Marie-France Vallat

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.

Chemical force titration of plasma polymer-modified PDMS substrates by using plasma polymer-modified AFM tips.

Plasma polymerization has gained increasing attention in surface functionalization. We use here chemical force titration to characterize PDMS (polydimethylsiloxane) substrates modified by maleic anhydride-pulsed plasma polymerization. The coating is hydrolyzed to promote the formation of dicarboxylic acid groups. To enhance the variation of the adhesion forces as a function of pH, we use AFM tips modified in the same way as the substrates. The pH-dependent adhesion measurements are performed at different KCl concentrations. The dicarboxylic nature of the maleic acid groups clearly emerges from the force titration curves. The surface pK(a) values (pK(a1) = 3.5 +/- 0.5 and pK(a2) = 9.5 +/- 0.5) of the dicarboxylic acids are evaluated from low electrolyte concentration solutions. The values are shifted toward higher pK(a) values when compared to maleic acid in solution. The first pK(a) appears in the titration force curve for low salt concentration as a peak. This peak changes to a sigmoidal shape at higher salt concentrations. The appearance of a peak is attributed to the formation of strong hydrogen bonds between the tip and the substrate as reported in the literature. The effect of the ionic strength on the force curves is explained by the condensation of counterions on the carboxylate groups. At high pH, the adhesion force almost vanishes. On the approach, at high pH, one first observes repulsion between the tip and the substrate, which varies exponentially with the tip/substrate distance. The decay length of this repulsion force is in good agreement with theoretical predictions of the Debye length, attesting to the electrostatic nature of the interactions. We also find that the replacement of monovalent cation K(+) by the divalent cation Ca(2+) leads to significant changes in the force titration curve at high pH where the dicarboxylic groups are fully ionized. We observe that the adhesion force no longer vanishes at high pH but even slightly increases with pH, an effect that is explained by Ca(2+) ions bridging between two carboxylate groups.

Biomimetic cryptic site surfaces for reversible chemo- and cyto-mechanoresponsive substrates.

Chemo-mechanotransduction, the way by which mechanical forces are transformed into chemical signals, plays a fundamental role in many biological processes. The first step of mechanotransduction often relies on exposure, under stretching, of cryptic sites buried in adhesion proteins. Likewise, here we report the first example of synthetic surfaces allowing for specific and fully reversible adhesion of proteins or cells promoted by mechanical action. Silicone sheets are first plasma treated and then functionalized by grafting sequentially under stretching poly(ethylene glycol) (PEG) chains and biotin or arginine-glycine-aspartic acid (RGD) peptides. At unstretched position, these ligands are not accessible for their receptors. Under a mechanical deformation, the surface becomes specifically interactive to streptavidin, biotin antibodies, or adherent for cells, the interactions both for proteins and cells being fully reversible by stretching/unstretching, revealing a reversible exposure process of the ligands. By varying the degree of stretching, the amount of interacting proteins can be varied continuously.

Strengthening the Junction Between EPDM and Aluminium Substrate via Plasma Polymerisation

High interfacial strengths of elastomer to metal joints imply the formation of covalent bonds at the interface. It is proposed to deposit a plasma polymer coating containing double bonds on an aluminium substrate for the formation of crosslinks between the elastomer and the coating during peroxide crosslinking of the elastomer. Ethylene-co-propylene-co-diene terpolymer (EPDM) is considered here. Maleic anhydride is polymerised and reacted with allylamine to lead to an amide functionalised plasma polymer which is heated to produce imide functional groups. Plasma polymer coated aluminium/EPDM joints were evaluated by a peel test and the locus of failure was identified by several techniques: SEM, infrared spectroscopy, wettability, and XPS.

Adhesive behavior of polyurethane-based materials

The role of prepolymer on the interfacial strength of polyurethane-based assemblies is examined for two hydroxyl-terminated (telechelic) polybutadienes, R45M and R45HT. Although it is known that both prepolymers lead to very different adhesive behavior, the reasons that explain this are not understood. In this study, we show that two main effects have to be considered: the length of the free chains, which can diffuse across the interface during the formation of the joint and the relative reactivity of the various hydroxyl groups of the chains and chain extenders with the isocyanate. The R45M chains are more efficient connectors than the R45HT chains. Migration of chain extenders and isocyanate over large distances creates a thick interphase where physical crosslinks appear. This interphase is thicker for R45M-based polyurethane showing the role of the chain reactivity.

