Maxime Pras

Jean-François Gérard

Luana Golanski

Guilhem Quintard

; Jannick Duchet- Rumeau

Key Role of the Dispersion of carbon nanotubes (CNTs) within Epoxy Networks on their ability to Release

Keywords: Carbon nanotube, epoxy networks, releasing

Carbon nanotubes (CNT) reinforced nanocomposites represent an unique opportunity in terms of designing advanced materials with mechanical reinforcement and electrical and thermal conductivities improvements. However, toxic effects of these composites on human health still have been studied and very soonly some regulations on CNT and on composites based on CNT will be enacted. That is why CNT releasing during nanocomposites lifecycle must be controlled. As the releasing depends on the interfacial strength that is stronger between CNT and polymer compared to CNTs in a CNTs agglomerate, two dispersion states, one poorly dispersed versus another well dispersed are generated and finely described. So the main aim of this study is to check if CNT dispersion state has an influence on the CNT potential releasing in the nanocomposite. To well tailor and characterize the CNT dispersion state in the polymer matrix, the electronic microscopies (SEM and TEM) but also the rheological analysis are carried out to well identify if CNTs are isolated, in bundles or in agglomerates. When the dispersion state is known and controlled, its influence on the polymerization kinetic and on mechanical properties is discussed. It appears clearly that in the case of good dispersion state, strong interfaces are generated linking the isolated nanotubes with the polymer whereas the CNT cohesion in an agglomerate seems much more weak and it does not provide any improvement to the polymer matrix. Raman spectroscopy is relevant to analyze the interfacial properties and allows the relationship with the releasing ability of nanocomposites, i.e.

CNTs poorly dispersed in the matrix are more readily released when compared to well dispersed nanocomposites. The tribological tests confirm from released particles granulometry and observations that a CNT dispersion state sufficiently achieved in the nanocomposite avoids single CNT releasing under those solicitations.

Introduction

Carbon nanotubes (CNTs) always have been very attractive nanofillers because of their remarkable properties. CNTs exhibit an extraordinary large specific surface area depending on diameter combined to an exceptionally high stiffness and electrical conductivity [START_REF] Iijima | Helical microtubules of graphitic carbon[END_REF]. As a result, CNTs have a real potential for processing the polymer composites and for getting mechanical reinforcement but also thermal and electrical conductivities improvement while making the materials more lightweight [START_REF] Gojny | Influence of different carbon nanotubes on the mechanical properties of epoxy matrix composites-A comparative study[END_REF][START_REF] Ma | Dispersion and functionalization of carbon nanotubes for polymer-based nanocomposites: a review[END_REF][START_REF] Song | Influence of dispersion states of carbon nanotubes on physical properties of epoxy nanocomposites[END_REF]. However the toxicological effects of CNTs on human health are still under study regarding morphological similarities with asbestos and other small fibers [5-6-7]. A release of nanotubes can occur not only during the CNTs production but also during the use of manufactured nanocomposites [START_REF] Köhler | Studying the potential release of carbon nanotubes throughout the application life cycle[END_REF]. Tailoring the releasing of isolated CNT during the nanocomposites lifecycle and especially during the different steps of handling (like friction, mechanical abrasion or breakage) is strongly required for a responsible development of these NTC nanofilled polymers materials [START_REF] Nowacka | Potential release scenarios for carbon nanotubes used in composites[END_REF][START_REF] Petersen | Potential Release Pathways, Environmental Fate, And Ecological Risks of Carbon Nanotubes[END_REF]. Up to now, the risk caused by the exposure to CNTs has been hard to evaluate by lacking of data about the impacts on human health and the environment. Precautionary measures have been recommended to protect the workers, users and the environment. The potential release of CNTs from composite materials as a result of cutting, drilling, sanding, grinding and UV-light weathering has been investigated [START_REF] Petersen | Methods to assess the impact of UV irradiation on the surface chemistry and structure of multiwall carbon nanotube epoxy nanocomposites[END_REF]. Most of them reported the particles generated were predominantly micron sized with protruding CNTs but generally without isolated NTCs [START_REF] Kovochich | Review of techniques and studies characterizing the release of carabon nanotubes from nanocomposites : Implications for exposure and human health risk assessment[END_REF][START_REF] Golanski | Release-ability of nanofillers from different nanomaterials (toward the acceptability of nanoproduct)[END_REF][START_REF] Ogura | Kishimoto Potential release of carbon nanotubes from their composites during grinding[END_REF]. Only Schlagenhauf et al showed the release of freestanding individual CNTs from CNTs-embedded nanocomposites in addition of abraded particles from the epoxy matrix and agglomerates of CNTs during an abrasion process [START_REF] Schlagenhauf | Release of carbon nanotubes from an epoxy-based nanocomposite during an abrasion process[END_REF]. Nanocomposites are processed by a wide variety of mixing protocols and include a range of formulations that can impact how CNTs are integrated and encapsulated within the composite. Kohler et al showed that the likelihood and form of release of nanotubes were determined by the way CNTs are incorporated into the material [START_REF] Köhler | Studying the potential release of carbon nanotubes throughout the application life cycle[END_REF]. In order to understand the toxicological potential of handling CNT-containing composites, characterizing and quantifying the amount of free CNTs versus matrix-bound ones is relevant. A good dispersion state of the nanotubes in the matrix is required to increase the composite performances but what about the impact of this dispersion state on the CNT releasing process in the environment ? Interactions between a CNT and the surrounding polymer matrix and between CNTs in nanotubes agglomerates are significantly different. Several literature works review the different methods to evaluate and measure the interfacial stress, i.e. IFSS, between a CNT and a polymer matrix and between two CNTs. Even if very different values of IFSS are reported depending on the type of measurement approach, computational simulations versus experimental microdebonding tests, these ones are high are due to strong physical and chemical bonds at the CNT/polymer interface [START_REF] Barber | Measurement of carbon nanotube--polymer interfacial strength[END_REF][START_REF] Frankland | Molecular simulation of the influence of chemical cross-links on the shear strength of carbon nanotube-polymer interfaces[END_REF][START_REF] Xu | Mechanical properties and interfacial characteristics of carbon-nanotube-reinforced epoxy thin films[END_REF][START_REF] Wagner | Nanotube-polymer adhesion: a mechanics approach[END_REF][START_REF] Duncan | Measurement of the critical aspect ratio and interfacial shear strength in MWNT/polymer composites[END_REF].

An average value of 100 MPa can be kept in mind against only 1 MPa for the interfacial stresses existing between two CNTs side by side (such as encountered in CNT agglomerates) which corresponds to the stress necessary to overcome Van der Waals interactions [START_REF] Kelly | Physics of graphite[END_REF][START_REF] Suhr | Viscoelasticity in carbon nanotube composites[END_REF][START_REF] Yu | Controlled Sliding and Pullout of Nested Shells in Individual Multiwalled Carbon Nanotubes[END_REF]. As a result, this study consisted in i) generating different dispersion states of CNT within an epoxy matrix from masterbatch in order to get either individually dispersed CNT or CNT agglomerates and ii) finely characterizing those dispersion states thanks to different multiscale approaches in order to well understand the impact of the nanocomposite morphology on its ability to release particules during its lifetime.

Materials and Methods

2.1-Materials

The epoxy prepolymer is the diglycidyl ether of Bisphenol A LY556 from Huntsman Co. (DGEBA, n = 0.15). The hardener chosen the diaminodiphenylsulfone denoted as DDS from Aldrich is added with a stoichiometric ratio aminohydrogen to epoxy equal to 1. This hardener leads to a network with a high Tg equal to 220°C after curing 2 hours at 150°C and 3 hours at 220°C. The CNT used in this study are multiwall carbon nanotubes, MWNT, provided by Arkema under the form of a DGEBAbased masterbatch (25 wt% MWNT). The CNTs have a mean outer diameter of 12 nm and a mean inner diameter of 5 nm. Their length is from 100 nm to several µm (average length of about 800 nm).

