Fast, Light-Responsive, Metal-Like Polymer Actuators Generating High Stresses at Low Strain

Abstract

•Polyethylene functionalized photo-responsive azobenzene dyes•Lightweight polymeric actuators with metal-like stress responses•Oscillating stress wave responses generated in soft actuators Lightweight polymeric actuators are traditionally not considered in applications such as robotics that demand high stress generation. A typical distinction is made between soft and hard actuators differing in modulus, actuation stress, and actuation strain: for example, hard actuators generate high stresses (20–500 MPa) at low strain, although being relatively heavy. Here, we report ultra-drawn ultra-high molecular weight polyethylene (UHMWPE) films doped with azobenzene dyes offering excellent mechanical properties and photo-mechanical stability combined with fast, remote response to (linearly polarized) light. These lightweight actuators bridge the gap between electrically driven hard and light-driven soft actuators generating stresses exceeding 60 MPa at low strain (∼0.06%). Emerging device architectures, such as domestic robotics, stimuli-responsive valves, or pressure controllers, can significantly benefit from these polymer-based actuators strong enough to perform important tasks. Producing lightweight polymeric actuators able to generate high stresses typical of hard metals and/or ceramics remains challenging. The photo-mechanical responses of ultra-drawn ultra-high molecular weight polyethylene (UHMWPE) actuators containing azobenzene photo-switches with symmetrically attached polyethylene (PE) side chains are reported. Long PE side chains promote dispersion within the apolar UHMWPE matrix, and the ultra-drawn films are highly aligned. The ultra-drawn azobenzene-doped UHMWPE films have high Young's moduli (∼100 GPa) and are viscoelastic at room temperature at strains below 1%. The photo-mechanical response of the films is fast (<1 s), showing a high specific actuation stress response (>6 × 104 Pa (kg m−3)−1) to UV or visible light at a low strain (∼0.06%). The actuator responds to rotating linearly polarized light, causing a photo-induced stress wave response. Such rapid, high-stress, low-strain, photo-mechanical responses are unique in soft polymer systems with physical values approaching hard metals/ceramics. Producing lightweight polymeric actuators able to generate high stresses typical of hard metals and/or ceramics remains challenging. The photo-mechanical responses of ultra-drawn ultra-high molecular weight polyethylene (UHMWPE) actuators containing azobenzene photo-switches with symmetrically attached polyethylene (PE) side chains are reported. Long PE side chains promote dispersion within the apolar UHMWPE matrix, and the ultra-drawn films are highly aligned. The ultra-drawn azobenzene-doped UHMWPE films have high Young's moduli (∼100 GPa) and are viscoelastic at room temperature at strains below 1%. The photo-mechanical response of the films is fast (<1 s), showing a high specific actuation stress response (>6 × 104 Pa (kg m−3)−1) to UV or visible light at a low strain (∼0.06%). The actuator responds to rotating linearly polarized light, causing a photo-induced stress wave response. Such rapid, high-stress, low-strain, photo-mechanical responses are unique in soft polymer systems with physical values approaching hard metals/ceramics. Actuators can generate mechanical stress, strain, and work in response to chemical, thermal, electrical, hydraulic, magnetic, pneumatic, and/or light stimuli.1Harris K.D. Bastiaansen C.W.M. Lub J. Broer D.J. Self-assembled polymer films for controlled agent-driven motion.Nano Lett. 2005; 5: 1857-1860Crossref PubMed Scopus (101) Google Scholar, 2Küpfer J. Finkelmann H. Nematic liquid single crystal elastomers.Die Makromol. Chem. Rapid Commun. 1991; 12: 717-726Crossref Google Scholar, 3Lipton J.I. Angle S. Banai R.E. Peretz E. Lipson H. Electrically actuated hydraulic solids.Adv. Eng. Mater. 