New 2-in-1 Polyelectrolyte Step-by-Step Film Buildup without Solution Alternation: From PEDOT-PSS to Polyelectrolyte Complexes

Although never emphasized and increasingly used in organic electronics, PEDOT-PSS (poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate)) layer-by-layer (lbl) film construction violates the alternation of polyanion and polycation rule stated as a prerequisit for a step-by-step film buildup. To demonstrate that this alternation is not always necessary, we studied the step-by-step construction of films using a single solution containing polycation/polyanion complexes. We investigated four different systems: PEDOT-PSS, bPEI-PSS (branched poly(ethylene imine)-poly(sodium 4-styrene sulfonate)), PDADMA-PSS (poly(diallyl dimethyl ammonium)-PSS), and PAH-PSS (poly(allylamine hydrochloride)-PSS). The film buildup obtained by spin-coating or dipping-and-drying process was monitored by ellipsometry, UV-vis-NIR spectrophotometry, and quartz-crystal microbalance. The surface morphology of the films was characterized by atomic force microscopy in tapping mode. After an initial transient regime, the different films have a linear buildup with the number of deposition steps. It appears that, when the particles composed of polyanion-polycation complex and complex aggregates in solution are more or less liquid (case of PEDOT-PSS and bPEI-PSS), our method leads to smooth films (roughness on the order of 1-2 nm). On the other hand, when these complexes are more or less solid particles (case of PDADMA-PSS and PAH-PSS), the resulting films are much rougher (typically 10 nm). Polycation/polyanion molar ratios in monomer unit of the liquid, rinsing, and drying steps are key parameters governing the film buildup process with an optimal polycation/polyanion molar ratio leading to the fastest film growth. This new and general lbl method, designated as 2-in-1 method, allows obtaining regular and controlled film buildup with a single liquid containing polyelectrolyte complexes and opens a new route for surface functionalization with polyelectrolytes.

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.

Relationships between structural properties of vapour-deposited metallic films on to polymer and their relevant adhesive performance

The use of ultrasonic vibrations to characterize the adhesion of ultra-thin metal layers on polymer films is reported. The aim of the study was to analyse and relate the microstructural characteristics of the metallic film to the ultimate practical adhesive properties of the metal-polymer system as measured by an ultrasonic test. The sample studied was formed by thermal deposition of an aluminium layer on to a poly(ethylene terephthalate) film: Al/PET. The adhesive strength of Al/PET laminates was studied in relation with the deposition rate of the metal, its thickness, the structure of the grains and the amount of metal oxide formed at the interface, as probed by transmission electron microscopy (TEM) and secondary ion mass spectroscopy (SIMS). From these results, a correlation between the microstructure of the interface, the bulk morphology of the metallic layer and the overall adhesive performance of the (Al/PET) laminates is proposed.

Adhesion in EPDM joints: Role of the interdiffusion mechanism on interfacial co‐crosslinking

The purpose of this study was to understand the relationship between the mechanism of interdiffusion of the polymer chains across the interface and the formation of crosslinks in the interfacial zone when two elastomer sheets are joined and crosslinked. It is commonly accepted that the strength of the interface thus obtained is related to the number of interlinks that are created in the molecular interphase. This number generally is considered as equal to the number of crosslinks determined in the bulk. Ethylene-copropylene-codiene polymer (EPDM) does not follow this general law. The slow diffusion of the chains at the interface may be responsible for the peculiar behavior observed. In order to separate the two mechanisms responsible for the interfacial strength, diffusion, and crosslinking, two crosslinking procedures, namely peroxide crosslinking at high temperature and electron beam crosslinking at room temperature, have been used. This latter procedure allows control of the diffusion depth. It has been shown that diffusion of EPDM chains is indeed occurring at a much slower rate than expected, leading to less efficient co-crosslinking in the interfacial zone.

Interphases in ethylene–vinyl acetate copolymer/steel sandwiches

The properties of a polymer near an interface with a substrate can be different from the bulk properties. To characterize the interphasial zone, the influence of the thickness of a polymer inserted between two steel sheets is carried out. The chosen polymer is a semi-crystalline ethylene–vinyl acetate copolymer with different amounts of vinyl acetate. Dynamic mechanical spectroscopy measurements were performed directly on the assemblies using a three-point flexure test in order to characterize the mobility of the amorphous phase. The crystalline properties were analyzed by differential scanning calorimetry. The mechanical transition temperature, Tmech, corresponding to the temperature at which the loss factor goes through a maximum was examined. The results show that at high thicknesses Tmech remains constant. However, when the polymer thickness decreases, Tmech increases greatly, indicating a decrease of mobility of the chains. This effect is seen whatever the vinyl acetate content. The crystalline properties are also modified with a higher proportion of small crystals for thin layers. For interfacial energy-minimization reasons, the vinyl acetate groups of the copolymer chains are oriented toward the polar steel surface. These orientation phenomena probably induce some reorganization of the phases, leading to more crystals that constitute physical ties, reducing the mobility of the amorphous phase.