The tensile properties of strain‐crystallising vulcanisates. I. A new theory to explain strengthening

The tensile properties of conventional and peroxide vulcanisates were studied over a range of crosslink densities at room temperature and at 90°C. At 90°C the tensile strength and elongation at break of vulcanisates of lower crosslink density are superior to those at room temperature, while for vulcanisates of higher crosslink density the reverse applies. When strain‐induced crystallites form, they act as crosslinks shortening chains within the network. Shortened chains have lower entropies and a larger force is required for their continued extension, i.e., for a further reduction in their entropy. It is proposed that because these stiffer chains resist deformation, other less stiff chains are preferentially extended. This alters the network deformation pattern, forcing more chains to become load bearing and delaying the development of taut chains or chain sequences. Thus the formation of strain‐induced crystals leads to the slope of the stress–strain curve rising rapidly. At elevated temperatures the rate of nucleation of strain‐induced crystallites is slower but data on stress–strain curves obtained with different temperature programs show that, while strain‐induced crystallization is essential for the development of high tensile strength, delaying their formation to higher elongations is advantageous for high tensile properties. In vulcanisates of higher crosslink density the rate of crystallization at high temperatures becomes too slow. Rupture occurs before strain‐induced crystallites can form and protect the network by altering the network deformation pattern. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1562–1569, 2006