Light-responsive block copolymer vesicles based on a photo-softening effect

We report the synthesis and study of two series of amphiphilic block copolymers (BCPs) of which the hydrophilic block is either poly(acrylic acid) (PAA) or poly(N,N-dimethylacrylamide) (PDMA) and the hydrophobic block is a side-chain liquid crystalline polymer (SCLCP) bearing biphenyl mesogens in majority and azobenzene mesogens in minority, namely, a random copolymer of 6-[4-(4-cyanophenyl)phenoxyl]hexyl acrylate with 6-[4-(4-methoxyphenylazo)phenoxyl]hexyl acrylate (P(BiPA-co-Azo)). Samples of both BCPs, PDMA-b-P(BiPA-co-Azo) and PAA-b-P(BiPA-co-Azo), could form large vesicles in aqueous solution with a SCLCP membrane. The BCPs were designed for investigating photoinduced order-disorder transition of the mesogens confined inside the vesicle membrane as a result of the trans–cisphotoisomerization of azobenzene and the LC cooperative effect. From photo-optical measurements on a BCP vesicle solution, we found evidence that a photoinduced LC order–disorder transition could occur inside the vesicle membrane in aqueous solution. Moreover, using a pH-sensitive fluorescent probe, we studied the photo-softening effect on the proton transfer across the vesicle membrane. Similar to plasticization of the vesicle membrane upon addition of a good solvent in aqueous solution, photo-induced softening could increase the rate of proton diffusion from inside to outside of the vesicle through the SCLCP membrane.