Rakhi Sood

Anna Donnadio

Stefano Giancola

Aurélien Kreisz

Deborah J Jones

Sara Cavaliere
email: sara.cavaliere@umontpellier.fr

-Triazole-Functionalized Polysulfone Synthesis through Micro-wave-Assisted Copper-Catalyzed Click Chemistry: a Highly Proton Conducting High Temperature Membrane

Keywords: microwave-assisted synthesis, click-chemistry, copper-catalyzed azide-alkyne cycloaddition, 1,2,3-triazole functionalized polysulfone, high temperature proton exchange membrane fuel cells, phosphoric acid doped membranes : yield point, b: breaking strength, El: elongation at break

published or not. The documents may come

Introduction

The 1,2,3-triazole functionality demonstrates high chemical stability towards severe hydrolytic, oxidizing, and reducing conditions, even at elevated temperatures, as well as aromatic character and the ability to form hydrogen bonds, and antioxidant, anti-microbial and medicinal properties, amongst others. [START_REF] Heravi | Recent Applications of Click Reaction in the Syntheses of 1,2,3-Triazoles[END_REF][START_REF] Totobenazara | New Click-Chemistry Methods for 1,2,3-Triazoles Synthesis: Recent Advances and Applications[END_REF][START_REF] Thibault | A Versatile New Monomer Family: Functionalized 4-Vinyl-1,2,3-Triazoles via Click Chemistry[END_REF][START_REF] Dubey | A Click Chemistry Strategy to Synthesize Geraniol-Coupled 1,4-Disubstituted 1,2,3-Triazoles and Exploration of Their Microbicidal and Antioxidant Potential with Molecular Docking Profile[END_REF][START_REF] Hein | Copper-Catalyzed Azide-alkyne Cycloaddition (CuAAC) and beyond: New Reactivity of Copper(i) Acetylides[END_REF] Due to these features, materials functionalized with 1,2,3-triazole are of interest for a variety of industrial and medicinal applications. Since the remarkable development of the copper-catalyzed azide-alkyne cycloaddition (CuAAC) modular ligation by Sharpless 6 and Meldal [START_REF] Tornøe | Peptidotriazoles on Solid Phase: [1,2,3]-Triazoles by Regiospecific Copper(I)-Catalyzed 1,3-Dipolar Cycloadditions of Terminal Alkynes to Azides[END_REF] , the CuAAC reaction has been extensively used as a tool for the elaboration of various polymers with advanced macromolecular design, including telechelic polymers, block copolymers, dendrimers etc.. [START_REF] Tornøe | Peptidotriazoles on Solid Phase: [1,2,3]-Triazoles by Regiospecific Copper(I)-Catalyzed 1,3-Dipolar Cycloadditions of Terminal Alkynes to Azides[END_REF][START_REF] Meldal | Cu-Catalyzed Azide -Alkyne Cycloaddition[END_REF][START_REF] Iha | Applications of Orthogonal "Click" Chemistries in the Synthesis of Functional Soft Materials[END_REF][10][START_REF] Lutz | 3-Dipolar Cycloadditions of Azides and Alkynes: A Universal Ligation Tool in Polymer and Materials Science[END_REF][START_REF] Johnson | Construction of Linear Polymers, Dendrimers, Networks, and Other Polymeric Architectures by Copper-Catalyzed Azide-Alkyne Cycloaddition "Click[END_REF][START_REF] Bock | CuI-Catalyzed Alkyne-Azide "Click" Cycloadditions from a Mechanistic and Synthetic Perspective[END_REF][START_REF] Fournier | Clicking Polymers: A Straightforward Approach to Novel Macromolecular Architectures[END_REF][START_REF] Alonso | Copper Nanoparticles in Click Chemistry[END_REF][START_REF] Dervaux | Heterogeneous Azide-Alkyne Click Chemistry: Towards Metal-Free End Products[END_REF][START_REF] Hashemi | In Situ Prepared CuI Nanoparticles on Modified Poly(styrene-Co-Maleic Anhydride): An Efficient and Recyclable Catalyst for the Azide-Alkyne Click Reaction in Water[END_REF][START_REF] Nemati | Magnetic Nano-Fe3O4@TiO2/Cu2O Core-Shell Composite: An Efficient Novel Catalyst for the Regioselective Synthesis of 1,2,3-Triazoles Using a Click Reaction[END_REF][START_REF] Hosseinnejad | Computational Studies on the Regioselectivity of Metal-Catalyzed Synthesis of 1,2,3 Triazoles via Click Reaction: A Review[END_REF][START_REF] Fazeli | Green and Facile Synthesis of 1,4-Disubstituted 1,2,3-Triazoles via a Click Reaction of α-Bromo Ketones, [bmim]N3 and Terminal Acetylenes Microwave Irradiation[END_REF][START_REF] Billiet | Step-Growth Polymerization and "click" Chemistry: The Oldest Polymers Rejuvenated[END_REF][START_REF] Mirsafaei | In Situ Prepared Copper Nanoparticles on Modified KIT-5 as an Efficient Recyclable Catalyst and Its Applications in Click Reactions in Water[END_REF] To extend the range of applications of polymers with the 1,2,3triazole functionality, it is crucial to extend the library of materials which capitalize on the important properties of 1,2,3triazole, by varying structural parameters including the nature of the polymer backbone, pendant chain structures and triazole-ring substituents. This will permit more precise understanding of the interplay of structure and properties in order to develop high performance materials with properties fine-tuned according to the requirements of the application. Recently, synthesis of 1,2,3-triazole functionalized poly-vinyls [START_REF] Thibault | A Versatile New Monomer Family: Functionalized 4-Vinyl-1,2,3-Triazoles via Click Chemistry[END_REF] , acrylates [START_REF] Sood | 1,2,3-Triazolium-Based Poly(acrylate Ionic Liquid)s[END_REF][START_REF] Sood | Triethylene Glycol-Based poly(1,2,3-Triazolium Acrylate)s with Enhanced Ionic Conductivity[END_REF] , ionenes [START_REF] Mudraboyina | 1,2,3-Triazolium-Based Poly(ionic Liquid)s with Enhanced Ion Conducting Properties Obtained through a Click Chemistry Polyaddition Strategy[END_REF] and siloxanes [START_REF] Akbey | 1H Solid-State NMR Investigation of Structure and Dynamics of Anhydrous Proton Conducting Triazole-Functionalized Siloxane Polymers[END_REF] through CuAAC chemistry have been reported for application in dye sensitized solar cells, fuel cells, batteries and electrochromic devices. Ideally, the CuAAC reaction is carried out at room temperature and produces high yields in short reaction times [START_REF] Kappe | Click Chemistry under Non-Classical Reaction Conditions[END_REF] . However, in some cases, it has been found to be very slow and to require elevated temperatures and/or long times for its completion. [START_REF] Kappe | Microwave Dielectric Heating in Synthetic Organic Chemistry[END_REF] The advent of Microwave-Assisted Organic Synthesis (MAOS) has enabled not only significantly accelerated rate of chemical transformations but also cleaner products, with higher yields, by avoiding unwanted side reactions. [START_REF] Kappe | Click Chemistry under Non-Classical Reaction Conditions[END_REF] Similarly, the implementation of microwave heating in CuAAC reactions has led to remarkable improvement in the yields and the reaction times. [START_REF] Kappe | Click Chemistry under Non-Classical Reaction Conditions[END_REF][START_REF] Appukkuttan | Microwave-Assisted Cycloaddition Reactions[END_REF] In particular, microwave assisted-CuAAC (MA-CuAAC) has been successfully utilized for organic synthesis, supra-molecular chemistry and synthesis of carbohydrates as well as oligonucleotide analogues. [START_REF] Kappe | Click Chemistry under Non-Classical Reaction Conditions[END_REF][START_REF] Gawande | Microwave-Assisted Chemistry: Synthetic Applications for Rapid Assembly of Nanomaterials and Organics[END_REF] Nonetheless, the application of microwave heating in macromolecular synthesis remains very limited up to now.

