Article pubs.acs.org/Macromolecules
Poly(N,N-diallylazacycloalkane)s for Anion-Exchange Membranes Functionalized with N‑Spirocyclic Quaternary Ammonium Cations Joel S. Olsson, 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: The alkaline stability of organic cations tethered to anion-exchange membranes (AEMs) is essential for the long-term performance of alkaline membrane fuel cells and electrolyzers. Here, we have prepared and studied the thermal and alkaline stability of a series of polyelectrolytes functionalized with N-spirocyclic quaternary ammonium (QA) cations. N,N-Diallylazacycloalkane quaternary salts were readily synthesized by diallylation of pyrrolidine, piperidine, azepane, and morpholine. These monomers were employed in radicalinitiated cyclo-polymerizations to obtain the target poly(N,Ndiallylazacycloalkane)s. 1H NMR spectroscopy revealed that the stability of the polyelectrolytes in 2 M KOD/D2O solutions critically depended on the ring size and the absence of additional heteroatoms in the ring. Thus, poly(N,N-diallylpiperidinium) showed the highest alkaline stability, with only minor signs of degradation at 120 °C after 14 days, while the polyelectrolytes based on the morpholine and azepane rings clearly degraded via both Hofmann elimination and ring-opening substitution already at 90 °C. Cross-linked water nonsoluble AEMs were prepared by copolymerizing N,N-diallylpiperidinium chloride with methylbenzyldiallylammonium groups tethered to poly(phenylene oxide). These transparent and mechanically robust AEMs reached high OH− conductivities, above 0.1 S cm−1 at 80 °C. The present work demonstrates the high alkaline stability of suitably configured N-spirocyclic QA cations, which will open up new prospects for readily accessible high performance polyelectrolytes and membranes. conductivity over time, especially at temperatures above ∼60 °C. Several successful strategies to significantly enhance the stability of organic cations under alkaline conditions have been reported recently.11−21 In these strategies the cationic moieties have been especially designed to protect the cation sterically, to incorporate conformational restrictions which increase the energy level of the transition state or to introduce inductive effects to lessen the risk of hydroxide attack. Important examples include sterically protected imidazolium11,12 and phosphonium13−15 cations, cycloaliphatic QAs,16−19 and the introduction of alkyl spacer units in-between the polymer backbone and the QA cations.7,20,21 Nevertheless, there is still a great need to design, synthesize, and explore new organic cations with enhanced alkaline stability. N-Spirocyclic QA cations constitute a special class of ions in which two cycloaliphatic rings are fused by a cationic nitrogen center. Recently, the remarkable stability of certain Nspirocyclic QA model compounds in 6 M NaOH at 160 °C was reported.16 The piperidine-based 6-azonia-spiro[5,5]undecane showed the highest stability with a half-time of 110 h. Under the same conditions, the pyrrolidine-based 5-azonia-
1. INTRODUCTION There is currently a great need for ionic polymers that are molecularly designed and synthesized to form thin and durable high-performance membranes with specific mechanical and transport properties.1−4 These membranes are crucial components in electrochemical energy storage and conversion devices such as batteries, fuel cells, and electrolyzers.1−4 Alkaline anionexchange membrane fuel cells (AEMFCs) are efficient and environmentally benign power sources which may potentially operate with comparatively inexpensive metal catalysts such as Ni and Ag.1 This offers great advantages in relation to protonexchange membrane fuel cells operating under acidic conditions. These fuel cells are in a highly developed state today but depend on expensive and rare platinum group metal catalysts. AEMFCs are reliant on anion exchange membranes (AEMs) which combine long-term alkaline stability with high hydroxide ion conductivity.5,6 Because of the highly basic and nucleophilic character of the hydroxide ion, standard organic cations such as benzyltrimethylammonium (BTMA) tend to degrade rather quickly via for example β-hydrogen Hofmann elimination, direct nucleophilic substitution at an α-carbon, and elimination via ylide formation.7−9 Moreover, links along polymer backbones which are activated by electron-withdrawing groups may be cleaved via hydroxide ion attack.10 Altogether, these degradation reactions will result in critical losses of mechanical strength, ion exchange capacity, and © XXXX American Chemical Society
Received: January 23, 2017 Revised: March 17, 2017
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DOI: 10.1021/acs.macromol.7b00168 Macromolecules XXXX, XXX, XXX−XXX
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Scheme 1. Synthesis of N-Allylpiperidine, N,N-Diallylpiperidinium Chloride, and Its Corresponding Polyelectrolyte (Upper) as Well as the Structures of the Additional N-Spirocyclic QA-Functionalized Polyelectrolytes of the Present Study (Lower)
2. EXPERIMENTAL SECTION
