Mechanism of the Crystallization of High Polymers

In this article a conception of the mechanism of crystallization is given, on the basis of which, it is suggested the origin of spherulites might be explained. A tentative explanation of several related phenomena is based on the new concept. Crystallites are conceived to be small collections of strongly oriented polymer which, on fusing, exhibit marked shrinking in the direction of orientation. Particles of amorphous material therefore stretch out in a certain direction during crystallization, as the result of which there is substantial contraction in the other directions. This is termed “auto-orientation.” As the growing crystallite has to be fed from surrounding material, an orienation is initiated in the latter at right angles to the first crystallite. As a result of this orientation, crystallites are formed there which are oriented Perpendicular to the first crystallite. In their turn, these crystallites induce orientation in the surroundings and in this way crystallization becomes autocatalytic in character. Spherulites owe their origin to the fact that the various sectors in a symmetrical spherical order mutually cancel out their stretching and shrinking tendencies. In highly crystalline polymers with rapid nuclear formation there first comes into existence numerous radomly oriented nuclei which proliferate to small crystallites. Wherever these small crystallites come to be arranged in a suitable position, their relative tensions will be cancelled out and they will grow into a spherulite. Simultaneously small crystallites which are in an unfavorable position will be consumed by the growing spherulite, or else oriented in the correct direction and incorporated. As spherulites commonly measure from 10 to 50 microns, compensation between the tensions of the various sectors has to take place at long distance. At higher temperatures, compensation is as a rule found at short distances, i.e., crystallites which are at right angles to each other are in close proximity. Therefore “spherulites” varying individually come into existence as the crystallites are subject to other influences in addition to auto-orientation. At very low temperatures the primary small crystallites are too stable to develop into spherulites. The mechanism of crystallization as thus conceived implies that the melting point of a polymer depends not only upon the dimensions and perfection of the crystallites, but also upon the specific arrangement of the crystallites, i. e., the degree in which they stabilize each other by a mutual compensation of tensions. It also helps to explain the following facts which have been noted in experiments performed with natural rubber: (1) The typical S-shape of the crystallization curve and the constancy of the product of induction period and maximum rate of crystallization. (2) The fact that natural rubber always crystallizes under nonequilibrium conditions. (3) The failure of rubber in latex to crystallize and the fact that the rate of crystallization in films produced from latex is poorly reproducible.