UCST Phase Transition of Azobenzene-Containing Random Copolymer in an Ionic Liquid

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Abstract

We present azobenzene-containing random copolymers exhibiting upper critical solution temperature (UCST) phase separation in an ionic liquid (IL). A series of well-defined random copolymers comprising N-isopropylacrylamide (NIPAm) and 4-phenylazophenyl methacrylate (AzoMA) were successfully prepared by reversible addition–fragmentation chain transfer (RAFT) polymerization, using S-1-dodecyl-S′-(α, α′-dimethyl-α″-acetic acid)trithiocarbonate as a RAFT agent. The UCST phase separation temperature depends on the photoisomerization state of azobenzene, as well as on the AzoMA composition in the random copolymers. Turbidity measurements of the random copolymers (P(AzoMA-r-NIPAm)s) in 1-ethyl-3-methylimidazolium bis(trifluoromethane sulfone)amide ([C2mim][NTf2]) confirmed that the phase separation temperature of trans-azobenzene (P(trans-AzoMA-r-NIPAm) significantly increased, whereas that of P(cis-AzoMA-r-NIPAm) slightly decreased, with an increase in the composition of AzoMA in the copolymer. This result implies that nonpolar trans-AzoMA behaves as a solvato-phobic comonomer for the majority NIPAm monomer, whereas polar cis-AzoMA is relatively solvato-philic. The phase separation temperature difference between trans- and P(cis-(AzoMA29.1-r-NIPAm70.9) was as large as 43 °C. The triad sequence distribution of P(AzoMA-r-NIPAm) indicates that NIPAm copolymerization with AzoMA proceeds in a somewhat blocky manner. The trend of a significantly increasing phase separation temperature in P(trans-AzoMA-r-NIPAm) is qualitatively rationalized by the π–π stacking interaction among neighboring azobenzene side chains. Finally, a reversible photoinduced phase transition in an IL is demonstrated by utilizing the large phase separation temperature differences between the trans and the cis polymers in the IL.

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