spiro[4.4]nonane displayed a half-time of 28 h, while the standard BTMA reached a mere 4.2 h.16 The resistance of the N-spirocyclic QA cations against elimination and ring-opening substitution reactions was ascribed to the geometric constraints of the rings on the transition states, which then require unfavorable bond angles and lengths. Hence, the balance between ring strain and the transition-state energies will determine the stability and which of the possible degradation reactions that will be the most harmful.16 We have previously reported on the synthesis of poly(arylene ether sulfone)s functionalized with various bis-N-spirocyclic QA moieties. These dicationic sites were formed via cycloquaternization reactions involving 2,3,5,6-tetrakis(bromomethyl)phenylene units in the polymer backbone and pyrrolidine, piperidine, and azepane, respectively.17 AEMs cast from these polymers showed a high OH− conductivity, reaching above 0.1 S cm−1 at 80 °C. Nevertheless, 1H NMR spectroscopy indicated that the AEMs degraded through ringopening substitutions at the benzylic positions and via cleavage of ether links in the backbone after immersion in 1 M NaOH at 40 °C.17 Consequently, the ring size did seemingly not influence the degradation rate. More recently, we have synthesized N-spirocyclic QA ionenes by cyclo-polycondensation of tetrakis(bromomethyl)benzene and dipiperidines.18 These “spiroionenes” were found to be stable over more than 1800 h in 1 M KOD/D2O at 80 °C and even displayed a reasonable stability at 120 °C under the same conditions. Blend AEMs based on spiroionenes and polybenzimidazole showed OH− conductivities up to 0.12 S cm−1 at 90 °C.18 In the present work we have synthesized and investigated a series of fully aliphatic N-spirocyclic polyelectrolytes. Four different N,N-diallylazacycloalkane quaternary salts were first synthesized by diallylation of pyrrolidine, piperidine, azepane, and morpholine, respectively. In the next step, these monomers were cyclo-polymerized, forming N-spiro centers during the radical-mediated propagation. The high ionic content of the polyelectrolytes rendered them water-soluble and impractical for direct membrane formation. In order to demonstrate the feasibility to prepare functional AEMs, and N,N-diallylpiperidinium chloride monomer was copolymerized with diallylmethylbenzylammonium cations attached to poly(phenylene oxide) to form cross-linked membranes. These cross-linked AEMs were characterized with respect to water uptake and OH− conductivity.
2.1. Materials. Pyrrolidine (≥99.5%, Sigma-Aldrich), piperidine (99%, Sigma-Aldrich), azepane (99%, Sigma-Aldrich), morpholine (99%, Sigma-Aldrich), diallylmethylamine (97%, Sigma-Aldrich), allyl bromide (98%, Fluka), allyl chloride (98%, Acros), diethyl ether (reagent grade, VWR), N-methyl-2-pyrrolidone (NMP, reagent grade, Acros), ammonium persulfate (98%, Sigma-Aldrich), N-bromosuccinimide (99%, Acros), azobis(isobutyronitrile) (98%, SigmaAldrich), poly(2,6-dimethyl-1,4-phenylene oxide) (PPO, SigmaAldrich, Mn = 20 kg mol−1, MwMn−1 = 2.3), poly(diallyldimethylammonium chloride) (PDADMAC) solution (low Mw, 35 wt % in H2O), and PDADMAC solution (medium Mw, 20 wt % in H2O) were all used as received. 2.2. Synthesis of N,N-Diallylazacycloalkane Quaternary Ammonium Salts. The synthesis of the N,N-diallylazacycloalkane quaternary ammonium bromide and chloride salts was performed via diallylation of different heterocyclic amines in two separate steps (Scheme 1), based on the method reported by De Vynck and Goethals.22 The synthesis of N,N-diallylpyrrolidinium chloride (DAPyrCl) is described below as an example. First, N-allylpyrrolidine was prepared by monoallylation of pyrrolidine. Allyl bromide (10.9 mL, 1.05 equiv) was added dropwise to a stirred mixture of pyrrolidine (20.0 mL, 2 equiv) and diethyl ether (20 mL) at 0 °C. After 30 min, the temperature was allowed to reach room temperature, and the mixture was left overnight under stirring. After removal of pyrrolidinium bromide by filtration and subsequent evaporation of the solvent and vacuum distillation, 10.9 g of N-allylpyrrolidine was obtained (82% overall yield). The same method was employed to prepare N-allylpiperidine, N-allylmorpholine, and N-allylazepane at 80, 70, and 83% overall yields, respectively. The obtained products were used in the next step without further purification. In the next step, DAPyrCl was prepared by quaternization of Nallylpyrrolidine using allyl chloride. A volume of 10 mL (1.25 equiv) of allyl chloride was added to a solution of 10.9 g (1 equiv) of Nallylpyrrolidine in 10 mL of NMP at room temperature. The solution was heated to 75 °C under stirring. After 48 h it was cooled to room temperature, and 20 mL of diethyl ether was added to the reaction vessel to ensure that all the product precipitated before filtration and washing with diethyl ether to obtain 15.8 g of DAPyrCl (overall yield 86%). N,N-Diallylpyrrolidinium bromide (DAPyrBr) was prepared in a 97% overall yield using the same method with the exception that the second step was performed at room temperature and that the mixture was cooled to 0 °C during the addition of allyl bromide. The methods described above were also used to prepare the chloride salts of N,N-diallylpiperidinium (DAPipCl, overall yield 88%), N,N-diallylmorpholinium (DAMorCl, 93%), and N,N-diallylazepanium (DAAzeCl, 97%). In addition, the corresponding bromide salts (DAPipBr, DAMorBr, and DAAzeBr) were prepared at 97, 94, and 95% overall yields, respectively. All the monomers were extremely hygroscopic and were readily soluble in water and methanol. They were insoluble in hydrocarbons, acetone, and diethyl ether but had a limited solubility in NMP and acetonitrile. B
