1 Introduction

Over the past 2 decades, there has been a steadily growing interest in poly(ionic liquid)s (PILs) as they interestingly gather the unique properties of ionic liquids (e.g. enhanced thermal, chemical, electrochemical and ion conducting properties as well as switchable solubility) with those of polymer materials (e.g. tunable mechanical, viscoelastic, film-forming properties through extensive macromolecular design possibilities) [1,2,3,4,5]. PILs can be assembled from wide libraries of cationic moieties (e.g. ammonium, pyridinium, pyrrolidinium, imidazolium, phosphonium, thiazolium, and more recently 1,2,3-triazolium and 1,2,4-triazolium…) and complementary anionic moieties (e.g. halides, carboxylates, sulfonates, phosphates, inorganic fluorides, perfluorinated sulfonimides…) allowing an almost unlimited variety of combinations in addition to the broad capacities offered by advanced macromolecular engineering. The potential of PILs has been already demonstrated in many applications including catalysis [6,7,8], dye sensitized solar cells [9], thermoresponsive polyelectrolytes [10], electrochromic devices [11], gas separation membranes [6, 12,13,14], self-assembled colloids [15,16,17], sensors and actuators [18, 19], electrolyte-gated transistors [20, 21], as well as polymer electrolytes for energy production and storage [6, 22,23,24,25]. Poly(1,2,3-triazolium ionic liquid)s (PTILs) are a recent addition to broad PIL library [2, 26,27,28,29,30,31]. They are generally synthesized via copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC), the most widespread example of the click chemistry philosophy [32]. Its experimental simplicity, reliability and robustness provides both ease of synthesis and broad structural design inspired from the extensive literature combining CuAAC with macromolecular engineering technologies [33,34,35,36]. Previously described PTILs include anionic polystyrene having covalently tethered TFSI anions and 1,2,3-triazolium counter cations [37], as well as cationic poly(meth)acrylates [38, 39], polyvinyl esters [40], and main chain PTILs having a broad range of counter anions [2, 26,27,28,29,30,31]. Several PTILs with enhanced properties have shown promising performances in key-enabling technologies such as gas separation membranes [41, 42], electrochromic devices [43, 44], and as antibacterial or antifungal materials [45, 46].

