Mechanical Response of Thermally Annealed Nafion Thin Films - ACS

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Mechanical Response of Thermally Annealed Nafion Thin Films Bradley R. Frieberg,*,† Kirt A. Page,†,§ Joshua R. Graybill,† Marlon L. Walker,‡ Christopher M. Stafford,† Gery R. Stafford,† and Christopher L. Soles† †

Materials Science and Engineering Division and ‡Materials Measurement Science Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States S Supporting Information *

ABSTRACT: Perfluorinated ionomers, in particular, Nafion, are a critical component in hydrogen fuel cells as the ion conducting binder within the catalyst layer in which it can be confined to thicknesses on the order of 10 nm or less. It is well reported that many physical properties, such as the Young’s modulus, are thickness dependent when the film thickness is less than 100 nm. Here we utilize a cantilever bending methodology to quantify the swelling-induced stresses and relevant mechanical properties of Nafion films as a function of film thickness exposed to cyclic humidity. We observe a factor of 5 increase in the Young’s modulus in films thinner than 50 nm and show how this increased stiffness translates to reduced swelling or hydration. The swelling stress was found to increase by a factor of 2 for films approximately 40 nm thick. We demonstrate that thermal annealing enhances the modulus at all film thicknesses and correlate these mechanical changes to chemical changes in the infrared absorption spectra. KEYWORDS: Nafion, thin film, mechanical properties, cantilever bending, thermal annealing, swelling stress

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decades.2,4−6 The internal morphology is known to be driven by the processing and environmental conditions, namely, the humidity and temperature. During normal operation of a hydrogen fuel cell, the PEM is exposed to a variety of humidity conditions. These changes in humidity can influence both the water content and the connectivity of the hydrophilic clusters, which control the proton conductivity. Additionally, the mechanical properties of Nafion are also known to be sensitive to both the humidity and the temperature. 7−11 The aforementioned variability in the structure and properties of Nafion may have influences on the long-term stability and performance of the membrane electrode assembly (MEA). There are three main modes of failure in a PEM: mechanical fatigue, chemical degradation, and electrical shorting.1 Mechanical fatigue is a result of the swelling stresses caused by humidity cycling during fuel cell operation. Chemical degradation occurs when the radicals created in the electrodes attack the ionomer

INTRODUCTION

One of the long-standing challenges to the commercialization of proton exchange membrane (PEM) fuel cells is their longterm durability and longevity.1 In a working fuel cell, the PEM is sandwiched between two catalyst layers, which are supported on either side by gas diffusion layers. The function of the PEM layer is to conduct protons between the electrodes, resist the passage of reactant gases, and insulate against electrical current. The most widely used PEM materials are perfluorosulfonic acid (PFSA) ionomers due to their high proton conductivity, chemical stability, mechanical strength, and resistance to permeation of uncharged gases.2 Dupont’s Nafion3 is the most widely studied PFSA ionomer, which consists of a hydrophobic polyfluoroethylene backbone with hydrophilic perfluorinated vinyl ether side chains, each terminated with a sulfonic acid. Due to the hydrophilic nature of the side chains and hydrophobic main chain, the sulfonic groups tend to aggregate, leading to a nanophase-separated morphology.2 The morphology and structure of Nafion and its relation to the PEM performance have been well studied over the past couple © XXXX American Chemical Society

Received: September 29, 2016 Accepted: November 14, 2016 Published: November 14, 2016 A

DOI: 10.1021/acsami.6b12423 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

hydration cycling.15 These preliminary studies quantified the humidity dependence of the Young’s modulus of a 200 nm thick film Nafion film and showed that it was different from a bulk membrane that is tens of micrometers thick. These measurements revealed that the Nafion films behave elastically when cycling between two relative humidities (RH) of 0% and 85% RH after the film was initially conditioned. Here we significantly extend upon these initial measurements and explore in detail a series of Nafion films ranging in thickness from 30 to 200 nm, critical length scales where thin film confinement effects become prominent. We quantify the swelling-induced stresses in these films for a range of humidity conditions between 0% and 90% RH. These measurements reveal that the reduced solubility and diffusivity of water in Nafion as a function of decreasing film thickness can be directly correlated with an increased stiffness or modulus in the thin films. These measurements also reveal an intriguing effect of thermal annealing that appears to be correlated with chemical changes in the Nafion films, potentially cross-linking the film, and increasing the resistance to hydration. The measurement technique described in this article is universally applicable to thin films and can be used to characterize other novel PEM materials as well as polymer thin films exposed to other external stimuli.