DOI: 10.1021/acs.macromol.7b00168 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules 2.3. Cyclo-Polymerization. The N-spirocyclic polyelectrolytes were prepared via radical-initiated cyclo-polymerization of the N,Ndiallylazacycloalkane quaternary ammonium salts. An amount of 2−3 g of respective ionic monomer and ammonium persulfate was dissolved in deionized water to concentrations of 60−70 and 2 wt %, respectively. Air was removed from the solution by four freeze−thaw cycles under an argon/vacuum atmosphere. The polymerization was then carried out at 50 °C for 18−24 h, after which the viscosity of the reaction solution increased significantly. Before precipitation in acetone, the polymer solution was diluted with deionized water to enable the use of a pipet. The precipitate was dissolved in fresh deionized water and again precipitated in acetone to remove residual monomers; yields ranged between 50 and 75%. The polyelectrolytes were very hygroscopic and were found to be soluble only in water. The samples were designated by adding the prefix “P” to the respective monomer designation. 2.4. Structural Characterization. The structure of the monomers and polyelectrolytes was determined by NMR spectroscopy. 1H NMR, 1 H−1H COSY, and 1H−13C HSQC spectra were obtained using a Bruker DR X400 spectrometer at 400.13 MHz using CDCl3 (δ = 7.26 ppm) or DMSO-d6 (δ = 2.50 ppm), D2O (δ = 4.79 ppm), or 1 M KOD in D2O, where the polymer signals are shifted upfield by 0.06− 0.08 ppm. The intrinsic viscosity was determined using an Ubbelohde viscometer with the polyelectrolytes dissolved in aqueous 1 M NaCl solutions at 30 °C. For each sample the efflux time was measured four times at four different concentrations. The reduced (ηred) and inherent (ηinh) viscosities were calculated as ηred =
ts tb
−1 (1)
c
() ts tb
ln ηinh =
corresponding to 10−15 wt % of the total weight was added. The concentrated solution was then left under nitrogen flow during at least 2 h before it was heated to 75 °C on a hot plate under a nitrogen atmosphere to conduct the polymerization while removing the residual NMP. The resulting AEMs were immersed in water and gently peeled from the dish. Subsequently, the membranes were ion-exchanged to the Br− form at 60 °C for 2 days. Finally, the membranes were stored in deionized water for at least 48 h at room temperature prior to analysis. The average membrane thickness was approximately 60 μm. 2.7. Thermal Characterization. The thermal decomposition of the polyelectrolytes and AEMs was studied by thermogravimetric analysis using a TA Instruments TGA Q500. The samples were first dried at room temperature under vacuum for at least 24 h. Prior to analysis, the samples were preheated at 120 °C for 60 min in the TGA to remove traces of water. The measurement was performed under a nitrogen atmosphere from 50 to 600 °C at a heating rate of 10 °C min−1. The decomposition temperature (Td,95) was determined at 5% weight loss. 2.8. Determination of Ion Exchange Capacity and Water Uptake. The ion exchange capacity (IEC) of the cross-linked AEMs in the Br− form was determined by Mohr titrations. Samples were dried at 50 °C under vacuum for at least 24 h before being weighted and immersed in 0.2 M aqueous NaNO3 (25.00 mL) for 48 h. The resulting solutions were titrated with 0.01 M aqueous AgNO3 using K2CrO4 as colorimetric indicator. In order to measure the water uptake, AEMs in the Br− form were first dried under vacuum at 50 °C for 24 h and then weighted to obtain the dry weight (WBr−). Next, the membranes were immersed in 1 M NaOH under a nitrogen atmosphere during 48 h to ensure complete ion-exchange and the membranes were repeatedly washed with degassed deionized water. The absence of excess OH− ions was confirmed by a constant neutral pH of the washing solution. Subsequently, the samples were equilibrated in degassed deionized water overnight at 20, 40, 60, and 80 °C before obtaining the weight of the swollen membranes (W′OH−). The dry weight of the membranes in OH− form (WOH−) was calculated based on the titrated IEC values and WBr−. The water uptake (WU) was finally calculated as
c
(2)
where ts is the efflux time for the polymer solution with concentration c and tb is the efflux time for a blank sample. The intrinsic viscosity, [η], was estimated by extrapolating ηred and ηinh to c = 0 and calculating the average intersection with the y-axis. The molecular weight (M) was calculated using the Mark−Houwink equation [η] = KM a
WU =
′ − − WOH− WOH × 100% WOH−
(4)
The hydration number (λ), defined as the number of water molecules per QA group, was calculated as
(3)
with constants K = 4.7 × 10−3 mL g−1 and a = 0.83, obtained for PDADMAC solutions in 1 M aqueous NaCl solutions at 30 °C.23 2.5. Preparation of PPO Functionalized with Diallylmethylbenzylammonium Chloride Groups. PPO was functionalized with pendant diallylmethylbenzylammonium groups in order to prepare water nonsoluble AEMs based on the DAPipCl monomer. In the first step, a PPO sample with a degree of bromination of 15% (i.e., with 15 bromobenzyl groups per 100 repeating units of the PPO) was prepared using AIBN and NBS, as described previously.21 Next, the bromobenzyl-functional PPO was reacted with methyldiallylamine under homogeneous conditions. A 200% molar excess of the amine was added to a 5 wt % solution of the polymer in NMP. The reaction solution was stirred in a sealed vessel during 4 days at 40 °C before precipitating the product in diethyl ether. After several washes in diethyl ether, the polymer was dried under vacuum at ambient temperature. The complete displacement of the bromine atoms was confirmed by 1H NMR spectroscopy. 2.6. Membrane Preparation. In order to demonstrate the potential to prepare water nonsoluble AEMs based on the Nspirocyclic polyelectrolytes, DAPipCl was copolymerized together with the diallylmethylbenzylammonium-functionalized PPO in a reactive membrane casting process. The cationic PPO (0.0370 g) was dissolved in a 1:1 (v:v) mixture of methanol and NMP, together with a calculated amount (0.0234 g) of DAPipCl. The homogeneous solution was poured into a Petri dish and heated to 60 °C. After 2 h, when the methanol had evaporated, the solution was cooled to room temperature and an amount of AIBN
