Bilayer Bending Actuators Based on Semicrystalline Polyurethanes with Large Thermal‐Expansion Coefficients

Abstract

Thermoresponsive polymer‐based bilayer bending actuators rely on the dissimilar thermal expansion of the polymers that form the two layers. To maximize the heat‐induced change in curvature, the thermal expansion shall be large in one and small in the other material. Semicrystalline polymers display a large nonlinear thermal expansion across their melting transition, but as their mechanical integrity is lost upon melting, they cannot readily be used in bilayer actuators. To overcome this limitation, segmented polyurethanes (PUs) with crystallizable polyethylene glycol (PEG) soft segments and hard segments formed by the reaction of 1,6‐hexane diisocyanate and 1,4‐butanediol (BDO) is developed. The latter serve as physical cross‐links and inhibit flow at temperatures where the domains formed by the PEG segments have melted. The molecular weight of the PEG segments and the hard‐segment content in the polymer are systematically varied. As the nonlinear expansion of the PEG–PUs is associated with the melting of the crystalline PEG domains, the thermal expansion of these materials is correlated with their crystallinity and is highest for the polymers with the lowest BDO content and the highest PEG molecular weight. Electro‐thermally controlled bilayer bending actuators based on the new materials display high deflection and low switching temperatures.

Modular Design of a Polymer‐Bilayer‐Based Mechanically Compliant Worm‐Like Robot

“…The PEG-PU employed in this study was designed based on a recently reported approach to create polymers displaying large coefficients of linear thermal expansion (CLTE) in a narrow temperature range. [44,45] The polyethylene glycol (PEG) blocks form semicrystalline domains, while the hard blocks containing the urethane groups act as physical crosslinks. The polymer softens and a large nonlinear thermal expansion is observed upon heating and melting of the PEG crystals, but the material liquefies only at a much higher temperature, where the urethane hard phase dissociates.…”

“…The specific material utilized here was synthesized according to a recently reported protocol. [ 45 ] Based on the reported structure‐property relations of PEG‐PUs, and with the goal to optimize thermal expansion, the PEG‐PU employed here was synthesized by the reaction of hydroxyl‐terminated PEG with a number‐average molecular weight ( M n ) of 10 kg mol −1 , 7 wt.% 1,4‐butanediol (BDO) as chain extender, and 1,6‐hexane diisocyanate. The resulting PEG‐PU had an M n of 41 kg mol −1 , as determined by size‐exclusion chromatography.…”

“…Previous experiments with electrothermally driven bilayer bending actuators have shown that an energy of ≈0.108–0.149 J mm −3 is required to fully melt the crystalline PEG‐PU domains to achieve full actuation. [ 45 ] In addition, it was found that this process becomes more energy efficient at lower heating rates. [ 45 ] Using the lower boundary as a requirement for the activation of an individual bilayer, the non‐slip constraint, and the curvatures predicted by the modified Timoshenko model, an analysis of a theoretical six‐segment worm as a function of θ and PEG‐PU layer thickness was performed.…”

“…[ 45 ] In addition, it was found that this process becomes more energy efficient at lower heating rates. [ 45 ] Using the lower boundary as a requirement for the activation of an individual bilayer, the non‐slip constraint, and the curvatures predicted by the modified Timoshenko model, an analysis of a theoretical six‐segment worm as a function of θ and PEG‐PU layer thickness was performed. The results show that for a given L o , the velocity and COT (Figure 1c; Figure S4, Supporting Information) can be optimized with a PEG‐PU layer thickness that affords the highest curvature or inverse R o (Figure 1a) and by selecting a θ of 90° (Figure S4, Supporting Information).…”

“…The durability of individual PEG‐PU/PI bilayer bending actuators has previously been investigated with no significant loss in blocking force over 250 actuation cycles. [ 45 ] Furthermore, SEM micrographs of the bilayer cross‐section show no indication of delamination or microcrack formation after 250 cycles (Figure S7, Supporting Information). The full recovery of the axial and radial strains required a bit more time (≈150 s recovery time, Figure 3d) than cooling to ambient temperature (Figure 3a,b), suggesting that the final stage of the recovery process depends on the crystallization and not cooling rate.…”