Two-phase Model for Structure of Polymers

Abstract

ATTEMPTS to relate polymer properties to structure have usually involved the assumption of a two-phase structural model: crystalline regions embedded in an amorphous matrix. The crystalline and amorphous regions in the structure are each presumed to have well-defined properties which do not vary as the proportions change in different polymers. With this model, the variables available to compare different samples of the same polymer are: crystallite size, shape and orientation correlation; orientation effects in the amorphous material; and the relative amounts of crystalline and amorphous material present. The last parameter mentioned is termed ‘degree of crystallinity’, and it is measured in various ways: by techniques involving X-ray diffraction, density, infra-red, nuclear magnetic resonances, chemical reactivity, and swelling measurements. The degree to which the results of these techniques correlate is a measure of the validity of the two-phase model. Farrow and Ward¹ found little correlation between crystallinity measurements on polyethylene terephthalate fibres by X-ray diffraction, density and infra-red techniques. Similar comparisons between crystallinity measurements on polypropylene by various techniques were reported by Quynn, Riley, Young and Noether² with similar inconsistencies. Nuclear magnetic resonance studies of polyethylene and polypropylene fibres by Hyndman and Origlio³ indicated the existence of a distinct phase intermediate in degree of order between the crystalline and amorphous phases. Sobue and Tabata⁴ found X-ray evidence for polymorphism in polypropylene.