Microfluidic exploration of the phase diagram of a surfactant/water binary system.

We investigate the behavior of a binary surfactant solution (AOT/water) as it is progressively concentrated in microfluidic evaporators. We observe in time a succession of phase transitions from a dilute solution up to a dense state, which eventually grows and invades the microchannels. Analyzing these observations, we show that, with a few experiments and a limited amount of material, our microdevices permit a semiquantitative screening of the equilibrium phase diagram as well as a few kinetic observations.

Pearling instabilities in water-dispersed copolymer cylinders with charged brushes.

We investigate the structural behavior of a poly(styrene)-block-poly(acrylic acid) diblock copolymer which forms hexagonally-packed PS cylinders (C-phase) in the melt state. The water dispersion of this structure provides hairy cylinders which comprise a PAA swollen cylindrical brush with a height h tunable via its degree of ionization and the ionic strength in the solution, and a water-free, PS cylindrical core of constant radius R(C). Such system constitutes an "out-of-equilibrium" frustrated model system: the selective swelling of the PAA brush results in a frustration of the interface curvature, which the ratio h/R(C) allows to quantify. Upon heating at a temperature higher than the glass transition temperature of the PS core, the glassiness of the core is relieved and the mechanical constraints arising from the selective swelling of the structure can be relaxed: the cylinders undergo a cylinder-to-sphere transition upon annealing at high temperature, when above a frustration threshold h/R(C) approximately 1.8. Thanks to a careful mapping of the transition diagram, an undulating cylindrical morphology (UC) is identified between unchanged cylinders ( h/R(C) approximately 1.8) and spheres ( h/R(C) < or = 2.0), which appears to result from a Rayleigh-like pearling instability of the copolymer cylinders.

Water-dispersed lamellar phases of symmetric poly(styrene)-block-poly(acrylic acid) diblock copolymers: model systems for flat dense polyelectrolyte brushes.

We investigate the static properties of a water-dispersed lamellar ( L) phase formed in the melt state with a nearly symmetric poly(styrene)-block-poly(acrylic acid) (PS- b-PAA) diblock copolymer. The PAA brush is considered as a model flat polyelectrolyte ( PE) brush of controlled surface density. Thanks to small-angle X-ray scattering, its behavior in water is studied as a function of (i) its ionization, through the pH of the dispersions which is increased by an addition of a known amount of a base, i.e. sodium hydroxyde NaOH, and (ii) in the presence of a monovalent salt, i.e. sodium chloride NaCl, of concentration C(S). At low pH, we find that the brush effectively behaves as a neutral brush. At high pH, the brush is in the so-called "osmotic regime", in which all sodium counterions are trapped within the brush volume and stretch the chains via an osmotic effect. The properties of such a brush in the presence of a monovalent salt, confirm this result, showing a C(S)(-1/3) dependence in the brush height L(O), in agreement with mean-field predictions. The L(O)- C(S) profiles at different ionizations give access to the actual brush internal charge fraction f. The results are found to be in very good quantitative agreement with experimental measures found in the literature, and can be completely and quantitatively described by Oosawa's approach to counterion condensation in a semi-dilute to concentrated solution of charged, rod-like chains.