λ=
′ − − WOH−) 1000 × (WOH IEC × WOH− × M H2O
(5)
2.9. Conductivity Measurements. The hydroxide ion conductivity of fully hydrated AEMs immersed in deionized and degassed water was measured in a sealed cell between −20 and 90 °C by employing a Novocontrol high resolution dielectric analyzer V 1.01S at 50 mV and 10−1−107 Hz. The membranes in Br− form were first ion exchanged to hydroxide form and washed to remove excess hydroxide ions as described above. The membranes were kept in degassed water under nitrogen prior to the measurements. 2.10. Small-Angle X-ray Scattering. Small-angle X-ray scattering (SAXS) of the polyelectrolytes in the Cl− form was measured after equilibration at 33% relative humidity (controlled by a saturated MgCl solution) at 25 °C, employing a SAXSLAB ApS system (JJ-Xray, Denmark) combined with a Pilatus detector. The scattering vector (q) was calculated as
q=
4π l sin 2θ
(6)
where 2θ is the scattering angle and l is the wavelength of the Cu Kα radiation (1.542 Å). The characteristic separation length (d) was then calculated using Bragg’s law: d= C
2π q
(7) DOI: 10.1021/acs.macromol.7b00168 Macromolecules XXXX, XXX, XXX−XXX
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Table 1. Properties of the Polyelectrolytes
2.11. Evaluation of Alkaline Stability. The chemical stability of the polyelectrolytes under alkaline conditions was determined using 1 H NMR spectroscopy. Solutions of the polyelectrolytes (7 wt %) in 2 M KOD/D2O were contained in sealed vessels and stored in a temperature-controlled oven. The evaluation at 90 °C were conducted with polypropylene vessels, while those at 120 °C required pressuresustainable borosilicate glass containers with Teflon inserts (glass tubes from Ace Glass, Inc., and Teflon inserts custom-made by Prototypverkstaden, Lund). For each polyelectrolyte, samples were extracted after 1, 3, 7, and 14 days of storage. The samples were first diluted to 1 M KOD by addition of D2O in order to limit shimming errors and to improve resolution, before analysis by 1H NMR spectroscopy as described above. In addition, the alkaline stability of PDAPipCl in KOD/CD3OD/D2O was investigated at 90 °C using the custom-made container described above. A 2 M KOD/CD3OD/D2O solution containing 7 wt % of PDAPipCl was prepared by diluting 1 g of 40 wt % KOD in D2O to 3.5 mL using CD3OD. The structural degradation of the sample was analyzed by 1H NMR spectroscopy after dilution to 1 M KOD by addition of CD3OD.
polyelectrolyte PDAPyrCl PDAPipCl PDAMorCl PDAAzeCl PDADMAC
IECa [mequiv g−1] 5.91 5.46 5.40 5.07 6.98
(5.33) (4.96) (4.91) (4.63) (6.19)
Td,95b [°C] 336 318 218 263 263
[η]c [dL g−1] 0.43 0.22 0.44 0.37 0.16
(0.06) (0.07) (0.07) (0.07)
Md [kg mol−1] 59 26 61 49 17
(5.7) (6.4) (6.8) (6.3)
Theoretical IEC in the OH− (values within the parentheses for the Cl− form). bEvaluated by TGA under N2 at 10 °C/min. cIntrinsic viscosity measured in 1 M aqueous NaCl solution at 30 °C of polyelectrolytes synthesized in the Cl − form (values within parentheses for the polyelectrolytes prepared in Br− form). d Calculated from [η] using Mark−Houwink constants for PDADMAC solutions (ref 21). a
3. RESULTS AND DISCUSSION 3.1. Polyelectrolyte Synthesis and Characterization. Radical-mediated cyclo-polymerization of allylic QA salts was
Figure 2. TGA traces of the polyelectrolytes in the Cl− form recorded under N2.
the effect of the spirocyclic arrangement on the alkaline stability. Cyclo-polymerization of N,N-diallylazacycloalkane QA salts offers a straightforward pathway to N-spirocyclic QA polyelectrolytes using inexpensive starting materials and reagents. By varying the structure of the cycloaliphatic amine used in the monomer synthesis, both the ring size and the presence of additional heteroatoms can be varied to tune the properties. The N,N-diallylazacycloalkane QA monomers of the present study were prepared by two separate monoallylations of pyrrolidine, piperidine, azepane, and morpholine (Scheme 1). A one-step monomer synthesis is feasible; however, it results in a mixture of salts from which it is difficult to isolate the pure cyclic monomer. As expected, allyl bromide was more reactive than allyl chloride in the reactions. On the other hand, the monomers in the chloride form gave higher molecular weights in the subsequent polymerizations. Hence, both the bromide and chloride form of the monomers were prepared. 1H NMR spectra confirmed the successful monomer syntheses (parts a and b of Figures S4−S7), which proceeded in high yields between 70 and 83% in the first allylation and between 88 and 97% in the second on the scale of 10−16 g. The monomers were then employed in radical-initiated cyclo-polymerizations where the intramolecylar cyclization step produced different spirocyclic ring arrangements depending on the cycloaliphatic ring of the monomer (Scheme 1). The polymerizations were performed in water solutions of the monomers (60−70 wt %) using ammonium persulfate as initiator at 50 °C. During the
Figure 1. 1H NMR spectra of the five different polyelectrolytes in the Cl− form dissolved in D2O. The cis and trans isomerism is shown in part f.
