Letter pubs.acs.org/macroletters
Aromatic Polymers Incorporating Bis‑N‑spirocyclic Quaternary Ammonium Moieties for Anion-Exchange Membranes Thanh Huong Pham and Patric Jannasch* Polymer & Materials Chemistry, Department of Chemistry, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden S Supporting Information *
ABSTRACT: We have prepared and studied a new class of anion-conducting membrane materials functionalized with Nspirocyclic quaternary ammonium (QA) cations formed via cycloquaternization reactions involving pyrrolidine, piperidine, and azepane, respectively. These cations were introduced in pairs, adjoined through fused phenyl rings along poly(arylene ether sulfone) backbones. Despite their bulkiness, the bis-Nspirocyclic QA moieties efficiently formed ionic clusters in anion-exchange membranes (AEMs) and showed thermal stability up to 309 °C, as well as a reasonable alkaline stability. The hydroxide ion conductivity of the AEMs increased with decreasing ring size, and a fully hydrated pyrrolidine-based AEM reached a conductivity of 110 mS cm−1 at 80 °C. The results of this study indicate new synthetic pathways to high-performance AEMs based on N-spirocyclic QA groups.
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materials constitute a new class of membrane materials for potential use in alkaline fuel cells and other advanced electrochemical applications. Introducing N-spirocyclic QA groups into aromatic polymers presents a significant challenge and calls for new synthetic pathways. In the present work we have incorporated N-spirocyclic QA groups into poly(arylene ether sulfone) (PAES) by using a straightforward synthetic route comprising polycondensation, bromination, and cycloquaternization (Scheme 1). We are aware that PAES has a tendency to degrade under alkaline conditions.1 Moreover, the present spirocyclic arrangements contain benzylic sites, unlike the compounds analyzed by Marino and Kreuer.9 However, we still selected this synthetic strategy because it offered clear advantages to conveniently form the N-spirocyclic QA groups. In the first step of the synthesis, a K 2CO3-mediated polycondensation of 4,4′-dichlorodiphenylsulfone (DCDPS), tetramethylhydroquinone (4MHQ), and bisphenol A (BPA) in dimethylacetamide at 165 °C resulted in the precursor PAES (Mn = 62 kg mol−1, MwMn−1 = 1.9). 4MHQ was selected as the comonomer in order to introduce two pairs of adjacent benzylic methyl groups on the same aryl ring in the polymer backbone (Scheme 1). After quantitative benzylic bromination, the resulting pairs of adjacent benzyl bromide groups were employed in cycloquaternization reactions with a secondary cyclic amine to form a pair of N-spirocyclic QA groups adjoined through the fused benzene rings. The content of these bis-Nspirocyclic moieties in the PAES, and hence the ion exchange capacity (IEC), was precisely controlled by the feeding ratio of
here is currently a considerable interest in the molecular design and synthesis of robust cationic polymers, mainly because of their great potential as anion exchange membranes (AEMs) for fuel cells, electrolysis, batteries, and water purification and other electrochemical processes and systems.1,2 Each application demands a specific set of AEM properties which usually includes high stability and anionic conductivity under the operating conditions of the particular system.1−7 Typically, AEMs are based on high-performance aromatic polymers such as polysulfones, polyketones, polystyrenes, polyethers, and polyphenylenes covalently functionalized with quaternary ammonium (QA) cations such as benzyl trimethylammonium (BTMA).1−7 Although leading to a high initial conductivity, recent studies have demonstrated that these cations may degrade quite rapidly under alkaline conditions.8,9 In the quest for readily accessible and alkaline-stable cations, polymers functionalized with alternative N-based cations including alkyl trimethylammonium,10−17 guanidinium,18−20 cycloaliphatic ammonium such as morpholinium,21,22 as well as aromatic ones including pyridinium23,24 and different imidazolium25−29 cations have been prepared and investigated. Very recently, Marino and Kreuer systematically investigated the alkaline stability of a large number of different model QA cations in NaOH concentrations up to 10 M and at temperatures reaching 160 °C.9 Among all the cations, they especially identified aliphatic N-spirocyclic QA compounds such as 6-azonia-spiro[5.5]undecane to be exceptionally stable. This was primarily ascribed to the high-transition-state energy of the degradation reactions, which stabilizes these cations against both eliminations and substitutions.9 As far as we know, the preparation and properties of AEMs based on N-spirocyclic QA as the anion-exchange group have not yet been reported in the literature. Consequently, these © XXXX American Chemical Society
Received: September 24, 2015 Accepted: November 16, 2015
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DOI: 10.1021/acsmacrolett.5b00690 ACS Macro Lett. 2015, 4, 1370−1375
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ACS Macro Letters Scheme 1. Synthetic Pathway to PAES-spiro-pyr, PAES-spiro-pip, and PAES-spiro-aze via Polycondensation, Benzylic Bromination, and Cycloquaternizationa
a
Key: (i) K2CO3, DMAc, 165 °C, (ii) NBS, AIBN, 1,2-dichlorobenzene, 110 °C, (iii) pyrrolidine, piperidine, or azepane, DIPEA, NMP, 60 °C.
