E Sutter

T T M Tran

C Fiaud

Erosion-corrosion of copper in cooling water systems at 60°C using an impinging liquid jet and Electrochemical Quartz Crystal Microbalance

Keywords: EQCM, Impinging liquid jet, RDE, copper 1 Back Back

When water flows over copper, turbulence may be created at different parts of metal surface (such as bends, tees, sudden changes in section), and may be sufficient to cause breakdown of the surface layer, even with a solution that is free from suspended solids. An impinging liquid jet directed at normal incidence onto a copper sample was used in this work. It was generated using a recirculating solution of 10 -3 M Na 2 SO 4 , continuously deaerated and heated at 60°C. Two types of copper samples were investigated: a bulk copper electrode was first anodised to form a copper oxide layer, before submitting it to the impinging jet. The polarisation resistance was then measured as a function of the time of impinging. In an other test, an electrolytic copper layer was deposited on an EQCM, then polarisation resistance and mass change were followed as a function of the time of impinging. The results are discussed from electrochemical and mass change measurements, and from scanning electronic microscopic observations. Comparisons are made with the variation of the polarisation resistance measured as a function of time, in the same solution, using a rotating disk electrode, at which no turbulence is assumed to occur

INTRODUCTION.

Distinction between flow-accelerated corrosion and erosion-corrosion is generally made [START_REF] Efird | Flow Induced Corrosion" and J. Postlethwaite and S. Nesic "Errosion-Corrosion in Single and Multiphase Flow[END_REF]. Flow-accelerated corrosion is defined as increased corrosion from increasing mass transfer, as a result of the increasing flow velocity of a fluid over a surface. Erosion-corrosion results from the impingement of second-phase particles contained in the fluid (e.g. solid particles, gas bubbles in aqueous flow), as a result of the flow of fluid over the surface. On copper samples, oxide layers are easily damaged by flows. The removal of the film by a flow enhances the corrosion rate of the underlying metal and may be accompanied by erosion of the underlying metal. The relative roles of corrosion and erosion is then neither easy to measure, nor easy to estimate. In the present work, the effect of fluid velocity on the corrosion of copper in an aqueous solution is measured and discussed over a wide range of fluid velocities, using two different flow geometries: a rotating disc electrode and an impinging liquid jet directed at normal incidence onto a copper sample. According to [START_REF] Syrett | [END_REF][START_REF] Schlichting | Boundery Layer Theory[END_REF], the comparison between both methods is allowed by taking into account a geometry-independent fluid flow parameter such as the wall shear stress (τ w ). This parameter can be calculated from empirical hydrodynamic equations developed for each experimental method. The rate of corrosion can then generally be correlated to the wall shear stress, according to: R corr = a τ w b , a and b depending on the metal and the composition of the solution.

EXPERIMENTAL

During all the experiments, measurements were performed in a 10 -3 M Na 2 SO 4 solution at 60°C. The pH of this solution was 6.2. When necessary the solution was deaerated through nitrogen bubbling during at least 15 minutes before any experiment. In some experiments the pH of the solution was adjusted by addition of NaOH ( until pH of 7 or 8) or by addition of H 2 SO 4 (until pH 5). For electrochemical measurements a PAR 273 (PRINCETON APPLIED RESEARCH) potentiostat was used. A saturated calomel electrode (SCE) was used as reference and platinum as the counter electrode. Most electrochemical measurements consisted of measuring the polarisation resistance Rp in the potential range between E corr -0.015V and E corr + 0.015V as a function of the immersion time.

RDE studies were performed on polycrystalline Cu samples or on oxidised copper samples with 0.8 cm diameter. The disc was embedded in a resin and in a PTFE holder. The disc assembly was rotated using a RADIOMETER BM-EDI101 setup. Before each measurement, the copper disc was mechanically polished with SiC up to grade 1200, rinsed with deionised water and air dried. On some samples measurements were performed on anodically oxidised copper samples. The oxide layer was formed in an airsaturated 10 -3 M Na 2 SO 4 solution through polarisation of the copper disc at 0.2V /SCE during 30 min. The thickness of the oxide layer was estimated by coulometry to be about 70 nm.

The general arrangement of the jet impingement test cell is shown in Figure1. The fluid jet was produced using a glass cylinder and a nozzle with 2 mm diameter. The flow density was 59.15 cm 3 s -1 , corresponding to an axial flow velocity of 18.8 m s -1 . The working electrode consisted of an oxidised copper disk with a 2.5 cm diameter or of a copper deposit on a quartz used for electrochemical quartz microbalance experiments.

In the case of the oxidised copper disk, a copper oxide layer of about 70 nm thickness was anodically formed using a galvanostatic method (1mA.cm -2 during 60 min). In the case of the copper deposit at the surface of an electrochemical quartz microbalance, a copper layer was electroplated from a saturated CuSO 4 solution on gold electrode with a surface of 1.37cm 2 , on a 5 MHz, AT-cut quartz crystal (MAXTEK Co). The applied plating current was -30 mA cm - 2 during 300s. The mass change was then 2500 µg cm -2 , corresponding to a copper thickness of about 2.8 µm. Figure 2 shows a set of cathodic polarisation curves obtained on copper in air saturated 10 -3 M Na 2 SO 4 solution at 60°C for different rotation rates of the copper disk. When plotted as a function of the square root of the disk rotation (ω 1/2 ), the diffusion-limited current densities vary linearly. This behaviour is indicative of a diffusion-limited process for the oxygen reduction, at the surface of copper, which can be described by the Levich equation.

The polarisation resistances were measured for the same rotation rates and are reported in Figure 3. The relation between Rp and the corrosion current density i corr was experimentally determined using the anodic Tafel slope obtained at Ω = 0rpm in the same solution. The relation Rp = B /i corr was established in this condition, with B = 0.026. This value was used throughout this work for the determination of the corrosion current density, assuming that B does not vary with the hydrodynamic conditions. The variations of the corrosion rate in Figure 3.were calculated from i corr values using Faraday's law. The comparison between Figures 2 and3 shows that two different behaviours have to be distinguished in the air saturated solution: at low rotation rates (Ω < 1000 rpm), the effect of an increasing velocity is to increase the surface concentration of oxygen and consequently to increase the corrosion rate of copper. At higher rotation rates (Ω > 1000 rpm), the corrosion rate gets practically independent of fluid velocity. It is noticeable that the value of i corr is always lower than the value of i lim , for all the tested rotation rates, indicating that the mechanism of copper corrosion is under mixed control. For rotation rates over 1000 rpm, the rate of mass transfer gets much higher than the rate of charge transfer at the surface of the metal leading, for the overall corrosion process, either to activation control or to control by the rate of diffusion of copper ions through a cuprous oxide film. This last assumption has been verified and reported by several authors [5,6], since it is well known that the corrosion resistance of copper depends mainly on the formation of a protective layer of Cu 2 O. The formation of such a layer at the open-circuit potential depends on the pH, the oxygen concentration, the temperature of the solution and the flow velocity. Many authors have shown that the growth of Cu 2 O in aqueous solutions of pH close to 7 occurs during a hundredth of hours before reaching a constant value [5,7,8]. Thickening and reinforcement of the oxide layer with increasing immersion time generally lead to an increase of the Rp value and consequently to a decrease of the corrosion rate with time. It has been shown that in flowing solutions, cuprous oxide films are thinner than in stagnant liquid [5,9]. There are two possible mechanisms for this behaviour: if cuprous oxide forms through a dissolutionprecipitation mechanism, the stirring of the solution would slow down, at the surface of copper, the concentration of soluble Cu(I) species necessary for the nucleation of Cu 2 O. The other hypothesis is that the oxide film is stripped from the metal surface by the action of hydrodynamic shear force. To distinguish between these two hypotheses, oxide layers were first anodically formed at copper samples in stagnant conditions of the electrolyte, before activating the rotation of the electrode.

-2,5 -2,0 -1,5 -1,0 -0,5 0,0 -1,4 -1,2 -1,0 -0,8 -0,6 -0,4 -0,2 0,0 E (V/

II. Influence of the flow on the behaviour of a pre-oxidised copper surface.

The variation of Rp value with time was measured in deaerated 10 -3 M Na 2 SO 4 solution on copper samples previously polarised in the anodic potential range. The thickness of the anodic oxide layer was about 70 nm. The solution was deaerated to reduce the contribution of the mass transfer to the overall mechanism. Two tests were performed simultaneously: in the first test the electrode was still (Ω =0 rpm). In the second test the electrode was rotated at 5000 rpm, but rotation was stopped during the electrochemical measurement of Rp, in order to make the measurements under the same hydrodynamic conditions. According to hydrodynamic equations reported in the literature, at 5000 rpm, the radial velocity of the flow at the outer part of the disc is 2.1 m s -1 , which corresponds to a Reynolds number of 8373 and a shear stress of about 16.4 N m -2 .

For both conditions (Ω =0 rpm, and Ω =5000 rpm), the corrosion rate sharply decreased during the first hours of immersion, from about 20 µm year -1 down to a value between 2 and 3 µm year -1 in the two cases. This decrease is probably due to a reinforcement of the oxide layer or an increase of its thickness during the first hours of immersion. When Ω =0 rpm the rate of corrosion kept roughly constant during the whole duration of the test (28 days). When Ω =5000 rpm, the corrosion rate suddenly increased after 11 days of rotation, then slowed down again, reaching the value of 9 µm year -1 . This sudden increase was attributed to mechanical removal of the protective film after 11 days, followed by its slow reformation, but although leading to a less protective film. Many authors agree with the fact that in the presence of a surface film on copper, the corrosion rate increases with the flow velocity and that the determination of the overall mass transfer coefficient is a reliable way of predicting the corrosion rate. The overall mass transfer coefficient K is related to the mass transfer coefficient in the diffusion boundary layer K d and to the mass transfer coefficient K f in the film by [10]:

f d K 1 K 1 K 1 + =
It is generally admitted that the mass transfer coefficient in the film varies during the first hours of exposure (when the film is built up), then reaches an almost constant value after 24 hours of exposure. To our knowledge, results concerning exposure times over 100 hours have not been reported. The new result in this work is that oxide films on a copper surface can be stripped from the surface several days after the steady state is reached, as shown in Figure 4 after 11 days of exposure. The time after which removal of oxide can be evidenced by Rp measurements will be called t erosion .

The effect of pH of the solution.

In Figure 5, copper samples previously polarised in the anodic potential range were exposed to solutions of different pH, and Rp values were followed as a function of time for two rotation rates: Ω = 0 rpm and Ω = 5000 rpm. As previously the rotation was stopped during the measurements. In each experiment, the exposure was stopped when t erosion was reached. In solutions of pH between 5 and 7, the value of t erosion increased with increasing pH. In solutions with pH 5.0, t erosion was already reached after 9 days of exposure, and oxide seemed difficult to form again. Oxide was damaged after 11 and 59 days respectively at pH 6.0 and 7.0, whereas no Rp increase was observed after 80 days of exposure to the solution of pH 8. For the last observation, two interpretations are possible: either the oxide film is not removed by the flow, or the rate at which the film can form again by other competing reactions at pH 8 is higher than the rate of film removal. In the two hypotheses, corrosion rates stay at a relatively low value. The increasing value of t erosion when pH value increases is in agreement with the higher stability of the oxide layer when pH gets more basic. The determination of t erosion seems thus to be an accurate method for classifying the mechanical resistance of oxide layers on copper, as a function of the parameters of the electrolyte, in conditions of low hydrodynamic intensity (small shear stress).