Fluorinated polymers such as poly(vinylidene fluoride) (PVDF) and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) are key materials widely used in the fields of energy storage and production (e.g. as binders for electrode materials in lithium ion batteries, and polymer solar cells [47,48,49]). Although many PIL structures contain fluorine atoms assessed through the introduction of perfluorinated counter-anions (e.g. tetrafluoroborate (BF4), hexafluorophosphate (PF6), bis(trifluoromethylsulfonyl)imide (TFSI), bis(pentafluoroethylsulfonyl)imide (BETI), or bis(fluorosulfonyl)imide (FSI), etc…) [1,2,3,4,5, 28], only limited amount of studies have reported on PILs having main-chain or side-chain perfluorinated segments. Previous examples rely either on step-growth polymerization, chain-growth polymerization or post-polymerization chemical modification strategies. Améduri et al. reported the first example of fluorinated PILs by free radical statistical copolymerization of diallyldimethylammonium chloride (DADMAC) and chlorotrifluoroethylene (CTFE) [50]. Ragogna et al. reported on UV-curable formulations based on the free radial copolymerization of hexane diol diacrylate with a series of styrenic ILMs having pendent highly fluorinated phosphonium groups [51]. Detrembleur et al. developed amphiphilic fluorinated PIL diblock copolymers by cobalt-mediated radical polymerization-induced self-assembly [52]. The first polyimidazolium block is hydrophilic due to the presence of oligo(ethylene glycol) pendent groups while the second polyimidazolium block is a statistical distribution of N-vinyl-3-ethyl-imidazolium bromide and N-vinyl-3-perfluorooctyl-imidazolium bromide units. The last example of chain growth polymerization for the elaboration of fluorinated statistical and block copolymers was reported by Binder et al. relying on the radical addition fragmentation chain transfer copolymerization of an imidazolium acrylate with 2,2,2-trifluoroethyl acrylate and subsequent chain extension with pentafluorostyrene to obtain ABA fluorinated triblock copolymers having PIL outer blocks [53]. Liu et al. reported the AA + BB step growth polymerization by N-alkylation of bisimidazole and bishalides having tetrafluorobenzene units [54]. Chen et al. developed the AA + BB step growth polymerization by Suzuki coupling between 2,2′-(9,9-bis(6-bromohexyl)-9H-fluorene-2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) and either 2-fluoro-1,4-diiodo-benzene or 1,2,4,5-tetrafluoro-3,6-diiodo-benzene followed by post-polymerization N-alkylation reaction with trimethylamine to afford the corresponding bistrialkylammonium-containing fluorinated conjugated PILs [55]. Mulhaupt et al. reported on a multistep post-polymerization chemical modifications strategy to obtain hyperbranched fluorine-containing PILs [56]. Hydroxy-functionalized poly(3-ethyl-3-hydroxymethyloxetane) obtained by cationic ring-opening polymerization of 3-ethyl-3-hydroxymethyloxetane was modified sequentially by (1) tosylation, and (2) N-alkylation with 1-(n-1H,1H,2H,2H-perfluorooctyl)imidazole to obtain a perfluorohexyl-containing outer shell. Hu, Chen et al. developed the synthesis of fluorinated poly(aryl ethers) containing pendent trialkylammonium groups by post-polymerization N-alkylation of trimethylamine with bromide-functionalized poly(aryl ethers) obtained by AA + BB condensation copolymerization between 9,9′-dihydroxyfluorene, 4,4′-bisphenol and decafluorobiphenyl [57]. Ho et al. have developed the post-polymerization chemical modification of PVDF-HFP to introduce different contents of pendent n-butyl imidazolium iodides and used them as electrospun membranes for quasi-solid dye-sensitized solar cells [58]. Finally, Améduri et al. prepared 1,2,3-triazolium-based fluorinated vitrimers by concomitant AA + BB step growth polyaddition and N-alkylation of poly(1,2,3-triazole) intermediates using perfluorinated derivatives [59].

Aiming at consolidating the library of PTILs with enhanced ion conducting properties by incorporating perfluoroether units and establishing a fine structure/properties relationship, we report herein the synthesis of fluorinated poly(1,2,3-triazolium)s by AA + BB CuAAC polyaddition of a dialkyne perfluoroether with two diazides followed by N-alkylation of the 1,2,3-triazole groups of the poly(1,2,3-triazole) intermediates. Discussion of their structure/properties correlations compared to previously developed PTILs relies on their characterization by 1H, 13C and 19F NMR spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and broadband dielectric spectroscopy (BDS).

2 Experimental Section

2.1 Materials

Sodium hydride (NaH, 99%), 18-crown-6 (99%), diisopropylethylamine (DIPEA, 99%), copper(I) iodide triethylphosphite (CuIP(OEt)3, 97%), N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA, 99%), N-methyl bis(trifluoromethylsulfonyl)imide (CH3TFSI, 90%), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI, 99.95%), lithium perchlorate (LiClO4, 99.99%), lithium hexafluorophosphate (LiPF6, 98%), lithium trifluoromethanesulfonate (LiSO3CF3, 99.995%) as well as all other reagents and solvents were purchased from Merck and used as received. 1H,1H,11H,11H-Perfluoro-3,6,9-trioxaundecane-1,11-diol (98%) was purchased from abcr and used as received. 1,12-diazido-dodecane 2 [60] and α,ω-diazido-tetra(ethylene glycol) 3 [61], were synthesized as previously described.

