Shape-Persistent Liquid Crystal Elastomers with Cis-Stable Crosslinkers Containing Ortho-Methyl-Substituted Azobenzene

Abstract

Shape-persistent cis-isomer-stable azobenzene-functionalized liquid crystal elastomers (azo-LCEs) were prepared using an azobenzene crosslinker with two methyl substitutions at the ortho positions of azobenzene [4,4′-di(4-(acrylolylhexyloxy) benzoyloxy)-2,6-dimethylazobenzene (DADO)] via thiol–acrylate Michael addition photopolymerization. Ultraviolet–visible (UV–vis) and nuclear magnetic resonance (NMR) spectroscopies revealed that the methyl substituents on DADO rocked its cis conformation under UV irradiation and significantly delayed the visible (laser)-light-induced cis-to-trans isomerization. Raman spectroscopy and small-/wide-angle X-ray scattering analyses indicated that the cis-isomers in DADO-linkage-containing monodomain azo-LCE (azo-MLCEDADO) were not readily converted to the trans conformation under visible (laser)-light irradiation but transformed easily upon mechanical drawing. The azo-MLCEDADO film exhibited a low-plateau modulus and high elongation because of the weak intermolecular interactions caused by its bulky methyl substituents, leading to considerable UV-irradiation-induced contraction (up to 50%) and suspended weight-induced recovery to its original length. These cis-stable azo-MLCEDADO films can be used in shape-persistent actuators in soft robotics, haptics, and biomedical device fabrication.