When telechelic polymers (poly-ethyleneoxide grafted at both ends with an aliphatic chain) are added to a microemulsion of decane droplets in water stabilized by a surfactant film they can link the droplets and thus introduce an effective attractive interaction between the droplets, eventually leading to a phase separation. Starting from a neutral microemulsion we show that this attractive interaction can be offset by adding minute quantities of an ionic surfactant, thus introducing a repulsive Coulombic interaction. This opens up the possibility of 'weighting' an unknown effective attraction against a well known repulsion. The phase behaviour and the small angle neutron scattering spectra of different samples illustrate this point. The spectra are simulated assuming the interaction potentials to be the sum of attractive and repulsive contributions and a good agreement is found between the experimental and simulated spectra in the neutral microemulsion, in the charged microemulsion and in the charged and connected microemulsion.

The crystal structure of N-methylacetamide (C3H7NO), M-r = 73.095, has been determined from single-crystal neutron diffraction data at two temperatures, 250 and 276 K, above and below the previously reported phase transition located at 274 K in this work. Crystal data: 250 K [276 K]: space group Pn2la [Pn2(1)m], a = 9.671(2) [4.878(1)] Angstrom, b = 6.613(6) [6.567(1)] Angstrom, c = 7.218(1) [7.332(2)] Angstrom; V = 465.7(6) [234.9(5)] Angstrom (3); D-n = 1.043 [1.034] g.cm(-3), R(F-2) = 0.168 [0.134], wR(F-2) = 0.062 [0.051], S = 1.18 [1.11]. This new investigation of the structure of N-methylacetamide was undertaken in order to assess a recent suggestion based on inelastic neutron scattering spectroscopy that transfer of the amide proton along the peptide hydrogen bond may be responsible for the vibrational anomalies. While we found no evidence for proton transfer along the N-H...O hydrogen bond (d(NH) = 1.025(17) Angstrom, and d(H...O) 1.856(14) Angstrom for at T = 250 K) at either temperature evidence for some molecular disorder is present in accord with our previous C-13 NMR studies. In addition, we find short intramolecular contacts between the amide hydrogen atom and those on both neighboring methyls, which may well affect the vibrational properties of the respective molecular groups.

To study the viscoelastic properties of certain complex fluids which are described in terms of a multiconnected transient network, we have developed a convenient model system composed of microemulsion droplets linked by telechelic polymers. The phase behavior of such systems has two characteristic features:  a large monophasic region which consists of two subregions (a fluid sol phase and a viscoelastic gel phase) separated by a percolation line and a two-phase region at low volume fraction with separation into a dilute sol phase and a concentrated gel phase. From the plausible origin of these features we expect them to be very similar in different systems. We describe here the phase behavior of four different systems we prepared in order to vary the time scale of the dynamical response of the transient network:  they consist of the combination of two oil (decane) in water microemulsions differing by the stabilizing surfactant monolayer (cetylpyridinium chloride/octanol or TX100/TX35) and of two telecheli...

Step strain experiments and dynamic light scattering measurements are perfomed to characterize the dynamic behavior of an o/w droplet microemulsion into which is incorporated a telechelic polymer. At sufficient droplet and polymer concentrations, above the percolation threshold, the system is viscoelastic and its dynamic structure factor shows up two steps for the relaxation of concentration fluctuations: the fast one is dominated by the diffusion but the slower one is almost independent of the wave vector. The terminal time of the stress relaxation tR and the slow time of the dynamic structure factor tS are both presumably controlled by the residence time of a sticker in a droplet: consistently, tR and tS are of the same order, they both vanishes at the percolation threshold according to power laws but with different exponents. We discuss these features in terms of deviations at the transition, from the usual mean field description of the dynamics of transient networks.

In this paper we examine the effective interactions introduced between the droplets of an oil in water microemulsion upon progressive addition of hydrophobically modified water soluble poly(ethylene oxide) (PEO) using essentially small angle neutron scattering. To discuss the relative importance of decoration and bridging of the droplets, we compare analogous samples with addition of a PEO grafted at both extremities with hydrophobic C12H25 chains (PEO-2m) or addition of a PEO grafted at one extremity only with a C12H25 chain (PEO-m). PEO-m or PEO-2m adsorb onto the droplets via their hydrophobic extremities, and the droplets are found to retain their form and size upon addition of up to 40 hydrophobic C12H25 chains per droplet. When the volume fraction of droplets is less than about 10%, the effective interactions introduced by PEO-m or PEO-2m are found to be very different:  PEO-m introduces a repulsive interaction, while PEO-2m introduces an effective attractive interaction. This attractive interaction...

We report on a novel telechelic associating polymers that self-assemble in solution into starlike flowers in the dilute regime and develop a fully connected network of flowers above some threshold concentration f. Small-angle neutron scattering has been used to investigate the form and structure factors of the starlike aggregates, whereas linear rheology was performed to identify the viscoelastic features of the temporary network. These findings are compared to those obtained on an associating polymer which is half of the telechelic in molecular weight and monofunctionalized (no bridging). Although the neutron scattering signatures are the same, the latter solutions remain Newtonian and fluid even for f ? f. [S0031-9007(98)08005-3]

We present an inelastic neutron scattering study of the low frequency vibrational dynamics of CsC60. In its polymeric phase, two main excitations are observed at 0.65 and 1.15 THz. The Q-dependence of their structure factor allows to confirm their assignment to twisting modes. For quenching-prepared samples, the modes shift at lower frequencies in agreement with the formation of dimers. We compare our results to previous Raman data and time of flight measurements of Renker et al. on RbC60 and derive a general description of the vibrational dynamics of alkali-doped C60 phases.