From Ionic Liquid Epoxy Monomer to Tunable EpoxyâAmine Network

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From Ionic Liquid Epoxy Monomer to Tunable EpoxyAmine Network: Reaction Mechanism and final properties Sébastien Livi, Charline Chardin, Luanda Chaves Lins, Nour Halawani, Sebastien Pruvost, Jannick Duchet-Rumeau, Jean-Francois Gerard, and Jerome Baudoux ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06271 • Publication Date (Web): 10 Jan 2019 Downloaded from http://pubs.acs.org on January 11, 2019

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ACS Sustainable Chemistry & Engineering

From Ionic Liquid Epoxy Monomer to Tunable Epoxy-Amine Network: Reaction Mechanism and final properties Sébastien Livi1*, Charline Chardin2, Luanda C. Lins3, Nour Halawani1, Sébastien Pruvost1, Jannick Duchet-Rumeau1, Jean-François Gérard1 and Jérôme Baudoux2*

1Université

de Lyon, CNRS, UMR 5223, Ingénierie des Matériaux Polymères, INSA Lyon, F69621 Villeurbanne, France

2Laboratoire

de Chimie Moléculaire et Thio-organique, ENSICAEN, Université de Normandie, CNRS, 6 boulevard du Maréchal Juin, 14050 Caen, France.

3

Université de Lyon, INSA de LYON, MATEIS UMR CNRS 5510, Bat L. de Vinci, 21 Avenue Jean Capelle, 69621 Villeurbanne Cedex, France. Correspondence to: Sébastien Livi (E-mail: [email protected]) Jérôme Baudoux (E-mail: [email protected])

1

ACS Paragon Plus Environment

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ABSTRACT In this work, new (multi)functional-dedicated polymer materials were designed and processed from the co-polymerization between novel imidazolium ionic liquid monomer (ILM) with conventional polyetheramine denoted Jeffamine D230. First, a facile and robust synthetic route was investigated in order to design polyfunctional imidazolium monomers bearing an aromatic ring and two epoxy functions at the end of aliphatic chains. Then, the main mechanisms of epoxy opening leading to polymerization with different kinetics were modeled through the reaction between monofunctional epoxy and aliphatic mono- and diamines by using “in situ” NMR spectroscopy. Finally, the monomer molecular structure-network architecture-physical properties relationships of the resulting IL-modified epoxy networks were investigated. As a consequence, epoxy networks with a glass transition temperature of 55 °C and with enhanced properties such as thermal stability (> 300°C), storage modulus of 700 MPa at room temperature, a ionic conductivity (4*10-4 S.m-1 for 70°C) combined with an hydrophobic character of their surface (33 mJ.m-2) were prepared.

KEYWORDS. Ionic Liquids, Epoxy Network, Polymerization Reaction Mechanisms, Ionic Conductivity

INTRODUCTION In recent years, the emergence of global challenges such as environmental stewardship as well as the production, storage, and energy conversion have prompted researchers to develop new synthetic routes for designing advanced and sustainable (multi)functional polymer materials. The 2


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use of new selected ionic liquid monomers (ILMs) and their polymerization to generate poly(ionic liquid)s (PILs) was considered as an innovative route to design new polymer materials with novel physical properties1-6 for potential applications in energy storage, electrochemical devices2-3, separation membranes4-5 and biomedical applications6. In fact, the unique properties of ILMs such as high ionic conductivity, high thermal and electrochemical stability, low vapor pressure, and non-flammability make them excellent candidates to overcome these challenges. Nevertheless, for many applications, solid or quasi-solid ion conductive polymeric materials are preferred and fit in a better way the practical requirements compared to fluids especially from the viewpoint of possible leakage during use. For these reasons, the majority of PILs or block copolymers containing PIL segments reported in the literature have been designed by conventional controlled radical polymerization techniques such as reversible additionfragmentation chain-transfer polymerization (RAFT) or atom-transfer radical polymerization (ATRP), or by “click” chemistry through the copper-catalyzed azide−alkyne cycloaddition (CuAAC) modular ligation. Despite a good conﬁnement of ionic liquid inside polymer matrix7-12, these synthetic routes are complicated and require many purification steps in order to prepare PILs with controlled molar-masses. However, the resulting polymer materials have poor final properties such as the mechanical performances mainly limiting their applications to energy or separation membranes3-5. Thermosetting polymers are interesting hosts for ionic liquids and especially epoxy-based networks according to the numerous architectures of epoxy networks which could be designed and their easy preparation methods as well as their excellent thermo-mechanical, thermal stability, and adhesive properties13-16. Thus, the market for thermosetting resins is widely 3



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dominated by the use of the Bisphenol A diglycidyl ether epoxy prepolymer (DGEBA) resulting from a synthesis that requires toxic and carcinogenic compounds, i.e. Bisphenol A and epichlorohydrin. To enhance these main features, a new approach was developed by introducing cationic groups covalently attached to the polymer network. Recently, the use of a new ionic epoxy monomer having a quaternary ammonium salt structure was reported by Matsumoto et al17-18.

The

ionic

monoepoxide,

i.e.

glycidyltrimethylammonium

bis(trifluoromethanesulfonylimide), was copolymerized with poly(ethylene glycol) diglycidyl ether by using -amino terminated PEG as a curing agent17. According to this route, the cationic ammonium moiety was introduced in the network and TFSI counter anion was considered as the mobile ion.