Back

Jet impingement test [11][12][13][14] consists of a liquid jet impinging on a stationary specimen (Figure 1). This technique is recommended when reliable high-turbulence conditions must be simulated. The flow field characteristic of a circular jet impinging on a flat plate with the jet's central axis normal to the plate is illustrated in Figure 6. In region A, the flow is essentially laminar near the plate, the axial velocity is independent of radial position, whereas the radial velocity increases linearly with radial distance [13,14]. Region B is a region of rapidly increasing turbulence, extending from r/ro = 2 to r/ro = 4 (ro being the nozzle radius, and r the distance from the centrum ) In region C, the flow rate and turbulence decay rapidly.

In the present experimental set-up, according to hydrodynamic equations reported in the literature, the jet Reynolds number is 37675, and the wall shear stress calculated for r/ro=3 (in the middle of zone B) is about 1030 N m -2 .

Rp measurements on copper discs

Attempts were first performed on copper discs with diameter of 25 mm, previously anodised to form an oxide layer of 70 nm. Rp values were then measured as a function of time of exposure to the jet in the deaerated 10 -3 M Na 2 SO 4 . (As for the RDE experiments, the impingement was stopped during Rp measurements). The corrosion rate first decreased during the two first days, then reached a constant value of 8 µm year -1 . After 22 days of exposure, no sudden increase of Rp was evidenced. Nevertheless SEM observation of the copper sample after 22 days of exposure showed locally a highly damaged surface at a radial distance from the centrum between 2 and 3 mm. Two possible explanations for the non variation of Rp with time are possible: either the flow velocity is high enough to impede any oxide formation and consequently any oxide removal, or the ratio between the surface affected by the turbulent flow (about 0.38 cm 2 between r/r o = 2 and r/r o =4) and the surface of the disk (about 2,54 cm 2 ) in contact with the electrolyte is to low. The geometry of the system should then be modified to ensure that the entire copper surface only lays within the turbulent zone (zone B). Mass change measured by using EQCM.

Since the measurements of Rp alone were not able to evidence any oxide breakdown, a gravimetric method using a quartz crystal microbalance was applied. EQCM is a powerful tool to study very small amounts of mass changes [15,16]. Copper layers of about 2.8 µm thickness were first electroplated on the surface of quartzes before exposure to the deaerated 10 -3 M Na 2 SO 4 solution at 60°C. Figure 8 shows mass changes measured as a function of time of exposure in two cases: one in the stagnant solution and the other under an impingement jet.

In the stagnant solution, mass change is first negative, becoming positive after 96 hours of exposure and then decreasing again with a rate of 4.5µm year -1 . The mass increase observed after 96 hours can be attributed to the formation of copper oxides and (or) hydroxides. Under the impinging jet the mass decreases from the beginning of the exposure, reaching a rate of corrosion of 32 µm year -1 during the first hundred hours of exposure. After this period, the rate slowed down to a value close to 10 µm year -1 . Obviously, the effect of the flow is to hinder the formation of a copper oxide layer and to increase the rate of copper dissolution. SEM examination of the surface of the quartz showed a high damaged surface with zones striped of copper but also of the gold sub-layer, indicating that in these hydrodynamic conditions, flow -induced corrosion and erosion both occurred at the surface of the quartz.

Corrosion rates are here calculated taking into account the whole surface in contact with the electrolyte (1.37 cm 3 ). Since the turbulent flow only affects a surface of about 0.38 cm 3 , the rates are likely to be underestimated.

CONCLUSION

The analyses presented in this work must be considered as preliminary ones. Two different methods were used to evidence the influence of flow velocity on the rate of corrosion or erosion-corrosion of copper in cooling water systems at 60°C. For low shear stress values, in conditions of laminar flow, the increased corrosion rate, compared to the rate in stagnant water, is first due to increased mass transfer at the surface of copper and copper oxide.

Measurements of Rp values as a function of the exposure duration of rotating disk electrodes showed that, even for low shear stress values of the fluid, removal of the oxide layer occurs after several days, enhancing the rate of copper dissolution. Use of RDE and determination of the time necessary of striping off the film (t erosion ) is thus a convenient way to evaluate the mechanical resistance of the oxide film as a function of the parameters of the electrolyte. For higher flow velocities, in the turbulent shear stress range, the surface of the copper electrode exposed to a impinging jet must be optimised and adapted to the size of the nozzle, to allow measurements of Rp variations. Copper deposits on an EQMB are damaged under the jet and the measured mass variations are at least 10 times higher than in a stagnant solution. SEM observations of the surface indicate that in these severe hydrodynamic conditions, the whole mass loss has to be partly attributed to erosion of the copper deposit.

Figure 1 .

 1 Figure 1. Schematic diagram of the jet impingement test cell

Figure 2 : 1 )Figure 3 :

 213 Figure 2: Cathodic i=f(E) curves of copper in air-saturated 10 -3 M Na 2 SO 4 at 60°C

Figure 4 :

 4 Figure 4: Corrosion rate of a pre-oxidised copper electrode as a function of the exposure time in deaerated 10 -3 M Na 2 SO 4 solution at 60°C.

Figure 5 :Figure 6 :

 56 Figure 5: Corrosion rate of a pre-oxidised copper electrode as a function of the exposure time in deaerated 10 -3 M Na 2 SO 4 solution at 60°C of different pH values.

Figure 7 :

 7 Figure 7: SEM examination of a copper disc after 22 days of exposure to a jet impingement

1 V

 1 = 59,15 cm 3 .s-1

Figure 8 :

 8 Figure 8: Mass variation of the copper deposit as a function of time (10 -3 M Na 2 SO 4 , deaerated, 60°C)

Acknowledgements:

The authors acknowledge the financial support provided by EDF (Electricité de France)


Clémence Nadal

Stéphane Gineste

Olivier Coutelier

Audrey Tourrette

Jean-Daniel Marty
email: jean-daniel.marty@univ-tlse3.fr

Mathias Destarac
email: mathias.destarac@univ-tlse3.fr

A

Keywords: self-assembly, thermoresponsive, pH-sensitive T

come

1

A deeper insight into the dual temperature-and pH-responsiveness of poly(vinylamine)-b-poly(N-isopropylacrylamide) double hydrophilic block copolymers

Introduction

Due to their wide range of application, amphiphilic copolymers represent more than ever a vast area of research. When dispersed in aqueous media, amphiphilic block copolymers display opposite behaviour, resulting in the reorganization of the two blocks to fit their affinities and self-assemble [START_REF] Kim | Functional nanomaterials based on block copolymer selfassembly[END_REF][START_REF] Xu | A Review on Colloidal Self-Assembly and their Applications[END_REF][START_REF] Samaddar | An engineering insight into block copolymer self-assembly: Contemporary application from biomedical research to nanotechnology[END_REF]. Alternatively, stimuli-responsive polymers need a trigger to assemble in solution in response to different stimuli such as temperature, electrical or magnetic field and mechanical stress, pH, ionic strength or addition of a chemical [START_REF] Khimani | Self-assembly of stimuli-responsive block copolymers in aqueous solutions: an overview[END_REF][START_REF] Wei | Stimuli-responsive polymers and their applications[END_REF].

Among them, thermoresponsive polymers which display Lower-or Upper Critical Solution Temperature (LCST or UCST), are of special interest because of their high potential in drug delivery and tissue engineering [START_REF] Doberenz | Thermoresponsive polymers and their biomedical application in tissue engineering -a review[END_REF][START_REF] Ward | Thermoresponsive Polymers for Biomedical Applications[END_REF]. Most reported LCST polymers are poly(vinylcaprolactam) (PVCL) [START_REF] Cortez-Lemus | Poly(N-vinylcaprolactam), a comprehensive review on a thermoresponsive polymer becoming popular[END_REF],

ethylene glycol-based copolymers including poly(oligo(ethylene glycol) methyl ether (meth)acrylates) [START_REF] Badi | Non-linear PEG-based thermoresponsive polymer systems[END_REF][START_REF] Constantinou | Thermoresponsive gels based on ABC triblock copolymers: effect of the length of the PEG side group[END_REF][START_REF] Steinhauer | Block and Gradient Copolymers of 2-Hydroxyethyl Acrylate and 2-Methoxyethyl Acrylate via RAFT: Polymerization Kinetics, Thermoresponsive Properties, and Micellization[END_REF][START_REF] Xiang | End group polarity and block symmetry effects on cloud point and hydrodynamic diameter of thermoresponsive block copolymers[END_REF], and polyacrylamides, e.g poly(N-isopropylacrylamide) (PNIPAM) [START_REF] Laschewsky | Tailoring of Stimuli-Responsive Water Soluble Acrylamide and Methacrylamide Polymers[END_REF][START_REF] Halperin | Poly(N-isopropylacrylamide) Phase Diagrams: Fifty Years of Research[END_REF]. The presence of pendant amide groups and their ability to form hydrogen bonds, as well as the hydrophobic/hydrophilic balance between the carbon backbone and the polar amide groups is responsible for the LCST behavior of acrylamides in solution [START_REF] Chen | A new thermoresponsive polymer of poly(N-acryloylsarcosine methyl ester) with a tunable LCST[END_REF]. The LCST-type phase transition of PNIPAM is induced by a coil-to-globule transition induced by dehydration of polymer chains at a temperature around 32°C [START_REF] Heskins | Solution Properties of Poly(N-isopropylacrylamide)[END_REF]. The incorporation of hydrophilic or hydrophobic comonomers in PNIPAM allows fine-tuning of the cloud point temperature (Tc) [START_REF] Sistach | Thermoresponsive Amphiphilic Diblock Copolymers Synthesized by MADIX/RAFT: Properties in Aqueous Solutions and Use for the Preparation and Stabilization of Gold Nanoparticles[END_REF][START_REF] Xiao | Amphiphilic block copolymers with aldehyde and ferrocenefunctionalized hydrophobic block and their redox-responsive micelles[END_REF]. For example, when NIPAM monomer is statistically copolymerized with hydrophilic monomers such as acrylamide, Tc increases up to about 45 °C when the polymer chain comprises 18 mol % of acrylamide, whereas Tc decreases to about 23 °C when 20 mol % of hydrophobic N-tert-butylacrylamide is incorporated into the polymer chain [START_REF] Hoffman | Really smart bioconjugates of smart polymers and receptor proteins[END_REF]. Although this effect is less pronounced than for statistical copolymers, it can still be observed in block copolymers. Double-hydrophilic LCST block copolymers for instance usually have an upper-shifted LCST [START_REF] Read | Mechanistic insights into the formation of polyion complex aggregates from cationic thermoresponsive diblock copolymers[END_REF].

