Eléonore Mathis
email: eleonoremathis29@gmail.com

Marie-Laure Michon

Claude Billaud

Pauline Grau

Anthony Bocahut

Caroll Vergelati

Didier R Long
email: didier.r.long@gmail.com

Thermoset modified with polyethersulfone: characterization and control of the morphology

INTRODUCTION

High-performance epoxy matrices have been used for decades in fiber-reinforced composites for aerospace industry. It is adopted today also by the automotive industry. Because of their light-weight, they are used to replace metals. Thermoset resins show high stiffness and strength as well as good thermal and chemical stability. However, their high cross-link density make them very brittle and of poor impact and crack growth resistances. Their toughening by the addition of thermoplastic depends directly on the final morphology. By monitoring the micro-structure of the cured system, the toughness can be optimized for composite applications [START_REF] Pascault | Thermosetting polymers[END_REF][START_REF] Gotro | Thermosets, Encyclopedia of Polymer Science and Technology[END_REF]. For polymer blends, it was shown in the literature that a significant improvement of the impact resistance is obtained when tougheners have phase ABSTRACT Thermoset (TS) epoxy resins can be toughened with a thermoplastic (TP) for high-performance applications. The final structure morphology has to be controlled in order to achieve high mechanical properties and high impact resistance. Four polyethersulfone-modified epoxy resins are considered. They consist of different epoxy monomer structure (TGAP, triglycidyl-p-aminophenol and TGDDM, tetraglycidyl diaminodiphenylmethane) and a fixed amount of thermoplastic, they are cured with two different amounts of curing agent. A reaction-induced phase separation occurs for all formulations generating morphologies, different in shapes and scales. The aim is to control the final morphology and in particular its dominant length scale. This morphology depends on the phase separation process, from the initiation to its final stage. The initiation relies on the relative miscibility of the components and on the stoichiometry between epoxy and curing agent. The kinetics depends on the viscosity of the systems. The different morphologies are characterized by electron microscopy or neutron scattering. Dynamic mechanical analysis allows confirming the presence of a phase separation even when it is not observable by electron microscopy. Vermicular morphologies with few hundreds nanometer width are obtained for the systems containing the TGAP as epoxy monomer. Systems formulated with TGDDM presents morphologies on much smaller scale of order a few tens of nanometers. We interpret the different sizes of the morphologies as a consequence of a larger viscosity for the TGDDM systems as compared to the TGAP ones rather than by a latter initiation of phase separation. separated to form a morphology at the submicron scale [START_REF] Wu | Control of intrinsic brittleness and toughness of polymers and blends by chemical structure: A review[END_REF][START_REF] Ma | Phase morphology and mechanical properties of polyetherimide modified epoxy resins: A comparative study[END_REF]. Specifically in polyethersulfone/epoxy blends, better fracture toughness was observed for systems presenting bicontinuous morphology [START_REF] Jiang | Enhanced mechanical and thermal properties of monocomponent high performance epoxy resin by blending with hydroxyl terminated polyethersulfone[END_REF][START_REF] Martinez | Phase separation in polysulfonemodified epoxy mixtures. Relationships between curing conditions, morphology and ultimate behavior[END_REF].

Since the work of Bucknall and Partridge in 1983 [START_REF] Bucknall | Addition of polyethersulphone to epoxy resins[END_REF], polyethersulfones are used as toughener in epoxy systems [START_REF] Pascault | Rubber-and thermoplastic-modified polymer networks. Phase separation process induced by polymerization and polycondensation[END_REF][START_REF] Oyanguren | Development of bicontinuous morphologies in polysulfone-epoxy blends[END_REF]. Toughening was obtained with samples exhibiting a two-phase morphology. The TP content was found to be one of the most important factor influencing final morphologies and related mechanical properties [START_REF] Pascault | Thermosetting polymers[END_REF]. Upon increasing the TP content, the final resin structure can evolve from seaisland morphology (TP-rich nodules dispersed in TS-rich matrix) to phase inverted morphology (TS-rich domains dispersed in a TP-rich matrix) [START_REF] Rico | Phase separation and morphology development in a thermoplastic-modified toughened epoxy[END_REF]. Increasing the thermoplastic molecular weight leads to a phase separation at lower epoxy conversion [START_REF] Emmerson | Phase separation and mechanical properties of epoxy/thermoplastic blends[END_REF][START_REF] Rico | Influence of the molecular weight of a modifier on the phase separation in an epoxy thermoset modified with a thermoplastic[END_REF]. Rosetti et al. [START_REF] Rosetti | Polyether Sulfone-Based Epoxy Toughening: From Micro-to Nano-Phase Separation via PES End-Chain Modification and Process Engineering[END_REF] showed that a hydroxyl reactive end-group on the thermoplastic leads to finer morphologies as it may react with epoxy group, compared to non-reactive thermoplastic of the same molecular weight [START_REF] Blanco | Influence of a selected hardener on the phase separation in epoxy/thermoplastic polymer blends[END_REF]. The phase separation occurs also later during the curing process. In another study, it was shown that the presence of reactive end-group on TP favoured a stronger interface between TP and epoxy and improve the toughness of the material [START_REF] Ballout | Specific influence of polyethersulfone functionalization on the delamination toughness of modified carbon fiber reinforced polymer processed by resin transfer molding[END_REF]. The relative miscibility of the components in the initial systems is also described as a parameter of choice that influences the phase separation behaviour [START_REF] Pascault | Rubber-and thermoplastic-modified polymer networks. Phase separation process induced by polymerization and polycondensation[END_REF][START_REF] Blanco | Influence of a selected hardener on the phase separation in epoxy/thermoplastic polymer blends[END_REF]. The miscibility of the components is characterized by the Flory-Huggins χ parameter [START_REF] Kurata | Thermodynamics of Polymer Solutions[END_REF].

Enhancing toughness properties such as impact resistance by the control of the final resin morphology still remains a scientific challenge. We propose a methodology to control the final morphology by combining various approaches with regard to the affinity of the constituents characterized by their χ parameters, the viscosity of the systems, the kinetics of epoxy curing and the gel point determination. We consider four different polyethersulfone/epoxy based formulations. Systems are prepared with two different epoxy monomers: the TGAP, triglycidyl-p-aminophenol or the TGDDM, tetraglycidyl diaminodiphenylmethane, and with two different amounts of curing agent. The influence of the initial resin composition on the onset temperature of phase separation, as well as on the phase separation kinetics is discussed. A soluble thermoplastic technology is used for preparing the resin. The chosen polyethersulfone is soluble in the thermoset precursors and phase separates upon curing [START_REF] Williams | Reaction-induced phase separation in modified thermosetting polymers[END_REF]. Phase separation evolves until the gelation state of the system which stops the evolution of the morphology [START_REF] Kim | Phase separation and apparent phase dissolution during cure process of thermoset/thermoplastic blend[END_REF][START_REF] Kim | A new time-temperature-transformation cure diagram for thermoset/thermoplastic blend: tetrafunctional epoxy/poly(ether sulfone)[END_REF]. After the gel point, the compositions of the thermoplastic-rich phase and of the epoxy-rich phase may still evolve. To study these effects, the curing reaction is characterized in situ through thermal and rheological analyses. Cure kinetics, gel point temperatures and phase separation mechanisms are studied. Morphology of the cured systems are analyzed by electron microscopy and further characterized by dynamic mechanical thermal analysis and small-angle neutron scattering if on the nano-scale (< 100 nm).

TGAP-systems result on vermicular morphology with few hundreds nanometer width. TGDDMsystems result on small nodular morphologies as the high viscosity of the resin system slows down the growth of nodules. We discuss the effect of the components affinity and of the viscosity of the four different formulations on the final morphology of our samples.

EXPERIMENTAL

Materials

The epoxy resin precursors were kindly supplied by Huntsman Advanced Materials, Basel (Switzerland): triglycidyl-p-aminophenol (TGAP, Araldite MY0510), tetraglycidyl diaminodiphenylmethane (TGDDM, Araldite MY721) and 4, 4'-diaminodiphenylsulfone (4, 4'-DDS, Aradur 9664-1). From supplier data, TGDDM has an epoxy equivalent weight of 111 -117 g eq -1 and a viscosity of 3000 -6000 cP (at 25 °C). TGAP has an epoxy equivalent weight of 95 -107 g eq -1 and a viscosity of 550 -850 cP (at 25 °C). The modifier is a hydroxyl-functionalized polyethersulfone (PES, PES-5003P from Sumitomo) with a number average molecular weight of 24,000 g mol -1 and a Tg of 232 °C.

Determination of interaction parameters

As introduced by Pascault in 1995 [START_REF] Pascault | Rubber-and thermoplastic-modified polymer networks. Phase separation process induced by polymerization and polycondensation[END_REF], the initial miscibility of epoxy monomers with modifier influences the final morphology. In order to compare the miscibility behavior of blends in their initial state, Flory interaction parameters [START_REF] Flory | Principles of Polymer Chemistry[END_REF] between two components are calculated. Polymer solubility parameters are estimated by using the group contribution theory. The Hoy method [START_REF] Hoy | The Hoy tables of solubility parameters[END_REF] is used to tabulate the group contributions. This method assumes a similarity between the solubility parameter of a polymer and the solubility parameter of its repeat unit. The solubility parameter δ of a component is determined by summing the different contributions of its chemical groups, i.e. the dispersive contribution, the polar contribution and the hydrogen bonding contribution. The corresponding values for each group are provided in the Tables of the Hoy method [START_REF] Hoy | The Hoy tables of solubility parameters[END_REF]. The interaction parameters between two components are calculated by using Equation 1. χ 1-2 is the Flory interaction parameter, δ 1 and δ 2 are the Hoy solubility parameters in for each component in (J cm -3 ) 1/2 , V 1 and V 2 are their respective volume in cm 3 mol -1 , R is the gas constant in J mol -1 K -1 and T is the temperature in K.

