Article Dans Une Revue Science Année : 2019

Shape memory nanocomposite fibers for untethered high-energy microengines

Cécile Zakri
Karl Kratz
  • Fonction : Auteur
Andreas Lendlein
  • Fonction : Auteur
Philippe Poulin

Résumé

Classic rotating engines are powerful and broadly used but are of complex design and difficult to miniaturize. It has long remained challenging to make large-stroke, high-speed, high-energy microengines that are simple and robust. We show that torsionally stiffened shape memory nanocomposite fibers can be transformed upon insertion of twist to store and provide fast and high-energy rotations. The twisted shape memory nanocomposite fibers combine high torque with large angles of rotation, delivering a gravimetric work capacity that is 60 times higher than that of natural skeletal muscles. The temperature that triggers fiber rotation can be tuned. This temperature memory effect provides an additional advantage over conventional engines by allowing for the tunability of the operation temperature and a stepwise release of stored energy. M iniature engines or motors are of practical interest for emerging applications ranging from microrobotics, lab-on-a-chip technology, and smart textiles, to microelectromechanical systems and miniaturized medical devices (1). Making large-stroke, high-speed, high-energy rotating micro-engines showing simplicity and robustness has long remained challenging. Different mechanisms and materials have been sought to provide torsional rotations, such as shape memory alloys (2), piezoelectric ceramics (3), and electro-active polymers (4). The most promising rotating performances have been achieved using the concept of twisted fibers (5), as the insertion of the twist amplifies the strokes and work capacities compared with those of nontwisted or noncoiled fibers. Following the demonstration of electrochem-ically driven motors based on twist-spun carbon nanotube (CNT) yarns (6), highly coiled CNT yarns have emerged. The helical topology enables a conversion of the yarn volumetric expansion into tensile contraction and torsional untwist, delivering high gravimetric work capacities (7-10). The volume change is driven by the expansion of infiltrated guest materials in response to heat (7), liquid adsorption (8), or by the surface tension of liquids diffused through gaps of hierarchically arranged helical CNT fibers (10). Nevertheless, the difficulty and high cost of fabricating CNT muscles has restricted their applications. Alternatively, simple and low-cost engines can be made by twisting polymer fibers. For example, the twisted rubber band used in airplane toys untwists because of the entropic elasticity of polymer chains (11). However, this type of engine suffers from a low energy density owing to the low Young's and shear moduli of elastomers. Twisted nylon-6,6 fibers contract and untwist in response to thermal expansion with a high energy density but necessitate large temperature changes (12). Buckled sheath-core rubber muscles that operate electrically have also been proposed as rotary motors (13). Here, we show that the twist insertion into shape memory nanocomposite fibers is efficient to create hook-free and high-energy microengines. Because of the energy storage capability (14) and the triggering of the untwist by a small increase in temperature above the switching temperature T sw (which is in the range of an involved thermal transition) by environmental heating, these multi-functional micromachines can work untethered. We start with polyvinyl alcohol (PVA)-based shape memory polymer (SMP) fibers with high strength and toughness (15), in which crystallites form the permanent netpoints. A 2-cm-long, 40-mm-diameter PVA fiber (Fig. 1A) was first heated to the programming temperature T d~1 00°C (which is above the glass transition temperature T g~8 0°C). Isobaric twist [under conditions of constant tensile force provided by a weight of 1.2 g attached to the fiber end (unless otherwise noted, the weight is fixed at 1.2 g)] was then inserted into this fiber at 7500 turns per meter of fiber length, with a rotation speed of 60 revolutions per minute (rpm). Afterwards, the twisted SMP fiber was quenched to room temperature T r to fix the coiled structure (Fig. 1B). The coiled structure can be retained without being hooked because of the glassy nonequilib-rium conformation of the helically configured polymer chains. Upon reheating the twisted fiber to a temperature above T sw (14), the one end-tethered fiber rotated its free end to revert to its original, equilibrium shape via untwist (Fig. 1C). A practical example of the programming and untwist of the twisted fiber in response to heat (shape memory effect) is shown in movie S1. The energy that a rotating engine provides via shape recovery is a function of the energy absorbed during the twist programming at T d. The polymer fibers can be stiffened by the inclusion of reinforcing nanofillers and can be made more efficient for high-energy microengines. We prepared single-walled carbon nanotube (SWNT)-and graphene oxide (GO)-doped PVA fibers by using a wet-spinning method upon injection of a PVA-SWNT dispersion or PVA-GO solution in an aqueous solution of Na 2 SO 4 as a coagulating bath (16). The wet-spun composite fibers have 5 weight % (wt %) SWNT and 5 wt % GO nano-particles (15). Quantitative characterizations were achieved by measuring the stress needed to stretch or the torque needed to twist the fibers at T r. The pure PVA fiber shows high toughness and a tensile Young's modulus of 4.9 GPa at T r (Fig. 1D). The incorporation of SWNTs or GO decreases the strain to failure but increases the Young's mod-ulus up to 13.5 and 12.5 GPa, respectively. Both nanofillers have nearly the same reinforcement efficiency on the tensile properties. By contrast, a greater torque is needed to twist the PVA-GO fiber to a given angle compared with that needed to twist pure PVA and PVA-SWNT fibers (Fig. 1E). By considering the fiber to be a non-coiled cylinder, the shear stress and shear strain can be calculated. The elastic shear moduli are deduced from the ratio of shear stress and shear strain at small twist angles (shear strain < 0.02) (fig. S1). Note that at higher strains, plastic deformation takes place, followed by coiling at extreme twist levels. Nevertheless, the exact onset of coiling could not be directly measured from the shear stress-shear strain curves. The SWNT and GO nanoparticles boost the shear modulus (~1.3 GPa) of PVA fiber up to 2.3 and 6.8 GPa, respectively. Relative to SWNTs aligned along the fiber axis (17), GO has a more pronounced effect on the improvement of torsional properties because of its two-dimensional (2D) structure. The shear is applied perpendicular to the nano-tube orientation direction but remains within the plane of 2D GO platelets. The nanosheets allow the fiber to sustain a high torque under twist (fig. S2). We therefore chose the strong PVA-GO fibers to investigate in more depth the torsional rotation of shape memory microengines. We investigated the effect of programming temperature T d on the torsional untwist for PVA-GO fibers. A higher torque is needed to twist the fibers at a lower T d , indicating that more torsional mechanical energy is stored during programming (Fig. 2A). When reheating the fiber in free load conditions at a rate of 5°C/min (unless otherwise noted, the heating rate is fixed at 5°C/min), higher full rotations are generated for fibers that have been initially programmed at lower T d (Fig. 2B). A maximum RESEARCH Yuan et al., Science 365, 155-158 (2019)
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Dates et versions

hal-02398750 , version 1 (12-06-2024)

Identifiants

  • HAL Id : hal-02398750 , version 1

Citer

Jinkai Yuan, Wilfrid Neri, Cécile Zakri, Pascal Merzeau, Karl Kratz, et al.. Shape memory nanocomposite fibers for untethered high-energy microengines. Science, 2019. ⟨hal-02398750⟩

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