부산대학교 도서관

Thermal deformations and polymorphic transformations of even long chain monoclinic normal paraffins CH and CH (homological purity 97-99%) are studied. Thermal deformations and temperature limits of the existence of crystalline (1 M and 2 M), low-temperature rotationally crystalline triclinic ( Tc), monoclinic ( M), and orthorhombic ( Or) and high-temperature rotationally crystalline hexagonal ( H) phases of these n-paraffins are estimated by the character of changes in their diffraction patterns and unit cell parameters (thermal X-ray diffraction). The molecular structure and conformational composition of these n-paraffins in different phase states are determined (IR spectroscopy). In order to determine the temperatures of phase transitions differential scanning calorimetry is also applied. The data of all three methods are in good agreement with each other. The initial sample of CH n-paraffin at room temperature is characterized by a two-layer monoclinic 2 M crystalline modification. When the melt is rapidly cooled, it crystallizes in monolayer monoclinic 1 M and orthorhombic Or modifications (two-phase mixture 1 M+ Or). The initial sample of CH n-paraffin at room temperature is characterized by the monolayer monoclinic modification 1 M. When the melt is rapidly cooled, as well as CH n-paraffin, it crystallizes in monolayer monoclinic 1 M and orthorhombic Or modifications (two-phase mixture 1 M+ Or). It is found that both n-paraffin undergo irreversible polymorphic transformations from the crystalline state cryst to the low-temperature rotationally crystalline state rot.1. For CH n-paraffin the transition is performed by the scheme 2 M → M → Or, and for CH n-paraffin it is performed by the scheme 1 M → Tc. Both paraffins do not undergo the transition to the high-temperature rotationally crystalline state rot.2 (hexagonal phase H), similarly to monoclinic CH n-paraffin studied previously. These and previously obtained data allow us to consider that as the molecular chain length increases, differences in the polymorphic modification of initial crystalline phases cease to be significant and the sequence of polymorphic transformations is mainly determined by the length of the molecule. [ABSTRACT FROM AUTHOR]