The introduction of facile and efficient MA-CuAAC chemistry for the production of high performance aromatic thermoplastics bearing a highly stable basic functionality would be an elegant strategy for the development of functionalized materials for a variety of applications. For instance, due to the presence of a pendant heterocyclic group, such polymers are expected to be proton conducting and could find application as membrane materials in Proton Exchange Membranes Fuel Cells (PEMFC) operating under quasi-anhydrous conditions at temperatures above those where perfluorosulfonic acid ionomers such as Nafion ® or Aquivion ® can be used. Although practicable at lower temperatures, water management in a fuel cell at temperatures higher than 100 °C has a significant impact on the viability of the membrane electrode assembly (MEA) and on overall operation of the system. Over the last decade, new concepts have evolved involving alternative proton carriers that mark a move towards reducing the need for high levels of hydration. By replacing water as a proton carrier by Nheterocycles such as imidazole [START_REF] Schuster | Toward a New Type of Anhydrous Organic Proton Conductor Based on Immobilized Imidazole[END_REF][START_REF] Kreuer | Imidazole and Pyrazole-Based Proton Conducting Polymers and Liquids[END_REF] , pyrazole [START_REF] Kreuer | Imidazole and Pyrazole-Based Proton Conducting Polymers and Liquids[END_REF] , benzimidazole or triazole [START_REF] Subbaraman | 4,5-Dicyano-1H-[1,2,3]-Triazole as a Proton Transport Facilitator for Polymer Electrolyte Membrane Fuel Cells[END_REF] , the conductivity is expected to be less dependent on water content, since proton transfer between heterocycles is an intrinsic property of these systems. The relevance of these concepts was shown elegantly by Kreuer, in particular in pioneering work on imidazole -sulfonated poly(ether ketone) type ionomers. [START_REF] Kreuer | Membrane Materials for PEM Fuel Cells: A Microstructural Approach[END_REF] Similarly, Zhou and co-workers described membranes of sulfonated polysulfone impregnated with 1H-1,2,3-triazole as solvent. [START_REF] Zhou | Promotion of Proton Conduction in Polymer Electrolyte Membranes by 1H-1,2,3-Triazole[END_REF] Immobilization of the heterocycle on a polymer is essential to avoid leaching from the polymer matrix over the course of time. A number of model compounds have been described, in which heterocycles are linked through ethylene oxide oligomer [START_REF] Schuster | Toward a New Type of Anhydrous Organic Proton Conductor Based on Immobilized Imidazole[END_REF][START_REF] Schuster | Proton Mobility in Oligomer-Bound Proton Solvents: Imidazole Immobilization via Flexible Spacers[END_REF] or polysiloxane [START_REF] Scharfenberger | Anhydrous Polymeric Proton Conductors Based on Imidazole Functionalized Polysiloxane[END_REF] backbones. Steps towards increasing the concentration of charge carriers have been made by radiation induced grafting of polymeric 1-vinylimidazole on to ethylenetetrafluoroethylene (ETFE) [START_REF] Schmidt | Proton Conducting Membranes Obtained by Doping Radiation-Grafted Basic Membrane Matrices with Phosphoric Acid[END_REF] and by free radical induced polymerization of vinylimidazole on to divinylbenzene modified polybenzimidazole [START_REF] Guan | Preparation and Characterisation of Proton Exchange Membranes Based on Crosslinked Polybenzimidazole and Phosphoric Acid[END_REF] .

Steps towards increasing nanophase separation between the heterocycles and the polymer backbone have been described in recent work on the development of novel copolymers in which heterocycle is immobilized as a pendant group on a chlorotrifluoroethene backbone [START_REF] Frutsaert | Synthesis and Properties of New Fluorinated Polymers Bearing Pendant Imidazole Groups for Fuel Cell Membranes Operating Over a Broad Relative Humidity Range[END_REF][START_REF] Campagne | Novel Blend Membranes of Partially Fluorinated Copolymers Bearing Azole Functions with Sulfonated PEEK for PEMFC Operating at Low Relative Humidity: Influence of the Nature of the N-Heterocycle[END_REF][START_REF] Campagne | New Semi-IPN PEMFC Membranes Composed of Crosslinked Fluorinated Copolymer Bearing Triazole Groups and sPEEK for Operation at Low Relative Humidity[END_REF][START_REF] Campagne | Anhydrous Proton Motion Study by Solid State NMR Spectroscopy in Novel PEMFC Blend Membranes Composed of Fluorinated Copolymer Bearing 1,2,4-Triazole Functional Groups and sPEEK[END_REF] . Guiver et al. have developed aromatic polymers functionalized with 1,2,3-triazole and 5-amino-benzotriazole groups for Alkaline Fuel Cell (AFC) and Direct Methanol Fuel Cell (DMFC) applications respectively. [START_REF] Li | Acid-base Blend Membranes Consisting of Sulfonated Poly(ether Ether Ketone) and 5-Amino-Benzotriazole Tethered Polysulfone for DMFC[END_REF][START_REF] Li | Towards High Conductivity in Anion-Exchange Membranes for Alkaline Fuel Cells[END_REF] Sanghi et al. elaborated composite membranes of 1H-1,2,3-triazole functionalized polysiloxanes in combination with trifluoroacetic acid/phosphoric acid and reported proton conductivity of 1.6 mS/cm at 180 °C for a membrane doped with 2.5 moles of phosphoric acid per polymer repeat unit. [START_REF] Sanghi | Hybrid Inorganic-Organic Proton Exchange Membranes Containing 1H-1,2,3-Triazole Moieties[END_REF] The nature of heterocycle and its substituents can have significant impact on thermal, mechanical, anti-oxidant properties. [START_REF] Nulwala | Synthesis and Characterization of Isomeric Vinyl-1,2,3-Triazole Materials by Azide-Alkyne Click Chemistry[END_REF][START_REF] Green | Alkyl-Substituted N-Vinylimidazolium Polymerized Ionic Liquids: Thermal Properties and Ionic Conductivities[END_REF][START_REF] Akutsu | Preparation and Characterization of Novel Aromatic Polyimides Having 4 , 5-Di ( 1 , 3-Phenylene ) Imidazole Structure[END_REF] In this work, we have developed microwave assisted-CuAAC to synthesize 1,2,3-triazole functionalized polysulfones with various aliphatic and aromatic triazole ring substituents (propyl, heptyl and phenyl) in order to validate the feasibility and versatility of the synthesis route as well as to establish the interplay of structure-properties for the resulting polymers, which were characterized using 1 H- [START_REF] Bock | CuI-Catalyzed Alkyne-Azide "Click" Cycloadditions from a Mechanistic and Synthetic Perspective[END_REF] C NMR, DOSY-NMR and FTIR-ATR spectroscopies as well as differential scanning calorimetry (DSC) and thermogravimetric analysis coupled with mass spectroscopy (TGA-MS). Those with high degrees of 4-heptyl-1,2,3-triazole and 4-phenyl-1,2,3-triazole functionalities were cast into membranes and doped with phosphoric acid and characterized for their proton conduction and mechanical properties. To the best of our knowledge, MA-CuAAC is here employed for the first time as a synthetic route to afford 1,2,3-triazole functionalized polysulfones and their phosphoric acid doped membranes.

EXPERIMENTAL SECTION

Materials. Chlorotrimethylsilane (TMCS, C3H9ClSi, ≥97.0%), p-formaldehyde powder (HO(CH2O)nH, 95%), tin tetrachloride (SnCl4, 98%), sodium azide (NaN3, ≥99%), diisopropylethylamine (DIPEA, [(CH3)2CH]2NC2H5, 99%), 1-pentyne (CH3CH2CH2C≡CH, 99%), 1-nonyne (CH3(CH2)6C≡CH, 99%), phenylacetylene (C6H5C≡CH, 97%), copper(I) iodide triethylphosphite (CuI•P(OEt)3, 97%), N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA, [(CH3)2NCH2CH2]2NCH3, 99%), phosphoric acid solution (H3PO4, 85 wt% in H2O) and all solvents were purchased from Aldrich and used as received. Polysulfone granules (Mn=80 KDa and Mw=102 KDa) were purchased from Goodfellow. 26 wt% solution of poly[2,2'-(mphenylene)-5,5'-bibenzimidazole] (PBI; Mw=35 KDa) in dimethylacetamide was purchased from PBI Performance Products.

Characterization methods. NMR spectra were recorded on a Bruker Avance HD 400 MHz spectrometer equipped with a BBFO (Broad band fluorine observation) probe in DMF-d6 using tetramethylsilane (TMS) as reference residual hydrogenated solvent. Peak multiplicities are designated as m = multiplet, s = singlet, t = triplet, br = broad. Attenuated total reflection-infrared (ATR-IR) spectra were recorded with Horiba Jobin Yvon Labram Aramis instrument using a MCT Ir2 module detector and ATR 36x objective lens. Differential scanning calorimetry (DSC) measurements were performed under argon atmosphere using a Mettler Toledo DSC1 at a heating rate of 10 °C/min. Tg values were determined from the second heating cycle. Thermogravimetric analysis (TGA) was carried out under argon using a STA 449 F1 Jupiter (Netzsch) coupled with a mass spectrometer Aeolos III, at a heating rate of 10 °C/min. Acid doping level (ADL) i.e. number of phosphoric acid molecules per repeat unit of the polymer was determined using ion chromatography (Shimadzu HPLC apparatus equipped with a Shim-pack IC-A1 column and a conductivity detector; a flow rate of 1.5 mL min -1 , an injection volume of 45 μL, a column temperature of 40 °C; with the mobile phase consisting of an aqueous solution of a mixture of 1.5 mmol L -1 phthalic acid and 0.7 mmol L -1 diethylenetriamine for the detection of HPO4 2-anion). The in-plane proton conductivity of the acid-doped membranes was determined using a dry Fumatech MK3 impedance cell (equipped with a temperature controller) using a Keithley 2400 source meter in the temperature range 100-140 °C. Stress-strain mechanical measurements were performed using a Zwick Roell Z1.0 with a 200 N static load cell. The Young's modulus (the slope of the stress-strain curve in the elastic-deformation region), yield stress (maximum stress that can be developed without causing plastic deformation), stress at break (the tensile stress at the breaking point of the specimen), and elongation at break (the percentage increase in length that occurs before the sample breaks) were measured on rectangular (100 x 5 mm) strips of membrane, obtained using a cutting machine. The thickness of the membrane was in the range 25-30 µm. An initial grip separation of 20.000 ±0.002 mm and a crosshead speed of 30 mm min -1 were used. At least five replicate strips were analyzed. The data were treated by the TestXpert V11.0 Master software. Membrane preparation. Membranes were prepared by casting a solution of the polymer in chloroform or DMF on to a glass plate, followed by thermal treatment at 40 °C for 24 hours and then at 100 °C under vacuum for 2 hours to remove solvent and anneal the membrane. Phosphoric acid doped membranes were elaborated by immersion in 14.6 M aqueous solution of H3PO4 at ambient temperature and at 110 °C. The membranes were weighed at regular intervals (after being thoroughly blotted with paper) until stable weights were achieved. The acid uptake (AU%) of the membrane was calculated in percent on the dry polymer basis as the ratio between the phosphoric acid content (Wpa) and the polymer content (Wp) of the membrane using the following equation:

