Multifunctional Shape Morphing Elastomer With Liquid Metal Inclusions

A polymer composite having shape-morphing capabilities where the composite comprises a liquid crystal elastomer and liquid metal inclusions to improve thermal and/or electrical conductivity. The liquid metal inclusions are metals such as Gallium, alloys of Gallium, eutectic alloys, and other metals that have low melting points. The composite is soft and stretchable, while still retaining the shape-morphing characteristics of the liquid crystal elastomer. The composite is an electrical insulator, yet conductivity can be induced through mechanical pressure.

BACKGROUND OF THE INVENTION

The present disclosure is related generally to elastomers. More specifically, the disclosure is related to shape-morphing elastomer composites that are adapted to have improved electrical and thermal conductivity.

The development of soft robotics and wearable technologies have led to increasing demand for functional materials that can be powered with portable electronics. One such demand is for shape morphing materials. Liquid crystal elastomers (LCEs) are a promising functional material for these applications as they exhibit thermally induced reversible deformation, contracting with heating and expanding with cooling. LCEs may exhibit thermo-reversible strains of up to 400%.

While LCEs have the potential to meet the performance characteristics for these applications, implementation is difficult because these materials lack the electrical and thermal conductivity required for shape memory activation. To address these limitations, LCEs are typically embedded with rigid fillers that enhance conductivity. However, these filler particles degrade the mechanical properties and shape morphing capabilities of the LCEs. For example, some prior art has showed that the electrical resistance was too high for Joule heated actuation of an LCE composite using bulk carbon black filler until 15 wt. % carbon black was added. At 15 wt. % carbon black, the actuation strain reduced from 35.0% in the unfilled LCE to 5.2% in the LCE with filler. With a limit to the amount of filler that can be used, surface heating has also been utilized to induce shape changes in these materials. Surface heating elements are limited in applicability beyond thicknesses of a few hundred microns due to the low intrinsic thermal conductivity of LCEs. Ambient heating limits the speed, efficiency, and utility of LCEs in practical applications. Therefore, it would be advantageous to create a LCE composite with improved thermal and/or electrical conductivity.

BRIEF SUMMARY

According to embodiments of the present disclosure is a composite comprising a liquid crystal elastomer (LCE) having shape morphing functionality and a plurality of deformable liquid metal inclusions to provide thermal and/or electrical conductivity to the composite. The liquid metal inclusions can replace rigid fillers, eliminating the drawbacks associated with such fillers. Because the liquid metal inclusions are liquid at room temperature, they freely deform with the surrounding matrix when the composite is stretched. Moreover, the liquid metal inclusions do not interfere with the ability of the LCE to change shape and perform mechanical work in response to external stimuli. The composite achieves macroscopic shape change through a liquid crystal phase transition. With these features, the soft multifunctional composite is capable of sensing, mechanically robust electronic connectivity, and active shape morphing.

In a method of fabrication, liquid metal microparticles can be mixed into an uncured LCE matrix by shear mixing. The resulting composite is capable of Joule heated actuation with sufficient thermal conductivity to function in digital circuitry. Shape-morphing in the absence of an external load can be programmed into the LCE-LM composite through photoinitiated crosslinking to enable it to reversibly transition between pre-programmed morphologies through electrical or thermal stimulation. The LCE composite retains the ability to actively shape-morph even for cases when up to 50 vol. % (or about 83 wt. %) of the composite is filled with the mechanically passive LM droplets.