first reported by Butler and co-workers24,25 and was initially proposed to yield six-membered rings along the polymer backbone.26 However, careful analysis later revealed that fivemembered rings were formed.27 The polymerization mechanism consists of initiation, intramolecular cyclization, and linear propagation, yielding the cyclic polymer structure with a cis:trans ratio of 6:1.28 Today, poly(diallyldimethylammonium chloride) (PDADMAC) is produced industrially and used mainly as a flocculating agent in paper manufacturing and water treatment.28 In the present work a commercial PDADMAC sample was used as a benchmark material, primarily to assess D
DOI: 10.1021/acs.macromol.7b00168 Macromolecules XXXX, XXX, XXX−XXX
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latter ratio also applied to the commercial PDADMAC sample used in the present study. The molecular weight of the polyelectrolytes was found to strongly depend on the counterion of the monomer during polymerization. Just as reported previously for both cyclic and noncyclic monomers,29 the polymerization of the present cyclic monomers in the chloride salt form resulted in polyelectrolytes with a much higher viscosity than when the polymerization was carried out with the corresponding bromide salt under the same conditions. The intrinsic viscosity data collected in Table 1 show that the values for the samples prepared in the bromide form were below 0.1 dL g−1, while the intrinsic viscosity ranged from 0.21 to 0.46 dL g−1 for the polyelectrolytes prepared from the corresponding monomers in the Cl− form. Employing Mark−Houwink constants evaluated for PDADMAC solutions,21 the molecular weights of the present N-spirocyclic polyelectrolytes were estimated to between 26 and 61 kg mol−1 when prepared in the Cl− form, while those prepared in the Br− form were found to be much lower, around 6−7 kg mol−1 (Table 1). The 1H NMR spectra of the polyelectrolytes prepared from monomers in the bromide salt form were found to contain small additional signals, most probably originating from structural defects introduced during the polymerizations. These polyelectrolytes were also less water-soluble than those prepared in the chloride salt form. Because of these findings, the remaining work was focused on the polyelectrolytes prepared from monomers with chloride counterions. In order to study structure formation of the present polyelectrolytes in the solid state, SAXS measurements were performed after equilibration at 33% relative humidity. However, the resulting SAXS profiles showed no detectable scattering peaks in the q-range 0−8 nm−1, indicating the absence of any characteristic scattering distances in the range 0.8−10 nm (Figure S8). We have previously observed clear scattering peaks in this range after analyzing highly (hyper)sulfonated polysulfones with IEC values in the range 4.5−8 mequiv g−1.30 The absence of structure formation in the present case may be explained by the aliphatic nature of the spirocyclic arrangement of the cation which makes it more compatible with the aliphatic backbone. 3.2. Thermal Stability. The thermal decomposition of the polyelectrolytes was studied by TGA analysis under N2 and the resulting traces are shown in Figure 2. Decomposition seemed to occur in two steps, except in the case of PDAMorCl, which displayed three distinct steps. The value of Td,95 of PDAPipCl was 100 °C above that of PDAMorCl (Table 1). This large difference was probably due to the inductive effect of the heteroatom present in the morpholinium which destabilized the ring. The results further showed that the thermal stability decreased with increasing ring size, indicating that Td,95 was affected not only by ring strain but also by other factors. The thermal stability of the N-spirocyclic polyelectrolytes, excluding PDAMorCl, were either similar or substantially higher than that of the benchmark PDADMAC. 3.3. Alkaline Stability. The present N-spirocyclic polyelectrolytes may degrade by a number of different ring-opening reactions via substitution and Hofmann elimination mechanisms, as seen in Scheme 2. In order to assess the stability and investigate the degradation pathways under alkaline conditions, structural changes of the polyelectrolytes were monitored by 1 H NMR spectroscopy after being kept in KOD/D2O solutions at elevated temperatures. The spectra were all recorded in 1 M KOD/D2O to decrease shimming issues and to limit the signal
Scheme 2. Alkaline Degradation Pathways of PDAPip by Ring-Opening Reactions via Substitution (a, b) and Hofmann Elimination (c, d) and of PDADMAC by RingOpening Substitution (e) and α-Methyl Substitution (f)
polymerizations, the viscosity of the reaction solutions increased significantly to indicate the formation of highmolecular-weight polyelectrolytes. Subsequently, the products were purified by repeated precipitation in acetone from water solutions, followed by washing with fresh acetone. The total yields in the polymerizations were between 50 and 75%. The structure of the polyelectrolytes was confirmed by both 1 H NMR and 13C NMR spectroscopy. As expected, several characteristic signals were found in all 1H NMR spectra because of the structural similarities of the polyelectrolytes (Figure 1). These include signals a and b, at 0.9−1.6 and 2.1−2.6 ppm, respectively, originating from the aliphatic polymer backbone, as well as c and d at 3−4 ppm, arising from the protons in αpositions to the charged centers. The substantial splitting of signals a and c was caused by the fusion of the polymer backbone ethylene groups to the characteristic pyrrolidinium rings. This also produced cis−trans isomerism (Figure 1f), which gave rise to the dual b signals. Signals from additional −CH2− units in the rings were observed between 1.5 and 2.2 ppm, with the exception of those neighboring the ether link in morpholine, which were shifted to 4 ppm. Notably, the 1H NMR spectra indicated no traces of any allylic protons which would have appeared between 5.5 and 6 ppm (part c of Figures S1−S4). The data were in agreement with earlier findings, also confirming that the polymers contain configurational isomers of pyrrolidinium rings in the repeating units.27 By comparing the intensities of the signals at 2.5 and 2.1 ppm, the cis:trans ratios were determined to be between 4:1 and 5:1. This was quite close to the earlier reported ratio of 6:1 for PDADMAC. The E
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Figure 3. 1H NMR spectra of the polyelectrolytes after different storage times in 2 M KOD/D2O solutions at 120 °C: (a) PDAPyrCl, (b) PDAPipCl, (c) PDADMAC, (d) PDAAzeCl, and (e) PDAMorCl. The spectra were recorded in 1 M KOD/D2O solutions at room temperature and are normalized with respect to the downfield b signal. The region above the water signal has been excluded in part a, b, and c because of the absence of any new signals in this region.