Monofunctional epoxy lithium sulfonate salts were also

considered by the same research group18. In this case, the counter anion is inserted in the network structure and the lithium cation is the mobile ion18. Nevertheless, this synthetic route remains extremely complex. Although several works have already been reported about the synthesis of ILs based networks, very few studies were dedicated to the development of efficient and flexible route to design polyfunctional imidazolium monomers bearing an aromatic ring, i.e. having a molecular structure close to bisphenol-A-based epoxies, and specific epoxy functions with respect to the mobility of ions19-20. Very recently, Gin et al have developed ion-gel membranes based on diepoxy-functionalized imidazolium IL monomer cured with a triamine, i.e. tris (2aminoethyl)amine in order to investigate their CO2 sorption as well as their gas separation performances21-23. In addition, the authors have studied the effect of the stoichiometric ratio on the epoxy conversion with and without the presence of different amounts of conventional ILs. Moreover, Drockenmuller and Duchet et al have synthesized a new class of PILs based on 1,2,3 4


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triazolium structure covalently linked with aliphatic diamine denoted Jeffamine D2000 and have investigated the final properties of these networks including the thermo-mechanical properties and the ionic conductivity24. Thus, they have processed ion‐conducting epoxy–amine networks with ionic conductivity of 2 × 10−7 S.cm−1 at 30 °C combined with a glass transition of -52 °C. Despite an epoxy conversion of approximately 90% was reached, these networks displayed a low thermal stability (< 250 °C) due to the premature degradation of triazolium salts and a lower storage modulus of 0.5 MPa in the rubbery state at 15 °C24. With the aim of processing new sustainable epoxy networks with improved properties including ionic conductivity but not only and adapted for many different applications, our research group had recently developed a new powerful and clean methodology in order to oxidize alkenes directly and quantitatively on an imidazolium backbone by avoiding the use of highly toxic epichlorohydrin. In fact, we have demonstrated that dimethyldioxirane (DMDO) is an efficient oxidizing agent to prepare monoor diepoxides with acetone as sole inert and volatile by-product25. Based on this previous work, a novel diepoxide imidazolium monomer (ILM) at multi-grams scale was designed in order to produce more sustainable (multi)functional-dedicated polymer materials based on epoxy networks. In this work, the reactivity of the epoxy-functionalized imidazolium ionic liquid monomer with polyoxypropylene diamine denoted Jeffamine D230 was studied by DSC and FTIR. A special attention was paid to analyse the effect of the presence of ionic species introduced by the IL onto the reaction kinetics and to determine if the reaction proceeds from the same mechanisms than conventional copolymerization of diglycidyl prepolymers (such as DGEBA) and amines. Thus, the kinetics will be monitored by 1H and 13C NMR using the reported models in order to compare our results with the existing literature. 5



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Finally, the morphology as well as the final physical properties of the resulting networks such as the thermal stability, thermo-mechanical behavior, ionic conductivity were investigated by transmission electronic microscopy (TEM), thermogravimetric analysis (TGA), dynamical mechanical analysis (DMA), and dielectric spectroscopy (DEA).

EXPERIMENTAL Synthesis of ionic liquid monomer (ILM) The imidazolium salt 6 was synthesized from 4-bromophenol. In a first step, the etherification reaction between the alcohol and 4-bromo-1-butene (2 equiv.) was carried out in the presence of potassium carbonate (2.5 equiv.) to generate the compound 1 with excellent yield (96%). The aryl-imidazolium backbone was prepared by a copper-catalyzed arylation to create carbonnitrogen bond. To achieve this Chan-Lam coupling, the imidazole was added to the boronic acid 2 previously prepared by adding n-butyl lithium, aryl bromide 1, and triisopropylborate in THF. This Cu-catalyzed reaction is perfectly reproducible on several grams of reagents. After purification, 4-bromo-1-butene was added to imidazole 3 to afford imidazolium bromide 4 in quantitative

yield.

Anion

metathesis

of

this

salt

was

carried

out

with

bis(trifluoromethane)sulfonimide lithium salt. Finally, compound 5 was oxidized in the presence of freshly prepared dimethyldioxirane (2.8 equiv.) in acetone at room temperature. To note, this compound was also prepared by commercially available meta‐chloroperbenzoic acid (mCPBA). In this case, the procedure requires an excess of oxidizing reagent (4 equiv.) in acetonitrile at 40°C. In both cases, diepoxide 6 was obtained in an excellent yield and the purity of this salt was confirmed by 1H-, 13C-, 19F-NMR, infrared, and mass spectroscopy as well as by the assignment 6


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of 1H-,

13C-

and

19F-NMR

resonance peaks reported in the ESI (Figure S1, Figure S16-S21).

The general procedure for the synthesis of ILM was detailed in Figure 1. Br

(2 equiv) ( )2 K2CO3 (2.5 equiv)

OH

CH3CN, 80 °C, 72 h

Br

imidazole (1.2 equiv) CuI (4 mol%), CH3OH 60 °C, 5 h

O

N

n-BuLi (1.2 equiv) B(Oi-Pr)3 (1.5 equiv) THF, -78 °C, 1 h

Br

1 (96%)

O N

( )2

O (HO)2B

4-bromo-1-butene (2 equiv)

( )2

H2O, rt, 24 h

( )2 N

N O

5 (91%)

O

2

4 (98%) DMDO (2.8 equiv) Acetone, 12 h, rt

NTf2

N Br

3 (62%) LiNTf2 (1.1 equiv)

2 (62%)

( )2 N

CH3CN, 80 °C, 48 h

( )2

2

or mCPBA (4 equiv) CH3CN, 24 h, 40 °C

O ( )2 N

O

N NTf2

O 6 (89-92%)