𝑨𝑼 (%) = ( 𝑾𝒑𝒂 𝑾𝒑 ) * 𝟏𝟎𝟎
The dry weight of the polymer was determined by leaching out the phosphoric acid though immersion of the doped membrane in 50 ml of 0.1 M aqueous NaOH solution followed by drying the polymer at 80 °C under vacuum for 8 hours.The contribution of water uptake (WU%) in the total uptake was determined on the dry polymer (Wp) plus phosphoric acid content (Wpa) basis by comparing the wet weight of the membrane after immersion in aqueous phosphoric acid solution (Wt) and the weight of dry polymer plus phosphoric acid (Wp+Wpa) using the following equation:

𝑾𝑼 (%) = ( 𝑾𝒕 -(𝑾𝒑 + 𝑾𝒑𝒂) (𝑾𝒑 + 𝑾𝒑𝒂) ) * 𝟏𝟎𝟎
The mass as well as area and volume swelling of the phosphoric acid doped membranes were calculated by measuring the dimensions of the membranes before and after swelling in the H3PO4 solution and using the following equation:

𝑽𝒂𝒓𝒊𝒂𝒕𝒊𝒐𝒏 (%) = (𝑫𝒇 - 𝑫𝒊 𝑫𝒊 ) * 𝟏𝟎𝟎
where Di and Df stands for dimension of membrane before and after swelling respectively.

General Procedure for chloromethylation reaction. Synthesis of 2. A solution of polysulfone (1, 1 g, 2.25 mmol), p-formaldehyde (0.68 g, 22.5 mmol) and SnCl4 (0.06 g, 0.24 mmol) in chloroform (200 mL) was stirred under nitrogen atmosphere. [START_REF] Abu-Thabit | New Highly Phosphonated Polysulfone Membranes for PEM Fuel Cells[END_REF] Chlorotrimethylsilane (2.43 g, 22.5 mmol) was added drop-wise into this solution and the mixture was kept under reflux temperature of 50 °C for 12h and 36h to recover chloromethylated polysulfone 2a (DS=30 %, 0.89 g) and 2b (DS=89 %, 0.95 g) respectively after the reaction being quenched by slowly adding the reaction mixture into ethanol. The precipitated polymers were re-dissolved in chloroform (10% w/v solution) and re-precipitated in methanol followed by drying at 50 °C under vacuum for 12 hours. [START_REF] Heravi | Recent Applications of Click Reaction in the Syntheses of 1,2,3-Triazoles[END_REF] H NMR (400 MHz, DMF) δ(ppm): 8.05-6.90 (aromatic backbone), 4.75 (brs, 2H, CH2Cl), 1.70 (brs, 6H, C-CH3). [START_REF] Bock | CuI-Catalyzed Alkyne-Azide "Click" Cycloadditions from a Mechanistic and Synthetic Perspective[END_REF] General procedure for azidation reaction. Synthesis of 3: A mixture of 2a (1 g, 0.61 mmol of chloromethyl functionalities) and sodium azide (0.16 g, 2.43 mmol) was stirred in DMF (10 mL) at 60 °C for 24 h. [START_REF] Sood | 1,2,3-Triazolium-Based Poly(acrylate Ionic Liquid)s[END_REF] After this, the reaction mixture was centrifuged twice to separate remaining salts (excess of sodium azide and sodium chloride formed during the reaction) and the polymer was precipitated by pouring the solution drop-wise into methanol to recover azide-functionalized polysulfone 3a which was then dried at 50 °C under vacuum for 12 h (0.93 g). The same procedure was applied to 2b (1 g, 0.61 mmol) to obtain 3b (0.90 g). [START_REF] Heravi | Recent Applications of Click Reaction in the Syntheses of 1,2,3-Triazoles[END_REF] H NMR (400 MHz, DMF) δ(ppm): 8.05-6.90 (aromatic backbone), 4.46 (brs, 2H, CH2N3), 1.70 (brs, 6H, C-CH3). [START_REF] Bock | CuI-Catalyzed Alkyne-Azide "Click" Cycloadditions from a Mechanistic and Synthetic Perspective[END_REF] C NMR (100 MHz, DMF) δ(ppm): 117.90-161.82 (aromatic backbone), 49.75 (CH2N3), 42.27 (C-CH3), 30.43 (C-CH3).

General Procedure for MA-CuAAC. Synthesis of 4,5,6. A solution of 3a (100 mg, 0.062 mmol of azide functionalities), 1pentyne (I, 12.6mg, 0.19 mmol), CuI•P(OEt)3 (22 mg, 0.062 mmol) and DIPEA (8 mg, 0.062 mmol) in DMF (5 mL) was prepared in a glass recipient and then placed in a microwave reactor (Sairem MiniFlow 220SS) [START_REF] Sood | Triethylene Glycol-Based poly(1,2,3-Triazolium Acrylate)s with Enhanced Ionic Conductivity[END_REF] . The maximum power of the reactor was set at 20 W. The reaction was carried out at 120 °C for 30 minutes under constant stirring. Afterwards, the solution was slowly added into methanol and the precipitated polymer was washed with methanol several times followed by drying under vacuum at 50 °C for 24 h. To extract the catalyst, a solution of the dried polymer in chloroform (10 mL) and a solution of PMDETA (150 µL) in water (2 mL) were taken in a separating funnel, shaken well and left for a short time to settle out forming two layers. The aqueous phase forming the bottom layer turned blue due to complexation of the catalyst and was removed. The recovered organic phase was then taken and the procedure was repeated until the aqueous phase was colorless. Afterwards, the retrieved organic phase was concentrated by evaporation of half of the volume of the solvent followed by its drop-wise addition into methanol to precipitate the polymer. The precipitated polymer was then dried at 50 °C under vacuum for 12 hours to recover 1,2,3-triazole-functionalized polysulfone 4a (85 mg). An identical procedure was used combining 3a (100 mg, 0.062 mmol) with 1-nonyne (II, 11.5 mg, 0.093 mmol) and phenylacetylene (III, 9.5 mg, 0.093 mmol) yielding 5a (93 mg) and 6a (88 mg) respectively as well as combining 3b (100 mg, 0.19 mmol) with 1-nonyne (II, 34.6 mg, 0.28 mmol) and phenylacetylene (III, 28.5 mg, 0.093 mmol) yielding 5b (105 mg) and 6b (102 mg) respectively. The CuAAC reaction between 3a (100 mg, 0.062 mmol) and 1-pentyne (I, 12.6mg, 0.19 mmol)/phenylacetylene (III, 9.5 mg, 0.093 mmol) using conventional heating was also carried out for 20 hours using a protocol described earlier [START_REF] Dimitrov | Polysulfone Functionalized With Phosphonated Poly ( Pentafl Uorostyrene ) Grafts for Potential Fuel Cell Applications[END_REF][START_REF] Gaina | Chemical Modification of Chloromethylated Polysulfones via Click Reactions[END_REF] yielding 4a (57 mg) and 6a (51 mg) respectively.

Results and Discussion

Synthesis of 1,2,3-triazole functionalized polysulfones and their characterization 1,2,3-triazole functionalized polysulfone was synthesized by sequential chemical modifications of the initial polymer followed by microwave-assisted Copper-catalyzed Azide Alkyne Cycloaddition (CuAAC) reaction (Scheme 1). Firstly, chloromethylation of polysulfone 1 was carried out in CHCl3 by using chlorotrimethylsilane, p-formaldehyde and tin tetrachloride as previously described [START_REF] Avram | Polymers with Pendent Functional Groups. V. Thermooxidative and Thermal Behavior of Chloromethylated Polysulfones[END_REF] . The degree of substitution (DS) of the resulting chloromethylated polymer 2 was calculated from the ratio of the CH2Cl proton signal to that of the methyl proton signal in the 1 H-NMR spectrum (Table 1). After reaction at 50 °C for 12 and 36 hours, the degrees of substitution were 30 and 89% respectively, considering one functional group per repeating unit. Afterwards, 2 was modified out by reacting it with sodium azide in DMF at 45 °C for 48 hours to afford azide-functionalized polysulfone 3. The quantitative transformation of chloromethylated sites into azidomethylated sites was confirmed in the 1 H-NMR by the total disappearance of the signal of the methylene protons of CH2Cl at 4.75 ppm and the appearance of a resonance at 4.46 ppm characteristic of CH2N3, as well as by a change in the chemical shift of aromatic hydrogen, with respect to the methylated-functional group, from 7.65 ppm to 7.48 ppm (figure 1). The composition of the modified polymers was determined by elemental analysis through combustion and pyrolysis processes. Table 1 presents the experimental values determined by the analysis, which are fully consistent with those deduced from the DS.

