Jean-Charles Majesté

Rheological models based on the double reptation mixing rule: The effects of a polydisperse environment

We present a comparison of various rheological models based on the double reptation concept, which relates linear viscoelastic data to molecular-weight distribution in order to determine the most efficient way to describe polydispersity effects. We have used for this study a two-step process. First, we have performed a systematic comparison of the predictions of various models with experimental data already published in the literature for binary blends of polystyrene (PS) and polymethylmethacrylate (PMMA). Then, we have compared the values of the complex shear modulus derived from these molecular models with rheological data obtained on “commercial” polydisperse polymers (PS, PMMA, and high-density polyethylene). It is shown that only the two models which explicitly take into account the effects of the polydisperse surrounding of a macromolecular chain through “tube renewal” effects are able to describe correctly the polydispersity effects on zero-shear viscosity and steady-state compliance.

A comparative analysis of dielectric, rheological and thermophysical behaviour of ethylene vinyl acetate/BaTiO3 composites

Dielectric, rheological and thermal behaviours of ethylene vinyl acetate/barium titanate sphere (EVA/BaTiO3) composite materials were investigated in this work. Several composites were prepared for different volume concentrations (from 5% to 49%) and for two types of spheres sizes d: d = 9 µm and d = 105 µm. We show that the use of larger particles allows an increase of dielectric permittivity and thermal conductivity, particularly at high filler concentrations. A comparison between the thermal, dielectric and rheological behaviours of the composites is also reported in this paper. A linear dependence of the relative variation of thermal conductivity and dielectric permittivity, on the one hand, and thermal conductivity and Newtonian viscosity, on the other, was shown.

Molecular structure and gross melt fracture triggering

We report in this paper rheological and rheo-optical experiments on a linear and a star-branched statistical copolymer of styrene butadiene rubber (SBR). The aim of this work is to study the influence of molecular structure on gross melt fracture defect. First, the rheological behavior in shear and in elongation of both materials are investigated. Then, the experimental results obtained by various rheological techniques enable us to compare the level of apparent shear rates and elongational stresses applied to each polymer at the onset of flow instabilities. The critical apparent shear rates and elongational stresses are found to be higher for the star-branched SBR copolymer. Moreover, the shear behavior of solutions of SBR copolymers in toluene is investigated using classical and optical rheometry. These results allow us to compute their stress optical coefficients. Birefringence experiments performed with a specific two-dimensional slit die corroborate axisymmetric observations. The level of the first normal stress difference along the flow axis is higher for the star-branched SBR copolymer than for the linear one before gross melt fracture occurrence. Thus, all experiments illustrate the dependence of gross melt fracture defect on molecular structure.

A kinetic model for silica-filled rubber reinforcement

The competition between filler-filler interaction and filler-rubber interaction during the dispersion process of silica-filled rubber has been investigated. Several complementary techniques were carried out going from local observations of the dispersion to a global view given from linear and nonlinear rheological measurements in order to lead to a better estimation of the dispersion kinetics. It has been shown that reinforcement evolves with mixing time. A direct link between the bound rubber amount and reinforcement indicators was found, revealing a replacement of strong filler-filler interactions by weak rubber-filler ones. As a result, rheological reinforcement can be cast under the form of a universal power law by introducing an effective interacting surface between fillers. Finally, a kinetic model of the rubber reinforcement has been developed on the basis of the competition between filler dispersion mechanism and rubber physical adsorption.

Crosslinking in the melt of EVA using tetrafunctional silane: gel time from capillary rheometry

Crosslinking of ethylene vinyl acetate (EVA) copolymers can be performed in the melt by use of a transesterification reaction using tetrapropoxysilane as a crosslinking agent and dibutyl tin oxide as a catalyst [Polymer 43 (2002) 6085]. Whether the kinetics of the reaction can be suitably controlled is an important point that must be answered before any processing method such as extrusion or injection molding is considered. This investigation was carried out in a capillary rheometer. Various stoichiometric ratio and processing temperatures were studied. Viscosity was measured during the reaction and the evolution of the flow curves with reaction time was calculated. Gel point under shear conditions was considered to be achieved at the appearance of a yield stress on the flow curves. It was shown that an excess of propoxy groups slows down the reaction kinetics but until 120 °C, the gelling time is longer than 8 minutes which is long enough for processing with conventional techniques.

Mediating gel formation from structurally controlled poly(electrolytes) through multiple "head-to-body" electrostatic interactions.

Tuning the chain-end functionality of a short-chain cationic homopolymer, owing to the nature of the initiator used in the atom transfer radical polymerization (ATRP) polymerization step, can be used to mediate the formation of a gel of this poly(electrolyte) in water. While a neutral end group gives a solution of low viscosity, a highly homogeneous gel is obtained with a phosphonate anionic moiety, as characterized by rheometry and diffusion nuclear magnetic resonance (NMR). This novel type of supramolecular control over poly(electrolytic) gel formation could find potential use in a variety of applications in the field of electro-active materials.

Dielectric and thermophysical properties of Ethylene Vinyl Acetate/BaTiO3 composites

Dielectric and thermal behaviour of Ethylene Vinyl Acetate/Barium Titanate spheres (EVA/BaTiO3) composite materials were investigated in this work. Several composites were prepared for different volume concentration (from 5% to 49%) and for two types of spheres sizes d: d = 9 and d = 105. A simultaneous determination of the thermal conductivity and the diffusivity was achieved using a periodic method. The experimental results of the relative permittivity were obtained from capacitance measurements performed on square plate samples in the frequency range from 103 Hz up to 106 Hz.

Using the anisotropy of magnetic susceptibility to infer flow-induced orientation of anisotropic particles: feasibility and sensitivity

In this paper, the use of anisotropy of magnetic susceptibility (AMS) measurements has been investigated in order to check the sensitivity of this technique versus the flow conditions. The orientation of anisotropic magnetic particles during the flow of a polystyrene

Solvent-free preparation, characterization, and properties of SEBS–g–polycarbonate copolymers

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Phase inversion and co-continuity domain in immiscible polyethylene/polystyrene blends

magnetite blend through a capillary rheometer has been studied. Thanks to the magnetic properties of the magnetite, AMS measurements are possible. Different values of the filler concentration, viscosity, and shear rate were used. It is shown that the AMS technique is able to detect accurately particle orientation and that sensitivity decreases when increasing the concentration of magnetite grains. In addition, the rectangular-shaped sample imposed by the rheological device does not affect measures of AMS significantly. The results give reasons to consider that the measure of AMS is an accurate and sensitive method to access the mean local rheological behavior in filled or non-filled systems containing anisotropic tracers.