Figure 1. General procedure for the synthesis of ILM Materials All reagents were purchased from Sigma Aldrich, Alfa Aesar or TCI and were used without further purification and used as received. Solvents were used in RPE grade without further purification. Anhydrous solvents were obtained from a PURESOLV SPS400 apparatus developed by Innovative Technology Inc. In this work, all structures and properties of the materials used are summarized in Table 1. Epoxy prepolymer based on imidazolium ionic liquid monomer denoted ILM-[NTf2] which is liquid at room temperature, was synthetized according to the procedure described in Figure 1. A commercial polyetheramine denoted Jeffamine D230 from Huntsman was considered as co-monomer of the synthesized diepoxy. The diamine was added in the stoichiometric ratio i.e. epoxy to amino hydrogen equal to 1, for neat system. Table 1. Chemical structures, nomenclatures and properties of the materials used to design polyelectrolytes 7



Chemical structure and name

Supplier

Properties

ILM-[NTf2]

« Homemade »

Epoxy equivalent (EEW): 290.75 g eq-1 Tg = -46 oC

Jeffamine D230

Huntsmann

Amine hydrogen equivalent (AHEW): 60 g eq-1

ILR-[NTf2]

« Homemade »

Epoxy equivalent (EEW): 495.04 g eq-1 Tg = -48 oC

O N

O

N NTf2

O

N

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O

N NTf2

Epoxy network synthesis To prepare the network, ILM-[NTf2] and Jeffamine D230 were mixed considering the suitable stoichiometric ratio under stirring at room temperature during 30-40 minutes. The mixture was then degassed in an ultrasonic bath during 15-20 minutes and poured into silicone molds. Finally, ILM-[NTf2]/D230 was cured for 2 h at 80 °C and for 3h at 120°C. A photograph of the obtained sample is shown in Figure 2.

Figure 2. A photograph of the ILM-[NTf2] network

8


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Characterization methods Thermogravimetric Analyses (TGA) of the neat ionic liquid monomer and the resulting epoxy network were performed on a Q500 thermogravimetric analyzer (TA instruments). The samples were heated from 30 to 700 °C at a rate of 20 K min-1 under nitrogen flow. Differential Scanning Calorimetry analyses (DSC) of ionic liquid epoxy monomer reactive system and network were performed on a Q20 (TA instruments) from 20 to 250 °C. The samples were kept for 3 min at 250 °C to erase the thermal history before being heated or cooled at a rate of 10 K min-1 under nitrogen flow of 50 mL min-1. The DSC was calibrated for temperature and heat flow using Indium as reference material. Surface energy of epoxy network was determined from the sessile drop method using a GBX goniometer. From contact angle measurements performed with water and methylene diiodide as probe liquids on the samples, non-dispersive and dispersive components of surface energy were determined using Owens-Wendt theory26. Fourier Transform Infrared spectra (FTIR) were recorded using a Thermo Scientific Nicolet iS10 Spectrometer in a transmission mode. 1H-, 13C-,

and 19F-NMR spectra were recorded on a Bruker Avance III 400 MHz, 500 MHz or

AvanceNEO 600 MHz spectrometer. Samples were dissolved in an appropriate deuterated solvent (CDCl3). The chemical shifts (δ) are expressed in ppm relative to internal tetramethylsilane for

1H

and

13C

nuclei, and coupling constants are indicated in Hz.

Abbreviations for signal coupling are as follows: s=singlet; d=doublet; dd=doublet of doublets; t=triplet; q=quartet; quin=quintet; m=multiplet; br=broad signal. To assign the signals to the 9



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different proton and carbon atoms, as well as the relative stereochemistry of the cycloadducts, additional 2D NMR experiments (COSY, TOCSY, HSQC, HMBC) and NOESY experiments were performed. Dynamic mechanical analysis (DMA) was performed on rectangular samples with dimensions of 15 mm and 5 mm and a thickness of 1 mm using an ARES-G2 rheometer in torsion mode (TA Instruments) during heating ramps of 3 °C.min-1 from −80 °C up to 150 °C. All tests were performed in the linear viscoelastic region at a frequency of 1 Hz. Storage modulus G′, Loss Modulus G″, and loss factor tan δ were measured. Transmission electron microscopy (TEM) was performed at the Technical Center of Microstructures (University of Lyon) using a Phillips CM 120 microscope operating at 80 kV to characterize the morphology of epoxy networks. 60-nm-thick ultrathin sections of samples were obtained using an ultramicrotome equipped with a diamond knife and were then set on copper grids. Dielectric measurements were performed using a AMETEK Solatron analytical (Modulab XM MTS) dielectric spectrometer, equipped with a LakeShore 335 Temperature controller. The measurements were carried out from -80 °C to 120 °C under Helium atmosphere. The temperature was controlled by the use of liquid nitrogen circulating around the measurement chamber containing the sample. The samples used are of circular shape with a 3 cm diameter and a thickness of 1 mm. Two gold electrodes, of 25 mm in diameter, were sputtered on both sides using a Quorum Q300TD Sputter Coater. Dielectric measurements were done under isothermal conditions (temperature steps of 3°) in the 106 - 10-2 Hz frequency range (10 points per decade) applying Vrms = 5 V. The complex conductivity σ* is calculated from the equation: 10


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𝜎 ∗ (𝜔) = 𝜔𝜀0𝜀 ∗ (𝜔)

Eq (1)

Where the angular frequency ω = 2πf (f being the frequency), ε0 the vacuum permittivity and ε* the complex permittivity. The DC conductivity, σDC, was extrapolated from the real part of the conductivity 𝜎′(𝜔) = 𝜔𝜀0𝜀′′(𝜔) for ω → 0 where a plateau is observed.

