Anisotropic thermal strain measurement at the thermochromic phase transition in a polydiacetylene crystal

Abstract

The anisotropic thermal strains of a polydiacetylene crystal induced by a heat cycle were measured from macroscopic changes in the sample dimensions. Measurement accuracy was superior to that of the x-ray diffraction method. Linear thermal expansion in phase A and a strain induced by the phase transition between phases A and B were clearly observed in the a and c axes. The reflectivity change was also monitored at the exciton energy of phase A. Both results exhibited similar transition behavior with hysteresis. On the other hand, in the b axis expansion was much smaller than that of other axes, and no hysteresis was found at the phase transition. The results support the supposition that the phase transition is a structural change of side groups. The volume change between phases A and B was measured to be 1.9% at 410 K, which indicates the possibility of an optically driven actuator using photoinduced phase-transition phenomena.