χ 1-2 = (δ 1 -δ 2 ) 2 (V 1 V 2 ) 1/2 RT (Equation 1)
Results for a temperature of 120 °C are given in Table 1. No reaction is monitored at that stage yet. The calculation of χ assumes that the volume of the system is constant upon mixing. Note that interaction parameters are calculated for binary systems. A supplementary step could allow to obtain the phase diagram of ternary systems as developed by Kurata [START_REF] Kurata | Thermodynamics of Polymer Solutions[END_REF]. For our study, it allows to obtain qualitative comparisons between the systems A and B composed of epoxy monomers with different structures. It shows that TGAP is highly miscible with the TP (low χ value); on the opposite TGDDM is not a good solvent for the TP (high χ value).

From the determination of the interaction parameters of the components in the initial state, it is possible to determine the onset of phase separation between the epoxy oligomers in formation and the polyethersulfone. It is based on the principle that phase separation occurs when Nχ > 0.5, where N is the number of reacted monomers and χ the initial Flory interaction parameter. Upon curing through a stepwise polymerization, the oligomer molecular weight grows: the number of the repeat units per oligomer increases until reaching the gel point when the value of N tends to infinity. Gelation is a critical phenomenon described by percolation theory [START_REF] De Gennes | Scaling Concepts in Polymer Physics[END_REF][START_REF] Rubinstein | Polymer physics[END_REF]. Near gel-point, the size N of epoxy aggregates can be calculated by the following (Equation 2):

N~| p-p c p c | -1/σ (Equation 2)
where 𝜎𝜎 is a percolation exponent with the value of 0.45 for a system in dimension 3, 𝑝𝑝 𝑐𝑐 is the epoxy conversion at gel point and p is the epoxy conversion. Epoxy conversion at gel point is determined by using dynamic rheology as it will be explained in section 3.2. After calculation of N during epoxy curing, the epoxy conversion at phase separation is approximated. The results are shown in Table 4.

Sample preparation

Four Systems are prepared by dissolving at first the thermoplastic in the epoxy monomer of choice at 140 °C; 4 hours are needed for the TGDDM systems while only 2 hours are sufficient for the TGAP systems. The 4, 4'-DDS curing agent is then added to the homogenized blend of epoxy and TP at 120 °C. The mixture is further stirred to achieve a partial dissolution of the 4, 4'-DDS. Before curing, the samples are degassed under vacuum at 90 °C for 2 hours. Finally for all systems, similar cure profiles are used with first a temperature ramp of 1 °C/min up to 180 °C and a plateau maintained at 180 °C to achieve the resin full cure. For all systems, it is checked that no reaction occurs during degassing and that the DDS is dissolved, by running DSC analyses on non-degassed and degassed samples. The final degree of cure is determined by running DSC analyses on the cured sample and by comparing the residual heat flow to the total heat flow determined from the degassed samples. It evolves from 90 % for TGDDM-systems to 95 % for TGAP-systems.

Material characterization

Physical and chemical characterization of system

In-situ kinetic study of epoxy curing by DSC Thermal analyses are performed with a Q2000 Differential Scanning Calorimeter (TA Instruments). A sample of uncured resin (6 to 8 mg) is placed in an aluminum pan hermetically sealed and is analyzed from -50 to 310 °C at 1 °C min -1 under a nitrogen flow (50 mL min -1 ). The uncured resin glass transition temperature and the heat flow of the exothermic curing process are measured. Degree of epoxy conversion upon curing is calculated from the enthalpy of reaction of the exotherm peak (Equation 3). HT is the integrated area of the exotherm at temperature T, HTotal is the total area of the exotherm, i.e. the total enthalpy of reaction, and T0 is the temperature at the onset of the exotherm. 3) Kinetic studies are made in triplicate in order to determine accurately the degree of conversion upon curing.

α T = ∫ dH T dT T T 0 dT ∆H Total (Equation

Determination of gel point by dynamic rheology

Dynamic rheological measurements are performed by using an oscillation rheometer (ARES-G2 rheometer, TA Instruments) equipped with disposable parallel plate geometry of 25 mm in diameter. Gap is adjusted using a constant force, and auto-strain adjustment from 0.01 to 50 % is used which corresponds to the linear viscoelastic regime. Multiwave rheological measurements are used on a non-isothermal test from 80 to 200 °C at 1 °C min -1 . Storage modulus (G′), loss modulus (G″) and complex viscosity (η*) of melts are collected. Gel point is determined as the crossover of tan δ curves for several frequencies (data extracted from the multiwave experiment).

Phase separation characterization on cured system

Morphology observation by SEM

The morphology of cured samples is analyzed by Scanning Electron Microscopy by using a Zeiss Ultra 55 microscope. The TP-rich phase is revealed by the acid phosphotungstic staining of 300 nm thick cross-sections of cured specimens prepared by microtomy at room temperature. An acid etching is also used as a second method in order to confirm the morphology obtained by the staining method. Polished samples are soaked in a sulfuric acid solution which removed the TP-rich phase. All samples are metallized with Platinum prior being observed.

Morphology observation by TEM

The morphology of cured samples is analyzed by Transmission Electron Microscopy by using a MET JEOL 1400. Ultrathin cross-sections from 50 to 100 nm in thickness are prepared by microtomy at room temperature. The resin micro-structure is revealed by the same staining method.

Determination of phase separation by DMTA

Dynamic mechanical thermal analysis (DMTA) measurements are performed on cured resins by using a rheometer Scientific analyzer RSAG2 (Ta instrument). A part of resin disc has been cut in the dimensions 40 x 10 x 3 mm 3 . The mechanical relaxations were characterized by using the three-point bending method. After determining the linear regime, a strain limit is fixed (5E-3 %) and curves are recorded at 10 Hz during heating from 140 °C to 300 °C at 2 °C min -1 . Relaxation temperatures are determined from the peak of tan δ plot.

Characterization of morphology by small-angle neutron scattering

Small-angle neutron scattering (SANS) are carried out at beamline D11 at Institut Laue-Langevin (ILL, Grenoble, France). Three configurations are used: 5.5 m/ 5.3 Å, 28 m/ 5.3 Å, 37 m / 12 Å, covering a size range from 5 to 1.2E+3 nm. The sample cell is made of an aluminum mold of 20 mm in diameter where samples are poured after degassing. Resins are cured in an oven and quenched at different time of the curing cycle. Samples are then stored in freezer before being studied during the beam shift on the reactor. The observed scattering intensities are corrected for incident neutron spectrum, sample transmission and sample thickness, background scattering. The incoherent scattering is subtracted to the intensity as the invariant intensity at high q.

RESULTS AND DISCUSSION

In-situ kinetic study of epoxy curing

For all resin systems, the profiles of epoxy conversion are determined by DSC nonisothermal analyses at 1 °C min -1 to mimic the chosen curing cycle. Samples are heated up to 310 °C in order to achieve a complete epoxy conversion (α=1) and to determine the total area of the exotherm HTotal. It is then possible to plot the degree of cure along temperature (or time) by using Equation 3 (Figure 2). For each system, the gel point is determined by dynamic rheology as explained in section 3.2. The gel point is indicated by crosses in Figure 2. Differences between the cure kinetics of the 4 formulations are observable. Systems containing TGDDM (A1 and A2) have a slower kinetic than systems with TGAP throughout the curing process and particularly close to their gel points. TGDDM systems have a higher viscosity than TGAP systems (Figure 4) which may reduce the monomers and oligomers mobility and thus the kinetics of reaction. Moreover, TGDDM epoxy monomer has a functionality of 4 which may induce more steric hindrance on its reactive epoxy groups than TGAP which has a functionality of 3, particularly after some epoxy branched. Also because of its highest functionality, vitrification should appear earlier in systems A1 and A2 which then should reduce chain mobility. After vitrification, reaction kinetics is controlled by diffusion. As an indication of this change of kinetic control, the changes of slope on curves A1 and A2 are observable around 70 % of curing degree. Swanson et al. [START_REF] Swanson | Investigation of network development and properties in multifunctional epoxy resins using 3,3'-diaminodiphenylsulfone[END_REF] explained the same influence of the monomer functionality and the steric hindrance by comparing the kinetic of epoxy conversion by near-infrared spectroscopy of three systems with TGAP, TGDDM or difunctional epoxy (DGEBA). Another tendency in our results is that the stoichiometry of the system influences the cure kinetics whatever the monomer epoxy is. As expected, epoxy conversion is faster all along the curing process for systems with excess of curing agent (A2, B2) because more reactive amine groups are present and accessible for the reaction with epoxy groups.