RESULTS AND DISCUSSION Model reactions and reaction mechanisms between imidazolium ionic liquid monomer and amine According to the literature, the reaction between a primary amine and an epoxy prepolymer has been extensively studied13,27-28 and involves three main reaction paths, presented in Figure 3. O R'-NH2

O OH

R First reaction

R"

R' N

OH R

R

H N

OH

R Second reaction

R'

R

O R Third reaction

R'

R" N

R' N

OH R

OH

O R

R

Figure 3. Mechanisms proposed in the literature for the epoxy ring-opening reaction with a primary amine13 The polymerization process involves a nucleophilic addition of the nitrogen which takes place between the primary amine and the glycidyl group. The first step is followed by a second addition of the nitrogen with another epoxy group leading to the formation of a tertiary amine and hydroxyl groups. An etherification reaction may occur between the resulting hydroxyl groups and epoxy moieties in specific conditions. It is noteworthy that the etherification reaction is strongly dependent on the chemical nature of the amine used as well as the temperature of the 11



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polymerization29-31. In fact, numerous studies have highlighted that for stoichiometric ratio amino hydrogen-to-epoxy, the etherification reaction is not significant when aliphatic diamines such as the one used in this study, i.e. Jeffamine D230 or primary or secondary amines react with phenyl glycidyl ether (model reactions)27, 32-33. Moreover, this reaction has been demonstrated to be effective only when considering high curing temperatures (> 150 °C) with some aromatic amines which are known to have a low reactivity9,

13, 34-35.

An additional reaction denoted

homopolymerization, may also take place in the presence of Lewis bases such as imidazoles or tertiary amines 36-38, inorganic bases, or boron complexes39. In this work, this last reaction cannot be considered due to the steric hindrance of the tertiary amine which screens the catalytic effect of a potential initiator and to the absence of a Lewis base. In addition, the ionic liquid monomer reacts in the same conditions as for the conventional epoxy-amine system, i.e. for 2 hours at 80 °C followed by 3 hours at 120 °C but also during 5 hours at 160 °C without altering its chemical structure and without formation of carbene40. So far, few works reported the reaction mechanisms between an epoxy prepolymer (DGEBA) or cyanate esters with imidazolium or phosphonium ILs used as initiators of the polymerization 40-44.

Maka et al have demonstrated that the use of imidazolium ILs combined with basic counter

anion such as dicyanamide induced a thermal degradation of this IL at lower temperatures generating a stable H-heterocyclic carbon structure40. Thus, their proposed mechanism of the epoxy polymerization consists in three steps: i) formation of imidazole and its alkyl derivative, ii) imidazole acting as an initiator of anionic polymerization, and iii) formation of the resulting network as well as regeneration of alkylimidazole. Moreover, they have highlighted that the use of a non-basic anion such as BF4- anion did not lead to this mechanism40. The latter result has 12


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also been reported in other studies using fluorinated anions (BF4-, PF6-). However, this mechanism was ruled out for the formation of ILM-[NTf2]/D230 due to the high thermal stability and the non-basicity of [NTf2] but also after 1H and 13C NMR verification at higher temperatures. To our knowledge, no work in the literature reported the study of the mechanism between an ionic epoxy prepolymer and an amine hardener. For this reason, a special attention was paid to analyze the influence of the presence of ionic species beared by the ILM on the reaction kinetics in order to demonstrate if the reaction proceeds according to the same mechanisms as conventional copolymerization between non-ionic epoxy monomer such as DGEBA and amine co-monomers. Thus, model compounds were considered to understand the reaction mechanisms involved. In a first step, the epoxy ring opening kinetics of the ILM by a primary amine was studied. In order to achieve this task, a model reactant based on an ionic monoepoxide ILR[NTf2] was synthesized according to the procedure developed in a previous work25 (see ESIFigure S2, Figure S13-S15) and the butylamine was selected as an monofunctional aliphatic amine. The reaction mechanisms were investigated at different temperatures, i.e. at 40 and 80 °C (Figure 4). Thus, the different reactions could be envisioned for the real ILM-[NTf2]/D230 system.

13



N

O H 2N

N

N

T °C, Time

NTf2

Solvent-free condition

ILR-[NTf2]

Page 14 of 44

N H

N NTf2

OH P1

ILR-[NTf2] and/or

N N

N

N NTf2

OH

OH

N NTf2

N

N O

NTf2

N H N

C30H41N5O22+ (m/z: 251.66)

P2

HO

N NTf2

P3

Figure 4. Epoxy ring-opening reaction of ILR-[NTf2] with a primary amine In this case, the formation of products denoted P1 and P2 were clearly confirmed by NMR experiments showing no signal corresponding to the presence of P3.

These results are

corroborated by several studies that reported the reaction mechanism between phenyl glycidyl ether (PGE) used a model reactant and different types of amines45-47. Whatever the reaction temperature (RT, 40 °C or 80°C), the products denoted P1 and P2 (see Figure 4 and Figure S3, S4, S5 in the ESI) corresponding to the first and second amine-epoxy addition reactions are formed gradually over time. As indicated in Figure 5, the two reactions are competitive ones from the beginning of the reaction between the ionic monoepoxy ILR[NTf2] and the butylamine.

14


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N

O

N

Epoxide

NTf2

N

P1

N H

N OH

NTf2

N NN

N

N

O

P1

N H

N NTf2

Epoxide OH P2

OH

NTf2

N

N

OH N

N N

N

N

N

OH

P2

OH

Figure 5. Evolution of the formation of P1 and P2 products as a function of time at room temperature. Then, a commercially available diamine, i.e. the 2,2’-oxybis(ethylamine), was chosen as coreactant of the ionic monoepoxide ILR-[NTf2] in order to confirm the previous results. The reaction was investigated at 40 °C and for an amino hydrogen-to-epoxy ratio equal to 1. The reaction between ILR-[NTf2] and the 2,2’-oxybis(ethylamine) diamine is shown in Figure 6.