The azided-polysulfones 3a and 3b were used as precursors for the preparation of 1,2,3-triazole functionalized polysulfones. Microwave assisted chemistry is completely dependent on the ability of the components of the reaction mixture to absorb microwave energy and transform it into heat. The heating characteristics of the components in turn depend on their dielectric properties [START_REF] Kappe | Controlled Microwave Heating in Modern Organic Synthesis[END_REF] . In particular, the reaction medium plays a very significant role in efficient absorption of microwaves and rapid heating of the system. A system with high polarity is therefore likely to encourage instant sufficient heating by microwaves. In this regard, DMF exhibits satisfactory microwave absorption while also being a good solvent for azidedpolysulfone and thus it was chosen as the reaction medium for MA-CuAAC. Initially, a limpid solution of the azided polymer, alkyne (I: 1-pentyne or II: 1-nonyne or III: 1-phenylacetylene, CuIP(OEt)3 and DiPEA was prepared in DMF in a closed quartz recipient followed by exposure to microwave energy in the reactor for 30 minutes. The temperature of the system was controlled at 120 °C during the course of the reaction. The microwave reactor utilized in the work constantly provided the data concerning the absorbed as well as the reflected microwave power by the reaction system. All the reaction mixtures absorbed power up to 13-14 W to maintain the temperature while the maximum power was set at 20 W. After precipitation and removal of the catalyst, the extent of CuAAC reaction between 3a and I or II or III resulting in 4a or 5a or 6a respectively, was confirmed to be complete from the appearance of 1,2,3-triazole proton signal at 7.75 ppm, 7.78 ppm and 8.49 ppm for 4a, 5a and 6a respectively, the quantitative shift of the peak at 4.46 ppm for the methylene protons neighboring the azide of 3a to 5.58 ppm, 5.58 ppm and 5.69 ppm for the methylene protons neighboring N-1 position of the 1,2,3-triazole group of 4a, 5a and 6a respectively, and the change in position of the peak at 7.52 ppm corresponding to aromatic -hydrogen in 3a towards lower chemical shift, where it is superimposed on other aromatic proton peaks present between 7.27-7.37 ppm for 4a, 5a and 6a respectively (Figure 1). In addition, new peaks corresponding to the propyl, heptyl and phenyl moieties of the 1,4-substituted 1,2,3-triazole groups of 4a, 5a and 6a respectively are clearly observed with matching integration ratios (all the labels are displayed in figure 1). ). It is important to note that all the spectra recorded in this work were obtained for samples dissolved in deuterated-DMF instead of the commonly used CDCl3 since the former allowed very clear segregation of peaks corresponding to aromatic and 1,2,3-triazole protons which thus facilitated collection of evidence for the chemical transformations occurring at each step. Furthermore, in the ATR-FTIR a band observed previously at 2093 cm -1 for the azide group of 3a is absent in the spectra of 4a, 5a and 6a (Figure 2), which is also consistent with completion of the CuAAC reaction. High purity of the clicked polymers (4a, 5a and 6a) was ascertained by DOSY-NMR which, as a spectroscopic chromatography tool, helps in separating different species present in a mixture based on their diffusion coefficients, since each species exhibits a distinct diffusion coefficient, depending on its size and shape. For instance, the DOSY-NMR spectrum of 5a (figure 3) shows no species other than the clicked polymer itself and thus affirms its high purity. Also, we carried out the CuAAC reaction on azided polysulfone by using a conventional method so as to compare with the results obtained with the microwave-assisted route. The conventional method requires not only complicated preparation steps such as multiple freeze/thaw cycles and inert atmosphere throughout the reaction, in order to avoid any unwanted side reactions but also needs up to 20 hours for the completion of the reaction with lower yield (43% of 5a and 40% of 6a) of the final product. In order to validate the efficiency of MA-CuAAC for the synthesis of highly functionalized polymers, the same procedure was carried out on 3b (with DS=0.89) and 1-nonyne or 1-phenylacetylene leading to formation of 5b and 6b respectively with similar rapidity and high yields. The ATR-FTIR spectra of 5b and 6b confirm the completion of the CuAAC reaction through the disappearance of the band observed previously at 2093 cm -1 for the azide group of 3b (figure S1 of supplementary information). Thus, MA-CuAAC provides a very facile route to polymers bearing the 1,2,3-triazole functionality with a range of alkyl and aryl pendant structures.

The solubility behavior of azided-and 1,2,3-triazole-functionalized polysulfones remains similar to non-modified polysulfone (Table 2). All these polymers are soluble in polar aprotic solvents such as DMF, dimethylsulfoxide (DMSO) and dimethylacetamide (DMAc) while insoluble in polar protic solvents such as water, methanol (MeOH) and ethanol (EtOH) as well as non-polar solvents such as hexane, toluene with an exception of being soluble in chloroform;

The glass transition temperatures (Tg) of azided precursors 3 (a,b) and their 1,2,3-triazole derivatives 4-6 (a,b) were determined using Differential Scanning Calorimetry (DSC). PSU is an amorphous polymer with Tg of 188 °C. Azidation of PSU results in slight decrease in its Tg (Tg =170 °C for 3a and Tg =151 °C for 3b, Table 2) as previously reported. [START_REF] Gaina | Chemical Modification of Chloromethylated Polysulfones via Click Reactions[END_REF] Conversely, the Tg of 1,2,3-triazole functionalized polysulfone is considerably impacted by the nature of the substituent on the triazole group according to: heptyl<propyl<phenyl. The higher Tg of the phenyl substituted 1,2,3-triazole-polymer is likely to be due to its bulkier nature, imparting rigidity and resonance stabilization to the polymer structure, in comparison to alkyl pendant groups. [START_REF] Kunal | Role of Chemical Structure in Fragility of Polymers: A Qualitative Picture[END_REF][START_REF] Paul | The Use, Design, Synthesis and Properties of High Performance/High Temperature Polymers: An Overview[END_REF] Moreover, an increase in the chain length of the alkyl pendant group results in a decrease in the Tg of the polymer owing to decrease in intermolecular coupling (Tg =147 °C for 4a and Tg =138 °C for 5a). [START_REF] Kunal | Role of Chemical Structure in Fragility of Polymers: A Qualitative Picture[END_REF] Similar trends of decrease in Tg with increasing chain length of the pendant alkyl group have been reported for poly (alkylmethacrylates) [START_REF] Qin | Correlation between Dynamic Fragility and Glass Transition Temperature for Different Classes of Glass Forming Liquids[END_REF] and alkyl substituted poly N-vinylimidazolium [START_REF] Green | Alkyl-Substituted N-Vinylimidazolium Polymerized Ionic Liquids: Thermal Properties and Ionic Conductivities[END_REF] . Interestingly, the impact of the degree of 1,2,3-triazole functionalization on the Tg of the material also depends on the chemical nature of the substituent on the heterocycle. While increase in the degree of 4-heptyl-1,2,3triazole functionalization results in a decrease in Tg (Tg =138 °C for 5a and Tg =115 °C for 5b), 4-phenyl-1,2,3-triazole functionalization does not exhibit such sharp decrease (Tg =182 °C for 6a and Tg =176 °C for 6b). These results clearly underline the impact of pendant structure-stiffness/flexibility on the activation of the backbone segmental motion and thus influence on the Tg of the 1,2,3-triazole-functionalized-polymer.

The thermal stability of functionalized polysulfones was investigated using thermogravimetric analysis (TGA) coupled with mass spectroscopy to analyze the decomposition products generated at different steps during heating. Although the mass spectrum obtained during the course of the analysis was complex, it provided substantial information to understand the mechanism of thermal degradation of the modified polymers. Unmodified polysulfone starts decomposing only above 500 °C. Its functionalization by chlormethylation and azidation results in reduced thermal stability although conversion of azided sites into 1,2,3-triazole groups increases the temperature at which the polymer decomposes (figure 4, Table 2). Nevertheless, all the functionalized polysulfones remain thermally stable at least up to 200 °C. Two main steps of mass loss were observed for all the functionalized polymers. Azided-polysulfone exhibits a first mass loss at 210-230 °C (depending on the degree of functionalization) due to expulsion of nitrogen gas evidenced by a strong ion current peak for the species with m/z=28 (figure 4) leading to transformation of azided sites into nitrene sites, analogous to the thermal behavior of glycidyl azide. [START_REF] Kubota | Combustion Mechanism of Azide Polymer[END_REF] The second major weight loss observed above 340 °C is mainly due to the decomposition of the polymer backbone resulting in evolution of sulfur dioxide (m/z=64), phenol (m/z=94), toluene (m/z=91), benzene (m/z=78) and methane (m/z=16), in agreement with the analysis reported by Gabor et al. on the thermal decomposition of different chemically functionalized polysulfones (such as brominated, bromo-methylated, nitrated, carboxylated). [START_REF] Molnár | Thermal Degradation of Chemically Modified Polysulfones[END_REF] All the 1,2,3-triazole functionalized polymers 4-6 (a,b) undergo thermal degradation in two consecutive steps while the onset temperature depends on the nature of the substituent on the 1,2,3-triazole functionality and the effect is more clearly visible for polymers with higher degree of functionalization (figure 4; TGA thermograms of polymers with 30% degree of functionalization not shown). Generally, 1,2,3-triazoles undergo decomposition of the triazole ring at elevated temperatures (300-600 °C) forming a carbene and eliminating nitrogen gas. 60 6b begins to decompose at 370 °C with elimination of nitrogen gas (m/z=28) which can be attributed to the initial decomposition of 1,2,3-triazole ring while for 5b thermal decomposition begins around 300 °C with elimination of nitrogen gas and alkyl fragments (m/z=43,57) signifying initial decomposition of 1,2,3-triazole ring along with its alkyl substituent group. The decomposition of the 1,2,3-triazole ring is quickly followed by decomposition of the polymer backbone for both 5b and 6b generating similar by-products observed as in the case of the azided polymer. The better thermal stability of 6b can be attributed to the steric shielding provided by the phenyl group to the 1,2,3-triazole ring. Similar impact of alkyl and phenyl substituents on the thermal stability of poly (alkylphenylpropiolate) has been reported. [START_REF] Lam | Helical Conjugated Polymers: Synthesis, Stability, and Chiroptical Properties of Poly(alkyl Phenylpropiolate)s Bearing Stereogenic Pendants[END_REF] Phosphoric acid doped 1,2,3-triazole functionalized polysulfone membranes

