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

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.