Sophie Bistac

Adhesion and friction of PDMS networks: molecular weight effects.

The objective of this work is to find relations between adherence and friction behaviors of elastomer networks. The chosen approach is based on the parallel study of the initial molecular weight (i.e., the degree of cross-linking) dependence of both adherence and friction. The polymers used are cross-linked polydimethylsiloxane (PDMS) and the substrate is a smooth glass plate. The experimental procedure uses both friction (pin on disk tribometer) and adhesion (tack test) measurements, associated with surface analysis and mechanical and rheological characterizations. Tack results show that high molecular weight PDMS exhibits the greater adherence energy. This can be explained by the role of both chain length and free and pendant chains: more numerous and longer free chains favor the substrate wetting (at a molecular scale) and increases the energy dissipation during separation (extraction and reptation mechanisms). However, friction results indicate a higher friction resistance for low molecular weight PDMS. This result could be quite surprising. An explanation based on interfacial sliding properties of free and pendant chains can be proposed. Elsewhere, for the lower molecular weight polymer, elastic contact present during friction is able to act as a forced wetting, constraining the network and consequently leading to a greater energy dissipation.

Crystalline modifications of ethylene-vinyl acetate copolymers induced by a tensile drawing: effect of the molecular weight

The influence of a tensile drawing on the crystalline organisation of ethylene-vinyl acetate copolymers (EVA) of different molecular weights has been investigated by differential scanning calorimetry (d.s.c.). The consequence of the crystalline organization changes on the mobility of the amorphous phase has been also investigated, through the study of the glass transition. The results show that more disorganized crystals appear after the tensile test, specially for the higher molecular weight. These more disorganized crystals constitute physical ties reducing the mobility of the amorphous chains which link them, as shown by the significant increase of the glass transition temperature of these high molecular weight EVA after tensile drawing.

Durability of steel/polymer adhesion in an aqueous environment

Abstract In this work, the durability of steel/polymer/steel assemblies in various liquids (water, salt, acid and basic solutions) is studied. Two steel substrates are compared: one without any surface treatment and the other one with a phosphatizing surface treatment. The polymer is an ethylene–vinyl acetate copolymer (EVA) grafted with maleic anhydride. The wedge test is used in order to quantify the steel/polymer adherence before and during ageing. The results show that a rapid delamination is observed for the non-treated steel/EVA assemblies in all the studied liquid. A cathodic delamination process is proposed to explain the delamination mechanism. A good ageing resistance is observed for the phosphatized steel/EVA assemblies, not only due to the corrosion resistance of the steel substrate but also due to the insensibility of the interfacial steel/polymer bonds of the studied liquids.

Relationship between polymer viscoelastic properties and adhesive behaviour

Abstract The adherence level of steel/polymer/steel assemblies has been evaluated by using a wedge test, performed at different wedge introduction rates. Polymer viscoelastic properties have also been investigated through tensile and relaxation tests performed at various solicitation rates. The aim of this study was to analyse the influence of the solicitation rate on both adherence level and tensile and relaxation behaviour in order to understand the effect of polymer viscoelastic properties on adherence energy, especially its rate sensitivity. The final objective was to establish a quantitative correlation (taking into account the contribution of adhesive viscoelastic properties) in order to obtain a constant adherence value, independent of the introduction rate.

Solvent retention in solution-cast films of PMMA: study by dielectric spectroscopy

Abstract Conventional PMMA was dissolved in two different solvents (chloroform and toluene). Solid polymeric films were formed after casting of the solutions on a substrate (aluminium) and evaporation of the solvent at room temperature during several days. The dielectric properties of the solid films were studied by dielectric spectroscopy. The evolution of the loss tangent (tan δ ) versus temperature was analysed. The results show that the tan δ peak, corresponding to the α -transition (glass transition), is shifted towards the lower temperatures, as compared with the bulk polymer obtained by heating press, without any solvent. The magnitude of the shift depends on the type of solvent. These results indicate that, in all cases, solvent molecules are retained and cannot be completely evaporated at room temperature, even after a long period. Solvent molecules are still trapped inside the PMMA films, but their effect on the polymer chains mobility differs in function of the nature of the solvent. Explanations based on acid–base interactions are proposed to try to understand this phenomenon. This work illustrates the interest of dielectric spectroscopy for studying in situ the properties of thin cast polymer films.

