Highly Durable and Tough Liquid Crystal Elastomers

Abstract

Liquid crystal elastomers (LCEs) are soft materials that exhibit interesting anisotropic and actuation properties. The emerging applications of thermally actuatable LCEs demand sufficient mechanical durability under various thermomechanical cycles. Although LCEs are tough at room temperature, they become very brittle at high temperature (above their actuation temperature), which can cause unexpected failure. We demonstrate a strategy to improve the high temperature fracture and fatigue properties of LCEs by designing interpenetrating polymer networks using a second polyurethane network. By selecting the appropriate composition of the polyurethane networks, the high temperature fracture and fatigue properties of LCEs were significantly enhanced, while retaining their actuation properties. The strategy from this work will help fabricate LCE-based actuators that are tough and durable at high temperatures and under cyclic loading.

Slidable Cross-Linking Effect on Liquid Crystal Elastomers: Enhancement of Toughness, Shape-Memory, and Self-Healing Properties

“…Liquid crystal elastomers (LCEs) undergo spontaneous shape transformations in response to various external stimuli, such as heat, light, and humidity, due to a change in the molecular ordering. − The large actuation capability up to 400% strain along with an anisotropic molecular order of LCEs has been harnessed to create a unique smart material platform and a variety of interesting devices, including soft robots, − switchable surfaces, − microfluidics, smart adhesives, dampers, , and biomedical substrates. − LCEs are lightly cross-linked through robust covalent networks, which impart rubber-like properties to them and enable effective reversible actuation. To further enhance their mechanical properties and actuating performances, LCEs consisting of dual networks have also been developed. − Recently, various types of dynamic covalent bonds that allow LCE network rearrangement under heat or light stimuli have also been introduced, and such systems are expected to offer several benefits over conventional LCEs, such as reprocessing, shape-reconfiguration, and self-healing properties. − However, the cross-linking points in the LCEs developed to date remain statically fixed and thus could not be translated in the absence of external stimuli.…”

Programmable Shape Change in Semicrystalline Liquid Crystal Elastomers

“…LCE materials generating higher blocking stress have been reported. To our knowledge, there are no reported LCEs capable of generating this high blocking stress at such low temperature values and over such a narrow temperature range. ,,, Actuation strain of an unconstrained semicrystalline LCE sample was also measured using an iso-force test (Figure C). On heating up to 50 °C, LCE films exhibit an actuation strain (contraction) of 0.56 ± 0.07% along the nematic director.…”

“…Cross-link density can also be increased to increase the elastic modulus of the material, but this approach greatly compromises the magnitude of the stimulus response the system can exhibit . Interpenetrating liquid crystalline polymer networks have been developed that significantly enhance the mechanical properties of the LCE. − Taking advantage of this approach, Yang and co-workers reported on interpenetrating polymer networks to achieve a blocking stress of 2.53 MPa and work capacity of 1267.7 kJ/m 3 . Inducing semicrystallinity in LCEs is another approach to enhance stiffness and achieve tough, higher-performance LCEs. ,− Kim et al reported on a crystallizable thiol–acrylate LCE material with blocking stress and actuation work capacity of 1.3 MPa and 730.5 kJ/m 3 , respectively .…”

Mechanically Strong, Wet Adhesive, and Self-Healing Polyurethane Ionogel Enhanced with a Semi-interpenetrating Network for Underwater Motion Detection

“…In addition, constructing an interpenetrating network (IPN) structure is considered another effective strategy to improve the mechanical properties of gel materials. 29 Compared with fully interpenetrating networks, semi-interpenetrating network (semi-IPN) gels can provide a fast response to environmental stimuli and higher tensile properties due to the absence of restricted interpenetrating elastic networks. 30 Consequently, it is worth exploring how to introduce PU and semi-IPN structures into the ionogel system to effectively enhance the mechanical performance and durability.…”