H 2N N

O

N NTf2

O

O

NH2

40 °C Solvent-free condtion

N

N

N NTf2

ILR-[NTf2]

P4

OH HO

2

N

NTf2 N

Figure 6. Ionic monoepoxy ring-opening reaction by 2,2’-oxybis(ethylamine). In this case, the product designated as P4 was obtained with the presence of a small proportion of by-products. The molecular formula of this tetramer M4+ was confirmed by High Resolution 15



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Mass Spectrometry (HRMS) which measured m/z = 241.1407 corresponding to C56H72N10O5 (see Figure S6 in the ESI). Moreover, several experiments were carried out to evaluate potential side reactions. In presence of an excess of isopropyl alcohol, the ionic monoepoxide ILR-[NTf2] being completely inert up to 80°C, we concluded the poor reactivity of the secondary alcohols for the epoxide ring-opening reaction of P4. Thus, we investigated that quaternary ammonium species could be formed under particular conditions. To confirm this assumption, a model reaction between ILR-[NTf2] and triethylamine was tested at room temperature. After 10 h, NMR analysis confirmed a complete consumption of the epoxide to form new species measured at m/z = 158.6239 by HRMS corresponding to imidazolium-ammonium salt with a molecular formula of C19H31N3O (M2+). Subsequently, Dimepranol which mimicks the hydroxyl amino groups of P2 was investigated to corroborate the ammonium formation during the ring opening reaction. At 40°C, the use of this co-reactant of ionic monoepoxide led to the formation of a quaternary ammonium (P5 compound-Figure 7) which can be evidenced on the NMR spectrum by deshelding of the N-Methyl group in concordance with literature and our previous result with trimethylamine (see Figure S7 in the ESI). Once again, the NMR result was confirmed by HRMS with the presence of M2+ at m/z = 159,6126 corresponding to the molecular formula C18H29N3O2.

N

O

N NTf2

N

OH

OH 40°C

Solvent-free condition

N

N

N O P5

ILR-[NTf2]

Figure 7. Reaction between ILR-[NTf2] and a tertiary aminoalcohol (dimethylaminoisopropanol) 16


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To conclude about the presence of ammonium functionality, we monitored a sequence from a secondary amine, i.e. diethylamine, and ILR-[NTf2] for an aminohydrogen-to-epoxy ratio equal to 1 (Figure 8). After complete consumption of the epoxyde, an excess of ILR-[NTf2] was added to generate the ammonium salt and the crude mixture was analyzed by NMR and HRMS.

N

O

N NTf2

ILR-[NTf2]

N H Solvent-free condition

N

N

N

ILR-[NTf2]

OH P6 N

N

N OH P7

N

N

O

by-products

Figure 8. Reaction of ionic monoepoxide with tertiary aminoalcohol, P6, resulting from the initial reaction of ILR-[NTf2] with diethylamine During the second step of this sequence, the deshelding of the P6 ethyl groups was observed and HRMS confirmed the presence of an ammonium M3+ at m/z = 158,6239 corresponding to the molecular formula C30H42N5O2 (see Figure S8-S12). In summary, all these experiments support the epoxide ring-opening reaction by nitrogen atoms exclusively. Furthermore, we noted the good reactivity of a tertiary amine in the vicinity of an alcohol which was able to open the epoxy ring leading to the formation of a quaternary ammonium. As a conclusion, these results showed very clearly that the first amine-epoxide reaction addition reaction takes place and is followed immediately and in parallel by the second addition reaction generating tertiary amines. Moreover, the NMR results also confirm the absence of the P3 product corresponding to the etherification reaction in agreement with the various results 17



Page 18 of 44

reported in the literature for conventional epoxy-amine systems. In addition, a presence of byproducts was also observed during the reaction of the ionic monoepoxide with 2,2'oxybis(ethylamine).

From a study considering three model systems, i.e. reaction of

triethylamine, dimepranol and diethylamine with different amounts of ionic monoepoxy, the formation of an ammonium during the epoxy-amine reaction was also evidenced. Thus, the mechanism of reaction between an ionic epoxy and a primary amine could be suggested (Figure 9): R'

R' N

O

N

N

80°C

NTf2

NH

R'-NH2 OH

N NTf2

very poor reactivity

OH 2

R' N 80°C to 120°C

OH

N NTf2

R' N

N

N

(fast)

NTf2

(very fast)

Followed by :

N

N

80°C

N

N

(minor reaction)

OH NTf2

2

O 2

N

N

NTf2

Figure 9. Proposed mechanism for epoxy ring-opening reaction of a monoepoxy with a primary amine Curing behavior of ILM-[NTf2]/D230 system: Reactivity study (DSC) and evaluation of the epoxy conversion (FTIR)

The curing behavior of ILM-[NTf2]/D230 system was investigated by differential scanning calorimetry (DSC) in order to highlight the ability of ILM to be considered as new epoxy prepolymer by measuring the enthalpy of reaction at the exothermic peak (Figure 10). 18


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Figure 10. DSC thermograms of ILM-[NTf2]/D230

The use of Jeffamine D230 as a conventional curing agent of ionic liquid monomer (ILM) leads to an exothermic peak with a maximum located at 103 °C (for the heating rate considered in this study, i.e. 10 K.min-1). Compared to a conventional epoxy prepolymer (DGEBA n=0.15) reacted with the same aliphatic amine (Jeffamine D230), that shows an exothermic peak temperature happening at higher temperatures, i.e. 120 °C48-49, the ionic liquid monomer seems to be very reactive. Furthermore, the temperature range of the exothermic peak (from 50 to 195 °C) is found to be very similar to the conventional epoxy-amine system (45 °C-180 °C)48. Nevertheless, a notable difference was observed on the system considered in this study. In fact, the enthalpy of reaction of the DGEBA-D230 system is higher compared to the one of ILM[NTf2]/D230. Various authors have measured for DGEBA-D230 system an enthalpy of reaction of about 550-600 J.g-1 compared to 230 J.g-1 for ILM-[NTf2]/D23050-51. Secondly, the presence 19