Determination of gel point

Gel point is often determined as the crossover of the elastic (G') and loss (G'') moduli for isothermal experiments [START_REF] Sellak | Elaboration et caractérisation d'une résine thermodurcissable conductrice[END_REF] as well as nonisothermal experiments [START_REF] Voleppe | Interdiffusion and phase separation upon curing in thermosetthermoplastic interphases unravelled by the characterization of partially cured systems[END_REF] for a fixed frequency test. It is a convenient approach for some network polymers for which tanδ = G"/G' =1 but it is not a general definition of the gel point as explained by Winter [START_REF] Winter | Analysis of Linear Viscoelasticity of a Cross-Linking Polymer at the Gel Point[END_REF], who recommends to consider the tanδ curves crossover (obtained from different frequency tests) for determining the gel point. Indeed this critical transition does not depend on the frequency of the test contrary to the vitrification phenomena which is frequency test dependent [START_REF] Winter | Can the gel point of a cross-linking polymer be detected by the G′ -G″ crossover?[END_REF]. Gel point according to Winter criterion was determined by Cicala [START_REF] Cicala | Influence of 1,5-naphthalenediamine on thermomechanical properties of epoxy/copolyethersulphone blends[END_REF] by plotting tanδ as a function of the time using a multiwave test. We apply the same method but in dynamic regime, in order to mimic in situ the standard epoxy curing cycle chosen for our resin for safety reasons. We saw in DSC kinetic study that most epoxy curing reaction occurs between 150 and 180 °C. We applied a temperature ramp at a rate of 1 °C min -1 , using a parallel plate geometry. The multiwave rheological measurement allows to apply the sum of a fundamental frequency and different harmonics each having individual strain amplitudes in one single test. The sum of the strain amplitudes require to stay into the linear viscoelastic regime. Then the individual responses at each discrete frequency can be obtained through a discrete Fourier transform. Results of the multiwave tests with 1, 3, 6, 10 Hz as discrete frequency for systems B1 are plotted in Figure 3. The temperature at the gel point is determined as the tan δ crossover and the corresponding degree of cure is calculated from previous DSC experiments. Degree of cure at gel point is also calculated from the Flory-Stockmayer equation [START_REF] Flory | Principles of Polymer Chemistry[END_REF] (Equation 4), where f e and f a are the epoxy and amine functionalities respectively and s the stoichiometry. This calculation only considers the functionality but not the possible difference in their respective reactivity and it excludes the possibility of intramolecular reaction. 4) All dynamic rheology results are indicated in Table 3, as well as the theoretical degree of cure at gel point for comparisons. Since steric hindrance effects may affect the reactivity of the different functional group in our system, and intramolecular reactions may occur, the experimental results assume larger values than those predicted by the Flory-Stockmayer theory, except for A1 and B2 which seem to agree more with these predictions. Experimental and theoretical results show that gel points occur at lower epoxy conversion for TGDDMsystems (A1 and A2) which can be explained by its higher functionality: the growing molecules build a percolating network faster. It is noticeable that the value of tanδ at gelation is not always equal to 1. As defined by Winter and Chambon [START_REF] Winter | Analysis of Linear Viscoelasticity of a Cross-Linking Polymer at the Gel Point[END_REF], it depends on the resin composition. The tanδ value provides information on the network rigidity: tanδ increases with the network stiffness. Thus, systems containing TGAP (B1, B2) epoxy monomers show a structure at the gel point with a higher stiffness than TGDDM systems (A1, A2), which is consistent with a higher epoxy conversion at gel point for those systems. The determination of the gel point upon curing has to be discussed in relation with the phase separation onset in order to access the extent of phase separation and to understand the finally obtained morphology.

α gel = � s (f e -1)(f a -1) � 1/2 (Equation
Onset of phase separation can be determined by dynamic rheology. It has been often studied by running isothermal experiments. Cicala et al. [START_REF] Cicala | Influence of 1,5-naphthalenediamine on thermomechanical properties of epoxy/copolyethersulphone blends[END_REF][START_REF] Cicala | Study on epoxy/thermoplastic blends based on the addition of a novel aromatic block copolymer[END_REF] compared viscosity fluctuations in different epoxy/coPES blends cured at different temperatures. For 15 wt % of thermoplastic, they recorded a sudden decrease in viscosity attributed to the demixing of a highly viscous TP. The same change of slope of the viscosity profiles is clearly observed in our study for the systems B1 and B2 at a constant temperature for all frequencies. The viscosity profile results from two contributions: the formation of a TP-rich phase (decrease in TP content present in the epoxy phase formed coincidentally) and the formation of the epoxy network (increase in molecular weight). For a better visualization, we plot the storage modulus curves in Figure 4, for the 10 Hz experiments. The gel point determined by the tanδ crossover is indicated by a cross and the approximate onset of phase separation is indicated by an arrow with the corresponding epoxy conversion. However no clear changes in viscosity are observable in systems A1 and A2. It may indicate that no phase separation occurs or that the contribution of phase separation on the viscosity is less important than the contribution of the network development. Note that the signature of a phase separation in dynamic rheology is accessible if the experiment occurs in a certain window of study which depends on the test frequency, the size of the interface, the viscosity ratio between the two phases. It is explained below that two phases are detected in all cured systems by DMTA, so that phase separation is confirmed for all systems but not discerned on systems A by rheology only.

FIGURE 4 Evolution of storage modulus along curing for all systems

In order to further explain the resulting morphology of the different systems, a theoretical estimate of epoxy conversion at phase separation based on scaling model of gelation is used (Equation 2). It allows for the calculation of the number of monomers branched at phase separation (when Nχ > 0.5) and therefore to determine the corresponding epoxy conversion value. The results are shown below in Table 4. It is important to note that the end-group of the PES5003P can react with epoxy as studied by Cheng et al. [START_REF] Cheng | Morphologies and mechanical properties of polyethersulfone modified epoxy blends through multifunctional epoxy composition[END_REF] and mentioned by Rosetti [START_REF] Rosetti | Polyether Sulfone-Based Epoxy Toughening: From Micro-to Nano-Phase Separation via PES End-Chain Modification and Process Engineering[END_REF].

Thus an epoxy-PES block copolymer is formed which is more soluble with epoxy than the PES. This enhanced compatibility delays the phase separation. So epoxy conversions at phase separation may be higher than those calculated above. Nevertheless we observe that the theoretical estimates regarding the epoxy conversion at phase separation for B1 and B2 are close to but slightly lower than the experimental values (0.41 and 0.37 respectively). Thus we assume that the epoxy conversion at phase separation calculated for A1 and A2 are also conservative estimates.

Resulting morphology of the epoxy/polyethersulfone blends

After curing, the morphology of all formulations is characterized. The reaction-induced phase separation in TP/TS blends is due to the increase of the molecular weight of the growing thermoset. The morphologies observed which have a definite length scale and are homogeneous on larger scales are relevant to spinodal decomposition rather than nucleation and growth mechanisms. Spinodal decomposition occurs when the system goes into the unstable region of its phase diagram [START_REF] Rico | Influence of the molecular weight of a modifier on the phase separation in an epoxy thermoset modified with a thermoplastic[END_REF][START_REF] Girard-Reydet | Reactioninduced phase separation mechanisms in modified thermosets[END_REF]. The resulting morphology may be cocontinuous or sea-island if a percolation-tocluster mechanism permits to reduce the interfacial energy of the system [START_REF] Oyanguren | Development of bicontinuous morphologies in polysulfone-epoxy blends[END_REF][START_REF] Emmerson | Phase separation and mechanical properties of epoxy/thermoplastic blends[END_REF]. Scanning Electron Microscopy (SEM) on cured systems after chemically labelling the TP-rich phase permits to bring out clearly the phase separation in systems B. TP-rich phase is dispersed in the epoxy matrix. In Figure 5 the morphologies obtained for systems B are presented. Systems B show vermicular morphologies which can also be described as discontinuous morphologies. We propose that this type of morphology is due to a swelling of TP-noduli by the TGAP which is highly compatible with the thermoplastic so that the amount of TP-noduli is close to the percolation threshold [START_REF] Rubinstein | Polymer physics[END_REF]. Size wise, it is interesting to compare the width of the vermicular morphology which is of sub-micron scale, while the lengths can cover a few micrometers when nodules coalesce. The average width of the PES-rich domains is about 250 nm for the formulation cured at a stoichiometry of 0.9 (B1), while 150 nm width is observed for the formulation with amine excess (B2). Furthermore the nodules of the B2 resin appear to have less coalesced. Rosetti and al. [START_REF] Rosetti | Polyether Sulfone-Based Epoxy Toughening: From Micro-to Nano-Phase Separation via PES End-Chain Modification and Process Engineering[END_REF] obtained vermicular morphologies for epoxy systems (blends of di and tri-functional epoxies) modified with 13 wt % of a similar polyethersulfone but with non-reactive end-groups. The scales are slightly smaller in our systems which can be explained by the presence of the -OH end-group on the polyethersulfone which may react with epoxy [START_REF] Pascault | Thermosetting polymers[END_REF] and so increase its compatibility and delayed phase separation. Also, it is noteworthy that the staining method permits to obtain information on the inside of TP-rich phase at high magnification. In system B2, nodules of epoxy are observable with a diameter of about 50-60 nm. We interpret this effect as a consequence of a secondary phase separation within TP-rich domains when curing goes on. It is likely that this second phase separation process takes place by nucleation and growth. To improve the characterization of the morphology, Transmission Electron Microscopy (TEM) is performed on cured systems after chemically staining the TP-rich phase. The observations are done under magnificence from 10k to 300k. Detailed images of the morphologies of systems B1 and B2 are obtained (Figure 8). Some insight morphologies are again supposed. FIGURE 8 TEM images of B1 and B2. The TP-rich phase is chemically stained and appears as dark domain. Scale bar 100 nm. However using staining method or dissolution method, no phase separation is observed in systems A. Possibilities are that there is no phase separation so that a homogeneous system is obtained or that morphologies are too small or not contrasted enough to be observable. Size wise, the morphologies for A1 and A2 seem to be in the scale limit of the SEM apparatus. The picture below shows A1 at high magnification after TP-chemical staining (Figure 9). Even if we could guess small light nodules (TPrich phase) around 20 -30 nm in a black background (continuous epoxy-rich phase), it is not convincing and the interfaces are not well defined. So that SEM observation was also tried for A1 and A2 systems but it was not successful (Figure 10). Again some darker zones of a few nanometers are visible on the images but we cannot conclude that it indicates the morphology. FIGURE 10 TEM images of A1 and A2. Scale bar 100 nm.

We think that the two phases are not pure enough, so that both phases are partially chemically stained by the phosphotungstic acid. It does not permit to bring a sufficient contrast. Thus another investigation on the phase separation is proposed by DMTA and neutronscattering.

For these systems it was not possible to resolve two different Tg's by DSC because the transitions temperatures are very close to each other. For epoxy/TP systems DMTA was used in different studies in the literature in order to detect the temperature of α-relaxations during ramp temperature test using a three-point bending method [START_REF] Emmerson | Phase separation and mechanical properties of epoxy/thermoplastic blends[END_REF] [33] [START_REF] Blanco | Influence of a selected hardener on the phase separation in epoxy/thermoplastic polymer blends[END_REF] or a single cantilever bending mode [START_REF] Cicala | Effects of mixing di-and tri-functional epoxy monomers on epoxy/thermoplastic blends[END_REF]. In our study, experiments are run on the different systems during a heating ramp at 2 °C min -1 with 10 Hz frequency applied. Tan δ plot shows evidence of phase separation for the four systems cured with a temperature ramp up to a 180 °C plateau. Results for systems A1 and A2 are represented below (Figure 11). A small shoulder is noticeable on the α-relaxation of the TS-rich phase which indicates the αrelaxation of the TP-rich phase. Indeed, two Gaussian peaks were necessary to fit the relaxations at high temperature with acceptable resolution (error <7 %) when comparing the area under the experimental curve and the area under the fitted curve. From the fitted curves two relaxation temperatures are given for each curve corresponding respectively to the TP-rich phase and the TS-rich phase. A1 shows 258.3 °C and 275.9 °C and A2 262.0 °C and 274.5 °C respectively. The curing temperature is 180 °C.

