Molecular-Motion, Thermal-Expansion, and the Phase-Transitions in Paraffins: A Model for Polymers

Abstract

Owing to their simple chemical structure paraffins are generally regarded as representing a system which is especially suitable for a model study of the thermal behavior of oligomeric and polymeric solids. Extensive investigations on the properties of n-tritriacontane (C33H68) provide an example. For this system an identification of several types of motional mechanisms become possible. Their presence leads to peculiarities in the thermal expansion and to the occurrence of solid-solid phase transitions.
Paraffins and polyethylene exhibit a marked anisotropy in the lateral thermal expansion. An analysis of the molecular background can be based on a modified Grüneisen theory which connects expansion coefficients with elastic constants and Grüneisen constants. The Grüneisen constants describe anharmonic properties of the lattice vibrations and can be derived from a temperature dependent measurement of the frequencies of a number of Raman active lattice modes. The analysis shows that the observed anisotropy is mainly a consequence of the uniaxial orientation of the strong thermal pressure originating from the rotational and torsional oscillations of the chains.
In the case of C33H68, the transition from the well ordered crystalline state to the melt occurs in several consecutive steps. Three solid-solid phase transitions occur before melting and each one leads to a steplike decrease in the degree of order. Applying a combination of x-ray and spectroscopic techniques it was possible to follow on a molecular level the changes in the state of order by specifying the defect structure. It is found that coupled 180°-jumps, flip-flop screw jumps, and defect diffusion along a chain set in successively, the onset of each mechanism of motion being accompanied by a phase transition.
The measured melting entropy is indicative for a coiled chain conformation in the melt with slight deviations from a random coil owing to the occurrence of limited short range order. This follows from theoretical considerations.