Stimuli-responsive double-hydrophilic block copolymers are attractive materials since they can be fully miscible in water until a stimulus such as temperature or pH is applied, inducing their selfassembly. Hence, PNIPAM-based thermoresponsive DHBCs were extensively studied for their ability to form thermally responsive micelles, where PNIPAM forms the core above LCST. [START_REF] Luo | 100th Anniversary of Macromolecular Science Viewpoint: Poly(N-isopropylacrylamide)-Based Thermally Responsive Micelles[END_REF] PNIPAM-b-PEG copolymers were indeed used as lipophilic drug nanocarriers for which a decrease of temperature below the LCST induces the disassembly of core-shell structure and consequently a drug release. [START_REF] Topp | Thermosensitive Micelle-Forming Block Copolymers of Poly(ethylene glycol) and Poly(N-isopropylacrylamide)[END_REF][START_REF] Zhang | An, Thermoresponsive Micellization of Poly(ethylene glycol)-b-poly(Nisopropylacrylamide) in Water[END_REF][START_REF] Chen | Core cross-linked double hydrophilic block copolymer micelles based on multiple hydrogen-bonding interactions[END_REF] Attaching a positively or negatively charged block to a neutral polymer is another way to tune its self-assembly by using pH and/or ionic strength as stimuli [START_REF] Hales | Using polyelectrolyte block copolymers to tune nanostructure assembly[END_REF]. Commonly used pHsensitive polymers are poly(acrylic acid) (PAA), poly(methacrylic acid) (PMAA) and poly (2-(dimethylamino)ethyl methacrylate) (PDMAEMA) [START_REF] Constantinou | Thermoresponsive Multiblock Copolymers: Chemistry, Properties and Applications[END_REF]. Poly(vinylamine) (PVAm) displays a good solubility in water over a large range of pH [START_REF] Xu | A Review on Colloidal Self-Assembly and their Applications[END_REF][START_REF] Samaddar | An engineering insight into block copolymer self-assembly: Contemporary application from biomedical research to nanotechnology[END_REF][START_REF] Khimani | Self-assembly of stimuli-responsive block copolymers in aqueous solutions: an overview[END_REF][START_REF] Wei | Stimuli-responsive polymers and their applications[END_REF][START_REF] Doberenz | Thermoresponsive polymers and their biomedical application in tissue engineering -a review[END_REF][START_REF] Ward | Thermoresponsive Polymers for Biomedical Applications[END_REF][START_REF] Cortez-Lemus | Poly(N-vinylcaprolactam), a comprehensive review on a thermoresponsive polymer becoming popular[END_REF][START_REF] Badi | Non-linear PEG-based thermoresponsive polymer systems[END_REF][START_REF] Constantinou | Thermoresponsive gels based on ABC triblock copolymers: effect of the length of the PEG side group[END_REF][START_REF] Steinhauer | Block and Gradient Copolymers of 2-Hydroxyethyl Acrylate and 2-Methoxyethyl Acrylate via RAFT: Polymerization Kinetics, Thermoresponsive Properties, and Micellization[END_REF] and a polyelectrolyte behavior, which aroused the interest of multiple research teams [START_REF] Pelton | Polyvinylamine: A Tool for Engineering Interfaces[END_REF]. Mori et al. have reported for the first time the synthesis of PVAm with controlled macromolecular characteristics from the polymerization of N-vinylphthalimide (NVPI) by reversible addition-fragmentation chain transfer (RAFT) polymerization followed by the deprotection of phthalimido groups with hydrazine. By using this method, the synthesis of PVAm homopolymer [START_REF] Maki | Controlled RAFT Polymerization of N-Vinylphthalimide and its Hydrazinolysis to Poly(vinyl amine)[END_REF], alternating copolymers with other monomers such as NIPAM [START_REF] Maki | Controlled Synthesis of Alternating Copolymers by RAFT Copolymerization of N-Vinylphthalimide with N-Isopropylacrylamide[END_REF], and finally PVAm-b-PNIPAM and zwitterionic (PVAm-b-PALysOH) block copolymers were reported as well as their physicochemical behavior in aqueous solution [START_REF] Maki | Synthesis of Amphiphilic and Double-Hydrophilic Block Copolymers Containing Poly(vinyl amine) Segments by RAFT Polymerization of N-Vinylphthalimide[END_REF][START_REF] Kanto | Mixed Polyplex Micelles with Thermoresponsive and Lysine-Based Zwitterionic Shells Derived from Two Poly(vinyl amine)-Based Block Copolymers[END_REF]. The self-assembly and stability properties of polyplex micelles obtained from the complexation of anionic DNA with these block copolymers was more specifically studied [START_REF] Kanto | Mixed Polyplex Micelles with Thermoresponsive and Lysine-Based Zwitterionic Shells Derived from Two Poly(vinyl amine)-Based Block Copolymers[END_REF].

As depicted in Figure 1, depending on pH and temperature each block in PVAm-b-PNIPAM copolymer should present different protonation states or conformations in solution. Also, intra-and inter-block hydrogen bonding interactions between amine, ammonium and amide groups are expected. At pH below pKa, the PVAm block is in the highly hydrophilic ammonium form, whereas at pH well above pKa, the amine form is predominant. Concerning the PNIPAM block, it is in a hydrated form below its critical temperature and upon heating undergoes a coil-to-globule transition due to partial dehydration of PNIPAM chains. These structural modifications will condition the solution properties of the polymers. The objective of this study is therefore to focus on the aggregation behavior of PVAm-b-PNIPAM block copolymers as a function of pH and temperature in order to bring out a deeper understanding of their complex self-assembly behaviour [START_REF] Kanto | Mixed Polyplex Micelles with Thermoresponsive and Lysine-Based Zwitterionic Shells Derived from Two Poly(vinyl amine)-Based Block Copolymers[END_REF]. PVPIn-b-PNIPAMm was carried out according to a previously reported procedure which is provided in ESI [START_REF] Glaria | Understanding the Role of ω-End Groups and Molecular Weight in the Interaction of PNIPAM with Gold Surfaces[END_REF].

Hydrazinolysis of P VPI10-b-PNIPAM50. The hydrazinolysis procedure was adapted from Mori et al. [START_REF] Maki | Controlled RAFT Polymerization of N-Vinylphthalimide and its Hydrazinolysis to Poly(vinyl amine)[END_REF] and carried out as follows: in a round-bottom flask was dissolved the PVPI10-b-PNIPAM50 copolymer (1.05 g, 0.646 mmol phthalimide units) in 12mL of a methanol/dioxane mixture with a 2/1 volume ratio. Hydrazine monohydrate (646 mg, 12.9 mmol) was introduced and the mixture degassed by bubbling argon for 1 h. The flask was equipped with a reflux condenser and the mixture was kept under reflux (85 °C) for 6 h. The heating was then stopped and solvents were removed under vacuum. The resulting oil was dissolved in MeOH/aqueous HCl (0.5M) with a 2/1 volume ratio. The PVAmn-b-PNIPAMm copolymer was finally precipitated in diethyl ether, filtered off, dried under vacuum and dialyzed at pH > 9 against ultrapure water for 4 days. The final copolymer was recovered by lyophilization and used as such for physicochemical characterizations.

Methods.

Nuclear Magnetic Resonance (NMR). 1 H-NMR spectra were recorded at 300 MHz on a Bruker Advance 300 MHz spectrometer. 1 H-NMR in temperature was carried on a Bruker Advance 400 MHz spectrometer. DMSO-d6 was used for all PVPI-containing samples, D2O was used for PVAm-based copolymers.

pH-titration. The pKa of the PVAm10-b-PNIPAM50 copolymer was determined by pH-titration of a polymer solution at a concentration equal to 3 mg/mL solubilized in 0.15 mol•L-1 KCl(aq). Prior to titration, 20 µL of a 0.1 mol•L-1 HCl(aq) solution was added to ensure complete protonation of amine functions. The initial pH of titrated solution was equal to 4). Titration was then performed with a 0.1 mol•L-1 KOH(aq) solution.

Size Exclusion Chromatography (SEC).

Samples were prepared in DMF-LiBr (10 mM) at 5 mg mL -1 and filtered on a PTFE 0.45 µm filter. SEC injections were performed with a 1.0 mL.min -1 flow rate on a Dionex UltiMate 3000 UHPLC pump from Thermo Scientific equipped with two Tosoh columns (α-2500, 8.0 x 300 mm and α-3000, 8.0 x 300 mm). Columns were thermostatted at 55 °C in an oven.

Multidetection was provided from a refractive index detector (RI) Wyatt Optilab rEX, a UV detector Varian ProStar 325 UV and a multi-angle light scattering (MALS) detector Wyatt MiniDawn TREOS.

Absolute molar masses and dispersities were determined using RI and MALS detection. All data were processed with Astra7 software from Wyatt Technology.

Dynamic Light Scattering (DLS). Multi-angle DLS experiments were performed on a 3D LS spectrometer from LS instruments (Switzerland). Working with a laser at 660 nm, this instrument recorded the light scattered at angles between 15 and 150° (typically more than ca 60 values). UV-vis spectroscopy. Transmittance of polymer aqueous solutions was recorded at 500 nm and different temperature with a Cary 100 Bio spectrophotometer at different heating/cooling rates (0.5, 1, 2.5 and 5 °C⋅min -1 ). Cloud point temperatures were calculated from the determination of inflexion point of each transmittance curve, extrapolated to 0 °C⋅min -1 .

Differential Scanning Calorimetry (DSC).

The thermal properties of the polymers (in solution and in bulk) were determined by DSC using a Mettler Toledo DSC 1 STARe System Thermal Analysis calorimeter equipped with a Gas Controller GC200. Solution samples were sealed in impermeable crucibles of 120 µL. Transition temperatures were taken at the top of the peak on the thermogram on heating and extrapolated the values obtained at different heating rates to 0 °C/min. The variation of enthalpy was measured as the temperature increased at a rate of 1 °C/min.

Small-Angle X-ray Scattering (SAXS)

. SAXS experiments were performed on a XEUSS 2.0 bench equipped with an internal copper source. X-ray beam energy was 8 keV. Samples were prepared at 10 %w/v at pH 5 and 11. Measurements were acquired during a heating ramp between 20 and 50 °C.

Data were processed using the software SasView. q is the scattering wave vector defined as q = (4π/λ) sin ϑ/2, λ being the wavelength (λ ∼ 1 Å) and ϑ the scattering angle.

Transmission Electron Microscopy (TEM). TEM images were acquired on a Hitachi HT7700 device.

Samples were prepared in thermostatted conditions (25 °C and 45 °C) by depositing a drop of the previously thermostatted polymer solution (0.5 mg/mL) on copper carbon grids (FCF400-CU from Electron Microscopy Sciences). Samples were let to dry then a drop of contrasting agent solution (uranyl acetate) was deposed as well.

Results and discussion

Figure S2

). PVAmm-b-PNIPAMn copolymers were purified by dialysis to remove unreacted macro-CTA from the diblock. A summary of the characteristics of PVAMm-b-PNIPAMn is reported in Table 1 below.

