Liquid Crystallinity In Responsive Soft Materials: Embedding Intelligence Via Anisotropy

Jiaqi Liu, University of Pennsylvania


Liquid crystal (LC), known for its application in LC displays, is a state of fourth matter that has intrinsic anisotropy as it possesses both the fluidic property of liquid and the long-ranged order of crystals. Different from responsiveness of soft materials with isotropic networks such as hydrogels, shape memory polymers and dielectric elastomers, which is usually programmed by specific structural designs, polymer networks containing LC molecules with intrinsic anisotropy can realize inhomogeneous and reversible deformation as well as programmability. In this thesis, we mainly focus on understanding the self-assembly of LCs and applying their anisotropy in elastomers or hydrogels for applications including soft robots, programmable surface patterns mimicking pollen grains, responsive photonic crystals, and stretchable electrodes.We first couple the anisotropy of LCs with the elasticity of elastomers into LC elastomers (LCEs) and show fabrication of meter-long LCE composite filaments as muscle-like actuators by extrusion. The introduction of 4 wt% cellulose nanocrystals (CNCs) facilitates the alignment of LC mesogens along the filament long axis, and the incorporation of 2 wt% carbon nanotubes (CNTs) without aggregation improves the mechanical property, work capacity and actuation speed. Based on the Joule heating effect of copper wires and the photothermal effect of CNTs, the LCE/CNT composite filaments show both IR-light and electrothermal responsiveness with the electrothermal response time of 10 s, and the locomotion of composite filaments is demonstrated in various soft robotic systems. Then, we utilize LC anisotropy as a template to program surface patterns on LCE microparticles, including holes, stripes and spikes that resemble the surface patterns on pollen gains. The phase separation between LC oligomers (LCOs) and organic solvent (dichloromethane) as well as the simultaneous LCO chain segregation creates a director field distortion, which competes with homeotropic anchoring at the droplet interface and serves as a template to dictate the final patterns. By tuning the surface anchoring (planar vs. homeotropic), LCO composition and chain length heterogeneity, we can influence the phase separation process and tune the types of surface patterns. Further, we explore the self-assembly of LCs into the cholesteric phase and their switchable structural colors via two approaches. Approach I utilizes the evaporation-induced chiral assembly of CNCs. By controlling the evaporation rate, co-assembling with depletion agent polyethylene glycol (PEG), and tuning the evaporation substrate, CNC films with structural colors are obtained. In approach II, we fabricate cholesteric LCEs by adding a chiral dopant, which could exhibit tunable reflection color by mechanical force or an electric field when acting as dielectric elastomer actuators (DEAs). Finally, we apply the concept of LC anisotropy to fabricate conductive hydrogels from poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). Several methods are used to fabricate PEDOT:PSS hydrogels, including DMSO treatment, chemical crosslinking by (3-glycidyloxypropyl)trimethoxysilane (GPS), and forming interpenetrating networks with poly(acrylic acid) (PAA). After introducing CNCs and manually shearing, the PEDOT:PSS/CNC composite film exhibits an enhanced conductivity along the alignment direction. In addition, the PEDOT:PSS/PAA hydrogels with CNCs can be stretched up to 305% with an average sheet resistance of 120  in the wet state. The integration of anisotropy with isotropic polymer networks will potentially offer new insights to engineer next generation responsive and multi-functional soft materials.