DETAILED DESCRIPTION

According to embodiments of the disclosure is a composite100comprising a liquid crystal elastomer (LCE)101and a plurality of liquid metal inclusions102. The LCE101is a type of polymer having shape morphing abilities and is formed from a polymer backbone and a mesogenic group111. A spacer112and crosslinker113may be included in the LCE101.FIG. 1shows the structure of the composite100, with the molecular, microscale, and macroscale ordering of the composite100depicted. As shown inFIG. 1, the liquid metal droplets102(or microparticles) are dispersed within the LCE matrix101in a random, uniform pattern. That is, the composite100shown inFIG. 1is homogeneous. Each liquid metal inclusion102has a roughly spherical shape and generally comprises an oxide skin (<10 nm) with a liquid interior. In one example embodiment, the shape of the liquid metal microparticles102can range from spherical to ellipsoidal with dimensions on the order of ˜4-15 μm. However, the size of the droplets102can vary depending on the intended application and may have a range of a few to several hundred microns. Depending on the type of liquid metal used in the composite100, the formation of the oxide skin can occur naturally during the fabrication process and aids in mixing without resorting to the use of an emulsifying agent. For example, EGaIn oxidizes in air to form a ˜1-3 nm thick Ga2O3skin.

FIG. 2shows an alternative view of the LCE101and its chemical components110/111, with the LCE101morphing capabilities demonstrated in cool/heat cycles. In one example embodiment, the mesogen111comprises 4-bis-[4-(3-acryloyloxypropypropyloxy) benzoyloxy]-2-methylbenzene (RM257, Wilshire Technologies) and the spacer112is 2,2-(ethylenedioxy) diethanethiol (EDDET) and the crosslinker113is pentaerythritol tetrakis(3-mercaptopropionate) (PETMP). A person having skill in the art will recognize that LCEs101can be made from a variety of components and the embodiment identified above is one such example. The mesogens111order anisotropically and order is disrupted above the nematic-to-isotropic transition temperature to enable macroscopic shape change.

The liquid metal inclusions102are freely deformable, as shown inFIG. 3. The images depicted inFIG. 3show the composite100in an unstrained state with spherical/ellipsoidal liquid metal inclusions102and in a strained state, where the liquid metal inclusions102have elongated along the direction of the strain. Due to the deformable nature of the liquid metal inclusions102, the stress-strain characteristics of the composite100are comparable to unfilled LCEs. InFIGS. 4A-4C, the composite100demonstrates: (i) a linear response for small strains; (ii) semi-soft elasticity due to reorientation of the liquid crystal director as strain increases; and (iii) an elastic regime before breaking. Composites100across a range of liquid metal loading are deformable, with average maximum extensions >150%, as shown inFIG. 5. In several embodiments, the average tensile moduli of the composite ranges between 0.2 and 1.2 MPa for all loadings (i.e. percentage of liquid metal102in the composite100), characteristic of the compliance of the LCE and composite100(seeFIG. 5). Both the unfilled LCE and 50 vol. % composite100had similar storage moduli across the temperature range measured by dynamic mechanical analysis (FIG. 6). Generally, the mechanical properties of the LCE matrix101are not drastically influenced by the presence of liquid metal inclusions102.

To maximize composite functionality, the liquid metal microparticles102should not inhibit the shape changing characteristics of the LCE101. As determined through dynamic mechanical analysis, the nematic-to-isotropic transition temperature of the composite100is not influenced by liquid metal inclusions102, occurring at 65.5±3.2° C. for the unfilled LCE and 64.7±2.7° C. for the 50 vol. % composite100(FIG. 6) and corresponding with a local minimum in the storage modulus. In this example, the transition temperature is within 1-2% of the unfilled LCE. A 50 vol. % composite100could reversibly extend to and retract from 1.62±0.10 times its original length, retaining >90% of actuation capabilities relative to an unfilled LCE at the same stress (1.74±0.01 times its original length). As the applied stress increases, the reversible change in length increases, a property typical for LCE shape-change. That the liquid metal microparticles102are functionally passive with respect to mechanical properties and actuation highlights a unique aspect of the composite100—the LCE matrix101can deform with the deformable liquid metal inclusions102and retain intrinsic shape-changing capabilities.

