Novolac-based poly(1,2,3-triazolium)s with good ionic conductivity and enhanced CO2 permeation

Novolac-based poly(1,2,3-triazolium)s with 1,2,3-triazolium side groups spaced by oligo(ethylene glycol), a new kind of poly(ionic liquid) membrane, was prepared via the well-known Click chemistry (1,3-dipolar cycloaddition reaction). The thermal properties, ionic conductivity and gas permeation performance of these self-standing membranes were investigated. The obtained membranes exhibit glass transition temperatures ranging from −1 °C to −7.5 °C, and a temperature at 10% weight loss above 330 °C. These membranes have good ionic conductivity (σDC up to 5.1 × 10−7 S cm−1 at 30 °C under anhydrous conditions) as compared with the reported 1,2,3-triazolium-based crosslinked polymers. And they could be potentially used for CO2 separation as they exhibit enhanced CO2 permeability up to 434.5 barrer at 4 atm pressure.


Introduction
Poly(ionic liquid)s (PILs) ideally contain many combined properties of ILs (e.g. tunable solubility, thermal stability, ionic conductivity, etc.) with intrinsic polymer properties (e.g. processability, adhesion, lm-forming properties, etc.). 1 Since ILs are known to possess high CO 2 sorption, PIL membranes for CO 2 permeation applications have gained substantial attention. 2 PILs are supposed to form tight and stable matrices, which are more energy efficient and environmentally benign compared with the commercial separation materials. 3 Generally, the IL moieties provide good CO 2 selectivity and other functional groups (e.g. ether segments, aromatic group, etc.) in PIL membranes could further improve CO 2 permeability. PILs membranes with imidazolium or pyrrolidinium cations based on various backbones (e.g. polyvinyl, polybenzimidazole, polyurethanes, etc.) have already proven their capability in CO 2 separation. [4][5][6][7][8] But, the 1,2,3-triazolium cations based PILs (poly(1,2,3-triazolium)s, TPILs) used for gas separation has just emerged. 5, 9 1,2,3-Triazoliums, the ionization products of 1,2,3-triazoles, are attracting great interest as a type of new potential electrolytes for the reason that the well-known "Click" chemistry (Huisgen 1,3-dipolar cycloaddition reaction) renders the syntheses and functionalization of 1,2,3-triazoles easy and exible. Up to now, numerous TPILs have been reported. 12 For example, through bringing in triethylene glycol (TEG) spacers, main-chain TPILs with bis(triuoromethylsulfonyl)imide (TFSI À ) anions have shown relatively high ionic conductivity (2.0 Â 10 À5 S cm À1 ) with a low glass transition temperature (T g ) of À35 C. 10 Hyperbranched TPILs with oligo(ethylene glycol) (OEG) terminal groups exhibited ionic conductivity 7.7 Â 10 À6 S cm À1 with T g of À14.9 C. 11 Several side-chain TPILs were also investigated, for instance, poly(vinyl ester 1,2,3-triazolium) with TFSI À anions displayed ionic conductivity of 9.2 Â 10 À7 S cm À1 with T g of À16 C while polyacrylates with 1,2,3triazolium side groups spaced by TEG groups displayed higher ionic conductivity of 1.1 Â 10 À5 S cm À1 with lower T g of À36 C. 12,13 It can be concluded that the TFSI À is the best candidate anion for conductive materials, meanwhile, the introduction of 1,2,3-triazolium in the side groups spaced by rich ether groups are effective ways to improve the ionic conductivity of the obtained TPILs. Unfortunately, some of the previously reported TPILs were unable to be used as gas separation membranes due to their brittle or viscous nature resulting from the exible polymer structure.
To promote innovative applications in diverse elds, 1,2,3triazolium-based crosslinked polymers are emerging. 14,15 For example, 1,2,3-triazolium-based epoxy-amine networks and polyether-based 1,2,3-triazoliums were recently reported and both of them show satisfying ionic conductivity (up to 2.0 Â 10 À7 S cm À1 , 10 À6 S cm À1 , separately). 9,16 However, only the latter have been evaluated as gas separation membranes, for the rst time. In view of the fact that innovation and breakthrough is still in need in both the crosslinked TPILs and their applications in CO 2 separation elds, we attempt to design and prepare new selfstanding crosslinked TPILs membranes by introducing rigid benzene rings and 1,2,3-triazoles in the polymer structure.

