Reversible Thermosalient Effect of Butyl Paraben Cocrystal for Thermal to Mechanical Energy Conversion

Abstract

Thermosalient crystals that exhibit reversible mechanical motion during temperature changes and phase transitions have potential for application in smart sensors, actuators, or drug delivery. During the phase transition of thermosalient crystals, energy conversion occurs, while thermomorphism produces physical work through continuous and nearly instantaneous directed forces. Here, we report for the first time a reversible, single crystal to single crystal phase transition from one polymorph to another of a butyl paraben cocrystal formed with isonicotinamide. This crystal exhibited highly anisotropic lattice changes and reversible crystal shape deformation by the rotation of functional groups in one component, and the stretching of hydrogen bonds between the two components. The slip, molecular rotation, and hydrogen bond stretching caused significant modulation between layers before and after the phase transition, accompanied by anisotropic changes in the crystal unit cells. These changes lead to crystal movements of rotating, splitting, and jumping. In addition, we succeeded in preparing a reversible thermomechanical actuator that generated a push force more than 450 times greater than the weight of the crystal, thereby enabling the cocrystal to act as an actuator for the reversible movement of the glass plate. When the temperature became over 373.15 K, the crystal expanded instantaneously to approximately 2.7% of its original length. When the temperature dropped below 353.15 K, the crystal contracted back to its original length. This cyclic process can rapidly display temperature changes by current. The structural changes during thermosalience processes helps to understand thermo-responsive molecular crystals.