Crystallized Rubber
Abstract In recent years various attempts have been made to crystallize natural rubber by some means other than by stretching. In the present paper the results of these experiments are described, together with some new observations, in order to settle the present status of the problem. The processes of crystallization are divided into three groups and discussed individually. 1. Spontaneous crystallization under conditions which vary but little from the normal conditions. 2. Crystallization from solution after purification. 3. Crystallization of a solution of purified rubber by cooling to a low temperature. It is a generally known fact that after storage for a year, plantation rubber becomes stiff, inelastic, and opaque. A rubber altered in this way is known as “frozen” rubber, because this change has been observed most frequently after storage in a cold place. The x-ray investigations of Katz and Bing have rendered it certain that during “freezing” there is crystallization of the rubber, because when subjected to x-rays frozen rubber shows crystalline interferences from which the same crystal lattice is calculated as that which is formed on stretching, except that the degree of orientation in frozen rubber differs from that of stretched rubber. This is seen in x-ray diagrams, where the nearly point interferences of the stretched samples lie on the same Debye-Scherrer circles as those of frozen rubber. There have been rather exhaustive investigations on those changes in frozen rubber which appear on warming, viz., during transformation of the crystalline to the amorphous state (the “fusion process”), and which are characterized by disappearance of the crystal interferences and by a discontinuous change in the heat content, specific gravity, hardness, and light absorption.