2.2-Nanocomposite processing

In order to get the proper weight fraction of CNT in the nanocomposite, the masterbatch is diluted in the DGEBA prepolymer to get 1 wt% in the DGEBA prepolymer leading to 0.7 wt% of CNT in the final nanocomposite. Different processing conditions have been applied to generate varying dispersion states, i.e. from large MWNT agglomerates, as in the masterbatch before dispersion, to nearly individually dispersed nanotubes. Two tools with different shearing rates have been compared : i) a rotary blade Turbotest Rayneri (with a speed of rotation set at 2000 rpm) and ii) a 3 roll-mill Exakt 80E.

2.3-Characterization of the dispersion state

2.3.1.In DGEBA prepolymer-CNT suspensions

Investigating the rheological properties of nanofilled suspensions is known to be a powerful method to understand the nanofillers organization into a liquid medium such as DGEBA prepolymer.

Frequency sweep tests were performed from 100 to 0.1 rad/s using an AR1000 (TA) rheometer with a cone-plate geometry in the linear viscoelastic region of DGEBA/MWNT suspensions.

In nanocomposites at the solid state

The morphology of nanocomposites, i.e. cured systems, is observed by Scanning Electronic Microscopy (SEM) using a Philips XL 20 microscope and by Transmission Electronic Microscopy (TEM) using a 1200EX Jeol microscope. Specimens for TEM observations have been prepared using ultramicrotomy at room temperature.

2.4-Cure kinetics

2.4.1.Epoxy conversion rate and activation energy of reaction

In order to determine if CNTs have an influence on the epoxy conversion rate during the polymerization of epoxy-amine reactive systems, cure kinetics were carried out during 2 hours at 150°C (first step of the curing cycle). The extent of reaction for given times was calculated from the exothermal peak of residual reaction of polycondensation, ΔHr (J/g), compared to the total heat of reaction, ΔHt (J/g). According to the Kissinger method, the activation energy of the condensation reaction can be deduced from differential scanning calorimetry experiments, DSC, at different heating rates, i.e. 2.5, 5, 10 and 15 K.min -1 , from 30 to 350°C in order to get a complete reaction.

Calorimetric measurements have been performed on the Q20 calorimeter from TA Instruments.

Gelation time determination

Multifrequency time sweep tests were performed on ARES with a plate/plate geometry to determine gel times of the epoxy/amine systems with or without CNT from the isofrequency crossover point of the tan δ curves during an isothermal curing (at 150°C).

2.5-Nanocomposite characterization

Mechanical behavior

Flexural modulus, stress at break and toughness of nanocomposites were measured and discussed as a function of dispersion states. The flexural properties have been measured at 25°C on a 2/M device from MTS. Flexural Young's modulus and stress at break have been determined considering three points bending tests on machined samples (75 x 15 x 3 mm 3 ). The fracture toughness has been evaluated at 25°C in order to determine critical stress intensity factor KIc on single edge notched samples (SEN) in three points bending mode.

Microscopy and Raman spectroscopy to analyze the stress transfer

Nanocomposites samples were fractured into liquid nitrogen for SEM (Scanning Electronic Microscopy) fracture analysis (Philips XL 20). Plastic deformation at crack tip was also analysed using TEM (Transmission Electronic Microscopy) on a 1200EX Jeol.

Micro-Raman measurements were realized on the nanocomposites having different dispersion states under compression. The spectra for the CNT were recorded using a Jobin Yvon Raman system operating at 514 nm.

2.6-Characterization of particles release

A tribological process represented in Figure 1 and already described in a previous paper was developed in order to analyze airborne particles which were released by the nanocomposite under mechanical wear [START_REF] Golanski | Release-ability of nanofillers from different nanomaterials (toward the acceptability of nanoproduct)[END_REF]. The nanomaterial is disposed in a clean glove box using a light air flow filtered with an H14 HEPA filter leading to a very low background level of nanoparticles (5-10 cm -3 ). In order to simulate two different mechanical wear, a circular brush in steel is used to simulate abrasion and a steel rake to simulate scratching. The normal force applied on the sample by the tool is controlled thanks to an analytical balance. A load of 50 g is applied on each sample with a speed of 1500 rotation per minute during 30 seconds of solicitation followed by 30 seconds of "rest". The 'airborne particles characterization part is composed of a particles collection device on a TEM grid (VTT filtration device), an OPC (optical particle counter) Grimm 1.109 which is spectrometer giving airborne particles granulometric measurement from 250 nm to 35 µm (optical diameter) and an ELPI (electrical low pressure impactor) which gives the airborne particles granulometric measurement from 7 nm to 10 µm (aerodynamic diameter). In this work only the ELPI granulometric results will be presented since the OPC granulometric results were not sensitive enough to provide information at the nanometric scale (lower limit at 250 nm is not appropriate to detect isolated CNT).

3-Results and discussion

3.1-Structuration of carbon nanotubes

Generating different dispersion states of CNT in a controlled way remains challenging. Tuning the processing parameters was the chosen pathway to generate extreme morphologies, i.e. aggregated vs dispersed. The organization of CNT within the epoxy material was studied : i) before curing, i.e. as the CNT-based masterbatch is dispersed in the DGEBA prepolymer and ii) after curing, i.e. in the solid state nanocomposites.

Structuration of CNT in the DGEBA-CNT suspensions

The rheology of suspensions is very sensitive to the CNT dispersion state (individually dispersed vs aggregated) [START_REF] Advani | Rheology of multiwall carbon nanotube suspensions[END_REF][START_REF] Ma | The rheological modelling of carbon nanotube (CNT) suspensions in steady shear flows[END_REF][START_REF] Li | Correlations between Percolation Threshold, Dispersion State, and Aspect Ratio of Carbon Nanotubes[END_REF]. In the case of a very good dispersion, the formation of a percolated network can be highlighted from the appearance of a plateau for storage modulus G' at low frequencies in the linear domain which is a signature of an elastic response at a given volume fraction. The changes in the G' dependence with frequency in the low frequencies region, i.e. the exponent of the power law f(w) decreases from 2 (liquid-like) to 0 (gel-like), could be the way to determine clearly the percolation threshold for different volume fractions of CNT. Thus, the rheological signatures of different suspensions (prepared with the same CNT weight percentage) can be compared in terms of dispersion state [START_REF] Advani | Rheology of multiwall carbon nanotube suspensions[END_REF][START_REF] Ma | The rheological modelling of carbon nanotube (CNT) suspensions in steady shear flows[END_REF][START_REF] Li | Correlations between Percolation Threshold, Dispersion State, and Aspect Ratio of Carbon Nanotubes[END_REF]. Different dispersion states of 1 wt% CNT in DGEBA prepolymer while varying dispersion tools were studied from 100 to 0.1 rad/s in dynamic mode (Figure 2). As expected the increase of the mixing time combined with the use of a high shear tool enhances the dispersion efficiency in the prepolymer. The rheological behavior of the initial DGEBA prepolymer displays a Newtonian behavior (storage modulus G' proportional to ω²) whereas a solid-like behavior is evidenced as the prepolymer is filled with 1 wt% of CNT, i.e. G' becomes independent on the frequency at low frequencies. This evolution of G' at low frequencies is more pronounced as the 3 roll-mill calander is used. The storage modulus values at the lowest frequency, i.e. at 0.1 rad/s, as well as its dependence at low frequencies according to the power law, G' ∼ ω ∆ , are reported in Figures 2 and3 as a function of the dispersion conditions. The same conclusions can be made from loss modulus (G'') dependence in frequency.

