Molecular Interpretations of Modulus and Swelling Relations in Natural Rubber Cross-Linked by Dicumyl Peroxide

Abstract

Abstract A survey of published experimental work on the modulus of natural rubber crosslinked by dicumyl peroxide permits a comparison with the results and molecular interpretations obtained in recent NBS work. Excellent agreement was found among values of the shear modulus G at the same crosslinking when the crosslinking is calculated from the amount of decomposed dicumyl peroxide. The types of deformation included torsion as well as uniaxial extension and compression. G increases linearly with crosslinking (except at the lowest degrees) with a slope from 5 to 15 percent greater than that predicted by the simple statistical theory. Data of Mullins demonstrated that at each degree of crosslinking the value of G is intermediate between 2C1 and 2(C1+C2) where C1 and C2 are the Mooney-Rivlin constants. Measurements of equilibrium swelling at a given degree of crosslinking are in reasonable agreement with each other. However, the entropy components of the modulus and the sub-chain density calculated from swelling measurements are appreciably greater than those calculated from crosslinking or from direct mechanical measurements. They increase linearly with crosslinking. It is concluded that the number of sub-chains effective in limiting swelling is greater than that effective in direct mechanical measurements.