PVAmm-b-PNIPAMn composition

DPVAm/DPNIPAM th DPVAm/DPNIPAM NMR (a) Mn,NMR (g mol - In the following, the block copolymer solutions were prepared at three different pH values: one highly above pKa (pH 11) when the PVAm block is fully deprotonated (α ~ 0), the second around pKa wt.% compared to the values assessed at 5 wt.%. Figure 4a presents the transmittance curves measured at 500 nm at different heating and cooling rates. A hysteresis phenomenon whose amplitude depends on the chosen heating/cooling rate is observed. This phenomenon is related to the difficulty of swelling compact aggregates formed at higher temperatures due to intra-chain entanglement. [START_REF] Kobayashi | Chelating ability of poly(vinylamine): effects of polyamine structure on chelation[END_REF] Extrapolation of the estimated inflection on heating at null speed [START_REF] Yin | Effect of the microstructure of n-butyl acrylate/N-isopropylacrylamide copolymers on their thermo-responsiveness, self-organization and gel properties in water[END_REF] enables the determination of the cloud point temperature Tc at 39.8°C (Figure S3 in ESI). This value is, as expected, slightly higher than the value of PNIPAM homopolymer (i.e. 32 °C) due to the presence of the PVAm hydrophilic block. [START_REF] Yin | Effect of copolymer composition of controlled (N-vinylcaprolactam/N-vinylpyrrolidone) statistical copolymers on formation, stabilization, thermoresponsiveness and catalytic properties of gold nanoparticles[END_REF] Tc values equal to 36.4 and 36.0 °C were obtained on heating thanks to DSC and DLS experiments respectively at (Figure S4-S5). It should be noted that for these low molecular weight polymers, the study of thermal properties by UV-Vis spectroscopy leads to a systematic overestimation of Tc values compared to those obtained from DSC or DLS measurements.

Such effects can be attributed to the need for these techniques to obtain aggregates of sufficient size to be detectable. Thus, even as the dehydration of the polymer chain begins, as evidenced by DSC measurements, the formation of aggregates of sufficient size able to scatter light at a sufficient level requires a certain amount of time and/or a greater dehydration level. Cloud points determined by these different characterization methods are given in Table S2 and in Table 2. 35°C the shifting of the correlation curve towards longer correlation times clearly indicates the appearance of larger objects or aggregates. Both the loss in intensity observed above 45°C and the shape of the correlation curves suggest a poor colloidal stability of these objects and a sedimentation phenomenon (the precise determination of the morphology and size of aggregates will be further investigated in the next section). In order to have a further understanding of the mechanism behind this transition, 1 H NMR analysis was performed with a temperature increase from 20 °C to 55 °C (see In order to highlight the influence of temperature on each block, the peak integrals corresponding to NIPAM monomer units (a and c) and backbone methylene protons of both blocks(b) were measured. The ratio (c/a) corresponding to PNIPAM solely remains roughly constant whatever the temperature and the measured value of 0.16 is in good agreement with the theoretical value equal to 0.17. The evolution of the integration of the characteristics peaks of PNIPAM relatively to the one of the copolymer backbone was then studied by following c/b and a/b ratio. Initial ratio values at 20 °C were equal to 0.3 and 1.9 respectively and were in good agreement with theoretical ratios estimated from the composition of the block copolymer (0.28 and 1.7 respectively). Nevertheless, once Tc was reached a significant decrease of both ratios was measured. This suggests that PNIPAM blocks are present in more dehydrated regions of the aggregates than PVAm, which limits the relaxation phenomenon.

Thermoresponsiveness at pH near and lower than pKa (pH 8 and 5)

At pH 5, the PVAmm block is fully protonated (Figure 3). UV-Vis transmittance profiles at heating and cooling ramps of 0.5 °C/min are shown in Figure S6. PVAm10-PNIPAM50@pH11 and PVAm10-PNIPAM50@pH5 present very different cooling profiles. Whereas the transmittance of PVAm10-PNIPAM50@pH11 falls from 100 % transmittance to 0 % with a sharp transition, PVAm10-PNIPAM50@pH5 did not reach full opacity when heated at 0.5 °C/min. This could be ascribed to the formation at pH 5 of smaller aggregates which results in a decrease of scattered light intensity. In addition, the evolution of transmittance as a function of temperature, indicating a slower and more difficult solubilization of dehydrated PNIPAM in water at pH 11 than for lower pH. Lastly, the measured values of Tc at different pH tend, as expected, to decrease with an increase of pH. Above pKa at pH 11, the polyamine block is fully deprotonated and is indeed less hydrophilic than the its cationic version, resulting in a decrease of Tc from 42.2 °C at pH 5 to 39.8 °C (see Table S1). At an intermediate pH, PVAm10-PNIPAM50@pH8 exhibited expected heating and cooling profiles with sharp slopes during both phases. DSC measurements revealed that the variation of enthalpy associated with the phase transition were -8.14 and -10.37 J per gram of PNIPAM at pH 5 and 8 respectively. At pH 11 it was reduced to -2.98 J/g suggesting a less energetically consuming dehydration which is consistent with a lower Tc value at pH 11 than pH 5. Consistently with turbidity measurements, scattering experiments showed notable differences between copolymer solutions at pH 5 and 11 (see experimental ratio is close to the theoretical one (0.3 vs 0.28). With the increase of temperature, this ratio then gradually decreased to finally reach a value of 0.17 above Tc regardless of the pH value.

This suggests that, whatever the pH, aggregates are formed with dehydrated PNIPAM blocks exhibiting more restricted motion than the PVAm ones. The zeta potential was then measured below and above Tc at different pH value (see Table 2). Under pKa at pH 5.6, the ξ-potential increased significantly between 25 °C and 45 °C from 8.9 to 27.4 mV.

During the transition PNIPAM dehydrated and curled up on itself, exposing charged PVAm blocks to the outer surface of objects. The same transition occurred at pH above pKa but as the PVAm block was fully deprotonated no change in ξ-potential was noticeable between 25 and 45°C (-7.4 and -11.9

mV respectively).

Influence of the molar mass of PVAm-b-PNIPAM copolymer on thermoresponsive properties

To evaluate the influence of chain length on physicochemical properties, all previous characterizations were also performed on the PVAm20-b-PNIPAM100 block copolymer. It should be noted that copolymers in this study are homothetic in their composition. Tc values obtained from different characterization techniques were gathered in Table 2 (see also Table S1). It appeared that Tc was not significantly affected by the copolymer chain length. At pH 11, scattering experiments evidenced the formation of large objects that have a tendency to precipitate as already observed for PVAm10-b-PNIPAM50. Nevertheless, from turbidimetry measurements comparatively to PVAm10-b-PNIPAM50, a sharper cooling transition is observed for PVAm20-b-PNIPAM100 copolymer (Figure S8). 1 H NMR studies as a function of temperature did not show noticeable differences for polymers of different chain lengths (see Figure 5 and Figure 8 for comparison). At pH 6, turbidimetry and scattering experiments (Figure S8) also demonstrated the formation of large colloids above Tc but with a smaller size than at pH 11. Consequently, the scattered light remained low above Tc and transmittance remained above 0. ξ-potential measurements of PVAm20-b-PNIPAM100 showed the same trend as a function of temperature and pH than for the shorter PVAm10-PNIPAM50. To conclude, apart from kinetic considerations on the hydration/dehydration mechanism, the block copolymer chain length did not significantly affect Tc values and the structure of the observed colloids. Tc values obtained from the different methods were reported in Table 2b and were generally higher

Colloid characterization

The study of the cloud point transition had already highlighted significant differences in the behaviour of PVAm10-b-PNIPAM50 at pH 5 and 11. More precisely it appears that more defined and smaller nano-objects tend to be formed at pH lower than pKa (Figure S9 of ESI). To confirm this, multi-angle DLS, TEM and SAXS experiments were carried out at pH 5 and 11.

At pH 11, the evolution of hydrodynamic diameter as a function of temperature was first analysed by multi-angle DLS (Figure 9a). Below Tc, a hydrodynamic diameter equal to 116 ± 60 nm is evaluated.

Above Tc, Dh reached a maximum value of 361 nm at 40 °C and then decreased to 150 ± 10 nm at 55

°C. TEM analysis performed on samples deposited at 25°C showed entangled elongated non-spherical structures (Figure S10a) whose partial aggregation might result on the aggregates observed with DLS.

At 45 °C, TEM images showed large well-defined spherical objects of 58 ± 10 nm (Figure 9b and corresponding size distribution in Figure S10). At this temperature, the absence of very large aggregates in dried samples might suggest that the observed colloids through scattering experiments might result from the aggregation of smaller colloids without reorganization of polymer chains between them. This mechanism is similar to the one observed for other families of block copolymers involving the formation of mesoglobules [START_REF] Read | Mechanistic insights into the formation of polyion complex aggregates from cationic thermoresponsive diblock copolymers[END_REF][START_REF] Yin | Effect of the microstructure of n-butyl acrylate/N-isopropylacrylamide copolymers on their thermo-responsiveness, self-organization and gel properties in water[END_REF] and enables the fast disruption of aggregates below Tc. The formation of mesoglobules might be promoted here by the dehydration induced by the increase of temperature. Interestingly as can be seen in Figure 9a, depending on the targeted temperature, different hydrodynamic diameters were measured above Tc: the higher the temperature, the smaller the mesoglobules. 10). Scattering curves showed an increase of I(q) from 20 °C to 35 °C followed by a decrease at higher temperatures. In the middle q region, I(q) followed a power law with I(q) ~ q -α with a coefficient α of 2.13 at 25 °C and 3.03 at 45 °C, suggesting a densification of aggregates. This observation is in accordance with DLS results which showed the aggregation of preexisting smaller aggregates above Tc. Moreover, SAXS confirmed the uncontrolled behaviour of this aggregation phenomenon as no defined morphologies could be identified. Therefore, at pH 11, block copolymers exist as aggregates generated by the interaction between PNIPAM and PVAm block. Increasing the temperature above Tc induced the formation of larger, denser and poorly defined aggregates. room temperature self-assembly could be induced by PVAm ability to form inter-and intra-hydrogen bonds [START_REF] Kanto | Mixed Polyplex Micelles with Thermoresponsive and Lysine-Based Zwitterionic Shells Derived from Two Poly(vinyl amine)-Based Block Copolymers[END_REF]. This suggests that at 25°C large and poorly defined colloids measured by DLS could be obtained from the aggregation through hydrogen bonding of smaller block copolymer aggregates.