Dispersion state characterization in the final nanocomposite

The final morphology at the solid state of nanocomposites results from the freezing of the dispersion state existing on uncured DGEBA/MWNT dispersion and potential changes of the dispersion state during the 3D growing of the epoxy network during cure. Actually, the polymerization process may stabilize the dispersion state obtained before adding the amine hardener or in the opposite may modify the morphology. In order to analyze the CNT dispersion into the thermoset matrix, several methods could be used such as electron microscopies SEM and TEM which could allow to get precise information like aggregate size distributions, interparticles distances etc. For the nanocomposites having the poorest dispersion state, SEM microscopy reveals the presence of aggregates and even of agglomerates which have mean sizes around a few tens of micrometers ( As reported in a previous paper dedicated to nanoclay-based nanocomposite morphologies, the dispersion needs to be defined at different scales : from large micrometer size aggregates or agglomerates to nanoscale which is the scale of the individual nanofillers [START_REF] Vermogen | Evaluation of the structure and dispersion in polymer-layered silicate nanocomposites[END_REF]. TEM (Transmission Electronic Microscopy) is a powerful method to achieve nanometric scale characterizations. These discriminating parameters which are relevant to quantify the dispersion states highlight that the Rayneri mixer is very relevant to improve the dispersion for short times (DD : +100 % and µagglo% : -50 % after 5 hours) but does not enhance drastically the dispersion state for very longer times of mixing. Residual micron-size agglomerates are very difficult to break and only the use of 3

roll-mill calander allows to decrease µagglo% significantly. By the combination of these analysis methods at different scales, extreme dispersion states have been generated and well characterized (with a DD from 5 to 75 and µagglo% from 70 % to 2 %).

3.2-Impact of Carbon nanotubes dispersion state on the crosslinking kinetic

As montmorillonite [START_REF] Pluart | H Epoxy /Montmorillonite nanocomposites : influence of organophilic treatment on reactivity, morphology and fracture properties[END_REF][START_REF] Wang | Clay-Polymer Nanocomposites Formed from Acidic Derivatives of Montmorillonite and an Epoxy Resin[END_REF] or nanosilica [START_REF] Altmann | The effects of silica fillers on the gelation and vitrification of highly filled epoxy-amine thermosets[END_REF], carbon nanotubes are well known to have a catalytic effect on the epoxy-amino condensation reactions by the presence of hydroxyl groups on external walls [START_REF] Wu | Calorimetric study of the effect of carbon fillers on the curing of epoxy[END_REF][START_REF] Tao | X Effects of carbon nanotube fillers on the curing processes of epoxy resin-based composites[END_REF][START_REF] Xie | Cure kinetics of carbon nanotube/tetrafunctional epoxy nanocomposites by isothermal differential scanning calorimetry[END_REF]. As expected, this effect is more pronounced when the volume fraction of CNT increases or when this volume fraction is well dispersed. Let's consider the effects of CNT on the condensation kinetics performed on the reactive system based on DGEBA/DDS/MWNT with 0.7 % of MWNT by weight.

3.2.1.Epoxy conversion rate

Thanks to isothermal DSC measurements, the extent of reaction and the reaction rate were calculated (Figure 6) as proposed by Xie works [START_REF] Xie | Cure kinetics of carbon nanotube/tetrafunctional epoxy nanocomposites by isothermal differential scanning calorimetry[END_REF].

Hr Ht

Ht ∆ + ∆ ∆ = α With :
α is the advance of reaction (epoxy conversion)

ΔHt is the heat evolved up to a given time (J.g -1 )

ΔHt + ΔHr is the total heat of reaction obtained from the isothermal and dynamic scan (J.g -1 )

After 2 hours at 150°C (first step of the curing cycle), the reaction kinetics for systems prepared with the 3 roll-mill calander pre-dispersion step is faster compared to the unfilled reactive one. On the other hand, the filled mixing without any dispersion step displays a reactivity quite similar to the neat epoxy one and even slightly lower. A clear difference in the cure kinetics is highlighted during the first step of the crosslinking reaction. The extent of reaction clearly depends of the MWNT dispersion state. After 2 hours at 150°C, α is higher than 80 % when the 3 roll-mill was used and is equal to 71 % in the case of a poor dispersion state. In this case α is even lower than the extent of reaction of the unfilled reactive system. An agglomerate-based morphology induces a downturn of the reaction kinetics whereas a good dispersion state has a catalytic effect on the crosslinking reaction.

It could be envisioned that some epoxy prepolymer remains confined in the MWNT agglomerates leading to a local modification of the epoxy/amine stoichiometric ratio, i.e. impacting reaction kinetics.

Activation energy of reaction in the presence of CNT

The Kissinger method [START_REF] Kissinger | Reaction Kinetics in Differential Thermal Analysis[END_REF] allows to determine the activation energy required for crosslinking reaction [START_REF] Hayaty | Cure kinetics of a glass/epoxy prepreg by dynamic differential scanning calorimetry[END_REF] from differential scanning calorimetry analyses (DSC) performed at different heating rates.

R Ea

T d T q d p p - = ) / 1 ( ²) / (ln( (4)
With :

q is the heating rate (K.min -1 )

R is the constant of gases

Tp is the temperature at the maximum of the exothermic peak (K)

Ea is the reaction activation energy (J.mol -1 )

Figure 7 gives the DSC traces evidencing the exothermic peaks of the DGEBA/DDS systems for the different heating rates.

Figure 7. DSC analyses on reactive epoxy/amine (E/A) systems for different heating rates

The polymerization activation energies as a function of dispersion degrees of NTC within the epoxy/amine reactive system are reported in Table 2. The activation energy of the reactive system is lower in presence of MWNT whatever the dispersion state compared with the neat epoxy-amine matrix. This decrease of the activation energy is enhanced by a better quality of dispersion (up to 9% of difference between the dispersion obtained with the 3 roll-mill calander and the system without pre-dispersion).

Gelation time

Gelation time determination provides information about the catalytic effect of carbon nanoparticles and their influence on the changes in physical behavior of the reactive system. The formation of a Figure 8 shows that the finest the dispersion, the shorter the gel time. On the opposite, an agglomerated state leads to a longer gel time than one measured on the unfilled reactive system. This observation can be explained once again by the confinement of a small quantity of prepolymer within MWNT agglomerates that does not participate to the growing of the network, i.e. local changes of the stoichiometric ratio are generated in and close to the CNT aggregates. As a conclusion, it appears that the MWNT dispersion state has a pronounced influence on the epoxyamine system polymerization. The catalytic effect of CNT is promoted by a good dispersion state whereas a poor dispersion induces a downturn of the crosslinking reaction.

3.3-Impact of CNT dispersion on the mechanical behavior of nanocomposites

The nanocomposites having a well-defined dispersion state were selected to study their mechanical behavior. Indirect and direct analyses were carried out in order to understand the stress transfer from matrix to CNT and between CNT in the aggregates or agglomerates.

Macroscopic analyses

3 point bending tests carried out on nanocomposites are reported in Figure 9 and 10. As reported in the literature [START_REF] Kovochich | Review of techniques and studies characterizing the release of carabon nanotubes from nanocomposites : Implications for exposure and human health risk assessment[END_REF][START_REF] Wagner | Nanotube-polymer adhesion: a mechanics approach[END_REF][START_REF] Kelly | Physics of graphite[END_REF][START_REF] Paiva | Y Mechanical and morphological characterization of polymer-carbon nanocomposites from functionalized carbon nanotubes[END_REF], the mechanical properties are very sensitive to the dispersion state. Thus a poor CNT dispersion state (DD = 4.9) leads to a decrease of more than 20 % for the Young's modulus whereas the best dispersion state (DD = 74.5) leads to an increase of 10% compared to the neat epoxy network one. The stress at break is also very sensitive to the dispersion state since a decrease of 40 % with the poorest dispersion state and an increase of 35 % in the case of a good dispersion state are reported. The increase of the Young's modulus and of the flexural stress at break associated to a good dispersion state of CNT can be explained by an efficient stress transfer at the CNT/polymer interface as the CNT surfaces are fully in interaction with the epoxy network. On the other hand, the significant decreases of these properties as the polymer matrix is filled with micronic MWNT agglomerates can be explained by the poor cohesion of agglomerates that is only governed by the Van der Waals interactions between MWNT. The fracture toughness of nanocomposites ( Figure 11) is also very sensitive to the dispersion state. KIC decreases of 50 % with a poor dispersion state and increases up to 22% for the best dispersion state (reached with 3 roll-mill) compared to the neat epoxy network. A good dispersion state is required to take benefit from multiwall carbon nanotubes on the mechanical properties of nanocomposites. Otherwise, an agglomerated morphology can strongly reduce the mechanical properties of neat epoxy network. The decrease of mechanical properties of nanocomposites compared to the neat network ones can be directly linked to the non-polymerized DGEBA prepolymer confined in the CNT agglomerates and/or to nonstoichiometric zones, as reported previously from reaction kinetics analyses. This reinforcing effect of CNT can be explained by the "crack-front pinning" process at the crack tip [START_REF] Paiva | Y Mechanical and morphological characterization of polymer-carbon nanocomposites from functionalized carbon nanotubes[END_REF]. In such a process, the crack-front length increases as the crack meets rigid inclusions. As a consequence, this effect is magnified as the number of obstacles increases, i.e. in the case of a good dispersion state. If the MWNT are agglomerated, the number of objects decreases and their cohesion being weak (Van der Waals interactions), secondary cracks are generated.

