One‐Pot Synthesis of Melt‐Processable Supramolecular Soft Actuators

Abstract The application of reprocessable and reprogrammable soft actuators is limited by the synthetic strategies, 3D‐shaping capabilities, and small deformations. In this work, melt‐processable supramolecular soft actuators based on segmented copolymers containing thiourethane and liquid crystal segments have been prepared via sequential thiol addition reactions in a one‐pot approach using commercially available building blocks. The actuators demonstrated immediate, reversible response and weightlifting capabilities with large deformations up to 32 %. Through exploiting the supramolecular cross‐links, the material could be recycled and reprogrammed into 3D actuators and welded into an actuator assembly with different deformation modes. Our work offers a one‐pot synthesis and straightforward melt‐processable approach to prepare supramolecular soft actuators with large deformations that can be reprocessed and reprogrammed into arbitrary 3D shapes.

Synthetic procedure A reaction vessel (100 mL) was charged with diacrylate mesogens 1 and 2 in DMAc (50 wt%) and allowed to stir under an inert atmosphere at 50 °C until fully dissolved. The solution was cooled to room temperature, and dithiol chain extender 3 was added while stirring, followed by nucleophilic catalyst 4 (0.1 wt%). The resulting reaction mixture was allowed to react at room temperature for 2 h. Afterward, diisocyanate 5 in DMAc (50 wt%) was added to the oligomer mixture immediately followed by base catalyst 6 (0.1 wt%) and allowed to stir at room temperature for 15 minutes. During this time, the mixture became viscous, and additional DMAc (30 wt%) was added to the prepolymer mixture. Next, dithiol 7 was added dropwise, and after complete addition, the reaction mixture was heated at 60 °C and allowed to react overnight. The crude mixture was poured into cold Et2O (500 mL) while stirring vigorously, and the polymer precipitated over time. The product was added to fresh Et2O (200 mL) and stirred overnight. The solvent was decanted, and the final polymer was dried at 40 °C under vacuum affording a white solid (≥97% recovery). The molar ratios and formulations of the synthesized thermoplastic PTU LCEs can be found in the Supporting Information (Table S2, S3).
Characterization FTIR spectra were recorded on a Varian 670 IR spectrometer equipped with an attenuated total reflectance (ATR) sampling accessory using a diamond crystal over a range of 4000-650 cm -1 with 50 scans per spectrum and a spectral resolution of 4 cm -1 . All spectra were recorded at room temperature unless stated otherwise. The obtained spectrums are processed with Varian Resolutions. Gel permeation chromatography (GPC) was performed on a Waters HPLC system equipped with a PSS PFG (8 × 50 mm, 7 µm) and two PFG linear XL columns (8 × 300 mm, 7 µm) in series. 1,1,3,3,3-hexafluoro-2-propanol (HFIP) with potassium trifluoroacetate (20 mM) at 35 °C was used as mobile phase supplied at a flow rate of 0.8 mL min -1 . The samples were prepared in HFIP with potassium trifluoroacetate (20 mM) and toluene (20 mM) at room temperature. The molecular weights were determined using a refractive index detector relative to poly(methyl methacrylate) standards. DSC measurements were performed using a TA Instruments Q1000 DSC instrument with hermetic T-zero aluminum sample pans. All scans were conducted with 10 ± 1 mg polymer over a temperature range from -50 to 200-210 °C at heating and cooling rates of 10 °C min -1 under nitrogen atmosphere. The second heating and cooling cycles were used to determine the enthalpies and transition temperatures of all samples. TGA was carried out on a TA instruments Q50 instrument with 4 ± 0.5 mg polymer over a temperature range from 28 to 800 °C and a heating rate of 5 °C min -1 . DMA was performed on samples (8 × 5.3 × 0.4 mm 3 ) cut from compression-molded films with a TA Instruments Q800 apparatus in vertical tension mode. The thermographs were measured from -50 to 250 °C with a heating rate of 5 °C min -1 , a single 1 Hz oscillating frequency, 10 µm amplitude, and 0.01 N preload force. Stress-strain curves were obtained with a Lloyd-Ametek EZ20 tensile testing machine using a 500 N load cell. The strain is defined as (l−L)/L where L is the initial length, and l is the length at a particular time. Dog-bone specimens with a cross-sectional area of 2 × 0.4 mm 2 (W × T) were cut from compression-molded films and uniaxially elongated at an elongation rate of 10 mm min -1 with a gauge length of 20 mm until failure. X-ray scattering measurements were performed on a Ganesha lab instrument equipped with a Genix-Cu ultralow divergence source that generates X-ray photons with a wavelength and flux of 0.154 nm and 1 × 10 8 photons s -1 , respectively. Diffraction patterns were obtained using a Pilatus 300 K silicon pixel detector with 487 × 619 pixels of 172 × 172 µm 2 . Silver behenate was used as a calibration standard. The sample-to-detector distance was 89 mm for wide-angle (WAXS) configurations, whereas, for medium-angle (MAXS), the detector was operated at 439.5 mm. The collected data were reduced and analyzed using a custom Python script with the PyFAI software package. D-spacings were calculated using the equation d = 2π/q. The orientational order parameters were calculated from the diffraction patterns using the Kratky method. [1] POM was carried out with a Leica DM2700 M microscope and crossed polarizers. Isotropization temperatures were determined on drop-casted samples from HFIP (15 ± 1 mg mL -1 ) using a Linkam THMS600 microscope stage with a heating and cooling rate of 10 °C min -1 .

