MICROPARACRYSTALS - THE INTERMEDIATE STAGE BETWEEN CRYSTALLINE AND AMORPHOUS

It has been established that "microparacrystals" are the building blocks of materials in the intermediate stage between crystalline and amorphous. A simplified review of the Theory of Paracrystallinity will be given. The analytical procedure for the determination of the paracrystalline distortion parameter, g(h k l), and evidence of the existence of paracrystalline structure in various types of materials will be included. g-values > 0 and < 15% characterize the world of paracrystalline materials, which range from catalysts (g congruent-to 1%) to polymers (g congruent-to 3%), graphites (g congruent-to 6%), glasses (g congruent-to 12%) and molten metals (g congruent-to 15%). Extensive studies of g and NBAR, the average number of netplane layers which build up the paracrystal, have led to a fundamental empirical relation alpha* = N1/2BAR g congruent-to 0.15. This relation implies physically that the growth of paracrystals in a particular material is controlled appreciably by the level of g in the netplane structure. Examples on the impact of g on the behaviour of materials will be given. Relationships between g, the crystallinity, and the mechanical properties in polymers will be discussed. The results of our recent investigations on high-modulus Kevlar fibres will be used to demonstrate these relationships.