Synthesis of PN-OEG through CuAAC
To a solution of PN resin

Synthesis of [PN-OEG] + I À
To a solution of PN-OEG (1.71 g, 3 mmol of triazole groups) in 50 mL CH 3 CN, CH 3 I (1.42 g, 10 mmol) was added, and the mixture was stirred at 45 C for 3 d. The mixture was concentrated and precipitated three times in diethyl ether and dried in vacuum to get [PN-OEG] + I À (2.03 g, yield 95%) as a yellow solid. 1

Preparation of the crosslinked NPTAm membrane
A stoichiometric mixture of [PN-OEG] + TFSI À (1.15 g, 2 mmol of alkyne) and p-xylyene diazide (0.19 g, 1 mmol) was dissolved in DMF (5 g), and then was stirred at 70 C for 2 h, following by casting the concentrated mixture onto a glass plate and levelling it with a stainless steel scraper, which had been preheated to 70 C. The glass plate was placed onto a horizontal platform. Then, the system was sequentially cured (70 C/3 h + 80 C/3 h + 120 C/2 h + 150 C/4 h). Aer that, the heating oven was turned off and the whole system was gradually cooled to room temperature. The membrane, named as NPTAm-1, was obtained by immersing the glass plate in water and was then dried at 100 C for 0.5 h for further use.

Characterization
Spectroscopic and thermal characterizations. All NMR spectra data were obtained on a Bruker Advance 400 MHz Spectrometer (Bruker, USA) using tetramethylsilane (TMS) as an internal standard in DMSO-d 6 . FT-IR spectrum measurements were carried out on a Nicolet iS10 FTIR spectrophotometer (Thermo Scientic, USA) in the region of 4000-400 cm À1 using KBr pellets. TGA were conducted on a TGA/DSC 1 (Mettler Toledo, Switzerland) under nitrogen at a heating rate of 10 C min À1 . DSC was performed in a nitrogen atmosphere on a TA Q2000 analyser (TA, USA). The samples were rst heated from 40 C to 150 C and held at 150 C for 2 min to eliminate the thermal history, then cooled to À30 C, and nally heated again from À30 C to 150 C. The heating or cooling rate remained 20 C min À1 and T g values were recorded during the second heating cycle.
Ionic conductivity measurements. The ionic conductivity was measured using a high-resolution Alpha-Analyzer (BDS, Novocontrol GmbH, Germany) assisted by a Quatro temperature controller under nitrogen. The samples were placed between two polished brass electrodes and heated at 110 C for 4 h under a ow of pure nitrogen. At the same time, the dielectric properties were measured to monitor the equilibration process of the sample. Frequency sweeps were then performed isothermally from 10 MHz to 0.1 Hz by applying a sinusoidal voltage of 0.1 V ranging from 110 C to À30 C in steps of 20 C. The temperature was controlled by heating the sample under a ow of pure nitrogen, which could exclude oxygen and humidity in the test chamber.
Gas permeation measurements. The gas permeation properties of the membranes were measured by a standard variable volume method at upstream gas pressure of 4 atm pressure at 25 C according to the literature 20 (Fig. S1 †).
The gas permeability (P) was determined from eqn (1): where q is the inltration capacity of the gas passing through the membrane, L is the membrane thickness, Dp is the differential pressure of feed and permeate side and A is the effective membrane area. The CO 2 /N 2 selectivity (a CO 2 /N 2 ) was calculated from eqn (2): where P CO 2 is the gas permeability of CO 2 and P N 2 is the gas permeability of N 2 . The data averaged from 3 samples for each membrane.

