Materials as Machines.

Machines are systems that harness input power to extend or advance function. Fundamentally, machines are based on the integration of materials with mechanisms to accomplish tasks-such as generating motion or lifting an object. An emerging research paradigm is the design, synthesis, and integration of responsive materials within or as machines. Herein, a particular focus is the integration of responsive materials to enable robotic (machine) functions such as gripping, lifting, or motility (walking, crawling, swimming, and flying). Key functional considerations of responsive materials in machine implementations are response time, cyclability (frequency and ruggedness), sizing, payload capacity, amenability to mechanical programming, performance in extreme environments, and autonomy. This review summarizes the material transformation mechanisms, mechanical design, and robotic integration of responsive materials including shape memory alloys (SMAs), piezoelectrics, dielectric elastomer actuators (DEAs), ionic electroactive polymers (IEAPs), pneumatics and hydraulics systems, shape memory polymers (SMPs), hydrogels, and liquid crystalline elastomers (LCEs) and networks (LCNs). Structural and geometrical fabrication of these materials as wires, coils, films, tubes, cones, unimorphs, bimorphs, and printed elements enables differentiated mechanical responses and consistently enables and extends functional use.

Localizing genesis in polydomain liquid crystal elastomers.

Programming the local orientation of liquid crystal elastomers (LCEs) is a differentiated approach to prepare monolithic material compositions with localized deformation. Our prior efforts prepared LCEs with surface-enforced spatial variations in orientation to localize deformation when the LCEs were subjected to directional load. However, because these surface alignment methods included regions of planar orientation, the deformation of these programmed LCEs is inherently directional. The absence of macroscopic orientation in polydomain LCEs results in uniform, nonlinear deformation in all axes (omnidirectional soft elasticity). Here, we exploit the distinct mechanical response of polydomain LCEs prepared with isotropic or nematic genesis. By localizing the polydomain genesis via masked photopolymerizations conducted at different temperatures, we detail the preparation of main-chain, polydomain LCEs that are homogeneous in composition but exhibit spatially localized programmability in their mechanical response that is uniform in all directions.

Mechanotropic Elastomers.

Liquid crystal elastomers (LCEs) are anisotropic polymeric materials. When subjected to an applied stress, liquid crystalline (LC) mesogens within the elastomeric polymer network (re)orient to the loading direction. The (re)orientation during deformation results in nonlinear stress-strain dependence (referred to as soft elasticity). Here, we uniquely explore mechanotropic phase transitions in elastomers with appreciable mesogenic content and compare these responses to LCEs in the polydomain orientation. The isotropic (amorphous) elastomers undergo significant directional orientation upon loading, evident in strong birefringence and x-ray diffraction. Functionally, the mechanotropic displacement of the elastomers to load is also nonlinear. However, unlike the analogous polydomain LCE compositions examined here, the isotropic elastomers rapidly recover after deformation. The mechanotropic orientation of the mesogens in these materials increase the toughness of these thiol-ene photopolymers by nearly 1300 % relative to a chemically similar elastomer prepared from wholly isotropic precursors.

All-Optical Control of Shape.

Photoresponsive liquid crystal elastomers (LCEs) are a unique class of anisotropic materials capable of undergoing large-scale, macroscopic deformations when exposed to light. Here, surface-aligned, azobenzene-functionalized LCEs are prepared via a radical-mediated, thiol-acrylate chain transfer reaction. A long-lived, macroscopic shape deformation is realized in an LCE composed with an o-fluorinated azobenzene (oF-azo) monomer. Under UV irradiation, the oF-azo LCE exhibits a persistent shape deformation for >72 h. By contrasting the photomechanical response of the oF-azo LCE to analogs prepared from classical and m-fluorinated azobenzene derivatives, the origin of the persistent deformation is clearly attributed to the underlying influence of positional functionalization on the kinetics of cis→trans isomerization. Informed by these studies and enabled by the salient features of light-induced deformations, oF-azo LCEs are demonstrated to undergo all-optical control of shape deformation and shape restoration.

Functional, composite polythioether nanoparticles via thiol-alkyne photopolymerization in miniemulsion.

Thiol-yne photopolymerization in miniemulsion is demonstrated as a simple, rapid, and one-pot synthetic approach to polythioether nanoparticles with tuneable particle size and clickable functionality. The strategy is also useful in the synthesis of composite polymer-inorganic nanoparticles.