Page 20 of 44

of two superimposed exotherms at 110 °C and 160 °C can be identified as the evidence of a twostep mechanism and/or different reaction mechanisms40. In addition, two endothermic peaks were also obtained at 240 °C and 260 °C. These multiple peaks (exothermic and endothermic) could be attributed to the simultaneous presence of tertiary amines corresponding to the second amine-epoxy addition reaction and quaternary ammonium as previously demonstrated on model systems (see Figure 8). According to the literature dedicated to the thermal dissociation of quaternary ammonium salts used as surfactant agents of cationic clay such as montmorillonite (MMT) in order to prepare epoxy nanocomposites, Park et al have shown that the presence of endothermic peak at 230 °C is attributed to the degradation of the hydrocarbon chain of the ammonium salt52. On the DSC curve, degradation phenomena occurring at highest temperatures can be explained by the reduced mobility of the quaternary ammonium which is confined in the epoxy network. Based on the previously proposed mechanisms but also on the literature reported on quaternary ammonium used as modifiers agents of MMT, the formation of tertiary and quaternary amines during the polymerization of ILM is confirmed. In order to confirm the curing mechanisms of ionic epoxy compounds with amines as well as to quantify the conversion of epoxide groups in the real conditions, i.e. for 2h at 80 °C followed by 3h at 120 °C, the FTIR spectroscopy is a powerful tool. Thus, it was used to follow the polymerization reaction at different times between the ionic species (ILM) and the diamine (Jeffamine D230). The ILM-[NTf2]/D230 system was analyzed in the wavelength region of 800 – 1300 cm-1 in order to record the evolution of the absorption bands at 914 cm-1 and 1184 cm-1

20


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corresponding to epoxy group and C-0 group of the ether linkage, respectively. Then, the conversion of the epoxide groups was determined from the following equation41, 53:

X% 

A0  At  100% At

Eq (2)

where A0 and At are the ratio between the area of two absorption peak at 914 cm-1 and 1184 cm-1 (A914/A1184) of the system at the beginning (t = 0) and at a given reaction time t, respectively. In Figure 11, the evolution of the conversion of epoxy group versus curing time was shown:

Figure 11. Epoxy group conversion as a function of reaction time from FTIR spectra for ILM[NTf2]/D230 In this case, the copolymerization of Jeffamine D230 with ILM-[NTf2] monomer led to a conversion of epoxy group higher than 96% after 2h at 80 °C and 3h at 120 °C. This result highlights the excellent reactivity between the two monomers corroborating the results obtained 21



Page 22 of 44

by DSC (see Figure 10). As shown in Figure 11, a progressive evolution of the conversion of epoxy functions was observed. In fact, as demonstrated on the model substrates, the first step at 80 °C led to a high conversion percentage (76%) from the three possible reactions, mainly from the primary and the secondary amine-epoxy addition reactions. Then, the second step at 120 °C was sufficient to initiate the reaction of the remaining epoxy functions with the transformation of tertiary amines into quaternary ammoniums. Compared to the literature on similar systems, these results look promising. Indeed, McDanel et al obtained only epoxy conversion included between 60 % and 75 % for stoichiometries of amino-hydrogen-to ratios of 3:1 and 3:221, 23. Other authors on triazolium-based systems observed similar final epoxide conversions for low temperatures of curing (10 h at 60°C, 10 h à 80°C and 3 h at 110 °C) due to the premature degradation of the 1,2,3-triazolium-based PILs. In summary, DSC and FTIR analyses showed the important reactivity between ionic epoxy monomer and amines compared to similar epoxy-amine systems. In addition, based on the reaction mechanisms reports in this work, we have suggested that the crosslinks of resulting network are mainly tertiary amines and quaternary ammoniums. Nevertheless, the networks contain some unreacted epoxy-functionalized species.

Thermomechanical properties of ILM-[NTf2]/D230: Determination of the relaxation temperature (Tα) and approximation of the average molecular weight between crosslinks (Mc)

22


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The dynamic mechanical properties of ILM-[NTf2]/D230 network were investigated. Storage modulus (G’) and loss factor (tan δ) curves were reported in Figure 12. The viscoelastic characteristics of the ionic epoxy network as well as the ones of the DGEBA/D230 system are given in Table 2.

Figure 12. Evolution of storage modulus (G’) and loss factor (tanδ) as a function of temperature for ILM-[NTf2]/D230 The dynamic mechanical spectroscopy (DMS) highlighted only one sharp well-defined relaxation peak (Tα) at 55 °C suggesting an epoxy network with homogeneous structure13, 49. Other complementary techniques have been used such as DSC and Transmission Electron Microscopy (TEM) (see ESI-IV) to analyze the morphology of the resulting network. As expected from DMS analysis, the first method showed that the ILM-[NTf2]/D230 network exhibits only one glass transition temperature at 52 ° C. TEM micrographs clearly highlighted the homogeneous nature of the epoxy network. Compared to a conventional epoxy-amine 23