Preparation of NPTAm membranes through CuAAC reaction and 1,3-dipolar cycloaddition
As shown in Fig. 1, PN-OEG was rstly obtained through Cu(I)catalyzed azide-alkyne cycloaddition reaction (Click chemistry) by adding the catalyst of CuSO 4 /NaVc to the solution of PN and OEG-N 3 in DMF. Next, PN-OEG was alkylated by CH 3 I to get the triazolium iodide polymers, [PN-OEG] + I À , following an anion metathesis reaction performed by exchanging the iodide anion (I À ) with TFSI À . Aer being washed through precipitating the mixture into deionized water for several times, no AgI formed when the water phase was tested with AgNO 3 , which showed that the generated LiI had been fully removed, and [PN-OEG] + TFSI À was obtained. Membrane fabrication is based on Huisgen 1,3-dipolar cycloaddition between p-xylyene diazide and [PN-OEG] + TFSI À and the membrane was cured to 150 C to ensure the complete polymerization. Three different membranes were prepared by manipulate the ratio between PN and OEG-N 3 as 1 : 0.4, 1 : 0.6 and 1 : 0.8, and thus the number of side-chain 1,2,3-triazolium and OEG moieties together with the crosslinking density were easily changed. The obtained membranes were observed to be self-standing as shown in Fig. 1. NPTAm-3 membrane (thickness 150 mm, wide 0.7 cm), as an example, could easily sustained an applied load of at least 20 g, that is, tensile force of a minimum of 1.87 Â 10 5 Pa. Additionally, 19 F NMR spectra of [PN-OEG] + TFSI À (Fig. S2 †) clearly showed a single peak, which was further evidence for the completion of anion exchange reaction. FTIR analysis was used to investigate the reaction between pxylyene diazide and [PN-OEG] + TFSI À (Fig. S3 †). Compared to the spectrum of [PN-OEG] + TFSI À , the red shi of the 1,2,3-triazole absorption peak from 3120 cm À1 to 3128 cm À1 and the Fig. 1 Syntheses of the crosslinked NPTAm membranes and the selfstanding ability test. disappearance of the -C^Cgroup absorption peak at 2122 cm À1 are indicative of the formation of NPTAm-1 through the polymerization based on 1,3-dipolar cycloaddition reaction between the alkyne groups in [PN-OEG] + TFSI À and the azido groups in p-xylyene diazide.

Thermal properties of the NPTAm membranes
Generally, ion transport in polyelectrolytes is related with segmental motion in the vicinity of conducting ions, and a low T g promotes the transportation of ions. 11,21 The thermal properties of the NPTAm membranes were investigated by DSC and TGA (Fig. 3, Table 1). All the samples exhibit a single transition corresponding to the glass transition temperature (T g ) values of À1.0 C for NPTAm-1, À4.6 C for NPTAm-2 and À7.5 C for NPTAm-3. NPTAm-3 showed the lowest T g , indicating the lowest crosslinking density and inversely most exible oligo(ethylene glycol) side groups. It could be further conrmed from the DMA result (Fig. S4 †) that NPTAm-3 exhibited the lowest storage modulus in the rubbery state (E 0 ¼ 16.9 MPa and 5.2 MPa at 25 C for NPTAm-1 and NPTAm-3, respectively). However, these NPTAm membranes showed remarkably high T g values as compared with previously reported crosslinked TPILs having TFSI À counter-anions (T g ranges from À52 to À65 C). 9,16 The rigid benzene rings and 1,2,3-triazoles in the backbone along with the p-p stacking and hydrogen bonding character of numerous 1,2,3-triazolium groups 22 may account for the higher T g values. TGA results indicate that all the membranes have good thermal stabilities above 330 C (T d10 ), which are in the upper range of values previously reported TPILs (145-371 C). 5