2.2 Characterization Methods

1H (300, 400 or 500 MHz), 13C (100 or 125 MHz) and 19F (282 or 470 MHz) NMR spectroscopy experiments were carried out with Bruker DRX 300, DRX 400 or DRX 500 spectrometers. Spectra were obtained with a 5-mm QNP probe at 363 K. Chemical shifts (δ) are given in ppm in reference to DMSO-d6 or CDCl3 for 1H and 13C NMR, and to CHCl2F for 19F NMR. Differential scanning calorimetry (DSC) was performed on a DSC Q23 (TA Instruments) using a heating rate of 10 °C/min under helium atmosphere. Tg values were measured during the second heating cycle. Thermogravimetric analysis (TGA) was performed with a TGA Q500 (TA Instruments) at 20 °C/min under helium atmosphere. Ionic conductivities (σDC) were measured using a high resolution Alpha-Analyzer (Novocontrol GmbH) assisted by a Quatro temperature controller. A solution (ca. 120 mg in 0.5 mL) of PTIL 6 was solvent cast onto a platinum electrode (Ø = 0.5″) and annealed at 110 °C/1 mmHg for ca. 24 h in order to remove solvent and water residuals. The supported polymer film was then placed over a second platinum electrode (Ø = 1″) using 200 µm thick Teflon spacers to build-up a measurement cell in a parallel plate configuration with a well-defined distance between electrodes. A further annealing under a flow of pure nitrogen during 4 h at 110 °C was performed in the cryostat of the dielectric spectrometer while monitoring the time evolution of the complex conductivity function σ*(ω,T) = σ′(ω,T) + iσ″(ω,T). Ionic conductivity measurements were started once a constant electrical response was attained, thus affording well equilibrated sample as well as reliable and reproducible experimental data. Frequency sweeps were then performed isothermally from 10 MHz to 0.1 Hz by applying a sinusoidal voltage of 0.1 V over temperatures ranging from 110 to − 70 °C, in steps of 10 °C. The temperature was controlled by heating the sample under a flow of pure nitrogen in order to prevent the presence of oxygen and humidity in the measurement chamber. The thermal stability was set to be better than 0.1 °C in absolute values with relative variations less than 0.2 °C/min. The same procedure was followed for PTIL 7, and for samples including PTIL 7 with additional lithium salts (i.e. 5 wt% LiTFSI, 5 wt% LiSO3CF3 and 5 wt% or 15 wt% LiClO4).

2.3 Synthesis of Poly(1,2,3-triazole)s 4 and 5

2.3.1 Synthesis of α,ω-Dialkyne Perfluoroether 1

Sodium hydride (2.92 g, 122 mmol) was added in small portions to a stirred solution of 1H,1H,11H,11H-perfluoro-3,6,9-trioxaundecane-1,11-diol (10.0 g, 24.4 mmol) and 18-crown-6 (100 mg, 0.40 mmol) in dry THF (350 mL) maintained at 0 °C under argon. After hydrogen was entirely emitted, a solution of propargyl bromide (10.9 mL, 90.0 mmol) in dry THF (150 mL) was added dropwise. The mixture was allowed to stir overnight at ambient temperature and after neutralization of residual NaH by methanol (10.0 mL), the mixture was extracted with dichloromethane (CH2Cl2, 2 × 150 mL). The separated organic layer was dried using MgSO4, filtered and evaporated under reduced pressure to give the crude product. The crude product was then purified by column chromatography (eluent petroleum ether) to afford the title product 1 as a yellow liquid (8.50 g, 71.7%). 1H NMR (500 MHz, CDCl3, δ): 4.31 (d, J = 2.0 Hz, CF2CH2OCH2C≡CH, 4H), 3.93 (t, J = 9.5 Hz, CF2CH2OCH2C≡CH, 4H), &2.52 (t, J = 2.0 Hz, CF2CH2OCH2C≡CH, 2H). 13C NMR (125 MHz, CDCl3, δ): 124.34-114.20 (OCH2CF2OCF2CF2OCF2CF2OCF2CH2O), 77.65 (CF2CH2OCH2C≡CH), 76.17 (CF2CH2OCH2C≡CH), 67.62 (CF2CH2OCH2C≡CH), 59.13 (CF2CH2OCH2C≡CH). 19F NMR (470 MHz, CDCl3, δ): − 77.52 (OCH2CF2OCF2CF2OCF2CF2OCF2CH2O, 4F), − 88.62 to − 88.74 (OCH2CF2OCF2CF2OCF2CF2OCF2CH2O, 8F).