2016; 18: 1710-1715Crossref Scopus (18) Google Scholar, 4Pilz da Cunha M. van Thoor E.A.J. Debije M.G. Broer D.J. Schenning A.P.H.J. Unravelling the photothermal and photomechanical contributions to actuation of azobenzene-doped liquid crystal polymers in air and water.J. Mater. Chem. C. 2019; 7: 13502-13509Crossref Google Scholar, 5Miriyev A. Stack K. Lipson H. Soft material for soft actuators.Nat. Commun. 2017; 8: 1-8Crossref PubMed Scopus (282) Google Scholar, 6Yamada M. Kondo M. Mamiya J.I. Yu Y. Kinoshita M. Barrett C.J. Ikeda T. Photomobile polymer materials: towards light-driven plastic motors.Angew. Chem. Int. Ed. 2008; 47: 4986-4988Crossref PubMed Scopus (763) Google Scholar Both organic and inorganic materials have been extensively explored for developing actuators for applications in soft and hard robotics, respectively.7Boyraz P. Runge G. Raatz A. An overview of novel actuators for soft robotics.Actuators. 2018; 7: 48Crossref Scopus (4) Google Scholar,8Huber J.E. Fleck N.A. Ashby M.F. The selection of mechanical actuators based on performance indices.Proc. R. Soc. A. Math. Phys. Eng. Sci. 1997; 453: 2185-2205Crossref Scopus (387) Google Scholar Rigid-bodied inorganic ceramics and/or metals are generally stiff materials (E ≫ 1 GPa) that generate high stresses at low strains (ε < 1%) in less than 1 s upon applying an electrical stimulus. In contrast, softer materials (E < 1 GPa) are primarily made of viscoelastic macromolecules with exactly the opposite properties: that is, low stresses at high strains (ε ≫ 1%). As an example, metal-based actuators typically have an actuation stress of 20–500 MPa at a strain of 0.009%–0.3% while soft actuators, such as human muscle, hydrogels, and liquid crystalline systems, exhibit typical stress values of 0.06–4 MPa at 2%–90% strain.8Huber J.E. Fleck N.A. Ashby M.F. The selection of mechanical actuators based on performance indices.Proc. R. Soc. A. Math. Phys. Eng. Sci. 1997; 453: 2185-2205Crossref Scopus (387) Google Scholar, 9Kondo M. Sugimoto M. Yamada M. Naka Y. Mamiya J.I. Kinoshita M. Shishido A. Yu Y. Ikeda T. Effect of concentration of photoactive chromophores on photomechanical properties of crosslinked azobenzene liquid-crystalline polymers.J. Mater. Chem. 2010; 20: 117-122Crossref Google Scholar, 10Banerjee H. Suhail M. Ren H. Hydrogel actuators and sensors for biomedical soft robots: brief overview with impending challenges.Biomimetics. 2018; 3: 15Crossref Scopus (76) Google Scholar Finding lightweight alternatives for tasks commonly performed by hard robotics, which are normally constructed from heavy inorganic parts and actuated via electric stimuli, is challenging. Using light as stimulus is attractive for allowing untethered, controlled actuation. Here, we opt for fabricating lightweight, robust macromolecular actuators based on ultra-drawn ultra-high molecular weight polyethylene (UHMWPE), which are capable of generating high stresses at low strains using local light exposure, bridging the gap between electrically driven hard and light-driven soft actuators. UHMWPE, a viscoelastic soft material, has outstanding metal-like mechanical properties after solid-state drawing at elevated temperature to ultra-high draw ratios.11Anandakumaran K. Roy S.K. John Manley R.S. Drawing-induced changes in the properties of polyethylene fibers prepared by gelation/crystallization.Macromolecules. 1988; 21: 1746-1751Crossref Scopus (35) Google Scholar The resulting fibers and films contain highly oriented macromolecules, largely present in the form of chain-extended crystals, and attain maximum Young's moduli of 100–180 GPa and tensile strengths of 3–5 GPa.12Gorshkova I.A. Andreeva G.N. Savitskii A.V. Frolova I.L. Properties of oriented fibers of high-molecular-weight polyethylenes.Mech. Compos. Mater. 