Soft Actuators Based on Spin‐Crossover Particles Embedded in Thermoplastic Polyurethane

Abstract

Recently, soft actuators have been increasingly considered for applications in various technological fields, such as soft robotics, artificial muscles, wearable technologies, medical devices, and smart textiles. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] These devices are inspired by human muscles, which are agile, reconfigurable, physically adaptive, lightweight, and multifunctional. The development of new stimuli-responsive materials, including the optimization of their properties and their integration into actuator systems, plays a central role in this endeavor. [15][16][17][18][19][20][21][22] Among the different kinds of functional materials used in soft actuators, the most studied are electroactive polymers, gels, piezopolymers, shape memory polymers, and soft magnetic composites. [23][24][25][26][27][28][29][30][31][32] Each material has pros and cons and none of them can fulfill the numerous and sometimes even disparate technological requirements (e.g., mechanical compliance, high work density, efficiency, reliability, speed, accuracy, scalability, controllability). Depending on the physical mechanisms underlying the actuation, they can be operated by different stimuli, including electric fields, temperature, pressure, magnetic field, humidity, or light irradiation, but electrical current or tension are the most relevant due to their ease of use. [33,34] Molecular spin-crossover (SCO) complexes of iron(II) ions exhibiting substantial volume change upon stimuli-induced switching between their high-spin (HS) and low-spin (LS) states appear promising candidates in this context. [35,36] Indeed, the SCO phenomenon is not only accompanied by a large spontaneous strain [37] (up to 22 vol% [38] ) and associated large strain energy densities (up to a few tens of J cm À3[36] ), but provides also unique multifunctionality (concomitant change of mechanical, optical, caloric, and magnetic properties), multiresponsiveness (to temperature, pressure, light, and chemical stimuli), and large compositional and morphological versatility. [39][40][41] To enhance their processability, SCO compounds are frequently embedded into polymer matrices to form particulate composites. [42,43] Based on this approach, conventional bending bimorph actuators were recently fabricated in our team using SCO@polymer composites by means of solvent casting, [44] spray coating, [45] blade casting [46,47] and 3D printing. [48] Since nearly all SCO@polymer composite materials are electrically insulating, in practice, they are used in combination with a conducting material to achieve electrothermal actuation. [44,46]

Dynamical Mechanical Analysis and Micromechanics Simulations of Spin-Crossover@Polymer Particulate Composites: Toward Soft Actuator Devices

“…The results allow us to reach two important conclusions. First, we confirmed our previous hypothesis , in that a soft (respectively stiff) matrix enhances the effective transformation strain (respectively stress), which is clearly demonstrated here by comparing soft TPU and stiff P­(VDF-TrFE) matrices. We also show that these responses can be quantitatively predicted using appropriate micromechanics homogenization methods, which allow the impact of particle shape, orientation, and concentration on the effective mechanical behavior of the composite upon the SCO phenomenon to be assessed also.…”

“…In agreement with our previous results, 21,57 TEM observations confirmed that compound 1 forms well-defined, rodshaped particles with lengths of 1−2 μm and an aspect ratio of ca. 10, whereas particles of 2 display irregular and polydisperse morphology with an average particle size of ca.…”

“…(See Scheme S1 in the Supporting Information, SI, for the chemical structures.) The SCO@P­(VDF-TrFE) and SCO@TPU composites were blade cast with different particle concentrations (0, 15, 30, and 50 wt %, see Table ) using the protocols reported in refs and , respectively.…”

“…The dispersion of SCO particles inside polymers not only increases the processability of the materials but can also be the source of unprecedented properties thanks to the strain-driven coupling between the particles and the matrix. Hence, efforts into SCO composite mechanics have focused on utilizing the spin transition phenomenon in two waysgenerating movement in actuators − and creating strain-coupled hybrid materials that combine the characteristics of the constituents to achieve novel functionalities, − notably, transduction between heat and electrical energy. − …”

“…We choose these matrices for their substantially different elastic moduli (∼1 GPa vs ∼10 MPa), which is a crucial parameter for optimizing the actuating properties ( vide infra ). In addition, these polymers and their composites can be easily processed by blade casting, affording homogeneous particle dispersions within large sample volumes, which are required for mechanical testing. , It is also worth noting that the selected polymers do not exhibit any significant thermally induced phenomena (e.g., relaxations or phase transitions) near the SCO temperatures of 1 and 2 , avoiding thus potential complications from overlapping mechanical spectroscopy signals. This issue is particularly important for the P­(VDF-TrFE) matrix, which is a ferroelectric polymer exhibiting intricate electromechanical properties on its own.…”