The inclusion of liquid microparticles102alters the thermal and electrical transport properties of the composite100, as compared to an un-filled LCE. The thermal conductivity of the composite100increases relative to the thermal conductivity of the unfilled LCE as liquid metal microparticles102loading increases. The thermal conductivities of liquid metal102loadings of 0, 10, 20, 30, and 50 vol. % are 0.24±0.02, 0.32±0.04, 0.45±0.02, 0.62±0.09, and 1.70±0.16 W m−1K−1, respectively (FIG. 7). As with other liquid metal embedded elastomers, the thermal conductivity increased along the loading direction as the liquid metal microparticles102elongate. The liquid metal/LCE composite100holds about 50-60% strain in a stress-free state due to reorientation of the liquid crystal director after being stretched. The thermal conductivity along the direction of strain increased for the unfilled LCE due to the change in liquid crystalline ordering from a polydomain to a monodomain. The average thermal conductivity of the unfilled LCE increased to 0.48±0.02 W m−1K−1(FIG. 7) along the direction of strain and was as high as 2.48±0.46 W m−1K−1for a 50 vol. % composite100. (FIG. 7). For comparison, the average thermal conductivities perpendicular to the direction of strain were 0.25±0.003 W m−1K−1for the LCE and 1.86±0.10 W m−1K−1for the 50 vol. % composite100. Improvements in thermal conductivity and heat dissipation are important for Joule heated actuation but also wearable electronics and soft machines that interface conductors and insulators with electronic devices, which may overheat with use.

After fabrication, the composites100are inherently electrically insulating. Electrical conductivity can be induced through a process of mechanically controlled liquid metal sintering, which comprises a process applying a pressure to the composite100to rupture the liquid metal droplets102, which then coalesce to form electrically conductive pathways120. Stated differently, the process irreversibly forms percolating networks of liquid metal102. Depending on where pressure is applied, conductivity can be limited to selective circuit traces120or active throughout the entire composite100. For example, mechanical pressure can be applied only along a path between electrical components, forming a wire-like trace120(as shown inFIG. 8). Electrical conductivities for a 50 vol. % composite100where the entire sample was mechanically sintered was 1×104to 2×104S/m. The normalized change in resistance can be monitored as a function of strain to evaluate electromechanical coupling for the 50 vol. % composite100(FIG. 8). There is negligible electromechanical coupling up to 60% strain when compared to the change in normalized resistance expected according to Ohm's Law. Conductive traces120can be self-healing through mechanisms in which mechanical damage activates the formation of new conductive traces120. Conductivity is stable when 1 V is applied for >65,000 seconds (18 hours), which is important for applications that require long-term Joule heating.

The composite's electrical conductivity enables the creation of internally Joule heated actuators, transducers for touch sensing, and circuit wiring for surface-mounted electronic components. Joule heated linear actuation of the composite can be excited at rates faster than 2 Hz and cycled to 50% reversible strain 15,000 times at 0.007 Hz (and 2.5% reversible strain >100,000 times at 1 Hz while still retaining >90% of its original shape-change).

In one example fabrication method, the composite100can be made by melting Gallium (Ga) in a 100° C. oven and, after allowing the Ga to cool without resolidifying, shear mixing to form Ga microparticles102. The Ga microparticles102are then mixed with a LCE101and cured. Using this process, solid composites100can also be fabricated by solidifying the Ga microparticles102after curing the LCE101. While Ga is described as the liquid metal102, other low-melting point metals can be used, including eutectic gallium indium (EGaIn), gallium-indium-tin (Galinstan), Indalloy, NewMerc, mercury, and other metals that are liquid at or near ambient temperature.

In another example fabrication method, the composite100can be synthesized by mixing 75 wt. % Gallium (Ga) with 25 wt. % Indium (In) at 100° C. until the In is completely dissolved, forming an eutectic alloy which will form the liquid metal inclusions102. The alloy is added to an uncured LCE mixture101before adding a catalyst by shear mixing with a stirrer 500 RPM for 30 seconds. After adding catalyst, the composite100is cured for >5 hours before placing the composite100in a 85° C. vacuum oven for >8 hours. The resulting composite100has liquid metal inclusions102in the form of droplets of ca. 200-500 μm in size.