1987; 23: 243-246Crossref Scopus (1) Google Scholar Per unit weight, these lightweight materials outperform ceramics and metals, and therefore are used in demanding applications including life protection.13Madej-Kiełbik L. Błaszczyk W. Osiewała-Just L. Łandwijt M. Struszczyk M.H. Kucińska-Król I. Ballistic armor for special applications.Text. Res. J. 2015; 85: 1800-1808Crossref Scopus (2) Google Scholar Tethered, temperature-responsive UHMWPE-based actuators have been reported; however, by making these polymers light responsive their practical usage can be extended, enabling fast, untethered actuation.14Haines C.S. Lima M.D. Li N. Spinks G.M. Foroughi J. Madden J.D.W. Kim S.H. Fang S. Jung de Andrade M. Goktepe F. et al.Artificial muscles from fishing line and sewing thread.Science. 2014; 343: 868-872Crossref PubMed Scopus (615) Google Scholar To allow the UHMWPE to wirelessly respond to light, we synthesized photo-responsive azobenzene dyes with polyethylene (PE) side chains and incorporated them in the ultra-drawn films. The long aliphatic side chains of the dyes facilitate dispersion within the UHMWPE matrix, and the dyes become highly dichroic after solid-state drawing. This incorporation results in a fast (<1 s) mechanical response to light, generating exceptionally high specific actuation stresses (>6 × 104 Pa (kg m−3)−1) at a strain of 0.06% in the stretched films. The mechanical response to light originates from the negative thermal expansion coefficient of the ultra-drawn UHMWPE in combination with the photo-thermal properties of the azo polymer.15Govaert L.E. Lemstra P.J. Deformation behavior of oriented UHMW-PE fibers.Colloid Polym. Sci. 1992; 270: 455-464Crossref Scopus (36) Google Scholar As a result, we generate mechanical values typical of hard robotics utilizing an ostensibly soft robotic material. Two azobenzene dyes with long PE chains were synthesized: C78-OH-AZO and C78-AZO (Figure 1A). The side chains are expected to reside in interfacial regions, possibly partially integrated into the crystal lattice, and/or in the amorphous regions of the apolar PE matrix.16Uznanski P. Kryszewski M. Thulstrup E.W. Polarized absorption spectroscopy of trans-azobenzene and trans-stilbene in stretched polyethylene films.Spectrochim. Acta Part A Mol. Spectrosc. 1990; 46: 23-27Crossref Scopus (34) Google Scholar,17Wang C. Weiss R.G. Thermal cis → trans isomerization of covalently attached azobenzene groups in undrawn and drawn polyethylene films. Characterization and comparisons of occupied sites.Macromolecules. 2003; 36: 3833-3840Crossref Scopus (35) Google Scholar It is anticipated that the C78-OH-AZO dye has a short cis isomer lifetime, acting primarily as a photo-thermal dye, while in case of C78-AZO films the photo-mechanical effect will also contribute, as the lifetime of the cis isomer is relatively long.18Bandara H.M.D. Burdette S.C. Photoisomerization in different classes of azobenzene.Chem. Soc. Rev. 2012; 41: 1809-1825Crossref PubMed Scopus (1468) Google Scholar The PE chains were attached via K2CO3 base activated alkylation of 4,4′-dihydroxyazobenzene or (E)-4-((4-hydroxyphenyl)diazenyl)benzene-1,3-diol using iodopolyethylene alkylating reactant (PE-I) (Mw ∼ 1220 g/mol, on average C78-I, 90% purity) with 4,4′-dihydroxyazobenzene or (E)-4-((4-hydroxyphenyl)diazenyl)benzene-1,3-diol. The Synthesis section details analytical data for the applied PE-I material, and reports on the synthesis and analytical characterization of both C78-AZO and C78-OH-AZO; high-temperature 1H nuclear magnetic resonance (NMR) and matrix-assisted laser desorption ionization time-of-flight mass spectroscopy (MALDI-TOF-MS) spectra are included, as well as high-temperature gel-permeation chromatography (HT-GPC) chromatograms (Figures S1–S4). Interpretation of the 1H NMR data indicate estimated purities of C78-AZO and C78-OH-AZO of about 73% and 61% (see Synthesis), respectively. The HT-GPC chromatograms prove conversion of iodopolyethylene PE-I into species with about a doubled molecular weight, in line with expectations. The MALDI-TOF mass spectra show arrays of peaks with masses attributable to C78-AZO (Figure 1B) as