Developments in self‐reinforced elastomers

Abstract

AbstractUntil recently it has been believed that either stress‐crystallization or the presence of reinforcing filler is necessary for the development of high strength in elastomers. Now it has been observed that gum vulcanizates of isoprene/acrylonitrile copolymers have tensile strengths in the order of 3000–4000 psi in the absence of fillers. In this case crystallization does not occur and the strength is associated with the ability of the polymer molecules to become highly oriented on stretching. Similar observations have been made with ABA block copolymers polystyrene‐polybutadiene‐polystyrene which are highly elastomeric in the absence of chemical crosslinks. The elastomeric properties and high strength of the block copolymers are attributed to the mutual incompatibility of the A and B blocks which leads to phase separation on a microscopic scale. The effective network structure can be temporarily destroyed by heating the polymer above the softening temperature of the resinous blocks which makes the material readily processable. The initial decrease in retractive force with increasing temperature up to 60–70°C is attributed to an increase of mobility of the resinous domains in the rubbery matrix. At higher temperatures the crosslinking effectiveness decreases markedly due to softening of the resinous regions. The process of yielding, hysteresis effects, and delayed elastic creep arise because of viscous forces which oppose the motions of the colloidal polystyrene domains. The strength and effectiveness of physical crosslinks in block copolymers shows a similar temperature dependence as has been observed for other filled and unfilled vulcanizates. The process of self‐reinforcement in isoprene‐acrylonitrile copolymers and polystyrene‐polybutadiene‐polystyrene block copolymers is attributed to viscous effects which make possible the alignment of a large number of polymer chains in the stretching direction.