Viscoelastic and Ultimate Tensile Properties of Styrene-Butadiene-Styrene Block Copolymers

A study was made of the stress—strain and ultimate properties in simple tension of an elastomeric styrene—butadiene—styrene block copolymer (Kraton 101) and also of a similar material (Thermolastic 226) that contains about 35% plasticizer as well as inorganic pigments. Stress—strain data were obtained at crosshead speeds from 0.02 to 20 in./min at temperatures from − 40 to 60° C. The relaxation rate, derived from the data at constant extension rates, was about 8% per decade of time for both materials at temperatures from − 40 to about 40° C and at extensions from about 20% up to 400%. Above − 30° C, the shift factor log aT was found to vary linearly with temperature. These findings indicate that the time and temperature dependence of the mechanical properties results primarily from the plastic (or viscoelastic) characteristics of the styrene domains. The tensile strength for Kraton 101 below 40° C is somewhat greater than 4000 psi, sensibly independent of extension rate and temperature. For the highly plasticized Thermolastic 226, the tensile strength at an extension rate of 1.0 min−1 increases from 2200 psi at 0° C to 3600 psi at − 40° C. Above 40° C for Kraton 101 and above 0° C for Thermolastic 226, the tensile strengths are dependent on extension rate and temperature owing to the increased ductility of the styrene domains. The high strength of these materials results from the uniformly dispersed styrene domains of colloidal dimensions. To obtain a crack of sufficient size to satisfy an energetic criterion for self-sustained high-speed propagation, domains must be disrupted. The plastic characteristics of the domains have a controlling effect on crack growth and thus on the ultimate properties of the materials. The strength and extensibility of other elastomers are considered in relation to those of the block copolymers.