well as for C78-OH-AZO. Ultra-drawn composite films were prepared as follows. UHMWPE, antioxidant, and azobenzene dyes were suspended and dissolved (2% [w/v]) in xylene at 130°C, cast in an aluminum tray, and dried under a fume hood for several days. The solution-cast, unstretched films were colored vibrant yellow (C78-AZO) or orange (C78-OH-AZO) depending on the presence or absence of the electron-donating hydroxy substituent. Solid-state drawing (at 120°C) of these films was performed to a draw ratio (DR) of 60, resulting in highly dichroic films: the oriented molecules predominantly absorb light along the stretching direction (Figures 2A and 2B).19Pucci A. Tirelli N. Ruggeri G. Ciardelli F. Absorption and emission dichroism of polyethylene films with molecularly dispersed push-pull terthiophenes.Macromol. Chem. Phys. 2005; 206: 102-111Crossref Scopus (35) Google Scholar,20Capaccio G. Crompton T.A. Ward I.M. Drawing behavior of linear polyethylene. II. Effect of draw temperature and molecular weight on draw ratio and modulus.J. Polym. Sci. Polym. Phys. Ed. 1980; 18: 301-309Crossref Google Scholar The azobenzene molecules exhibit typical azobenzene absorption characteristics, indicating excellent miscibility (Figure S5). The azobenzene dye alignments were determined by measuring the order parameter (S), calculated usingS=Apar−AperpApar+2⋅Aperp,(Equation 1) where Apar and Aperp are the absorbance for light polarized parallel and perpendicular to the drawing direction, respectively: totally random orientations result in S = 0 while perfect alignment is described by S = 1.21Uznanski P. Kryszewski M. Thulstrup E.W. Linear dichroism and trans → cis photo-isomerization studies of azobenzene molecules in oriented polyethylene matrix.Eur. Polym. J. 1991; 27: 41-43Crossref Scopus (35) Google Scholar After baseline correction for reflection, S values were found to be ≥0.7 for both C78-AZO-doped and C78-OH-AZO doped films (Table S1). However, determining the order parameter is strongly influenced by the baseline correction.22Dirix Y. Tervoort T.A. Bastiaansen C. Optical properties of oriented polymer/dye polarizers. 2. Ultimate properties.Macromolecules. 1997; 30: 2175-2177Crossref Scopus (42) Google Scholar As a control, a hydroxyazobenzene derivative with short aliphatic side chains (C12-OH-AZO) was also incorporated into UHMWPE. UV-visible spectroscopy and X-ray diffraction evinced poor miscibility of the C12 derivative within the PE matrix. Even at low doping content (≤0.10 mol %), absorption spectra show a blue shift of the absorption maximum, revealing probable H-type aggregation of this azo dye (Figures S6A and S6B).23Song X. Perlstein J. Whitten D.G. Supramolecular aggregates of azobenzene phospholipids and related compounds in bilayer assemblies and other microheterogeneous media: structure, properties, and photoreactivity.J. Am. Chem. Soc. 1997; 119: 9144-9159Crossref Scopus (181) Google Scholar At higher doping content (0.22 mol %) the dye was found to form relatively large crystals during the casting procedure (Figure S6C).24Palmans A.R.A. Eglin M. Montali A. Weder C. Smith P. Tensile orientation behavior of alkoxy-substituted bis(phenylethynyl)benzene derivatives in polyolefin blend films.Chem. Mater. 2000; 12: 472-480Crossref Scopus (49) Google Scholar Dispersing the azobenzene derivatives—up to 0.22 mol %—did not affect the storage modulus E′ of the ultra-drawn UHMWPE host polymer (Figure S7A). At DR = 60, the crystallinity, orientation, and degree of chain extension increased significantly, leading to dynamic tensile moduli around 100 GPa.12Gorshkova I.A. Andreeva G.N. Savitskii A.V. Frolova I.L. Properties of oriented fibers of high-molecular-weight polyethylenes.Mech. Compos. Mater. 