Selective Decrosslinking in Liquid Crystal Polymer Actuators for Optical Reconfiguration of Origami and Light‐Fueled Locomotion

The ability to optically reconfigure an existing actuator of a liquid crystal polymer network (LCN) so that it can display a new actuation behavior or function is highly desired in developing materials for soft robotics applications. Demonstrated here is a powerful approach relying on selective polymer chain decrosslinking in a LCN actuator with uniaxial LC alignment. Using an anthracene‐containing LCN, spatially controlled optical decrosslinking can be realized through photocleavage of anthracene dimers under 254 nm UV light, which alters the distribution of actuation (crosslinked) and non‐actuation (decrosslinked) domains and thus determines the actuation behavior upon order‐disorder phase transitions. Based on this mechanism, a single actuator having a flat shape can be reconfigured in an on‐demand manner to exhibit reversible shape transformation such as self‐folding into origami three‐dimensional structures. Moreover, using a dye‐doped LCN actuator, a light‐fueled microwalker can be optically reconfigured to adopt different locomotion behaviors, changing from moving in the laser scanning direction to moving in the opposite direction.

Die selektive Entkopplung der Polymerketten in einem flüssigkristallinen Polymernetzwerk(LCN)‐Aktuator wird beschrieben. Die räumlich kontrollierte optische Entkopplung verändert die Verteilung der aktuierbaren (vernetzt) und nicht aktuierbaren Domänen (entkoppelt) und bestimmt das Aktuationsverhalten bei Ordnungs/Unordnungs‐Phasenübergängen. Durch die Einsatz eines farbstoffdotierten LCN‐Aktuators kann ein lichtbetriebener Mikroläufer optisch rekonfiguriert werden.

Keywords: Farbstoffe und Pigmente; Flüssigkristalle; Fortbewegung; Polymere; Weiche Aktuatoren

Liquid crystal polymer networks (LCNs, including liquid crystal elastomers) have been gaining more and more interest for use as smart actuators for applications including energy generators, motors, fluid propellers, and in particular, soft robotics. Generally, an LCN actuator is prepared by first aligning the mesogens in a certain way (using mechanical force or surface effect or electric/magnetic field) and then fixing the LC alignment through polymer chain crosslinking. By controlling the LC alignment (e.g. uniaxial vs. splayed vs. twisted) or the distribution of crosslinked area, or by using "patterned" stimulation, LCN actuators can realize complex, reversible shape change upon order–disorder phase transitions of the mesogens. Among the important issues on LCN actuators towards practical applications, is the development of new methodologies that facilitate the preparation of diverse actuators capable of programmable or on‐demand shape change and even locomotion using the same liquid crystal polymer (LCP) without the need for organizing the LC alignment in different ways. Recognizing the importance of chain crosslinking, which memorizes the original LCN shape in ordered phases and enables the actuation reversibility, controlling crosslinking has been the center of attention in many studies. The most explored strategy is to replace covalent bonds with dynamic covalent bonds for chain crosslinking, by using transesterification reactions, disulfide metathesis,[21] and exchangeable carbamate bonds,[22] to name a few. In practical terms, if an existing LCN actuator is to be reconfigured to exhibit a different shape change, it must be reprocessed, generally remoulded by hot‐compression, before preparing a new actuator. Another reported approach consists in photocrosslinking aligned mesogens in selected areas.[23] Since reversible actuation occurs only in the crosslinked areas, depending on how they are organized, complex shape changes can be achieved. In practice, after inscription of actuation domains, the LCN is equilibrated in the isotropic state and subjected to "flood" photocrosslinking to ensure the actuation stability. This approach means that, similar to the use of dynamic crosslinking, different actuators can be prepared from the same LCP, but cannot be made from an existing actuator through reconfiguration.