Thus small-angle neutron scattering has been chosen to investigate nanometer scales. Results for systems A1 and A2 of samples quenched at 20 % of epoxy conversion and of samples after the curing process are presented below (Figure 12). At 20 % of epoxy conversion no morphology can be detected on the scattering data. Note that we estimate the onset of phase separation at around 10 % of epoxy conversion but it is possible that the scattering is not sufficient to identify two phases at this stage of the reaction. However the scattered intensity of the final cured systems indicates the presence of two phases corresponding to the well pronounced oscillation at q=0.026 Å -1 . From DMTA and smallangle neutron scattering results we conclude that systems A exhibit phase separation with characteristic size of 24 nm. It is consistent with other studies using TGDDM and polyethersulfone. Fernandez et al. [START_REF] Fernández | Influence of polyethersulfone modification of a tetrafunctional epoxy matrix on the fracture behavior of composite laminates based on woven carbon fibers[END_REF] did not see phase separation on TGDDM/DDM/PES system by SEM but DMA showed a small shoulder on the α-relaxation peak of the TS which confirmed that phase separation had occurred.

Cheng et al. studied TGDDM/TGAP/44'DDS/15 wt%PES with OH endgroup through different formulations (along different quantities of each monomer) [START_REF] Cheng | Morphologies and mechanical properties of polyethersulfone modified epoxy blends through multifunctional epoxy composition[END_REF]. Besides no phase separation was seen on SEM, they distinguished nanophases around 50 nm by Atomic Force Microscopy.

By combining the determination of solubility parameters in initial systems, in situ characterization of the systems along curing and investigation on the final morphologies, a contribution to the understanding of the complex behavior of epoxy/polyethersulfone systems is proposed. Phase separations mechanisms have to be understood according to two characteristics: the onset and the kinetics of phase separation. The onset of phase separation can be predicted by calculating the Flory interaction parameters in the initial system. If phase separation takes place early in the curing process thanks to high χ values, the morphologies have more time for growing before being arrested by gelation. However, the kinetics of phase separation itself is essential as regards the final morphology. Morphologies in more viscous systems grow more slowly than in less viscous systems. The viscosity is related to the T g's of the epoxy and of the thermoplastic and to the temperature at phase separation. In Figure 13 we plot the loss moduli of our different systems during curing (ramp 1 °C min -1 ). The gel points are indicated by crosses and the onsets of phase separation by squares.

FIGURE 13 Evolution of loss modulus along curing for all systems

Systems B phase separate at a higher epoxy conversion than systems A. The elapsed time between phase separation and gelation is about 4 min. We observed above that TP-rich domains in B1 are slightly coarser than in B2. Even if phase separation occurs for both systems at similar epoxy conversion, their kinetics are different. Indeed the viscosity of B1 at phase separation and beyond is almost one decade lower than in B2 which contains a larger quantity of curing agent. As a consequence, we assume that the kinetics of phase separation is faster in B1 than in B2 [START_REF] Pascault | Thermosetting polymers[END_REF].

Systems A (TGDDM) present a poorer miscibility in initial state than systems B. The corresponding value of the χ parameter between TGDDM and TP is 10 times larger than between TGAP and TP. Phase separation takes place earlier during curing. The epoxy conversion was approximated at 10 % which corresponds to a dimerization of the system. Half of epoxy molecules are attached to an amine. We estimate the time lag between phase separation and gel point to be about 24 min. The kinetics of phase separation is controlled by the high viscosity of the system. Loss moduli for a same epoxy conversion are compared between the different systems, using Figure 2 and 13.

At 20 % of epoxy conversion, the loss moduli for systems A1 and A2 are 300 Pa and 310 Pa respectively. At the same stage of curing the loss moduli for B1 and B2 are 63 Pa and 280 Pa respectively. At 30 % of epoxy conversion, the loss moduli for systems A1 and A2 are 3900 Pa and 1260 Pa respectively. At the same stage of curing the loss moduli for B1 and B2 are 180 Pa and 860 Pa respectively. We observe thus that the viscosities of A-systems are higher than those of B-systems at the same stage of curing which may be the cause of the smaller sizes of final morphologies in A-systems as compared to B-systems.

CONCLUSIONS

A methodology for controlling final morphologies in thermoplastic/epoxy blends is applied on four formulations. It starts with the determination of the relative miscibility of the components in initial systems (interaction parameters between epoxy and TP). Then it addresses a kinetic study of epoxy curing by DSC and a thermodynamic study by dynamic rheology with gel point determination and phase separation estimation. The viscosities and the temperatures at phase transition in the different systems are compared. The final morphologies are observed by SEM and TEM. A combination of DMTA analysis and neutron-scattering study allows determining the size of morphologies of a few tens of nanometers in some samples which are not observable by electron microscopy.

Different morphologies are obtained according to the chosen epoxy monomers and the amount of curing agent, all systems having the same fraction of thermoplastic. Systems composed of TGDDM present nodules of 40 nm whereas systems containing TGAP display vermicular morphologies of characteristic width 250 nm and a few micrometers in length.

The structure of the epoxy monomer used in the formulation influences the onset of phase separation. TGDDM separates earlier during the epoxy curing. At the opposite, TGAP is more miscible with the polyethersulfone and the homogeneous polymer solution is stable until a higher degree of epoxy conversion. The epoxy monomer structure (either tri-or tetrafunctional) controls the gel point and the stopping of the phase separation process. An important parameter is the viscosity of the systems which controls the kinetics of phase separation. TGDDM systems have a higher viscosity than TGAP systems which leads to smaller length scales for the final morphology. Furthermore the stoichiometry also influences the viscosity of systems. The B2 formulation has a higher viscosity than B1 and displays a finer morphology. This study puts forward a combination of methods for investigating both the onset and duration of phase separation and the kinetics of morphology growth of epoxy/polyethersulfone systems.

In conclusion, the morphologies obtained in this study are controlled by the affinity between components and the viscosity of the systems. These parameters will be more systematically studied in future works.

FIGURE 1

 1 FIGURE 1 Structure of the resin components.

FIGURE 2

 2 FIGURE 2 Evolution of the degree of cure along temperature ramp at 1 °C min -1 . The point marked by a cross represents the gel point.

FIGURE 3

 3 FIGURE 3 Storage and loss moduli (a) and tan δ curves (b) for 4 frequencies for system B1.

FIGURE 5

 5 FIGURE 5 SEM observations of systems B. The TPrich phase is chemically stained and appears as bright domain. Scale bar 2 µm.Systems B have also been prepared for SEM observation by acid etching method in which the thermoplastic is dissolved, in order to confirm the observed morphologies.

FIGURE 6

 6 FIGURE 6 SEM observation of the systems B. The TP-rich phase is dissolved by acid etching method. Scale bar 2 µm.

FIGURE 7

 7 FIGURE 7 SEM observations of system B2 at high magnification. Scale bar 200 nm.

FIGURE 9

 9 FIGURE 9 SEM observations of system A1 at high magnification. Scale bar 200 nm

FIGURE 11

 11 FIGURE 11 DMTA of A1 and A2.

FIGURE 12

 12 FIGURE 12 SANS intensity of systems A1 and A2, before and after phase separation.

TABLE 1

 1 Flory interaction parameters in initial uncured systems.

	System	Epoxy monomers	Thermoplastic	Flory interaction parameter χ at 120 °C
	A B	TGDDM TGAP	PES-5003P	0.27 0.03

TABLE 3

 3

		Gel point characterization for all
	systems
		Gel point	Tan δ	Degree	Theoretical
	System	temperature	at gel	of cure at	degree of
		(°C)	point	gel point	cure at gel

TABLE 4

 4 Estimation of epoxy conversion at phase separation based on scaling model of gelation.

	System	Epoxy conversion at phase separation
	A1	0.08
	A2	0.11
	B1	0.40
	B2	0.34

GRAPHICAL ABSTRACT

Eléonore Mathis, Marie-Laure Michon, Claude Billaud, Pauline Grau, Anthony Bocahut, Caroll Vergelati, Didier R. Long 1 THERMOSET MODIFIED WITH POLYETHERSULFONE: CHARACTERIZATION AND CONTROL OF THE MORPHOLOGY.

The control of morphology is challenging to the control of mechanical properties in polyethersulfonemodified epoxy resin for composites application. A combination of methods for characterizing the kinetic and thermodynamic behavior as well as the final morphology is applied on four systems. Significant differences in final morphology size and type is assigned to the influence of the interaction parameter and the viscosity.


Sagar B Sharma

Vincent Maurice
email: vincent.maurice@chimieparistech.psl.eu

Lorena H Klein

Philippe Marcus
email: philippe.marcus@chimieparistech.psl.eu

In situ scanning tunneling microscopy study of 2mercaptobenzimidazole local inhibition effects on copper corrosion at grain boundary surface terminations

Keywords: Intergranular corrosion, corrosion inhibition, 2-mercaptobenzimidazole (MBI), Copper, in situ STM, Nanometer scale

New insight on local inhibition effects of 2-mercaptobenzimidazole (MBI) on early stage intergranular corrosion of copper in hydrochloric acid solution is reported from in situ analysis at the nanometer scale and comparison with 2-mercaptobenzothiazole (MBT) effects in the same pre-adsorption and corrosion testing conditions.

Macroscopic cyclic voltammetry analysis, including grains and grain boundary (GB) network, showed a passivation-like behavior in the Cu(I) oxidation range, specific to MBI since not observed with MBT and assigned to the anodic formation of a surface film of Cu(I)-MBI reaction products protecting against dissolution.