1987; 23: 243-246Crossref Scopus (1) Google Scholar Films with a lower draw ratio (20 and 40, respectively) were also prepared, resulting in thicker, less crystalline films that absorbed more light with lower tensile moduli than the ultra-drawn films.25Litvinov V.M. Xu J. Melian C. Demco D.E. Möller M. Simmelink J. Morphology, chain dynamics, and domain sizes in highly drawn gel-spun ultrahigh molecular weight polyethylene fibers at the final stages of drawing by SAXS, WAXS, and 1H solid-state NMR.Macromolecules. 2011; 44: 9254-9266Crossref Scopus (65) Google Scholar,26Balzano L. Coussens B. Engels T. Oosterlinck F. Vlasblom M. van der Werff H. Lellinger D. Multiscale structure and microscopic deformation mechanisms of gel-spun ultrahigh-molecular-weight polyethylene fibers.Macromolecules. 2019; 52: 5207-5216Crossref Scopus (9) Google Scholar The mechanical responses of the azobenzene-doped UHMWPE films to UV and visible light illumination were examined during stress-relaxation experiments at strain levels ranging from ε ∼ 0.25% to 1%, and revealed the composite films as being linear viscoelastic at room temperature up to at least 1% of strain (Figure S7B). The setup illustrated in Figure S8 was used to measure stress generation within the composite films at a constant strain of 1%. At t = 0 s, the films are strained, and the stress (σ) initially spikes and is measured as a function of time: stress relaxation by the polymer chains results in a decay of the stress, eventually reaching an apparent steady state.27van der Werff H. Pennings A.J. Tensile deformation of high strength and high modulus polyethylene fibers.Colloid Polym. Sci. 1991; 269: 747-763Crossref Scopus (47) Google Scholar Repeated periodic light exposures (10 s on/off light switching between 100 and 400 s) at constant intensity demonstrated a consistently fast superimposed stress response (<1 s) on top of the relaxation-governed stress decay in the azobenzene-doped films, which disappeared at a similar timescale after switching off the light source (Figures 3A and 3B). It is most surprising that the photo-induced (σphoto) response of these ultra-drawn composite films having ∼100 GPa tensile moduli occurs in less than 1 s for both C78 dye-doped films. Remarkably, fast stress generation and relaxation also occurred regardless of whether the 365- or 405-nm light-emitting diode (LED) was used. Subsequent 10-s exposures using increasing intensities between 400 and 670 s increased the stress response (Figures 3C and 3D). By increasing the LED intensity the film is exposed to increasing temperatures, thereby lowering the relaxation modulus G′(t), resulting in a faster relaxation response, which is evidenced by the progressive decay in average stress.28Vapaavuori J. Laventure A. Bazuin C.G. Lebel O. Pellerin C. Submolecular plasticization induced by photons in azobenzene materials.J. Am. Chem. Soc. 2015; 137: 13510-13517Crossref PubMed Scopus (52) Google Scholar, 29Leblans P.J.R. Bastiaansen C.W.M. Govaert L.E. Viscoelastic properties of UHMW-PE fibers in simple elongation.J. Polym. Sci. Part B Polym. Phys. 1989; 27: 1009-1016Crossref Scopus (31) Google Scholar, 30Govaert L.E. Bastiaansen C.W.M. Leblans P.J.R. Stress-strain analysis of oriented polyethylene.Polymer (Guildf). 1993; 34: 534-540Crossref Scopus (46) Google Scholar The observed stress decay, however, did not interfere with reproducibility of the σphoto values, consistent with the observed linear viscoelastic response and the knowledge that the heat dissipation is extremely efficient in the highly aligned polymer chains, calculated Fourier and Biot numbers indicated uniform and rapid conductive heat transfer over the thickness, impeding accurate film-temperature measurements.4Pilz da Cunha M. van Thoor E.A.J. Debije M.G. Broer D.J. Schenning A.P.H.J. Unravelling the photothermal and photomechanical contributions to actuation of azobenzene-doped liquid crystal polymers in air and water.J. Mater. Chem. C. 2019; 7: 13502-13509Crossref Google Scholar,31Xu Y. Kraemer D. Song B. Jiang Z. Zhou J. Loomis J. Wang J. Li M. Ghasemi H. Huang X. et al.Nanostructured polymer films with metal-like thermal conductivity.Nat. Commun. 