Water dynamics in ionomer membranes by field-cycling NMR relaxometry

Magnetic Resonance Imaging 25 (2007) 501 – 504

Water dynamics in ionomer membranes by field-cycling NMR relaxometry Jean-Christophe Perrina, Sandrine Lyonnarda, Armel Guillermoa, Pierre Levitzb,4 a

Structures et Proprie´te´s d’Architectures Mole´culaires, UMR 5819 (CEA-CNRS-UJF), DRFMC/SPrAM, CEA-Grenoble, 38054 Grenoble Cedex 9, France b Physique de la Matie`re Condense´e, Ecole Polytechnique-CNR, 91128 Palaiseau Cedex, France

Abstract The dynamic behavior of water within two types of ionomer membranes, Nafion and sulfonated polyimides, has been investigated by field-cycling nuclear magnetic relaxation. This technique, applied to materials prepared at different hydration levels, allows to probe the proton motion on a time scale of the microsecond. The NMR longitudinal relaxation rate R 1 measured over three decades of Larmor angular frequencies x is particularly sensitive to the host–water interactions and thus well suited to study fluid in restricted geometries. In pdynamics ffiffiffiffi the polyimide membranes, we have observed a strong dispersion of R 1(x) following closely a 1= x law in a low-frequency range

(correlation times from 0.1 to 10 As). This is indicative of a strong interaction of water with binterfacialQ hydrophilic groups of the polymeric matrix (wetting situation). On the contrary, in the Nafion, we observed weak variations of R 1(x) at low frequency. This is typical of a nonwetting behavior. At early hydration stages, the proton–proton inter-dipolar contribution to R 1(x) evolves logarithmically, suggesting a confined bidimensional diffusion of protons in the microsecond time range. Such an evolution is lost at higher swelling where a plateau related to 3D diffusion is observed. D 2007 Elsevier Inc. All rights reserved. Keywords: Ionomer membranes; Nafion; Sulfonated polyimides; Fast field-cycling relaxometry; NMRD; Water dynamics; Confined motions; Water diffusion; Porous media

1. Introduction Nowadays, ionomer membranes are extensively studied, notably for polymer electrolyte fuel cell (PEFC) applications. These membranes have the property to be good ionic conductors when hydrated. The hydration state of the membranes is characterized by the number of water molecules per ionic group k. A considerable effort has been made in the last years to understand the proton conduction processes as a function of k and thus the water’s role in enhancing conductivity, with the clear objective to improve the performances in view of further industrial application. Presently, the perfluorinated ionomer membranes stand for the best materials for PEFC applications: among them, the so-called Nafion is the most popular (Fig. 1A). Its hydrophobic polytetrafluoro-

4 Corresponding author. E-mail address: [email protected] (P. Levitz). 0730-725X/$ – see front matter D 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.mri.2007.01.002

ethylene backbone carries fluorether side chains terminated by hydrophilic sulfonic acid groups. Although the Nafion exhibits remarkable performances and notably an unchallenged protonic conductivity under operating conditions in fuel cell power-supplied prototypes, it has numerous drawbacks (high cost, low glass transition temperature T g) that strongly encourage the development of alternative and competitive materials. Polyaromatic compounds, such as sulfonated polyimides (Fig. 1B), are one of these new promising systems with lower cost synthesis and highly tunable properties through a wide choice of chemical units [1]. In both cases, ionomer membranes exhibit a nanoscale separation between hydrophilic and hydrophobic domains [2] creating a multi-connected bporeQ network available for water sorption. A strong difference concerns the structural evolution with the relative humidity (RH): Nafion membranes exhibit a strong swelling, the sulfonated polyimides do not. Moreover, at low RH, the macroscopic conductivity of Nafion is much higher than that of sulfonated polyimides. In this paper, we report the results obtained by NMR relaxometry (NMRD) on Nafion and polyimide

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Fig. 1. (A) Nafion 112 (X = 7, IEC = 0.91 meq/g) and (B) sulfonated polyimide 5/5 (X = 5, Y = 5; IEC = 1.98 meq/g).

membranes equilibrated at various relative humidities (from almost dry to saturated).