The role of free chains in adhesion and friction of poly(dimethylsiloxane) (PDMS) networks

The aim of this study is to analyse the influence of some molecular parameters (crosslinking density, presence of free chains) on both adhesion and friction properties of elastomer networks. The polymers used are polydimethylsiloxane (PDMS) networks with different initial molecular weights. Networks were studied before and after extraction of free chains in toluene. The substrate is a smooth glass plate. The experimental procedure uses both friction (pin-on-disk tribometer) and adhesion (tack test) measurements, associated with surface analysis (static and dynamic wetting). Adherence energy increases with PDMS molecular weight. Friction measurements results, however, exhibit a higher friction stress for low molecular weight PDMS. The influence of chain length is discussed and some molecular mechanisms are proposed to explain both friction and adhesion behaviors. The major role of pendant chains is underlined.

A new theoretical approach for the determination of molecular orientation persistence length of adsorbed nanofilms by FTIR reflectance spectroscopy

FTIR-Reflectance experiments have proved to be a powerful tool for the determination of molecular orientation in thin films adsorbed onto highly reflecting metals. We propose an original method for determination of the persistence length of molecular orientation in the film. This approach is based on the fact that molecular orientation persists only over a given distance from the geometrical interface. This distance is called the persistence length of molecular orientation. We then suppose that the nanofilm adsorbed is stratified and consists of an oriented layer (in the near-interface region) plus an isotropic one. Correlation between infrared reflection absorption band intensities and simulated band intensities allows experimentators to determine accurate molecular orientation and persistence length of orientation of a considered functional group. This is accomplished by using various IR reflection angles and p-polarization state of the incident IR wave. Film thickness and complex refractive index spectra, n(ν) - i.k(v), are only needed to deduce calculated specular reflectance intensities.

Investigation of Chemical Interactions at the Steel/Polymer Interface by FT-IR Diffuse Reflectance Spectroscopy

Interactions between a steel surface and an ethylene–vinyl acetate copolymer grafted with maleic anhydride have been investigated by Fourier transform infrared (FT-IR) diffuse reflectance spectroscopy. The failed surfaces obtained after a mechanical separation of the polymer/steel assemblies have been analyzed. The results show that a new peak appears, characteristic of a carboxylate group. A model system composed of maleic anhydride deposited on iron plates has been studied in order to consider more specifically the reactivity of maleic anhydride. Peaks characteristic of metallic carboxylate complexes are present in the spectrum. A two-step mechanism has been proposed: the opening of the anhydride cycle by a hydrolysis reaction, leading to the formation of a carboxylic diacid, followed by the reaction of the acid with some oxidized metallic elements present at the metal surface. This study underlines the contribution of FT-IR reflectance techniques to the understanding of the adhesion mechanisms.

Mechanisms of Interfacial Degradation of Epoxy Adhesive/Galvanized Steel Assemblies: Relevance to Durability

This study reports on the durability of galvanized steel joints bonded with toughened epoxy adhesive formulations under the influence of temperature and moisture aging. The influence of hydrothermal aging on the substrate and adhesive properties was first determined and then correlated to the durability of bonded joints. The mechanical properties of the adhesively bonded joints were assessed by a shear stress-strain analysis on single lap joints. Experimental results have shown a loss of adherence during aging. Scanning electron microscopy, fourier transform infra-red spectroscopy, and differential scanning calorimetry were conducted to analyze precisely the locus of failure and the failure faces after separation of the interface. Consequently, physical and chemical modifications of the adhesive and surface corrosion of the substrate were observed. A three-step mechanism has been proposed to explain the loss of adhesion during aging. The first step was attributed to adhesive and interface hydrolysis of the lap-shear samples Highly stressed zones, the second step to hydrolysis of the centre of the bonded joint and initiation of corrosion. The last one is due to an important growth of corrosion products which favours rupture in a weak boundary layer at the metal/polymer interphase. The aging model proposed bears out that the higher the corrosion resistance of the metal surface the lower the loss of adhesion under a wet environment.

Crack propagation in adhesively bonded steel assemblies

The adhesive strength of steel/polymer/steel assemblies has been measured by using a wedge test. An original device developed in the laboratory allows the introduction of the wedge into the assembly at a controlled speed and for the crack propagation to be followed with a camera-equipped microscope. The aim of this study was to analyse the influence of the wedge introduction speed V, both on the adherence energy G and of the crack propagation rate. The results show that firstly, when the introduction speed increases the adherence energy globally increases and a transition is observed on the G versus V curve. Secondly, when the wedge introduction rate is increased, the global crack propagation rate is reduced. Furthermore, a two step evolution is observed for the crack propagation. During the first step, the crack propagates rapidly. The crack velocity is then slower during the second step, and is inversely proportional to the wedge introduction speed.