Near-Infrared Chromophore Functionalized Soft Actuator with Ultrafast

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A Near Infrared Chromophore Functionalized Soft Actuator with Ultrafast Photoresponsive Speed and Superior Mechanical Property Li Liu, Mei-Hua Liu, Lin-Lin Deng, Bao-Ping Lin, and Hong Yang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b06410 • Publication Date (Web): 08 Aug 2017 Downloaded from http://pubs.acs.org on August 8, 2017

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Journal of the American Chemical Society

A Near Infrared Chromophore Functionalized Soft Actuator with Ultrafast Photoresponsive Speed and Superior Mechanical Property Li Liu, Mei-Hua Liu, Lin-Lin Deng, Bao-Ping Lin, and Hong Yang* School of Chemistry and Chemical Engineering, Jiangsu Province Hi-Tech Key Laboratory for Bio-medical Research, Jiangsu Key Laboratory for Science and Application of Molecular Ferroelectrics, State Key Laboratory of Bioelectronics, Southeast University, Nanjing, 211189, China.

Supporting Information Placeholder ABSTRACT: In this manuscript, we develop a two-step acyclic diene metathesis in-situ polymerization/crosslinking method to synthesize uniaxially aligned main-chain liquid crystal elastomers with chemically bonded near-infrared absorbing four-alkenyltailed croconaine-core crosslinkers. Due to the extraordinary photothermal conversion property, such a soft actuator material can raise its local temperature from 18 to 260 oC in 8 seconds, and lift up burdens 5600 times heavier than its own weight, under 808 nm near-infrared irradiation.

In the past decade, near infrared (NIR) light stimulated soft actuators with wide application prospects in tissue engineering,1 biomedical devices2 and robotic technology,3,4 have attracted extensive scientific attention, in particular because the longwavelength NIR light, compared with ultraviolet/visible light, could efficiently penetrate through biomaterials which rendered NIR the best biocompatible remote stimulus control.5 However, the real application of NIR-responsive soft actuators was plagued by two key limitations: moderate photo-actuation speeds and poor mechanical properties, which were intrinsically caused by the exclusive, flawed physical blending preparation method of NIRresponsive soft actuators. Specifically, all of the previously reported NIR-responsive soft actuators including liquid crystal elastomers (LCEs),6-8 hydrogels9 and other polymeric materials,10 were prepared by physically doping into polymeric matrices, a variety of photothermal conversion reagents (such as carbon nanotubes,11-16 graphenes,17-19 metal nanoparticles,20-25 conjugated polymers26,27 and organic dyes28-31), which could absorb NIR light, convert photon into thermal energy and further trigger the phase transitions, thermal expansions or swelling property variations of polymeric materials.32-34 The main drawback of this physical blending strategy was the poor solubilities of photothermal reagents in organic polymeric matrices, which resulted in a dilemma that lowering the doping percentages of photothermal reagents would decrease the photo-actuation speeds of the soft actuators, while increasing the doping percentages would induce the inhomogenous dispersions of photothermal reagents and phase segregations, consequently sacrifice the mechanical properties of the soft actuators.

To address this solubility limitation, scientists have developed several intelligent optimization solutions. For instance, Terentjev group12 have used the π-π stacking effect of a pyrenefunctionalized polysiloxane-based LCE material to increase the dispersion concentration of carbon nanotubes from 0.4 wt% to 3 wt%. Compared with inorganic thermal-conductive fillers, small organic NIR dye molecules would have better solubilities in polymeric matrices. Following this common sense, our group recently decorated a thiophene-croconaine unit35 with two long alkyl tails to prepare a mesogenic NIR dye30 which achieved high solubility in LC molecules. However if the concentration of NIR dye was increased to more than 5 wt%, dye leaking out of LCE matrix occurred meanwhile the ion pairs of the thiophene-croconaine units would interfere with radical polymerizations. Inspired by the landmark works of incorporating azobenzene chromophore into LCE networks,36,37 the ultimate solution is to chemically bond NIR absorbing chromophore units into soft actuators. However, to our surprise, there have been barely any reports using this strategy, possibly due to two challenging technique issues: (1) NIR dyes were relatively rare species, it was difficult to design a highly soluble NIR absorbing chromophore bearing polymerizable, crosslinkable functional groups. (2) Most of NIR dyes, such as indocyanines,38 squaraines39 and croconaines,40 possessed cation-anion pairs which could in some extent interfere with radical polymerizations or platinum-catalyzed hydrosilylation reaction, which were widely used in synthesizing soft actuators.6-8,11-31,41-44To address the above challenges, herein we report the first example of chemically bonding a NIR absorbing chromophore crosslinker into a main-chain LCE soft actuator material prepared by acyclic diene metathesis polymerization (ADMET). As shown in Figure 1, the LCE material was comprised of an α,ωdiene LC monomer, 4-undec-10-enyloxy-benzoic acid 4-dec-9enyloxy-phenyl ester (Y1709), Grubbs 2nd generation catalyst, and a newly designed four-armed NIR absorbing crosslinker, 2,5bis[(1-dec-9-enyl-undec-10-neyl-4-carboxylate-piperidyl-amino) thiophenyl] croconium (YHD796C), whose relative weight percentage reached to ca. 17 wt%. The synthetic routes of LC monomer Y1709 and the crosslinker YHD796C were illustrated in Scheme S1, the detailed experimental procedures were described in the supporting information.

