Stéphane Marais

Effect of chemical treatments on water sorption and mechanical properties of flax fibres.

In this work, in order to improve the adhesion between a polyester matrix (unsaturated polyester resin) and flax fibres (Linum usitatissimum L.) and to increase their moisture resistance, chemical surface treatments have been used. These different treatments were performed with maleic anhydride (MA), acetic anhydride (Ac), silane (Si) and styrene (S). The modified flax fibres were characterized by means of infrared spectroscopy and surface energy analysis. The effect of these treatments on water sorption was investigated by using a gravimetric static equilibrium method. Water sorption isotherms were derived from kinetic data. The Park model based on the three sorption modes: Langmuir, Henrys law and clustering, was successfully used to simulate the experimental sorption data. It was found that the (Ac) and particularly (S) treatments reduced overall water uptake of flax fibres. We show that tensile modulus, breaking strength and breaking strain depend on the chemical treatment used.

Thermal and mechanical properties of bio-nanocomposites reinforced by Luffa cylindrica cellulose nanocrystals.

Cellulose nanocrystals have been prepared by acid hydrolysis of Luffa cylindrica fibers. The acid-resistant residue consisted of rod-like nanoparticles with an average length an diameter around 242 and 5.2nm, respectively (aspect ratio around 46). These cellulose nanocrystals have been used as a reinforcing phase for the processing of bio-nanocomposites using polycaprolactone (PCL) as matrix. To promote interfacial filler/matrix interactions the surface of cellulose nanocrystals was chemically modified with n-octadecyl isocyanate (C(18)H(37)NCO). Evidence of the grafting was supported by infrared spectroscopy and elemental analysis. X-ray diffraction analysis was used to confirm the integrity of cellulose nanocrystals after chemical modification. Both unmodified and chemically modified nanocrystals were used to prepare nanocomposites. The thermal properties of these materials were determined from differential scanning calorimetry and their mechanical behavior was evaluated in both the linear and non-linear range.

Polyepichlorhydrin Membranes for Alkaline Fuel Cells: Sorption and Conduction Properties

Polymer electrolytes, using a poly(epichlorhydrin-allyl glycidyl ether) copolymer as matrix, are shown to perform well in alkaline fuel cell electrolyte. An anion-conducting network is obtained by the incorporation of cyclic diamines, 1,4-diazabicyclo[2.2.2]octane (DABCO) and 1-azabicyclo[2.2.2]octane (quinuclidine). The physicochemical and electrochemical characteristics are evaluated. The best conductivity of 1.3.10 (-2) S/cm is obtained at 60 degrees C and a relative humidity of RH = 98%. Ionic conductivity is particularly sensitive to relative humidity. To gain insight into the OH (-) conduction mechanism and the role of water, sorption measurements versus water activity, differential scanning calorimetry, and NMR measurements are carried out.

Diffusion of water through various polymer films: a new high performance method of characterization

Abstract Diffusion of water in polymer films has been characterized by using a new high performance and reproducible method based on permeation measurements. With this method, which uses a fast response water-specific sensor based on dew point measurement, it is possible to determine, from the transient step, the time lag water diffusion coefficient D L and, when the steady state is reached, a mean water permeability coefficient P . In this work we have emphasized the water concentration dependence of diffusion coefficient D . After calibration, the technique yields accurate values of P , consistent with literature data. By testing various polymers, different behaviours with respect to water have been observed, particularly with low density polyethylene (LDPE) which shows significant hydrophobic properties.

Permeation of water through polar and nonpolar polymers and copolymers: Determination of the concentration-dependent diffusion coefficient

The diffusion and permeation properties of liquid water through different polar and nonpolar polymers and copolymers were studied with a highly sensitive permeameter. The transient permeation fluxes through the polar polymer films could be fitted well only with an exponential equation for the diffusivity concentration dependence; this empirical exponential equation represented the diffusion plasticization effect of water on the materials. For the hydrophobic polyolefins, this exponential equation was no longer valid, and another form of the equation was empirically found to account for the reduction of the water diffusivity with the extent of the permeation. Such a negative plasticization effect might be attributed to the formation of water clusters in the polyolefins. The values of the diffusion coefficient of water in the dry polar polymers were smaller than those in dry polyolefins, but the opposite behavior was found for the permeability because it was much more favorable for water sorption in the polar polymers than in the hydrophobic polyolefins. For the ethylene–vinylacetate copolymers, the plasticization effect of water on its own diffusion was negative for the sample with a low vinyl acetate (VA) content; it became nil at 19 wt % VA and positive at higher VA contents. This increase in the extent of the water sorption with the increase in the VA content led to a steady increase in the water permeability in the poly(ethylene-co-vinylacetate) copolymers.

