Jeong Jae Wie

Voxelated liquid crystal elastomers

Making small actuators more effective Liquid-crystal molecules orient locally in response to external fields. When long-chain liquid-crystalline molecules are crosslinked together, changes in local orientation can lead to significant volume changes. Ware et al. made efficient microactuators that can change their shape from flat to three-dimensional structures (see the Perspective by Verduzco). By patterning volume elements so that each has a different preferred alignment for the liquid-crystalline molecules, they could fine-tune the volume changes. Science, this issue p. 982; see also p. 949 Liquid crystal elastomers are spatially patterned to create microactuators with controlled local volume changes. [Also see Perspective by Verduzco] Dynamic control of shape can bring multifunctionality to devices. Soft materials capable of programmable shape change require localized control of the magnitude and directionality of a mechanical response. We report the preparation of soft, ordered materials referred to as liquid crystal elastomers. The direction of molecular order, known as the director, is written within local volume elements (voxels) as small as 0.0005 cubic millimeters. Locally, the director controls the inherent mechanical response (55% strain) within the material. In monoliths with spatially patterned director, thermal or chemical stimuli transform flat sheets into three-dimensional objects through controlled bending and stretching. The programmable mechanical response of these materials could yield monolithic multifunctional devices or serve as reconfigurable substrates for flexible devices in aerospace, medicine, or consumer goods.

Torsional mechanical responses in azobenzene functionalized liquid crystalline polymer networks

Soft materials capable of both planar and flexural–torsional responses could enable the development of soft robotic elements that emulate the dexterity and functionality of a multitude of creatures in the animal kingdom. Here, we examine the response of azobenzene-functionalized liquid crystal polymer networks (azo-LCNs) specifically focusing on realizing large magnitude flexural–torsional responses observed as out-of-plane twisting or coiling. Towards this end, azo-LCNs were prepared in either the twisted nematic (TN) or hybrid orientations. The characterization of the flexural–torsional photomechanical responses is complimented with examination of thermomechanical properties. The diverse range of tailorable photomechanical responses is shown to be strongly dependent on the alignment of the nematic director to the film geometry and the actinic light intensity.

Photomotility of polymers

Light is distinguished as a contactless energy source for microscale devices as it can be directed from remote distances, rapidly turned on or off, spatially modulated across length scales, polarized, or varied in intensity. Motivated in part by these nascent properties of light, transducing photonic stimuli into macroscopic deformation of materials systems has been examined in the last half-century. Here we report photoinduced motion (photomotility) in monolithic polymer films prepared from azobenzene-functionalized liquid crystalline polymer networks (azo-LCNs). Leveraging the twisted-nematic orientation, irradiation with broad spectrum ultraviolet–visible light (320–500 nm) transforms the films from flat sheets to spiral ribbons, which subsequently translate large distances with continuous irradiation on an arbitrary surface. The motion results from a complex interplay of photochemistry and mechanics. We demonstrate directional control, as well as climbing.

Thermally and Optically Fixable Shape Memory in Azobenzene-Functionalized Glassy Liquid Crystalline Polymer Networks

Thermally and optically fixed shape memory is examined in glassy, azobenzene- functionalized liquid crystalline polymer networks (azo-LCN) in the twisted nematic (TN) geometry. The thermal and optical responses of two materials with a large difference in crosslink density are contrasted. The crosslink density was reduced through the inclusion of a monoacrylate liquid crystal monomer RM23. Reducing the crosslink density decreases the threshold temperature of the thermally-induced shape change and increases the magnitude of the deflection. Surprisingly, samples containing RM23 also allows for retention of a complex permanent shape, potentially due to differentiated thermal response of the pendant and main chain mesogenic units of the azo-LCN material.

Photopiezoelectric composites of azobenzene-functionalized polyimides and polyvinylidene fluoride.

Light is a readily available and sustainable energy source. Transduction of light into mechanical work or electricity in functional materials, composites, or systems has other potential advantages derived from the ability to remotely, spatially, and temporally control triggering by light. Toward this end, this work examines photoinduced piezoelectric (photopiezoelectric) effects in laminate composites prepared from photoresponsive polymeric materials and the piezoelectric polymer polyvinylidene fluoride (PVDF). In the geometry studied here, photopiezoelectric conversion is shown to strongly depend on the photomechanical properties inherent to the azobenzene-functionalized polyimides. Based on prior examinations of photomechanical effects in azobenzene-functionalized polyimides, this investigation focuses on amorphous materials and systematically varies the concentration of azobenzene in the copolymers. The baseline photomechanical response of the set of polyimides is characterized in cantilever deflection experiments. To improve the photomechanical response of the materials and enhance the electrical conversion, the polyimides are drawn to increase the magnitude of the deflection as well as photogenerated stress. In laminate composites, the photomechanical response of the materials in sequenced light exposure is shown to transduce light energy into electrical energy. The frequency of the photopiezoelectric response of the composite can match the frequency of the sequenced light exposing the films.

Reconfigurable Antennas Based on Self-Morphing Liquid Crystalline Elastomers

Pattern- and frequency-reconfigurable antennas are developed using metalized liquid crystalline elastomers that can change their geometries from flat to helical shapes. The pitch angle and the number of turns in this transformation are controlled by temperature. An external heat source is applied to reshape the geometry of these antennas, thereby reconfiguring their performance. The antennas are characterized using simulations and measurements.

The contribution of hydrogen bonding to the photomechanical response of azobenzene-functionalized polyamides

Photomechanical effects in materials can directly convert light stimulus into mechanical work. The magnitude of the photogenerated work is directly associated with the material stiffness (modulus). Here, we report the synthesis of an azobenzene-functionalized polyamide (azoPA) and demonstrate its room-temperature photomechanical response. The comparatively large photomechanical response of this high stiffness material is enabled by light-induced, azobenzene-assisted dissociation of inter-chain hydrogen bonding within the polymer backbone. The photomechanical response of the azoPA is directly contrasted with an analogous azobenzene-functionalized polyimide (azoPI) to illustrate the influence of intermolecular interactions.

Novel reconfigurable antennas using Liquid Crystals Elastomers

In this paper, normal and axial mode reconfigurable antennas are considered. Shape memory polymer actuators called Liquid Crystal Elastomers (LCEs) are used to dynamically change antennas. Specifically, a controlled heat source is utilized to the geometry of antennas. The proposed antennas were fabricated to fixed states to allow measurements to be performed and illustrate the antenna reconfigurability between normal and axial mode. Measurements are compared with simulations.