2.3.2 General Procedure for the AA + BB CuAAC Polyaddition: Synthesis of 4

A solution of α,ω-dialkyne perfluoroether 1 (0.25 g, 0.51 mmol), 1,12-diazido-dodecane 2 (0.13 g, 0.51 mmol), CuIP(OEt)3 (4 mg, 0.01 mmol) and diisopropylethylamine (66 mg, 0.51 mmol) in DMF (2.0 mL) was stirred in the dark at 60 °C for 72 h. The crude reaction mixture was diluted with CH2Cl2 and precipitated three times in diethyl ether (Et2O). The product was dissolved in chloroform (CHCl3, 10 mL) and washed several times with 10 mL of a 5 mg/mL aqueous solution of PMDETA until the water phase remained colourless. The collected organic solution was dried with MgSO4, filtered and concentrated under reduced pressure to afford the title product 4 as a yellow viscous oil (329 mg, 86.7%). 1H NMR (300 MHz, DMSO-d6, δ): 8.12 (s, CF2CH2OCH2C=CH, 2H), 4.68 (bs, CF2CH2OCH2C=CH, 4H), 4.31 (t, J = 4.5 Hz, NCH2CH2(CH2)8CH2CH2N, 4H), 4.10 (bs, CF2CH2OCH2C=CH, 4H), 1.77 (bs, NCH2CH2(CH2)8CH2CH2N, 4H), 1.17 (m, NCH2(CH2)8CH2CH2N, 16H). 13C NMR (100 MHz, DMSO-d6, δ): 138.25 (CF2CH2OCH2C=CH), 130.11 (t, J = 30 Hz, CF2CH2OCH2C=CH), 121.21–118.47 (OCH2CF2OCF2CF2OCF2CF2OCF2CH2O), 68.53 (CF2CH2OCH2C=CH), 60.81 (CF2CH2OCH2C=CH), 53.98 (NCH2CH2(CH2)8CH2CH2N), 38.43–25.87 (NCH2CH2(CH2)8CH2CH2N). 19F NMR (282 MHz, DMSO-d6, δ): − 76.70 to − 78.74 (OCH2CF2OCF2CF2OCF2CF2OCF2CH2O, 4F), − 88.18 to − 89.08 (OCH2CF2 OCF2CF2OCF2CF2OCF2CH2O, 8F).

2.3.3 Synthesis of 5

The general procedure for AA + BB CuAAC polyaddition was applied to α,ω-dialkyne perfluoroether 1 (750 mg, 1.54 mmol), α,ω-diazido-tetra(ethylene glycol) 3 (376 mg, 1.54 mmol), CuIP(OEt)3 (11 mg, 0.03 mmol) and diisopropylethylamine (199 mg, 1.54 mmol) in DMF (6.0 mL) to yield PT 5 as a yellow viscous oil (997 mg, 89.1%). 1H NMR (500 MHz, DMSO-d6, δ): 8.08 (s, CF2CH2OCH2C=CH, 2H), 4.68 (bs, CF2CH2OCH2C=CH, 4H), 4.50 (t, J = 5.0 Hz, NCH2CH2OCH2CH2O, 4H), 4.08 (t, J = 9.5 Hz, CF2CH2OCH2C=CH, 4H), 3.78 (t, J = 5.0 Hz, NCH2CH2OCH2CH2O, 4H), 3.47 (t, J = 2.0 Hz, NCH2CH2OCH2CH2O, 4H), 3.41 (t, J = 3.5 Hz, NCH2CH2OCH2CH2O, 4H). 13C NMR (125 MHz, DMSO-d6, δ): 142.45 (CF2CH2OCH2C=CH), 122.76 (t, J = 278 Hz, OCH2CF2OCF2CF2OCF2CF2OCF2CH2O), 124.61 (CF2CH2OCH2C=CH), 69.45 (NCH2CH2OCH2CH2O), 69.39 (NCH2CH2OCH2CH2O), 68.52 (NCH2CH2OCH2CH2O), 67.49 (t, J = 30 Hz, CF2CH2OCH2C=CH), 64.33 (CF2CH2OCH2C=CH) 49.27 (NCH2CH2OCH2CH2O). 19F NMR (282 MHz, DMSO-d6, δ): − 76.74 to − 76.78 (OCH2CF2 OCF2CF2OCF2CF2OCF2CH2O, 4F, 4F), − 88.18 to − 88.64 (OCH2CF2 OCF2CF2OCF2CF2OCF2CH2O, 8F).