Ionic conductivity of the NPTAm membranes
The temperature dependence of the anhydrous ionic conductivity of these NPTAm membranes was investigated by BDS. As an example, Fig. 4A describes the frequency (u) dependence of the conductivity (s 0 ) at temperature ranging from À30 to 110 C for NPTAm-3. A plateau in the s 0 value between two characteristic frequency (f E , f EP ) could be observed for all membranes samples when the temperature was above 10 C (Fig. S5 †). This plateau corresponds to the direct current conductivity (s DC ), which associates with the appearance of the ionic conduction character. Considering the correlation between the charge transport of the ionic species and the molecular mobility of the polymer chain, the evolution of s DC with reciprocal temperature for all NPTAm membranes follows a typical Vogel-Fulcher-Tammann (VFT) dependence, and thus the experimental results tted with the VFT eqn (3).
where s N is the ionic conductivity in the limit of high temperature, B is the tting parameter related to the activation energy of ionic conduction, and T 0 is the Vogel temperature. (Fig. 4B) As shown in Table 1, NPTAm-3 showed the highest ionic conductivity (s DC at 30 C) of 5.1 Â 10 À7 S cm À1 compared with NPTAm-1 (2.8 Â 10 À8 S cm À1 ) and NPTAm-2 membranes (7.9 Â 10 À8 S cm À1 ). The enhanced ionic conductivity of NPTAm-3 could be attributed to the high dissociation and mobility of the maximum TFSI À anions promoted by the largest number of side-chain OEG as discussed above on T g s. Except from the crosslinked polyether-based TPIL (s DC up to 3.9 Â 10 À6 S cm À1 ), 9 the ionic conductivity of NPTAm-3 is slightly high for a reported crosslinked TPILs (s DC ranges from 2.2 Â 10 À11 to 2.0 Â 10 À7 S cm À1 ), 14,16,23 clearly demonstrating the structural advantage of introducing the conductive ions in the side groups spaced by exible ether groups with TFSI À as counter-anions.

Gas separation of the NPTAm membranes
The gas separation performances of the NPTAm membranes are summarized in Table 2. The membranes show CO 2 permeability of 264.7-434.5 barrer and CO 2 /N 2 selectivity of 18.4-12.3. Compared to NPTAm-1, NPTAm-3 had higher CO 2 permeability but decreased CO 2 /N 2 selectivity, following a traditional tradeoff. Fig. 5 shows the comparison of the separation performances of this work with the reported data of other crosslinked TPILs membranes. 9 The CO 2 permeability is relatively enhanced. Firstly, there were higher density of side-chain OEG and 1,2,3-triazolium moieties inside the NPTAm membranes. The numerous OEG provided plentiful polar ether groups, which are efficient CO 2 -philic units and could efficiently improve CO 2 affinity. Secondly, the presence of aromatic groups in the backbone signicantly improves the CO 2 uptake. 6 Although both work could not reach Robeson's upper bound, they convince crosslinked TPILs membranes as promising separation materials and optimization of their structure through Click chemistry (1,3-dipolar cycloaddition reaction) will endow them further enhancement in both the permeability and selectivity.

Conclusion
Crosslinked poly(1,2,3-triazolium)s membranes with 1,2,3-triazolium in side chain spaced by oligo(ethylene glycol) were prepared through "Click" chemistry (Huisgen 1,3-dipolar cycloaddition reaction). These self-standing membranes show good thermal properties with T g s ranging from À1.0 to À7.5 C and T d10 above 330 C. These membranes have good ionic conductivity with s DC at 30 C up to 5.1 Â 10 À7 S cm À1 . The structure of rigid benzene ring in the backbone with 1,2,3-triazolium spaced by exible OEG as side groups contribute to enhanced CO 2 permeation, up to 434.5 barrer at 4 atm pressure.

Conflicts of interest
There are no conicts to declare.