As a conclusion, it appears that as a small amount of MWNT is well dispersed in the matrix, the mechanical properties are significantly improved compared to the unfilled polymer but they can be seriously affected if the dispersion state is not good enough. In the first case, these are the CNT/polymer interactions which govern the behavior of well-dispersed CNT-based nanocomposites whereas in the second case, this is the level of interaction between CNT that governs the behavior of poorly dispersed CNT-based nanocomposites.

Analysis of interfacial interactions CNT/CNT vs CNT/matrix

Interfacial interactions with CNT can be also analyzed more locally from scanning electronic microscopy (SEM) and transmission electronic microscopy (TEM) on fracture surfaces. From those electron microscopy observations, the interactions between CNT/CNT (in agglomerates)

and CNT/polymer appear clearly different, i.e. isolated CNT will require more energy to be extracted from surrounding polymer than ones confined in an agglomerate.

Another experimental method to measure the level of the interaction between a CNT and the polymer matrix is the micro-Raman spectroscopy. Previous works showed that the Raman spectra of carbon fibers were sensitive to the stresses induced in the fibers along their axis [START_REF] Galiotis | Strain dependences of the first-and second-order Raman spectra of carbon fibres[END_REF][START_REF] Lachman | Raman Response of Carbon Nanotube/PVA Fibers under Strain[END_REF]. Thus the stress transfer between polymer matrix and CNT can be assessed by considering the shift of G' band (2,700 cm -1 ) in the nanocomposite submitted to a compression or traction load ( Figure 14). Nanocomposites with very different dispersion states were placed under compression (1 % and 2 % of strain) and the G' band location was followed as a function of the MWNT dispersion state (as shown in Figure 15). In case of well-dispersed CNT based nanocomposite under compression, a slight shift of the G' band is observed which means that MWNTs sustain compression thanks to a good stress transfer. In the case of nanocomposite with poorly dispersed CNT, no shift of the G' band is observed whatever the strain applied to the nanocomposite. MWNTs in agglomerates probably tend to be reorganized under the matrix deformation and only few CNTs sustain compression. Nevertheless, let's note that the G' band location in both extreme dispersions is very different even without applying compression. This can be explained by the polymer shrinking during the crosslinking applying an additional compression on individually dispersed MWNT.

3.4-Impact of CNT dispersion on the potential release ability

Let's consider now the effect of the dispersion of fillers on the potential release ability during the nanocomposite life. The analysis of released particles investigated after simulating the wear undertaken by the material by using two tools either a brush or a rake is reported under form of size distribution in Figure 16 and 17. A releasing of ultrathin particles can be noted with a maximum diameter lower than 100 nm. The tool used has a non negligible influence on the releasing. The rake allows to release a higher concentration of nanoparticles (22600 cm -3 ) with a much lower diameter (7-28 nm) in comparison with the brush (6000 cm -3 ). This may be explained by the friction coefficient that is higher with the rake than with the brush applying an higher equivalent tensile stress from the Von Mises yield criterion. The hardness of material has also an influence on the stress applied by the tool on the sample and consequently on the releasing. The stiffness and the hardness of the glassy epoxy networks is prone to generate more particles with lower size (Wohlleben et al [START_REF] Wohlleben | On the Lifecycle of Nanocomposites: Comparing Released Fragments and their In-Vivo Hazards from Three Release Mechanisms and Four Nanocomposites[END_REF]).

4-Conclusion

This paper aims at generating different dispersion states but in a controlled way of carbon nanotubes into an epoxy-amine network. Two extreme morphologies are designed by varying the dispersion tools, one with micron size agglomerates of aggregated carbon nanotubes characterized by a high micron size agglomerates percentage and another one with individual carbon nanotubes embedded into epoxy matrix, characterized by a high dispersion degree (DD). The dispersion state of carbon nanotubes has an outstanding effect on the network build up : the gelation is catalyzed and the conversion is enhanced when the carbon nanotubes are well dispersed. In the other case, epoxy prepolymer can be confined into the carbon nanotubes aggregates locally modifying the epoxyamine stoichiometry preventing or delaying a total conversion. A well dispersed morphology is the key architecture to reinforce the mechanical resistance of nanocomposite. Well embedded in the surrounding polymer, the carbon nanotubes ensure a good load transfer towards the matrix. On the contrary, a poor dispersion state with micron size CNT agglomerates has quite no influence on the crack direction. If the crack front founds agglomerate on its path it just goes through considering the weak agglomerate cohesion. There is no strong load transfer between CNT in agglomerate and the mechanical properties of composites containing micronic agglomerates of MWNT can be much lower than the initial properties of the unfilled polymer. After showing the huge influence of the NTC dispersion on the polymerization mechanisms at macromolecular scale and on the macroscopic behavior of nanocomposites, these works established a link between the nanocomposites morphology and the potential release of carbon nanotubes during the nanocomposite life. We checked experimentally that less energy is required to extract a CNT from an agglomerate than to extract a CNT individually dispersed in the polymer matrix.

Figure 1 .

 1 Figure 1. Representation of the wear simulation device in order to analyze airborne released particles

Figure 2 .

 2 Figure 2. Evolution of storage modulus (G') as a function of frequency, 0.1 % of dynamic strain at 25°C, for DGEBA/MWNT suspensions (1 wt%) with different mixing conditions

Figure 4 -

 4 Figure 4-right). The comparison between nanocomposites having different states of dispersion ( Figure 4) shows clearly the efficiency of the 3 roll-mill calander to avoid micron-size agglomerates that are observed after a dispersion of 5 hours with Rayneri rotary blade which is much less effective.

Figure 4 .

 4 Figure 4. SEM micrograph of unfilled epoxy network (left), epoxy/DDS/MWNT nanocomposite (0.7 wt%) after a 5 h dispersion with Rayneri (right) and epoxy/DDS/MWNT nanocomposite (0.7 wt%) after a dispersion with 3 roll-mill calander (below)

Figure 5 .

 5 Figure 5 . TEM micrographs of epoxy/DDS/MWNT nanocomposites (0.7 wt%) after a 5h dispersion with Rayneri (left) and after a dispersion with the 3 roll-mill calander (right)

Figure 6 .

 6 Figure 6 . Extent of reaction (α) vs. time for a DGEBA DDS reactive system : without MWNT (continuous line) and filled with MWNT (0.7% in weight) with a different dispersion state (slashed line for the poor dispersion state and dotted line for the good dispersion state) at 150°C

  chemical three-dimensional network leads to the appearance of an elastic response during polymerization. The gelation time corresponds to the transition from a liquid state to a gel, experimentally measured by chemio-rheological analyses from the crossover of the loss factor tanδ vs. time during a multi frequency time sweep. Gel times are measured at 150°C on the reactive systems based on DGEBA/DDS filled with 0.7 % of MWNT by weight tuned as a function of dispersion state.

