Phase transitions, relaxations, and properties of high polymers

Abstract

AbstractStarting with a nondefective bundle of macromolecules (the ideal crystal) one gets a low‐defective bundle (the real crystal) by introduction of stable defects (kinks, torsional defects, jogs and folds) which are compatible with the intra‐ and intermolecular potential. The cooperative statistical treatment of this bundle reveals – under a certain condition – a first order transition, connected with a jump in defect‐concentrations. This transition corresponds in most cases to the melting of the polymer, in some polymers (e. g. trans‐polybutadiene) it splits up into a solid state transition (large kinkblocks without torsions) and the melt transition (small kinkblocks, torsional defects, jogs and folds). In the case of polyethylene the bundle model has been shown to explain quantitatively the transition data (Tm, ΔHm, ΔVm) as well as the expansion coefficient and the compressibility of the melt together with their dependence on static pressure. The calculated short‐range order also is in accordance with the X‐ray and electron diffraction data. By introduction of cooperatively arranged gauche‐areas (for planar molecules) the meander model is established which guarantees the isotropy of the melt. Assuming the bundle diameter (calculated to about 50 Å which is in the range of the observed superstructure in amorphous polymers) to be constant during crystallization a lamellae‐structure results in which the meander thickness is determined by the crystallization temperature. This two phase meander model is in accordance with the experiment and implies the key to the understanding of molecular motions in polymers. This will be shown by a quantitative molecular interpretation of the relaxation processes (secondary, main and crystalline relaxation) in a polymer as well as of its deformation behaviour (paraelasticity, plasticity).