Shear-induced structural and thermodynamic phase transitions in micellar systems.

In this contribution a methodology to compute and classify shear-induced structural and phase transitions in surfactant/water mixtures from rheological measurements is presented. Non-linear rheological experiments, considering variations in surfactant concentration and temperature, are analyzed. In particular, the parameters of the BMP (Bautista-Manero-Puig) model, obtained from the fitting of the shear stress versus shear rate data, which are functions of surfactant concentration and temperature, allow classifying structural and phase transition boundaries. To test this methodology, we consider the analysis of the shear-induced structural and phase transitions of two micellar systems, cetyltrimethylammonium tosylate (CTAT)/water as a function of CTAT concentrations and Pluronics P103/water as a function of temperature. We found that the CTAT/water system presents a first-order phase transition at 30 ° C, and around 31 to 32 wt.% from isotropic to nematic phases, whereas a 20 wt.% Pluronics P103 aqueous micellar solution has two second-order (structural) phase transitions, one from spherical to cylindrical micelles at 33.1 ° C, and another one from cylindrical micelles to a nematic phase at 35.8 ° C and one first-order phase transition around 37.9 ° C at high shear rates near to the cloud point previously reported. The proposed methodology is also able to identify the instability regions where the wormlike micelles are broken, producing the typical shear banding behavior.

Effect of monomer dosing rate in the preparation of mesoporous polystyrene nanoparticles by semicontinuous heterophase polymerization.

The semicontinuous heterophase polymerization of styrene in the presence of cross-linking and porogen agents was carried out. Latexes with close to 20% solid content, which contained mesoporous nanoparticles with 28 nm in average diameters, up to 0.5 cm3/g in porosity and 6-8 nm in pore diameters were obtained. By varying the monomer dosing rate over the micellar solution, an unexpected direct dependence of instantaneous conversion on the monomer dosing rate was found. This was ascribed to the higher average number of radicals per particle attained in the polymerization at the higher dosing rate, which in turn would arise from the higher gel percentage in the polymer. It is believed that the cross-linked chains prevent encounters between radicals, delaying the bimolecular termination reactions and allowing the existence of more than one radical inside the particles, which in turn increases the propagation rate.

Critical phenomenon analysis of shear-banding flow in polymer-like micellar solutions. 1. Theoretical approach.

The shear-banding flow in polymer-like micellar solutions is examined here with the generalized Bautista-Manero-Puig model. The coupling between flow and diffusion naturally arises in this model, which is derived from the extended irreversible thermodynamic formalism. The limit of an abrupt interface is treated here. The model predicts a dynamic master steady-flow diagram, in which all data collapse at low shear rates. Moreover, the model predicts that a nonequilibrium critical line is reached upon decreasing the shear-banding intensity parameter of the model, which corresponds to increasing temperature, increasing surfactant concentration, or varying salt-to-surfactant concentration ratio. By employing nonequilibrium critical theory and the concept of dissipated energy (or extended Gibbs free energy), a set of symmetrical reduced stress versus reduced shear-rate curves are obtained similar to gas-liquid transitions around the critical point. In addition, the nonequilibrium critical exponents are derived, which follow the extended Widom's rule and the extended Rushbroke relationship, but they are nonclassical.