Michel Nardin

A round-robin programme on interfacial test methods

Abstract A round-robin programme has been undertaken to assess the compatibility in the micromechanical techniques used to evaluate the interfacial shear strength of the fibre/matrix bond in composite materials. The tests selected for evaluation were the single-fibre pull-out test, the microdebond test, the fragmentation test and the micro-indentation test. Twelve laboratories were invited to participate in this programme. Each laboratory was supplied with Caurtaulds XA fibre in the untreated condition and with a standard surface treatment, and a quantity of epoxy resin, hardener and catalyst, all from the same batch. Some laboratories were supplied with composite bars made with the same materials. A common cure cycle was chosen for sample preparation. Each laboratory conducted the tests to its own procedures. The results showed that the scatter within each laboratory was acceptable but the scatter between laboratories for a particular test was high. The results are discussed and possible explanations are presented for these observations. The indications are that the fundamental procedures used in each laboratory are sound. The results also suggest that there is great potential for achieving standard procedures and reducing the inter-laboratory scatter. A further round-robin programme is proposed to generate test protocols.

Behavior of human osteoblastic cells on stoichiometric hydroxyapatite and type A carbonate apatite: Role of surface energy

To determine the role of physicochemical characteristics of the surface of dense ceramics on osteoconduction, we studied the proliferation and differentiation of human trabecular (HT) osteoblastic cells, extracellular collagenous matrix production, and biologic apatite formation on stoichiometric hydroxyapatite (HA) and type A carbonate apatite (CA). The surface physicochemical characteristics (composition, roughness) of HA and CA carefully were determined by Fourier-transformed infrared, X-ray photoelectron, and Raman spectroscopies, and by FTIR microscopy, before and after cell culture. On both HA and CA substrates, HT cells attached, proliferated, and differentiated. Cell proliferation did not differ on HA and CA. However, the initial cell attachment and spreading of HT cells were much lower on CA compared to HA. Physicochemical and biologic analyses showed that collagenous synthesis by HT cells after 6 weeks of culture also was lower on CA than on HA. Quantitative histologic analysis confirmed that the collagenous matrix production was lower on CA than on HA. Measurement of wettability showed that the polar interaction energy with water was significantly lower on CA than on HA. The lower cell attachment and collagen production on CA compared to HA clearly were related to the low affinity of HT cells for the CA surface. This study shows that the surface energy of the biomaterial greatly influences the initial cell attachment and spreading of human osteoblastic cells at the surface and affects collagenous matrix deposition on the biomaterial. This suggests that the enhancement of polar components of the surface of dense biomaterials may improve osteoblastic cell attachment and, thereby, osteoconduction.

Osteoclast adhesion and activity on synthetic hydroxyapatite, carbonated hydroxyapatite, and natural calcium carbonate: Relationship to surface energies

This study investigates the adhesion, cytoskeletal changes, and resorptive activity of disaggregated rat osteoclasts cultured on polished slices of three biomaterials: crystalline synthetic hydroxyapatite (HA), carbonated hydroxyapatite (C-HA), and natural calcium carbonate (C). The surface chemistry of each substrate was defined by X-ray diffraction and IR spectroscopy, surface wettability by the dispersive, and the polar components of the surface energies. Osteoclast adhesion was modulated by the polar component of the surface energy: fewer (p < 0.01) osteoclasts adhered to C-HA (97 +/- 20/slice, surface energy 9 +/- 5 mJ/m2) than to HA (234 +/- 16/slice, surface energy 44 +/- 2 mJ/m2) or to C (268 +/- 37/slice, surface energy 58 +/- 0.5 mJ/m2). Actin rings, which are the cytoskeletal structure essential for resorption, developed on all three materials. The area of the actin ring, which is resorbed by local acidification, and the osteoclast area, which reflects osteoclast spreading, were both greater in osteoclasts cultured on HA and C-HA than in those cultured on C. C was resorbed, but HA and C-HA were not. Thus, the surface energy plays an essential role in osteoclast adhesion, whereas osteoclast spreading may depend on the surface chemistry, especially on protein adsorption and/or on newly formed apatite layers. Resorption may be limited to the solubility of the biomaterial.

Effect of dynamic contact angle on capillary rise phenomena

Abstract Capillary rise experiments of different liquids in glass capillaries and in columns of packed powders were carried out. The analysis of this rise was performed according to the classical Washburn’s equation in which the calculation of a constant term is needed in order to be able to determine contact angle of the considered liquid on the capillary wall or powders. However, it was observed that this constant term apparently varies as a function of the liquid used, in contradiction with Washburn’s approach. A more fundamental study of alkane rise into glass capillaries was carried out showing that this apparent variation is due to the variation of contact angles, which can take large values (up to 60°) as a function of velocity of the liquid front, although their expected value is 0°. Therefore, in the case of powders, different approaches to determine the real constant term with respect to particle size are proposed. Consequently, the use of Washburn’s equation for the determination of contact angles of liquids on these powders is also discussed.

