Polymorphic transformations of C 26 H 54 and C 28 H 58 n -paraffins as typical rotator substances

Thermal deformations and polymorphic transformations of two long-chain evennumbered normal paraffins C 26 H 54 and C 28 H 58 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 C 26 H 54 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 1 M cryst forms (two-phase mixture Tc cryst + 1 M cryst ). The starting C 28 H 58 n -paraffin sample is a double-layer monoclinic modification 2 M cryst at room temperature. Crystallization from hexane or slow cooling of a melt leads to a monolayer monoclinic form 1 M cryst . Thermal deformations and temperature ranges of existence of the crystalline forms ( Tc cryst , 1 M cryst , and 2 M 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.