degradation pathways of PDAMorCl and PDAAzeCl, the temperature of the alkaline solutions was increased to 120 °C. The resulting 1H NMR spectra are shown in Figure 3. To clearly illustrate the degree of degradation, the spectra were overlaid and normalized with respect to the downfield b signal. As seen in Figure 3, the 1H NMR spectra of PDAPyrCl, PDAPipCl, and PDADMAC showed only very small changes after 14 days of storage, indicating a remarkable alkaline stability under these harsh conditions. The spectra of all these three polyelectrolytes exhibited minor decreases in the intensity of signals c and d, arising from the α-protons. Taking into account the lack of signals corresponding to Hofmann elimination products (which would have appeared in the region downfield to the water peak), this indicated a loss of ionic groups exclusively via substitution reactions (Scheme 2). In the spectra of PDAPyrCl and PDAPipCl, shown in Figures 3a and 3b, respectively, the presence of open-chain products from ring-opening substitution reactions was indicated by the increase of the signal intensity at 1−1.5 ppm. In contrast, this region remained rather unaffected in the spectrum of PDADMAC (Figure 3c), which implied that no, or only very
shift in comparison with corresponding spectra recorded in neutral D2O solutions. Initially the alkaline stability of the polyelectrolytes was evaluated in 2 M KOD/D2O at 90 °C. Under these conditions the 1H NMR spectra of PDAPyrCl, PDAPipCl, and PDADMAC indicated no detectable changes after 14 days storage (Figure S7). In contrast, PDAMorCl and PDAAzeCl showed clear signs of degradation after the same time. The spectrum of PDAMorCl indicated a clear reduction of the signal from the protons on the α-carbons in the morpholinium ring at 3.4 ppm as well as the appearance of a number of new small shifts at 3.5, 3.7, and 6.3 ppm. In the case of PDAAzeCl, new signals were observed at 3.5 and 5.7 ppm. The results indicated that the 7-membered ring, with less conformational restrictions, had a lower alkaline stability than the spirocyclic systems with pendant 5- and 6-membered rings. Moreover, the inductive effect of the oxygen atom seemingly destabilized the morpholine-based cation. These findings essentially follow the TGA results which showed low values of Td,95 for both PDAAzeCl and PDAMorCl (Table 1). In order to further evaluate the alkaline stability of PDAPyrCl, PDAPipCl, and PDADMAC and to clarify the F
DOI: 10.1021/acs.macromol.7b00168 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Scheme 3. Preparation of AEMs Based on Cross-Linked NSpirocyclic QA Polyelectrolytes via (i) Benzylic Bromination of PPO, (ii) Quaternization Using Diallylmethylamine, and (iii) Cyclo-Polymerization of DAPipCl and Diallylmethyl QA Tethered to PPOa
methyl substitution reactions (Schemes 2e and 2f, respectively). The former reaction would result in primary −OH groups in the polymer structure. Unfortunately, emerging signals from −CH2OH groups would be expected to overlap the signals at 3 ppm. Similarly, signals arising from corresponding groups in PDAPyrCl and PDAPipCl (Scheme 2a,b) would also overlap with already existing signals. Still, by expanding the spectrum of PDAPyrCl (Figure S9), a small overlapping signal was seen to emerge at 3.4 ppm. This corresponded very well with the formation of a 1-butanol moiety (Scheme 2b) and thus indicated degradation via ring-opening substitution of the pendant ring. Potential signals originating from −CH2OH groups formed by ring-opening substitution reactions on backbone pyrrolidinium rings would be very difficult to observe in the spectra of PDAPyrCl, PDAPipCl, and PDADMAC because of severe signal overlap (Figures S9−S11). An estimate of the level of degradation after 14 days in 2 M KOD/D2O at 120 °C was made based on the spectra. The decrease of signals c and d was estimated in relation to the intensity of the downfield b signal for PDADMAC and PDAPipCl. In the case of PDAPyrCl, signals c and d were instead related to the large signal at 2.03 ppm because of overlapping signals around 2.3 ppm. The decay of signal c was estimated to ∼10% for PDAPipCl and to ∼20% in the cases of PDADMAC and PDAPyrCl. For signal d the decrease was ∼3% for PDAPyrCl and PDAPipCl and ∼28% for PDADMAC. This indicated that PDAPipCl possessed the highest alkaline stability of the five polyelectrolytes. As expected, both PDAAzeCl and PDAMorCl degraded quite fast in 2 M KOD/D2O at 120 °C. The solutions darkened after a few days, and precipitates were observed in the solution of PDAAzeCl within 72 h. The 1H NMR spectra of PDAAzeCl and PDAMorCl recorded after storage at 120 °C, shown in Figures 3d and 3e, respectively, exhibited many similarities. This included an emerging signal above 5 ppm, which was consistent with the formation of unsaturated bonds to indicate degradation via β-elimination (Scheme 2c,d). The new signals around 3.5 ppm were ascribed to −CH2OH protons and hinted degradation via ring-opening substitution. Loss of ionic groups via ring-opening reactions was further verified by the increase of signals in the low-ppm region. The degradation of the polyelectrolytes presumably occurred mainly through reactions on the pendant rings because of the significant differences in the alkaline stability of the five investigated polyelectrolytes. Absolute