The challenge of reconfiguring a single LCN actuator to achieve on‐demand shape changes has been addressed in a few recent studies. On one hand, a pH‐rewritable LCN actuator was demonstrated using a polymer containing pH‐responsive azomerocyanine that can be reversibly converted into hydroxyazopyridinium.[24] Since the two molecular species absorb light of different wavelengths, by patterning their distribution in the LCN through pH change, various actuation behaviors can be activated by light at wavelengths that excite one of the chromophores. On the other hand, a single LCN containing exchangeable epoxy acid bonds for crosslinking and doped with carbon nanotubes was shown to allow near‐infrared (NIR) light‐assisted reshaping for the actuator reconfiguration.[25] Upon irradiation, the local temperature of the vitrimer can rise to activate the transesterification, making it possible to mechanically deform the irradiated areas and thus alter the LC orientation in the actuator. More recently, an optical reconfiguration method was proposed for an LCN actuator bearing azobenzene mesogens and containing a dye.[26] It consists of two steps: firstly, locally tuning the cis‐isomer content by UV light, and then activating the shape change using red light through the photothermal effect. By patterning the cis isomers differently in the LCN, different shape changes can be obtained using a single specimen. Though simple and effective, to repeat the same shape transformation, blue light needs to be used to convert all cis isomers back into the trans form, and the same UV patterning of cis isomers should be applied.

Herein, we demonstrate a new approach for optical reconfiguration of a single LCN actuator, not only for changing its shape transformation as in the previous studies,[[24]] but also for changing locomotion behavior, which is unprecedented. The process is based on controlled photo‐decrosslinking, which requires only the use of light at one wavelength, conducted at room temperature and involves no mechanical deformation or reshaping of the polymer. We show that through optical reconfiguration, the LCN can be used to prepare origamizers which, in contrast with conventional origami polymers, can fold and unfold under stimulation to switch between a planar sheet and various targeted three‐dimensional (3D) shapes. Most interestingly, using a dye‐doped LCN actuator, a light‐fueled microwalker was optically reconfigured to reverse its locomotion direction with respect to laser scanning. This first‐time achievement is a significant step forward in optical reconfiguration of LCN actuator functions.

The approach is shown in Figure 1 a. A stretched strip of LCN with uniaxial LC orientation is uniformly crosslinked. This actuator displays reversible contraction (in the disordered isotropic phase) and extension (in the ordered LC phase) along the initial LC orientation direction. If the crosslinking can be removed in selected regions by light, different reversible shape change should be obtained upon order–disorder phase transitions as a result of an imbalanced contractile or extensional force. As illustrated by the example, if one side of the strip is decrosslinked, the new actuator should exhibit bending and unbending, and if the other side is further decrosslinked in a patterned way, the actuator is reconfigured to deform between wavy and flat shapes. Here we mention that photo‐decrosslinking was conducted at room temperature. Since the decrosslinking does not change the ordered phase (LC alignment and chain extension), when heated to the isotropic state for thermal relaxation, the whole strip contracts and remains flat, while the shape transformation takes place on subsequent cooling to the LC phase. The same reconfiguration process was used in all cases. Figure 1 b shows the liquid crystal polymer (LCP) designed to validate the approach (synthesis and characterization are detailed in the Supporting Information). It contains biphenyl units as the majority, as mesogens in the main chain, and anthracene pendant groups in the minority (5 mol % with respect to mesogens) for reversible photocrosslinking. Indeed, anthracene is known for reversible photodimerization and photocleavage of dimers under UV irradiation at two different wavelengths, and enables the initial uniform crosslinking and the subsequently conducted selective decrosslinking of the LCN. As seen in Figure 1 c, the reversible photoreaction in the LCP film was confirmed by the UV‐vis spectra. Upon exposure to 365 nm UV light (80 mW cm−2), the absorption peaks of anthracene groups at 333, 356, 368, and 387 nm decrease in intensity over exposure time, indicating the continuous formation of dimers. Upon subsequent irradiation by 254 nm UV light (12.6 mW cm−2), the intensities of all peaks increase with increasing the irradiation time, indicating the cleavage of the dimers. Figure 1 d shows DSC heating and cooling curves of the LCN. It displays a low glass transition temperature (Tg) near room temperature and a LC‐isotropic phase transition (TLC‐iso) at 63 °C (on heating). To prepare an LCN actuator with uniaxially aligned mesogens (monodomain), typically, a compression‐moulded LCP strip (thickness: 0.2 mm) was stretched at 51 °C (in LC phase) to 400 % strain, and after removing the external force, 365 nm UV light was applied to uniformly photocrosslink the strip. Such a LCN actuator can realize reversible contraction and elongation upon heating above and cooling below the TLC‐iso, respectively (see Figure S5 a in the Supporting Information), with the length of the strip increased by 42 % when going from the isotropic to LC phase. While the actuation remains unchanged upon repeated heating‐cooling cycles, a non‐crosslinked monodomain sample shows negligible reversible shape change (see Figure S5 b). Finally, presented in Figure 1 e are the plots of order parameter of mesogens in an LCN, calculated from X‐ray diffraction (XRD; see Figures S6 and S7), versus temperature and two‐dimensional (2D) diffraction patterns in the LC and isotropic phase. In the LC phase, the wide‐angle reflection indicates uniaxial orientation of mesogens (nematic order) along the strain direction, while the small‐angle reflection indicates ordered smectic layers with the layer normal aligned along the strain. In the isotropic phase, as expected, both nematic and smectic orders disappeared. The observation from 2D‐XRD patterns are corroborated by the order parameter of mesogens which decreases from S=0.71 at room temperature with increasing temperature within the LC phases before dropping to near zero in the isotropic state.

