Mechanical and Civil Engineering Seminar
Theoretical, computational, and experimental characterization of nematic elastomers
PhD Thesis Defense
Nematic elastomers are programmable soft materials that display large, reversible, and predictable deformation under an external stimulus such as a change in temperature or light. They are composed of a lightly crosslinked polymer network with stiff, rod-like liquid crystal molecules incorporated within the polymer chains. In thermotropic nematic elastomers, the liquid crystals undergo a continuous and reversible phase transition between the randomly oriented isotropic state and the highly oriented nematic state. Further, there is a direct thermo-mechanical coupling between the underlying temperature-responsive orientational order of the liquid crystal molecules and the macroscopic shape change of the surrounding elastomer chains. Finally, these materials display an unusually soft behavior. These remarkable properties make them promising materials for applications in aerospace as deployable structures and skins, in biomedical engineering as a soft pump, and in communications as the actuation mechanism in a reconfigurable antenna. Motivated by these applications, this thesis discusses the theoretical, computational, and experimental characterization of nematic elastomers.
We begin by investigating an example of actuation that takes advantage of the programmable, soft nature of these materials as well as instabilities associated with large deformation. We outline the multi-stable equilibrium solutions to a cylindrical balloon subjected to internal inflation, the material's microstructure formation due to this deformation, and its use as a soft pump with large ejection fraction, which involves a snap-through instability. Then we extend the Agostiniani-DeSimone-Dolzmann relaxed energy to a generalized Mooney-Rivlin constitutive relation and study four examples of Ericksen's universal deformations -- the inflation of cylindrical and spherical balloons, the cavitation of a disk, and the bending of a block.
We then move beyond the modeling of ideal materials and present a new constitutive relation for isotropic-genesis polydomain nematic elastomers. It is based on internal variables that describe the fine-scale domain patterns and evolve according to a kinetic process with dissipation. We discuss the model's implementation in the commercial finite-element software, ABAQUS, and study the problem of torsion of a cylinder. We identify an interesting instability at large torsional strains as a result of the Poynting effect. Finally, we present the design of a thermo-mechanical tensile setup and the experimental results for strain-rate dependence and temperature-dependence of samples that we synthesize in-house. https://caltech.zoom.us/j/83125707697?pwd=ZHVDUS9yZlJRUng1UHczd1dKb3pBQT09
Please virtually attend this thesis defense: Zoom Link: https://caltech.zoom.us/j/88331437327?pwd=RW42RWJWbnh1RTNTTjZ4cFJybHIvQT09