UV-enhanced Ordering in Azobenzene-Containing Polystyrene-block-Poly(n-Butyl Methacrylate) Copolymer Blends

Reversible photocontrol on the ordering of block copolymers through the disorder-to-order transition (DOT) is important to promote long-range laterally ordered nanostructures over a macroscopic scale by "photo-combing", similar to the classic zone refining used to produce highly pure, large single crystals. In this report, we have synthesized two deuterated polystyrene-b-poly(n-butyl methacrylate-r-2-(4-(phenylazo)benzoate)ethyl methacrylate) (PS-d8-b-P(nBMA-r-AzoEMA)) block copolymers containing photoisomerizable azobenzene functionalities. These two block copolymers and their binary mixtures exhibit an entropy-driven lower DOT (LDOT). Upon UV irradiation, the photoisomerization of azobenzene led to a gain in entropy, originating from conformational changes and density variations. Such an increase in entropic contributions significantly enhanced the ordering, thereby driving the phase-mixed copolymer blends toward a fully ordered state, as evidenced by small-angle neutron scattering (SANS) and specular neutron reflectivity (NR). This process could also be reversed by exposure to visible light via photoinduced cis−trans isomerization of azobenzene.