2.4 Synthesis of Poly(1,2,3-triazolium)s 6 and 7

2.4.1 General Procedure for N-alkylation of 1,2,3-Triazole Groups: Synthesis of 6

A solution of PT 4 (400 mg, 0.54 mmol of 1,2,3-triazole groups) and CH3TFSI (800 mg, 2.71 mmol) in acetonitrile (5 mL) was stirred for 48 h at 85 °C. The solution was evaporated under vacuum to give a crude product which was then dissolved in acetonitrile, precipitated three times in Et2O and evaporated under vacuum to afford PTIL 6 as a brown viscous oil (712 mg, 99.0%). 1H NMR (300 MHz, DMSO-d6, δ): 8.9 (s, CF2CH2OCH2C=CH, 2H), 4.99 (bs, CF2CH2OCH2C=CH, 4H), 4.58 (t, J = 7.5 Hz NCH2CH2(CH2)8CH2CH2N, 4H), 4.28 (bs, CF2CH2OCH2C=CH, 4H), 4.22 (s, CH3N, 6H), 1.90 (bs, NCH2CH2(CH2)8CH2CH2N, 4H), 1.23 (m, NCH2CH2(CH2)8CH2CH2N, 16H). 13C NMR (100 MHz, DMSO-d6, δ): 139.28 (CF2CH2OCH2C=CH), 130.30 (CF2CH2OCH2C=CH), 124.04,122.45,117.98 (OCH2CF2OCF2CF2OCF2CF2OCF2CH2O), 119.38 (q, J = 320 Hz, CF3SO2N), 68.40 (t, J = 30 Hz, CF2CH2OCH2C=CH), 61.15 (CF2CH2OCH2C=CH), 53.46 (NCH2CH2(CH2)8CH2CH2N), 38.28 (CH3N), 28.91–28.29 (NCH2CH2(CH2)8CH2CH2N). 19F NMR (282 MHz, DMSO-d6, δ): − 76.74 to − 76.83 (OCH2CF2 OCF2CF2OCF2CF2OCF2CH2O, 4F), − 78.75 (s, CF3SO2N, 12F), − 87.85 to − 88.40 (OCH2CF2 OCF2CF2OCF2CF2OCF2CH2O, 8F).

2.4.2 Synthesis of 7

The general procedure for N-alkylation was applied to 5 (600 mg, 0.88 mmol of 1,2,3-triazole groups) and CH3TFSI (1.21 g, 4.10 mmol) in acetonitrile (7.0 mL) to yield PTIL 7 as a dark brown viscous oil (1.06 g, 98.6%). 1H NMR (500 MHz, DMSO-d6, δ): 8.87 (s, CF2CH2OCH2C=CH, 2H), 5.01 (bs, CF2CH2OCH2C=CH, 4H), 4.79 (t, J = 5.0 Hz, NCH2CH2OCH2CH2O, 4H), 4.27 (bs, CF2CH2OCH2C=CH, 4H), 4.24 (s, CH3N, 6H), 3.90 (t, J = 4.5 Hz, NCH2CH2OCH2CH2O, 4H), 3.54 (t, J = 5.0 Hz, NCH2CH2OCH2CH2O, 4H), 3.46 (t, J = 4.0 Hz, NCH2CH2OCH2CH2O, 4H). 13C NMR (100 MHz, DMSO-d6, δ): 138.91 (CF2CH2OCH2C=CH), 130.39 (CF2CH2OCH2C=CH), 122.44, (t, J = 348 Hz, OCH2CF2OCF2CF2OCF2CF2OCF2CH2O), 119.37 (q, J = 320 Hz, CF3SO2N), 69.49 (NCH2CH2OCH2CH2O), 69.64 (NCH2CH2OCH2CH2O) 68.14 (t, J = 30 Hz, CF2CH2OCH2C=CH), 67.31 (NCH2CH2OCH2CH2O), 60.80 (CF2CH2OCH2C=CH), 53.18 (NCH2CH2OCH2CH2O), 38.01 (CH3N). 19F NMR (282 MHz, DMSO-d6, δ): − 76.86 (s, CF2CH2O, 4F), − 78.80 (s, CF3SO2N, 12F), − 87.92 (t, OCF2CF2OCF2CF2OCF2CH2O, 8F).

