Basic characteristics of uniaxial extension rheology

We have carried out continuous and step uniaxial extension experiments on monodisperse and bidisperse styrene-butadiene random copolymers (SBR) to demonstrate that their nonlinear rheological behavior can be understood in terms of yielding through disintegration of the chain entanglement network and rubber-like rupture via non-Gaussian chain stretching leading to scission. In continuous extension, the sample with bidisperse molecular weight distribution showed greater resistance, due to double-networking, against the yielding-initiated failure. An introduction of 20 % high molecular weight (10 6 g/mol) SBR to a SBR matrix (2.4×10 5 g/mol) could postpone the onset of non-uniform extension by as much as two Hencky strain units. In step extension, the bidisperse blends were also found to be more resistant to elastic breakup than the monodisperse matrix SBR. Rupture in both monodisperse and bidisperse SBR samples occurs when the finite chain extensibility is reached at sufficiently high rates. It is important to point out here that these results along with the concept of yielding allow us to clarify the concept of strain hardening in extensional rheology of entangled polymers for the first time.