4MHQ:BPA. By comparing the ratio between 1H NMR signal a, originating from the methyl protons of 4MHQ, and signal b, arising from the isopropylidene protons of BPA, the composition was determined to be x = 0.49, which coincided very well with the target value, x = 0.5 (Figure 1a). Thermogravimetric analysis (TGA) of the PAES indicated a decomposition temperature of Td,95 = 396 °C under N2 (Figure S1), and analysis by differential scanning calorimetry (DSC) showed a glass transition temperature at Tg = 237 °C. The benzylic bromination of PAES was achieved using Nbromosuccinimide (NBS) and azobis(isobutyronitrile) (AIBN) and yielded the brominated derivative PAES-Br. 1H NMR analysis of PAES-Br indicated complete conversion by the total disappearance of the signal of the methyl groups at ∼2.0 ppm and the simultaneous appearance of a new signal at ∼4.5 ppm, arising from the protons of the newly formed benzyl bromide groups. After bromination, the value of Td,95 decreased to 281 °C, probably due to the decomposition of the unstable (reactive) benzyl bromide groups. Finally, PAES-Br was employed in cycloquaternization reactions with pyrrolidine, piperidine, and azepane, respectively, to produce the bis-N-spirocyclic QA derivatives PAES-spiro-pyr, PAES-spiro-pip, and PAES-spiro-aze according to Scheme 1. Typically, QA groups are introduced into polymers via Menshutkin reactions involving benzyl or alkyl halide groups
in the polymer structure and an excess of a tertiary amine, e.g., trimethylamine, to ensure complete conversion. Following this route, we have previously used PAESs containing benzylbrominated di-, tri-, and tetramethylhydroquinone residues to introduce precisely two, three, and four QA groups, respectively, on single phenyl rings along the backbone.30 However, in the present case our aim was to perform cycloquaterizations to lock the cationically charged nitrogens in spirocyclic ring systems. Hence, a modified approach involving secondary cyclic amines was employed in one-pot, two-step reactions. In the basic medium provided by the nonnucleophilic Hü n ig’s Base N,N-diisopropylethylamine, (DIPEA), the cyclic amine was first reacted with only one of the adjacent benzyl bromide groups by nucleophilic attack, thereby forming a tertiary amine attached to the polymer backbone. Next, an intramolecular Menshutkin reaction between this tertiary amine and the neighboring benzyl bromide group resulted in the cycloquaterization and formation of the desired N-spirocyclic QA group. To ensure complete conversion in both steps, the reaction conditions had to be carefully controlled. In the first step, it was necessary to ensure that the secondary cyclic amine reacted with precisely one of the two neighboring benzyl bromide groups on each side of the aryl ring, leaving the other available for the subsequent cycloquaterization. A large excess of the cyclic amine would 1371
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Figure 1. 1H NMR spectra of neat PAES (a), PAES-Br (b), and the bis-N-spirocyclic QA derivatives PAES-spiro-pyr (c), PAES-spiro-pip (d), and PAES-spiro-aze (e). 1372
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ACS Macro Letters Table 1. Properties of AEMs Cast from PAES Functionalized with Bis-N-spirocyclic QA Moieties polymer
IECtheorya [meq g−1]
IECtitrationa [meq g−1]
Td,95b [°C]
qmaxc [nm−1]
dc [nm]
water uptaked [wt %]
λOH-d
PAES-spiro-pyr PAES-spiro-pip PAES-spiro-aze
1.98 (1.76) 1.92 (1.72) 1.87 (1.68)
