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-rNIPAm) (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 lowtemperature 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. INTRODUCTIONIonic liquids (ILs) are ambient temperature molten salts and have attracted considerable attention because of their unique properties such as nonvolatility, nonflammability, thermal and chemical stability and high ionic conductivity. 1-8 Composite materials consisting of polymers and ILs can realize useful materials such as polymer electrolyte membranes, gas separation membranes and catalytic membranes. 9,10 Furthermore, we previously focused on stimuliresponsive polymers in ILs. 11,12 For example, poly(N-isopropylacrylamide) (PNIPAm) shows an upper critical solution temperature (UCST) phase transition in certain ILs, which is a completely opposite phenomenon to that observed in aqueous solutions. 13 In contrast, certain polymethacrylates 14-16 or polyether derivatives 17-19 are found to exhibit lower critical solution temperature (LCST) phase behavior in ILs. Phase transitions of polymers in nonvolatile and thermally stable ILs can afford long-term stable smart materials without solvent evaporation. Recently, we developed photoresponsive polymers in ILs by copolymerization of an azobenzene-containing monomer, 4-phenylazophenyl methacrylate (AzoMA), with either NIPAm 20 or benzyl methacrylate 21 as the main monomer.Self-assembly of block copolymer in an IL is another fascinating topic from the materials science point of view. 22-26 Block copolymer membranes or dilute solutions exhibit useful nanostructures due to intramolecular segregation. Lodge and coworkers reported polybutadiene-block-poly(ethylene oxide) (PB-b-PEO) block copolymer selfassembly in 1-butyl-3-methylimidazolium hexafluorophosphate ([C 4 mim]PF 6 ). 27 They found that PB-b-PEO self-assembled into micelles with an IL-insoluble PB core surrounded by P...

Photo-healable ion gel with improved mechanical properties using a tetra-arm diblock copolymer containing azobenzene groups

“…The water content of [C 4 mim]PF 6 was determined by a Karl Fischer titration and was less than 10 ppm. A(B) 2 (A block: PEG, B block: P(AzoMA-r-NIPAm)) and A(B) 4 (A block: tetra-PEG, B block: P(AzoMA-r-NIPAm)) polymers were synthesized according to previously reported procedures (Scheme 1) [30,38]. The number average molecular weights of the indicated segments are derived from the data presented in Table 1.…”

“…In this paper, we firstly compare the mechanical properties between ion gels consisting of [C 4 mim]PF 6 and either the tetra-arm diblock copolymer (A(B) 4 ) or the triblock copolymer (A(B) 2 ), in which the A segment is PEG and the B segment is P(NIPAm-r-AzoMA) (Scheme 1). The A(B) 4 block copolymer was prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization from a tetra-PEG macro-initiator according to a reported procedure [38]. For comparison, the A(B) 2 block copolymer was also prepared by RAFT polymerization from a bifunctional PEG initiator, following a reported procedure [30].…”

Advanced Materials Based on Polymers and Ionic Liquids

“…Because light is easily switchable, the above mentioned random copolymer could be utilized in stimuli‐responsive smart materials. To this end, we introduced a P(AzoMA‐ r ‐NIPAm) random copolymer segment into block copolymers and investigated their resultant phase transition behavior in ILs . Our recent results on a photo‐responsive ion gel of an ABA triblock copolymer consisting of the random copolymer as the A segment and IL‐soluble PEO as the B segment, (i. e., P(NIPAm‐ r ‐AzoMA)‐ b ‐PEO‐ b ‐P(AzoMA‐ r ‐NIPAm)), is highlighted.…”

Small-angle X-ray scattering from the concentrated bulk phase separated from an amphiphilic block-copolymer solution

“…Block copolymers consisting of a hydrophilic and a thermosensitive block or of two thermosensitive blocks form self-assemblies in water upon heating. Extensive studies have been performed recently on the temperature-induced self-assembly of such block copolymers, in view of to their potential applications in drug delivery systems, [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] Block copolymers studied so far consist predominantly of a poly(N-isopropylacrylamide) (PNIPAM) block and a poly(ethylene glycol). [1][2][3][4][5][6][7] With the advent of controlled polymerization techniques, it has become possible to prepare hydrophilic/thermosensitive block copolymers consisting of various kinds of blocks, e.g., PNIPAM-b-poly(acrylic acid), 9 PNIPAM-b-poly(glycidol), 10 PNIPAM-b-poly(N-vinyl-2-pyrrolidone), 11 , and doubly thermosensitive block copolymers consisting of two thermosensitive blocks, e.g., block copolymers of vinyl ethers with different pendant groups, 12 block copolymers of polystyrenics and polyacrylates with pendant oxyethylene moieties, 13 and block copolymers of poly(acrylamide)s bearing different alkyl side chains.…”