Use of plasma polymerization to improve adhesion strength in carbon fiber composites cured by electron beam.

Maleic anhydride plasma polymer was deposited at the surface of carbon fibers and functionalized with vinyl and thiol groups to improve its adhesion strength with an acrylate matrix cured by an electron beam. A characterization of the fiber surface properties was done before and after coating (topography, surface chemistry, and surface energy). Sharp improvements of the interfacial shear strength (+ 120%), measured by a micromechanical test derived from the pull-out test, were obtained and, to the best of our knowledge, never reported before. The values were close to the ones obtained with a thermal cure. The comparison of this approach with other types of surface treatments (oxidation, grafting of coupling agents) enabled the establishment of a general strategy for the improvement of the interfacial adhesion in carbon fiber composites cured by an electron beam and potentially the improvement of their mechanical properties. This strategy is based on a high surface density of functionalities that are generating covalent bonding during the polymerization of the matrix and on the insertion of a polymer layer strongly attached to the fiber surface and acting as a buffer between the fiber surface and the matrix to counteract the generation of stress in the interphase.

In situ determination of the thermodynamic surface properties of chemically modified surfaces on a local scale: an attempt with the atomic force microscope.

We have monitored deflection-distance curves with an atomic force microscope (AFM) in contact mode, with a silicon nitride tip, on chemically modified silicon wafers, in the air. The wafers were modified on their surface by grafting self-assembled monolayers (SAMs) of different functional groups such as methyl, ester, amine, or methyl fluoride. A chemically modified surface with a functionalized hydroxyl group was also considered. Qualitative analysis allowed us to compare adhesive forces versus chemical features and surface energy. The systematic calibration procedure of the AFM measurements was performed to produce quantitative data. Our results show that the experimentally determined adhesive force or thermodynamic work of adhesion increases linearly with the total surface energy determined with contact angles measured with different liquids. The influence of capillary condensation of atmospheric water vapor at the tip-sample interface on the measured forces is discussed. Quantitative assessment values were used to determine in situ the SAM-tip thermodynamic work of adhesion on a local scale, which have been found to be in good agreement with quoted values. Finally, the determination of the surface energy of the silicon wafer deduced from the thermodynamic work of adhesion is also proposed and compared with the theoretical value.

New approach to characterise physicochemical properties of solid substrates by inverse gas chromatography at infinite dilution. I. Some new methods to determine the surface areas of some molecules adsorbed on solid surfaces.

This study was divided into three different parts that tried to give a new contribution to determine and quantify more precisely the superficial properties of solid substrates (polymers and/or oxides) obtained by inverse gas chromatography (IGC) at infinite dilution. It criticised certain of the classical methods and relationships used to determine some physico-chemical properties of a solid and gave new methods and equations that can be more precise and more realistic. Part I developed some new methods to determine the surface areas of some molecules adsorbed on solids. A theoretical calculation of surface areas of molecules adsorbed on solid substrates was proposed by using some theoretical models. Two new methods to determine the surface areas of polar molecules were developed. The first one consists of the determination of surface areas by coupling two classical methods, the dynamic contact angle and IGC techniques. The second new and general method, using the IGC technique, assumes that the specific interactions between a polar molecule and a solid proposed by Papirer and Schultz are the same. A new equation was obtained to determine the surface areas of polar molecules.

New approach to characterise physicochemical properties of solid substrates by inverse gas chromatography at infinite dilution. II. Study of the transition temperatures of poly(methyl methacrylate) at various tacticities and of poly(methyl methacrylate) adsorbed on alumina and silica.

The determination of the temperature transitions in polymers and more particularly when polymers are adsorbed on oxides is very important in many industrial processes. In this second part, we used inverse gas chromatography (IGC) at infinite dilution to determine the second order transition temperatures of poly(methyl methacrylate) (PMMA) adsorbed (or not) on alumina or on silica. Three types of PMMA were used: atactic (a), syndiotactic (syndio) and isotactic (iso). The IGC technique allowed to obtain the net retention volume Vn and the dispersive component of the surface energy gamma(s)d for various theoretical models of molecules, using the results of Part I. By plotting RT ln Vn as a function of (1/T) or gamma(s)d as a function of the temperature T, we proved the presence of three particular temperatures that correspond respectively to the transition temperature relative to beta-relaxation (Tbeta), the glass transition temperature (Tg), and the liquid-liquid transition temperature or order-disorder transition (T1.1). Results obtained in this part allowed us to show the effect of the tacticity of PMMA and the recovery fraction of polymer adsorbed on alumina or silica on the transition temperatures.

New approach to characterise physicochemical properties of solid substrates by inverse gas chromatography at infinite dilution. III. Determination of the acid-base properties of some solid substrates (polymers, oxides and carbon fibres): a new model.

In this paper, the inverse gas chromatography (IGC) technique was used to calculate the acid-base superficial characteristics of some solid substrates such as oxides, polymer and polymer adsorbed on oxide. The acid-base constants were calculated for many solids: Monogal, MgO, ZnO, SiO2 and Al2O3, four different carbon fibres and polymers such as poly(methyl methacrylate) (PMMA) at various tacticities adsorbed on alumina or silica. The determination, by IGC, of the specific free enthalpy deltaG(a)sp of adsorption corresponding to the specific interactions of polar molecules with the solid, as a function of the temperature, allowed to obtain the specific enthalpy deltaH(a)sp and specific entropy deltaS(a)sp. Knowing deltaH(a)sp of the various polar molecules, we were able to determine the acidic constant K(A) and basic constant K(D), the two constants characterizing solid substrates like PMMA, PMMA-SiO2 or PMMA-Al2O3, using the following classical expression: - deltaH(a)sp = K(A)DN + K(D)AN where DN and AN are respectively the electron donor and acceptor numbers of the solid substrates. This study showed an important effect of the tacticity on the acid-base properties. On the other hand, we proved that the previous relation was not correct in many cases and especially for some oxides (as MgO, ZnO and Monogal) and carbon fibres. We proposed a new relationship by adding a third parameter K reflecting the amphoteric character of the solid according to: - deltaH(a)sp = K(A)DN + K(D)AN - KDN x AN.