Catalyst-free, highly sensitive and adjustable photo-responsive azobenzene liquid crystal elastomers based on dynamic multiple hydrogen bond

Abstract

Photo-responsive liquid crystal elastomers (LCEs) containing azobenzene components have received great attention and are in great demand gap for the development of smart materials. Herein, we have prepared a novel azobenzene photo-responsive liquid crystal elastomer (HAzo-LCEs) through simultaneously introducing azobenzene molecules and 2-ureido-4-pyrimidone (UPy) units into main chain of polyurethane molecule. Utilizing polyurethane system into this HAzo-LCEs has a trade-off between the exceptional mechanical performance and quick response. Especially, the transformation of photo-responsive HAzo-LCEs from polydomain to monodomain liquid crystal structures has been achieved under the catalyst-free and low temperature conditions by exploiting the dynamic nature of multiple hydrogen bond between UPy units. This synthetic method breaks the dilemma that the conventional photo-responsive LCEs require the preparation of monodomain liquid crystal structures at high temperatures or with the help of a large number of catalysts. Importantly, the HAzo-LCEs sample can bend rapidly in response to the UV light, and the maximum bending angle of the sample could reach 144°. Intriguingly, the blended HAzo-LCEs sample can rapidly straighten and finally extended under green light, showing the high sensitivity and flexible adjustability. It is valuable to integrating the high sensitivity, flexible adjustability and complete reversibility into one photo-responsive elastomers of this HAzo-LCEs. Furthermore, the multiple hydrogen bonds from UPy units allows the HAzo-LCEs to regulate its dynamics and stability by increasing the number and binding sites through the advantages of adjustable strength, high selectivity and abundant formation modes, providing great design and application scope for photo-responsive liquid crystal elastomers. This design route of non-toxic PU-based HAzo-LCEs provides a novel approach to broaden photo-responsive LCEs application in sensor and actuator.