3 Results and Discussions

3.1 Synthesis and Physical Properties of PTs 4 and 5

α,ω-Dialkyne perfluoroether monomer 1 was first synthesized in 71.7% yield by alkylation reaction between 1H,1H,11H,11H-perfluoro-3,6,9-trioxaundecane-1,11-diol and propargyl bromide (Scheme S1). The structure and purity of 1 were confirmed by 1H, 13C and 19F NMR spectroscopy (Fig. S1–S3). PTs 4 and 5 (Scheme 1) were then synthesized by AA + BB CuAAC polyaddition accordingly to the previously optimized reaction conditions, i.e. 20 wt% of dialkyne 1 and diazide 2 or 3 in N,N-dimethylformamide (DMF) using 1 equiv. DIPEA and 0.05 equiv. CuIP(OEt)3 as catalytic system [30]. After 72 h of reaction at 60 °C, the obtained polymers were purified by precipitation in Et2O and copper catalyst was removed by liquid/liquid extraction using CHCl3 and aqueous solutions of PMDETA. Solubility study was then performed with several solvents (Table S1). PTs 4 and 5 are insoluble in water, n-heptane, Et2O and toluene (PhCH3) while they are soluble at 10 mg/mL in CHCl3, CH2Cl2, ethyl acetate (EtOAc), tetrahydrofuran (THF), acetonitrile (CH3CN), DMF, and acetone. However, only PT 5 is soluble at 1 mg/mL in methanol (MeOH) and dimethylsulfoxide (DMSO).

Scheme 1
scheme 1

Synthesis of PTILs 6 and 7

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1H NMR spectroscopy (Fig. 1) confirmed the structure of PTs 4 and 5 by the presence of the characteristic signal of the 1,2,3-triazole protons at 8.12 and 8.08 ppm for 4 and 5, respectively. Moreover, the absence of signals corresponding to propargyl and azidomethyl chain ends proved that polymers having relatively high polymerization degrees (Xn) were obtained. DSC confirmed that PT 4 containing dodecyl segments is semi-crystalline with a melting temperature (Tm) of 68 °C and a glass transition temperature (Tg) of − 21 °C (Fig. S4). Conversely, PT 5 containing tetra(ethylene glycol) (TEG) segments is amorphous with a Tg of − 32 °C (Table 1). The presence of the perfluoroether segment affords a decrease of Tm and/or Tg values compared to previously reported perhydrogenated PT analogues having either an undecanoyl (Tm = 107 °C and Tg = − 9 °C) or a triethylene glycol (Tg = − 23 °C) spacer between 1,2,3-triazole groups [26]. However, Tg values of are higher than those reported by Habas et al. for semi-crystalline and amorphous fluorinated PTs obtained from the AA + BB CuAAC polyaddition of a longer perfluoropolyether dialkyne and different fluorinated diazides (Tg values below − 103 °C) [62, 63]. TGA results (Fig. S5) show that PTs 4 and 5 undergoes 10% weight loss (Td10) at 335 and 340 °C, respectively, which is in par with PT analogues having an undecanoyl or a triethylene glycol spacer (i.e. Td10 = 340 and 345 °C, respectively) [26].

Fig. 1
figure 1

1H NMR (25 °C, DMSO-d6) of PTs 4 and 5

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Table 1 Physical properties of PTs 4, 5 and PTILs 6,7
Full size table