2019; 10: 1-8PubMed Google Scholar,32Varghese S. Fredrich S. Vantomme G. Prabhu S.R. Teyssandier J. De Feyter S. Severn J. Bastiaansen C.W.M. Schenning A.P.H.J. Epitaxial growth of light-responsive azobenzene molecular crystal actuators on oriented polyethylene films.J. Mater. Chem. C. 2020; 8: 694-699Crossref Google Scholar Additionally, light absorption by the azo dyes induced heat, which generated a contraction of the highly oriented PE chains.33Porter R.S. Weeks N.E. Capiati N.J. Krzewki R.J. Concerning the negative thermal expansion for extended chain polyethylene.J. Therm. Anal. 1975; 8: 547-555Crossref Scopus (22) Google Scholar Increasing the concentration of C78 azo dyes from 0.06 to 0.22 mol % within UHMWPE increased the photo-induced stress. Figures 4A and 4B display the responses to UV and visible light illumination as a function of the azobenzene concentration, revealing that increased C78 azobenzene content in the UHMWPE matrix indeed results in higher photo-induced stress (σphoto). Illumination with a 365-nm LED source showed identical stress responses: ∼46 and ∼48 MPa for 0.22 mol % of C78-AZO and C78-OH-AZO, respectively. In contrast, excitation with a 405-nm LED resulted in diverging σphoto values for the different azobenzene derivatives. Remarkably, by incorporating 0.22 mol % of C78-OH-AZO exceptionally high mechanical stresses could be generated, even exceeding 60 MPa, the response being similar to a metallic linear actuator. At equal doping levels, the hydroxy-substituted azobenzene outperforms the C78-AZO. The absolute magnitude of the stresses depends on total light absorption of the azobenzene dye. Hydroxy substitution of the azobenzene dye (C78-OH-AZO) shifts the absorption spectra of the polymer films to the red (Figure 2), resulting in more overlap with the 405-nm LED emission spectrum. The performances of the ultra-drawn films were compared by calculating the absorbed energy per square centimeter from the absorption and LED emission spectra. In Figures 4C and 4D the stress generated is shown as a function of the absorbed energy per excitation source at an equal dye concentration. Both C78 photo-switches perform similarly when illuminated with UV light, consistent with the nearly identical absorption of 365-nm light. Utilizing a more powerful 405-nm LED source favors the C78-OH-AZO dye, which absorbs nearly 200 mW cm−2. Photo-induced stresses were found to scale with increasing dye concentration, regardless of the dye (Figures S9A–S9D). Remarkably, the nature of the dye did not influence the efficiency (photo-induced stress per absorbed mW cm−2), as both generated large photo-induced stresses at identical absorbed energy levels.34Sánchez-Ferrer A. Finkelmann H. Opto-mechanical effect in photoactive nematic main-chain liquid-crystalline elastomers.Soft Matter. 2013; 9: 4621-4627Crossref Scopus (35) Google Scholar Solid-state stretching of UHMWPE films or fibers significantly alters the structure of the crystalline and amorphous domains. At higher draw ratios UHMWPE increases in crystallinity, which leads to a smaller amorphous fraction.25Litvinov V.M. Xu J. Melian C. Demco D.E. Möller M. Simmelink J. Morphology, chain dynamics, and domain sizes in highly drawn gel-spun ultrahigh molecular weight polyethylene fibers at the final stages of drawing by SAXS, WAXS, and 1H solid-state NMR.Macromolecules. 2011; 44: 9254-9266Crossref Scopus (65) Google Scholar,26Balzano L. Coussens B. Engels T. Oosterlinck F. Vlasblom M. van der Werff H. Lellinger D. Multiscale structure and microscopic deformation mechanisms of gel-spun ultrahigh-molecular-weight polyethylene fibers.Macromolecules. 