Exploiting Phase Transitions In Polymer Bilayer Actuators

In many biological systems, the process of motion involves mechanically morphing materials, which change their mechanical properties and/or shape in response to a control signal. In the past two decades, significant progress has been made toward mimicking the underlying designs and resulting functions in artificial materials and systems. Polymer-based bilayer bending actuators, in which the dissimilar thermal expansion of two materials is exploited, represent widely investigated actuation elements, but one limitation of this design approach is that the thermal expansion coefficient of polymers is generally small. Herein, it is shown that this problem is mitigated using a thermoplastic polyurethane elastomer with a crystallizable soft segment. The domains formed by the latter reversibly melt and crystallize in a convenient temperature range (30-60 degrees C), whereas the hydrogen-bonded urethane hard segments serve as physical crosslinks that provide mechanical integrity and elastic behavior beyond this temperature. Melting the polyester results in a large nonlinear thermal expansion and thus actuation, in a narrow temperature range. The electrically controlled bilayer actuators are created by combining this material with cellulose acetate and integrating resistive heating electrodes. Optionally these devices are equipped with a thermally controlled supramolecular polymer adhesive "gripper."