Interestingly, at higher temperature smaller colloids were observed with an average hydrodynamic diameter Dh = 70 ± 5 nm (at 55°C). Consistently with correlation functions showed in Figure 6, polydispersity decreased with an increase of temperature, suggesting a more organized state above Tc, and a random one below Tc. In TEM, at 45 °C, spherical particles were visualized with a mean diameter of 24 ± 3 nm (Figure 12, right). As discussed in the previous section, the formation of better defined nano-objects at higher temperature might result from the aggregation of amphiphilic PVAm10-b-PNIPAM50, PNIPAM being hydrophobic above Tc. The presence at the surface of these colloids of positively charged PVAm blocks evidenced by positive zeta-potential measurements can prevent their further aggregation. aggregates at T <Tc to nano-objects above Tc, with still some residual aggregates as I(q) did not stabilize in the small q region. The comparison of SAXS profiles at pH 5 and 11 at 25 °C and 45 °C (Figure 13b) clearly showed the difference of behaviour as a function of the protonation degree of the PVAm block. In the middle q region, at pH 5 signals of the PVAm10-b-PNIPAM50 copolymer followed the power law I(q) ~ q -α , with α decreasing from 2.4 at 25 °C to 1.72 at 45 °C. Assuming a dense core/hydrated shell model an average diameter of 30 nm was obtained for the inner core of PNIPAM suggesting a thickness for PVAM shell equal to 20 nm (i.e. an overall diameter of 70 nm).

Assuming a density for the dense PNIPAM core equal to 1.1 g/cm 3 (ρPNIPAM) and knowing the average molar mass of PNIPAM (MPNIPAM) and Avogadro constant (NA) an aggregation number equal to 1600 was calculated using the following equation: = . .

. . [START_REF] Sistach | Thermoresponsive Amphiphilic Diblock Copolymers Synthesized by MADIX/RAFT: Properties in Aqueous Solutions and Use for the Preparation and Stabilization of Gold Nanoparticles[END_REF] In conclusion at pH 5, whereas poorly defined aggregates were observed for T < Tc, an increase of temperature above Tc led to the formation of better defined nano-objects. At high temperature, scattering experiments along with NMR results and zeta potential measurements suggest the formation of colloids comprising a PNIPAM dehydrated core with an outer charged shell issued from protonated PVAm with an average aggregation number of 1600 polymer chains per nano-object.

Finally, the SAXS study was also performed on PVAm20-PNIPAM100 and revealed a very similar behaviour with aggregates at pH 11 and the transition from aggregates to self-assembled nanoobjects at pH 5 over the critical transition (Figure S11). On the basis of the obtained results at pH 5 and pH 11, a schematic phase diagram of the aggregates morphologies of PVAm10-b-PNIPAM50 as a function of pH and temperature can be proposed as depicted in Figure 14.

Figure 1 .

 1 Figure 1. PVAmm-b-PNIPAMn structure and schematic representation as a function of cloud point temperature and pKa

  Typical acquisition time for each angle was set to 30 s. Five correlograms were recorded for each angle. All DLS data were then analyzed using a laboratory made software program (named M-STORMS). Cumulant or NNLS (Non-Negative Least Squares) methods led to an estimation of the decay rate Γ for each angle. The linear dependency of Γ versus the square of the scattering vector q ( Γ=q^2.D ) gives the diffusion coefficient of the nano-objects and therefore their radius through the Stokes-Einstein equation: D= (kT)/(6πηRh) where T is the temperature and η the viscosity of the solution. The size error was estimated by the standard deviation of all the calculations (with cumulant and NNLS methods, ca 10-12 values per experiment) for all similar experiments (4 experiments). Multi-angle DLS experiments were performed in dilute condition (0,01%w/v) to prevent sedimentation and multi-scattering phenomena to occur.Mono-angle DLS measurements were conducted using a Zetasizer Nano-ZS (Malvern Instruments, Ltd, UK) with integrated 4 mW He-Ne laser at λ = 633 nm. The light scattering intensity (at 173°) was measured with instrumental parameters set to constant values for all the samples. The correlation function (measured twice for each sample) was analyzed via the cumulant method to get the Zaverage size of the colloids. The standard deviation in the intensity and Z-average size are found for each sample respectively around 2% and 1%. Samples were prepared at 1 %w/v in ultrapure water.

3. 1 .

 1 Synthesis of PVAmn-b-PNIPAMm diblock copolymer Poly(vinyl amine)-b-poly(N-isopropylacrylamide) copolymers PVAmm-b-PNIPAMn with controlled molar mass and well-defined structures were synthesized as follows: a two-step RAFT polymerization strategy (Figure 2) based on the work of Mori and coworkers [28,30] was used starting from the RAFT polymerization of N-vinyl phthalimide (NVPI) with xanthate XA1 control agent, to form a first PVPI block terminated with a xanthate functionality PVPIm-X. The polymerization of NIPAM in the presence of a PVPIn-X macro-RAFT agent led to the formation of PVPIm-b-PNIPAMn-X diblock copolymer.

Figure 2 .

 2 Figure 2. Synthesis pathway of PVAm10-b-PNIPAM50 via a 2-step RAFT polymerization to obtain PVPI10-b-PNIPAM50-X, followed by post-polymerization treatmentPVPI10-X and PVPI20-X were obtained with NVPI conversion above 80 %. From these macro-CTAs, homothetic PVPI10-b-PNIPAM50-X and PVPI20-b-PNIPAM100-X copolymers were synthesized. NIPAM conversion was above 97 % for both copolymerizations. Chain extensions were confirmed by SEC analysis (FigureS1). Mn values of respectively 6900 g mol -1 and 15400 g mol -1 were obtained from SEC-MALS analysis. The terminal xanthate group was then removed by radical reaction with dilauroyl peroxide in propan-2-ol leading to PVPIm-b-PNIPAMn copolymers. Finally, PVPIm-b-PNIPAMn was reacted with an excess of hydrazine and converted into PVAmm-b-PNIPAMn.1 H-NMR confirmed the deprotection of phthalimido groups with the disappearance of aromatic protons around 8 ppm (see

1 ) 2 . 5 [ 27 ,Figure 3 )Figure 3 .

 1252733 Figure 3. ■ Evolution of the pH and ▶ corresponding protonation degree (according to Henderson-Hasselbalch equation) of aPVAm10-b-PNIPAM50 polymer solution (3mg/mL) titrated with a KOH solution (0.1 mol.L -1 ).

(pH 8 )

 8 with an intermediate protonation state (α between 0.2 et 0.8) and the last one well under (pH 5) so that PVAm is mostly protonated (α ~ 1). Each solution was characterized over a range of temperature between 25 °C and 55 °C. The homothetic PVAm10-b-PNIPAM50 and PVAm20-b-PNIPAM100 copolymers were studied. The solutions were noted as PVAmn-b-PNIPAMm@pHx with respectively n, m the number-average degree of polymerization of the blocks and x the pH of the solution. For each method, samples were prepared in ultrapure water (or D2O when mentioned) at different concentrations and targeted pH of 5, 8 and 11. pH adjustment was made with diluted HCl and NaOH solutions (0.1 mol•L-1 ). The effective pH of each sample is indicated in TableS1of ESI.Thermoresponsiveness at a pH value higher than pKa (pH 11).The behaviour of PVAm10-b-PNIPAM50 polymer solutions was studied as a function of temperature thanks to UV-vis, DSC, light scattering and 1 H NMR techniques for concentrations ranging from 0.1 to 5 wt.%. Whatever the technique used, a sharp and reversible phase transition was observed. It should be noted that for most polymers, there is no significant difference in Tc values measured at different concentrations. Only PNIPAM homopolymers shows a significantly different value at 0.1

Figure 4 . 13 DLS

 413 Figure 4. a) Turbidimetry and DLS measurements as a function of temperature of an aqueous solution of PVAm10-b-PNIPAM50@pH11 (0.1%w/v and 1%w/v respectively). a) Transmittance monitored at 500 nm at different heating and cooling rates ( 0.5 -1 -2 -5 °C/min), b) Evolution of correlation functions on heating between 25 °C and 55 °C.

Figure 5 )

 5 Figure5). First, a significant increase of chemical shift was measured as a function of temperature.Moreover, even if signals remained visible from 20 °C to 55 °C, the measured intensity dropped down along with temperature. Indeed, upon heating partial dehydration hampered chain mobility, inducing a shift of signals towards lower fields and a loss of the signal intensity[START_REF] Yin | Effect of copolymer composition of controlled (N-vinylcaprolactam/N-vinylpyrrolidone) statistical copolymers on formation, stabilization, thermoresponsiveness and catalytic properties of gold nanoparticles[END_REF][START_REF] Tokuhiro | NMR study of poly(N-isopropylacrylamide) gels near phase transition[END_REF][START_REF] Burba | Salt Effects on Poly(N-isopropylacrylamide) Phase Transition Thermodynamics from NMR Spectroscopy[END_REF][START_REF] Spěváček | Application of NMR Spectroscopy to Study Thermoresponsive Polymers[END_REF]. The peaks did not completely disappear, probably due to the presence of residual water inside the aggregates.

Figure 5 . 1 H

 51 Figure 5. 1 H NMR spectra of PVAm10-PNIPAM50 block copolymer as a function of temperature (from 20 °C to 55 °C) and the corresponding evolution of ▲ a/b, • c/b ra=o and ■ c/a integration ratios block copolymer in D2O at 1%w/v).

Figure 5b depicts the

  Figure 5b depicts the evolution of c/a, c/b and a/b integration ratios as a function of temperature.

Figure 6 and

 6 Figure 6 and Figure 4b respectively). Correlation functions of PVAm10-PNIPAM50@pH5 are representative of objects with a larger polydispersity at 25 °C than at 45 °C (from 0.7 at 25 °C to 0.06 at 45 °C) suggesting a transition from an uncontrolled state of randomly formed objects to more organized objects. Interestingly above Tc, the estimated size of the aggregates formed at pH 5 from Zav values (100 nm) are found to be significantly smaller than the ones estimated at pH 8 and 11 (in the µm range).

Figure 6 .

 6 Figure 6. DLS measurements as a function of temperature of a 1%w/v aqueous solution of PVAm10-b-PNIPAM50@pH5 : evolution of correlation functions on heating between 25 °C and 55 °C.

Comparative 1 H

 1 NMR studies as a function of temperature were further performed at pH < pKa, pH ~ pKa and pH > pKa and are presented in Figure7at 25 °C and 45 °C (see also FigureS7). It is first noteworthy that the intensity of amide protons of PNIPAM at 8.0-8.3 ppm was higher at pH 11. This could be ascribed to a slower exchange phenomenon than at pH 5 and 8. Moreover, the observed trends as a function of temperature are very similar whatever the pH. At 20 °C, the measured c/b

Figure 7 . 1 H

 71 Figure 7.1 H NMR of PVAm10-PNIPAM50 aqueous solutions: evolution of integration ratio c/b (as defined in Figure5) as a function of temperature between 20 °C and 55°C at pH 5 (blue), pH 8 (red) and pH 11 (green).

Figure 8 . 1 H

 81 Figure 8.1 H NMR spectra of PVAm20-PNIPAM100 block copolymer as a function of temperature (from 20 °C to 55 °C): evolution of ▲ a/b, • c/b and ■ c/a integration ratios (a,b and c are defined in Figure5), block copolymer in D2O at 1%w/v).

at pH 5

 5 than pH 11 as expected considering the stronger hydrophilicity of protonated PVAm compared to the deprotonated version. Tc values were not remarkably influenced by the molar mass of polymers and neither were ζ-potentials. Whatever the pH, PVAm-b-PNIPAM copolymers remain thermoresponsive. Nevertheless, pH strongly influenced the kinetics of aggregation/disassembly and it seems that at low pH, smaller and more defined aggregates are prone to be formed.1 H NMR analysis also evidenced the modification of polymer motion induced by the change of temperature that might result in the formation of large objects with PNIPAM blocks located in regions with restricted motion capability. In the following section, based on these first results, we will investigate the formation of colloidal structures induced by the change of temperature through TEM, multiangle DLS and SAXS experiments.