Figure 8 .

 8 Figure 8. Gel time evolution as a function of the mixing conditions for the reactive system epoxy/amine/MWNT (0.7 wt%) at 150°C

Figure 9 .Figure 10 .

 910 Figure 9. Young's modulus as a function of MWNT dispersion degree in the DGEBA/DDS/MWNT nanocomposites (0.7 wt%)

Figure 11 .

 11 Figure 11. Fracture toughness (Critical stress intensity factor KIC) as a function of the MWNT dispersion degree in the DGEBA/DDS/MWNT nanocomposites (0.7 wt%)

Figure 12 .

 12 Figure 12. SEM Analysis on fractured surfaces of DGEBA/DDS/MWNT nanocomposites (0.7 wt%) with a poor dispersion state, DD = 4.9 (left) and with a good dispersion state, DD = 74.5 (right)

Figure 13 .

 13 Figure 13. TEM micrographs on the fracture surfaces of DGEBA/DDS/MWNT nanocomposites (0.7 wt%).above) after dispersion using a 3 roll-mill calander, below) without pre-dispersion

Figure 14 .

 14 Figure 14. Raman spectrum of a MWNT-based agglomerate in DGEBA/DDS/MWNT nanocomposite (0.7 wt%, DD = 4.9)

Figure 15 .

 15 Figure 15. G' band shift as a function of the compression strain of DGEBA/DDS/MWNT nanocomposite (0.7 wt%) and of two different dispersion states (DD=4.9 and DD=74.5)

Figure 16 .Figure 17 .

 1617 Figure 16. Granulometric distribution of released particles during the use of the steel brush on DGEBA/DDS samples (ELPI data)

Figure 18 .

 18 Figure 18. Released Particule after wear of DGEBA/DDS/MWNT (0.7 wt%) nanocomposite with a bad dispersion, DD = 4,9.

Table 1 .

 1 No pre-dispersion means have been used to break the agglomerates before adding the amino hardener.

Table 1 .

 1 Dispersion

degree (DD) and micron size agglomerates percentage (µagglo%) for the different DGEBA/DDS/MWNT nanocomposites (0.7 wt%) processed in different conditions (standard deviation in brackets)

No pre-dispersion 5 h Rayneri 20 h Rayneri 160 h Rayneri 240 h Rayneri 3 roll-mill

	DD	4.9 (1.4)	10.6 (3)	28 (10.3)	42 (18.7)	44.9 (13.1) 74.5 (15.8)
	µagglo%	73.3 (32.3)	36.9 (20.8) 16.1 (11.5)	14 (16.9)	13.4 (2.3)	2.2 (3.6)

Table 2 -

 2 Polymerization activation energies of the DGEBA/DDS reactive systems filled with MWNT (0.7 % by weight) for different dispersion degrees (DD)

	DD	Ea (kJ.mol -1 )
	Neat	71.9
	4.9	71.1
	10.3	70.7
	28.35	69.4
	47.6	69.4
	74	65.9


Cécile Robert

Doing politics and pretending not to. The Commission's role in distributing aid to Eastern Europe

INTRODUCTION

Academic research on the European Union has undergone a series of evolutions, one of which is the increasing attention given to the European administration. This concern is twofold : on one hand, the growing number of studies on EU policies has chiefly led to acknowledging the decisive role played by the Commission in setting the agenda and elaborating European public policy making [START_REF] Christiansen | A maturing bureaucracy ? The role of the Commission in the policy process[END_REF][START_REF] Cini | The European Commission. Leadership, organisation and culture in the EU administration[END_REF][START_REF] Cram | The Commission[END_REF][START_REF] Fouilleux | Idées, institutions et dynamiques du changement de politique publique. Les transformations de la politique agricole commune[END_REF][START_REF] Nugent | The leadership capacity of the European Commission[END_REF][START_REF] Peters | Euro-politics. Institutions and Policy-making in the 'New' European Community[END_REF][START_REF] Smith | L'intégration communautaire face au territoire. Les fonds structurels dans les zones rurales en France, en Espagne[END_REF][START_REF] Jourdain | La France et la politique de recherche communautaire[END_REF] ; on the other hand, observations made lately with a more anthropological perspective on European institutions looking at the new meanings ascribed to notions of administration and policy in Brussels, revealed their specificity in comparison to existing administrative models [START_REF] Abélès | Approche anthropologique de la Commission, Bruxelles: Rapport rédigé pour le compte de la cellule de prospective Abélès M[END_REF][START_REF] Abélès | La Commission européenne, du compromis culturel à la culture du compromis[END_REF][START_REF] Bellier | La question technocratique : de l'invention d'une figure aux transformations de l'action publique[END_REF].

Denying the political dimension of the Commission's workobserved in the strategies it implements in order to enforce both its own understanding of issues to be considered and solutions it propoundsis thus out of question. Yet few works draw a parallel between this political activity and the distinct position held by the European commission in the European institutional system, but to stress the fact that here again, texts are betrayed by realities. The fact that it may be simultaneously deprived of democratic legitimacy while dependent upon political circumstances allowing for its work and investment to translate into matter, does not prevent the commission from exerting a decisive influence upon the European Union's decision making process and the definition of broad orientations in European construction. In another way, anthropological studies have highlighted the constraints of its institutional position, particularly its incapacity to claim political legitimacy, and shown how these constraints are apprehended in the European Commission. But they have not systematically questioned the effects of these representations on the daily practices of Commission officials and the way they build public action.

Standing precisely at the crossroad of both approaches, we shall attempt to further their reciprocal results and interrogations. Once admitted that the classical opposition between administration and politics cannot in a precise manner account for the particular roles of the European Commission and its institutional partners (Parliament, Council), trying and understanding the resilience of this opposition in the actors' discourses and perceptions as well as its consequences on the strategies and behaviours they adopt, appears momentousi.

In this view, this paper shall aim at analysing how the Commission relates to the political dimension of its action, starting from two complementary hypothesis. (1) The Commission constantly seeks out to play a political role, yet without accessing to the political legitimacy required to endorse such a role. It is therefore involved in acts seeking to legitimise the power it handles, and simultaneously to mask the political dimension of its activity. ( 2 This paper addresses and pictures out both hypothesis in two parts. First, observing and describing the different dimensions taken by the European Commission's relationship to politics, as it manifests itself in the discourse the institution and its members produce about themselves and about their partners in the EU ii . Secondly, dwelling on the European policy towards Central and Eastern European countries as an illustration [START_REF] Robert | La fabrique de l'action publique communautaire. Le programme PHARE 1989-1998, enjeux et usages d'une politique européenne incertaine[END_REF], we shall endeavour to reveal to what an extent and how the Commission's relationship to politics inspires and structures the daily practices of civil servants and leads the European Commission to selecting particular types of policy making.

I. THE EUROPEAN COMMISSION: ITS RELATIONSHIP (S) TO POLITICS

The European Commission's relationship to politics is particular and manifold. More precisely, we shall here support an hypothesis which considers the activity of the Commission as political in nature, although this institution and actors who belong to it are striving, for different reasons, to camouflage this reality. This paper hereafter examines two distinct and complementary aspects of the effort made to shrug off politics: how civil servants construct their professional identity, constituted from the claim to specific dispositions, in opposition to the characteristics supposedly attributed to the political counterparts of the Commission -MEPs, Council personnel and members; what modes of legitimisation are favoured by the European administration in order to justify and account for its action in the European political system.

With regards to researches carried out on the EU in the past two decades, the Commission's political role and ambition today appear unquestioned and unquestionable [START_REF] Christiansen | Les tensions de la gouvernance européenne : l'administration politisée et la responsabilité multiple de la Commission européenne[END_REF]. In particular, specialists on EU public policy making have established that the European administration often plays a decisive role in the different steps surrounding reforms and new measures, from agenda setting to their implementation, during which its work stretches much further than the mere execution of rules and decisions adopted by the Council [START_REF] Eichener | Social dumping or Innovative regulations ? Processes and Outcomes of European Decision-Making in the Sector of Health and Safety at Work Harmonisation[END_REF]. Besides, numerous studies carried out on the Commission's personnel have shown the growing place and importance taken by political carriers and resources [START_REF] Donnelly | The College of Commissioners and their cabinets[END_REF]Joanna, Smith 2003;Page 1997;Smith 1996[START_REF] Smith | Commissaire européen, un homme politique sans métier ?[END_REF].