Fabrication procedures
First, the polymer was dried at 60 °C for at least 1 h before processing. Then, the material was loaded homogenously into a mold (20 × 40 × 0.1 mm 3 ) and covered with polytetrafluoroethylene (PTFE) protection sheets (T = 0.12 mm) on both sides. The mold was heated to 200 °C in a Collin P200E press and subjected to five breath cycles with a mold pressure of 50 bar. The final compression molding process was performed at 100 bar and 200 °C for 2 minutes, whereafter the mold was immediately quenched to room temperature to form polydomain polymer films. Dog-bone shaped specimens (35 × 2 × 0.4 mm 3 ) were cut from compression-molded polymer films and uniaxially strained at room temperature by using a custom-made stretching instrument until elongation reached 100%. The strained samples were then heated to 130 °C for 30 minutes and subsequently cooled to room temperature while remaining strained. It was noticed that during cooling, the aligned samples spontaneously elongated above the initially applied strain. Finally, the aligned LCEs were annealed at room temperature for at least 48 h before characterization and testing. For recycling, a film was cut into small pieces and compression-molded according to the previously described procedure. The twisted actuator was obtained by fixing one end of an aligned LCE while twisting and subsequently fixing the other followed by heating it to 130 °C for 30 minutes, after which it was cooled to room temperature. Similarly, a uniaxial region was fixed in between a left-and right-handed twisted segment by using two tweezers during programming. Welding was performed by overlapping the end of two actuators and heating it to 200 °C for 2 minutes.

Actuation measurements
Thermal actuation measurements were performed by placing aligned samples on a black anodized aluminum sheet on top of a hotplate. The samples were heated from 30 to 110 °C by gradually increasing the temperature in intervals of 10 °C. Afterward, the samples were allowed to cool to room temperature. All samples were subjected to a full heating and cooling cycle to erase the thermal history before the actuation measurement. Photographs were taken at each temperature using a camera (Olympus OM-D E-M10 Mark III), and the obtained images were analyzed using ImageJ. Weightlifting tests were conducted with various weights attached to the sample with a paper clamp (1.26 g) and heated to around 80 °C using a heat gun. Actuation measurements of reprogrammed actuators were performed in an oven with a window allowing for recording the deformations upon heating and cooling. Actuation strain was measured on a TA Instruments Q800 DMA by monitoring the sample length as a function of temperature. Samples (8 × 1.3 × 0.3 mm 3 ) were measured in controlled force mode from -50 to 110 °C at a heating rate of 5 °C min -1 under initial bias stress of 250 kPa (constant force). All samples were heated to 110 °C for 3 minutes before the measurement to ensure the thermal history was erased. The corresponding work capacity was calculated from Equation 1, considering that the bias stress depends on the cross-sectional area of the samples, which changes upon actuation. [2] Work capacity = = ∆ = [ ] (1)  [8] ; c Irreversible deformation requiring prestraining after each deformation cycle.   Figure S1. FTIR spectra of polydomain PTUs S1-S5. Figure S2. C═OH-bond/C═Ofree ratio as a function of hard segment content.  Figure S3. DSC thermographs of the PTUs S1-S5. The second heating run is shown. Figure S4. POM images of PTUs S2-S5. Upon heating, the birefringence disappears over a wide temperature range (60 to 120 °C) that reappears upon cooling, although showing thermal hysteresis. Poor solubility of PTU S1 prevented sample fabrication. Figure S5. TGA profiles of PTUs S1-S5.   Figure S7. 1D WAXS diffractograms of polydomain PTUs S1-S5. Figure S8. 2D WAXS diffractograms of polydomain PTUs S1-S5. The observed orientation is attributed to the sample preparation and handling. Figure S9. 2D WAXS diffractograms of programmed PTUs S1-S5. The molecular director of the aligned materials is horizontal. Figure S10. Azimuthal profile of the 2D WAXS diffractogram at q = 10.0-10.5 nm -1 for PTU S1. Figure S11. Azimuthal profiles of the 2D WAXS patterns at q = 14.3-14.6 nm -1 for PTUs S1-S5. The order parameter was obtained by fitting a Kratky function over the azimuthal angle by a custom-made script. Figure S12. 1D MAXS diffractograms of polydomain PTUs S1-S5. An additional peak is observed at q = 1.37 nm -1 originating from the spacing between the mesogens within the soft segment (d = 4.6 nm). For samples with larger LC segment lengths, the signal is stronger. Figure S13. Stress-strain curves at room temperature of pristine and remolded samples.

Supplementary Video Captions
Video S1. Cyclic thermal actuation of thermoplastic PTU S5 while lifting 5g.
Video S3. Thermal actuation of the reprogrammed twisted actuator.
Video S4. Thermal actuation of the reprogrammed patterned actuator.
Video S5. Thermal actuation of the reprogrammed and welded twisted actuator.