Amphiphilic block copolymers bearing strong push–pull azo chromophores: Synthesis, micelle formation and photoinduced shape deformation

Abstract

In this work, a series of amphiphilic diblock copolymers bearing strong push–pull type azo chromophores was synthesized through post-polymerization azo-coupling reaction scheme. The copolymers (P(CNAZOm-b-MAAn)), composed of 2-(N-ethyl-N-(4-(4′-cyanophenylazo)-phenyl)amino)ethyl methacrylate (CNAZO) and methacrylic acid (MAA) blocks, were obtained through four-step reactions. Firstly, precursor diblock copolymers (P(EMAm-b-tBMAn)) were obtained through sequential two-stage ATRP reactions of 2-(N-ethyl-N-phenylamino)ethyl methacrylate (EMA) and tert-butyl methacrylate (tBMA). Then, 4-amino-4′-cyanoazobenzene chromophores were introduced by azo-coupling reaction of P(EMAm-b-tBMAn) with diazonium salt of 4-aminobenzonitrile. Finally, P(CNAZOm-b-MAAn) was obtained through selective hydrolysis of the tert-butyl ester linkages in the tBMA blocks. Three block copolymers with the same CNAZO block length (m=100) and different MAA block lengths (n=5, 13, 23) were prepared by this method. The polymer and copolymers prepared in the process were characterized by GPC, 1H NMR, UV–vis, DSC and TGA measurements. Results show that P(CNAZOm-b-MAAn) forms spherical micellar aggregates by gradually increasing the water content in THF/H2O mixtures. The diameters of the spherical aggregates are related to the composition of the block copolymers and the water-adding rate. The block copolymer with larger molecular weight of the hydrophilic MAA block forms the aggregates with the smaller average size. The increase of the water-adding rate also shows an effect to reduce the diameters. Upon irradiation with a linearly polarized Ar+ laser beam, the spherical aggregates can be elongated in the light polarization direction. The deformation degree shows an almost linear dependence on the light irradiation time in the testing period. The deformed aggregates can recover the original spherical shape after thermal annealing at a temperature above Tg of the block copolymer.