In this respect the PHARE programme of technical assistance to the Eastern and Central European States represents a clear example. First of all its genesis owes a great deal to Jacques Delors, chairman of the Commission, who mustered resources found in a whole array of relations built during the course of his political carrier to convince and compel his partners outside and inside the EU to entrust the European administration with western aid to Eastern Europe (Deloche-Gaudez, [START_REF] Lequesne | La Commission européenne entre autonomie et indépendance[END_REF][START_REF] Niemann | The PHARE programme and the concept of spillover : neofunctionalism in the making[END_REF]. Beside revealing this institution's ability to put to the fore its own understanding of issues to be considered, such institutional activism as displayed by the Commission's representatives in the process of the creation of PHARE shows that the Commission, just like the Council and the Parliament, acts according to its own purposes. These objectives more often than not amount to the willingness to expand its own institutional prerogatives (Mak, Guignier in this book): thus, obtaining the management of the PHARE programme was perceived by those who sponsored it, as a means for the European administration to gain access to the so far restricted domain of EU Foreign policy.

Next, the room for manoeuvre left to the Commission by the successive regulations of Councils concerning the PHARE programme, who often merely indicate that PHARE is designed for aiding Eastern states in their effort of economic restructuring iii , a contrario emphasises the creative dimension of the Commission's management. In fact it amounts to permitting the Commission to define itself the priority sectors of Western aid, the nature of this assistance, conditions of its implementation, as well as the personnel in charge of it, i.e. companies and consultants entrusted with running project in the field.

Finally, progressive institutionalisation of the programme throughout the 1990s and the fact that it was clearly identified as one of the main instruments of European policy to applicant states, must both be matched with the Commission's capacity to 'hinge up technical and political patterns', in other words its ability to enlist technical knowledge and aptitudes to produce solutions which also take into account political imperatives and constraints to which public choices are subjected (forward-looking management of conflict between member States, anticipating symbolic consequences of its activity on relationships with Eastern Europe, and/or their electoral impact.) (Robert 2001a).

If the fact that activity of the European Commission is political in nature is now fully acknowledged, another characteristic of this institution is less frequently emphasised by researchers: the discourse the European administration produces about itself and its action tends to systematically lessen, let alone deny, the political dimension of its work, and more generally to institute 'a-politicism' as the raison d'être of this institution and the very basis of its legitimacy.

On the one hand, 'a-politicism' is exhibited as a personal disposition, better presented as a whole set of know-how and behaviours, exclusively belonging to the Commission's civil servants. Interviews carried out with members of the Commission in the course of our research tend to show how, when referring to their professional identity, they make constant allusion to MEPs and Council members. This allows them to claim a whole pack of personal qualitiesneutrality, independence, disinterestedness, scientific knowledge, technical competence, objectivity, ability to think in the long term -set against the 'defaults and weaknesses' of the 'politics'national interest and selfishness, ignorance and remoteness from the field, pretence, inconsistency and incoherence, short term and electoral vision. As illustrated in the following extract, actors of the Commission draw from this contrasted picture to apprehend and lend meaning to the daily tasks of implementing PHARE, and make (Bellier 1999: 47) On the other hand, the claim to 'a-politicism' also translates in the discourse put together by the European Commission to describe its activity and support the relevance of its proposals.

What sets it apart from its institutional partners (Parliament, Councils, Member States) does not apparently dwell in the fact that its activity might come down to purely technical, juridical or administrative tasks, but in the efforts it spends to present it as such, and appear itself as a mere tool to attend social forces and immanent principles. The Commission's effort of repression of the political dimension in its work relates to two types of operations -hereafter artificially differentiated, but most frequently mixed up in reality:

-the concealment of the creative dimension in its work, through denying the extend of interpretative activity and available room for manoeuvre in the implementation of the Council's decisions and European rules; -the obliteration of the 'ideological' dimensioni.e. based upon a reference to values and involving choice makingof its positions and suggestions, by presenting them as dictated by a certain rationale (juridical, technical) and /or compelled by European common interest.

Examples taken in the European policy towards Eastern European countries yet again help to illustrate this disposition. First, the presentation made of the programming activity of the Commission in annual PHARE reports-handed out to the Council, the Parliament and widely distributed in EU public relations officessuggests that European civil servants are mere underlings, and that the Council's declaration and regulations are guidelines precise enough to set the frame and content of their mission. Beside, these reports never hint that certain choices may possibly be made: programming appears as a simple process both well controlled and objectivable because based on seasoned techniques (particularly the so-called 'demanddriven' technique which consists in co-operating with the beneficiary of aid) and relying upon a 'scientific' vision of transition. Thus identification of so-called priority sectors is presented as the twin outcome of preferences expressed by partner States, and as the practical exercise of a whole constituted knowledge on changes felt in these countries. This latter observation reminds of conclusions reached in studies carried out in the case of other European policies, especially in Research [START_REF] Jourdain | La Commission européenne et la construction d'un nouveau modèle d'intervention publique[END_REF] or in programmes meant for South American States (Le [START_REF] Naëlou | Politiques européennes de développement avec les pays du Sud[END_REF] and which all uncover a process of naturalisation of choices made during the implementation of these projects, or constant reference to technical rationality presented as the ultimate and insurmountable justification of decisions made by the Commission.

It falls out of our subject in the present paper to mention every explicative factors behind the Commission's relationship to politics. Suffice to emphasise that it can be recognised as a means for the European administration to solve some of the inner contradictions relating to its institutional position. Resort to law and technical issues to make natural its positioning allows the Commission to justify its prerogatives and proposals without calling upon a political argument, prohibited anyway for want of democratic legitimacy. Qualities of independence, disinterestedness and expertise, as assumed by officials, come to attest of the capacity and legitimacy of those men and women, non elected and belonging to various nationalities, to define where Europe's general interest lies. Furthermore, European civil servants' relationship to politics, their conflicts of legitimacy with Member States manifest common features with representations and behaviours typical of national administration staff, well studied by sociology and political science researchers [START_REF] Duran | Penser l'action publique[END_REF]De Baecque, Quermonne 1982 ;[START_REF] Grémion | Profession : décideurs. Pouvoir des hauts fonctionnaires et réforme de l'Etat[END_REF][START_REF] Suleiman | Les hauts fonctionnaires et la politique[END_REF]). These similarities can perhaps find an explanation in the history of administration and more precisely in the types of legitimacy on which it rests. Yet it may be suggested that such conflicts are of a particular nature in the case of the European administration. Institutional rivalries first gain in acuteness, owing to the larger range of prerogatives handled by the European administration compared to national oneswhile facing more attacks and scepticism. Its legitimacy is therefore more demanding and more fragile. Besides the link made in Brussels between politics and the defence of national interests exacerbates such tensions between administration and politics, and allows European civil servants, rather than national ones, to claim a monopoly in handling "European thinking" and in defining a common European interest. The Commission's exhibited a-politicism, to the extent that it is at the roots of its existence and autonomy as an institution and conditions both affirmation and practice of its authority, coerces this institution into permanently producing evidences of its a-politicism, and leads it to perceive itself as such. If discourses and representations prevailing within the European administration on politics partly fulfil this function, we should point out in the following part of our paper, that these cannot be taken independently from the political practices which produce them. In other words, the Commission's privileged modes of legitimisation determine, at least partly, the shape and content of its action in the European political system.