Page 24 of 44

network (DGEBA/D230), a lower glass transition temperature was obtained. Indeed, numerous works reported Tg close to 90 ° C for the DGEBA/D230 network49-51. This difference is related to the epoxy monomer architecture having a single aromatic group and a spacer arm of a longer length on both sides of the imidazolium ring as well as to the higher molar mass between crosslinks (Mc) compared to DGEBA. Thus, a better mobility and flexibility of ILM[NTf2]/D230 -based network was observed whereas the steric hindrance of DGEBA prevents chain mobility resulting in an increase of the Tg. Previously, the residual concentration of unreacted epoxy groups was estimated by infrared spectroscopy. If a complete conversion of the ILM during the curing schedule is considered, the molar mass between crosslinks, Mc, of the resulting epoxy network can be calculated using the rubber elasticity theory presented in the following equation: Mc = ρ0RT/G’

Eq (3)

where G’ (Pa) represents the equilibrium modulus at the rubbery plateau determined at T= Tα + 30 K and R is the gas constant (8.314 J.K-1.mol-1). All these data values were also presented in Table 2. Table 2. Thermomechanical data of epoxy networks denoted DGEBA/D230 and ILM[NTf2]/D230. Sample

Tα (°C)

Mc (g.mol-1)

G’ (MPa)

ILM-[NTf2]/D230

55

890

3.1

DGEBA/D230

90

590-630

-

24


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For ILM-[NTf2]/D230, an average molar mass between crosslinks of 890 g. mol-1 was experimentally calculated whereas for the DGEBA/D230 system, experimental values included between 590 g. mol-1 and 630 g. mol-1 were reported49, 54. These values are consistent with the evolution of the glass transition temperatures of the two respective networks indicating the formation of a homogeneous network in terms of crosslinking density. In fact, for DGEBA/D230 network, the restricted motion of the chain ends led to a higher Tg. This phenomenon has been widely described in the literature on thermosets. In the opposite, the higher Mc for ILM[NTf2]/D230 induces a higher ductility of the network. In conclusion, the thermomechanical properties of the ionic epoxy-based network by DMS revealed the formation of a homogeneous network with a glass transition temperature of 55 °C.

Thermal stability of ILM-[NTf2]/D230 Thermogravimetric analysis (TGA) was used to determine the thermal behavior of the resulting network denoted ILM-[NTf2]/D230. The evolution of the weight loss in function of the temperature is shown in Figure 13.

25



Page 26 of 44

Figure 13. Weight loss as a function of temperature (TGA in black, DTG in blue) of ILM[NTf2]/D230: (heating ramp 20 K.min-1, N2 atmosphere)

An excellent thermal stability of ILM-[NTf2]/D230 is clearly highlighted compared to conventional DGEBA/D230 network53. However, two degradation temperatures evidenced by maxima close to 290 °C and 440 °C are determined. As previously discussed, this epoxy network presents the same topology as one synthesized for a conventional epoxy-amine network, i.e. resulting from copolymerization of an epoxy monomer and a diamine. The occurrence of two degradation temperatures can be explained by the proposed reaction mechanisms (Figure 9). In fact, these two consecutive degradations were attributed to the degradation of quaternary ammonium (295 °C) during epoxy polymerization and to the 26


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degradation of the network (440 °C), respectively. In this case, the mobility of the quaternary ammonium is significantly reduced due to its insertion by covalent bonding in the formation of the epoxy network (see Figure 9). Thus, it can be considered as in “confined” situation in the epoxy network. According to the literature on the use of quaternary tetraalkylammonium salts as modifying agents of cationic clay, especially montmorillonite (MMT), it was observed that the ammonium salts showed a progressive thermal decomposition corresponding to the presence of physisorbed species on the layered silicates (intrinsic thermal stability of pure salt) and to the organic species intercalated between clay layers55-57. In both cases, ammonium salts are confined and therefore, the degradation of the quaternary ammonium formed during the polymerization of the ILM should have a similar degradation to the ammonium salt confined between clay layers5657.

In conclusion, these results confirm the different reactions proposed previously during the epoxy polymerization with the formation of 25-30 wt% of quaternary ammonium determined by using TGA.

Surface energy of ILM-[NTf2]/D230

Surface energy of epoxy networks were investigated by the sessile drop method. Then, the non-dispersive and dispersive components of the ILM were determined by using Owens-Wendt method from the contact angles with water and methylene diiodide (Table 3). Table 3. Contact angle and surface energy of ILM-[NTf2]/D230 network measured by sessile drop method. 27



Network

θ water (°C)

θ (CH2I2) (°C)

γdispersive (mJ.m-2)

Page 28 of 44

γnondispersive (mJ.m-2)

γtotal (mJ.m-2)

ILM-[NTf2]/D230

101.7

51.7

33.3

0.1

33.4

DGEBA/D230[60]

72

50

27.5

9.2

37

Compared to DGEBA/D230 network, the use of imidazolium-based ionic liquid monomer led to a reduction in surface energy from 37 mJ/m-2 to 33 mJ/m-2. This decrease can be explained by the strong reduction of the polar component from 9.2 mJ/m-2 to 0.1 mJ/m-2

inducing a

hydrophobic behavior to the epoxy network, i.e. similar to polyethylene (PE). These results are attributed to the intrinsic hydrophobic nature of the ILM combined with [NTf2] anion58-59. Various authors such as Coutinho et al have demonstrated that ionic liquids have surface energies of about 29-32 mJ/m-258-59. These results are in agreement with the literature as some authors have also shown that the incorporation of ionic liquids as additives of thermoplastic matrices or as initiators of the anionic polymerization of the epoxy prepolymer generate a significant increase in hydrophobic behavior60-61. In summary, these promising results can open new perspectives in the preparation of hydrophobic coatings by replacing conventional epoxy prepolymers by such an ionic epoxy monomer.