3.2 Synthesis and Physical Properties of PTILs 6 and 7

PTILs 6 and 7 were then obtained by selective N-alkylation at the N-3 position of the 1,2,3-triazole groups of PTs 4 and 5 using CH3TFSI in CH3CN at 85 °C for 48 h. After purification by precipitation in Et2O, the structures of PTILs 6 and 7 were confirmed by careful assignment of 1H, 13C and 19F NMR spectra. 1H NMR of 6 and 7 (Fig. 2) shows the quantitative N-alkylation of 1,2,3-triazole groups with the appearance of the N-3 methyl signal at ca. 4.2 ppm with a 3:1 ratio according to the 1,2,3-triazolium signal at ca. 8.9 ppm as well as the total disappearance of the 1,2,3-triazole signal at ca. 8.1 ppm which characteristic of the PT precursors. Moreover, significant shifts were observed for the chemical displacements of most signals of the protons neighbouring the 1,2,3-triazole/1,2,3-triazolium groups (Fig. S6 and S7). 13C NMR (see e.g. Fig. S8 for PT 5 and PTIL 7) have also been used to further confirm the quantitative nature of the N-alkylation reaction: the peak corresponding to 1,2,3-triazole signal at 124.98 ppm disappeared, while new signals at 38.01, 130.39 and 119.38 ppm were ascribed to the carbon atoms of the N-3 methyl and 1,2,3-triazolium groups as well as the TFSI counter-anions, respectively. Furthermore, N-alkylation of the 1,2,3-triazole groups was also corroborated by the appearance of the trifluoromethyl signal from the TFSI counter-anions at − 78.75 ppm in the 19F NMR spectra (Fig. S9). All the above evidence a successful and quantitative N-alkylation reaction at the N-3 position of the 1,2,3-triazole groups of PTs 4 and 5.

Fig. 2
figure 2

1H NMR (25 °C, DMSO-d6) of PTILs 6 and 7

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As their neutral precursors 4 and 5, PTILs 6 and 7 are insoluble in H2O, heptane, Et2O and PhCH3 while they are soluble at 10 mg/mL in acetone, THF, CH3CN, and DMF (Table S1). However, as generally observed, several changes in solubility occur after N-alkylation of the 1,2,3-triazole groups [2]. Indeed, conversely to their neutral precursors 4 and 5, PTILs 6 and 7 are insoluble in CH2Cl2, CHCl3 and EtOAc. Finally, while PT 4 is only partially soluble at 1 mg/mL in MeOH and DMSO, corresponding PTIL 6 is fully soluble at 10 mg/mL in these solvents. Moreover, whereas dodecyl-containing PT 4 is semi-crystalline, PTIL 6 is amorphous with a significant decrease of Tg from − 21 to − 38 °C, respectively. Conversely, N-alkylation of the 1,2,3-triazole groups has only a limited impact on tetra(ethylene glycol)-containing derivatives as Tg values slightly increase from − 32 to − 29 °C for PT 5 and PTIL 7, respectively (Fig. S4). These differences are mainly due to the loss of crystallinity of PTIL 6 after N-alkylation of 4, while both PT 5 and PTIL 7 are amorphous. It is worth noting that the presence of more bulky and less flexible perfluoroether segments affords an increase in Tg compared to previously reported perhydrogenated PTIL analogues having an undecanoyl or a triethylene glycol spacer between the 1,2,3-triazolium groups (Tg = –68/− 35 °C and − 38/− 29 °C for fluorinated and hydrogenated analogues, respectively) [26]. TGA measurements (Fig. S5) demonstrate the good thermal stability of PTILs 6 and 7 that exhibit Td10 values above 340 °C (Table 1). These values are comparable to those reported for PTIL analogues having an undecanoyl or a triethylene glycol spacer between the 1,2,3-triazolium groups (Td10 = 370 and 340 °C, respectively). However, it is surprising that Td10 values of PTILs 6 and 7 are ca. 5 °C higher than for PTs 4 and 5 (Td10 = 335 and 340 °C, respectively) while thermal stability generally decreases after N-alkylation of the 1,2,3-triazole groups [2]. This is most probably a positive consequence of the presence of the perfluoroether segments.

