Shear banding in entangled polymers in the micron scale gap: A confocal-rheoscopic study

Recent shear experiments in well-entangled polymer solutions demonstrated that interfacial wall slip is the only source of shear rate loss and there is no evidence of shear banding in the micron scale gap. In this work, we experimentally elucidate how molecular parameters such as slip length, b, influence shear inhomogeneity of entangled polybutadiene (PBD) solutions during shear in a small gap H ∼ 50 μm. Simultaneous rheometric and velocimetric measurements are performed on two PBD solutions with the same level of entanglements (Z = 54) in two PBD solvents with molecular weights of 1.5 kg mol^-1 and 10 kg mol^-1 that possess different levels of shear inhomogeneity (2b_max/H = 17 and 240). For the PBD solution made with a low molecular weight PBD solvent of 1.5 kg mol^-1, wall slip is the dominant response within the accessible range of the shear rate, i.e., up to the nominal Weissenberg number (Wi) as high as 290. On the other hand, wall slip is minimized using a high molecular-weight PBD solvent of 10 kg mol^-1 so that bulk shear banding is observed to take place in the steady state for Wi > 100. Finally, these findings and previous results are in good agreement with our recently proposed phase diagram in the parameter space of apparent Wi versus 2b_max/H suggesting that shear banding develops across the micron scale gap when the imposed Wi exceeds 2b_max/H [Wang et al., Macromolecules, 2011, 44, 183].