The polymers with high degree of 1,2,3-triazole functionality bearing heptyl (5b) and phenyl (6b) triazole ring substituents were cast into membranes using DMF as solvent. Membrane thickness, after removal of solvent and annealing, was 30 µm. Membranes were immersed in 14.6 M phosphoric acid solution (85 wt% in water) at ambient temperature and at 110 °C for different time periods (depending on the type of the membrane) until saturation in acid uptake was achieved. The Acid Doping Levels (ADL, number of phosphoric acid molecules per repeat unit of the polymer) of the doped membranes were determined by ion chromatography (Table 3). When acid doping was carried out at room temperature, the acid doping level for 5b membrane was observed to reach a maximum value of 2 after 72 hours, while almost no acid uptake occurred for membrane 6b under these conditions. This behavior of 6b was unexpected, and can be attributable to high rigidity/steric hindrance imparted by phenyl pendant ring-structure to the polymer structure inhibiting any interactions of the triazole ring with phosphoric acid. At 110 °C, the ADL of 5b reached at its maximum of 4.9 molecules H3PO4 per polymer repeat unit after only 1 hour. Uptake of phosphoric acid by 6b membrane was slower, but nevertheless achieved an ADL of 5.0 after 48 hours immersion. Since the membranes were immersed in an aqueous solution of phosphoric acid, it's equally interesting to evaluate the contribution of phosphoric acid and water in the total uptake. Thus, in terms of acid and water uptake (AU% and WU% respectively) for these doped membranes swollen at 110 °C, 5b exhibited an uptake of 70 % of phosphoric acid and 12% of water while 6b exhibited an uptake of 74 % of phosphoric acid and 18% of water. The two contributions of the acid and water uptakes roughly correspond to the weight percent of water and phosphoric acid present in the 14.6 M aqueous solution of phosphoric acid though 6b seems to be slightly more hydrophilic than 5b. Moreover, membrane 6b exhibits much lesser area swelling in comparison to than 5b for very similar ADL, while volume swelling is the same for both (Table 3). These results clearly underline significant impact of nature of 1,2,3-triazole substituent on acid uptake kinetics of the final membrane.

The proton conductivities obtained with 1,2,3-triazole functionalized membranes (5b and 6b) were compared to that of obtained with polybenzimidazole (PBI). A 10 wt% solution of PBI in dimethylacetamide was prepared in a closed vessel to cast a membrane with initial thickness of 52 µm which was then dipped in 14.6 M phosphoric acid solution at 25 °C for 24 hours to obtain an ADL of 6.5. The dependence of the proton conductivity of phosphoric acid doped 5b and 6b on temperature is shown in Figure 5, where it is compared with that of phosphoric acid doped polybenzimidazole (PBI). The conductivity of both the acid-doped 1,2,3-triazole functionalized membranes increases with increasing temperature ( at 110 o C of 5b= 29 mS/cm and of 6b= 22 mS/cm). The higher conductivity of 5b than 6b could be related to the lower Tg of the former allowing improved segmental motion and thus improved conductivity. The ADL and thus the proton conductivity of acid doped 1,2,3-triazole functionalized polymers is lower than that of H3PO4/PBI. However, PBI contains two imidazole groups per repeat unit while 5b and 6b possess only 0.89 1,2,3-triazole moiety per repeating unit in this case which results in higher acid uptake and thus proton conductivity of the former.

The mechanical properties of the membranes of 5b and 6b before and after doping with phosphoric acid were investigated (Table 3). 6b presents higher elastic modulus (Emod) than 5b (Emod of 5b= 1438 MPa and of 6b= 1622 MPa) which can be linked to the rigidity imparted by phenyl substituent of the 1,2,3-triazole ring to the polymer structure. After doping with phosphoric acid, as expected, 6b continue to present relatively higher elastic modulus and lower elongation at break (El) in comparison to 5b (Emod and El of 6b-5.0-H3PO4= 349.0 MPa and 11.3 % respectively; Emod and El of 5b-4.9-H3PO4= 332.7 MPa and 19.3 % respectively) owing to lower plasticization of the doped polymer matrix of the former due to it its structural rigidity (Table 3). Based on all the results, it can be inferred that by increasing the degree of functionalization to multiple 1,2,3-triazole groups per polymer repeating unit (through increase in chloro-methylation degree up to 2 functional groups per repeat unit) as well as by careful choice of the pendant ring substituent, the functional properties can be modulated according to the requirements of the application.

Conclusion

Application of microwave heating to macromolecular synthesis and modification holds great promise for developing straightforward, fast and high-yielding routes to high performance functional materials. We have successfully exemplified this through the synthesis of 1,2,3-triazole functionalized polysulfones bearing various alkyl and aryl ring substituents using microwave-assisted CuAAC chemistry. The degree of 1,2,3-triazole functionalization of the polymer and chemical nature of the ring substituent play critical roles in defining the final properties (thermal, mechanical, proton conduction) of the functionalized polymer. Polysulfone functionalized with 4-phenyl-1,2,3-triazole is characterized by higher Tg, Td and elastic modulus than those with 4-propyl-1,2,3-triazole and 4-heptyl-1,2,3-triazole functionalizations. However, the proton conductivity of phosphoric acid doped polysulfone bearing 4-heptyl-1,2,3-triazole is higher than that bearing 4-phenyl-1,2,3triazole due to lower Tg and higher chain mobility of the former. It is concluded that by carefully adjusting parameters such as degree of functionalization and ring substituents of 1,2,3-triazole functionality, properties and thus performance required from the material for a desired application can be achieved. a) From DSC. b) From TGA; * First major weight loss; ** Second major weight loss; n/o: not observed.

"++" indicates solubility at 10 mg/mL, "+" indicates solubility at 1 mg/mL, "~" indicates partial solubility at 0.1 mg/mL, while "-" indicates no detectable solubility even at 0.1 mg/mL

  C NMR (100 MHz, DMF) δ(ppm): 118.16-162.70 (aromatic backbone), 42.58 (C-CH3), 41.48 (CH2Cl), 30.60 (C-CH3).

4: 1 H

 1 NMR (400 MHz, DMF) δ(ppm): 8.05-6.90 (aromatic backbone), 7.75 (s, 1H, CN=CH), 5.58 (brs, 2H, CH2N), 2.46 (t, 2H, CNCH2CH2CH3), 1.70 (brs, 6H, C-CH3), 1.49 (brm, 2H, CNCH2CH2CH3), 0.80 (t, 3H, CNCH2CH2CH3).[START_REF] Bock | CuI-Catalyzed Alkyne-Azide "Click" Cycloadditions from a Mechanistic and Synthetic Perspective[END_REF] C NMR (100 MHz, DMF) δ(ppm): 118.15-162.65 (aromatic backbone), 148.19 (CN=CH), 122.41 (CN=CH), 48.66 (CH2N), 42.53 (C-CH3), 30.70 (C-CH3), 27.58 (CNCH2CH2CH3), 22.84 (CNCH2CH2CH3), 13.56 (CNCH2CH2CH3). 5: 1 H NMR (400 MHz, DMF) δ(ppm): 8.05-6.90 (aromatic backbone), 7.78 (s, 1H, CN=CH), 5.58 (brs, 2H, CH2N), 2.52 (t, 2H, CNCH2CH2CH2CH2CH2CH2CH3), 1.70 (brs, 6H, C-CH3), 1.52 (brs, 2H, CNCH2CH2CH2CH2CH2CH2CH3), 1.23 (brs, 8H, CNCH2CH2CH2CH2CH2CH2CH3), 0.80 (t, 3H, CNCH2CH2CH2CH2CH2CH2CH3). 13 C NMR (100 MHz, DMF) δ(ppm): 118.15-162.65 (aromatic backbone), 148.19 (CN=CH), 122.35 (CN=CH), 48.64 (CH2N), 42.52 (C-CH3), 30.70 (C-CH3), 29.15-31.94 (CNCH2CH2CH2CH2CH2CH2CH3), 25.57 (CNCH2CH2CH2CH2CH2CH2CH3), 22.73 (CNCH2CH2CH2CH2CH2CH2CH3), 13.96 (CN CH2CH2CH2CH2CH2CH2CH3). 6: 1 H NMR (400 MHz, DMF) δ(ppm): 8.05-6.90 (aromatic backbone + phenyl substituent of 1,4-substituted triazole ring), 8.49 (s, 1H, CN=CH), 5.69 (brs, 2H, CH2N), 1.70 (brs, 6H, C-CH3). 13 C NMR (100 MHz, DMF) δ(ppm): 118.15-162.65 (aromatic backbone + phenyl substituent of 1,4-substituted triazole ring), 148.21 (CN=CH), 121.77 (CN=CH), 48.96 (CH2N), 42.53 (C-CH3), 30.69 (C-CH3).

Scheme 1 . 9 IFigure 1 .

