Instability of dilute poly(ethylene-oxide) solutions

Using photon correlation spectroscopy we studied dilute solutions of polyethyleneoxide) in solvents such as methanol, pure water, or, a water isopropanol mixture. In alcohol the polymer is molecularly dispersed and the solution is stable over a period of time. On the other hand, aggregates seem to be present in water and to be time-dependent. For high values of the scattering wave vector K the inverse relaxation time is proportional to Kw where w = 2.95 which is in good agreement with the expected theoretical value for internal motions of the coil. At the lower K values the diffusion coefficient is found to evolve with time. This evolution is interpreted as a disentanglement of the initial aggregates.Nous avons étudié par diffusion de la lumière des solutions de poly(oxyéthylène) dans le méthanol, l'eau pure et un mélange eau-isopropanol. En solution alcoolique, le POE est moléculairement dispersé et ne présente pas d'anomalie de comportement. Dans l'eau, le polymère semble être sous une forme agrégée évoluant avec le temps. Aux grandes valeurs du vecteur de diffusion K, la fréquence caractéristique est proportionnelle à Kw avec w = 2,95, valeur très proche de la valeur 3 espérée théoriquement. Aux faibles valeurs de K, on obtient un coefficient de diffusion évoluant avec le temps. Cette évolution s'interprète comme un désenchevêtrement des agrégats initiaux

Shear thickening of halato-telechelic polymers in apolar solvents

Short-length polymers selectively end-capped with a metal carboxylato group at both ends, i.e, metal carboxylato-telechelic polymers, exhibit an unusual shear-thickening behaviour in apolar solvents, depending on the main experimental parameters that control the ion-pair association, such as metal cation, concentration and temperature. In contrast to the non-ionic precursors, metal carboxylato-telechelics are indeed responsible for a marked increase in the solution viscosity in a range of shear rates between 0.1 and 1000 s(-1). The shear-rate dependence of the solution viscosity has been analysed in a close relationship to metal counterion, polymer concentration, temperature and addition of polar compounds to the apolar solvent. As a rule, the dilatant behaviour depends strongly on the formation of polymer aggregates in relation to the mutual interactions of the ion pairs. A consensus is now emerging on the origin of the shear-thickening effect, which should be found in inter-aggregate associations rather than in a transition from intramolecular to intermolecular associations of the individual chains

An Examination of the Shear-Thickening Behavior of High-Molecular-Weight Polymers Dissolved in Low-Viscosity Newtonian Solvents

The anomaly of shear thickening at high shear rates can be observed under certain conditions for high molecular weight polymers dissolved in low-viscosity Newtonian solvents despite the fact that shear-thinning behavior is considered the norm for these fluids. The nature of the shear-thickening region of the flow curve is examined herein through the application of a recent rheological model that has the capability of quantifying not only the rheological properties of the material, but its internal microstructural state as well. The results of this examination provide a self-consistent explanation of the full flow characterization of this anomalous behavior, including both rheological and optical experimental measurements. The results presented herein suggest that the shear-thickening behavior is actually caused by the destruction of structures formed during shear at lower shear rates, not by their formation, as previously assumed. The linear birefringence and linear dichroism observed experimentally in correlation with the shear-thickening behavior are well described by the rheological model and give predictions in line with experimental measurements. Furthermore, quantitative predictions are made for rheological characteristic functions, such as the first and second normal-stress coefficients, for which experimental measurements for these solutions have not yet been made