A. A. Lefebvre

Microstructural changes in a colloidal liquid in the shear thinning and shear thickening regimes

The structure of a colloidal suspension under shear flow was studied by in situ small angle neutron scattering (SANS). This suspension exhibited shear thinning at low shear rates and shear thickening at high shear rates. Under quiescent conditions, the SANS profiles were azimuthally symmetric and contained a well-defined scattering maximum. This is due to local, liquidlike correlations between neighboring particles. Increasing shear rate lead to changes in the interparticle correlations. These changes are quantified by obtaining the anisotropic structure factor of the suspension under shear flow. We found an increased probability for the formation of inter-particle clusters in the gradient-vorticity plane. This results in an increase in the low angle scattering intensity in the flow direction, and the scattering peak, observed under quiescent conditions, is reduced to a shoulder. We found no evidence for a shear-induced phase transition in our experimental window. At low shear rates (γ), the microstructu...

Determination of critical length scales and the limit of metastability in phase separating polymer blends

Liquid–liquid phase separation in polymethylbutylene/polyethylbutylene blends near the metastability limit was studied using small angle neutron scattering (SANS). Our objective was to study the relationship between quench depth and Rc, the lower limit for the length scale of the structures formed during the early stages of the phase transition (nuclei). During the early stage of phase separation, the SANS profiles merged at a time-independent critical scattering vector, qc. We discuss different methods for estimating qc, and present arguments for the scaling relationship, Rc∼1/qc. The theory of Cahn and Hilliard predicts that in metastable blends Rc increases with increasing quench depth, and diverges at the spinodal. In contrast, our experimental measurements showed Rc increases with decreasing quench depth, and the location of the point where Rc diverges lies between the binodal and the spinodal. Some aspects of our results are addressed in recent theoretical work of Wang and Wood [J. Chem. Phys. 117, ...

Initial stages of nucleation in phase separating polymer blends

The initial stages of nucleation during liquid–liquid phase separation in mixtures of high molecular weight polymers was studied by time-resolved small angle neutron scattering. Phase separation was induced either by decreasing temperature or by increasing pressure. One of the blend components was labeled with deuterium to obtain sufficient scattering contrast between the components. The general features of nucleation were independent of quench depth and the nature of the quench (temperature quench versus pressure quench). The early stages of nucleation consisted of amplification of concentration fluctuations. During this stage, the scattered intensity (I) in the low scattering vector (q) limit was consistent with the Ornstein–Zernike equation. This enabled the determination of the characteristic length scale of the growing fluctuations, ξ. The I vs q behavior at intermediate scattering vectors (q>1/ξ) could be described by a power law (I∼q−d). We demonstrate the existence of a time–temperature superposit...

Critical length and time scales during the initial stages of nucleation in polymer blends

The initial stages of nucleation during liquid–liquid phase separation in polymer mixtures were studied by time-resolved small angle neutron scattering. The time required for nucleation vanishes exponentially as the stability limit (spinodal) is approached. The critical nucleus size decreases monotonically with increasing quench depth and remains finite at the spinodal. Our data differ qualitatively from theoretical predictions.

Fluctuation mediated phase separation in polymer blends near the limit of metastability

The evolution of the structure factor of off-critical polyolefin (polymethylbutylene/polyethylbutylene) blends quenched from the homogeneous state to states near the limit of metastability was studied by small angle neutron scattering. The Cahn–Hilliard–Cook theory was used to organize the data in terms of three time-independent parameters that depend on the scattering vector, q: S0(q), the initial structure factor, St(q), the terminal structure factor, and R(q), a kinetic parameter that indicates the time scale for the transformation from S0(q) to St(q). These three parameters change systematically with quench depth. Changes in the structure factor S(q,t) are only observed in the q values smaller than a critical scattering vector, qc. At small quench depths, qc is obtained because R(q)→0 as q→qc. At deeper quenches, qc is obtained because St(q)→S0(q) as q→qc. Scattering characteristics at q

Micelle formation of randomly grafted copolymers in slightly selective solvents

Amphiphilic surfactants, molecules with chemical moieties that interact differently with the solvating medium, are important for technological applications and ubiquitous in biology. Understanding how to control surfactant properties is, therefore, of wide-ranging importance. Using a combination of light scattering experiments and field theory, we demonstrate that the behavior of polymeric surfactants can be controlled sensitively by manipulating molecular architecture. We find that branched polymeric amphiphiles can be much better surfactants than traditional linear analogs. This is indicated by micelle formation in solvents that are very slightly selective for the backbone of the branched molecule. Our experimental and theoretical findings also suggest that, for a given chemistry and architectural class, surfactant properties of polymeric amphiphiles are very sensitive to subtle changes in architectural features. Specifically, we find that choosing a particular branching density optimizes the propensity...