 191 Scheme 1. Synthesis of 1,2,3-triazole functionalized polysulfones 4-6.

Figure 2 .Figure 3 .

 23 Figure 2. FTIR-ATR spectra of azide-functionalized polysulfone 3a and 1,2,3-triazole-functionalized polysulfones 4a-6a, with an inset image of the polymer before and after CuAAC reaction.

Figure 4 .

 4 Figure 4. TGA and ion current traces for species with m/z=28 of non-modified PSU 1 (long-dash line), azide-functionalized polysulfone 3b (short-dash line) and 1,2,3-triazole-functionalized polysulfones with pendant substituent i.e. 5b (solid line) and 6b (dotted line).

Figure 5 .

 5 Figure 5. Conductivity of 1,2,3-triazole-functionalized polysulfones with different pendant structures 5b (ADL: 4.9; solid triangle) and 6b (ADL: 5.0; solid square) and of PBI (ADL: 6.5; solid circle) as a function of temperature in a closed conductivity cell under dry condition.

Table 1 . Degree of substitution (DS) and elemental analysis of azide-functionalized polysulfone 3 and 1,2,3-tria- zole-functionalized polysulfones 4-6.

 1 Calculated from 1 H-NMR spectrum; Theoretical values (Theo.) were calculated based on DS values; Experimental values (Exp.) were determined by combustion and pyrolysis analysis

			Elemental Analysis
	Sample code	DS * (%)	C%		H%		O%		S%		N%
			Theo	Exp.	Theo	Exp. Theo. Exp.	Theo.	Exp. Theo. Exp.
			.		.
	1	0	72.55	72.71	5.12	5.98	14.88	n/a	7.44	7.92	0	0
	3a	30	70.64	68.89	5.06	4.32	14.32	14.18	7.16	6.98	2.82	2.73
	3b	89	67.24	64.89	4.95	3.83	13.34	13.67	6.67	6.41	7.79	7.85
	4a	30	71.27	68.55	5.26	4.61	13.82	14.06	6.91	6.73	2.72	2.59
	5a	30	71.78	69.77	5.58	4.65	13.30	12.27	6.66	6.06	2.65	3.10
	5b	89	70.95	68.40	6.44	5.05	10.84	10.61	5.42	5.01	6.33	6.09
	6a	30	72.02	n/a	5.02	n/a	13.52	n/a	6.76	n/a	2.66	n/a
	6b	89	71.52	69.48	5.10	4.31	11.22	12.06	5.61	6.52	6.55	6.44

*

Table 2 . Thermal and solubility properties of azide-functionalized polysulfone 3 and 1,2,3-triazole-functionalized polysulfones 4-6.

 2

Table 3 . Dimensional swelling behavior, mechanical properties and proton conductivities of 1,2,3-triazole-func- tionalized polysulfones (5b and 6b) and PBI before and after doping with phosphoric acid.

 3

	H2O,			DMF,
	MeOH, EtOH, Acetone, ACN	THF	CHCl3, CH2Cl2	DMAc DMSO, NMP

Acknowledgement Funding from the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013) / ERC Grant Agreement n. 306682 is acknowledged.

AUTHOR INFORMATION

Corresponding Author * sara.cavaliere@umontpellier.fr Present Addresses † Anna Donnadio, Department of Pharmaceutical Sciences, University of Perugia, Via del Liceo 1, 06123 Perugia, Italy.

Author Contributions

The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript.

Funding Sources

ABBREVIATIONS

CuAAC, copper-catalyzed azide-alkyne cycloaddition; PEM, polymer electrolyte membrane; PEMFC, Proton Exchange Membranes Fuel Cells; PA, phosphoric acid.


Rémi Bèges
email: rbeges@laas.fr

Fabrice Caignet

André Durier

Christian Marot

Marise Bafleur

Nicolas Nolhier

Practical Transient System-level ESD Modeling -Environment Contribution

A methodology for building a transient model of an analog system is detailed. It does not require proprietary knowledge for integrated circuits (IC). At IC level, it combines a protection structure characterization and behavioral modeling with a core description. A specific test board is developed with a smart voltage regulator to validate the methodology. A system model is assembled to perform powered-on transient ESD simulations. A soft-failure criterion is chosen and the prediction of trends in soft-failure generation is carried out. The strong influence of the electrical environment is demonstrated through this case study.

I. Introduction

Performing system level reliability prediction against an Electro-Static Discharge (ESD) stress remains a very challenging topic. Being able to do so could bring further improvements to existing protection approaches [START_REF]White Paper "System Level ESD, Part I: Common Misconceptions and Recommended Basic Approaches[END_REF], while reducing costs. In the case of analog devices, the lack of freely available transient IC models makes the task difficult. For digital circuits, IBIS files [START_REF]Input Output Buffer Information Specification[END_REF], meant for signal integrity simulations, could form the beginning of an answer but are not entirely suitable [START_REF] Monnereau | Behavioral-modeling methodology to predict Electrostatic-Discharge susceptibility failures at system level: An IBIS improvement[END_REF].

Several studies were performed in the past about IC modeling and system level ESD simulations [START_REF] Monnereau | Behavioral-modeling methodology to predict Electrostatic-Discharge susceptibility failures at system level: An IBIS improvement[END_REF][5] [8][13]. This preliminary work laid the basis for behavioral modeling based on TLP extraction of IC parameters. Validations were performed using simple injection scenarios (TLP and IEC 61000-4-2) [START_REF] Besse | ESD system level characterization and modeling methods applied to a LIN transceiver[END_REF] and soft failures were studied on digital chips. Other investigations [START_REF] Bertonnaud | IEC System Level ESD Challenges and Effective Protection Strategy for USB2 Interface[END_REF] [START_REF] Li | An application of utilizing the system-efficient-ESD-design (SEED) concept to analyze an LED protection circuit of a cell phone[END_REF] were conducted in the past about hard failures of non-powered systems. The propagation of a stress from the input connector of a board to the input pin of an IC was studied and improved design rules (System Efficient ESD Design -SEED) were defined.

In the continuity of this work, this paper addresses a new case of a system designed around a mixedsignal IC (a smart voltage regulator) that contains digital, analog and power functions. The final goal is the prediction of soft-failure robustness of the device. In order to study soft-failures, it is required that the device is powered-on and placed in regular operating conditions. The integrated circuit model must be able to work under such novel condition. For that matter, the IC modeling technique is extended in order to perform powered-on simulations. Before doing softfailure prediction, the ESD propagation through the system is simulated. This intermediate step aims at reproducing in simulation the voltage variations on the IC output when an input is exposed to an ESD.

The complete test setup is modelled using a modular approach. Each element of the setup is well characterized and modelled separately. This approach is quite flexible and enables mixing different types of models (structural, behavioral, physical, etc.) with different implementation techniques (SPICE, VHDL-AMS, Verilog-AMS, etc.). All these blocks are then assembled following a classical hierarchical modeling technique.

In order to validate this technique, a specific test board has been developed around a smart voltage regulator (see part II). The IC is placed on a board with all the passive devices recommended by the IC manufacturer to ensure proper functioning, as it would be in a real application. This chip is a smart regulator supplying a reset-out signal that indicates the loss of the regulation function. The raise of this flag is considered here as the soft-failure criterion.

An important aspect highlighted throughout this document is the tremendous impact of the environment when trying to achieve accurate systemlevel simulations. The impact of the main elements of the board and the test setup will be detailed in parts V and VI.

Different ESD stresses are applied to validate this approach in part VII. First, a TLP is used as a stress generator in order to test the model in a "simple" injection scenario, to perform a first evaluation of the modeling work. Two more tests are then conducted, using a standard IEC 61000-4-2 ESD gun [START_REF]measurement techniques -Electrostatic discharge immunity test[END_REF] and pulse 3B from the ISO 7637-2 automotive standard [START_REF]Road vehicles -Electrical disturbances from conduction and coupling -Part 2: Electrical transient conduction along supply lines only[END_REF].

Ultimately, it is demonstrated in section VIII that some trends on soft-failure robustness can be predicted.

II. Test Board

The test board is build around an automotive low dropout (LDO) smart voltage regulator IC. For an input voltage between 4.4 and 45V, it supplies a regulated 3.3V output voltage with a maximum current of 400mA. The regulator also provides a digital open collector output that triggers low when the regulated voltage is outside the +/-3% range around 3.3V. The raise of this flag is considered to signal a soft-failure. The test setup (see schematic Fig. 1) is a printed circuit board with all the passive devices (decoupling capacitors, pull-up resistors, etc.) needed for the IC to properly operate. An injection network (Fig. 1) is used to superimpose the impulse on the DC supply voltage. This is based on the Direct Power Injection (DPI) standard injection setup [START_REF]Electromagnetic Compatibility (EMC), Integrated Circuits, Measurement of Electromagnetic Immunity 150 kHz to 1 GHz -Part 4: Direct RF Power Injection Method[END_REF]. A 5.6 uH inductor in series with a Schottky diode ensures that no transient current will be injected inside the DC supply. The 100 nF capacitor isolates the stress generator from the DC supply. The capacitor value is chosen high enough to not filter the stress pulse and provide correct isolation from the DC supply. The system is implemented on an FR4 double-sided PCB. Bottom face is a ground plane and all traces are 110 matched.

The board is then connected to a test setup (Fig. 2), for stress injection and measurements. All cables are 50  matched, and the ground connection is provided by the stress generator. 1M oscilloscope inputs are used for measurements.