1.80 (1.62) 1.67 (1.51) 1.64 (1.49)
308 309 274
1.3 1.3 1.3
4.8 4.8 4.8
53 (17) 53 (14) 45 (14)
18 19 17
IEC in the OH− form (values within the parentheses for the Br− form). bEvaluated by TGA under N2. cEvaluated by SAXS in Br− form. dMeasured at 20 °C in OH− form (values within the parentheses for the Br− form). a
thus lead to an overreaction, resulting in polymers bearing only the cyclic tertiary amine instead of the desired N-spirocyclic QA group. Consequently, the excess of the cyclic amine was limited to only 10%. Mixtures of the cyclic amine and the catalyst DIPEA were added dropwise to an N-methyl-2-pyrrolidone (NMP) solution of PAES-Br in order to establish a temporary shortage of the amine initially in the reaction. In addition, the polymer concentration in the reaction mixture was kept at 1 wt % to suppress potential intermolecular polymer−polymer reactions and favor the intramolecular cycloquaternization. Under the optimized reaction conditions, PAES-Br was successfully transformed into the Br− form of PAES-spiro-pyr, PAES-spiro-pip, and PAES-spiro-aze. As seen in the 1H NMR spectra of these polymers in Figure 1c, d, and e, respectively, the signal at 4.5 ppm assigned to the benzyl bromide protons disappeared entirely, and new signals from the α-protons of the spiro nitrogens appeared at ∼3.7 and ∼4.8 ppm (signal a and g, respectively). Additional signals from the cycloaliphatic methylene protons were found between 1.5 and 2.1 ppm (signal h and i) depending on the cyclic amine used in the synthesis. Importantly, no signals corresponding to any tertiary cyclic amine intermediates were observed. The complete conversion in the cycloquaternizations was confirmed by taking into account the integrated signals a and c, the latter originating from the protons in ortho-positions to the sulfone bridges of the PAES backbone. The cationic content thus corresponded to theoretical IECs of 1.98, 1.92, and 1.87 mequiv g−1 for PAESspiro-pyr, PAES-spiro-pip, and PAES-spiro-aze, respectively (Table 1) Flexible, transparent, and mechanically stable AEMs of the polymers in the Br− form were cast from 5% solutions in NMP. The Br− contents of the membranes were determined by Mohr titrations and were found to correspond to ∼90% of the theoretical IEC evaluated by 1H NMR analysis (Table 1). The reason may be the rather low water uptake of these AEMs in the Br− form (14−17 wt %), which may limit the exchange of the ions during the titrations. PAES-spiro-pyr and PAES-spiropip were found to have an excellent thermal stability under N2 atmosphere, with Td,95 values well exceeding 300 °C (Table 1, Figure S1). These decomposition temperatures were much higher than in similar PAES functionalized with conventional BTMA groups, which show Td,95 values between 230 and 270 °C.30 On the other hand, PAES-spiro-aze displayed Td,95 = 274 °C which was significantly below the values of the other two AEMs but still in level with the BTMA-functionalized PAESs. The ability of the bis-N-spirocyclic QA moieties to phase separate from the PAES backbone and form ionic clusters was studied by small-angle X-ray scattering (SAXS) measurements of dry AEMs in the Br− form. The formation of these clusters is crucial in order to develop a percolating water-containing channel system with high anion conductivity upon hydration.2 As seen in Figure 2, all three membranes showed a distinct ionomer peak at qmax ∼ 1.3 nm−1, which corresponded to a characteristic distance between the ionic clusters of d = 4.8 nm.
