Measurements of freezing‐point depression to evaluate rubber network structure. Crosslinking of natural rubber with dicumyl peroxide

Abstract

An experimental approach based on the freezing‐point depression of a solvent in a swollen gel has been developed to characterize the structure of rubber networks. This property depends on the conditions required for the formation of crystalline nuclei, which are limited by the elastomer network restrictions. Information about the functionality, spatial distribution, and number of crosslinks can be obtained by the use of this easy and ready experimental method. Application of the tube model of rubber elasticity in the uniaxial stress–strain experiments of natural rubber samples vulcanized with dicumyl peroxide yields the characteristic parameters of the rubber networks, which are in concordance with the network structures predicted by the freezing point method. Finally influence of vulcanization conditions in network structure and its relationship with the mechanical properties was evaluated. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 544–556, 2007

Uncertainties in the Determination of Cross-Link Density by Equilibrium Swelling Experiments in Natural Rubber

“…Note that peroxidecured systems can be assumed to have a certain fraction of highly functional cross-links or clusters due to radical chain reactions. 112 Nevertheless, we also observe some deviation from linearity at very high cross-link density. One reason could of course be that very short network may show deviations from the assumed Gaussian deformation behavior (note that such rubbers are in fact technologically unimportant).…”

“…Also, the microstructure of very highly cross-linked elastomers might be different; e.g., higher network chain polydispersity or more pronounced spatial heterogeneities may arise, maybe as a consequence of the mentioned peculiarities of radical cross-linking. 112 The chain order distribution in these systems is in fact not as narrow as in sulfur-cured samples, sometimes with a clear bimodality, i.e., a small fraction with higher local cross-link density. Details on this topic will be the subject of a separate publication.…”

Inhomogeneities and Chain Dynamics in Diene Rubbers Vulcanized with Different Cure Systems

“…It is important to note that some other factors that we kept invariant, such as the vulcanization temperature, could also have an important influence on the cross-links distribution. 16 For sulfur-based systems, it appears that most homogeneous rubbers are obtained for natural rubber, but note that we cannot fully exclude at this point that differences in the local spin dynamics among the individual protons in the different monomers, or microstructural variations (trans-1,4 or vinyl units) could also account for this small difference, as these factors also affect the shape of the analyzed NMR signal function. 59 A clear trend is the one related to the crosslink density, where a minimum (high homogeneity) is found at cross-link densities of around 2 Â 10 -4 mol/g.…”

“…There are several factors that favor the addition of peroxides: peroxide concentration, vulcanization temperature and polymer backbone structure. 15,16 In this work, the two first two factors are invariant; therefore, the different behavior must again be ascribed exclusively to the chemical structure of polymer. There are three characteristics that support this supposition:…”

ESR investigation of NR and IR rubber vulcanized with different cross-link agents

“…IR and NR) via abstraction of the more labile allylic hydrogen atoms or via addition to the double bonds, forming polymer radicals. In the simplest cross-linking reaction (ignoring numerous other side reactions that do not generate junctions between rubber chains), two competing termination reactions may take place [8,[24][25][26]. On one hand, recombination of two polymer radicals to form a tetra-functional carbon-carbon cross-link and on the other hand, addition of one polymer radical to a double bond of another rubber chain giving rise to a cross-link and a new active free radical.…”