III. Simulation flow for systemlevel ESD

In this part, the global approach for achieving system level ESD simulation and robustness prediction is presented. The simulation flow employed is introduced in Fig. 3. This is a modular approach where each module can be described separately using any language (SPICE, VHDL, VHDL-AMS, Verilog, etc.).

The principle is to have a library of well defined models, and to assemble them together according to the board and setup configuration.

For a practical in-lab testing, where the test setup is fixed and each element is well characterized, this flow is particularly suitable, as only the IC model and board schematic needs to be changed when testing a new product. The application of this simulation flow to a real-world application, where the electrical environment of the board is not mastered, is not considered in this work. This is relying on the fact that test standards define testing conditions that should be representative of the real-world electrical environment of the device under test.

The IC model is divided in multiple blocks as detailed in IV. Package, ESD protections and core functions are modelled separately then connected together. A characterization and modeling technique for protection structures is presented, that requires no proprietary information on the chip. The method used to model the IC core and package parasitic will be detailed. The integrated circuit model is the most complex to implement, but that does not mean it has the biggest influence on the simulation. The printed circuit board is in the second level in the hierarchy. The schematic, the chip model and offthe-shelf models for PCB traces and passive devices are combined together. Several interactions between those elements are detailed in V.

Finally, at the test setup level (section VI), this study presents the work done for electrical environment modeling (transmission lines, pulse generator, oscilloscope inputs).

The influence of most elements of the setup and root causes for simulation errors and measurement artefacts are detailed through sections IV, V & VI. It is applied to input, output and ground pins, by pairs of two pins. Voltage regulation and reset functions are modeled as well, as described in IV.b.

IV. The IC Model

a. Pin-to-pin model (ESD protections)

A pin-to-pin I(V) model is developed for ESD protections. It is a behavioral black-box model, meaning that no design knowledge of the structure existing between the two pins is needed. An ESD structure, a power structure (power transistor, power clamp, etc.) or a parasitic structure are treated the same way.

First, a classical 100 ns (300 ps risetime) TLP characterization is performed (Fig. 5). An I(V) curve for each pair of pins is extracted on a unpowered device. Using TLP-extracted I(V) curves to describe ESD protection structures and pin-to-pin responses has been previously introduced in several papers [START_REF] Monnereau | Building-up of system level ESD modeling: Impact of a decoupling capacitance on ESD propagation[END_REF][6] [START_REF] Hyatt | TLP Measurements for Verification of ESD Protection Device Response[END_REF] and is considered a valid approach. Each characteristic is then approximated by a piecewise linear curve, forming a behavioral pin-topin model. For simulation purposes, a two-terminal VHDL-AMS component has been developed to translate the I(V) characteristic. The code handles snapbacks and uses a numerical state-machine (see figure 6) as described by N. Monnereau [3][8].

There is no timing information in the implementation of the ESD I(V) module, the voltage across the component can change instantly, leading to unrealistic behavior and raising convergence issues when performing a simulation of the structure.

To solve this problem, a small capacitor is connected in parallel of each module. Physically, such capacitor is meaningful because a real ESD protection always shows an equivalent parallel capacitor, defining the rate at which the voltage can change across its terminals. It slows down the voltage changes across the protection structure, allowing the simulation to converge during snapbacks. The only issue with that approach is to not account for the overvoltage that can be generated during the triggering of the ESD protection (Fig. 7). In this work, this did not showed up as an issue. This point can be fixed in VHDL-AMS by adding a delay between the moment the state machine goes into snapback and the moment the current starts flowing. In reality, ESD protections do not immediately conduct current before a conduction delay. During that delay, the voltage keeps increasing, causing an overvoltage. When the current through the protection becomes high enough, voltage falls down to the TLPextracted quasistatic bias point.

b. Core modeling

A behavioral model was developed from scratch with information extracted from the datasheet. This is a far from ideal approach, but it requires once again no proprietary knowledge of the chip. The voltage regulation is implemented by the combination of a simple SPICE level 1 MOS transistor (M1) and a Proportional Integral Derivative (PID) controller [START_REF] Åstrom | PID Controllers: Theory, Design, and Tuning[END_REF] (see Fig. 8).

Basically, the PID module computes the error between the desired and actual output voltage, and acts on the grid-source voltage in order to reduce that error. It is essential to limit the grid-source voltage supplied by the PID module in order to limit the maximum output current to the value given in the datasheet. This is done in the PID module implemented in VHDL-AMS.

The maximum output current I max , in conjunction with the output capacitor of value C out , gives the maximum rate of increase of the output voltage Vout/t = I max /C out (in V/s). In this model, the maximum rate of decrease of the output voltage does not depend on the regulator itself, but on the output load. Indeed, when the output voltage is too high, Vgs is clamped to 0 V and the current through the MOS is null. The output capacitor being charged up too high, the only way to lower the output voltage is to discharge the capacitor through a load. This is an assumption made on the actual unknown behavior, but because the voltage regulation is slow compared to an ESD it does not seem to have a great impact on simulation accuracy.

Finally, the values for the three parameters P, I and D will determine how the current varies between those two boundaries [0 mA ; 400 mA] (values from the datasheet), and most importantly, will provide stability to the control loop if correctly tuned.

A first order low-pass filter constituted by [R1;C2] (Fig. 8) was added to limit the rate of change of V GS . The C1 capacitor accounts for the gatecoupling effect during ESD. Because the voltage regulation is slow in comparison to an ESD event, this rough model of the function is enough to achieve accurate results at the ESD timescale.

The RESET function is a simple range comparator without any timing. The chosen voltage range for the model is the nominal output specified in the IC datasheet (+/-2%). Outside of that range, a numerical flag is raised. This is a very simplistic approach compelled by the lack of knowledge on the actual function. However it is already sufficient to produce acceptable results for soft-failure simulation (section VIII).

c. Package model

An RLC network models the package of the device. The equivalent capacitance (from IBIS [START_REF]Input Output Buffer Information Specification[END_REF]) to ground is neglected because supposed of extremely low value. The series inductance is the only element that can have a significant impact on the waveforms. Typical values are considered close to a few nH [START_REF] Pavier | Understanding the effect of power mosfet package parasitics on vrm circuit efficiency at frequencies above 1mhz[END_REF], which is enough to induce a significant voltage spike across its terminals during transient current injection (i.e. TLP) [START_REF] Monnereau | Building-up of system level ESD modeling: Impact of a decoupling capacitance on ESD propagation[END_REF].

The combination of all these blocks form the complete IC model introduced previously (Fig. 4). It can be used to simulate the injection of an electrical fast transient (TLP, IEC 61000-4-2, etc.) directly on the bare chip.

V. Board Modeling a. Passive Devices

For pulses with rise-time close to the nanosecond, High-Frequency (HF) effects of passive elements are not negligible. The use of perfect models for capacitors and inductors lead to unrealistic simulations as presented in Fig. 9.

b. PCB traces

The study was conducted for 110  matched PCB traces, but it can apply to other impedances as well.

Figure 10: PCB "masking" effect configuration PCB traces induce a delay proportional to their length (~33 ps/mm for 110 ). In the case where a PCB line, on the propagation path of a stress, separates a load from a measurement point A (see Fig. 10 and11), a residual peak can be generated on the measurement. Even very short lines (< cm) are affected.

Indeed when a stress propagates towards the load (see Fig. 10), it first hits the measurement point A. The potential at point A starts rising. After a time corresponding to the delay of the PCB trace, the pulse reaches the load. The voltage settles to the load bias point, and this voltage change propagates back toward the measurement point A, reaching it after a second PCB delay. During this interval of time, the voltage at the measurement point A was exclusively set by the original pulse, and not dependent on the load. In Fig. 11 and 12 the load (a capacitor) is considered perfect and does not show any HF behavior. The spike is purely due to the PCB trace. In reality this delay-induced peak and HF-effects previously described in V.a. could cumulate.

Taking this effect into account is essential when the delay of the PCB trace is not negligible against the stress signal risetime. The ratio between both time values will determine the amplitude of the generated spike.

On the other hand, in our setup, other PCB traces not involved in this type of configuration were kept short (< 5 cm) and did not seem to impact the measurements.

VI. Environment Modeling a. Cables

Because ESDs can last a few hundred nanoseconds, propagation delay of coaxial cables (about 5ns/m for a 50 Ω cable) is not negligible in a simulation. Such cables, by introducing a delay between two reactive loads or impedance breaks, can induce important oscillations with a period of about twice the cable delay.

There are several solutions for modeling a lossless coaxial cable or PCB trace. The most popular model is the distributed LC ladder. The issue with this model is the huge amount of elements required for dealing accurately with long transmission lines and short risetimes. In the case of a TLP, there can be several orders of magnitude between line's delay and LC's element delay (that should be much smaller than signal risetime for accuracy), leading to a massive amount of LC elements and very long simulation times.

There is a computationally much more efficient solution when losses are negligible. In [START_REF] Branin | Transient analysis of lossless transmission lines[END_REF], Branin details an analytical solution to the transient analysis of a transmission line. It provides a true mathematical solution, rather than the approximation performed with the LC ladder, and is perfectly accurate for uniform lossless transmission lines. As a consequence, this analytical model guarantees that risetimes will not be deteriorated. Last but not least, simulation time is not dependent on the transmission line delay because the delay is simply a parameter in the model's equations. This is the solution that was chosen for cables and PCB traces, as it enables much more accurate results and shorter simulation times. This model is implemented in VHDL-AMS with an option to preload the line at a user-defined voltage, which is very practical for TLP simulations and reduces computation time.

b. Probes

Building a proper oscilloscope probe model is also very helpful to achieve proper correlation between measurement and simulation. An important characteristic to take into account in the simulation is the actual bandwidth of the probe. In some cases, it can be lower than the sampling frequency of the oscilloscope. Adding a simple 1 st order low-pass filter with similar bandwidth in the probe model seems to be sufficient.