2019; 52: 5207-5216Crossref Scopus (9) Google Scholar,35Ohta Y. Murase H. Hashimoto T. Structural development of ultra-high strength polyethylene fibers: transformation from kebabs to shishs through hot-drawing process of gel-spun fibers.J. Polym. Sci. Part B Polym. Phys. 2010; 48: 1861-1872Crossref Scopus (45) Google Scholar In other words, drawing the films significantly imposes conformational constraints on the azobenzene photo-switches, the cores of which are expected to reside in the non-crystalline amorphous regions, restricting trans-cis photo-isomerization and favoring cis-trans back isomerization.17Wang C. Weiss R.G. Thermal cis → trans isomerization of covalently attached azobenzene groups in undrawn and drawn polyethylene films. Characterization and comparisons of occupied sites.Macromolecules. 2003; 36: 3833-3840Crossref Scopus (35) Google Scholar,36Tanchak O.M. Barrett C.J. Light-induced reversible volume changes in thin films of azo polymers: the photomechanical effect.Macromolecules. 2005; 38: 10566-10570Crossref Scopus (160) Google Scholar The cis C78-AZO isomer could only be identified in modestly drawn—DR = 20—films containing 0.22 mol % of the C78-AZO (Figure S10) suggesting that the cis isomer lifetime is reduced in the ultra-drawn polymer films. Therefore, we hypothesize that photo-excitation of both azobenzene photo-switches are predominantly generating heat, which is transferred via the highly oriented aliphatic side chains embedded in the matrix. The well-dispersed azobenzene derivatives thus reside in non-crystalline amorphous regions, possibly anchoring the PE tails to the crystal faces, or in the (aligned) amorphous regions, not distorting the UHMWPE lattice (Figure S11) but simply conducting heat to the extended-chain PE matrix and generating large mechanical stresses.37Friebel J. Ender C.P. Mezger M. Michels J. Wagner M. Wagener K.B. Weil T. Synthesis of precision poly(1,3-adamantylene alkylene)s via acyclic diene metathesis polycondensation.Macromolecules. 2019; 52: 4483-4491Crossref PubMed Scopus (4) Google Scholar The fact that no photo-induced stress is observed in water for either film supports the hypothesis that both C78 dopants act as photo-thermal agents (Figure S12).4Pilz da Cunha M. van Thoor E.A.J. Debije M.G. Broer D.J. Schenning A.P.H.J. Unravelling the photothermal and photomechanical contributions to actuation of azobenzene-doped liquid crystal polymers in air and water.J. Mater. Chem. C. 2019; 7: 13502-13509Crossref Google Scholar The photo-thermal response of the azobenzene depends on the miscibility with the UHMWPE matrix, yet despite poor miscibility the short C12 side chain azobenzene still attained a photo-induced stress of ∼30 MPa (Figures S13A and S13B). In fact, even the commercially available photo-thermal 2-(2H-benzotriazol-2-yl)-4,6-ditertpentylphenol (BZT) dyes could be embedded into ultra-drawn UHMWPE, enabling stress generation in response to UV light (Figure S13C). However, stress generation using this photo-thermal dye was found to be less efficient than for azobenzene-doped films (Figure S13D). This experiment reveals that stress generation using UV or visible light exposure does not necessarily require good dispersion within the PE and that good miscibility is crucial in realizing the exceptionally high stresses. To our knowledge, macromolecular actuators that operate at low strain values capable of generating such high stresses (σphoto > 60 MPa) have never been reported previously. These novel, lightweight films show untethered, fast responses to local UV and/or visible light exposure while maintaining extremely high storage moduli. Additionally, periodic light exposure of azobenzene-doped films has shown excellent photo-mechanical stability over at least ∼2,600 cycles without any observable loss in photo-induced stress (Figures S14 and S15). Moreover, the highly aligned dyes swiftly respond to rotating linearly polarized light (Figure 5A), causing a photo-induced stress wave response. As the polarization state equals the drawing direction of the dichroic films, i.e., at 0° and 180° maxima in actuation stress are found (Figure S16), this fast, oscillating mechanical response enables the creation of periodic stresses under constant light intensities. The specific actuation stress is calculated by dividing the photo-induced stress by the density of the material:Specificactuationstress=σactuationρ.