We have reexamined the fluorescence polarization anisotropy of single polymer chains of the prototypical conjugated polymer poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) isolated in a poly(methyl methacrylate) (PMMA) matrix employing improved synthetic samples that contain a much smaller number of tetrahedral chemical defects per chain. The new measurements reveal a much larger fraction of highly anisotropic MEH-PPV chains, with >70% of the chains exhibiting polarization anisotropy values falling in the range of 0.6-0.9. High anisotropy is strong evidence for a rod-shaped conformation. A comparison of the experimental results with coarse grain, beads on a chain simulations reveals that simulations with the usual bead-bead pairwise additive potentials cannot reproduce the observed large fraction of high polarization values. Apparently, this type of potential lacks some yet to be identified molecular feature that is necessary to accurately simulate the experimental results.

We analyze the stability of the plane Couette flow of a Newtonian fluid past an incompressible deformable solid in the creeping flow limit where the viscous stresses in the fluid (of the order eta_{f}VR ) are comparable with the elastic stresses in the solid (of the order G ). Here, eta_{f} is the fluid viscosity, V is the top-plate velocity, R is the channel width, and G is the shear modulus of the elastic solid. For (eta_{f}VGR)=O(1) , the flexible solid undergoes finite deformations and is, therefore, appropriately modeled as a neo-Hookean solid of finite thickness which is grafted to a rigid plate at the bottom. Both linear as well as weakly nonlinear stability analyses are carried out to investigate the viscous instability and the effect of nonlinear rheology of solid on the instability. Previous linear stability studies have predicted an instability as the dimensionless shear rate Gamma=(eta_{f}VGR) is increased beyond the critical value Gamma_{c} . The role of viscous dissipation in the solid medium on the stability behavior is examined. The effect of solid-to-fluid viscosity ratio eta_{r} on the critical shear rate Gamma_{c} for the neo-Hookean model is very different from that for the linear viscoelastic model. Whereas the linear elastic model predicts that there is no instability for H

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The stability of the plane Couette flow of a viscoelastic fluid adjacent to a flexible surface is analyzed with the help of linear and weakly nonlinear stability theory in the limit of zero Reynolds number. The fluid is described by an Oldroyd-B model, which is parametrized by the viscosity , the relaxation time , and the parameter ␤, which is the ratio of solvent-to-solution viscosity; ␤ = 0 for a Maxwell fluid and ␤ = 1 for a Newtonian fluid. The wall is modeled as an incompressible neo-Hookean solid of finite thickness and is grafted to a rigid plate at the bottom. The neo-Hookean constitutive model parametrized by the shear modulus G, augmented to include the viscous dissipation, is used for the solid medium. Previous studies for the Newtonian flow past a compliant wall predict an instability as the dimensionless shear rate ⌫ = ͑V / GR͒ is increased beyond the critical value ⌫ c . The present analysis investigates the effect of fluid elasticity, in terms of the Weissenberg number W = G / , on the critical value of the imposed shear rate ⌫ c for various parameters. The fluid elasticity is found to increase ⌫ c , indicating the stabilizing influence of the polymer addition on the viscous instability. For dilute polymeric solutions with ␤ ജ 0.5, the flow is stable when the Weissenberg number is increased beyond a maximum value W max , and W max increases proportional to the ratio of solid-to-fluid thickness H. For concentrated polymer solutions and melts with ␤ Ͻ 0.5, the flow becomes unstable when the strain rate increases beyond a critical value for any large Weissenberg number. The weakly nonlinear analysis reveals that the bifurcation of the linear instability is subcritical when there is no dissipation in the solid. The nature of bifurcation, however, changes to supercritical when the viscous effects in the solid are taken into account and the relative solid viscosity r is large such that ͱ r / H Ͼ 1. The equilibrium amplitude and the threshold strain energy for the solid have been calculated, and the effect of parameters H, ␤, r , and interfacial tension on these quantities is analyzed.

The finite amplitude stability of a plane Couette flow over a deformable solid medium is analyzed with emphasis on the class of high Reynolds number ͑Re͒ modes, referred to as the wall modes, for which the viscous stresses are confined to a thin layer adjacent to the fluid-solid interface with thickness O͑Re −1/3 ͒ times the channel width in the limit Reӷ 1. Here, the Reynolds number is defined in terms of the top plate velocity V and the channel width R. Previous linear stability analyses have shown that the wall modes are unstable for Newtonian flow past a linear viscoelastic solid. In the present study, the analysis is extended to examine the weakly nonlinear stability of these unstable wall modes in order to determine the nature of bifurcation of the transition point to finite amplitude states. To account for the finite strain deformations, the flexible solid medium is described by a neo-Hookean elastic model which is a generalization of the commonly used linear constitutive model. The linear stability analysis provides the critical shear rate ⌫ c and the critical wavenumber in the axial direction ␣ c , where the dimensionless shear rate is defined as ⌫ = ͱ V 2 / G, where is the fluid density and G is the shear modulus of the elastic solid. The critical parameter ⌫ c for the neo-Hookean solid is found to be close to ⌫ c for the linear elastic solid analyzed in the previous studies. The first Landau constant s ͑1͒ , which is the finite amplitude correction to the linear growth rate, is evaluated in the weakly nonlinear stability analysis using both the numerical technique and the high Re asymptotic analysis. The real part of the Landau constant, s r ͑1͒ , is negative for the wall mode instability in the limit Reӷ 1 for a wide range of dimensionless solid thickness H, indicating that there is a supercritical bifurcation of the wall mode instability. The amplitude of the supercritically bifurcated equilibrium state is derived in the vicinity of the critical point. The equilibrium amplitude, in the form A 1e 2 / ͑⌫ − ⌫ c ͒, is found to scale as Re −1/3 in the limit Reӷ 1 and is proportional to H 2.3 for H ӷ 1 in the same limit.