GRAPH: a) Schematic of the approach to optically reconfiguring an LCN actuator through selective decrosslinking. The magenta regions represent crosslinked actuation domains, and the blue regions represent decrosslinked non‐actuation domains. b) Chemical structure of the anthracene‐containing liquid crystal polymer, and the reversible photodimerization (for crosslinking) and photocleavage (for decrosslinking) of anthracene groups under UV light irradiation at two wavelengths. c) UV‐vis absorption spectra of a liquid crystal polymer film exposed to 365 nm UV light for up to 10 min (on the left), and subsequently to 254 nm UV light for up to 100 min (right). d) DSC heating and cooling curves of LCN actuator. e) 2D‐XRD patterns of LCN in the LC and isotropic phase as well as change in order parameter (S) of mesogens upon heating from room temperature to the isotropic state (LCN strip placed horizontally), S being calculated from the 2D‐XRD patterns.

Starting with an initial, fully‐crosslinked LCN actuator, because the crosslinked regions can be selectively decrosslinked through photocleavage of the anthracene dimers, the actuator can be reconfigured with spatially distributed crosslinked and uncrosslinked areas. The former is an actuation domain that is responsible for the reversible shape change upon LC‐isotropic phase transition, while the latter is a non‐actuation domain displaying no reversible shape change. Thus, a given LCN actuator can be optically reconfigured to exhibit different shape changes by controlling the depth and shape of non‐actuation domains. We carried out an experiment, presented in Figure 2, to showcase the power of reconfiguration for tuning the shape morphing of one specimen. We used the 254 nm UV light (12.6 mW cm−2) to locally and gradually reconfigure a uniformly crosslinked sample that shows initially reversible contraction and elongation (Figure 2 a). First, the opposite sides of the two halves of the sample were each irradiated by 254 nm UV light for 2 hours under a nitrogen atmosphere. On cooling, both ends of the sample rolled toward the irradiated surfaces to form a "roll up‐roll down" shape (Figure 2 b). Then, knowing that shining the UV light on the same area can decrease the bending angle because of increased depth of the non‐actuation area (see Figure S8), the sample achieved thermally induced flat to "roll up‐bend down" reversible shape change when the right side was irradiated for another 2 hours (Figure 2 c), and flat to "bend up‐bend down" ("S"‐like) shape change after the left side was further irradiated in the same way (Figure 2 d). Subsequently, the other two sides (the unirradiated sides) were subjected to the 254 nm UV light through a photomask for decrosslinking in a rectangular region in sequence, the reversible changes between flat to "bend up‐wrinkle" (Figure 2 e), and flat to all‐wrinkle (Figure 2 f) were realized. In principle, the number of reconfigurable shapes is infinite, determined by the resolution in size of the spatially separated actuation (crosslinked) and non‐actuation (decrosslinked) domains. More examples of light‐reconfigurable LCN actuator are shown in Figure S9.

