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

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.