(Equation 2) For highly crystalline, ultra-drawn UHMWPE, a density of 975 kg/m−3 was used.40Wunderlich B. Davidson T. Extended-chain crystals. I. General crystallization conditions and review of pressure crystallization of polyethylene.J. Polym. Sci. Part A Polym. Phys. 1969; 7: 2043-2050Crossref Google Scholar At low strain (1%), the composite films photo-mechanically respond to UV or visible light by generating high specific actuation stresses of >6 × 104 Pa (kg m−3)−1. The features of our macromolecular actuators and other soft and hard actuators are presented in Figure 5B, which displays the specific actuation stress (per unit weight) as a function of actuation strain. Note that the azobenzene-doped materials are the only materials that generate stress in response to UV and/or visible light in Figure 5B; the rest are all temperature driven. Despite remote triggering of the system, light-to-heat conversion plays a crucial role in inducing stress in the azobenzene-doped materials. Among all the actuators, hard (depicted in gray/black) or soft (shown in white and green), the UHMWPE composites act as a unique hybrid soft-hard actuator.3Lipton J.I. Angle S. Banai R.E. Peretz E. Lipson H. Electrically actuated hydraulic solids.Adv. Eng. Mater. 2016; 18: 1710-1715Crossref Scopus (18) Google Scholar,5Miriyev A. Stack K. Lipson H. Soft material for soft actuators.Nat. Commun. 2017; 8: 1-8Crossref PubMed Scopus (282) Google Scholar,8Huber J.E. Fleck N.A. Ashby M.F. The selection of mechanical actuators based on performance indices.Proc. R. Soc. A. Math. Phys. Eng. Sci. 1997; 453: 2185-2205Crossref Scopus (387) Google Scholar, 9Kondo M. Sugimoto M. Yamada M. 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Mater. 2018; 30: 1-28Crossref Scopus (267) Google Scholar These lightweight UHMWPE composites could bend an inorganic high-modulus materials using visible light exposure: the film ends of a 0.22 mol % C78-OH-AZO composite were fixed to an aluminum strip using UV-curable glue, and visible light exposure resulted in reversible hinge-like bending of the strip thanks to the mismatch in thermal expansion (Video S1 and Figure S17).39Wang W. Rodrigue H. Kim II, H. Han M.W. Ahn S.H. Soft composite hinge actuator and application to compliant robotic gripper.Compos. Part B Eng. 2016; 98: 397-405Crossref Scopus (47) Google Scholar In device architectures as proposed by the groups of Haines, Wu, and Sutton,14Haines C.S. Lima M.D. Li N. Spinks G.M. Foroughi J. Madden J.D.W. Kim S.H. Fang S. Jung de Andrade M. Goktepe F. et al.Artificial muscles from fishing line and sewing thread.Science. 2014; 343: 868-872Crossref PubMed Scopus (615) Google Scholar,42Wu L. Jung de Andrade M. Saharan L.K. 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The PE chains of the dyes facilitated dispersion within the highly apolar PE matrix, dispersing much better in comparison with azobenzenes with shorter (n = 12) PE tails or the small-molecule UV absorber BZT, and resulted in well-aligned photo-responsive composites upon solid-state stretching. The viscoelastic macromolecules were found to have tensile moduli of ∼100 GPa, comparable with hard linear actuators. Regardless of the photo-absorber (C78, C12, or BZT) or excitation source (365- or 405-nm LEDs), large photo-induced stresses were generated in ultra-drawn (DR = 60) composite films. However, the increased miscibility of the C78 derivatives in the UHMWPE allowed the absorption of more incident excitation light, and during stress-relaxation experiments photo-induced stresses exceeding 60 MPa were obtained at a strain of ∼0.06%. The exceptionally high stresses generated by simple exposure to light in what normally would be considered a soft actuator material are similar to those found in light-insensitive hard actuators embodying a new class of fast-responding (<1 s) hybrid soft-hard linear actuators.