Indirect detection of structural changes of pluronic PE 6200/H2O system by rheological measurements

In this study the material behaviour of both a liquid and lamellar lyomesophase were investigated by rheometrical means in steady and oscillatory shear flow. Three different aqueous mixtures were analyzed in order to probe the structural changes occurring at low (32 wt.%), intermediate (52 wt.%) and high (74 wt.%) polymer concentration for the commercial Pluronic copolymer PE 6200 (BASF-Germany). The liquid phase at low and intermediate compositions appeared as a direct micellar solution whose aggregates grew progressively with increasing temperature. These mixtures showed a micellar to lamellar phase transition at ca. 45 ◦ C. The mixture at 74 wt.% polymer showed a larger liquid phase in the temperature interval 20–60◦C. This solution structure was not well defined since an expected reverse micellar organisation did not fit with experimental data. The lamellar phase of the PE 6200/water system appeared stable within the temperature range 45–60 ◦ C and was studied for the mixtures at low and intermediate compositions. The 32 wt.% mixture showed a defected lamellar phase that aligns macroscopically under shear and then gives rise to a planar lamellae to MLVs (vesicles) transition at a shearing higher than 1 s−1 . Mechanical spectra showed a sensitive frequency dependence of material moduli. Against, the 52 wt.% polymer mixture presented an ideal lamellar organisation showing a simple steady flow response under the overall shearing interval 0.1–100 s−1 . Its mechanical spectrum was typical of a weak-gel fluid (G′ ≈ G′′ ≈ νn and G′ > G′′ ) in agreement with the viscoelastic behaviour of typical lamellar phases of low molecular weight surfactants.