Near-infrared-responsive Copper Sulfide Nanoparticle/Liquid Crystal Elastomer Composites

We report a novel near-infrared-responsive photothermal conversion reagent/liquid crystal elastomer composite material. Nowadays, most of the near-infrared absorbing inorganic metal nanoparticles can poison platinum and other precious metal catalysts, due to the chemical properties of these metal nanoparticles or their stabilizers (such as long chain thiols, quaternary ammonium salts, etc.), which cause the failure to prepare inorganic metal nanoparticle/liquid crystal elastomer composites through classical Finkelmann two-step hydrosilylation reaction. In order to solve this problem, we apply a fast two-step thiol-ene photoaddition technique, by embedding near-infrared-responsive copper sulfide nanoparticles into liquid crystal elastomer matrix, and a composite film, based on copper sulfide nanoparticle/liquid crystal elastomer with near-infrared-responsiveness, is successfully fabricated for the first time. Water-soluble CuS nanoparticles are first synthesized, and oil-soluble CuS nanoparticles are prepared by exchanging the sodium citrate stabilizer with poly(3-mercaptopropyl-methylsiloxanes) (PMMS). The latter is mixed with PMMS, mesogenic monomers, crosslinkers and photoinitiators. The mixture is cast into a polytetrafluoroethylene mould and UV-illuminated at room temperature for 3 min to provide a pre-crosslinked liquid crystal elastomer composite film, which is further hung up and uniaxially stretched. The suspended pre-crosslinked liquid crystal elastomer film is UV-irradiated again at room temperature for another 15 min. to accomplish the second thiol-ene photo-crosslinking to obtain the composite film consisting of monodomain copper sulfide nanoparticle/liquid crystal elastomer. This composite material is thoroughly investigated using ultra-violet spectroscopy, transmission electron microscopy, polarized optical microscope, differential scanning calorimetry, and one-dimensional X-ray scattering. The experimental results demonstrate that this composite has a room-temperature nematic liquid crystal phase with a high-quality monodomain alignment. Most importantly, this composite film can perform a fast and reversible two-dimensional shrinking motion under irradiation of near infrared light with a wavelength of 980 nm