Observations on the X-Ray Structure of Rubber and the Size and Shape of Rubber Crystallites

Abstract

Vulcanized rubber is composed of a cross-linked network of chain molecules, segments of which are sufficiently free and mobile in localized regions to form a crystal lattice upon stretching. The crystallites thus formed represent an automatic molecular mechanism for re-enforcement and are analogous to particles of re-enforcing pigment which increase the modulus, strength, and tear resistance. These effects depend upon the number of particles present and their size and shape. X-ray determinations were made of the crystallite sizes in a series of vulcanized gum stocks using the Scherrer method but calculating the diffraction broadening by the formula proposed by Taylor. Evidence was found that the crystallite size distribution was heterogeneous and included small crystallites which broadened the base of the diffraction peaks. It could be shown that a high degree of crystallinity in a compound was associated with small crystallite size. Both of these factors combine to give high modulus stocks. For different cures with a given rubber compounding formula, there is a definite correlation between the amount of combined sulfur and the crystallite size. This indicates that irregularities in the structure caused by the combined sulfur tend to limit the crystallite growth. This interpretation is preferred although the experimental evidence is not decisive as to whether or not lattice distortion contributes to the width of the diffraction spots. Significant conclusions can be drawn from the work in regard to crystallite formation in stretched rubber and effects of crystallites on the physical properties.