during fuel cell operation. Electrical shorting is a result of topographical irregularities in the layers, which are exploited by overcompression and creep of the PEM materials, causing the electrodes to come into contact. When fully hydrated, Nafion membranes are known to swell by as much as 20% by volume in all directions. Because the membrane is physically constrained within a fuel cell, swelling leads to a buildup in stress which is believed to facilitate cracking, tearing, or delamination of the PEM. Once the membrane is structurally compromised, the reactant gases can leak through, resulting in chemical shorting and failure of the fuel cell device. Stresses can build up within the membrane material in many different ways. As the humidity changes, the PEM absorbs and desorbs water causing the membrane to swell and shrink, respectively. A number of studies have detailed the effects of exposing bulk Nafion membranes to hygrothermal cycling.1,11−15 When constrained, the membrane can experience both compressive and tensile stresses that approach and even exceed the yield strength of the material.13 The large strain energy experienced by the MEA leads to plastic deformation and irreversible damage. In addition to fluctuations in humidity, temperature variations can also lead to crack formation. However, quantifying these effects can be a challenge. It is generally intractable to quantify stresses in operando for an actual fuel cell membrane. Likewise, the catalyst layer is a complicated composite (see below) that makes mechanistic studies of the failure mechanism difficult. It is paramount to develop measurement techniques that quantify both the source and the magnitude of stress generation in model systems that are of direct relevance to the operating conditions in both the membrane and the catalyst layers of the MEA. While there has been a large body of work moving toward understanding the response of bulk Nafion membranes to hygrothermal stresses, there is still much to be understood with regard to thin films. Within the catalyst layer, the PFSA is used as an ionically conductive binder to maintain the electrochemical interface between the platinum and the carbon nanoparticles.16 A suspension of these components is typically cast or sprayed onto the bulk membrane to create the sandwiched MEA structure. Within this polymer nanocomposite electrode, the PFSA can be confined into domains that are on the order of tens of nanometers in thickness.17 It has been demonstrated that a number of PFSA properties such as water transport,18−24 proton transport,24−30 and polymer orientation31−35 are strongly influenced by the film thickness and the substrate material at these length scales.36 This implies that these changes are influential to the overall performance of a working fuel cell. A number of experimental techniques have also demonstrated that thin film confinement impacts the mechanical properties of the material when the film thickness is on the order of tens of nanometers.15,21,37 Page et al. recently demonstrated a confinement-driven increase in the modulus of Nafion thin films using a wrinkling-based metrology under ambient lab conditions.37 Nadermann et al. demonstrated an increase in modulus with film thicknesses thinner than 1 μm using nanoindentation at ambient conditions.21 These initial measurements demonstrate that thin film confinement effects are also important in Nafion. However, it is well known that the modulus of bulk Nafion is strongly dependent on the both temperature and humidity,7−11 which have yet to be mapped out in Nafion thin films. Page et al. recently introduced a cantilever bending methodology capable of in situ measurement of the biaxial stress generated within a Nafion film during

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EXPERIMENTAL SECTION

Materials and Sample Preparation. Nafion stock solution (20% by mass in a mixture of lower aliphatic alcohols and water; contains 34% by mass water) and anhydrous ethanol were purchased from Aldrich. The as-received Nafion stock solution was diluted in ethanol at various concentrations (1:3 to 1:20 by volume), and the mixed solutions were sonicated for 30 min to ensure the solutions were thoroughly dispersed. The (100) silicon wafers ((120 ± 10) μm thickness, double side polished) were purchased from University Wafer Inc. Prior to spin coating, the wafers were rinsed with acetone and toluene and then dried with compressed dry nitrogen. The wafers were then placed in a UV ozone cleaner for 20 min. In order to ensure that the backside of the wafer stayed clean during spin coating, a thick silicon wafer (525 ± 25) μm thickness, single side polished, also purchased from University Wafer Inc. was first placed on the spin coater chuck. A drop of 2-propanol was placed onto the thick wafer, and capillary forces were used to adhere the thin wafer onto the surface of the thick wafer. The wafers were allowed to sit for 2 min to ensure sufficient adhesion. The Nafion suspension was then cast onto the thin wafer at a spinning rate of 314 rad/s (3000 rpm) for 45 s. These spin coating conditions and solution concentrations yielded dry polymer film thicknesses ranging from 30 to 175 nm, as measured by spectroscopic ellipsometry (M-2000, JA Woollam Inc.). Cantilever beams were cleaved from silicon wafers with dimensions of 5 mm × 50 mm. Samples were dried at ambient conditions for 24 h before annealing in an oven under vacuum for 2 h at 25, 60, or 140 °C. These annealing temperatures were chosen to be representative of an as-cast membrane, a membrane operating below the alpha transition of Nafion, and a membrane operative above the alpha transition of Nafion, respectively. The oven was then turned off and allowed to cool to room temperature while under vacuum. The samples were subsequently further dried by flowing dry air over the sample for 3 h immediately before testing and tested within 24 h of annealing. Cantilever Bending Measurements. The deflection of a cantilever beam was measured as a function of relative humidity in situ using an experimental setup outlined elsewhere.15,38 The measured deflection was then used to determine the change in biaxial stress within the Nafion films at each condition. The curvature of the cantilever substrate was monitored by reflecting a HeNe laser off the polymer-free side of the polished silicon substrate onto a position sensitive detector (PSD). The change in deflection of the laser beam is B