Electrochemical scanning tunneling microscopy analysis revealed net intergranular dissolution, mitigated by the imperfect protection provided by the anodically formed MBI layer. It also showed local accumulation of reaction products in the GB surface regions, blocking preferential dissolution. For random GBs, blocking by local accumulation of reaction products was dominant, in agreement with the expected higher reactivity of these GBs generating more Cu(I) ions under anodic polarization and thus less efficiently protected by the anodically formed MBI layer. For Coincidence Site Lattice (CSL) boundaries, mitigated net dissolution was more frequently observed. Coherent twins showed equally efficient inhibition in the GB surface region than on adjacent grains. MBI inhibition was less efficient than MBT inhibition with more Cu(I) reaction products generated on the grains to form a surface film and their preferential local accumulation more frequently observed in the GB surface regions.

Introduction

Intergranular corrosion is a major form of degradation for polycrystalline metallic materials. It initiates locally at the surface termination of grain boundaries (GBs), and eventually propagates to attack the GB network, which can compromise the entire microstructure after penetration in the sub-surface. This form of localized corrosion can be combated by engineering polycrystals having the GB network the most resistant to corrosive attack. GB engineering is based on the knowledge of the relationships between crystallographic type and energy of grain boundaries and their susceptibility to intergranular corrosion, as developed from studies of GB engineered materials submitted to intergranular corrosion tests and studied with microstructural analytical techniques .

One efficient and widely applied mean to mitigate corrosion is to add inhibitors to the aggressive environment contacting the material. On copper and its alloys, azole derivatives such as the widely used and most studied benzotriazole (BTAH, C6H5N3) but also mercaptoazoles derivatives such as 2-mercaptobenzothiazole (MBT, C7H5NS2) and 2-mercaptobenzimidazole (MBI, C7H6N2S) have been shown to be effective corrosion inhibitors [START_REF] Antonijevic | Copper corrosion inhibitors. A review[END_REF][START_REF] Antonijevic | Films formed on copper surface in chloride media in the presence of azoles[END_REF][START_REF] Petrovic Mihajlovic | Copper corrosion inhibitors. period 2008-2014. a review[END_REF][START_REF] Shahrabi | Corrosion inhibition of copper in sulphuric acid by some nitrogen heterocyclic compounds[END_REF]. The MBT and MBI molecules, the latter being studied in this work, contain sulfur and nitrogen atoms that act as active sites that enable chemisorption to the metal substrate by strongly bonding, individually or concomitantly, to copper atoms, and thus form a stable and protective molecular film at the solid/liquid interface [START_REF] Shahrabi | Corrosion inhibition of copper in sulphuric acid by some nitrogen heterocyclic compounds[END_REF][START_REF] Niamien | Correlation between the molecular structure and the inhibiting effect of some benzimidazole derivatives[END_REF][START_REF] Finsgar | 2-mercaptobenzimidazole as a copper corrosion inhibitor: Part i. long-term immersion, 3D-profilometry, and electrochemistry[END_REF][START_REF] Milosev | The roles of mercapto, benzene and methyl groups in the corrosion inhibition of imidazoles on copper: I. experimental characterization[END_REF][START_REF] Zerjav | Protection of copper against corrosion in simulated urban rain by the combined action of benzotriazole, 2-mercaptobenzimidazole and stearic acid[END_REF][START_REF] Chadwick | Electron spectroscopy of corrosion inhibitors: Surface film formed by 2-mercaptobenzothiazole and 2-mercaptobenzimidazole on copper[END_REF][START_REF] Tooru | An X-ray photoelectron spectroscopic study of several metal complexes of 2mercaptobenzimidazole and 2-mercaptobenzoxazole[END_REF][START_REF] Xue | The formation of an effective anti-corrosion film on copper surfaces from 2-mercaptobenzimidazole solution[END_REF][START_REF] Xue | Surface reaction of 2-mercaptobenzimidazole on metals and its application in adhesion promotion[END_REF][START_REF] Xue | SERS and IR studies of the reaction of an oxidized surface and an etched surface of copper with 2-mercaptobenzimidazole[END_REF][START_REF] Perrin | Characterization and mechanism of direct film formation on a Cu electrode through electro-oxidation of 2-mercaptobenzimidazole[END_REF][START_REF] Trachli | Protective effect of electropolymerized 2-mercaptobenzimidazole upon copper corrosion[END_REF][START_REF] Izquierdo | Scanning microelectrochemical characterization of the anti-corrosion performance of inhibitor films formed by 2mercaptobenzimidazole on copper[END_REF][START_REF] Finsgar | 2-mercaptobenzimidazole as a copper corrosion inhibitor: Part ii. surface analysis using X-ray photoelectron spectroscopy[END_REF][START_REF] Wu | 2-mercaptobenzimidazole films formed at ultra-low pressure on copper: adsorption, thermal stability and corrosion inhibition performance[END_REF]. For MBI, the formation of a polymeric film has been proposed [START_REF] Chadwick | Electron spectroscopy of corrosion inhibitors: Surface film formed by 2-mercaptobenzothiazole and 2-mercaptobenzimidazole on copper[END_REF][START_REF] Xue | The formation of an effective anti-corrosion film on copper surfaces from 2-mercaptobenzimidazole solution[END_REF][START_REF] Perrin | Characterization and mechanism of direct film formation on a Cu electrode through electro-oxidation of 2-mercaptobenzimidazole[END_REF][START_REF] Trachli | Protective effect of electropolymerized 2-mercaptobenzimidazole upon copper corrosion[END_REF][START_REF] Zhang | Inhibition of copper corrosion in aerated hydrochloric acid solution by heterocyclic compounds containing a mercapto group[END_REF] with the presence of Cu reaction products in the film -Cu-MBI [START_REF] Xue | The formation of an effective anti-corrosion film on copper surfaces from 2-mercaptobenzimidazole solution[END_REF] or Cu2-MBI [START_REF] Chadwick | Electron spectroscopy of corrosion inhibitors: Surface film formed by 2-mercaptobenzothiazole and 2-mercaptobenzimidazole on copper[END_REF] organo-metallic complexes have been suggested -conditioned by the initial surface state of copper, metallic or oxidized, prior to formation of the protective film [START_REF] Xue | The formation of an effective anti-corrosion film on copper surfaces from 2-mercaptobenzimidazole solution[END_REF], and also by the reactivity of copper during the film formation.

The efficiency of corrosion inhibitors to mitigate intergranular corrosion and the relationship between GB crystallographic type and inhibition mechanism remains to be investigated.

Recently, this was studied on copper by investigating the effects of MBT on early stage intergranular corrosion at the surface termination of grain boundaries, i.e. before penetration in the sub-surface region and propagation [START_REF] Sharma | Local inhibition by 2mercaptobenzothiazole of early stage intergranular corrosion of copper[END_REF]. Electro-Chemical Scanning Tunneling Microscopy (ECSTM) was used to study in situ in HCl acid solution the local alterations of the topmost surface of the metallic material at the nanometer scale. Among high angle GBs that are susceptible to intergranular corrosion in the absence of inhibitors [START_REF] Lin | Influence of Grain Boundary Character Distribution on Sensitization and Intergranular Corrosion of Alloy 600[END_REF][START_REF] Kim | Grain Boundary Character Distribution and Intergranular Corrosion Behavior in High Purity Aluminum[END_REF][START_REF] Shimada | Optimization of grain boundary character distribution for intergranular corrosion resistant 304 stainless steel by twininduced grain boundary engineering[END_REF][START_REF] Lehockey | On the relationship between grain boundary connectivity, coincident site lattice boundaries, and intergranular stress corrosion cracking[END_REF][START_REF] Xia | Effect of single-step strain and annealing on grain boundary character distribution and intergranular corrosion in Alloy 690[END_REF][START_REF] Jones | Sensitisation behaviour of grain boundary engineered austenitic stainless steel[END_REF][START_REF] Ralston | Effect of Grain Size on Corrosion: A Review[END_REF][START_REF] Kumar | Methods for making alloy 600 resistant to sensitization and intergranular corrosion[END_REF][START_REF] Martinez-Lombardia | In Situ Scanning Tunneling Microscopy Study of the Intergranular Corrosion of Copper[END_REF][START_REF] Bettayeb | Nanoscale Intergranular Corrosion and Relation With Grain Boundary Character as Studied In Situ on Copper[END_REF], only coherent twinswhich are 3 coincidence site lattice (CSL) boundaries with a {111}-oriented GB plane -did not show preferential intergranular reactivity compared to adjacent grains, which was assigned to the intrinsic resistance of these GBs also observed without the inhibitor [START_REF] Vinogradov | On the Corrosion Behaviour of Ultra-Fine Grain Copper[END_REF][START_REF] Ralston | Effect of Grain Size on Corrosion: A Review[END_REF][START_REF] Martinez-Lombardia | In Situ Scanning Tunneling Microscopy Study of the Intergranular Corrosion of Copper[END_REF][START_REF] Bettayeb | Nanoscale Intergranular Corrosion and Relation With Grain Boundary Character as Studied In Situ on Copper[END_REF]. Low  Coincidence Site Lattice (CSL) boundaries were observed to be protected by the pre-adsorbed MBT layer against net preferential dissolution whereas high  CSLs and random GBs were observed to react preferentially compared to grains. For these intrinsically more reactive boundaries, net dissolution, mitigated by the pre-adsorbed MBT layer, was observed as well as protection resulting from preferential formation of reaction products in the GB regions. For random GBs, protection by reaction products was more frequently observed but less stable upon repeated anodic cycling applied to force dissolution.

Here, we report new insight for MBI and its local inhibiting effects at the surface termination of grain boundaries of various crystallographic types. ECSTM and CV (cyclic voltammetry) analysis was applied to microcrystalline copper in HCl aqueous acid solution in which no stable copper oxide is formed. Comparative analysis of the results obtained here with MBI with those obtained with MBT [START_REF] Sharma | Local inhibition by 2mercaptobenzothiazole of early stage intergranular corrosion of copper[END_REF] in the same pre-adsorption and corrosion testing conditions provides deeper understanding on local inhibition mechanisms as well as their dependence on GB type and inhibiting molecule.

