT between two 60 kg/mol PMMAb-P(BnMA-s-AzoMA) polymers, one with no azobenzene and one with 5 mol % AzoMA statistically copolymerized in the PBnMA, at 35 wt % in [C 1 mim][TFSI] was 80 °C (see Figure S27), a slightly larger difference than that between MAB and MBsA here. 20 This suggests that the midblock AzoMA is partially shielded from the solvent, and ordering will instead be driven by the PBnMA block.…”

“…Molar mass distributions for the polymers were obtained using SEC equipped with refractive index (Wyatt Optilab rEX) and multiangle light scattering detectors (Wyatt DAWN Helios II) with THF as the eluent. The d n /d c of the polymers in THF was calculated using a weighted average of the d n /d c values from M (0.086 mL/g), B (0.144 mL/g), and AzoMA (0.259 mL/g), where the weight percent monomer in the polymer was calculated from the 1 H NMR spectra . All 1 H NMR spectra and SEC traces can be found in the Supporting Information.…”

“…CTA end groups of all three block copolymers were removed by aminolysis followed by a Michael addition of methyl acrylate, as previously described. 20 In a representative reaction, MBsA-CTA (4 g, 0.18 mmol) was dissolved in tetrahydrofuran (70 mL) and stirred in a round-bottom flask (250 mL). Tris(carboxylethyl)phosphine hydrochloride (0.078 g, 0.27 mmol) was dissolved in deionized water (0.7 mL) and added to the reaction.…”

Concentration and Temperature Dependence of the Interaction Parameter and Correlation Length for Poly(benzyl methacrylate) in Ionic Liquids

“…The ionic liquids 1-butyl-1-methylpyrrolidinium bis­(trifluoromethylsulfonyl)­imide ([BMP]­[TFSI]) and 1,3-dimethylimidazolium TFSI ([MMIM]­[TFSI]) were purchased from IoLiTec GmbH (Heilbronn, Germany) and dried under vacuum for 48 h prior to use. 1-Ethyl-3-methylimidazolium TFSI ([EMIM]­[TFSI]) and 1-butyl-3-methylimidazolium TFSI ([BMIM]­[TFSI]) were synthesized via ion exchange in water as previously reported and dried under vacuum prior to use . Representative 1 H and 19 F NMR spectra of the ionic liquids in DMSO-d 6 are shown in Figures S3–S6.…”