Microfluidic preparation of liquid crystalline elastomer actuators
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Abstract
The unique thermomechanical properties of liquid crystalline elastomers (LCEs) allow the reversible shape change of these materials towards an external stimulus, which provides suitable material properties for actuator applications and artificial muscles. The microfluidic processing of LCEs enables an effective shear force induced alignment of the liquid crystalline molecules (mesogens) in ordered director fields and features the preparation of variously shaped LCE microparticles.
In the present work, capillary based microfluidic devices are optimized for the fabrication of advanced LCE actuators. For the first time, actuating LCE Janus particles are synthesized in various shapes via microfluidics and their stimuli-responsive properties are studied at the nematic-isotropic phase transition via polarized optical microscopy (POM) and wide angle X-ray scattering (WAXS). Furthermore, dual temperature-responsive Janus particles are presented, which contain a lower critical solution temperature (LCST) hydrogel part beside a hydrophobic actuating LCE part to allow the independent shape change of each part at different temperature ranges and solvent environments. The amphiphilic character of these Janus colloids is utilized in a specially developed multi-step molding process for the self-assembly of rod-like Janus particles in well aligned monolayers at water/oil interfaces. By this approach, actuator functionalized surface devices of different shapes are obtained, which show collective and locally addressed actuation of the LCE covered surface during the phase transition.
In addition, a main-chain liquid crystalline monomer system is adopted for the microfluidic synthesis of strongly elongating LCE particles via thiol-ene click chemistry. The study of the nematic-isotropic LCE phase behavior concerning the amount of a liquid crystalline crosslinker reveals tunable and completely reversible actuation properties of corresponding main-chain LCE particles. Furthermore, the different microfluidic preparation methods of variously shaped homogeneous, core-shell and Janus LCE particles are reviewed with special regard to the microfluidic device construction and processability of liquid crystalline monomers.
In this work, the addressed advances in the field of LCE actuator research open new possibilities for the development of future LCE applications, such as temperature-responsive composite materials or actuator functionalized surfaces with variable adhesive and wetting properties.