For high-speed oscilloscopes, the user often has a choice between a strong impedance (i.e. 1 MΩ) probe and a matched 50 Ω termination. In the case of a 1 MΩ probe, the strong impedance guarantees that only a tiny fraction of the transient current will be injected inside the oscilloscope. However, when connected to the circuit by the mean of a coaxial cable (50 Ω for example), the resulting impedance break generates unwanted reflections and spikes on the measurements.

Using a matched 50 Ω probe allows overcoming this issue and get a higher bandwidth. However, with such lower impedance, the probe is much less transparent to the rest of the circuit. The impedance offered by the IC input can often be higher than 50 Ω, directing the majority of transient current directly into the probe.

In the measurements (section VII), it was chosen to use a 1 MΩ probe and take the unwanted reflections into account in the simulation, but for future work an on-site pickup resistor (pick-off tee) combined with a 50 oscilloscope input will rather be used.

c. Generator model

Finally, when lacking an accurate pulse generator model, an excellent knowledge of the generator's electrical circuit has proven extremely useful. In the case of TLP generators, several elements such as attenuators or power diodes can significantly impact the signal waveform after the moment the pulse has been injected, because of reflection phenomena.

For ISO 7637-2 generator, none of these options were available. It has been characterized with high quality resistive or capacitive loads, and a model was derived from these measurements (see figure 13).

VII. Measurements/Simulations

A comparison between simulated and measured signals at the main input and output pins of the system is performed first with a TLP stress (see figure 14). The goal is to predict the signal waveform at the output of the IC when injecting an ESD on the input.

Results demonstrate that the model correlates very well with the measurements when a powered-on device is stressed by a TLP impulse sufficiently important to trigger ESD protections. This modeling technique gives accurate results even if the ESD model of the IC has been extracted in nonpowered conditions. On the input (Fig. 14), we see from 20 ns the capacitor charging up at constant current, minus the current deviated into the ESD structure. As the voltage increases, the current flowing through the ESD protection rises as well, slowing down the charge of the decoupling capacitor. On the output (Fig. 14), we can observe two damped oscillations. They are well reproduced in simulation because cable, probe and decoupling capacitor models are accurate.

In that case, the IC does not really impact the period of oscillations as one might first assume, it only has an influence on the amplitude (transmitted power from the input). The RESET flag is raised (triggered low), both in real and simulated conditions. The simulated reset curve is not displayed because it is simply a numerical flag that is also raised for a 600V TLP impulse. On Fig. 15, the reset output triggers low about 5 us after the stress is injected, which is the standard behavior defined in the datasheet.

Basically, the ESD event seems to be detected immediately by the reset comparator, but it takes 5 us to trigger the reset pin down. The flag is then held down during 12 ms, which is consistent with the datasheet. TLP turned out to be an excellent tool for adjusting the values of the different elements of the simulation. Its clean, squared pulse and 50 Ω matching ease both signal analysis and model debugging.

A similar test on the identical setup is conducted with an 8kV IEC 61000-4-2 ESD gun. Chiu's model [START_REF] Chiu | Simulation and measurement of ESD test for electronic devices[END_REF] is used to generate the ESD in simulation, with values slightly tuned for our in-lab ESD gun.

On the input, the first peak amplitude matches well with the measurement (figure 16). A strong oscillation is observed between -10 and 30 ns, mainly due to the in-lab gun imperfections and the cable used in the setup to drive the stress to the PCB. The oscillation is damped much quicker in the measurement, apart from that there is a good correlation between simulation and measurement on the input.

On the output (figure 16), the 8 kV gun stress induces an over-voltage of 60V on the 47 F capacitance, and the observed oscillation is well reproduced by the simulation. As a consequence, the RESET flag is raised. All these results confirm that the model behaves properly when confronted to an IEC 61000-4-2 pulse. Finally, the test is repeated with an ISO 7637-2 3B impulse (Fig. 17). Once again, oscillations with similar period to those in Fig 14 and 16 are observed, and correspond to measurement cables. In simulation, using the delay defined in the cable datasheet is enough to get the right period for these oscillations.

The results are in the same order of magnitude in terms of accuracy. On the output, amplitudes are a bit off and the dampening is slower, which may be explained by the lack of accuracy of the generator's model. The first peak on both input and output correlates well, showing that PCB effect, HF behavior of decoupling capacitors and generator risetime are well modelled.

Globally, the results are acceptable and demonstrate that this modeling work is valid at the ESD timescale in the case of an IC that hosts digital, analog and power functions.

VIII. An example of soft-failure main trend prediction

Once the transient behavior of the models was validated, an attempt to predict the minimal TLP level required to induce soft failures has been carried out. For that matter, the amount of identical 6.8 nF decoupling capacitors in parallel on the input was increased, and for each amount, the minimal TLP stress level triggering a reset was recorded (see top curve Fig. 18). The simulation has an important offset error but the curves are varying in the same manner and the trend fits with the measurement. Simulation and measurement agree on the fact that increasing the amount of decoupling capacitors on the input improves the robustness of the system against soft failures. The impact of the DC supply voltage on the softfailure robustness is also studied (see bottom curve Fig. 18). The DC voltage is varied and once again the minimal TLP level inducing a soft-failure is recorded. The curves are globally flat with an important static error. Once again, simulation and measurement agree that in this particular study case, the DC supply voltage has no influence on the soft-failure robustness of the system.

This two curves show the present limit of the model. In this work, it is so far not possible to predict accurately the soft-failure level, only a trend. This result however was achieved with no proprietary knowledge of the chip and in particular of the resetout function properties (timings, exact triggering levels, etc.).

IX. Conclusion

A working technique for modeling an integrated circuit (ESD protections and core functions) without any knowledge of its design has been presented. It enables transient system-level simulations of a powered-on device stressed by a positive pulse of several hundred nanoseconds.

The model of the system, built on a modular hierarchical approach, has been validated with TLP, IEC 61000-4-2 and ISO 7637-2. The analog output voltage and some trends for the reset are predictable by simulation. It was demonstrated that the environment impacts quite a lot the propagation of the stress, and must be accurately taken into account in the modeling.

For soft-failure prediction, this approach showed some limitations. Ultimately, the knowledge of the internal design could help developing a more accurate behavioral model of the chip that would enable softfailure prediction without disclosing proprietary information. This concept will be investigated further at the IC level and specifically at modeling core functions at the ESD timescale in order to enable true soft-failure prediction at the system level.

Figure 1 :

 1 Figure 1: Test PCB schematic

Figure 2 :

 2 Figure 2: Test setup schematic

Figure 3 :

 3 Figure 3: Proposed simulation flow

Figure 4 :

 4 Figure 4: The complete IC model

Figure 5 :

 5 Figure 5: TLP pin-to-pin measurements of the IC

Figure 6 :

 6 Figure 6: State machine overview for 1 pin/ 1 polarity

Figure 7 :

 7 Figure 7: ESD Protection induced overvoltage In simulation, the capacitor slows down voltage variations. When the voltage is high enough, the ESD protection switches on and absorbs current with no delay.In reality, ESD protections do not immediately conduct current before a conduction delay. During that delay, the voltage keeps increasing, causing an overvoltage. When the current through the protection becomes high enough, voltage falls down to the TLPextracted quasistatic bias point.

Figure 8 :

 8 Figure 8: Behavioral voltage regulation model

Figure 9 :

 9 Figure 9: Simulated response of perfect vs. real 47uF capacitor during TLP injection on the test boardHowever, it is unnecessary to use an HF model for every single passive element of the circuit. In the simulations presented in this document, only decoupling capacitors were modelled as such, apparently being the only ones to have a significant impact. An impedance analyser allows for easy and

Figure 11 :

 11 Figure 11: V(t) at node A (blue) original impulse (red) and load voltage (green)-the load is a capacitor

Figure 12 :

 12 Figure 12: delay masking effect on test board In simulation only the 5 mm long PCB trace specified in figure 13 was modelled, but not any other longer traces. The same transmission line model was employed for coaxial cables and is described in VI.a.

Figure 13 :

 13 Figure 13: Behavioral ISO 7637-2 3B generator model

Figure 14 :

 14 Figure 14: Measured vs Simulated Vin(t) & Vout(t) (600 V TLP impulse)

Figure 15 :

 15 Figure 15: Measured Reset(t) (600V TLP impulse) -Different timescale than figure 14 -Note : there is a 5us delay on x axis compared with figure 14.

Figure 16 :

 16 Figure 16: Vin(t) (top), Vout(t) (center) and Reset(t) for 8kV IEC 61000-4-2 injection

Figure 17 :

 17 Figure 17: Vin(t) (top) and Vout(t) (bottom) for ISO 7637-2 3B150V injection

Figure 18 :

 18 Figure 18: RST TLP voltage versus decoupling capacitance (top) and DC supply voltage (bottom)

Acknowledgements

The authors would like to thank Leonardo Di Biccari (ST Microelectronics) for his very valuable feedback on this work.