FRACTURE ENERGIES OF STYRENE-BUTADIENE-STYRENE BLOCK-COPOLYMERS .1. 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 solven ts: toluene, cyclohexane, and a mixture of tetrahydrofuran and methyl ethyl ketone (THF/MEK). Measurements of fracture energies of SBS speci mens were carried out at various temperatures and rates using two meth ods (i.e., a conventional tear test and a recently developed cutting t est). Effect of casting solvent on the fracture energy was investigate d as well. It was found that stick-slip tearing dominates at low tempe ratures (-50 similar to 20 degrees C). Tear strength increased with de creasing temperature. Eventually, a value of 180 kJ/m(2) was reached a t -70 degrees C, close to the glass transition temperature of polybuta diene phase. At temperatures higher than 20 degrees C, however, steady tearing was found and the tear strength gradually decreased with incr easing temperature. Tear strength was virtually zero at 100 degrees C, above the glass transition temperature of polystyrene phase. Effect o f temperature on tear strength is more pronounced than that of tearing rate. In contrast, the intrinsic strength of SBS block copolymers det ermined from cutting test remains unchanged, about 570 J/m(2), over a wide range of temperatures and rates. Specimens cast from THF/MEK solu tion 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. (C) 1997 John Wiley & Sons, In c.