2. Experimental section

strips are soaked in EDTA 0.015 mol/L at room temperature for 1 day and rinsed in distilled–deionized water at 808C for 2 h. This step was repeated two times. The efficiency of the protocol has been checked by electronic paramagnetic resonance. The sulfonated polyimide membranes were provided by LMOPS Laboratory (Vernaison, France). Strips of sulfonated polyimide were soaked in 500 ml of 0.5 mol/L sulfuric acid solution at room temperature for 4 h. They were then soaked in distilled–deionized water at 508C for 4 h and rinsed with fresh distilled water. Nafion and sulfonated polyimide strips were rolled inside 10-mm-diameter NMR tubes (typical weight of one sample in the dry state is 500 mg). A series of samples at different water content have been prepared by controlling the relative humidity (RH) inside the NMR tube. For doing so, a small glass tube containing well-chosen saturated salt solutions has been placed above the membrane in the 10-mm-diameter tube. The nature of the salt fixes the vapor pressure P/P 0 at room temperature and thus imposes a known RH that can be translated into k through the sorption isotherms. 2.3. NMR relaxometry experiments 1

2.1. Materials Fifth-micrometer-thick membranes have been used in this work. Nafion membranes with an ionic exchange capacity (IEC) of 0.9 meq/g (i.e., 0.9 mmol of SO3H per gram of polymer) and a density of about 2.1 g/cm3 have been chosen (Nafion 112, see Fig. 1A). In the case of polyimides, the polymers are composed of a hydrophilic sulfonated block based on naphthalenic structure and a hydrophobic block obtained with an oxydianiline monomer (see Fig. 1B). The repetition rate of the blocks is 5/5 (X = 5 and Y =5), corresponding to an IEC of 1.98 meq/g for a polymer density close to 1.4 g/cm3. If n s is the number of sulfonated groups and n p the total number of polar groups (including, for instance, carbonyl groups that are present in the polyimides), the ratio defined as f s = n s/n p is a chemical characteristic of each polymer (in the case of a fully perfluorinated polymer such as Nafion 112, f s =1). The ratio k =N/n s, with N being the total number of water molecules in the material characterizing the hydration state.

H NMRD and 2H NMRD experiments related to water molecules inside the membranes have been performed at 298 K with a Stelar Spinmaster-FFC2000 relaxometer. 3. Results and discussion 3.1. Sulfonated polyimides At all relative humidities, a strong R 1(x) dispersion is observed, following a x a trend, with a close to 0.5. As shown elsewhere [4], this is the signature of a noticeable interaction between the entrapped fluid and the polymeric interface, typical of a good wetting. Similar results were also observed for deuterated water, strongly suggesting that the

2.2. Sample preparation The Nafion 112 membranes were purchased from Dupont Company. Strips of Nafion 0.7 cm wide were cut from the raw sheet. To ensure a complete acidification, they were soaked in 800 ml of 2 mol/L hydrochloric acid solution at 808C for 2 h and rinsed in distilled–deionized water at 808C for 2 h. They were then soaked in 800 ml of 1 mol/L nitric acid solution at 808C for 2 h and rinsed in distilled–deionized water at 808C for 2 h. Paramagnetic impurities introduced during the manufacture of Nafion have been removed by applying a cleaning protocol close to the one described by MacMillan et al. [3]. The purification was achieved by chelation with ethylenediaminetetraacetic acid (EDTA). The

Fig. 2. Reduced coordinate dispersion law kR 1(x) for the sulfonated polyimide 5/5 at room temperature and at different humidities, showing the existence of a master curve at RH N 52%.

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magnetic interfacial fluctuations are related to the intradipolar interaction for 1H and the quadrupolar interaction for 2 H. Except at low relative humidity (RH = 52% and k =3.05), it is possible to obtain a master curve kR 1(x), as shown in Fig. 2. In this highly hydrated regime, it is possible to propose a fast exchange process between adsorbed water on polar groups and bbulky liquidQ water (Eq. (1)). Introducing the constant number of adsorbed water molecules per polar group N H = N a/n p, experimental data can be expressed as: R1 ðxÞ ¼

 NH  a R ðxÞ  Rb1 ðxÞ þ Rb1 ðxÞ kfs 1

ð1Þ

where R 1a(x) is the relaxation rate of the adsorbed phase and R 1b(x) that of the bulky phase. Experiments show that the quantity kR 1(x) is independent of the hydration level at low frequency, which corresponds to the case R 1a(x) NR 1b(x) leading to: NH a R ðxÞ ð2Þ fs 1 pffiffiffiffi The 1= x dispersion law, observed for the proton, looks very similar to what has been observed in rigid porous media such as Vycor glass [5] or colloidal glass [6], where the nuclear relaxation is mainly driven by surface reorientation mediated by bulk diffusional translations. However, the fact that interfacial region is diffuse rather than sharp in the polyimides membranes [1,7] needs to be taken into account for a more complete analysis [8]. kR1 ðxÞ

