Mechano-Actuated Light-Responsive Main-Chain Liquid Crystal Elastomers

Abstract

Herein, a highly mechano-actuated light-responsive main-chain liquid crystal elastomer (LCE) was prepared using the two-stage thiol–acrylate Michael addition–photopolymerization (TAMAP) method with a reactive mesogenic mixture. This reactive mixture included a newly synthesized azobenzene-containing diacrylate reactive mesogen 4,4′-di(4-(acrylolylhexyloxy) benzoyloxy) azobenzene (D6OA), which exhibited a long mesogenic structure around the azobenzene bond. The monodomain main-chain azobenzene-containing LCE (M azo-LCE) film was prepared by mechanical drawing and visible-light curing the polydomain azo-LCE (P azo-LCE) film synthesized using the TAMAP method. M azo-LCE film showed approximately 30% contraction in length because of the trans-to-cis isomerization caused by ultraviolet (UV) irradiation, but it did not exhibit response to visible light because the long mesogenic structure of D6OA has a high energy barrier with respect to the cis-to-trans isomerization caused by visible light. However, the simple mechanical drawing of the M azo-LCE film induced the cis-to-trans isomerization. The alternating UV irradiation and mechanical drawing resulted in reversible trans-to-cis and cis-to-trans isomerizations for more than 10 cycles. The cis-to-trans/trans-to-cis isomerization and structural changes during UV irradiation and mechanical drawing were investigated via Raman spectroscopy and wide- and small-angle X-ray scatterings using a synchrotron radiation source. The azobenzene-containing chains in the layer were changed such that the staggered chains with extended trans isomers in the tilted layer (smectic C) were transformed into the chains with kinked cis isomers in the parallel-packed layer via UV irradiation, which were back to the original staggered extended chains with extended trans isomers in the tilted layer through mechanical drawing. The cis-to-trans isomerization via mechanical drawing can be attributed to the location of the azobenzene linkage in the main-chain backbone, but this has been rarely reported. The high actuation and simple recovery of the main-chain M azo-LCEs provide many application possibilities in various fields such as robotics, prosthetics, microfluids, data storage, and surface relief gratings.