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Figure 2. Schematic illustration of the preparation protocol of a uniaxial aligned LCE film.

Figure 1. Chemical composition of LCE material described in this work. To prepare a uniaxial-aligned LCE material, we have tried thiol-ene polymerization45 and platinum-catalyzed hydrosilylation reaction,46,47 neither of which could function properly in confronting with a large quantity (8~10 mol%) of thiophene-croconaine crosslinker YHD796C. Alternatively, we found that the fouralkenyl-tailed thiophene-croconaine unit was compatible with Grubbs catalysts, and thus chose ADMET polymerization approach which has been only used by Mather group48 to synthesize LC copolymers with unsaturated backbones for further radical crosslinking to provide polydomain LCEs. Learned from two recent examples extending the application of Finkelmann’s classical two-step hydrosilylation crosslinking strategy46,47 to acrylatetype19 and thiol-ene-type45 LCEs, we developed a two-step ADMET in-situ polymerization/crosslinking method to fabricate uniaxial-aligned LCE soft actuator materials. The preparation protocol of the monodomain LCE film was schematically illustrated in Figure 2. Under nitrogen atmosphere, a mixture comprised of Y1709, YHD796C and Grubbs catalyst dissolved in a small amount of toluene was allowed to react at 60 o C for 3 h (Figure 2a) to realize the first partial-crosslinking purpose. The obtained polydomain LCE gel was sliced into a film ribbon (Figure 2b), which was hung in an oven (Figure 2c) and burdened with a heavy load (two counterweights, ca. 40 g) at 120 o C to achieve uniaxial stretching (Figure 2d). The resulting film ribbon was fixed on a glass slide (Figure 2e) and kept at 100 oC for 2 days, to accomplish the second full-crosslinking step to provide the desired monodomain LCE film ribbon (Figure 2f). Investigated by UV-vis spectroscopy as shown in Figure 3a, both the NIR crosslinker YHD796C and the LCE sample dispersed in dichloromethane exhibited an intense and sharp absorption peak centered at 796 nm. The mesomorphic property of the LCE sample was examined by differential scanning calorimetry (DSC, Figure 3b) and wide angle X-ray diffraction (WAXD, Figure 3c-f) experiments. At low temperature (< 91.7 oC), LCE sample showed a highly ordered smectic C phase, which gradually turned to a nematic phase with smectic C fluctuations49 until entering into the isotropic phase (above 116 oC). The order parameters of the sample recorded at 30 oC and 100 oC were 0.78 and 0.69 (Figure 3e,f) respectively, which indicated a high-quality uniaxial orientation of the mesogenic directors inside the LCE sample.

Figure 3. a) UV-vis absorption spectra of YHD796C (concentration = 3.75 ╳ 10-2 mg/mL, dissolved in CH2Cl2) and LCE sample (concentration = 0.09 mg/mL, swelled in CH2Cl2). b) DSC diagram of LCE sample. One-dimensional WAXD patterns of LCE sample on c) heating and d) cooling. Two-dimensional WAXD patterns of LCE samples at e) 30 oC and f) 100 oC The thermal-induced actuation property of the LCE film was first examined by directly heating the LCE sample on a hot stage.50,51 As demonstrated in Figure S5 and Movie S1, taking advantage of the main-chain end-on mesogenic structure,52,53 this LCE material could achieve at the nematic-to-isotropic phase transition a large and reversible shrinkage of around 110%, which outweighed most of previously reported side-chain LCE materials.11-31