quantification of the NMR signals was difficult due to the many overlapping signals. Still, it was possible to reasonably well identify the primary degradation pathways by comparing the intensity of the assigned degradation signals (Figures S12 and S13). Thus, PDAAzeCl was found to degrade primarily via Hofmann elimination (Scheme 2c), while substitution (Scheme 2b) and elimination (Scheme 2c) reactions seemed to occur at a similar rate in the case of PDAMorCl. All in all, PDAMorCl appeared to degrade faster than PDAAzeCl, while the amount of Hofmann elimination products seemed to be very similar for the two polyelectrolytes after 24 h. Our findings are much in line with previously reported results of low-molecular-weight model compounds. Studies of the degradation of 5-azoniaspiro[4.5]decane have shown that cleavage of the five-membered ring via substitution is favored over the six-membered one.31 Lillocci et al. have shown that the degradation rate of heterocyclic QAs is related to the ring strain and is thus significantly higher for the 5- and 7-membered rings
Key:(i) AIBN, NBS, 1,2-dichlorobenzene, 110 °C; (ii) NMP, 40 °C, 4 days; (iii) AIBN, NMP/methanol, 75 °C, NaOH (ion-exchange). The photographs illustrate the yellow-transparent and flexible character of membrane PPOPip1.7.
a
limited, degradation of the 5-membered ring occurred. In the same spectrum, the signal emerging at 2 ppm corresponded well with that of a methyl group of a tertiary amine. Notably, tertiary amines would be present after both ring-opening and G
DOI: 10.1021/acs.macromol.7b00168 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 2. AEM Properties AEM
IECtitrationa [mequiv g−1]
IECtheoreticala [mequiv g−1]
Td,95b [°C]
water uptakec [wt %]
λOH−d
σOH−e [mS cm−1]
PPOPip1.7 PPOPip2.3
1.72 (1.58) 2.34 (2.03)
2.0 (1.75) 2.46 (2.13)
285 258
51 101
18 25
11 54
IEC in the OH− form (values within parentheses for the Br− form). bMeasured by TGA under N2. cMeasured at 20 °C in OH− form. dMolar ratio H2O:OH−. eOH− conductivity measured at 20 °C with AEMs fully immersed in water.
a
∼4:1, v:v) at 90 °C. The 1H NMR spectra before and after storage during 48 and 168 h are shown in Figure S14. Signals emerging above 5 ppm reveal Hofmann β-hydrogen ringopening elimination, according to Scheme 2c,d. Most likely, ring-opening elimintation occurred in both five- and sixmembered rings, although the degradation of the latter rings seemed to be favored. In addition, ring-opening substitution reactions most probably also took place, but this was difficut to verify because of signal overlap. In conclusion, the results show that the conditions in 2 M KOD/CD3OD/D2O solution at 90 °C were more severe than in the 2 M KOD/D2O solution at 120 °C, where no Hofmann elimination reactions were detected. The findings raise questions concerning the relevance of alkaline testing in the presence of different organic solvents, which may lead to alternative degradation pathways and products.36 3.4. Preparation and Properties of Anion Exchange Membranes. All the N-spirocyclic polyelectrolytes were watersoluble because of their high ionic contents, indicated by their high IEC values (Table 1). Consequently, these materials need to be immobilized in order to be used as OH− conducting AEMs. In order to demonstrate the potential to prepare robust AEMs based on the polyelectrolytes, we developed a reactive casting strategy that utilized the ability of the diallyl QA monomers to copolymerize with diallylmethyl QA groups tethered to a polymer backbone, in this case poly(phenylene oxide) (PPO), to form cross-linked AEMs (Scheme 3). Hence, we first benzyl-brominated PPO to contain an average of 15 −CH2Br groups per 100 repeating unit of the PPO. The bromine atoms were then quantitatively substituted in Menshutkin reactions with diallylmethylamine to functionalize the polymer with the diallylmethylbenzyl QA groups (Figure S15). The modified PPO was then dissolved together with DAPipCl monomer in a 1:1 (v:v) mixture of methanol and NMP. After degassing and removal of most of the methanol, AIBN initiator was added and the polymerization was performed under a nitrogen flow at 75 °C. The cross-linked AEMs were flexible, transparent, and yellow, as seen in Scheme 3. The IEC of the AEMs was controlled by varying the concentration of DAPipCl in the polymerization mixture and two AEMs with IECs of 1.7 and 2.3 mequiv g−1 (designated PPOPip1.7 and PPOPip2.3, respectively) were prepared. The difference between the titrated and theoretical IEC values, seen in Table 2, was most probably caused by limited monomer conversions in these initial nonoptimized polymerizations. The value of Td,95 decreased with the concentration of PPO and was 285 and 258 °C for PPOPip1.7 and PPOPip2.3, respectively. The water content of AEMs greatly promotes ionic dissociation and the formation of percolating hydrated phase domains for efficient ion transport.2,3 As expected, the water uptake increased with the IEC value and the temperature, as seen in Figure 4a. The high IEC values of these nonoptimized AEMs resulted in a high water uptake, 51 and 101 wt %, respectively, at 20 °C. The OH− conductivity was measured by
Figure 4. Water uptake (a) and OH− conductivity (b) of the AEMs based on the cross-linked N-spirocyclic PDAPipCl under fully hydrated (immersed) conditions.