In an alternative fabrication method, and by way of further detail, the composite100is created according to the following steps. 12 g of 4-bis-[4-(3-acryloyloxypropypropyloxy) benzoyloxy]-2-methylbenzene is dissolved in 4.5 g of toluene by heating in an 85° C. oven. The saturated solution is allowed to cool to room temperature before adding 0.675 g of tetra-functional thiol crosslinking monomer, pentaerythritol tetrakis(3-mercaptopropionate) (PETMP) and 3.17 g of di-functional thiol spacer monomer, 2,2-(ethylenedioxy) di-ethanethiol (EDDET). For composites synthesized with a photoinitiator, 0.576 g of PETMP. and 2.88 g of EDDET are added along with 0.144 g of the photoinitiator 2-hydroxy-4-(2-hydroxyethoxy)-2-methylpropiophenone (HHMP). After vigorous mixing, the mixture is returned to the 85° C. oven for 10 minutes. 1.5 g of the catalyst dipropylamine (DPA diluted 1:50 by weight in toluene is added after allowing the uncured mixture to cool to room temperature. If air bubbles are present, the mixture can be placed under vacuum for 60 seconds. A catalyst is stirred into the mixture slowly, and. the viscous mixture is poured into high density polyethylene molds. The reaction proceeds for >5 hours until the composite forms a stiff gel before placing the molds in an 85° C. vacuum oven for >8 hours to completely remove the solvent. After fabrication, the composite100is an electrical insulator. However, as previously mentioned, the composite100can be mechanically sintered to create electrical conductivity for internal Joule heating by applying pressure to the surface of the composite100. After sintering, any liquid metal residue on the surface of the composite100can be cleaned with isopropyl alcohol.

The composites100that are electrically conductive can function as electrically responsive actuators that activate through Joule heating. Macroscopic deformation can be observed as a weight is lifted, and multiple actuation cycles of 8 s on and 35 s off were tracked, showing repeatability in both the actuator displacement and power consumption.FIG. 9shows measurements of how quickly the composite100can actuate by tracking the normalized stroke as a function of cycle frequency. The actuation time, i.e. the active time period of the duty cycle, was set to 1/9ththat of the cooling time. Cycling frequencies as high as 10 Hz with actuation times as low as 10 ms are perceptible. The normalized stroke increased up to 0.02 Hz, where it remained nominally constant at 0.5 (i.e. 50% output strain at about 10-15 kPa). Actuation times on the order of hundreds of milliseconds for actively heated LCEs have been previously reported, and actuation strains at a given frequency for the soft composite100are consistent with previously reported LCEs that use rigid fillers.

Cycling frequency in Joule heated composites is limited by heat dissipation to the ambient environment. Thermal diffusion in the composite has a time scale approximated by the sum of internal conduction and external convection time constants. For composites100in a medium with a high convective heat transfer coefficient (i.e. water), a 50 vol. % composite100has a thermal diffusion increased by a factor of 5.

The maximum specific work density of the composite100was 30.5 J/kg, comparable to the maximum work density of mammalian skeletal muscle, and corresponding to a normalized dead load that was 193 times the mass of the composite. For comparison purposes, the intrinsic work density of the LCE matrix when normalized with respect to the mass of only the stimuli-responsive matrix material (i.e. 17% of the composite mass) is estimated to be as high as 179 J/kg. This calculation assumes that the mechanically inactive liquid metal particles 102 do not contribute significantly to actuation. Such an assumption is consistent with mechanical analyses that show that the liquid metal microparticles102do not influence mechanical properties of the composite100.

Since Joule heating is uniform throughout the composite100, shape-change is resilient to damage. The composite100can continue to lift a load by internal Joule heating after sustaining significant damage. Furthermore, the composite100operates normally in cold environments and when struck with a hammer. The composite100can also sense and respond to damage since mechanical forces activate conducting pathways120. For example, conductive traces120to power an LED were induced in a 50 vol. % composite100. Mechanical damage causes an electrical short around the LED, and the electrical current is rerouted through the composite100. The rerouted current initiates Joule heated actuation of the LCE101and causes the composite100to contract. The mechanical damage must be substantial enough to rupture the liquid metal microparticles102, coalesce, and reroute percolation pathways120. The composite100can be stretched, deformed, and prodded without inadvertently activating new traces120. This autonomous damage response demonstrates the versatility in material functionalities that this composite100, with the composite capable of functioning as a transducer, conductor, and actuator.

