Fracture energies of styrene-butadiene-styrene block copolymers (I). Effects of rate,temperature, and casting solvent

Specimens of styrene-butadiene-styrene (SBS) block copolymers, Kraton D-1102, were prepared by solution casting using three different solvents: toluene, cyclohexane, and a mixture of tetrahydrofuran and methyl ethyl ketone (THF/MEK). Measurements of fracture energies of SBS specimens were carried out at various temperatures and rates using two methods (i.e., a conventional tear test and a recently developed cutting test). Effect of casting solvent on the fracture energy was investigated as well. It was found that stick-slip tearing dominates at low temperatures (−50 ∼20°C). Tear strength increased with decreasing temperature. Eventually, a value of 180 kJ/m² was reached at −70°C, close to the glass transition temperature of polybutadiene phase. At temperatures higher than 20°C, however, steady tearing was found and the tear strength gradually decreased with increasing temperature. Tear strength was virtually zero at 100°C, above the glass transition temperature of polystyrene phase. Effect of temperature on tear strength is more pronounced than that of tearing rate. In contrast, the intrinsic strength of SBS block copolymers determined from cutting test remains unchanged, about 570 J/m², over a wide range of temperatures and rates. Specimens cast from THF/MEK solution exhibit yielding phenomena when stretched. Moreover, they possess a relatively larger tear strength, compared to those cast from either toluene or cyclohexane solution. A more continuous polystyrene phase is expected to develop in THF/MEK as-cast specimens which is believed to account for the large tear strength. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2003–2015, 1997