Figure 2. SAXS profiles of the AEMs in Br− form. The data have been shifted vertically for clarity.
This value coincided with the d-spacing of AEMs based on PAES with precisely two BTMA groups placed on benzene rings along the backbone.30 Thus, despite their bulkiness and constituting an integral part of the PAES backbone, the bis-Nspirocyclic QA moieties efficiently formed ionic clusters in the AEMs. While there was no significant difference in characteristic distance, the scattering intensity of the AEMs increased with decreasing size of the ring. This observation may be explained by an increasing electrostatic shielding effect of a larger ring, which may impede the ionic clustering and/or the scattering efficiency of the clusters. It is critical that AEMs possess sufficient long-term thermochemical stability under the conditions set by the intended application. In particular, the stability of AEMs under alkaline conditions remains a major challenge in the development of fuel cell membranes. According to Marino and Kreuer, monocyclic QA ions may degrade by (i) nucleophilic substitution at the methyl group, (ii) ring-opening elimination, and (iii) ring-opening substitution under alkaline conditions.9 They found that for dimethylpiperidinium reaction (i) was the dominating pathway, while both reaction (i) and (iii) occurred for dimethylpyrrolidinium. In addition, the same authors reported that fully aliphatic N-spirocyclic QA compounds, based on both piperidine and pyrrolidine, respectively, degraded by ring-opening substitution at the α-carbon in 6 M NaOD in D2O at 160 °C.9 In the present case, the alkaline stability was evaluated by analyzing the polymers by 1H NMR spectroscopy after immersing the AEMs in 1 M aq. NaOH at 20, 40, and 60 °C, respectively. As seen in Figure 3, no distinct changes were observed in the 1H NMR spectrum of PAESspiro-aze after storage at 20 °C, which indicated good stability. 1373
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aromatic and the aliphatic regions indicating further degradation of the AEMs, possibly including backbone scission (Figure 3). The stability of the present N-spirocyclic QA groups was thus significantly lower than that observed for the fully aliphatic model compounds studied by Kreuer.9 Because of these findings we strongly suspect that the degradation in the present case mainly occurred through ring-opening substitution at the benzylic positions and that the new signals appeared due to benzyl alcohol and benzyl amine groups formed in the degradation reaction. However, further studies of model compounds are necessary to confirm this. The level of degradation observed for the N-spirocyclic QA groups was significantly lower than that observed for the corresponding AEMs based on PAES-containing phenyl rings functionalized with precisely four BTMA groups along the backbone, thus also containing four benzylic N-positions per ring (∼80% loss of the QA groups after 7 days in 1 M aq. NaOH at 40 °C).31 After locking the QA groups in the spirocyclic arrangement, the rotation of the Ar−CH2−QA bonds is restricted. This is likely to force the nucleophilic ringopening substitution attack to come from a more sterically unfavorable direction in comparison with the situation when the Ar−CH2−QA bonds can rotate freely as in the BTMA groups. In this context it should be mentioned that AEMs with just one BTMA group per ring usually show higher alkaline stabilities.32,33 A sufficient water concentration in the AEMs is critical for the formation of percolating water channels to enable anion conductivity. However, too high water concentrations will lead to high swelling degrees and are detrimental for the mechanical integrity of the AEMs. Figure 4a shows the water uptake of the present AEMs in both the Br− and OH− form. In the Br− form, the water uptake was moderate and increased only slightly, or seemingly not at all, with temperature. As expected, after conversion to the OH− form the water uptake of all three AEMs increased sharply. PAES-spiro-aze showed the lowest water uptake (46 wt % at 60 °C), while PAES-spiro-pip had similar water uptake as PAES-spiro-pyr (∼56 wt % at 60 °C).
