Molecular Motion In Crystals Research Articles

Rigid-body molecular motion in crystals. The centre of libration

It is shown that the concept of the molecular centre of libration in the description of the average rigid-body thermal behaviour of molecules is a useful approximation. A restriction is made on the full theory, making the centre of libration model a constrained version of the complete model. The relationship between the models is discussed. Some examples are chosen to demonstrate the closeness of the approximation. These results show that for data reaching a reliability factor of no better than 7% an analysis using the full theory might yield a meaningless result. Some data of much higher accuracy does, however, show the need for the full theory.

Molecular motion in polytetrafluoroethylene at cryogenic temperatures

Abstract Speculation is offered on the nature of the molecular motion in crystals of polytetrafluoroethylene at temperatures well below the 19°C transition. It is proposed that a specific type of defect in the chain, a helix reversal point, creates a mechanism whereby large dynamic angular displacements of chain segments about their long axes can occur without significantly affecting the X-ray diffraction data.

Mechanical Dispersion and Molecular Motion in Crystals of Polyethylene and Other Polymers

Dynamic mechanical loss of solution-grown polyethylene crystals is measured by the suspension method as a function of temperature and the crystalline dispersion (α dispersion) is observed above 40°C in just the same manner as in melt-grown polyethylene. In the temperature range of this dispersion, the ratio of crystal lattice constants b/a progressively changes toward that of a higher symmetry hexagonal form with increasing temperature. NMR line width of the broad component becomes gradually narrow with rising temperature in this temperature range. It is concluded from these reasons that the α dispersion of polyethylene should be ascribed to the rotational vibration of chain molecules in crystalline phase about c-axis. Similar behavior of low molecular weight paraffin crystals below the rotational transition point affords an additional confirmation of the above conclusion. Crystalline dispersions of other polymers are briefly discussed.

The study of molecular motion in crystals by infra-red spectroscopy

The study of molecular motion in crystals by infra-red spectroscopy