Experimental

The high purity microcrystalline copper samples were the same as in previous work [START_REF] Martinez-Lombardia | Scanning Electrochemical Microscopy to Study the Effect of Crystallographic Orientation on the Electrochemical Activity of Pure Copper[END_REF][START_REF] Martinez-Lombardia | In Situ Scanning Tunneling Microscopy Study of the Intergranular Corrosion of Copper[END_REF][START_REF] Martinez-Lombardia | In Situ Scanning Tunneling Microscopy Study of Grain-Dependent Corrosion on Microcrystalline Copper[END_REF][START_REF] Chen | Grain Boundary Passivation Studied by In Situ Scanning Tunneling Microscopy on Microcrystalline Copper[END_REF][START_REF] Chen | Local passivation of metals at grain boundaries: In situ scanningtunneling microscopy study on copper[END_REF][START_REF] Bettayeb | Nanoscale Intergranular Corrosion and Relation With Grain Boundary Character as Studied In Situ on Copper[END_REF][START_REF] Lapeire | Effect of Neighboring Grains on the Microscopic Corrosion Behavior of a Grain in Polycrystalline Copper[END_REF][START_REF] Lapeire | Structural Dependence of Gold Deposition by Nanoplating in Polycrystalline Copper[END_REF]. From previous microstructural characterization [START_REF] Martinez-Lombardia | In Situ Scanning Tunneling Microscopy Study of the Intergranular Corrosion of Copper[END_REF][START_REF] Martinez-Lombardia | In Situ Scanning Tunneling Microscopy Study of Grain-Dependent Corrosion on Microcrystalline Copper[END_REF], texture is nearly random and ∑3 CSL and random boundaries correspond to 66% and most of remaining 34% of the GB length, respectively. The surface was prepared by mechanical polishing (diamond spray, 0.25 μm final grade) followed by electrochemical polishing (15 s in 66% orthophosphoric acid at 3 V versus Cu counter electrode) in order to remove the cold work layer and reveal the GB surface terminations.

An Agilent Technologies ECSTM system (PicoSPM base, Keysight STM S scanner, PicoScan 2100 controller, PicoStat bi-potentiostat and Picoscan software) was used. ECSTM cell, cleaning and tip preparation have been detailed elsewhere [START_REF] Zuili | Surface Structure of Nickel in Acid Solution Studied by In Situ STM[END_REF][START_REF] Seyeux | In Situ STM Study of the Initial Stages of Growth and of the Structure of the Passive Film on Ni(111) in 1 mM NaOH(aq)[END_REF][START_REF] Seyeux | In Situ STM Study of the Effect of Chloride on the Growth Mechanism and Structure of the Passive Film on Nickel in Alkaline Solution[END_REF][START_REF] Maurice | In situ STM Study of the Surface Structure, Dissolution and Early Stages of Electrochemical Oxidation of the Ag(111) Electrode[END_REF][START_REF] Li | Structure and Morphology Modifications of Silver Surface in the Early Stages of Sulphide Growth in Alkaline Solution[END_REF]. The ECSTM cell exposes a working electrode area of 0.16 cm 2 to ~350 l of electrolyte. Two Pt wires served as counter electrode and pseudo reference electrode (+0.20 V/SHE). The 10 mM HCl(aq) (pH 2) electrolyte was prepared from ultrapure HCl and Millipore water (resistivity > 18 MΩ cm) and non-deaerated. MBI was dissolved from the powder physical state at a concentration of 1 mM.

After exposing the sample to the electrolyte at -0.45 V/SHE, below the open-circuit potential value of -0.35 V/SHE, the native oxide film formed after electropolishing was reduced by CV treament in the cathodic potential range as described before [START_REF] Martinez-Lombardia | In Situ Scanning Tunneling Microscopy Study of the Intergranular Corrosion of Copper[END_REF][START_REF] Bettayeb | Nanoscale Intergranular Corrosion and Relation With Grain Boundary Character as Studied In Situ on Copper[END_REF]. The cathodic peak associated with Cu(I) to Cu(0) reduction in the CVs was suppressed after two cycles, independently of the presence or absence of MBI in the solution during the cathodic reduction pre-treatment.

After reduction of the native oxide, the microcrystalline copper surface was imaged by ECSTM in the metallic state at -0.45 V/SHE to ensure localization of grain boundaries in the field of view. CV was also used to force anodic oxidation by cycling between the anodic and cathodic apexes of -0.18 and -0.60 V/SHE (1 mV s -1 ), respectively, like applied in our MBT study [START_REF] Sharma | Local inhibition by 2mercaptobenzothiazole of early stage intergranular corrosion of copper[END_REF]. Up to 4 cycles were applied and new ECSTM images of the same surface field of view were taken at -0.45 V/SHE after 2 and 4 cycles, thus enabling to follow the evolution of the local topography after electrochemical cycling. The images were acquired in the constant current mode. No filtering was used during acquisition and the recorded images were processed with the Gwyddion software [START_REF]Gwyddion -Free SPM (AFM, SNOM/NSOM, STM, MFM …) data analysis software[END_REF]. At the surface, the terminations of the GB planes correspond to the GB edges. These GB edges and their immediate vicinity are referred to as the GB surface regions, easily identified in the STM images. The local difference in topographic height between the bottom of the GB surface regions and the two adjacent grains, hereafter called GB depth, was determined by line profile analysis as previously described [START_REF] Chen | Grain Boundary Passivation Studied by In Situ Scanning Tunneling Microscopy on Microcrystalline Copper[END_REF][START_REF] Bettayeb | Nanoscale Intergranular Corrosion and Relation With Grain Boundary Character as Studied In Situ on Copper[END_REF][START_REF] Sharma | Local inhibition by 2mercaptobenzothiazole of early stage intergranular corrosion of copper[END_REF].

Results and discussion

Macroscopic MBI inhibition effects

Cyclic voltammetry was applied with the same protocol as for the ECSTM study in order to characterize the effects of MBI on the macroscopic corrosion behavior. The exposed surface fraction of the grains being very large compared to that of the GB network, the macroscopic electrochemical response can be assigned to the surface properties of the grains. Figure 1(a) shows CVs started in the metallic state at -0.45 V/SHE after reduction of the native oxide. At pH 2, no stable oxide is formed on copper and the Cu(I) ions dissolve as 𝐶𝑢𝐶𝑙 2

-in the investigated anodic potential range [START_REF] Strehblow | The Investigation of the Passive Behaviour of Copper in Weakly Acid and Alkaline Solutions and the Examination of the Passive Film by Esca and ISS[END_REF][START_REF] Feng | Corrosion Mechanisms and Products of Copper in Aqueous Solutions at Various pH Values[END_REF][START_REF] Beverskog | Pourbaix Diagrams for the System Copper-Chlorine at 5-100 C. SKI[END_REF][START_REF] Magnussen | In-Situ Atomic-Scale Studies of the Mechanisms and Dynamics of Metal Dissolution by High-Speed STM[END_REF]. In the absence of MBI, anodic dissolution is observed for E > -0.35 V/SHE. The cathodic peak observed in the reverse scan corresponds to the reductive deposition of dissolved Cu(I) [START_REF] Bettayeb | Nanoscale Intergranular Corrosion and Relation With Grain Boundary Character as Studied In Situ on Copper[END_REF][START_REF] Magnussen | In-Situ Atomic-Scale Studies of the Mechanisms and Dynamics of Metal Dissolution by High-Speed STM[END_REF][START_REF] Kruft | Chloride Adsorption on Cu(111) Electrodes in Dilute HCl Solutions[END_REF][START_REF] Vogt | In-Situ STM Study of the Initial Stages of Corrosion of Cu(100) Electrodes in Sulfuric and Hydrochloric Acid Solution[END_REF][START_REF] Wan | In Situ Scanning Tunneling Microscopy of Cu(110): Atomic Structures of Halide Adlayers and Anodic Dissolution[END_REF][START_REF] Li | In Situ STM Study of Chloride Adsorption on Cu(110) Electrode in Hydrochloric Acid Aqueous Solution[END_REF][START_REF] Hai Phan | Potential dependence of self-assembled porphyrin layers on a Cu(111) electrode surface: In-situ STM study[END_REF]. As discussed previously [START_REF] Sharma | Local inhibition by 2mercaptobenzothiazole of early stage intergranular corrosion of copper[END_REF], the equivalent thickness of reacting copper can be deduced from the total positive current density (26 885 𝐶 cm -2 ) using Eq. ( 1),

𝛿 = 𝑞𝑉 𝑚 𝑧𝐹 Eq. ( 1
)
where 𝑞 is the charge density, 𝑉 𝑚 the molar volume of metallic copper (7.1 cm 3 mol -1 ), 𝑧 the number of exchanged electrons (1), and F the Faraday constant. The obtained value is 19.7 nm, which indicates that a nanometer thick layer of material has reacted in these testing conditions.

The CVs obtained in the presence of MBI are superimposed in Figure 1(a) and enlarged in After pre-treatment in the presence of MBI in the solution, the CV shows a higher copper electrochemical activity, however markedly reduced compared to that measured in the absence of the inhibitor. In this case, MBI pre-adsorbs on copper covered by the native oxide and the reduction process occurs in the presence of a surface layer of MBI. As a result, the Cu atoms dissociated from oxygen by electro-reduction may be captured by the molecular surface layer, resulting in the formation of a barrier film less effective to block atomic transport across the interface as indicated by the residual electrochemical response in the Cu(I) oxidation range.

The anodic current also increases for E > -0.35 V/SHE indicating that the anodic oxidation of copper and its transport across the interface leading to dissolution is initiated in the same potential range as in the absence of MBI. However, the current increase is much slower due to the mitigating effect of the MBI molecular layer pre-adsorbed on the surface. With increasing potential, a peak is observed at -0.23 V/SHE. The anodic reaction decreases in intensity in a process similar to passivation. Copper transport and dissolution appear blocked by the anodic formation of a modified, compared to pre-adsorbed layer, but protective surface layer. The charge density transfer cumulated at the anodic apex is 2 292 𝐶 cm -2 . Using Eq. ( 1), this corresponds to 1.7 nm of copper having reacted and captured by the formation of the protective surface film grown at this stage (if one assumes no transient dissolution). Given that several monolayers of copper were consumed, it is suggested that this surface layer is 3D in thickness and constituted of Cu(I)-MBI reaction products, possibly organic-metallic complexes [START_REF] Chadwick | Electron spectroscopy of corrosion inhibitors: Surface film formed by 2-mercaptobenzothiazole and 2-mercaptobenzimidazole on copper[END_REF][START_REF] Xue | The formation of an effective anti-corrosion film on copper surfaces from 2-mercaptobenzimidazole solution[END_REF],

since grown in the Cu(I) oxidation range. It cannot be excluded that during the growth of the protective surface film some Cu(I) ions dissolved in the solution although no redeposition peak is observed in the subsequent cathodic scan.