than for the 6-membered one.32−34 The same group also reported that azepane-based QAs degrade primarily via ringopening β-elimination, while the pyrrolidinium QAs were predominantly cleaved by nucleophilic ring-opening substitution.33 The low stability of PDAMorCl compared to PDAPipCl can be attributed the inductive effect of the oxygen atom in the ring of the former polyelectrolyte.35 In some previous studies the alkaline stability was assessed in KOD/CD3OD/D2O solutions instead of KOD/D2O solutions, as in the present case.11,12 In the former cases, CD3OD was added mainly to accelerate the degradation rates and to ensure complete solubility of the cationic polymers or model compounds (and their degradation products) throughout the experiments. For comparison, we evaluated the alkaline stability of PDAPipCl in 2 M KOD/CD3OD/D2O (CD3OD:D2O, H
DOI: 10.1021/acs.macromol.7b00168 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Notes
electrochemical impedance spectroscopy with the AEMs fully hydrated (immersed) in a sealed two-probe cell. The conductivity data of PPOPip1.7 and PPOPip2.3 collected during heating from 20 to 80 °C reached very high values, 11 and 54 mS cm−1, respectively, at 20 °C and 31 and 101 mS cm−1, respectively, at 80 °C (Figure 4b). The high conductivity of the latter AEM was a consequence of the high IEC value and water content. The conductivities may be compared with those measured for polysulfones with bis-N-spirocyclic QA moieties.17 Having an IEC value of 1.7 mequiv g−1, the OH− conductivity of AEMs based on these polymers exceeded 100 mS cm−1 at 80 °C. The present results should be regarded as preliminary, and the AEMs need to be further tuned and optimized for high performance. For example, the QA cations were tethered to PPO via benzylic sites, which are known to have a limited alkaline stability.16 Hence, our future work on these materials will focus on the attachment of the QA cations via flexible alkyl side chains to provide a substantially higher stability.37
The authors declare no competing financial interest.
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REFERENCES
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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/acs.macromol.7b00168. Additional 1H NMR spectra, DSC and TGA traces, and SAXS profiles (PDF)
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ACKNOWLEDGMENTS
We thank the Swedish Energy Agency, the Swedish Research Council, and the Swedish Research Council for Sustainable Development for financial support. We are also grateful to Peter Holmqvist for assistance with SAXS measurements and data treatment.
4. CONCLUSIONS A series of aliphatic polyelectrolytes based on different Nspirocyclic QA cations were successfully synthesized by radicalinitiated cyclo-polymerizations of N,N-diallylazacycloalkane quaternary salts. These monomers were conveniently synthesized in high yields by diallylation of cycloaliphatic secondary amines. The alkaline stability of the polyelectrolytes was in general very high but was observed to decrease with ring strain and the presence of an additional heteroatom in the ring. Hence, poly(N,N-diallylmorpholinium) and poly(N,N-diallylazepanium) degraded already at 90 °C via Hofmann elimination and ring-opening substitution. On the other hand, poly(N,N-diallylpiperidinium) showed a high alkaline stability, with only minor degradation at 120 °C. In general, larger rings tended to degrade via Hofmann elimination, while smaller rings only degraded via ring-opening substitution reactions. A novel strategy to prepare AEMs was developed whereby N,Ndiallylazacycloalkane quaternary salts are copolymerized with methylbenzyldiallylammonium groups attached to poly(phenylene oxide). The cross-linked and water nonsoluble AEMs functionalized with N-spirocyclic QA cations reached high OH− conductivities. Our study demonstrated the high thermal and alkaline stability of suitably configured Nspirocyclic QA cations and will open up new opportunities to prepare high performance polyelectrolytes and membranes.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (P.J.). ORCID
Thanh Huong Pham: 0000-0002-2063-8461 Patric Jannasch: 0000-0002-1102-3959 I
DOI: 10.1021/acs.macromol.7b00168 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
Conductivity of Anion-Exchange Membranes. Macromolecules 2015, 48 (16), 5742−5751.
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DOI: 10.1021/acs.macromol.7b00168 Macromolecules XXXX, XXX, XXX−XXX
