Novel rheological features of molten SEBS copolymers: Mechanical relaxation at low frequencies and flow split

Poly(styrene)-poly(ethylene-co-butylene)-poly(styrene) (SEBS) triblock copolymers with a PS volume fraction φ{symbol} = 0.30 are investigated. The analysis of the loss tangent at low frequencies leads to define a mechanical relaxation associated to the blocking effect of ordered PS cylinders on PEB chains motion. Submitting SEBS to a shear flow at 250 °C an orientation of the cylinders is observed. The effect of the molecular weight and the cylinder alignment on the relaxation is studied. Continuous flow experiments, in torsion and capillary mode, reveal the presence of very severe surface cracks which cause the phenomenon called "flow split". The correlation between dynamic viscoelastic results and flow split is investigated, concluding that this is related to an entanglement-disentenglement process, rather to an alignment effect of PS cylinders.