Mechanical, surface and interfacial characterisation of pitch and PAN-based carbon fibres

Abstract The mechanical and surface characteristics of pitch and PAN-based carbon fibres were studied by tensile testing, XPS, SEM analysis and wetting measurements. The pitch-based fibres had two different geometries, with circular and ellipsoidal (ribbon-shaped) cross sections. Plasma oxidation was used to treat the surface of the fibres. The interfacial characteristics of untreated and treated fibres were measured by fragmentation tests of single filament composites. The effect of the surface treatment on the mechanical, surface and interfacial properties of the fibres was determined and correlated. It was shown that a relationship exists between the ability of the surface to transfer loads and its oxygen content. Finally, the influence of the non-axisymmetry on the interfacial parameters obtained in the fragmentation tests was assessed.

Effect of vinylacetate content on crystallinity and second-order transitions in ethylene—vinylacetate copolymers

Thermal characteristics of ethylene—vinylacetate (EVA) copolymers having vinylacetate contents ranging from 5 to 40 w/w % are studied by differential scanning calorimetry. It is first shown that EVA copolymers having a vinylacetate content lower than 30 w/w % obey the Flory and Burfield theories of copolymer crystallisation. The minimum sequence length of CH2 ethylenic entities required to participate in a crystalline lamella is also deduced. One can conclude that EVA copolymers represent cases of “total exclusion” of the noncrystallizable comonomer. Moreover, it is observed that when the vinylacetate content is increased, the relative quantity of polyethylene amorphous phase increases and the degree of crystallinity decreases; whereas the β transition temperature of a characteristic-oriented amorphous phase is kept constant. A phase model of ethylene-vinylacetate copolymers, based on an enrichment process of the interlamellar amorphous phase by polyethylene segments originating from the crystalline phase, at increasing vinylacetate content, is proposed.

Effect of ultrasounds on the electrochemical synthesis of polypyrrole, application to the adhesion and growth of biological cells.

In this study, a new way to synthesize polypyrrole films is presented. This original way consists in the electropolymerization of polypyrrole under high frequency ultrasonic irradiation on conductive fluorine-doped tin oxide surfaces. The polypyrrole films obtained are then compared, in terms of chemical structure and morphology, to polypyrrole films synthesized by standard electrochemical methodology. Next, these polymer films are tested as an alternative to biomaterials that are commonly used as cell culture substrates. Thus, the adhesion and growth of osteoblastics cells and microbial cells on polymer-modified surfaces are investigated by using qualitative observation and quantitative tests. These studies proved the non-toxicity of the polymer films for osteoblastic and microbial cells but also a different behaviour of osteoblastic cells and microbial cells with polypyrrole films.

Surface characteristics of commercial carbon fibres determined by inverse gas chromatography

Abstract The surface characteristics of carbonized and stabilized carbon fibres have been studied using inverse gas chromatography. First, the dispersive component of the surface energy of both types of fibres was determined. Secondly, using polar probes of known properties, acid-base surface characteristics of the fibres were obtained. It has been shown that the carbonized fibres exhibit an electron donating (base) character, whereas stabilized fibres are more amphoteric.

Variation of the surface energy characteristics of mica (muscovite) upon grinding

Abstract Inverse gas chromatography (IGC), at zero surface coverage, has been used to follow the variation of the surface free energy characteristics of muscovite samples after grinding either in water or in organic media. For initial mica, the value of γ S D measured through IGC, is in agreement with the values given by other methods (30 mJ/m 2 After grinding, γ S D increases and reaches a value of 90 mJ/m 2 when the operation is performed in methanol. At the same time, the acid/base surface properties change as qualitatively indicated by IGC. The variation in surface characteristics, upon grinding, may have significant consequences for the utilization of mica as a filler for composite materials.

Influence of thermal history on the results of fragmentation tests on high-modulus carbon-fibre/polycarbonate model composites

The present work reports a study of the influence of cooling rate on interfacial parameters determined from single-filament composites by the fragmentation test. The dependence of the longitudinal compression stress on the cooling rate was analysed and the corresponding compression strain was assessed by different techniques. On the basis of the results obtained it is proposed that a standard cooling rate should be used to compare data obtained in different laboratories.