Temperature and light-induced self-assembly changes of a tetra-arm diblock copolymer in an ionic liquid

Abstract

A tetra-arm diblock copolymer ([PEG-b-P(AzoMA-r-NIPAm)] 4) was synthesized by reversible addition-fragmentation chain transfer copolymerization of N-isopropylacrylamide (NIPAm) and 4-phenylazophenyl methacrylate (AzoMA), initiating from the ends of functionalized tetra-arm polyethylene glycol (tetra-PEG). The resulting tetra-arm diblock copolymer consists of two segments: tetra-PEG (the center block; compatible with 1-butyl-3-methylimidazolium hexafluorophosphate ([C 4 mim]PF 6)) and P(AzoMA-r-NIPAm) (the end blocks; temperature- and photosensitive compatibility with [C 4 mim]PF 6 due to photoresponsive upper critical solution temperature phase behavior). We found that [PEG-b-P(AzoMA-r-NIPAm)] 4 underwent high-temperature unimer and low-temperature micelle (upper critical micellization temperature (UCMT)) transitions in [C 4 mim]PF 6. The UCMT of the tetra-arm diblock copolymer depended on photoisomerization states of the azobenzene groups within the copolymer. The UCMT of the trans-form polymer in dark conditions was higher than that of the cis-form polymer under UV-light irradiation. We demonstrated photoinduced self-assembly changes of the tetra-arm diblock copolymer in [C 4 mim]PF 6 at a 'bistable' temperature. Reversible photoinduced unimer/micelle transitions were also demonstrated.