Liquid Crystal Elastomers for Actuators and Electronics
Liquid crystal elastomers (LCEs) are well-recognized for programmable, large strain, and reversible shape changes in response to external stimuli. However, so far, there are several issues preventing the use of this class of materials in practical engineering applications such as actuators and electronics. The first part of the dissertation focuses on synthesis and processing strategies to expand capabilities of LCEs for actuator applications. Engineering application of LCEs are often limited by poor static and dynamic mechanical properties, e.g., modulus (~10 MPa), toughness (~10 MPa), blocking stress (~500 kPa), and work capacity (~300 kJ/m3 ). Also, these materials require high temperatures (typically above 100 °C) to undergo shape change. This work enables significant improvement in mechanical properties of LCEs by combining liquid crystallinity and semi-crystallinity. By developing novel synthesis and processing methods, crystallized LCEs are capable of not only enhanced static mechanical properties, including modulus (~350 MPa) and toughness (~40 MPa) but also improved dynamic mechanical properties, including blocking stress (~1.3 MPa) work capacity (~730 kJ/m3 ). This work also describes two routes to create multi-responsive LCE actuators that overcome the need to externally heat the material to high temperatures. We show high speed (~380 rpm) torsional actuation in response to chemical stimuli. Moreover, we provide a facile way to create programmed LCEs and carbon nanotubes (CNTs) composites. The LCE/CNT composites utilize visible light or electricity to trigger high-speed bending (~1 s) or uniaxial actuation (work capacity ~100 kJ/m3 , 2.5 times higher than mammalian muscles). The second part of the dissertation discusses electronic applications of LCEs. As current micro-electronic fabrication requires 2D flat substrates for photolithography processing, resulting devices are limited in 2D geometry which has minimal strain tolerance. Also, polymer-based biomedical electronics, e.g., neural interfaces, have significant issue to achieve long-term reliable encapsulation in the physiological condition. This work enables to process electronics on programmed 2D LCE substrates, then morph to desired 3D structures. The 3D electronics on LCE substrates provide strain tolerance up to 100% of deformation. We further show various examples of 3D electronics including strain tolerant capacitors and temperature sensing antenna enabled by LCE substrates. In the end, we briefly discuss current and on-going research to utilize LCEs for reliable packaging for advanced biomedical devices, e.g., deployable neural probes.