In the reverse scan performed after reaching the anodic apex at -0.18 V/SHE, no cathodic peak is observed in the range of -0.3 to -0.50 V/SHE unlike in the absence of the inhibitor. This is consistent with the grown surface layer blocking anodic dissolution and thus the reductive deposition of copper. The cathodic current increase observed for E < -0.30 V/SHE may correspond to the electro-reduction of the surface film of Cu(I)-MBI reaction products. A cathodic charge density transfer of 2107 µ𝐶 cm -2 is obtained from the integration of the negative current density measured during cycling, which corresponds to an equivalent thickness of 1.55 nm of reacting copper. This value confirms the 3D thickness of the protecting surface layer grown by anodic oxidation of copper in the presence of the MBI inhibitor.

The CV previously obtained in the same testing conditions after pre-treatment in the presence of MBT [START_REF] Sharma | Local inhibition by 2mercaptobenzothiazole of early stage intergranular corrosion of copper[END_REF] is reproduced in Figure 1(b) for comparison. The surface is much less reactive than in the presence of MBI with no anodic peak indicative of anodic formation of a 3D layer constituted of Cu(I)-MBT reaction products. The total measured positive current density is 36 𝐶 cm -2 , corresponding to only a fraction of a monolayer of copper (0.03 nm equivalent thickness) having reacted [START_REF] Sharma | Local inhibition by 2mercaptobenzothiazole of early stage intergranular corrosion of copper[END_REF]. For the same conditions of pre-treatment of the surface, comparison between the two molecules shows higher anodic inhibition efficiency for MBT than for MBI. For MBT, the pre-adsorbed molecular layer nearly fully blocks the macroscopic anodic activity. It can be deduced that the surface of grains is fully protected by the interfacial molecular layer in the test conditions, and that the residual activity may be associated to the most reactive (i.e. imperfectly protected) sites, including the GB network. For MBI, the preadsorbed molecular layer only mitigates anodic oxidation, leading to a passivation-like behavior with the buildup of a protective 3D surface layer of Cu(I)-MBI reaction products. This surface layer forms at the surface of the grains as deduced from the macroscopic electrochemical response. It also forms in the GB regions as deduced from the local analysis of the topography variations discussed hereafter. The comparative analysis of the GB depth before and after the CV treatments in the 74 sites revealed 5 types of local intergranular behavior: i) unchanged GB depth, ii) repeated increase, iii) repeated decrease, iv) decrease followed by increase, and v) increase followed by decrease.

Local MBI inhibition effects at grain boundaries

The distribution of these observed behaviors is shown in Figure 4.

The 5 sites showing no specific local behavior caused by intergranular corrosion after 2 and 4

CVs are labelled 1 to 5 in Figure 4(a). The bar graph of the GB depth confirms no net variation of the depth of the GB region compared to the adjacent grains (Figure 4(b)). In the absence of inhibitors, such an absence of preferential local intergranular reactivity is typical of coherent twins (3 CSL with a {111}-oriented GB plane) [START_REF] Vinogradov | On the Corrosion Behaviour of Ultra-Fine Grain Copper[END_REF][START_REF] Ralston | Effect of Grain Size on Corrosion: A Review[END_REF][START_REF] Martinez-Lombardia | In Situ Scanning Tunneling Microscopy Study of the Intergranular Corrosion of Copper[END_REF][START_REF] Bettayeb | Nanoscale Intergranular Corrosion and Relation With Grain Boundary Character as Studied In Situ on Copper[END_REF][START_REF] Sharma | Local inhibition by 2mercaptobenzothiazole of early stage intergranular corrosion of copper[END_REF]. In the present case, it is observed along two parallel GBs separating a narrow sub-grain (sites 3 to 5), a configuration typical of 3 coherent twins. It is also observed for sites 1 and 2 also located along two parallel GBs and separating a wider sub-grain. At these sites, the absence of preferential local reactivity is thus assigned to the intrinsic intergranular properties of the 3 coherent twins rather than to a local effect of the MBI inhibitor. The surface layer of Cu(I)-MBI reaction products formed indifferently in the GB region and on the adjacent grains, and the 3 coherent twins did not react preferentially owing to their intrinsic resistance against intergranular corrosion (Figure 5(a)).

The 10 sites showing a repeated net increase of the depth of the GB region after 2 and 4 CVs are labelled 6 to 15 in Figure 4(a) and (c). In the absence of formation of a passive film, the increase of the GB depth is indicative of the preferential local consumption of the material by irreversible and faster dissolution than on the adjacent grains as previously observed for early stage intergranular corrosion of copper in 1 mM HCl(aq) in the absence of inhibitor [START_REF] Martinez-Lombardia | In Situ Scanning Tunneling Microscopy Study of the Intergranular Corrosion of Copper[END_REF][START_REF] Bettayeb | Nanoscale Intergranular Corrosion and Relation With Grain Boundary Character as Studied In Situ on Copper[END_REF]. In the present case, a surface layer of Cu(I)-MBI reaction products is formed as shown by the CV analysis. The formation of a thinner layer in the GB regions than on the adjacent grains would result in an increase of the GB depth. However, this is not consistent with the preferential reactivity expected in the GB regions. In contrast, the formation in the GB regions of a surface layer less efficiently mitigating anodic dissolution than on the adjacent grains is consistent with the more reactive character of the GBs, and would result in an increase of the GB depth owing to the preferential consumption of copper in the GBs regions (Figure 5(b)). Among the 10 GB sites where preferential although mitigated irreversible dissolution is observed, only site 12 can correspond to a 3 coherent twin (the same as for site 2). This coherent twin is possibly locally deviated from the perfect geometry since exhibiting local preferential dissolution [START_REF] Bettayeb | Nanoscale Intergranular Corrosion and Relation With Grain Boundary Character as Studied In Situ on Copper[END_REF]. Among This unstable behavior suggests that the thicker layer of Cu(I) reaction products formed in the GB regions first provides local protection against anodic dissolution, like observed on the grains by CV. However, this initial protection provided by the surface layer is not stable since altered during the subsequent CV treatments as shown by the increase of the GB depth resulting from net dissolution (Figure 5(d)). Among the GB sites where this behavior is observed, site 48 can be assigned to a 3 coherent twin of locally imperfect geometry like at site 12. Site 33 can be assigned to a low  CSL boundary, the same as site 16 discussed above. Site 42 belongs to a straight segment that can be assigned to a high  CSL boundary. Note that at these 3 sites the GB depth variations are small indicating little alterations of the local early intergranular corrosion behavior. All the other 31 sites belong to curved or locally curved boundaries assigned to random boundaries. Among them only a minority of 10 sites (37, 38, 40, 43, 44, 47, 50, 58, 61 and 62) show an increase of the GB depth after 4 CVs to a value higher that the initial value measured before cycling, indicating a pronounced alteration of the transient protection brought by the formation of the surface layer of reaction products.

We found 9 GB sites where the behavior was first characterized by an increase of the GB depth after 2 CVs and subsequently by a decrease after 4 CVs. They are labelled 66 to 74 in Figure 4(a) and (f). Like for sites 6 to 15 discussed above, we assign the increase of the GB depth measured after 2 CVs to net irreversible dissolution, less mitigated by the surface layer of Cu(I) reaction products at the GBs than on the adjacent grains. After 2 more CVs, the preferential accumulation of Cu(I) reaction products in the GB region decreases the GB depth owing to the formation of a thicker surface layer than on the adjacent grains. In this case, the repeated alterations of the surface layer brought by the CV treatment promote a delayed protection in the GB regions (Figure 5(e)). All GB sites except one (site 66) at which this behavior was observed belong to curved or locally curved boundaries characteristic of random GBs. Site 66 belongs to short and straight boundary, assigned to a low  CSL boundary.

Local MBI inhibition effects according to GB type

The 5 types of local intergranular corrosion behavior observed in this work in the presence of MBI and their occurrences according to GB crystallographic type are compiled in Table 1 and Figure 6. The distribution of behavior observed with MBT [START_REF] Sharma | Local inhibition by 2mercaptobenzothiazole of early stage intergranular corrosion of copper[END_REF] is also reported in Figure 6 for comparison.

In most cases and independently of the presence of the MBI or MBT inhibitors in the solution, the 3 coherent twins showed no preferential intergranular reactivity compared to the adjacent grains. This is in agreement with intrinsic resistance of these grain boundaries to intergranular corrosion [START_REF] Vinogradov | On the Corrosion Behaviour of Ultra-Fine Grain Copper[END_REF][START_REF] Ralston | Effect of Grain Size on Corrosion: A Review[END_REF][START_REF] Martinez-Lombardia | In Situ Scanning Tunneling Microscopy Study of the Intergranular Corrosion of Copper[END_REF][START_REF] Bettayeb | Nanoscale Intergranular Corrosion and Relation With Grain Boundary Character as Studied In Situ on Copper[END_REF], and thus equally efficient inhibition in the GB surface region than on adjacent grains can be concluded for the 3 coherent twins. Local preferential reactivity was observed in two cases despite the presence of MBI, likely owing to the local deviation of the grain boundary plane from the perfect geometry as observed previously in the absence of inhibitor [START_REF] Bettayeb | Nanoscale Intergranular Corrosion and Relation With Grain Boundary Character as Studied In Situ on Copper[END_REF]. In these cases, the anodically formed surface layer of Cu(I)-MBI corrosion products provided imperfect or only transient protection against anodic dissolution.