Thus far, the composite100discussed was cured without a preferential orientation of the nematic director; that is, the microstructure is polydomain, with liquid crystal moieties that are locally oriented but macroscopically isotropic. However, the composite100can also be irreversibly programmed using a photoinitiated post-curing process to lock in the molecular orientation of the LCE101after mechanical deformation. The appeal of photoinitiated programmability includes actuation in the absence of a bias-stress and controlled spatially-patterned shape-change that could see use in deployable and morphing structures. At zero-applied stress, a linearly programmed 50 vol. % composite100reversibly elongates to 1.53±0.09 times its contracted length, compared to 1.61±0.06 for a linearly programmed unfilled LCE101. Blocking stress at failure for a linearly programmed 50 vol. % composite100is 119±36 kPa, on the same order of magnitude as state-of-the-art multifunctional dielectric actuators (300 kPa) and natural muscle (100-350 kPa). Complex shape changes are possible with simple modifications to the strain field during programming, and the use of an opaque stencil mask during UV exposure.FIG. 10is a schematic for UV programming of the composite100. In this process, the composite100is stretched, exposed to UV light (mask is optional), and then heated above the nematic transition temperature. The process can be repeated as necessary for other stretch directions on the opposite side of the composite100.

FIG. 11illustrates another example of complex zero-stress shape change. The composite100undergoes shape-change from multiple half-cones to a flat geometry. Because the composites100are electrically conductive, such shape change can be induced by internal Joule heating. Joule heating can also be activated asymmetrically, enabling bistable and feasibly, multi-stable programmable structures. An example structure contracts on the side that is activated, and the monolithic composite100slides as each side contracts. The programmability of the LCE matrix101is not detrimentally disrupted by the presence of liquid metal102and thus is a beneficial additional functionality of the composite.

The impact of the liquid metal inclusions102on the composite100can be demonstrated by comparing a composite100with liquid inclusions102to one with solid Gallium inclusions. When solid Gallium microparticles are used, the composite100(composite (s) or (l) for solid or liquid gallium microparticles) was rigid with a tensile modulus >10× larger than the unfilled LCE and either liquid metal or liquid Ga-filled 50 vol. % LCE composite100. The composite (s) began to visually tear at less than 10% strain. Gallium microparticles102can be melted at a temperature less than the LCE nematic-to-isotropic transition. Upon melting the gallium microparticles102, the composite (l)100could extend to >50% strain. With further heating to above the transition temperature, the composite (l)100reversibly contracts since the inclusions102were in the deformable, liquid state. One aspect of utilizing Gallium vs. liquid metal is the ability to achieve zero-stress actuation without photoinitiated crosslinking. First, the composite (l)100is uniaxially extended and then cooled to solidify the Ga inclusions. By selectively Joule heating certain regions of the composite100, rigid and deformable domains can coexist in a monolithic composite. The deformable regions of the composite100can be selectively heated above the transition temperature by Joule heating. Since the rigid regions are constrained while the deformable regions attempt to contract, curvature and buckling is observed. Solid and liquid composites100reveal the mechanism by which high filler loading LCEs remain mechanically functional while also introducing an additional route to unique and arbitrary shape-morphing of the composites.

The invention may also broadly consist in the parts, elements, steps, examples and/or features referred to or indicated in the specification individually or collectively in any and all combinations of two or more said parts, elements, steps, examples and/or features. In particular, one or more features in any of the embodiments described herein may be combined with one or more features from any other embodiment(s) described herein.

Protection may be sought for any features disclosed in any one or more published documents referenced herein in combination with the present disclosure.

Although certain example embodiments of the invention have been described, the scope of the appended claims is not intended to be limited solely to these embodiments. The claims are to be construed literally, purposively, and/or to encompass equivalents.