3.2. The Nafion membrane The Nafion polymer being perfluorinated, in the Nafion– water system the 1H NMR signal arises only from water and the Field Induce Decay is mono-component. The 1H longitudinal relaxations are monoexponential and characterized by a single T1 at each frequency. Compared to the sulfonated polyimide profile, the Nafion exhibits (see Fig. 3) at all hydration levels less pronounced frequency dependence, which is the signature of nonwetting situation and weak surface interactions [4]. NMR relaxometry measurements allow to efficiently discriminate the water behavior in nonwetting membranes such as Nafion from that in wetting ones such as polyimides. In order to evaluate the relevance of any mode of relaxation, it is necessary to separate the interand intramolecular contributions to the dipolar relaxation. To do so, we first performed 2H NMRD experiments on the same Nafion membrane and at the same water content as for the 1H experiments. The spin-lattice relaxation of the deuterium nucleus arises from the fluctuation of the quadrupolar interaction between the spin and the electric field gradient along the O–D bond. This intense quadrupolar interaction being intramolecular, the relaxation rate R 1,Q(x) reduces to the orientational correlation function of the water molecule. Moreover, in the case of a spin equal to 1, the quadrupolar relaxation rate R 1,Q (x) obeys the same

Fig. 3. Different contributions to the proton spin-lattice relaxation inside the Nafion membrane at k = 3.4 in semilogarithmic coordinate. The intradipolar contribution is deduced from the 2H NMRD. The H-F hetero-dipolar contribution was obtained looking at highly diluted solution of H2O in D2O. The interdipolar 1H NMRD was obtained by application of Eq. (3).

equation as the one associated with the intramolecular contribution of the dipolar H-H relaxation. 2H NMRD and 1 H NMRD are strongly different. This suggests that 1H magnetic fluctuations involves both intra- and interdipolar interactions. Moreover, we prepared samples with deuterated water where a very small quantity of H2O was added. In such a case, 1H NMRD is essentially driven by the interdipolar coupling between proton in water proton and fluor nucleus located on the polymer backbone. From these different experiments, we extracted the inter-1H NMRD homodipolar contribution [9] according to 1  1  Rinter 1 ðH  HÞ ¼ R1 H  R1 H intra  Rinter 1 ðH  FÞ 1  R1 ð2 HÞ cR1 H  14:5

ð3Þ

 Rinter 1 ðH  FÞ The different contribution is shown in Fig. 3. At low hydration k =3.7, the R 1inter(H-H) x-dependence follows a logarithmic trend which is progressively replaced by a plateau as k increases. Logarithmic NMRD profiles are found in the case of dipolar relaxation by translational diffusion (interdipolar interactions) of a nonwetting liquid confined in a 2D porous media [10]. The existence of such a bidimensional geometry at very low water content is consistent with recent structural observations on the Nafion membrane. A dilution law close to / 1 (/ being the polymer volume fraction), typical of one-dimensional swelling of biaxial polymeric aggregates, has been reported in the k range of our study [11]. These elongated hydrophobic aggregates embedded in a continuous ionic medium are assembled into bundle [12] of typical size

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80 nm. Interesting enough, taking into account the water self-diffusion coefficient obtained by Quasi Elastic Neutron Scattering [13] at k =4, and the former bundle nominal size, we found that the corresponding correlation time of 2D selfdiffusion is about 10 As and the associated frequency of a few tenths of a kilohertz falls in the low-frequency range of our NMRD logarithmic profile. The drastic change in R 1(x) shape observed with a weak water uptake (k increasing from 3.7 to 5.6) can be understood also through the model which predicts that a weak increase of pore size induces rapid disappearance of the bidimensional behavior and the emergence of a plateau related to 3D diffusion. 4. Conclusion We have performed NMR relaxometry experiments on two kinds of ionomer membranes [9]: the Nafion and the sulfonated polyimides. Our results indicate that the technique is well adapted to the study of water dynamics in these hydrated systems in the range of a few nanometers and can provide valuable information to discriminate the molecular mechanisms associated with water motion as a function of the hydration state. In the case of polyimides, strong dispersions characteristic of good wetting have been observed. In the Nafion, the situation looks very different. The nonwetting behavior of the perfluorinated membrane has been evidenced at the scale of the microsecond through the observation of poorly dispersive relaxation rates. Moreover, a transition from 2D to 3D local diffusion when increasing the water content is consistent with the evolution of the intermolecular dipolar NMRD. The anisotropy of diffusion observed at low hydration level could be related to a local structural organization involving biaxial aggregates organized in lamellar domains at the nanometer scale. Interesting enough for Nafion, water molecules have a weak interaction with the polymer backbone but a high electric conductivity. This is perhaps a good compromise for ionomer membranes.

Acknowledgment We acknowledge O. Diat, G. Gebel, Y. Mare´chal and F. Volino for very helpful discussions about membrane properties and NMR relaxation.

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