We use confocal microscopy and particle image velocimetry to visualize motion of 250-300 nm. fluorescent tracer particles in entangled polymers subject to a rectilinear shear flow. Our results show linear velocity profiles in polymer solutions spanning a wide range of molecular weights and number of entanglements (8 Z 56), but reveal large differences between the imposed and measured shear rates. These findings disagree with recent reports that shear banding is a characteristic flow response of entangled polymers, and instead point to interfacial slip as an important source of strain loss. DOI: 10.1103/PhysRevLett.101.218301 PACS numbers: 47.57.Ng, 83.10.Kn, 83.80.Rs Flow properties of entangled polymers are important in myriad commercial processes for molding, extruding, and spinning plastic components. The Doi-Edwards (DE) theory provides the most successful molecular framework for understanding these properties. Developed around the reptation or ''tube'' model [1], this theory contends that a melt of entangled polymers responds affinely to instantaneous macroscopic deformations. The affine response is sustained by long-lived entanglements between molecules and causes the network of entanglements (tube) constraining any given molecule to orient and stretch in the same way as does the macroscopic melt. Polymer molecules trapped in the tube initially stretch and orient in synergy with their environment.DE predictions for step strain, oscillatory, and steady shear flows agree, sometimes quantitatively, with experiments [2][3][4][5][6]. A more controversial prediction is that under steady shear, the shear stress is a multivalued function of the imposed shear rate. Thus, simple shear flow is unstable to perturbations in shear rate and should produce shear banding [7,8]. Surprisingly, with the exception of entangled wormlike micellar fluids [9][10][11], banding is generally not observed in flows of entangled polymers at any shear rate. This implies that some other dynamic processes not taken into account by the theory must contribute to the fluid's response. Efforts to date have focused on understanding how convective acceleration of reptation [12][13][14][15][16][17], tube diameter shrinkage [18], and slip [19][20][21][22] near the shearing surfaces influence this prediction. All three processes eliminate or weaken the driving force for entangled polymers to shear band and, when integrated into the DE theory, lead to steady shear stress predictions that compare favorably with experiments using moderately entangled polymers.Recent velocity profile measurements using 10 m particles dispersed in entangled polybutadiene solutions show, for the first time, that entangled polymer systems do in fact appear to shear band [23][24][25]. Surprisingly, these studies find that shear banding occurs even in solutions with intermediate levels of entanglements, generally thought to be well described by DE theory with the aforementioned modifications. Parameters such as shear strain and shear rate, widely used to characterize sh...

Time-dependent shear stress versus shear rate, constitutive curve, and velocity profile measurements are reported in entangled polymer solutions during start-up of steady shear flow. By combining confocal microscopy and particle image velocimetry (PIV), we determine the time-dependent velocity profile in polybutadiene and polystyrene solutions seeded with fluorescent 150 nm silica and 7.5 μm melamine particles. By comparing these profiles with time-dependent constitutive curves obtained from experiment and theory, we explore the connection between transient nonmonotonic regions in the constitutive curve for an entangled polymer and its susceptibility to unstable flow by shear banding [Adams et al. Phys. Rev. Lett. 2009, 102, 067801-4]. Surprisingly, we find that even polymer systems which exhibit transient, nonmonotonic shear stress-shear rate relationships in bulk rheology experiments manifest time-dependent velocity profiles that are decidedly linear and show no evidence of unstable flow. We also report that interfacial slip plays an important role in the steady shear flow behavior of entangled polymers at shear rates above the reciprocal terminal relaxation time but has little, if any, effect on the shape of the velocity profile.

Two likely causes of Type C Damping in highly entangled polymers are interfacial slip and shear banding. To isolate these mechanisms, we use confocal microscopy and particle image velocimetry to visualize flow in a planar-Couette shear. Polybutadiene {,M^= 200K, I.IM) solutions with different entanglement densities (8 < Z < 56) are sheared in narrow gaps ~ 35 |im. Not only does the velocity at the boundaries violate the no-slip condition, but the velocity profiles are linear. This is inconsistent with shear banding. The measured shear rates and stresses are used to characterize interfacial slip.