II.'A-POLITICISM' IN ACTION: RESOURCES AND CONSTRAINTS FOR THE

COMMISSION

Case studies analysed in the second part of this work are drawn from the history of the UE preparation to Eastern enlargement throughout the1990s. Returning to the role played by the Commission in elaborating and the implementing the European policy towards Eastern European countries, we will discuss and illustrate the following hypothesis. At the beginning of the decade, the Commission's repression process concerning the political dimension of its action was activated as a resource, and made possible the whole process of both institutionalising such a policy and of achieving its leadership in the field of public policy making. More recently though, on some issues relating to enlargement, the Commission's apoliticism seemed on the contrary to play as a constraint, to which the Commission couldn't escape, and which significantly limited the scope of available positions and possible actions.

In the 1990s, the Commission apparently particularly well managed to benefit from the perspective of enlargement to re-negotiate its role and position within the political European system. Whereas in 1989 it successfully obtained responsibility for the PHARE programme, thanks to the closer linkage between the EU and Eastern Europe in this decade it goes on to rely on this early position to secure growing autonomy and power particularly with regards to conceiving and enacting European policy in the East (Robert 2001a). Minute examination of the strategies displayed by the Commission during this period shows that these strategies work out precisely on the denial of their political dimension.

First of all, by introducing itself as an expertneutral and independentand never positioning itself as a decider, the European administration succeeded in presenting and pressing forth its understanding of issues pertaining to Eastern and Central European States. A chronological perspective of the Council's activities aside with reports submitted by the Commission clearly shows that the former has often simply re-used, sometimes literally, proposals put forward by the latter. However, never did the Commission claim this policy to be its making, and the documents it publishes systematically present the Ostpolitik as issued from efforts and will of the sole omnipotent European Council, in its desire to integrate applicant countries. Besides keeping silent about its own contribution, the solutions it presses on totally brush aside its arbitrary aspects, in other words the idea that they might have resulted from choices. Notably the essentially juridical dimension given to the pre-adhesion strategy (based on the definition of the European Acquis, and its inclusion in the law of candidate States), made possible to introduce it as directly connected to the Treaties and founding texts, i.e. solely based on respect for the law and procedure. Such "juridicisation" apparently was key to the Commission's success with its partners, inasmuch as it depicted a temporary and eminently legitimate response to the expectations of applicant countries, it also avoided member States, who could not reach an agreement on the conditions of enlargement, the duty to take position on potently highly conflict-prone issues. Hence the co-production of the policy apparently rested upon a particular form of exchange between different European institutions. The Commission's autonomy in the making of this public action and the institutional gains it derives from it would have been connected to its ability to provide Member State with the possibility to endorse it. To do so, it repressed the political dimension of its action: first by ascribing responsibility to member States, second/to then make it acceptable to the publics it is designed for [START_REF] Roubieu | Des 'managers' très politiques. Les secrétaires généraux des grandes villes[END_REF].

Secondly, the Commission's relationship to politics seems to have partly determined the shape and the nature of relationships between those in charge of the implementation of the PHARE programme in DG1A, and their institutional partners: member State representatives in the PHARE Management committee, MEPs in charge of controlling expenditures of the assistance programme. These exchanges stand out for the technicisation of issues and debates that surrounded the process of programming. Technicisation for instance appears in the systematic translation into technical termsbelonging to some highly specialised sectorof objectives of assistance programmes and their mode of implementation. It responds to the Commission's concern to present its work as a rational implementation of competencies and 'scientific' unquestionable methods, whose respect is a guarantee of quality, neutrality, objectivity, indeed the unquestionable aspect of the projects it carries out in Eastern Europe.

Yet technicisation also allows the Commission to limit its partners' ability to influence, control, and possibly sanction its activity in this domain. By compelling the use of a technical language onto their correspondents, principally in the Management Committee, officials from PHARE take them into a field of competencies they reputedly control, and are often much more at ease with. Technicisation also renders more difficult to understand the programme's ground-level realities for national representatives and Members of European Parliament.

Perceiving and comprehending political issues outlined in the technical choices mentioned in the documents, require a certain degree of experience in the debates and controversies which structure the domains of public policy making [START_REF] Eichener | Social dumping or Innovative regulations ? Processes and Outcomes of European Decision-Making in the Sector of Health and Safety at Work Harmonisation[END_REF][START_REF] Grémion | Profession : décideurs. Pouvoir des hauts fonctionnaires et réforme de l'Etat[END_REF], Institut für Europaïsche Politik 1989). The Commission's effort of repression of the political dimension thus inspires not only discourses but also practices of its members, in their choice of particular modes of action, and the relationship to their counterparts. Besides, this 'enacted' a-politicism enabled the Commission to achieve a form of leadership in the policy designed for Eastern European States, and to preserve its autonomy in the making of this policy. If some aspects of this 'apoliticism', such as the official's claim to neutrality or independence, their legitimate competencies to define European general interest, have been presented by some studies as strategic resources, and offer an explanation for the Commission's success [START_REF] Abélès | Approche anthropologique de la Commission, Bruxelles: Rapport rédigé pour le compte de la cellule de prospective Abélès M[END_REF][START_REF] Lequesne | La Commission européenne entre autonomie et indépendance[END_REF][START_REF] Nugent | The government and politics of the European Union[END_REF], it should however be insisted upon that the Commission has not constantly benefited from such a stance on politics, and this didn't always work out as a means to expand its prerogatives and its range of competencies. Yet more precisely, it appears that under certain condition, the Commission can barely escape the necessity to silence the political dimension of its activity, although keeping silent limits the room for manoeuvre and available choices.

In this respect, a return to the debates that surrounded the Commission's definition of the acquis communautaire in the social domain appears particularly significant to us. In the process of preparation to enlargement, a group made up principally of DG1A officials in charge of the PHARE programme suggested that the acquis communautaire to be presented to applicant States should not solely consist in European rules, but should also take into account a number of political principles which haven't obtained juridical translation as yet, for instance the requirement of a high level of social security standards. This broad definition of the social acquis communautaire thus required to leave the domain of the law and take aboard a non juridical argument and legitimacy. DG5 officials' reaction to the suggestion and the subsequent decision made to 'bury the issue' and comply with a strictly and exclusively juridical definition are precisely the outcome of those constraints exerted upon the Commission in its relationship to politics. Indeed it seems that DG5 officials gave up on the defence of a more extensive understanding of the social acquis, to which they actually found much benefit, chiefly because such positioning would have led them to derogate to two unwritten rules governing the Commission with the European political system, connected to the necessity to affirm its 'a-politicism' [START_REF] Robert | Ressources juridiques et stratégies politiques. Analyse d'une controverse à la Commission européenne sur la dimension sociale de l'élargissement de l'Union[END_REF].

First of all, DG5 officials considered as unforeseeable and hazardous the defence of such a position that they were aware had little chance of reaching broad support among Council members. Obligation, for the Commission, to anticipate the reactions of its institutional partners [START_REF] Cram | Calling the Tune without paying the piper ? Social policy regulation : the role of the Commission in EC social policy[END_REF], Pierson 1996, Wendon 1998, Mak in this book) and thus avoid conflict or failure over a particular suggestion, is the price to pay for the exclusive qualities it asserts to be endowed with. These include its claim to ably integrate every single demands issued by the social bodyeven contradictoryin the process of building common interest: no more than a simple mediating body, deprived of any proper will and objectives and therefore of political project, its existence relies on its ability to stimulate consensus and produce settlement [START_REF] Abélès | La Commission européenne, du compromis culturel à la culture du compromis[END_REF][START_REF] Lamy | Choses vues de l'Europe[END_REF]. As underlined by one informant, French representative in the PHARE management committee, 'The Commission, who thinks itself repository of the Union's common interest, and rightly so, would not stand to find itself disavowed. So it does its utmost so that it does not happen. v ' Besides, the need felt by European civil servants to support the pertinence and relevance of their suggestions through reference to their inner technical quality and rational characteristics has devoid of meaning the very idea of conflict. While in the political realm, opposition exists on the very basis that protagonists do not share the same values, the Commission seems at least outwardly doomed to perceive opposition as a challenge to the technical qualities of a project and the independence and neutrality of its designers, and as a contention over its own ability to define and defend European common interest. In case of opposition from the Council, the Commission's very asset turns against itself: the rationale produced to fix as rational the position it supports allows for no failure to convince. To the extent that its argument posits a project validity on the very intrinsic qualities of the institution from which it originates, failure amounts to immediate disavowal of the Commission itself. Even though, within the administration, most officials are able to display an analysis which distances itself from the position approved by the Council, it seems as if a negative vote by representatives of Member States would invariably come down to a form of individual and collective failure.