Figure 9 .

 9 Figure 9. a) Multi-angle DLS analysis: evolution of the estimated hydrodynamic diameter as a function of temperature from 25 to 55 °C of a PVAm10-b-PNIPAM50@pH11 aqueous solution at 0.01%w/v.Measurements were carried at each temperature with a return to ambient temperature in between. b) TEM image of PVAm10-b-PNIPAM50@pH11 deposited on a coppercarbon grid from a solution at 0.1%w/v at 45 °C SAXS measurements were performed on PVAm10-b-PNIPAM50 at 1 %w/v as a function of temperature (Figure10). Scattering curves showed an increase of I(q) from 20 °C to 35 °C followed by a decrease

Figure 10 .

 10 Figure 10. Scattering curves of PVAm10-PNIPAM50 at pH 11 as a function of temperature from 20 °C to 50 °C At pH 5, the PVAm block is fully protonated. Average Dh values measured by multi-angle DLS as a function of temperature are plotted in Figure 11. PVAm10-b-PNIPAM50 spontaneously assembled at 25 °C into polydisperse objects with a 150 nm average diameter. In a dried state, spherical particles with a mean diameter of 19 ± 2 nm were observed by TEM (Figure 12, left). Mori et al. reported that

Figure 11 .

 11 Figure 11. Multi-angle DLS measurements: evolution of estimated hydrodynamic diameter as a function of temperature from 25 °C to 55 °C of a PVAm10-b-PNIPAM50 aqueous solution (1%w/v) at pH 5. Measurements were carried out at each temperature with a return to ambient temperature in between.

Figure 12 .

 12 Figure 12. TEM image of PVAm10-b-PNIPAM50 at pH 5 deposited on a copper-carbon grid from a solution at 0.1%w/v at 25 and 45 °C SAXS measurement was carried out to further investigate the morphology of aggregates. Scattering curve profiles as a function of temperature (Figure 13a) indicated a transition from fractal-like

Figure 13 .

 13 Figure 13. Scattering curves of a) PVAm10-b-PNIPAM50 at pH 5 as a function of temperature from 20 °C to 50 °C and of b) PVAm10-b-PNIPAM50 at pH5 (▲) and pH11 (•) at 25 °C (blue) and 45 °c (pink).

Figure 14

 14 Figure 14 Schematic representation of the proposed phase diagram based on scattering measurements and TEM images of PVAm10-b-PNIPAM50 at pH </ > pKa and T </> Tc

Table 1

 1 Main characteristics of PVAMm-b-PNIPAMn copolymers

Table 2 .

 2 Main physicochemical properties of PVAmm-b-PNIPAMn copolymers: Tc values determined from (a) turbidimetry measurements during heating ramp and extrapolated at 0 °C/min, (b) Z-av onset point in DLS heating ramp, (c) Derived Count Rate (DCR) onset point in DLS heating ramp and (d) DSC during heating ramp and extrapolated at 0 °C/min; (e) enthalpy variation per gram of PNIPAM obtained from DSC at different heating rates and extrapolated to 0°C/min; zetapotential values (associated standard deviations are given in supporting information)

	PVAmm-b-PNIPAMn properties

Conclusion

PVAmm-b-PNIPAMn copolymers were synthesized by hydrazinolysis of PVPIm-b-PNIPAMn precursors obtained by RAFT polymerization. The experimental compositions and molar masses of the obtained diblock copolymers were very close to PVAm10-b-PNIPAM50 and PVAm20-b-PNIPAM100 targeted structures. This family of copolymers has a double-hydrophilic character and is both pH-and thermoresponsive. This double feature brings them interesting aggregation properties which we studied in this work with a thorough physico-chemical characterization. First, the transition in temperature was studied by the determination of Tc at different pHs (i.e pH < pKa and pH > pKa) and different methods (turbidimetry, DLS, DSC). As expected, we showed that Tc is higher at low pH as the protonated PVAm block is more hydrophilic than its neutral analog. The comparison of PVAm10-b-PNIPAM50 and PVAm20-b-PNIPAM100 did not reveal major differences, a result that can be explained since PNIPAM is a type-II LCST polymer whose Tc is not affected by molar mass. Light and X-ray scattering experiments, along with TEM, confirmed this spontaneous aggregation at T < Tc and revealed polydisperse fractal-like aggregates with Dh of 150 nm at pH < pKa and 120 nm at pH > pKa.

Above Tc, the aggregation of copolymers is mostly influenced by the protonation state of the PVAm block. In its protonated form i.e. below pKa, core-shell particles were revealed with a dense PNIPAM core and a hydrophilic cationically charged and repulsive shell which prevented further aggregation and resulted in a monodisperse population. Above the pKa of PVAm, an aggregation of pre-existing aggregates, was observed. This study confirmed the pH-and thermoresponsive properties of PVAmb-PNIPAM diblock copolymers which make them great candidates for surface modification applications.

Aknowledgements

The authors would like to thank Barbara Lonetti and Christophe Mingotaud for their help in the discussion of SAXS and DLS results respectively.


Peng-Cheng Gao

Wan-Yu Tsai

Barbara Daffos

Pierre-Louis Taberna

Carlos R Pérez

Yury Gogotsi

Patrice Simon
email: simon@chimie.ups-tlse.fr.

Frédéric Favier

Graphene-like carbide derived carbon for high-power supercapacitors

Introduction

Carbon-based materials with high specific surface area (SSA) have been widely studied as supercapacitor electrode materials. This choice is led by their balanced electrochemical performances (including specific capacitance and long term cyclability), good (electro) chemical stability, reasonable electronic conductivity, ease of production and processability, low cost, etc. While activated carbon is the material of choice for commercial supercapacitors, carbide derived carbons (CDCs), [1,2] onion-like carbons (OLCs), [3,4] carbon nanotubes (CNTs), [5,6] and graphene [7,8] are currently considered for designing the next generation of pure carbon-based supercapacitors. Among these materials, CDCs are highly attractive as their microporous structure leads to high capacitance, high power and high energy performances. [2,9,10] The reported CDCs, however, mainly derive from commercial and/or bulk powders of metal carbides. Chlorination of these carbides results in carbons showing a microporous structure with a very narrow pore size distribution which can be tuned in the subnanometer range by the chlorination temperature. CDC-based electrodes however suffer from limited power capabilities at high current densities, especially when operated at low temperature or in viscous electrolytes. On the other hand, the chlorination procedure has a limited impact on the carbide powder morphology and the resulting microporous carbons retain most of the original shape and morphology of their carbide precursors (through a conformal or topotactic transformation). [1] 5

Graphene is another attractive carbon-based material thanks to its open 2D structure and amazing electronic conductivity. [11,12] Moreover, because of its high accessible surface area and good electrochemical stability, graphene and more specifically Reduced Graphene Oxide (RGO), is widely used as active material or conductive substrate/additive in energy storage devices. [7,8,12] In supercapacitors, graphene-based materials have shown remarkable specific capacitances as high as ~200 F•g -1 in aqueous electrolytes, [13,14] ~120 F•g -1 in organic electrolytes [15,16] and ~75 F•g -1 in ionic liquids. [16] Moreover, their low equivalent series resistances (ESR) and good electrochemical stabilities make graphene and RGO efficient substrates for pseudocapacitive [17][18][19] and battery [20,21] materials, by significantly improving both energy and power capabilities of the resulting composite electrodes. Recently, Ruoff et al. -drilled‖ micropores in the basal planes of graphene nanosheets by potassium hydroxide chemical activation. [22] The specific surface area (SSA) of the resulting activated graphene reached up to 3100 m 2 •g -1 . The specific capacitance of the resulting activated graphene was also enhanced to 160 F•g -1 in an organic electrolyte demonstrating the efficiency of this approach with respect to specific capacitance as well as energy density. [22] However, through the activation process, alkaline species and oxygen functional groups are regenerated at the graphene surface with a detrimental effect on the material electronic conductivity. The ESR of symmetric cells assembled with activated graphene-based electrodes was high (2.5 Ω•cm²) thus limiting the power performance.

In previous works, [23,24] we introduced an efficient route to synthesize mesoporous silicon carbides (SiCs) through a magnesio-thermal treatment of mesoporous silica/carbon composites at ~800 °C. In contrast with the actual carboreduction proceeding at temperatures above 1400 °C, such synthetic route does not consume any carbon from the pristine composite, preventing any material structure collapsing or sintering. [25] As a consequence, 5 as-prepared carbides perfectly inherit the mesoporosity from their silica/carbon precursors.

The corresponding CDC was further synthesized by chlorination of the prepared ordered mesoporous silicon carbide and electrochemically tested in a 1.5 M tetraethylammonium tetrafluoroborate (NEt 4 BF 4 ) in acetonitrile (AN) electrolyte. [26] Its hierarchical micro/mesoporous structure promoted very promising supercapacitive capabilities, especially power and energy densities, such as an outstanding rate capability (90% of capacity retention at 1 V•s -1 and a time constant of 1 s) and a high specific areal capacitance (0.5 F•cm -2 of electrode).

Recently, Yang et al. reported on a bottom-up approach to synthesize a 2D graphenebased material consisting in alternating layers made of mesoporous silica thin film and graphene. [27] Cetyltrimethylammonium bromide (CTAB), used as cationic surfactant, electrostatically adsorbs and self-assembles onto the surface of highly negatively charged graphene oxide (GO) in alkaline solution. Simultaneously, amphiphilic CTAB micelles strongly interact with tetraethoxysilane (TEOS) molecules, and provide the template for nucleation and growth of mesoporous silica at the surface of graphene oxide sheets. Furthermore, the porous silica film efficiently prevents the irreversible restacking of graphene. This sandwich-like silica and carbon nanocomposite is potentially an ideal precursor for the synthesis of nanosheet-like SiC by magnesio-thermal reduction. In the present work, the synthetic details and structural/morphological characteristics are given for 2D SiC prepared by magnesio-thermal reduction of silica/carbon nanosheets. CDCs obtained by chlorination of the prepared silicon carbide nanosheets are evaluated as supercapacitor electrode materials in 1.5 M NEt 4 BF 4 /AN organic electrolyte.

