A. Menelle

Thermoreversible gelation of syndiotactic polystyrene in benzene

Abstract The temperature-concentration phase diagram of syndiotactic polystyrene (sPS)/benzene gels establishes the existence of two compounds, C1 and C2, the stoichiometries of which, expressed as benzene molecules/monomer, are 4 1 (C1) and 1 1 (C2). C1 is an incongruently-melting compound as it transforms into C2 at about the boiling point of benzene. In both compounds the chains take on a 21 helical form. Neutron diffraction investigations confirm the existence of these compounds but also disclose that compound C1 possesses a novel crystalline form. Neutron scattering experiments reveal that sPS chains can be described by worm-like statistics in the molten state while still retaining a conformation close to the 21 helix. It is suggested that helix stabilization occurs by means of benzene molecules intercalated between the phenyl groups as previously proposed for isotactic polystyrene. A molecular model is put forward. The neutron scattering curves obtained in the gel state, although smeared by strong interchain scattering, are consistent with the absence of chain folding. The link between the fibrillar morphology of the gel and helix stabilization is discussed.

Upgrade of a curved multichannel neutron guide using supermirror coatings at the Laboratoire Léon Brillouin

Abstract Recent developments in supermir-ror coatings allow the manufacturing of neutron mirrors with high reflectivity (>90 percent) on large surfaces and an effective critical angle of twice the critical angle of natural nickel. In August 1994, the neutron guide G3bis at the ORPHEE reactor of the Laboratoire Leon Brillouin (LLB) in Saclay (France) was upgraded using supermirrors.

Liquid interfaces investigated by neutron reflectivity

Abstract Reflectivity in Saclay is a long story starting in 1984 with the first measurements [1] performed by B. Farnoux on a small angle scattering spectrometer. A few years later, a dedicated time offlight spectrometer called DESIR gave new results [2]. At that early time, focus was already made on liquid interfaces. In 1992, EROS, a similar multipurpose reflectometer, started operation [3]. In order to study magnetic layers, another polarized spectrometer using a constant wavelength was used. Called PRISM, it has been upgraded recently and presented in this magazine [4]. The DESIR prototype of the early days is now decommissioned and all the experiments on liquid interfaces are performed on the EROS instrument.