Lubricant Effect of Flexible Chain in the Photoinduced Motions of Surface-Grafted Liquid Crystalline Azobenzene Polymer Brush

Abstract

Poly(hexyl methacrylate)-block-poly[4-(10-methacryloxydecyloxy)-4′-pentylazobenzene] (PHMA-b-P5Az10MA) diblock copolymer brush was stepwisely synthesized on an initiator-immobilized quartz surface by atom transfer radical polymerization. In this chain design, a flexible PHMA chain was connected between a liquid crystalline (LC) azobenzene (Az) block and a solid substrate. The orientation and structural features of the smectic layer spacing of the azobenzene (Az) block at the outer surface were essentially the same as those of P5Az10MA homopolymer brush. Upon irradiation of 436 nm linearly polarized light (LPL) at a temperature of liquid crystalline (LC) state of the Az polymer, the block copolymer brush exhibited a significant improvement in the induction of in-plane optical anisotropy compared to a corresponding homopolymer brush as revealed by UV–vis spectroscopy and grazing angle incidence X-ray diffraction measurements. The temperature dependence of the optical and structural anisotropy and photoreorientation behavior unveiled a significant lubricant role of the flexible chain block for the improvement of photoresponsive motions. This work proposes an idea to decouple the photoinduced motions of the upper photoresponsive block from the restriction by the substrate through introduction of a “buffer spacer” chain.