Tear Strength of Styrene−Butadiene−Styrene Block Copolymers

Measurements of tear strength were carried out on specimens of styrene−butadiene−styrene (SBS) block copolymers prepared from three formation conditions: SBS I cast from toluene solution; SBS II and SBS III hot-pressed from molten state. Prior to hot press, SBS III were additionally melt processed in an internal mixer. Results showed that stick−slip tearing was observed at all given rates of tearing. SBS II and SBS III possessed similar tear strengths, ca. 24.2 ± 2.6 kJ/m 2, which was about 2 times larger than that for SBS I, ca. 14.3 ± 1.5 kJ/m 2 . Moreover, SBS III possessed the largest elastic modulus, ca. 6 times larger than that of SBS I whose elastic modulus was the lowest. Microstructure and properties of PS and PB phases were extensively studied using transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and thermal and dynamic mechanical analyses. SAXS and TEM results showed that the interdomain length and the diameter of PS cylinders were remained approximately similar, 28.6 ± 2.5 and 12.7 ± 2.5 nm, respectively, regardless of sample preparation. Relatively ordered microdomains of PS cylinders packed hexagonally were developed in SBS I, whereas a more continuous pack of interconnected PS cylinders was observed in SBS II and SBS III . On tearing, the presence of interconnected PS cylinders is more likely to blunt the crack tip, leading to a rough fracture locus which accounts for a larger energy required to fracture hot-pressed specimens. Compared to SBS II, the enhanced elastic modulus for SBS III was attributed to a shortening of the PB molecular segments between effective cross-linkers ( M c ) determined by equilibrium swelling of the network using isooctane. Changes of the molecular weight distribution of SBS III due to the stress-induced degradation of SBS molecules were revealed by gel permeation chromatography. It implies that original SBS molecular chains would undergo intensive shear stresses (friction) in the mixer and mechanochemical degradation of triblock components takes place, giving rise to the reduction of M c .