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Figure 4. a) The photo image of a LCE ribbon before and after NIR (808 nm) illumination. b) NIR illumination time vs the shape deformation (L/Liso) of the LCE ribbon. c) NIR illumination time vs temperature diagram of LCE samples. d) The photo image of a LCE film (4.3 mg) lifting up a load (ca. 24.44 g) under NIR illumination. e,f) Stress-strain diagrams of LCE film measured at varied temperatures. The photothermal-stimulated actuating behaviors of the LCE film were investigated by using a NIR light source (0.83 W.cm-2, center wavelength: 808 ± 3 nm). The experimental setup was shown in Figure 4a, a LCE ribbon hung up with a binder clip, could shrink under NIR illumination and expand to its original length after removal of NIR stimulus. The reversible contraction/elongation cycle was repeated 8 times as recorded in Movie S2, the detailed uniaxial shape deformations (L/Liso) of the LCE ribbon were plotted against the corresponding NIR illumination time. Herein, Liso and L0 are the minimum and maximum lengths of the LCE ribbon in the isotropic and glassy phases respectively, while L is the length of the ribbon measured at any temperature below the clearing point. As shown in Figure 4b, in each cycle, the average contraction time of the LCE ribbon was 4~5 seconds while the elongation recovery could be finished in ca. 2 seconds. The maximal shrinkage value equal to (L0 - Liso)/Liso, was determined as ca. 110%. Meanwhile, one delicate thermal imager equipment (FLUKE Ti90) was adopted to further inspect the local surface temperature variation of the LCE sample under NIR irradiation. As indicated in Figure 4c, the LCE surface temperature jumped over the LC-toisotropic transition (116 oC) in 2 seconds and could incredibly rise up from 18 oC to ca. 260 oC in less than 8 seconds, while the temperature of a control sample containing no YHD796C but a NIRinactive crosslinker (glyoxal bis(diallyl acetal), as shown in Figure S6) increased by less than 10 oC. Undoubtedly, this extraordinary ultrafast photoresponsive speed benefited from the high concentration (ca. 8.8 mol%, 17.1 wt%) of the NIR absorbing croconaine-core crosslinker YHD796C. The mechanical property of this LCE sample was further investigated on a dynamic mechanical analyzer (DMA Q800, TA Instrument) with a tension clamp for static stress-strain measurements, which were conducted along the mesogenic director orientation of the LCE film in a temperature range from 30 oC to 135 oC. As presented in Figure 4e,f, the longitudinal Young’s moduli of the LCE film were found in the range from 177.82 MPa to 9.62 MPa before its temperature entered into the nematic phase. When the temperature of LCE sample was raised to 115 oC which was close to the clearing point, the Young’s modulus gradually

decreased to 1.78 MPa. Most importantly, the Young’s moduli of this sample in the isotropic phase could maintain at ca. 1.4 MPa (1.43 MPa at 125 oC, 1.36 MPa at 135 oC). It is well known that the actual actuating performance of LCE material was determined by its isotropic-phase mechanical property indeed. Compared with the isotropic-phase Young’s moduli of traditional side-chain LCE soft actuators which were limited to 30~300 KPa,45-47,54-56 the performance of this novel main-chain LCE material increased by nearly one order of magnitude. This superior mechanical property might derive from the advantageous main-chain end-on mesogenic structure and the homogeneous distributed NIR chromophore units chemically bonded in the LCE matrices. Due to the superior mechanical property, this novel LCE soft actuator material had an outstanding heavy-lift capability. As demonstrated in Figure 4d and Movie S3, under NIR illumination, the LCE film (4.3 mg) easily lifted up heavy loads including one counterpoise and two binder clips (ca. 24.440 g, in total), which was ca. 5680 times heavier than the LCE film’s own weight, while some traditional side-chain LCE soft actuators containing physically embedded photothermal reagents could just lift up loads roughly 200-300 times of their own weights.26 In conclusion, aiming to chemically incorporate NIR activated photothermal reagents into polymeric matrices, we designed and synthesized a four-alkenyl-tailed croconaine-core crosslinker as the NIR absorbing chromophore, and further developed a two-step ADMET in-situ polymerization/crosslinking method to fabricate a uniaxial aligned main-chain LCE soft actuator material. Taking advantage of the highly concentrated, homogeneous distributed, chemically bonded NIR chromophore units and the main-chain end-on mesogenic skeletal structure, this novel LCE soft actuator material exhibited an ultrafast photoresponsive speed and superior mechanical property. We hope this design will pave the way for developing high-speed responsive, ultra-strong smart polymeric materials.

ASSOCIATED CONTENT Supporting Information



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Instrumentation information; syntheses and characterizations of Y1709, YHD796C and the corresponding LCE film; three NIRresponsive movies. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author *[email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This research was supported by National Natural Science Foundation of China (21374016), Jiangsu Provincial Natural Science Foundation of China (BK20170024), and the Open Research Fund of State Key Laboratory of Bioelectronics, Southeast University. We thank Prof. Er-Qiang Chen (Peking University) for his help with XRD measurements.

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Near-Infrared Chromophore Functionalized Soft Actuator with Ultrafast

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