DOI: 10.1021/acsami.6b12423 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces related to the curvature of the cantilever through the geometry of the setup

d − d0 1 = R − R0 2LD

optical anisotropy or gradients in the refractive index; however, they did not significantly improve the fit to the data, and the trends with film thickness and annealing were largely unaffected. The humidity within the sample chamber was controlled by flowing dry air through a bubbler of deionized water, similar to the cantilever experiments. Infrared Spectroscopy. The effect of thermal annealing on the chemical structure of Nafion was characterized using a Nicolet 6700 Fourier Transform Infrared (FTIR) spectrometer (Thermo Scientific) equipped with an external polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) accessory. The accessory included a photoelastic modulator operating at a frequency of 50 kHz and a half-wave retardation set at 2500 cm−1, a sample holder set at an incident angle of 70°, and a liquid nitrogen cooled mercury− cadmium−telluride (MCT) detector. Spectra were averaged over 100 scans at a resolution of 4 cm−1. No baseline correction was necessary.

(1)

where R is the radius of curvature, d is the displacement of the laser on the PSD, D is the distance between the cantilever and the detector, and L is the distance between the clamp on the cantilever and the center of the laser spot. The subscript 0 refers to the values at the initial conditions which are defined as the conditions in the dry state after conditioning. A schematic of the setup, demonstrating the necessary variables, is shown in Figure 1.

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RESULTS AND DISCUSSION Prior to the cantilever bending experiments, the samples were first exposed to a three-step conditioning procedure previously reported.15 After the initial conditioning procedure, it was demonstrated that the response of the cantilever would be elastic and repeatable.15 The samples were first dried for 3 h by flowing dry air through the chamber. Subsequently, the chamber was equilibrated at 80% RH for 3 h to condition the film. After the film has been hydrated, the sample chamber is subsequently purged with dry air for 1 h. Note that this conditioning procedure is applied after the different thermal annealing procedures are performed as described in the Experimental Section. An example of the cantilever response during this conditioning procedure is shown in the Supporting Information, Figure S1. As this technique measures the change in deflection of a single-point laser, it does not measure the absolute curvature of the cantilever. Therefore, in all cases the measured stress−thickness is arbitrarily set to 0 N/m when the film is initially dry. The initial dry state is determined as the average of the last 30 min of the 1 h drying step. The time values on the abscissa are scaled so that the initial drying step is represented as negative values, and t = 0 s is manually set to the time where the humidity change is initialized. Of particular interest to the work presented here is the elastic response after conditioning. As indicated in the previous work, this elastic response can be cycled and the stress response is path independent with respect to humidity, as shown in Figure S4.15 In order to obtain the steady-state stress−thickness values at different humidity values, the humidity was cycled from 0% RH to different humidity values in steps of approximately 10% RH shown in Figure 2a. The humidity was held constant at each step for 30 min in order for the film to equilibrate. Figure 2a shows the typical humidity cycling for a cantilever sample, and Figure 2b is the corresponding cantilever response. The stress− thickness values reach steady state within 5 min of the cell reaching the desired humidity. The humidity cycling procedure is used in this study because background drift of the cantilever is observed with time, where the stress at 0% RH generally drifts away from the 0 N/m starting value. This drift is baseline corrected by reporting the stress values at a given humidity to the subsequent stress value for the 0% RH baseline. Further description of the background drift is described in the Supporting Information. A similar response has been observed in a cantilever bending experiment using 25 μm thick Nafion films constrained on top of polyether ether ketone substrate44 and in the previous study of Nafion films on gold.15 The steadystate value is taken as the difference between the stress in the humidified state and the following dry state. The stress−

Figure 1. Schematic of the cantilever bending setup defining the variables used in the calculations. (a) Cantilever in a dry, reference position. (b) Cantilever in a humid, curved position. Note, the dimensions are not shown to scale; D is more than 10 times larger than L in the experimental setup. As the thickness of the film (hf) is much less than the thickness of h the substrate (hs), hf ≪ 1, the product of the biaxial stress, σBi, and s

film thickness, hf, of a bimaterial spherically bent strip is defined using the classic thin film Stoney’s equation39 σBihf =