GRAPH: Demonstration of the optical reconfiguration process on a single LCN actuator. The reversible switch between flat (in isotropic phase) and various shapes (in LC phase) upon photo‐decrosslinking in selected areas (blue) in succession: from a) elongation, to b) "roll up‐roll down", to c) "roll up‐bend down", to d) "bend up‐bend down" (S shape), to e) "bend up‐wrinkle", and to f) all‐wrinkle. The magenta regions represent crosslinked actuation domains, and the blue regions represent decrosslinked non‐actuation domains. Scale bars in photos: 0.5 cm. Photos of the flat strip in isotropic phase are not shown.

More interestingly, the optically reconfigured part (area that is unevenly decrosslinked along the thickness direction) can act as a flexible "hinge" to enable the planar LCN actuator to self‐fold into 3D origami structures under stimulation. Unlike other origamizers, here the self‐folding processes of one single LCN actuator can be programmed and reprogrammed through the two‐dimensional photo‐reconfiguration (see Movie S1). An example is shown in Figure 3. A pyramid‐like flat sheet with a mini‐incision (Figure 3 a) was photo‐reconfigured on both top (area 2) and bottom (area 1) sides (Figure 3 b). Upon thermally induced actuation (on cooling), the programmed hinges across the planar film rapidly bent towards the decrosslinked side, causing the flat sheet to self‐fold into an airplane‐like origami architecture. In the second reconfiguration step, after locally decrosslinking area 3 and 4 (Figure 3 c), the same flat sheet underwent folding into a sophisticated bull‐like origami structure during actuation. Similarly, the third reconfiguration step allowed the flat to frog‐like transformation to occur after further photo‐decrosslinking of areas 5 and 6 (Figure 3 d). It should be emphasized that all the transformations into 3D structures are reversible, making the process dynamic.

GRAPH: Sequential photo‐reconfiguration process on a 2D planar film of LCN actuator for thermally induced transformation into various sophisticated self‐folding 3D origami architectures: from a) an airplane‐like, to b) a bull‐like, and to c) a frog‐like structure. The cartoons show the patterned decrosslinked areas in each optical reconfiguration step (plane blue: top side, and shaded blue: bottom side). Corresponding 3D structures are given in the photographs (scale bars: 0.5 cm. See also Movie S1). Note: the original sample is yellowish and the appearance of the pinkish color is caused by using a pink pen to mark irradiation areas during the experiment.

