The Effect of Liquid Crystal Inclusions on the Mechanical Properties of Liquid Crystal Elastomers

Abstract

The synergy between materials with differing mechanical properties is an evolutionary adaptation for survival that pervades all of biology. Recognizing these masterstrokes of the natural world has inspired composite materials that enhance all aspects of quality of life. Composite design is particularly important for soft robots, which have advantages over their rigid-bodied counterparts for precision medicine, aquatic locomotion, and human interaction broadly. The relatively inferior mechanical properties of contemporary soft robots are not yet sufficient to replace hard-bodied robots and must be enriched for high load-bearing situations. Liquid Crystal Elastomers (LCEs) hold much promise as a candidate material for soft robotic bodies due to their rapid and reversible stimuli-responsive shape change. Solid fillers, interpenetrating polymer networks, and microstructural modulation have been employed to stiffen and toughen LCEs, yet these strategies substantially hinder extensibility or the liquid crystalline (LC) order. Liquid metal inclusions have recently been harnessed to profoundly increase the elastic modulus and toughness, though the isotropic droplets still compromise LC order. Ever elusive is a method for amplifying mechanical properties that elevate LC order without substantially compromising extensibility. In this thesis, a stiffened and toughened LCE composite is developed by doping with low molecular weight liquid crystal solvents. First, the influence of the nematogen 4-cyano-4'-pentylbiphenyl, 5CB, is studied. Through miscibility, thermal, and crystallographic studies, the enhanced mechanical properties are shown to emanate from strain-induced short-range smectic order (i.e., cybotacticity) and nanoscale phase separation of the LC solvent from the matrix. Uniquely, cybotacticity arises from components possessing no individual smectic ordering. Improvements of 570% and 370% in stiffness and toughness are conferred and extensibility only decreases by 20%. The first study is built upon by examining LCE modification with the smectogen 8CB (4-cyano-4′-octylbiphenyl). Markedly larger improvements are displayed in the stiffness (760%) and toughness (415%) while retaining 90% of the neat LCE’s elasticity. Strain-induced charge transfer is discovered as another factor responsible for the improved mechanical properties. Designing a stiffer, tougher, and lighter LCE with anisotropic liquids will facilitate the development of more effective soft actuators and attract more interest to the theory and application of liquid inclusion stiffening.