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This paper reports on the synthesis and characterization of a series of amphiphilic polymers which exhibit spectacular reversible shear-induced gelation in water. The systems consist of hydrophobically modified copolymers based on N,N-dimethylacrylamide and acrylic acid. The influence of molecular parameters such as the molecular weight and composition of the terpolymers on the rheological properties has been established. In the linear regime, the polymers behave like classical associating polymers: the sharp increase of the viscosity with concentration is the result of the transformation of intra- into intermolecular associations. Moreover, the dynamics of the physical gels obtained at high concentrations is tremendously slowed by the presence of alkyl side chains. Under shear the solutions display a Newtonian plateau at low shear rate followed by a shear-induced gelation. The latter is characterized in terms of a critical shear rate which sharply decreases as the polymer volume fraction increases. Thus, it appears that the shear-induced gelation may be controlled by adjusting the polymer molecular features. The networks built under shear closely behave like highly concentrated solutions at rest, suggesting a possible parallel between increasing shear and increasing concentration. This latter fact is reminiscent of the situation encountered with wormlike micelles. Shear flow could induce a transition between individual aggregates and a transient network as an increase in concentration does, thanks to the sharp modification of the electrostatic screening at the overlap concentration.

Cement pastes are used in various conditions of fluidity. This leads to the necessary introduction of polymers in order to control the rheology and to stabilize the slurry against segregation. This work describes the influence of hydrophobically modified acrylate polyelectrolytes on the rheological behavior of a tricalcium silicate paste at a constant solid volume ratio φ = 0.32.
© 2003 Society of Chemical Industry

Filled elastomers exhibit a complex dependence of their viscoelastic modulus as a function of both temperature and frequency. Otherwise, recent observations on thin polymer films have shown that their glass transition temperature depends on their thickness. On the basis of these recent results and on a recent model, we propose that the mechanical behavior of the filled elastomer is strongly influenced by a gradient of the glass transition temperature in the vicinity of the particles. This allows us to suggest a specific temperature−frequency superposition law for filled rubbers. This law seems to apply very successfully on two systems with different dispersion qualities, revealing the existence of a glass transition temperature gradient in the vicinity of the particles.

An associating amphiphilic polyelectrolyte was used as an emulsifier of the n-dodecane−water system. First, the dynamic rheological behavior of polydisperse concentrated direct emulsions with dispersed phase volume fractions up to 0.91 was investigated. Both the Princen and Kiss (J. Colloid Interface Sci. 1983, 91, 160; 1986, 112, 427) and Mason et al. (Phys. Rev. Lett. 1995, 75, 2051) approaches on emulsion elasticity give an adequate fitting of the rheological behavior leading to the conclusion that the emulsion elasticity mainly arises from droplet compression. Second, the shear-induced formation of ordered monodisperse emulsions was studied. More specifically, the conditions under which ordered structures are obtained by shearing polydisperse premixed emulsions within the two-parallel circular glass plates of a homemade shear apparatus were reported. The light scattering patterns, first-order ring, first-order diffuse ring with 6 bright spots, and second-order diffuse ring with 12 bright spots successively observed while shearing the emulsion samples reveal the progressive formation of ordered layers of hexagonal close-packed planes of oil droplets as the shear rate increases. The observation of the diffraction patterns also shows that the level of droplet organization decreases after cessation of flow thus suggesting that the droplets occupy well-defined spatial positions within a sample under shear as compared to a sample after cessation of flow. The radius of the droplets forming the colloidal crystal was systematically measured under shear as a function of the radial distance from the center of the glass slides, the gap spacing, the polymer concentration, and the oil content to follow the development of long-range ordering within the emulsions. The formation of the droplet crystal was found to depend only on the shear rate. Smaller droplets are obtained with increasing both the shear rate and the polymer concentration, and only a slight change in the droplet size could be detected upon varying the volume fraction of the dispersed phase. The results are in qualitative agreement with a coalescence−fractionation mechanism of the droplets under shear.

An emulsion is a dispersion of one fluid in another nonmiscible fluid, stabilized by a surfactant. Such a mixture is not at thermodynamic equilibrium, and some energy is needed to create the unfavorable interface. This energy is provided by a mechanical stirring. Due to the preparation process, the surfactant is shared among the oil/water interface (droplets surface) and the continuous phase. In this paper, we estimate the emulsification rate in relation with both the dispersed phase volume fraction and the mechanical stirring. For low volume fraction, this rate is very low showing that the mechanical stirring is inadequate, whereas it becomes more efficient (but still insufficient) for higher volume fractions. Using small-angle neutron scattering under shear and rheological measurements, we follow the surfactant distribution when applying a steady flow: small shear flows help the aging whereas high shear flows improve the fragmenting and hence the emulsification.