Effect of Homologous Impurity on the Solid-Solid Phase Transition of Normal-Alkane (Hexatriacontane) Crystal Studied by Time-Resolved X-Ray Measurements

The effect of impurities on the solid-solid phase transition of solution-grown normal-alkane (hexatriacontane) crystals was investigated using differential scanning calorimetry (DSC), time-resolved small-angle X-ray scattering (SAXS) and polarizing microscopy. Normal-dotriacontane was used in this work as a homologous impurity. Six kinds of crystals were grown using two different solution concentrations (S C) and three kinds of impurity fractions (I F). DSC results show that the temperature of the solid-solid phase transition decreases with I F and S C. Time-resolved SAXS measurements exhibit a dynamic change of the solid-solid phase transition where the onset temperature of the transition is also lowered by I F and S C. A streak-like pattern which was apparent in the low-angle scattering clarifies that flat-shaped defects parallel to the lamellar surface increase with I F and S C. The transition frontier observed by polarizing microscopy indicates that molecules of more than 104 lamellae cooperatively accompany the solid-solid phase transition. The onset temperature as measured by SAXS is more than 1°C higher than that measured by DSC. The deviation is explained by the time lag for growing the bulk (secondary) nucleation from local (primary) nucleation sites of the solid-solid phase transition.