3.3 Ionic Conductivity of PTILs 6 and 7

The temperature dependence of the ionic conductivities of PTILs 6 and 7 was investigated by broadband dielectric spectroscopy (BDS, Fig. S10). σDC values measured under anhydrous conditions are reported as a function of reciprocal temperature in Fig. 3. As generally observed for glassy materials, ionic conductivities of 6 and 7 above Tg follow a Vogel–Fulcher–Tammann (VFT) dependence. Temperature-dependent ionic conductivities of PTILs 6 and 7 were thus fitted with the VFT Eq. (1),

$$\sigma_{\text{DC}} = \sigma_{\infty } \times { \exp }\left( { - B/\left( {T{-}T_{0} } \right)} \right)$$
(1)

with σ∞ the ionic conductivity in the limit of high temperatures, B the fitting parameter related to the activation energy of ionic conduction, and T0 the Vogel temperature which generally ranges from 30 to 70 K below Tg. The parameters affording the best fittings of the experimental curves are reported in Table 1. PTILs 6 and 7 exhibit relatively high and comparable values of anhydrous ionic conductivity (σDC at 30 °C = 1.2 × 10−5 and 8.4 × 10−6 S/cm, respectively). This difference is consistent with the evolution of Tg values measured by DSC, i.e. Tg increases from − 38 to − 29 °C for 6 and 7, respectively. Obtained values are in the upper range of values previously measured for TFSI-containing PTILs which range from 1.0 × 10−7 to 6.7 × 10−5 S/cm [2, 64]. σDC of PTIL 6 (issued from dodecyl and perfluoroether units) is comparable to a previously reported triethylene glycol-based PTIL obtained by AB + AB CuAAC polyaddition and having a comparable Tg of − 35 °C [26]. However, σDC of PTIL 7 (issued from tetraethylene glycol and perfluoroether units) is lower than both PTIL 6 and above mentioned triethylene glycol-based PTIL (σDC at 30 °C = 8.4 × 10−6, 1.2 × 10−5 and 1.6 × 10−5 S/cm, respectively). This most probably means that the perfluoroether units are more compatible with dodecyl units than tetraethylene glycol ones. The impact of the addition of different lithium salts (i.e. 5 wt% of LiSO3CF3, 5 wt% of LiTFSI, or 5 and 15 wt% of LiClO4) was then investigated by BDS (Fig. S11 and S12) using PTIL 7 as ether units are known to enable a better solubilization and dissociation of lithium salts compared to e.g. aliphatic units found in PTIL 6. Indeed, the solubilization and compatibilization of an additional lithium salt is essential to afford an enhancement of ionic conductivity. While addition of 5 wt% LiSO3CF3 affords a ca. 40-fold decrease of σDC at 30 °C (σDC at 30 °C = 2.2 × 10−7 S/cm), ionic conductivity is almost unchanged by the addition of 5 wt% LiTFSI. However, addition of 5 wt% LiClO4 affords a twofold increase in σDC reaching 1.5 × 10−5 S/cm at 30 °C (Fig. S12). Further addition of 15 wt% LiClO4 induces a drastic ca. 115-fold decrease in ionic conductivity (σDC = 6.9 × 10−8 S/cm at 30 °C). This is most likely due to the aggregation and insufficient solubilization of such amount of LiClO4 in the PTIL matrix.

Fig. 3
figure 3

Direct current conductivity versus reciprocal temperature for PTILs 6 (solid squares) and 7 (open triangles)

Full size image

4 Conclusion

This paper successfully demonstrates the synthesis and characterization of two PTILs combining the attractive features of AA + BB CuAAC polyaddition and quantitative N-alkylation of 1,2,3-triazole groups together with the unique properties of perfluoroether materials. Although having Tg values above those that could be expected for fluorinated materials, obtained perfluoroether-based PTILs display high ionic conductivity with σDC at 30 °C in the 10−5 S/cm range. Moreover, thermal stability is positively impacted by the presence of fluorinated segments compared to other previously reported PTIL analogues. Both PTILs afford remarkably high ionic conductivity values although the dodecyl-based PTIL 6 affords a value ca. twice as high of tetraethylene glycol-based PTIL 7. LiClO4 proved to be the best additive as a ca. twofold increase in ionic conductivity was observed when 5 wt% were added to PTIL 7. In summary, our study provides an important insight into the structure/properties relationship of PTILs with main chain perfluoroether segments. The high level of ionic conductivity reached together with high content of fluorine atoms make these materials potential candidates for functional polymer electrolytes required for future lightweight and flexible diplays, solar cells, batteries and other electrochemical devices. The search for other fluorinated PTIL derivatives capable of more intensely improving ion transport without lithium salts is currently explored.