Failure may bear dire consequences notably on professional carriers, as noted by Mark Pollack: 'As one Commission official explained, having one's proposal referred from a committee to the Council can cast a long shadow over the career prospects of a young fonctionnaire […].' (Pollack 1997: 115).

Moreover, DG5 officials' refusal to support this extra-juridical definition of the acquis communautaire is due to the range of process of legitimisation available with regards to position of the Commission. Concerned with the creative dimension of its work, the Commission often presents its doings and sayings as the strict implementation of principles fixed elsewhere, be it law, science, or political statements approved by all of the Member States. Deprived of democratic legitimacy, the European administration cannot alone engage an explicitly political argument based on attachment to and defence of values such as the need for a high level of social security. In a broad context indeed characterised by the deafening silence of policy makers on the issue of the social dimension of Eastern enlargement, it appears that the Commission had no other choice than law. Even though law does not permit much justifying, so limited is the acquis in the social domain (Leibfried, Pierson 1998). As put by a DG5 civil servant: 'In a way it is an administrative tradition in the Commission, to be focusing on legal norms.

We cannot tell an applicant country: you should do so and so, if there is no legal basis, because that would imply that the Commission suddenly gives orders to third countries, on issues where it had no say within the Union, that would be somewhat schizophrenic and illogical, wouldn't it? So we have the Treaty, and all the secondary norms, these are the rules of the game, and we have to accept them, as they stand today. Yes or no, if yes then we may become a Member State. vi ' CONCLUSION To conclude with this analysis, we should like to make the subsequent two observations. The first deals with the status that can possibly be granted to the Commission's relationship to politics in this analysis. Through several instances mentioned in this paper, it appears that the repression of the political dimension is enlisted as a resource, and as such makes up a powerful tool for the legitimisation of the Commission itself and its activity. However close study of the European Ostpolitik also indicates that the Commission, in some particular political circumstances, is forced to rub off the properly political nature of its work and of its role, and that such a constraint determines and limits the actions it can undertake. The commission's 'a-politicism' cannot be apprehended simply as a choice, transient and connected to the expectation of potential gains. If it is sometimes so vigorously compelling, the reason is that it is closely tied up to the very position of the Commission in the European political system, and to the modes of legitimisation it can access. Its use also comes together into a kind of cumulative logic: when actors claim a whole set of qualities and dispositions relating to their a-politicism in order to justify of their existence and lend legitimacy to their projects, they are obviously coerced into behaving accordingly or at least pretend to; in short they must adjust to the requirements of the role they choose to play.

The second observation we shall make relates to the way the Commission communicates and strives to legitimise itself with national public opinions. One may indeed wonder if a good deal of obstacles met by Commission to exist in the daily reality of European citizens couldn't be connected to the Commission's relationship to politics. Isn't this process of repression a contradiction to the need to produce a discourse intended for European citizens, enabling them to better understand the role played by the European administration, and what it actually stands for? Doesn't it prevent the Commission from at least partly controlling its image with Member States nationals (Baisnée in this book) ? In other words, to what an extent are games of inter-institutional legitimisation complementary to and compatible with the process of legitimisation worked out by European institutions in relation to European citizens [START_REF] Abélès | En attente d'Europe[END_REF] ? Do not these contradictions mirror fundamental tensions of the European integration process, between the control retained by Member States and the direct responsibility of European institutions before the citizens, between "the political and legal nature of the European Union", and for the Commission between the requirement of being responsible before the Member States and European citizens, and its twin function as an executive body and a public administration [START_REF] Christiansen | Les tensions de la gouvernance européenne : l'administration politisée et la responsabilité multiple de la Commission européenne[END_REF]) ?

  ) This form of repression (refoulement) is substantiated on several modes: modes of action and management within the Commission (through resorting to technicisation and juridicisation), or the officials' self perception of their role and function in the Commission, within which the operation of defining what politics and what political officials should be takes a central place. The Commission's relationship to the political dimension of its own action may thus be considered as a relevant grid for understanding the dynamics working out the production of European public policy making.

  use of this classification to decipher their relationships with their institutional partners: 'When we worked together on the PHARE programme, Member States were always very careful about protecting [their own interest] (i.e. to offer European contracts to national consultants and enterprises). They played their part as member State, their role of defending national interest, and we [the Commission] have represented the interest of the European Community, as usual, and the interests of applicant countries. (…) The European Parliament members were very enthusiastic about PHARE programme, we have had to rationalise their mobilisation. For example, they wanted Romanian orphanages to become, overnight, as modern as in Western Europe. (…) We [the Commission] were here to reason with them and take actual needs of the Romanian people into account. To build luxury orphanages could have been a provocation for Romanian families. This story of the Romanian orphanages is a good instance of our usual work with EP members. iv ' Although perhaps more obvious among agents of the PHARE programme owing to the history of the office, such dispositions are however not a particular specificity of DGIA, but can be regarded as characteristics pervasive to the Commission's administrative culture, as shown by Marc Abélès and Irène Bellier: 'Les euro-fonctionnaires, quel que soit leur grade, sont censés procéder sur une base non émotionnelle et rester insensibles aux influences, tout en recevant les demandes de divers représentants qui leur servent en partie de source d'information. Théoriquement ils agissent rationnellement, ce qui désigne essentiellement une technique de choix, et impartialement, ce qui renvoie aux notions d'indépendance politique et éthique. Du point de vue endogène, les actes de la Commission et des ses agents, même imparfaits se fondent sur des critères quantifiables, évaluables, et s'inscrivent dans des cadres établis et contrôlables, contrairement et l'opposition est pertinente aux yeux des hauts fonctionnairesaux actes des hommes et des femmes politiques qui en étant soumis aux aléas des choix électoraux et non assurés de la pérennité, seraient enclins à 'céder à l'émotion.'

ii Our hypothesis is that this relation to politics is a common feature of the whole European administration staff. It doesn't imply an understanding of the Commission as an homogenous and monolithic institution : the numerous conflicts and rivalries between services and General directories, who also have different administrative cultures, political goals and ways of working have been well studied [START_REF] Cram | The European Commission as a multi-organisation : social policy and IT policy in the UE[END_REF][START_REF] Cini | La Commission européenne, lieu d'émergence de cultures administratives. L'exemple de la DGIV et de la DGXI[END_REF]). Nevertheless, this paper will not investigate those effects of internal fragmentation on the civil servant's relation to politic.

Suffice to say that this fragmentation can present a problem for the Commission to manage its relation to politics : to appear as a neutral and technical expert in the service of the European common interest, it has to find ways of regulating internal tensions and masking them to external observers (Baisnée in this book, Robert 2001b: 440-465).

iii Règlement (CEE) n°3906/89 du Conseil du 18 décembre 1989 relatif à l'aide économique en faveur de la république de Hongrie et de la république populaire de Pologne, [JO L375, 23 décembre 1989[JO L375, 23 décembre , pp.11-12], modified in 1991[JO L375, 23 décembre , 1992[JO L375, 23 décembre , 1993[JO L375, 23 décembre , 1995[JO L375, 23 décembre , 1996, but only to add new beneficiary countries.

iv Interview with a former 'directeur général' in DGIA, in charge of PHARE service. Paris, July 1999.

v Interview with a civil servant of the French Ministry of Foreign Affair, member of the PHARE management committee. Paris, November 1997.

vi Interview with a DG5's member, Brussels, November 1996.