Figure 3. 1H NMR spectra of AEMs cast from PAES-spiro-aze in Br− form before and after stability test in aqueous NaOH solution 1 M after 7 days at different temperatures. New signals appearing at ∼4.5 ppm are indicated by arrows.
However, after storage at 40 °C the spectrum showed two new signals at around 4.5 ppm, indicating partial degradation of the N-spirocyclic QA groups. The intensity of the new signals was observed to increase over time (SI, Figure S2). Also for PAESspiro-pyr and PAES-spiro-pip, two new signals appeared around 4.5 ppm after storage at 40 °C during 7 days (SI, Figure S3). No significant differences in the stability between PAES-spiropyr, PAES-spiro-pip, and PAES-spiro-aze were observed. Hence, the ring size had no apparent effect on the stability. After 7 days in 1 M NaOH at 40 °C, the intensity of the new signals at 4.5 ppm constituted ∼5% of the signal of the benzylic protons at 4.8 ppm (Figure 3 and SI, Figure S3). For the AEMs stored at 60 °C, the spectra showed significant changes in both the
Figure 4. Water uptake data (a) and Arrhenius conductivity plots (b) of fully hydrated (immersed) AEMs (symbols apply to both parts a and b). 1374
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The values did not change much between 20 and 60 °C but increased markedly between 60 and 80 °C. The temperature-dependent conductivity of Br− and OH− in fully hydrated AEMs was investigated by electrochemical impedance spectroscopy (EIS) using a two-probe cell. As expected, both the Br− and OH− conductivity increased with increasing temperature and increasing IEC, i.e., decreasing ring size (Figure 4b). Between −20 and 20 °C, the conductivity increased more sharply because of melting of ice. At 60 °C, the OH− conductivity of PAES-spiro-pyr, PAES-spiro-pip, and PAES-spiro-aze reached 52, 31, and 19 mS cm−1, respectively. Here, the apparent activation energy was 17−19 kJ mol−1 which was higher than reported by Pan et al. for BTMAmodified PAES membranes (13 kJ mol−1).34 At 80 °C, the AEM based on PAES-spiro-pyr reached an OH− conductivity of 110 mS cm−1. These high conductivities were facilitated by the efficient ionic clustering and phase separation of the Nspirocyclic QA groups during membrane formation. Upon hydration, this allowed for the formation of localized and percolating water-rich channels for effective ion transport. The Br− conductivity is generally significantly lower than the OH− conductivity because of the lower mobility of Br− in dilute solution and the lower water uptake of the AEMs in the Br− form.35 As seen in Figure 4b, the Br− conductivity of PAESspiro-pyr, PAES-spiro-pip, and PAES-spiro-aze was 3.6, 2.7, and 1.9 mS cm−1, respectively, at 60 °C. In conclusion, a series of PAES functionalized with Nspirocyclic QA groups were successfully synthesized by first introducing two pairs of benzyl bromide groups on benzene rings along the backbone and then employing these in a cycloquaternization reaction with different cycloaliphatic secondary amines. Despite the bulkiness of the spiro-centered cationic sites, these polymers efficiently formed ionic clusters during the casting of AEMs, leading to high anion conductivities. The AEMs showed high thermal stability and a reasonably high alkaline stability but were found to degrade at elevated temperatures. Most probably the N-spirocyclic QA groups mainly degraded through ring-opening substitution at the benzylic positions. Consequently, we are currently developing synthetic pathways to functionalize polymers with fully aliphatic N-spirocyclic QA groups without benzylic positions to enhance stability under basic conditions for this new class of AEM materials.
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ACKNOWLEDGMENTS We thank the Swedish Energy Agency for financial support. We are also grateful to Annika Weiber and Marc Obiols-Rabasa for assistance with SAXS measurements and data treatment.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.5b00690.
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Letter
A detailed experimental description, TGA data, and additional 1H NMR spectra from the alkaline stability evaluation (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 1375
DOI: 10.1021/acsmacrolett.5b00690 ACS Macro Lett. 2015, 4, 1370−1375
