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Voxelated liquid crystal elastomers

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Science  27 Feb 2015:
Vol. 347, Issue 6225, pp. 982-984
DOI: 10.1126/science.1261019

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

Abstract

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.

The dexterity, reversibility, and reconfigurability of complex shape or surface adaptivity within soft materials may enable enhancement and miniaturization of devices in medicine, robotics, and aerospace (1). Complex shape change and actuation has been reported in patterned hydrogels and through mechanical programming of carefully designed semicrystalline polymer networks (2, 3). The implementation of programmable shape change in applications in aerospace and other outlets requires the further development of soft materials that exhibit large stimuli-induced responses while affording local control of the magnitudes and directionality of the strain. Once realized, these shape-programmable materials could enable and extend the functionality of devices in applications from as simple as packaging to as complex as deployable and tunable antennas.

Liquid crystal elastomers (LCEs) are lightly cross-linked, ordered polymers that exhibit a reversible shape change in response to heat, light, or solvent. The alignment of LCEs into so-called monodomain or single-crystal orientations has primarily used stretching [analogous to the training of shape memory metals (4)] or magnetic fields. Uniaxially aligned LCEs exhibit dimensional changes (tensile strain) that can exceed 300% along the alignment direction in response to temperature changes (5). However, these alignment methods are limited in spatial control of orientation as well as resolution. Complex director profiles within LCEs are necessary to realize monolithic devices or functional substrates capable of nontrivial, programmable, reversible shape change (68). Such methods exist and have been used to generate complex and spatial variations in the director orientations of low-molar-mass liquid crystals as well as glassy liquid crystalline polymer networks (9, 10). However, the materials chemistries and procedures used to synthesize aligned LCEs have proven insensitive to these techniques. Here, we report on the development of a facile materials chemistry platform that is conducive to the surface alignment of liquid crystals. The sensitivity of the materials chemistry to surface alignment is combined with photoalignment of volumetric elements (also known as “voxels”) containing discrete domains of aligned LCEs. Enabled by the large strain inherent to LCEs, the sensitivity of the materials chemistry to surface alignment, and the optical patterning methods, we demonstrate programmable shape change and actuation in monolithic elements derived from a variety of complex director profiles.

To prepare spatially heterogeneous LCEs, we developed an optical patterning system in which the polarization of an irradiating 445-nm laser is dynamically controlled over an area as small as 0.01 mm2 (fig. S1). The optical setup was purposed to manipulate the local surface alignment of liquid crystalline cells prepared with commercially available, azobenzene-based photoalignment materials. The azobenzene dyes in the photoalignment material orient normal to the electric field vector of the linearly polarized light. Upon filling the cell with a nematic mixture of monomers, the director of the liquid crystal aligns to the local surface orientation of the photoalignment layer, which is translated through the sample thickness. Arbitrary and spatially complex patterns can be written into alignment cells with the system described in fig. S1 as illustrated in Fig. 1A, which presents a patterned liquid crystal cell that replicates the grayscale image of the first powered flight of the Wright brothers. The grayscale pixel values of the image are converted to the orientation angle of the surface-alignment pixels between 0° (dark) and 45° (bright). Upon filling the cell, the liquid crystalline monomers interact with the orientation of the pixelated surface-alignment layer to form oriented voxels. After polymerization, the grayscale image is retained and visible between crossed polarizers. In this example, 226 distinct director orientations are patterned into 21,350 voxels of material, each 100 by 100 μm in area and 0.0005 mm3 in volume. Each of these voxels can be thought of as local domain with a specific director orientation.

Fig. 1 Digital patterning of LCEs.

(A) Liquid crystals can be aligned point-by-point by altering surface conditions. (Top) An image (29) is digitized, and the grayscale value is converted to an alignment condition. Between crossed polarizers, the programmed optical rotation of the liquid crystal introduces light and dark regions. (B) Schematic of chemical structure of the main-chain LCE that can be surface-aligned. (C) Transmission of light through an LCE between crossed polarizers in the room-temperature nematic state and high-temperature paranematic state. (D) Biaxial actuation of homogeneously aligned LCE in the absence of mechanical load.