Not limited to shape morphing, the optical reconfiguration through selective decrosslinking can also be applied to light‐powered LCN microwalkers, imparting it with changing locomotion behavior. The example in Figure 4 shows how an LCN strip capable of walking along the laser scanning direction can be reconfigured to undergo a crawling movement opposite to the laser scanning direction. To achieve light‐induced rapid heating and cooling cycles required for light‐fueled locomotion process, a near‐infrared (NIR) dye, having strong photothermal response (chemical structure and absorption spectrum in Figure S10), was added into the LCN at 0.2 wt %. The temperature in the exposed area could rise to 70 °C (above TLC‐iso) within 6 seconds with a laser (λ=980 nm) intensity of 4.5 W cm−2, and cool below TLC‐iso upon removal of the laser within few seconds (see Figure S11). On this basis, laser irradiation could generate cyclic actuation and locomotion of the LCN actuator. For the first configuration pattern (Figure 4 b), one side of the crosslinked dye‐loaded sample was decrosslinked using 254 nm UV light, except the right end of the actuator for the purpose of adjusting the shape of the microwalker and friction during the locomotion. Upon cooling from the isotropic phase to room temperature, a caterpillar‐like microwalker was obtained. As the laser spot moved from right to left, the illuminated area of the walker (in isotropic phase) rapidly flattened. And with the laser spot leaving, the area cooled (in LC phase) and recovered the arch shape. Upon illumination, the untreated right end serves as a stationary point because of the higher friction originating from the larger contact area between the softened actuator and substrate, leading to the flattening of the film with the left edge being pushed forward. During the arch recovery, the left edge has a larger frictional force against its motion, and drags the right edge to move to left. By repeating the laser movement from right to left for the flattening/arching cycles, the actuator can walk from the right to left side, that is, along the laser scanning direction, at an average speed of 3.4 mm min−1. After that, this actuator could be reconfigured to a wrinkle‐like microwalker through selectively decrosslinking the actuation domain to generate a parallel fringe pattern (Figure 4 a). Unlike the previous one, this wrinkle‐like actuator can mimic crawling locomotion of worms and exhibits opposite locomotion direction at an average speed of 2 mm min−1 when the laser scans from right to left in the same manner (Figure 4 c). In this case, more evenly distributed contact points are present between the actuator and the substrate prior to irradiation. As the laser spot moves from right to left, the flattening and softening of the film on the right increases the contact surface and adhesion with the substrate, and squeezes the wrinkles on the left side. When the left side is exposed to a laser, its flattening pushes the recovering wrinkles in the previously irradiated area and induces a slight displacement of the actuator from left to right. The demonstrated ability of optically tuning the locomotion behavior is appealing for the control and adaptability of light‐powered soft robots.

GRAPH: Demonstration of optically reconfigurable locomotion of light‐fueled microwalker using dye‐doped LCN actuator. a) Schematic drawing of the reconfiguring process for converting the light‐walker into a light‐crawler. Schematic and photographs showing b) the walking of an arch‐shaped actuator along the laser scanning direction (from right to left) by reversible flattening/arching shape change as laser spot moves on, and c) the crawling of a wrinkle‐shaped actuator against the laser scanning direction through the reversible shape change after photo‐reconfiguring the same LCN strip (dimension: 9×1.35×0.55 mm). See also Movie S2. NIR light (980 nm) intensity: 4.5 W cm−2. Scale bars: 0.5 cm.

In conclusion, we have demonstrated a new approach that allows a single LCN actuator to be optically reconfigured in an on‐demand manner so that it can exhibit both versatile reversible shape changes and changing light‐powered locomotion behavior. The method consists of optically decrosslinking the actuator in selected regions to reorganize the actuation and non‐actuation domains that determine the actuation behavior. Using a rationally designed LCP containing anthracene moieties that can undergo reversible photodimerization for chain crosslinking and photocleavage of dimers for decrosslinking under 365 and 254 nm UV light irradiation, respectively, we showcased the power of this optical reconfiguration method. By successively changing the distribution of decrosslinked areas across a flat LCN actuator, not only reversible complex shape morphing can readily be realized, but origamizers could be fabricated to display reversible self‐folding into sophisticated 3D structures. Most interestingly, a light‐fueled microwalker made with a dye‐doped LCN has been optically reconfigured to reverse the locomotion direction with respect to the laser scanning direction. This achievement is the first of its type, and highlights the importance of the demonstrated optical reconfiguration method in developing LCN actuators for soft robotics applications.

Y. Zhao acknowledges financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) and le Fonds de recherche du Quebec: Nature et technologies (FRQNT). Z. Jiang thanks FRQNT and the China Scholarship Council (CSC) for awarding him a scholarship. Y. Xiao thanks the China Scholarship Council (CSC) for awarding her a scholarship. Dr. Franck Camerel (University of Rennes 1) is thanked for providing the NIR dye.

The authors declare no conflict of interest.

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