Materials and methods

Material synthesis

2D silica/carbon composites were prepared by electrostatic interactions between negatively charged GO and cationic CTAB, and further self-assembling in presence of TEOS. [27] GO was synthesized from graphite powder following a modified Hummers' method as described in a recent work. [28] In a typical synthesis of silica/carbon composite, 30 mg GO was exfoliated and dispersed in an aqueous solution containing 1 g CTAB (99%, Aldrich) and 40 mg NaOH (98%, Aldrich) under ultrasonic treatment for 3 h. The suspension was homogenized by vigorous stirring for 2 h at 40 °C. Then, 2 ml TEOS (98%, ACROS Organics) was added in, and the suspension aged by stirring for 12 h. The resulting solid was recovered by filtration, washed with warm ethanol (EtOH, denatured with 5% MEK, ACRO Organics), and dried. The dry powder was carbonized under argon flow at 800 °C for 3 h. The final composite was named CS-G-1. A second sample was prepared following the same synthetic route but using half the quantities of TEOS and CTAB, and named as CS-G-2.

The CS-G-1 and CS-G-2 samples successively underwent magnesio-thermal reduction and chlorination following the as previously described preparation routes. [23,24,26] The products from the magnesio-thermal treatment were named SiC-G-1 and SiC-G-2 and the corresponding chlorination products were named CDC-G-1 and CDC-G-2, respectively.

Material characterization

Transmission Electronic Microscopy (TEM) images were obtained using a JEOL 1200 EX2 TEM operated at 100 kV, and high resolution TEM (HR-TEM) images were taken using a JEOL JEM 2200 FS microscope operated at 200 kV. X-ray diffraction (XRD) measurements were performed on a Philips X'Pert diffractometer using CuKα1 radiation (λ= 1.5405 Å).

Raman spectra of the raw powders were recorded by using a LabRAM ARAMIS IR2 spectrometer with a He/Ne laser (λ = 633 nm). The samples were ground with a mortar and suspended in ethanol by ultrasonic treatment for 10 min (25 kHz D-78224 Singen). For TEM sampling, a droplet of the suspension was dropped onto a 200-mesh copper grid and left to dry in air. For HR-TEM sample, a 300-mesh carbon-coated copper grid was used instead.

5

Porosity characteristics were calculated from nitrogen (SiC-Gs) and argon (CDC-Gs) sorption isotherms measured at 77 K and 87 K, respectively, with a Micromeritics ASAP 2020 porosimeter. The specific surface area was estimated by using BET for each gases while the pore volumes and the pore size distributions were calculated from adsorption isotherms by using the QSDFT method. [29]

Electrochemical characterization

A slurry was obtained by mixing 95 wt.% of CDC-G powder with 5 wt.% of polytetrafluoroethylene (PTFE; 60 wt.% dispersion in water) using ethanol as the medium.

Electrode films of CDC-G-1 (thickness of ca. 150 μm) and CDC-G-2 (thickness of 300 μm) were fabricated with a mass loading of 2.6 mg•cm -2 and 6.2 mg•cm -2 , respectively. The apparent electrode density of CDC-G-1 and CDC-G-2 were calculated at 0.17 g•cm -3 and 0.20 g•cm -3 . The electrochemical characterization of the prepared CDC electrodes was conducted in 1.5 M NEt 4 BF 4 , (Acros Organics, CAS #429-06-1) in AN (Acros organics, CAS #75-05-8, H 2 O below 10 ppm) in a two-electrode Swagelok cell [30,31] assembled in a glove box under argon atmosphere, at room temperature. The electrode films were laminated onto pre-treated aluminum current collectors, [32] and two layers of 25 μm-thick porous cellulose were used as separator.

Electrochemical tests were performed using a multichannel VMP3 potentiostat/ galvanostat (Bio-Logic, France). CVs were recorded in a potential window from 0 to 2.3 V at various scan rates. EIS measurements were performed at the rest potential, using a sinusoidal signal of ±5 mV from 50 kHz to 10 mHz.

Results and discussion

The crystallographic characteristics of prepared SiC powders, SiC-G-1 and SiC-G-2, were evaluated by XRD diffraction as shown in Figure 1.

In the pattern depicted in Figure 1 pattern: Silicon, #00-026-1481). Silicon impurities are due to an excess of TEOS as precursor and, as a matter of fact, the silica in the SiO 2 /GO composite (CS-G-1, see experimental section for synthetic details) is approximately in a twofold molar excess versus GO. At the reaction temperature (800 °C), parts of silica and GO in CS-G-1 are reduced by Mg as SiO x vapors and solid RGO, respectively. These reduced species react according to Reaction (1) to give SiC. Meanwhile, the silica excess is directly converted to silicon (Reaction (2)).

SiO 2 /GO + 2Mg → SiC + 2MgO (1)
SiO 2 (excess) + 2Mg → Si + 2MgO (2)
SiC + 2Cl 2 → SiCl 4 + C (3)
For the SiC-G-2 sample (Figure 1 An extra diffraction, observed at 28.3° (2θ), is assigned to the (002) peak of graphite, mainly coming from the excess of GO in CS-G-2. Graphite is formed by reduction of GO and restacking of the resulting graphene during the magnesio-thermal reduction. The distinct crystal structures of the SiCs in the SiC-G-1 and SiC-G-2 samples can be assigned to the various SiC polytypes. In our previous report, SiC polytypes were revealed to alter from 3C-SiC to 6H-SiC along with the carbon/silica molar ratio in the composite precursor. [24] When the C/SiO 2 ratio is less than 1, SiC preferentially crystallizes in the 3C-SiC structure together with silicon impurities. 6H-SiC crystal structure appears when the C/SiO 2 ratio is close or equal to 1. respectively. For SiC materials, two active modes are expected in the 700-1000 cm -1 range; [33] they correspond to transverse (TO) and longitudinal (LO) modes, in the 750-850 cm -1 and 950-1000 cm -1 ranges, respectively. While the LO mode peak remains unchanged in shape and wave number, the TO mode range depends on the SiC allotrope considered. [33] From 3C-SiC to 6H-SiC, the TO peak is shifted towards lower wave numbers and split in two different peaks. Despite the unfavorable signal to noise ratio, Raman spectra presented in is about 0.42 nm, in agreement with those previously reported for a limited number of stacked graphene layers. [36] Additionally, the average pore size of CDC-G-2 sampled over 20 spots is about 0.8 nm (±0.05 nm) which is approximately equal to the pore size measured for CDC-G-1 and consistent with similar chlorination conditions used for both pristine carbides.

The above analyses of XRD, Raman and TEM data allow an overall synthetic mechanism In the low PSD region, for both SiC-G-1 and SiC-G-2, a weak adsorption peak appears at about 2.5 nm corresponding approximately to the pore size of mesoporous silica prepared by using CTAB as molecular template. [27] The presence of mesopores of this size originates from the localized conversion, through the magnesio-thermal reduction, of carbon located underneath the CTAB-templated mesoporous silica. However, most of the porous volume comes from the macropores formed by the random aggregation of the SiC graphene-like particles. As such, the SSAs of SiC-G-1 and SiC-G-2 calculated from BET method are limited to 68 m 2 •g -1 and 56 m 2 •g -1 , respectively.

The Ar adsorption/desorption isotherms and the pore size distributions calculated by the quenched solid density functional theory (QSDFT) method [29] of CDC-G-1 and CDC-G-2 are shown in Figure 4. For both CDCs, isothermal curves exhibit type II curves with H3 or H4 5 hysteresis loops characteristic of a combination of a large number of micropores together with a small number of mesopores. In the low relative pressure range (p/p 0 = 0.0~0.1), the strong adsorption comes from micropores opened in the graphene-like CDCs through chlorination treatments. Due to the generated micropores, the SSAs of CDC-G-1 and CDC-G-2 calculated by the BET method at 913 m 2 •g -1 and 872 m 2 •g -1 , respectively, were significantly enhanced compared to those of SiC-G-1 and SiC-G-2. In the PSD of both CDC-G-1 and CDC-G-2, a strong adsorption is centered at about 1 nm. This approximately corresponds to the pore size measured by TEM imaging for both samples. Moreover, there are two other adsorptions centered at 2.5 and about 0.5 nm, respectively. The mesoporosity at 2.5 nm corresponds to the weak adsorption measured for SiC-Gs as previously mentioned. Usually, the applicable pore diameter range of argon-sorption using QSDFT method is 0.35-8 nm. [29] Although there are no reports on the interlayer distance of graphene or graphene-based materials measured by argon-sorption using QSDFT method, in the present case, the PSD of CDC-G-1 centered at 0.53 nm is speculated as the distance between nanosheets. This extended interlayer distance in CDC-G-1 is in good agreement with the Raman conclusions demonstrating the low restacking degree of the nanosheets despite both magnesio-thermal reduction and chlorination. The PSD of CDC-G-2 locates at a smaller diameter of about 0.45 nm, also matching with the observed results from the TEM image (at 0.42 nm). These sorption results support the higher graphitic degree of CDC-G-2 compared to that of CDC-G-1.

The electrochemical behaviors of CDC-G-1 and CDC-G-2 were studied by cyclic voltammetry (CV) measurements and electrochemical impedance spectroscopy (EIS) in 1.5 M NEt 4 BF 4 /AN electrolyte. CV tests were performed between 0 and 2.3 V at various scan rates from 20 mV•s -1 to 5 V•s -1 . CV curves of both the CDC-G-1 and CDC-G-2 electrodes, as depicted in Figure 5 (specific capacitance versus cell voltage) and Figure S3 (current versus CDC-G-2 agree these porous volume discrepancies. These values are however slightly lower than those reported for CDCs in the literature, [2,9,37,38] but, considering carbon loadings of 2.6 mg•cm -2 and 6.1 mg•cm -2 , the areal capacitances of the electrodes using either CDC-G-1

or CDC-G-2 as active materials are quite attractive at 0.21 F•cm -2 and 0.4 F•cm -2 , respectively. To emphasize this point, a CDC-G-1 cell with a higher mass loading of 5.5 mg•cm -2 was tested in the same experimental conditions. The areal capacitance of this highloading CDC-G-1 cell reaches up to 0.39 F•cm -2 (as shown in Figure S4). This areal capacitance, nearly double that of the low-loading cell, is quite promising from the practical application point of view. However, due to the low bulk density of graphene-derived materials, the volumetric capacitance of the CDC-G electrodes are not that impressive, measured at 14 F•cm -3 and 10 F•cm -3 , respectively. By increasing the scan rate, the capacitive electrochemical signature of the CDC-G-1 electrode is preserved even at very high rate of 3 V•s -1 . At such scan rate, despite a marked ohmic behavior, the specific capacitance of the CDC-G-1 electrode is still 59 F•g -1 , a 76% retention of the capacitance at low scan rate (20 mV•s -1 ). Figure 6 shows a comparison of the power capabilities of selected carbonaceous materials, either CDC-based or graphene-like materials, from the change in relative capacitance (C/C 0 ) upon scan rate for electrodes with similar mass loading. The progressive 5 decay observed for both CDC-G-1 and CDC-G-2 relative capacitances is almost the same, demonstrating similar power performances. Except for the ordered mesoporous CDC which take an obvious advantage of its hierarchical porosity, C/C 0 decays of CDC-Gs are less than that of their homologous materials at the same scan rates. [14,22,26] The EDLC behavior of both electrodes is well kept at scan rates up to 500 mV•s -1 as they behave as efficiently as previously reported activated graphene (Figure 6). [22] The Nyquist plots of the CDC-G-1 and CDC-G-2 electrodes depicted in Figure 7 show distinct capacitive behaviors of porous block electrodes. From the high frequency range data analysis, the ESR of CDC-G-1 is measured at 0.45 Ω•cm 2 , close to those previously reported for CDC electrodes tested in similar conditions. [2,14] It is also smaller than those found in the literature for graphene, activated graphene and other CDCs. relaxation time τ 0 can be calculated from τ 0 =1/ƒ 0 , where ƒ 0 is the characteristic frequency of the cell obtained at the phase angle of -45°. τ 0 corresponds to the maximum on the dispersed energy curves and to the frontier between the capacitive and resistive behavior of the supercapacitor electrode. It is the time required to efficiently deliver the stored energy. [32,[START_REF] Miller | Pulse power performances of electrochemical capacitors: technical status of[END_REF] The low τ 0 values of CDC-G-1 and CDC-G-2 at 0.6 s and 0.9 s, respectively, demonstrate the high power performances of the graphene-like CDC electrodes. The mass loading (ML) of the active materials strongly affects the τ 0 value of the prepared electrode. CDC-G-1 electrodes with ML of 2.6 and 9.6 mg•cm -2 were prepared and tested in 1.5 M NEt 4 BF 4 /AN electrolyte.

