Polymorphic transformations of C26H54 and C28H58 n-paraffins as typical rotator substances

Thermal deformations and polymorphic transformations of two long-chain evennumbered normal paraffins C26H54 and C28H58 were studied by thermal X-ray diffraction (temperature step is tenths of a degree), infrared spectroscopy (temperature step is 1–5°), and differential scanning calorimetry (temperature step is 2° in a sample heating melt cooling mode. The samples are characterized by high homologous purity (99.0%) and belong to so-called “boundary” n-paraffins. The starting C26H54 n-paraffin sample is a triclinic modification at room temperature (Tc cryst). When quickly cooled, the melt crystallizes as the triclinic Tc cryst and monoclinic monolayer 1M cryst forms (two-phase mixture Tc cryst + 1M cryst). The starting C28H58 n-paraffin sample is a double-layer monoclinic modification 2M cryst at room temperature. Crystallization from hexane or slow cooling of a melt leads to a monolayer monoclinic form 1M cryst. Thermal deformations and temperature ranges of existence of the crystalline forms (Tc cryst, 1M cryst, and 2M cryst), low-temperature rotator crystalline orthorhombic form (Or rot.1), and high-temperature rotator crystalline hexagonal (H rot.2) phases of these n-paraffins were evaluated from changes in their diffraction patterns and unit cell parameters. The molecular structure and the conformational composition of these n-paraffins in different states were found from their IR spectra. Differential scanning calorimetry (DSC) was used to determine the phase transition temperature. Thermal X-ray diffraction, IR, and DSC data agree well with one another.