Seeking to enable both high actuation strain and complex surface alignment in a monolith, we prepared LCEs via a two-step synthetic procedure to produce poly(β-amino ester) networks (11, 12). Whereas the “Finkelmann method” to produce LCEs uses mechanical stretching to align the liquid crystal polymer and limits the complexity of alignment (13), this method allows for self-assembly to patterned surfaces. To enable surface alignment, we used a solvent-free, one-pot reaction that begins with low-viscosity precursors. After alignment, these monomers undergo a chain-extension reaction, forming main-chain nematic macromers that can be subsequently cross-linked, as schematized in fig. S2 (14). Cross-linking traps the alignment as dictated by the surface patterning into a low modulus and elastic solid. The resulting network structure is shown in Fig. 1B, in which the mesogen lies in the main chain of the polymer. After cross-linking, the birefringence of a surface-aligned uniaxially ordered elastomer is evident by monitoring light transmission between crossed polarizers (Fig. 1C) and through wide-angle x-ray scattering (fig. S3). At 200°C, the material is still birefringent (anisotropic), although the magnitude of the light transmission is greatly reduced at all angles. This remnant order is characteristic of a low-order paranematic state that arises from the constraint of cross-links within the network of certain LCEs (15). As evident in Fig. 1D, a spontaneous and reversible contraction on heating and expansion of 55% on cooling along the director is observed. Deformation of the LCEs is expected to be nearly volume-conserving, and as such, the two directions perpendicular to the director exhibit contraction upon cooling and expansion upon heating (16).

Here, we demonstrate that topological defects can also be imprinted in LCEs. A photograph between crossed polarizers of a voxelated LCE containing nine defects in a square array is shown in Fig. 2A. As depicted in the inset schematic to Fig. 2A, each +1 defect is a point around which the director varies azimuthally by 360°. Macroscopic azimuthal contraction (along the director) and radial expansion around each defect center lead to the emergence of cones. Remarkably, these features are more than 100 times taller than the initial film thickness of 50 μm, as seen in Fig. 2B. The apex of the cone is centered on the topological defect and is tipped with a stretched hemispherical cap with a radius less than three times the film thickness. The original patterned director profile is maintained in the film over many actuation cycles. This result suggests that recent theoretical and experimental results on highly cross-linked, liquid crystalline networks are also applicable to LCEs (17, 18).

Fig. 2 Topological defects, conical actuators.

(A) Representative photograph of LCE film with nine +1 topological defects between crossed polarizers. As indicated in the inset schematic, the director orientation varies azimuthally around the defect. (B) Upon heating, nine cones arise from the LCE film that reversibly flatten upon cooling. (C) Actuation occurs in the presence of loads tens of times larger than the actuator weight. (D) Quantification of specific work and stroke of a single actuating +1 defect.

Despite their highly compliant nature, the actuation of voxelated LCEs is not limited to an unloaded state (fig. S4). A representative photograph of an array of four defects lifting a load 147 times heavier than the weight of the actuator with a stroke of ~3000% is shown in Fig. 2C. This giant stroke arises from the amplification of the intrinsic shape change (~55% contraction) through a combination of localized stretching and delocalized bending. The performance of a single +1 defect as an actuator (along the axis of the cone) is quantified in Fig. 2D as a function of resisting load. Maximum specific work capacities of 2.6 J/kg and volumetric work capacities of 3.6 kJ/m3 were measured. As a point of reference, the specific work capacity is similar to some low-stroke, high-stress linear actuators, such as piezoelectric ceramic stacks (19). This work capacity can be attributed to the high energetic cost of preventing the emergence of Gaussian curvature at the center of the defect, which is equivalent to introducing stretch in a flat film (20). When normalizing to the 25-mm2 area of the defect (fig. S5), the actuation stress of a single +1 defect under the maximum applied load was 260 Pa. This value is not the blocking stress because the corresponding stroke is still >650%. The observed specific work capacities are far from the maximum reported values for organic tensile actuators. For example, coiled high-performance polymer and carbon nanotube fibers can generate >103 J/kg of work (21, 22). Despite these limitations in pure work capacity and stress generated, the voxelated LCEs are distinguished in the combination of moderate work capacity, the ease of shape programmability, and large stroke. These shape changes are not limited to thermal stimulus but can also be triggered by using chemical stimulus. Exposure to good solvents leads to the reduction of order in LCEs and also triggers complex reversible shape changes (fig. S6.). This feature may be particularly useful in active fluidics in which both linear displacement and shape of the active surface are critical to performance and could be controlled in situ.

