Crystallization in Natural Rubber. V. Chemically Modified Rubber

Abstract

Abstract Small amounts of crosslinking have been shown to reduce the rate of crystallization of natural rubber by large factors. It has also been inferred from measurements on peroxide and sulfur vulcanizates of various kinds that sulfur combined in forms other than crosslinks may retard the crystallization process efficiently. A simple banned volume mechanism has been proposed to account for the influence of crosslinking and sidegroup combination on the rate and final extent of crystallization, and has been shown to predict the form of the observed dependences for crosslinked rubber, and approximately the relative magnitudes, but good quantitative agreement was not obtained. Measurements are described below of the rate and final extent of crystallization in natural rubber which has been modified by the chemical combination of sidegroups to the rubber molecule. It has not proved possible to account quantitatively for the observed rates of crystallization, but it is thought that a description of the extremely large effects found to accompany slight chemical modifications will be of general interest. The preparation of the modified rubbers is described elsewhere. The substances added were six thiol acids; namely, monochlorothiolacetic, trichlorothiolacetic, thiolbenzoic, 1-thiolnaphthoic, thiolsalicylic, and thiol-stearic acids. The reactions were carried out in the solid phase by addition of the thiol acid to purified rubber on an open mill, in solution by addition of the thiol acid to a solution of purified rubber in benzene, and in the latex phase.