As shown in Figure S5, the calculated τ 0 increases from 0.4 s to 0.9 s while increasing the ML in CDC-G-1 electrodes from 2.6 mg•cm -2 to 9.6 mg•cm -2 . Besides, assuming that authors preferentially exhibit the best performances of their electrode materials, time constant is also a good marker to benchmark our CDC-Gs towards other devices reported in the literature.

Noteworthy, the τ 0 value of the CDC-G-1 cell is close to or even faster than those reported for 5 high power carbonaceous materials in organic electrolyte, such as multiwalled nanotube (0.7 s for 15 mg•cm -2 ML), [4] carbon onions (1 s for 15 mg•cm -2 ML) [4] and mesoporous CDCs (1 s for 7.3 mg•cm -2 ML). [26] Compared to commercial activated carbons operated in the same electrolyte, the τ 0 value of CDC-G-1 is more than 10 times shorter (10 s for a 15 mg•cm -2 ML). [32] This excellent power performances of CDC-G-1 are mainly assigned to its unique open structure offering free ion paths for the electrolyte to reach micropores together with high electronic conductivity of graphene nanosheets.

Conclusions

  (a), the XRD diffractions of SiC-G-1 can be assigned to two sets of diffraction planes. A first set of five peaks located at 35.5°, 41.4°, 60.0°, 65.5°, 71.7° and 75.1° (2θ), correspond to the [002], [220], [110], [103], [112] and [004] characteristic diffractions of the 3C-SiC F43m cubic structure, (PDF ref. pattern: Silicon carbide, #01-075-0254). Another set of diffractions centered at 26.3°, 41.3°, 75.2° and 89.7° (2θ) is assigned to the [111], [100], [110] and [200] silicon Bragg diffractions (PDF ref.

  After chlorination (Reaction (3)), only the characteristic peaks of carbon are shown in both XRD patterns of CDC-G-1 and CDC-G-2 (Figures 1(a) and (b)). For CDC-G-1, the broad peak in the 20° to 30° (2θ) range is assigned to amorphous carbon, and the small peaks located at 42.8°, 43.9° and 61.8° (2θ) are characteristic of graphite (PDF ref. pattern: carbon, #01-079-1469). Considering the integral area of the observed XRD features, the CDC-G-1 sample is mostly composed of amorphous carbon together with a few graphite impurities. The XRD peaks of CDC-G-2 located at 26.2°, 44.4°, 54.0° and 83.2° (2θ) are [002], [101], [004] and [112] graphite peaks, respectively (PDF ref. pattern: carbon, #01-075-1621). Unlike CDC-G-1, the whole CDC-G-2 sample presents some level of graphitization. Raman spectra of the prepared SiC-G-1 and SiC-G-2 samples are shown in Figure 1(c) and (d), and in Figure S1(a) in the wave number ranges from 1250 to 2000 cm -1 and from 700 to 1000 cm -1 ,

Figure

  Figure S1(a) show the expected discrepancies and confirm SiC-G-1 and SiC-G-2 as 3C-SiC

5 to be established as depicted in Figure 3 .

 53 The adsorption onto GO and self-assembling of CTAB/TEOS moieties lead, by hydrolysis of TEOS, to 2D sandwiched SiO 2 /GO nanosheets.During the magnesio-thermal reaction, generated SiO x vapors diffuse either perpendicularly and laterally into RGO layers.[24] The resulting SiCs inherit the nanosheet structure from their silica/GO composite precursors. Through the silicon oxidation/extraction by chlorination, massive micropores are opened in SiC to give CDC nanosheets.Textural structures of SiC-G and CDC-G materials were characterized by N 2 and Ar adsorption/desorption measurements, respectively. In FigureS2(a), N 2 sorption isotherms of SiC-G-1 and SiC-G-2 exhibit a type-III behavior with a H3-type hysteresis, characteristic of macroporous materials with a small amount of slit-shaped mesopores. The pore size distributions (PSD) of SiC-G-1 and SiC-G-2 were calculated from their adsorption branch as shown in FigureS2(b). Both the PSDs of SiC-G-1 and SiC-G-2 are broadly ranging from 7 to 100 nm. These pores are assigned to a random porous structure built on exfoliated nanosheets.

5

  cell voltage), show rectangular shapes characteristic of electrochemical double layer capacitor (EDLC) behaviors. The specific capacitance of the CDC-G-1 electrode calculated by using the integral area of the CV curve at 20 mV•s -1 is 80 F•g -1 . That of CDC-G-2 is lower, at 65 F•g -1 for the same scan rate. For porous carbonaceous materials, most of the capacitance (more than 80%) comes from the volume developed by pores ranging from 0.7 to 2 nm. The pore volumes of CDC-G-1 and CDC-G-2 in this pore size range (the dashed area under the curves in Figures4(b) and (d)) occupy 55% and 38% of total pore volumes (calculated from 0.45 to 10 nm), respectively. The specific capacitances presently measured for CDC-G-1 and

[ 2 , 9 ] 32 F

 2932 The low ESR as well as the decent specific capacitance of the CDC-G-1 electrode can be assigned to the remarkable nanostructure of CDC-G-1 with micropores about 1 nm in diameter, highly accessible to the electrolyte ions thanks to the 2D graphene-like open surface. For CDC-G-2, the high frequency range shows two loops which are related to charge transport phenomena, in the present case, more likely to the electron diffusion at the CDC-G2 grain boundaries. The corresponding ESR was measured at the intercept of the second high frequency loop at 0.55 Ω•cm². As compared to CDC-G-1, the high frequency part of the Nyquist plot for CDC-G-2 shows a shift of the real part of the impedance witch fit the observed more resistive CV.Nevertheless, EIS results for both CDC-G-1 and CDC-G-2 electrodes fit well with their capabilities at high scan rates, and support the potentially high power capability of these graphene-like CDC materials.A complex capacitance model, based on the modeling of the capacitance in real part C' (ω) and imaginary part C'' (ω) both as function of the frequency, was used to investigate the frequency behavior of the cells.[32] Figure 8(a) and (c) show the change of the real part of the 5 capacitance C' (ω) with frequency of the CDC-G-1 and CDC-G-2 cell, respectively. At low frequency, the observed plateaus corresponding to phase angles of approximately -86°, are characteristic of the purely capacitive behavior of the two cells.[32] Most of the capacitance of CDC-G-1 and CDC-G-2 cells is maintained up to frequencies of 0.6 Hz and 0.2 Hz, respectively. The -45° phase angle of CDC-G-1 is reached at about 2.4 Hz. In contrast, CDC-G-2 relatively lags behind since the phase angle reaches -45° at about 1.1 Hz. The corresponding capacitances are 0.09 F•cm -2 and 0.16 F•cm -2 for CDC-G-1 and CDC-G-2,respectively. These values should theoretically correspond to half of the total capacitances of the cells, in perfect agreement with capacitance measured by CV at 0.18 F•cm -2 and 0.•cm -2 for CDC-G-1 and CDC-G-2, respectively.

Figure 8 (

 8 Figure 8(b) and (d) show the imaginary part of the capacitance, C' (ω) vs. frequency. The

4 FFiguresFigure 1 .

 41 Figures

Figure 2 .

 2 Figure 2. TEM images of CDC-G-1 (a) and CDC-G-2 (b). The partial enlarged images of

Figure 3 .

 3 Figure 3. Synthetic mechanism from 2D sandwiched SiO 2 /C nanocomposites to SiC5

Figure 4 .

 4 Figure 4. Argon sorption isotherm curves (a) and corresponding pore size distributions (b) of

Figure 5 . 5 Figure 6 .

 556 Figure 5. CVs at various scan rates for CDC-G-1 (a) and CDC-G-2 (b) electrodes in 1.5 M

Figure 7 .

 7 Figure 7. EIS Nyquist plots for CDC-G-1 (a) and CDC-G-2(b) in 1.5 M NEt 4 BF 4 /AN. Mass loadings are 2.6 mg•cm -2 and 6.1 mg•cm -2 , respectively. Insets show data in the high-

Figure 8 .

 8 Figure 8. Real capacitance, C' (a) and imaginary capacitance, C'' (b) vs. the logarithm of

present commercial devices. In: Proceedings of the 8th seminar on double layer capacitors and similar energy storage devices, Deerfield Beach, US, 1998, pp.21-35.

Acknowledgements

P. -C.G would like to thank the Chinese Scientific Council for fellowship. W.-Y.T. was supported by the European Research Council (ERC, Advanced Grant, ERC-2011-AdG, Project 291543-IONACES). P. S. acknowledges the funding from the Chair of Excellence of the EADS foundation -Embedded multi-functional materials‖. C.P. and Y.G. are thankful to the U.S. Department of Energy, Office of Basic EnergySciences, Division of Materials 23

ABSTRACT

Two graphene-like carbide derived carbons (CDC-Gs) were produced by chlorination of SiC nanosheets obtained by magnesio-thermal reduction at moderate temperature of 5 silica/graphene oxide nanocomposites. These CDC-Gs were evaluated as supercapacitor electrode materials in an organic electrolyte. Starting from a low SiO 2 /GO ratio in the precursor, the resulting CDC-G nanosheets are composed of a few layers of graphite, partially coated with microporous CDC. In contrast, a high SiO 2 /GO ratio leads to micropores generated on the basal plane of individual carbon nanosheets. The latter CDC-G shows a remarkable high power capability with 76 % of retention of the initial capacity at scan rates up to 3 V•s -1 . Notably, the equivalent series resistance (ESR) and time constant of the cell were found to be extremely low at 0.45 Ω•cm 2 and 0.4 s, respectively, thanks to the unique 2D open surface and enhanced access to micropores. These features were attributed to the unique nanostructure of the microporous graphene.

Keywords

2D carbide; graphene; microporous CDC; high power supercapacitor; organic electrolyte