Carbon nanotube–cesium tungsten oxide–poly(N-isopropylacrylamide) hydrogel-based optical writing display

Abstract

Studies have attempted to improve thermal conductivity by complexing polymers. Via the complexing of photothermal materials with a heat-sensitive polymer, heat can be generated internally, thus resulting in efficient changes in the properties of the heat-sensitive polymer. This study fabricated an optical writing display to locally induce heat generation in poly(N-isopropylacrylamide) (pNIPAM), which is a heat-responsive material, using the photothermal effect of a near-infrared (NIR) absorbing material. Cesium tungsten oxide (CTO), an NIR-absorbing material, exhibits excellent properties due to its photothermal properties. By mixing carbon nanotubes (CNTs), the photothermal effect caused by NIR rays was strengthened and was more effectively controlled by providing thermal conductivity characteristics. The CNT–CTO hybrid absorbed the NIR laser, converted it into heat, and transmitted it to the adjacent pNIPAM network. When pNIPAM received heat, a hydrophilic–hydrophobic phase transition occurred, and a white pattern was observed owing to a change in the refractive index inside the hydrogel. Upon stopping the application of the NIR laser, rapid cooling occurred due to the high thermal conductivity of the CNTs, and the hydrophobic–hydrophilic phase transition was quickly achieved and restored. Therefore, based on the results of this study, optical writing displays using heat-responsive polymers containing photothermal particles and NIR lasers are expected to be applicable for information transmission in extreme environments, such as aviation, space, and the military, owing to the ability of lasers to travel in a straight line.