Ionic conductivity of ILM-[NTf2]/D230

For several years, many authors have investigated the development of gel- or solid-polymer electrolytes (GPE or SPE) for energy applications62-66. Thus, many polymers have been used as 28


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“host” such as polyethylene oxide (PEO), poly(vinylidene fluoride) (PVDF) and various poly(ionic liquid)s (PILs)62-64. Nevertheless, the majority of mono-cationic PILs based electrolytes display relevant ionic conductivities by using large quantities of ionic liquids or lithium salts (10 to 90 wt%) but mainly in the temperature range between 50 °C and 100 °C65-68. For example, Drockenmuller et al have designed 1,2,3 triazolium-based networks with ionic conductivities of 2.2*10-9 S.m-1 and 1.5*10-7 S.m-1 at 30 °C65-66. Others authors have developed poly(diallylammonium) chloride containing 10 wt% of IL with an ionic conductivity of 9*10-7 S.m-1 at 30 °C67. Recently, Porcarelli et al have synthesized new IL-like methacrylic monomers copolymerized with poly(ethylene glycol) methyl ether methacrylate and have obtained solid polymer electrolytes with ionic conductivities included between 1.9*10-4 S.m-1 and 2*10-3 S.m-1 depending of the copolymer composition combined with extremely low Tg (up to -61 °C) and for solid polymer electrolytes having Tg close to room temperature, an ionic conductivity < 10-9 S.m-1 was obtained68.

In order to determine the potential of this new epoxy network as a polymer electrolyte, the evolution of the ionic conductivity of ILM-[NTf2]/D230 at different temperatures as a function of the frequency is reported in Figure 14.

29



1.E-01 1.E-02 1.E-03 1.E-04 1.E-05 1.E-06

σAC' (S/m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 44

1.E-07 1.E-08 1.E-09 1.E-10

-78 5 35 80 121

1.E-11 1.E-12 1.E-13 1.E-14 1.E-15 1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

-22 20 50 101

1.E+06

Frequency (Hz) Figure 14. Ionic conductivity as a function of temperature of ILM-[NTf2]/D230: (From -80 °C to 120 °C under He atmosphere in the 106 - 10-2 Hz frequency range)

It is well-known in the field of polymer electrolytes or semi-conducting systems that ionic conductivity is highly dependent on frequency as well as temperature62-63. Moreover, an increase of the temperature causes an increase of the ionic conductivity which becomes less frequency dependent. In fact, ionic mobility being associated with temperature, a rise in temperature will have a significant impact on ionic conductivity. As can be seen in Figure 14, a plateau at low frequencies for 35 °C and 50 °C is displayed for ILM-[NTf2]/D230 which can be explained by a temperature higher (but close to) than the α relaxation (T α) associated to the glass transition temperature (Tg), i.e. 55 °C as observed by 30


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DMA (Figure 12). Thus, ionic conductivity values of about 7.39*10-8 S.m-1 and 4.53*10-6 S.m-1 were obtained, respectively. Then, for higher temperatures than the glass transition temperature of the ILM-[NTf2]/D230 network, ionic conductivity increased significantly to reach values close to 4* 10-4 S.m-1 at 70 °C and 2.4 * 10-3 S.m-1 at 100 °C, respectively. Compared to the literature, these results seem promising compared to 1,2,3 triazolium-based networks having low Tg or different solid polymer electrolytes having Tg above room temperature65-66. Even if this novel thermosetting polymer could be used in extreme conditions i.e. at high temperatures, a modification of the system is required in order to achieve the desired performance at room temperature. Thus, many optimization pathways can be studied such as i) modification of the imidazole skeleton by adding alcohol or ether functionalities and/or ii) the use of aliphatic amines with flexible segments in order to promote the mobility of [NTf2] and iii) the addition of different amounts of lithium salts or ILs. As a consequence, the excellent thermal stability of the ILM-[NTf2]/D230 network compared to various PILs reported in the literature can offer new perspectives in the energy field.

CONCLUSION In this work, new sustainable (multi)functional-dedicated polymer material was developed from a promising imidazolium ionic liquid monomer (ILM) issue of an easy and versatile synthesis at multi-grams scale and commercially available polyetheramine, denoted Jeffamine D230. Moreover, the use of this ILM compared to the conventional DGEBA prepolymer prevents the requirement of toxic and carcinogenic compounds such as Bisphenol A and epichlorohydrin. Initially, an original in-depth study of the reaction mechanisms involved was carried out for the 31



Page 32 of 44

first time on an ionic liquid epoxy monomer ILR-[NTf2] with amine models in order to provide a full understanding of the polymerization of ionic epoxy network formed. The architecturemorphology-properties relationships of ILM-[NTf2] were investigated. It was demonstrated that the copolymerization of ILM-[NTf2] and D230 leads to an homogeneous epoxy network with high thermal stability, an hydrophobic character and ionic conductivity of 2.4 * 10-3 S.m-1 at 100 °C combined with a glass transition temperature of 55 °C. In conclusion, the potential of this ionic liquid monomer as epoxy prepolymer in order to prepare more sustainable epoxy thermosets was established. Nevertheless, a lot of works are required to find the suitable association between ILM and amine or anhydride hardener in order to produce optimized epoxy networks according to the targeted applications. SUPPORTING INFORMATION The general experimental and analytical data, the procedure for the synthesis of ILR-[NTf2] and ILM-[NTf2], the mechanism part as well as the NMR spectrum of ILR-[NTf2] and ILM-[NTf2] are in the supporting information. ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support from Ministère de l'Enseignement Supérieur et de la Recherche (MESR) and from Groupement de Recherches Liquides Ioniques et PolymèreS (GDR LIPS-CNRS #5223).

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