The Effect of Gold Nanorod Doping in Optical-to-Mechanical Energy Conversion in Liquid-Crystal Elastomers

Abstract

Smart stimuli-responsive materials that can be externally triggered to undergo mechanical motions have a huge potential in applications ranging from artificial muscles and sensing to microrobotics. Among different classes of stimuli-responsive materials, photomechanical actuators based on liquid-crystal elastomers (LCEs) are of particular importance because they provide a route to control mechanical motions with light. Liquid-crystal elastomers exhibit a unique combination of anisotropic molecular order due to liquid crystallinity, and elasticity brought about by the polymer network. This coupling leads to the ability of free-standing LCE samples to change their shape reversibly after the application of external stimulus. Plasmonic nanoparticles, for example gold nano-rods, have the ability to absorb light strongly at different wavelengths, depending on their dimensions, which makes them a possible candidate to produce the light response needed to actuate LCEs. In the Thesis work three photoactuable liquid-crystal elastomers, which absorb light in different wavelengths, were studied. Infrared-absorbing gold nanorods, cross-linked UV-absorbing azobenzenes, and doped visible-absorbing azobenzenes, were used as light-absorbing moieties. The LCE that was used was acrylate-based and the sample films were prepared by applying well-developed liquid crystal-alignment technologies and photopolymerization. As a major part of this work, gold nanorods were synthesized and functionalized. Gold nanorods were synthesized with a seed-mediated growth method in water by using hexadecylcetyltrimethylammonium bromide as a surfactant and 5-bromosalisylic acid as an additive. Rods were also synthesized without the additive for comparison and it was found that nanorods synthesized with the additive had smaller size dispersion and better morphology. Gold nanorods were successfully functionalized and transferred to organic solutions with dodecanethiol when thiolated polyethylene glycol was used for pre-functionalization. The solubility of the functionalized gold nanorods into LCE mixture (comprising photopolymerizable monomer and cross-linker) was studied. However, functionalized gold nanorods were not soluble into the LCE matrix, whereas the two dyes showed a good solubility and polymerized films could be efficiently actuated with light. The main characterization methods used in this work were UV-Vis-NIR spectroscopy, transmission electron microscopy, and polarized optical microscopy.