Stress softening in styrene‐butadiene block copolymers: Rheo‐optical studies

Abstract

AbstractThe stress‐strain and the optical properties of styrene/butadiene block polymers indicate that at strains below those at which finite extensibility of the polybutadiene chains become Important, the mechanical properties of the polymers are determined by the combined effects of the elastic deformation of the polybutadiene phase and the degree of interaction between the polystyrene regions. The process of stress‐softening in styrene/butadiene block polymers is due to the break‐up of polystyrene regions, rather than to flow and rupture of weak crosslinks. Some additional softening occurs in the polybutadiene phase due to loosening‐up of the polybutadiene network structure. Specimens which were softened to the same degree of internal strain of the polybutadiene phase had similar stress‐strain properties when account was taken of the filler effect of the polystyrene regions. Block polymers which were prestretched to a strain below the finite extensibility of the polybutadiene chains exhibited a stable network structure. At large strains non‐Gaussian behaviour of the polybutadiene chains, probably combined with a recoverable plastic deformation of the polystyrene regions, determined the stress‐strain properties of the block polymers. The stress‐strain and the optical studies of the original and the softened block polymers indicate that under the experimental conditions described here the physical crosslinks formed by the polystyrene regions were stable.