Molecular Motion, Defect Structures and Phase Transitions in Oligomer Crystals

Abstract

Defect structures in the high temperature modifications of oligomer crystals can generally be discribed as a superposition of chain rotations, longitudinal shifts of the molecules and the occurrence of conformational defects. Applying a suitable combination of experimental techniques it is possible to discriminate between the different components of disorder. This is demonstrated in a discussion of results obtained on n-alkanes, cycloalkanes and perfluoro-n-alkanes.

Surface Crystallization and Phase Transitions of the Adsorbed Film of F(CF2)12(CH2)16H at the Surface of Liquid Hexadecane

“…In the condensed phase the surface density reaches values up to 5.2 μmol m -2 , corresponding to a mean area per molecule of 0.32 nm 2 . This value is about 15% larger than the van der Waals cross-sectional area of an all-trans CF 2 chain of 0.28 nm 2 , , which is consistent with the expectation for a condensed phase. Grazing incidence X-ray diffraction (GIXD) studies of the condensed phase are planned to elucidate the structure of this monolayer phase.…”

Correlation between DSC Curves and Isobaric State Diagrams. 2. Concentration-Dependence of Eutectic Peak Widths in DSC Curves

“…Instead of uniform segments stated, there are, for instance, small crystallites, frequently enclosed in a matrix of the other crystal, that may entail surface phenomena. Furthermore, since n-paraffin crystals always have a sheetlike structure and are built up of regularly stacked layers, 18 the supposition of isotropic segments cannot really be maintained. In the end, crystallization and thus the microstructure of the forming crystals are also markedly governed by crystallization kinetics, such as the cooling rate, the number and stability of crystal nuclei, and their rate of formation and growth.…”

A Structural X-ray Study on Semifluorinated Alkanes (SFA): SFA Revisited

“…n-Alkanes are known to pass through several solid modifications between room temperature and their melting points, and all phase transitions can be associated with the onset of a characteristic motion. 19 In the low-temperature phases the planar "zig-zag" carbon backbones are arranged in layers of triclinic or orthorhombic unit cells, and as thermal motion increases, the so-called "rotator phases" are formed. The latter term describes layered phases with hexagonal symmetry originating from a superposition of rotational, translational, and conformational disorder in the samples.…”

“…For the qualitative interpretation of the diffraction data, it is useful to recall the structure of the pure alkane and perfluoroalkane compounds in the solid state. n −Alkanes are known to pass through several solid modifications between room temperature and their melting points, and all phase transitions can be associated with the onset of a characteristic motion . In the low-temperature phases the planar “zig-zag” carbon backbones are arranged in layers of triclinic or orthorhombic unit cells, and as thermal motion increases, the so-called “rotator phases” are formed.…”