All other types of grain boundaries showed preferential reactivity. For the most reactive random GBs, protection by accumulation of reaction products was found to largely predominate protection by mitigated dissolution. For MBI, with which a passivation-like behavior of the grains is observed by CV, this mode of protection results from the formation of a thicker film of Cu(I) corrosion products in the GB region than on the adjacent grains. For MBT, with which no passivation-like anodic behavior of the grains is observed, this mode of protection results from the local accumulation of Cu(I) corrosion products in the GB region. With both inhibitors, the preferential formation of corrosion products in the GB region is consistent with more intense activity occurring at the surface terminations of random GBs, as expected from the ill-defined crystallographic character of these boundaries, and producing upon anodic polarization more Cu(I) corrosion products than in CSL boundaries. With MBI, as well as with MBT, the major part of the measured random GB sites did not show stable protection after repeated anodic polarization, showing the fragile protectiveness provided by the reaction products accumulated in the surface regions of the random GBs.

Much fewer occurrences of the locally observed intergranular behavior could be associated with CSL boundaries. For the high CSLs, protection by a thicker layer of Cu(I)-MBI corrosion products is also observed but mitigated dissolution by the anodically formed MBI layer is more frequent, in agreement with a lower reactivity of high CSLs compared to most random GBs.

A more balanced distribution of these two types of local inhibition effects was also observed with MBT. For low  CSLs, protection by a thicker layer of Cu(I)-MBI corrosion products is more frequent than mitigated dissolution by the MBI layer. This suggests that some CSL boundaries could be initially as reactive as random GBs for forming Cu(I)-MBI corrosion products. A larger fraction of the low CSLs than of random GBs would form a stable protective layer of corrosion products.

Conclusions

The local inhibiting effects of MBI on the initial stages of intergranular corrosion of copper

Figure 1 (

 1 Figure 1(b). The observed behavior depends on the presence or absence of MBI during the

Figure 2 (

 2 Figure 2(a) shows a typical local area at the topmost surface of microcrystalline copper as

the 9

 9 other sites, site 14 is located at a short and straight boundary, assigned to a low  CSL boundary. Sites 7 to 9 are located along straight segments of a GB whose morphology is locally straight or curved and assigned to a high  CSL. Sites 6, 10, 11, 13 and 15 belong to GBs with curved morphologies indicating their random character. For these high  CSL and random boundaries, local intergranular irreversible dissolution is observed but mitigated by the MBI surface layer given the relatively small increase of the GB depth.The repeated net decrease of the depth of the GB region after 2 and 4 CVs could be measured in 16 sites, labelled 16 to 31 in Figure4(a) and (d). The decrease of the GB depth is consistent with the local accumulation of reaction products, more important in the GB region because of the locally higher reactivity. Such a local GB behavior has been observed by ECSTM for copper passivated in 0.1 M NaOH(aq) and attributed to a locally thicker Cu(I) passive oxide film formed in the GB regions[START_REF] Chen | Grain Boundary Passivation Studied by In Situ Scanning Tunneling Microscopy on Microcrystalline Copper[END_REF][START_REF] Chen | Local passivation of metals at grain boundaries: In situ scanningtunneling microscopy study on copper[END_REF][START_REF] Bettayeb | Combined in situ microstructural study of the relationships between local grain boundary structure and passivation on microcrystalline copper[END_REF]. In 10 mM HCl(aq) + 1 mM MBI, the electrochemical response is indicative of a passivation-like behavior not observed in the absence of MBI. More copper would react in the GB region during anodic polarization leading to the formation of a thicker layer of Cu(I) reaction products than on the adjacent grains. The process would be repeated at each treatment cycle thus leading to the thickness increase of the film formed in the GB regions (Figure5(c)). This formation of a thicker film of Cu(I) reaction products than on the adjacent grains is observed at sites 16, 28 and 29 along straight GBs that can be assigned to low  CSL GBs. It is also observed at site 19 along the same straight GB segment as at sites 7 and 8 and corresponding to a high  CSL, thereby showing that the inhibitor effect may be dependent on the local structure along the same GB edge.Sites 17,[START_REF] Stratulat | Grain boundary structure and intergranular stress corrosion crack initiation in high temperature water of a thermally sensitised austenitic stainless steel, observed in situ[END_REF][START_REF] Mahesh | Role of Nanostructure in Electrochemical Corrosion and High Temperature Oxidation: A Review[END_REF][START_REF] Srinivasan | Near boundary gradient zone and sensitization control in austenitic stainless steel[END_REF][START_REF] Martinez-Lombardia | In Situ Scanning Tunneling Microscopy Study of the Intergranular Corrosion of Copper[END_REF][START_REF] Martinez-Lombardia | In Situ Scanning Tunneling Microscopy Study of Grain-Dependent Corrosion on Microcrystalline Copper[END_REF][START_REF] Chen | Grain Boundary Passivation Studied by In Situ Scanning Tunneling Microscopy on Microcrystalline Copper[END_REF][START_REF] Chen | Local passivation of metals at grain boundaries: In situ scanningtunneling microscopy study on copper[END_REF][START_REF] Bettayeb | Nanoscale Intergranular Corrosion and Relation With Grain Boundary Character as Studied In Situ on Copper[END_REF][START_REF] Antonijevic | Copper corrosion inhibitors. A review[END_REF] 30 and 31 all belong to random GBs characterized by curved morphologies. It is at such GBs that the largest decrease of the GB depth is observed (sites 21, 24, 30 and 31), which is consistent with more copper being consumed in these more reactive GBs leading to increased accumulation of Cu(I)-MBI reaction products with formation of a thicker film.

Figure 4 (

 4 Figure 4(a) and (e) shows the 34 GB sites, labelled 32 to 65, where, after 2 CVs, the behavior

  were studied in situ by ECSTM in 10 mM HCl(aq) acid solution. Macroscopic analysis by CV of the grains and GB network electrochemical response in the Cu(I) oxidation range confirmed that MBI inhibits the active dissolution of copper and showed that the inhibition efficiency markedly depends on the conditions of pre-adsorption of the molecule. If pre-adsorbed on the surface in the metallic state, anodic as well as cathodic activity is suppressed, most likely by formation of a homogeneous barrier molecular film efficiently blocking atomic transport across the interface. If pre-adsorbed on the surface covered by its native oxide and subsequently reduced, the residual activity is higher and characteristic of a passivation-like behavior assigned to the anodic formation of a protective surface film of Cu(I)-MBI reaction products.Local ECSTM analysis at the surface termination of the GB network showed no preferential intergranular corrosion of the 3 coherent twins compared to grains, in agreement with their intrinsic resistance to intergranular corrosion. All other types of GBs reacted preferentially compared to adjacent grains. They showed preferential dissolution mitigated by the imperfect protection provided by the anodically formed Cu(I)-MBI surface film, or preferential accumulation of Cu(I) reaction products protecting the GB surface regions against intergranular dissolution. Local GB protection by preferential accumulation of Cu(I) reaction products was observed to be transient, delayed or stable in the conditions of repeated anodic cycling applied to force anodic oxidation. For CSL grain boundaries other than 3 coherent twins, mitigated preferential dissolution occurred more frequently than blocking by accumulated reaction products, indicating more efficient protection by the anodically formed MBI surface layer. For the random boundaries, local protection by preferential accumulation of Cu(I) reaction products was dominant like previously observed with MBT. This is assigned to the intrinsically more reactive character of the random GBs, less efficiently protected by the anodically formed MBI layer and with the generation of more Cu(I) ions under anodic polarization resulting in the local accumulation of Cu(I)-MBI reaction products.Comparison with MBT effects studied in the same pre-adsorption and testing conditions shows less efficient inhibition by MBI, with more Cu(I) reaction products generated on the grains to form a surface film and the preferential accumulation of reaction products more frequently observed in the GB surface regions.

Figure 1

 1 Figure 1 CV analysis of macroscopic electrochemical behavior of microcrystalline copper in 10 mM HCl(aq) (ECTM cell, scan rate = 1 mV/s): (a) CVs recorded without and with 1 mM MBI in the solution. (b) Enlarged CVs obtained in the presence of MBI after cathodic pretreatment with and without MBI in the solution. In (b), the CV obtained in 10 mM HCl(aq) + 0.1 M MBT is shown for comparison.

Figure 2

 2 Figure 2 Topographic ECSTM images of microstructure obtained at E = -0.45 V/SHE in 10 mM HCl(aq) + 1 mM MBI (Z range ΔZ = 6 nm, tip potential Etip = -0.5 V/SHE, tunneling current It = 0.6 nA): (a) Initial surface state; (b) Surface state after 2 CV treatments; (c) Surface state after 4 CV treatments.

Figure 3

 3 Figure 3 Average topographic line profiles measured across the GB at site # 1 in Figure 2 after 0, 2 and 4 CV treatments.

Figure 4

 4 Figure 4 ECSTM evaluation of the intergranular behavior in the early stage corrosion of microcrystalline copper sites in 10 mM HCl(aq) + 1 mM MBI: (a) Topographic image with GB sites labelled 1 to 74; (b) Bar graph of the GB depth measured across the sites (labelled in white in (a)) showing no preferential reactivity after 2 and 4 CVs; (c) Bar graph of the GB sites (labelled in yellow in (a)) showing depth increase after 2 and 4 CVs; (d) Bar graph of the GB sites (labelled in dark blue in (a)) showing depth decrease after 2 and 4 CVs; (e) Bar graph of the GB sites (labelled in black in (a)) showing depth decrease and subsequent increase after 2 and 4 CVs, respectively. (f) Bar graph of the GB sites (labelled in light blue in (a)) showing depth increase and subsequent decrease after 2 and 4 CVs, respectively.

Figure 5

 5 Figure 5 Schematic illustration of observed MBI inhibiting effects on early intergranular corrosion of copper in acid electrolyte.

Figure 6

 6 Figure 6 Percentage of each type of early intergranular corrosion behavior observed on various types of grain boundaries for microcrystalline copper in 10 mM HCl(aq) + 1 mM MBI and comparison in 10 mM HCl(aq) + 0.1 mM MBT

Acknowledgments

This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (ERC Advanced Grant agreement No 741123).

Tables

Delayed protection by corrosion products

--1 --8

29