Abstract:
The polymorphic transformations of tricosane n-C$_{23}$H$_{48}$ upon heating were studied using synchrotron X-ray diffractometry and differential scanning calorimetry. The kinetics of the development of a sequence of solid-solid phase transitions was revealed. It was shown that the initial orthorhombic subcells of tricosane are gradually transformed into hexagonal ones, which is determined by the weakening van der Waals interaction between individual molecules and the occurrence of discrete rotation of molecules around their principal axes in the lamellae during thermal expansion of crystals. These transformations occur through the formation of a number of intermediate rotator phases $(R_V, R_I, R_{II})$. It was found that the phase transitions: ordered crystal $\to$$R_V$ and $R_I\to R_{II}$ are diffuse first-order phase transitions and develop according to a heterogeneous mechanism, and the phase transition $R_V\Rightarrow R_I$ refers to second-order transitions. The parameters of unit cells and their temperature changes in each of the observed polymorphic modifications of tricosane were determined.
Keywords:phase transitions, X-ray diffractometry, polymorphic transformations of tricosane.
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