Comparison between the Observed Density of Crystalline Rubber and the Density Calculated from X-Ray Data

Abstract

When unvulcanized rubber is stretched quickly, an orientation of structure occurs parallel to the direction of extension. This orientation is usually regarded as a form of crystallization, because of the optical and other phenomena which accompany it. It is also accompanied by a change of volume. When unstretched rubber is subjected to suitable temperatures below about 10° C, it too becomes crystalline. The crystallization of rubber is accompanied by an increase of density, and the simultaneous production of an x-ray diffraction pattern is usually interpreted as evidence that crystallization has occurred. A diffraction pattern of this kind results from regular repetition, within the crystal structure, of groups of atoms or molecules in a definite spatial arrangement. The smallest group from which the structure can be built up in this way is called the unit cell; and the dimensions can be computed from measurements of the pattern, provided it is possible to determine the crystal form. The pattern of stretched rubber consists of interference spots, and that of frozen rubber consists of rings, but both of their measurements have been shown to indicate the same unit cell. If, in addition, the number of atoms in the unit cell is known, the density of the material can be computed. If the computed density agrees with that found by direct measurement, the fact is a valuable indication of the correctness of the assumed structure on which the calculation of density was based. Accurate knowledge of the density of completely crystallized rubber would therefore serve both as a guide in studies of structure and as a measure of the degree of crystallization of any material under consideration. In the experimental work described in this paper, the rubber was crystallized as completely as possible. Its density was determined by recognized methods, and compared with the densities calculated from x-ray measurements.

Crystallization of unvulcanized rubber at different temperatures

“…The rates of crystallization observed in the present work are in reasonable agreement with observations of density by Bekkedahl [5] and Treloar [69] at 0 0 C, by Smith and Hanna [56] at -20 0 C, and by van Rossem and Lotichius [53] at 4 0 C and _10 0 C. Likewise, a similar rate can be calculated from observations of Young's modulus at 0 0 C made by Conant and Liska [16].…”

“…Values for this quantity, calculated from measurements of density or specific gravity reported by previous workers [4,5,8,14,19,31,39,53,55,56,72] generally lie within the same limits, although a few are somewhat lower. Holt and McPherson [32] found a volume change of about 1.85 percent on stretching a specimen of vulcanized rubber to an elongation of 700 percent at 25 0 C. Treloar [69), by stretching unvulcanized rubber at 0 0 C, observed a volume decrease of over 3 percent, the highest value reported.…”

sp2 carbon allotropes in elastomer matrix: From master curves for the mechanical reinforcement to lightweight materials

“…Moreover, density difference is expected to be not relevant for the calculations according to Equation (7). In the literature [77] it was reported that, in the case of NR, density value should be between that of amorphous and crystalline NR: these values range between 0.9 and 0.96. The variation of matrix density is below 6%, and this gives an uncertainty of the density of the composite even lower.…”