Using deuterium NMR spectroscopy in a Couette cell, we observe shear-induced nematic ordering in the concentrated wormlike-micelle system CTAB/D(2)O, and our results are qualitatively consistent with birefringence studies, and in exact quantitative agreement with the degree of order measured in neutron-diffraction measurements. The width of the nematic region depends on shear rate, as well as on the temperature proximity to the equilibrium isotropic-nematic transition. Comparison of the nematic order profiles with velocity profiles obtained under identical conditions shows quite clearly that the nematic state is not identifiable with a highly sheared, low viscosity layer, and we conclude that the process of shearing induces a nematic state of high viscosity, possibly associated with mesoscale ordering. We present a simple model in which transition from the high shear branch to the viscous nematic branch is counterbalanced by subsequent relaxation of nematic order.

The discrepancy between the width of birefringence bands and shear rate bands in the system CTAB/water is addressed using a combination of NMR spectroscopy and NMR velocimetry. In correspondence with birefringence measurements in the annular gap of a cylindrical Couette cell, 2 H NMR spectroscopy indicates a separation of fluid into distinct nematic and isotropic phases. By contrast the velocity profile exhibits an extremely narrow shear band, consistent with the predictions of the lever rule. Contrary to previous assumptions, the experimental evidence points to a nematic phase with high viscosity.

The dynamics of polymer chains confined in artificial tubes formed by the pores of a nanoporous material should display all the features predicted by the reptation model, provided that the polymer/wall interaction does not lead to adsorption effects. We have studied the segment diffusion behavior of linear polyethyleneoxide in the molten state in pores of well characterized cross-linked polyhydroxyethylmethacrylate matrices. These ''semi-interpenetrating networks'' were prepared in such a way that (10Ϯ2) nm thick cylindrical pores completely filled with polyethyleneoxide were produced. The measuring technique was fringe-field NMR diffusometry. The results are compatible with a power law for the mean squared displacement, ͗r 2 ͘ ϰt Ϫ0.4Ϯ0.1 M w Ϫ0.8Ϯ0.2 , where t is the diffusion time and M w is the weight average molecular mass. This is to be compared with the limiting law ͗r 2 ͘ϰt Ϫ1/2 M w Ϫ1/2 predicted by de Gennes, Doi, and Edwards for region III, that is, R ӶtӶ d of the tube/reptation model. In the frame of the experimental accuracy the conclusion is that the reptative diffusion mechanism applies to linear polymer chains of sufficient length relative to the diameter of the confining pores.

The localization and dynamics of fragrance compounds in surfactant micelles are studied systematically in dependence on the hydrophobicity and chemical structure of the molecules. A broad range of fragrance molecules varying in octanol/water partition coefficients P ow is employed as probe molecules in an aqueous micellar solution, containing anionic and nonionic surfactants. Diffusion coefficients of surfactants and fragrances obtained by Pulsed Field Gradient (PFG)-NMR yield the micelle/water distribution equilibrium. Three distinct regions along the log(P ow ) axis are identified: hydrophilic fragrances (log(P ow ) \ 2) distribute almost equally between micellar and aqueous phases whereas hydrophobic fragrances (log(P ow ) [ 3.5) are fully solubilized in the micelles. A steep increase of the incorporated fraction occurs in the intermediate log(P ow ) region. Here, distinct micelle swelling is found, while the incorporation of very hydrophobic fragrances does not lead to swelling. The chemical structure of the probe molecules, in addition to hydrophobicity, influences fragrance partitioning and micelle swelling. Structural criteria causing a decrease of the aggregate curvature (flattening) are identified. 2 H-NMR spin relaxation experiments of selectively deuterated fragrances are performed monitoring local mobility of fragrance and leading to conclusions about their incorporation into either micellar interface or micelle core. The tendencies of different fragrance molecules (i) to cause interfacial incorporation, (ii) to lead to a flattening of the micellar curvature and (iii) to incorporate into micelles are shown to be correlated.

No abstract