H. Hervet

Colloid & Polymer Science

AbstractCertain water soluble polymers may have a repulsive two-body interaction, but an attractiven-body interaction induced by certain “clustering” effects. In the bulk this may lead to a “n

Self-diffusion and undulation modes in a Smectic A liquid crystal

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The stick-slip transition in highly entangled poly( styrene-butadiene) melts

n point” in the phase diagram. Here, with polymer brushes, we construct the theoretical density profiles, using a local mean-field approximation. The brush often shows two layers (one dense near the wall, and one dilute), but the concentrations in both layers depend on the distance to the wall. The location of the interlayer boundary can be derived from a Maxwell construction.

Smectic H-smectic VI pretransitional effect by orientational ordering in TBBA

The incoherent scattering law for neutron quasi-elastic scattering in liquid crystals is calculated using a classical formalism including (anisotropic) self-diffusion, rotational motion along the long molecular axis and short time vibrational behaviour. Explicit expressions are given for oriented as well as for unoriented liquid crystals.

Wetting of stratified solids

High-resolution neutron quasi-elastic scattering measurements on an aligned smectic A sample of terephthal-bis-butyl-aniline are presented and analysed in terms of self-diffusion and undulation mode motions. (1) We find good agreement with the longitudinal and transverse diffusion coefficients D measured by Kruger et al. [5] using spin-echo techniques. (2) We conclude that self-diffusion alone is sufficient to explain both our line widths and line shapes. The undulation mode contribution to the broadening, if present, is too small to be seen in our range of wave vectors.

Statistics of « starburst » polymers

Abstract It is shown that high resolution incoherent neutron quasi-elastic scattering (NQES) may be used, via the measurement of the elastic incoherent structure factor (EISF), as a new tool to study molecular ordering in liquid crystals. The EISF is a bilinear combination of all the order parameters (P1 ) and its experimental value can be deduced from an intensity measurement. As an example, we measure from the NQES spectra, the EISF of powder smectic A TBBA at 184 [ddot]C. These data are analysed in terms of two microscopic models (molecules rotating around their long axis and (i) fluctuating about the director: model 1, (ii) precessing on a cone about the director: model 2). It is shown that from the powder results only, we cannot discriminate between these two models, but that model 2 can explain other published experimental results (in particular some Raman scattering of light data) whereas model 1 cannot. Some conjectures about model 2 are made.

Colloid & Polymer Science: Collapse of polymer brushes induced byn-clusters

Abstract We study the interfacial slippage of highly viscous polymer melts in a plane Couette cell by combining stress measurements and optical determination of sliding velocities (Fluorescence Recovery After Photobleaching). This enables us to observe the apparition of stick–slip instabilities in a narrow shear rate range whose limits are governed by the molecular characteristics of both the bulk and surface anchored polymer chains. A qualitative interpretation of these results will be given in terms of extraction and re-entanglement of the surface anchored chains from the bulk ones as well as a necessary criterion for the apparition of these instabilities. Experiments exploring the kinetics of the stick–slip regime appear to be a simple and promising way to investigate interfacial dynamics and to estimate the characteristic times involved.

Uniaxial rotational diffusion in the smectic B phase of TBBA observed by quasi-elastic neutron scattering

Abstract New neutron quasi-elastic scattering data on TBBA in its supercooled smectic H phase are presented and analyzed in terms of the elastic incoherent structure factor. Contrary to what was found in the normal H phase, the results support the idea of partial orientational ordering of the molecules. It is suggested that this ordering (of dynamical nature) is a precursory effect to the smectic H-smectic VI phase transition.