Abstract
Simultaneous measurements have been performed using differential scanning calorimetry, time-resolved small-angle X-ray scattering and X-ray television (TV) methods to study the solid-solid phase transition of the normal-alkane (hexatriacontane) single crystal. The low-temperature monoclinic form (M L) of hexatriacontane transforms to the high-temperature monoclinic form (M H) by two mechanisms. One mechanism is a direct transition from M L to M H by twin formation without passing any activated state. The other mechanism is a two-step transition through an intermediate orthorhombic state (O), where the molecules stand up from the M L state and then re-incline into the M H state. The subsecond time-resolved investigation by simultaneous measurements reveals dynamic variations during the transition. The former M L→M H transition appears at a lower temperature showing that twin formation requires lower energy, than the two-step process with the activated state. Continuous changing of intensities observed in the X-ray TV indicates that the latter M L→O→M H transition is caused by the coherent motion of the molecules.