NETWORK STRUCTURE AND MECHANICAL-PROPERTIES OF SULFUR-CURED RUBBERS

The effect of sulfur/accelerator amount (at fixed ratio) and accelerat or type (n-cyclohexyl-benzothiazole-2-sulfenimide, diphenylguanidine) on network parameters (network chain density, cross-linking density, t ube diameter, trapping factor) of styrene-butadiene copolymer networks is investigated via uniaxial stress-strain measurements up to large e xtensions. The mechanical properties are discussed in the framework of a non-Gaussian network model. It includes topological constraints for the network chains and the network junctions (cross-links and trapped entanglements). The model assumes a trapped entanglement contribution to the stress that does not vanish in the infinite strain limit. The two types of junctions (cross-links and trapped entanglements) are ass umed to fluctuate in a Flory-Kastner domain around their mean position , while the chain segments fluctuate in a conformational tube built up by the surrounding chains. The finite extensibility component of the measured stress is evaluated by using a series expansion of the invers e Langevin function. The typical network parameters are estimated from the Gaussian contribution to the stress. It is found that the cross-l ink contribution G(c) of the moduli changes approximately linearly wit h the concentration of cross-linking agent sulfur. Vanishing values of the moduli G(c) are found at finite concentrations of cross-linking a gent that are related to the gel point. The concentrations of cross-li nks at the gel points are almost identical for both systems of acceler ators whereas the estimated cross-linking efficiencies are different. In relation to the estimated structure parameters, some measured techn ically important mechanical properties (tensile strength, hardness, re bound resilience, dynamic losses) of the differently accelerated sulfu r-cured SBR networks are discussed qualitatively in the framework of t he proposed network model.