A theory of green strength in polyisoprene

It is shown that, at −25°C, natural rubber (NR) crystallizes more readily than synthetic polyisoprene (IR), the long induction period for nucleation in IR in particular being dependent on the carbon black loading. Both elastomers form bound rubber with carbon black but not with nonreinforcing fillers such as CaCO 3 and glass powder. At room temperature, in the presence of carbon black, NR has good green strength but not IR, while neither rubber has good green strength with nonreinforcing fillers. However, at 0°C CaCO 3 filled NR too has good green strength. A theory is proposed to account for the good green strenth characteristics found with certain compounds. In the model rubber molecules, already bound to the carbon black surface, are linked together, at low strains, via stress‐induced crystal lamellae, giving a three‐dimensional network in the compound. Such crystal lamellae are known to grow at right angles to the direction of strain from row nuclei formed at low strains. The coherence provided by the network permits the formation, at higher strains, of stress‐induced crystals in which polymer chains are now aligned in the direction of strain. This leads to an upturn in the stress–strain curve. In the absence of either bound rubber or of crystal lamellae, a long range network structure cannot form and extension of the sample continues at constant stress.