Programming of shape can be extended further by taking advantage of the mechanical multistability of stretched and bent polymer films. Because of the constant director pattern through the thickness of the film, each topological defect possesses inversion symmetry. On activation, the defect must break this symmetry and spontaneously choose an orientation, upward or downward. In Fig. 3A, we examine a strip of three +1 defects tiled into a rectangle. Assuming that each defect can independently be identified and assigned either an up or down configuration, a total of 23 shapes could arise from a single monolithic element. We observed that each defect can be directed by mechanically pressing on the vertex until the defect snaps through to the opposite orientation. The potential energy associated with the tip of the cone displacement is schematized in Fig. 3B. Temperature cycling does not alter the orientation of the defect once “programmed.” As a result, the actuator reversibly forms the selected state in subsequent temperature cycles. This memory is likely imparted by a small irreversible strain bias that can be seen in the room-temperature actuator after cooling (Fig. 3A, top). In the particular case of this actuator design, symmetry reduces the actual number of distinct shapes to three, each of which is shown in Fig. 3A. This is a singular route to multistable shapes and is distinct from the variety of methods used to prepare multiple temporary shapes in shape memory polymers, in that the shape change is reversible and only requires simple up/down mechanical training (23, 24).

Fig. 3 Mechanical multi-stability.

(A) Each individual defect can actuate either up or down, leading to three distinct shapes from a single actuator with three defects. The orientation of the defect is indicated with black and white triangles (B) The potential energy diagram illustrates the presence of two metastable states on heating a single actuating defect.

Three-dimensional displacement can also be generated by introducing nonuniform director profiles through the thickness, as demonstrated in Fig. 4. By patterning a 500-μm-wide twisted nematic [in which the orientation of the nematic director rotates by 90° across the sample thickness (Fig. 4A)] region (hinge) across an otherwise unpatterned cantilever, reversible bending of over 150° is demonstrated in fig. S7. Over the course of 100 temperature cycles, the performance of the hinge does not appreciably change (fig. S7). Localized bending is caused by a gradient in strain through the material thickness associated with the variation in the orientation of the nematic director, which is analogous to a bimetallic strip used in household thermostats (25, 26). The facets of the film are composed of unpatterned regions that do not exhibit a preferential actuation direction. When arrays of these hinges are combined, origami-like actuators can be fabricated. The Miura-ori pattern describes a series of mountain and valley folds that can be simultaneously folded or unfolded with a mechanical stimulus (27). The resulting structure is an example of a planar auxetic structure. This pattern arises naturally in the leaves of certain plants and has been used in deploying solar cells (28). As shown in Fig. 4B, we mimic this pattern using 84 twisted nematic hinges, as schematized in fig. S8. Heating the initially flat film (318 mm2) results in cooperative folding at the localized hinges, which correspondingly reduces the macroscopic area by five (Fig. 4C). Although inherently the material has a Poisson’s ratio of ~0.5, the Poisson’s ratio of the structure is negative throughout the actuation, with a final value of –3.

Fig. 4 Origami-inspired actuators.

(A) Schematic of an edge portion of the Miura-ori pattern with a localized twisted nematic region bounded by unordered regions. (B) At room temperature, the LCE film is flat and upon heating above 150°C collapses in a way reminiscent of Miura-ori. (C) A reversible 5× reduction in area is observed with a negative in-plane Poisson’s ratio.

A facile one-pot synthesis of a poly(β-amino ester) allows for the preparation of LCE sensitive to directed self-assembly of local alignment with voxel-by-voxel–level control of actuation direction and magnitude. The resulting film can be programmed to exhibit localized bending or stretching in response to an order-reducing stimulus. These localized actuations can be combined to generate monolithic actuators with giant stroke or shape deployment.

Supplementary Materials

www.sciencemag.org/content/347/6225/982/suppl/DC1

Materials and Methods

Supplementary Text

Figs. S1 to S8

Reference (30)

References and Notes

  1. Materials and methods are available as supplementary materials on Science Online.
  2. 1860–1952 Records of the War Department General and Special Staffs, Photograph No. 165-WW-7B-6, “Wright Brothers’ 1903 Aeroplane Kitty Hawk in First Flight,” National Archives at College Park.
  3. Acknowledgments: This research was completed at Wright-Patterson Air Force base with the support of the Air Force Research Laboratory and the Air Force Office of Scientific Research. The authors thank A. Harbach for experimental assistance and K. M. Lee for helpful discussions. Additional information is available in the supplementary materials and from the authors.
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