Study of transport of small molecules through ethylene-co-vinyl acetate copolymers films. Part A: Water molecules

Abstract The transport of liquid water through poly(ethylene-co-vinyl acetate) (EVA) films with 4.5%, 19%, 33%, 50% and 70% w/w VA contents, is studied by permeation measurements. A low density polyethylene LDPE, used as reference, is also studied. The diffusion of water is determined from transient permeation. For a better understanding of the role played by the structure and the surface of these copolymers whose crystallinity varies with the presence of VA groups, Differential Scanning Calorimetry and contact angle (surface free energy) measurements are performed. It is found for LDPE and 4.5% w/w VA membranes that the water diffusion coefficient decreases with increasing water concentration, while for 19% VA a constant diffusion coefficient is obtained. For membranes with 33, 50 and 70% w/w VA a diffusion coefficient which increases exponentially with the local water concentration is obtained. These results are explained by the formation at low VA content of small water clusters, clusters evidenced by means of IR spectrometry. On the other hand, the plasticization effect (concentration-dependent diffusion coefficient) is observed for VA content greater than 19% wt. VA is explained by the increase of the amorphous phase ratio, which allows an increase of the number of interactions between polar groups (carboxyl units in the statistical copolymer) and water molecules

Study of transport of small molecules through ethylene-co-vinyl acetate copolymers films. Part B: CO2 and O2 gases

Abstract The transport of gases (pure oxygen and carbon dioxide) through poly(ethylene-co-vinyl acetate) (EVA) of different VA contents, was studied by permeation measurements. Permeabilities, diffusivities, and solubilities were determined for EVA films containing 19, 33, 50 and 70% (w/w) VA. A low density polyethylene LDPE (used as reference with 0% VA) was also studied. The diffusion of oxygen was determined from transient permeation fluxes and the diffusion of carbon dioxide from time-lag permeation. In the case of gas permeation, whatever the VA content of the polymer used, the experimental transient fluxes are characterized by a constant diffusion coefficient for both O2 and CO2. In terms of gas permeability, compared with water permeability (Part A), these copolymers are characterized by very low coefficients, especially for oxygen. Assuming that D is constant, the O2 and CO2 solubility coefficient was then deduced and its value is shown to increase proportionally with the VA content in these rubbery copolymers (whose quantity of crystalline phases decreases and become nil for 70% VA). The H2O/CO2 and H2O/O2 ideal selectivity α increases with the VA content, in agreement with the increase in average polarity of the copolymer.

Diffusion and permeation of water through unsaturated polyester resins—influence of resin curing

Abstract The diffusion and permeation properties of liquid water through an isophthalic-maleic acid-propanediol-based copolyester resin were studied with a highly sensitive permeameter. The value of the parameters of the exponential dependence of the diffusion coefficient on the water concentration decreases with the curing extent, except that of the plasticization coefficient, which decreases after a post-curing at 120°C. Infrared-spectrophotometric and calorimetric data show that the curing involves mainly a polymerization of styrene during the curing at 25°C, and a crosslinking by the opening of the double bonds on the polyester chains in the subsequent curing step at 80°C. The water sorption by the resin results in a 20°C-decrease in the glass transition temperature of the water saturated resin, leading to the diffusivity enhancement by the plasticization effect.

Surface modification by low-pressure glow discharge plasma of an unsaturated polyester resin: effect on water diffusivity and permeability

Abstract Unsaturated polyester resin (UPR) films have been modified by a tetrafluoromethane microwave plasma. The treated surface morphologies investigated by contact angle, atomic force microscopy and permeation measurements show that the CF 4 plasma treatment decreases drastically the surface energy by increasing the hydrophobic character. The surface modification due to plasma fluorination decreases the water content diffusing through the UPR film and the time lag diffusion coefficient and thus, the sorption kinetic. From these results, it is clearly shown that the CF 4 plasma treated layer improves drastically the barrier effects.

Water diffusion and permeability in unsaturated polyester resin films characterized by measurements performed with a water-specific permeameter: Analysis of the transient permeation

A water-specific permeameter was developed to study water diffusion in an unsaturated polyester; these polyesters are often used as high-barrier materials. Low water enrichment of a dry sweeping gas is measured via the dew point temperature, with two hygrometers, one of which is used because of its fast response and the other because of its accuracy. This high-performance device is just as suited to pervaporation as to permeation tests and allows the transient and stationary fluxes to be characterized. At first sight, the experimental data seem in good agreement with D = D0eγC. However, a more thorough study has shown a time-dependence of D.