Eshs2 = ΔF 6(1 − vs)ΔR

(3)

where νs is the Poisson ratio of the substrate and the change in stress− thickness is denoted by ΔF. The mechanical properties for the bending of a thin narrow beam (L > 5w) cleaved from a (100) silicon wafer have previously been derived: Es = 130 GPa, vs = 0.28, and hs = 120 μm.40,41 The thickest film used in this study is 200 nm; therefore, hf ≈ 10−3 validating the use of the thin film approximation as it leads hi s

to less than 1% error in the calculation of the biaxial stress in the polymer film. Film Swelling Measurements. The swelling of the polymer films was monitored as a function of RH with a spectroscopic ellipsometer (M-2000, JA Woollam Inc.) equipped with a custom-designed 20 mL Teflon sample chamber described elsewhere.42 The instrument acquires the ellipsometric parameters of amplitude and phase of the reflected light as a function of wavelength from 193 to 1000 nm. The sample chamber limits the angle of incidence to 65° and holds samples up to (2 × 2) cm2 which are clamped in the chamber by Teflon clips with polypropylene screws. The instrument was run in dynamic mode so that sequential spectra of the ellipsometric parameters were acquired throughout the experiment. The data were modeled using the manufacturer-supplied CompletEASE software and corrected for the birefringence of the silica windows. The films were modeled as Si/ SiO2/polymer, where the polymer film was modeled with a Cauchy dispersion: n(λ) = A + B/λ2, and the native oxide on the silicon wafer was measured to be 1.61 ± 0.04 nm. The complex refractive index of a dry Nafion film was found to be comparable to water;43 therefore, it was reasonable to assume a uniform refractive index for a humidified film. Additionally, a number of more complex models were employed to approximate the optical properties of the humidified films, such as C

DOI: 10.1021/acsami.6b12423 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 3. Total out of plane swelling strain, εz, measured for 150 nm films that were as cast (blue squares) and annealed at 60 °C (purple circles) and 140 °C (green triangles). Error in film thickness measured as one standard deviation in the film thickness of the swollen films is smaller than the size of the symbols. Solid lines represent a guide to the eye.

strain in the z direction can be determined using eq 5. The values of the swelling strain reported here are consistent with values that have been reported in the literature.15,20 The swelling strain shows a strong dependence on the thermal annealing conditions, Figure 3. Compared to an as-cast sample, a Nafion film annealed at 140 °C exhibits a 65% reduction in swelling strain. Similar observations have previously been observed for bulk membranes and self-assembled films.27,36 This is typically attributed to increased crystallinity or the formation of a hydrophobic “skin” layer at the surface of Nafion as suggested by X-ray scattering and contact angle measurements.19,27,33 By combining eqs 4 and 5 the biaxial stress can be determined as a function of relative humidity, shown in Figure 4 for films of approximately 150 nm for several annealing conditions. It is apparent that the biaxial stress within the film is dependent on the thermal annealing conditions and that the stress generated at the highest humidity is in excess of the yield stress measured in uniaxial tension for bulk Nafion, which ranges between 8 and 15 MPa.8,45 However, as indicated in a previous publication, the film can be cycled more than 50 times

Figure 2. (a) Example of humidity cycling procedure used for the cantilever bending experiments. (b) Corresponding stress−thickness response of a 150 nm sample annealed at 140 °C. (c) Change in stress−thickness observed from 150 nm films that were as cast (blue squares) and annealed at 60 (purple circles) and 140 °C (green triangles). Uncertainty in ΔF measured as one standard deviation at a given humidity is smaller than the size of the symbols. Lines are drawn as guides to the eye.

thickness is shown as a function of humidity for several different 150 nm films. Each data set is the average of 3 runs on comparable samples of the same thickness and annealing conditions. The changes observed while at constant humidity are smaller than the symbols. One of the unique advantages of the cantilever bending technique over other thin film mechanics measurements is that one can determine the biaxial stress due to water absorption. Recalling from eq 3, the stress−thickness is the product of the biaxial stress and the film thickness

ΔF = σBihf

(4)

The stress in the film is assumed to be equibiaxial due to the isotropic nature within the plane of the film, due to film formation during spin coating. The film thickness as a function of humidity was measured using spectroscopic ellipsometry. The data are shown in Figure 3 in terms of the total out-ofplane swelling strain, εz, which is defined as εz =

hf − h0 h0

(5)

where h0 is the thickness of the Nafion film when the film is fully dried. Similar to the wafer curvature measurements, the humidity within the sample cell was increased in increments of 10% RH, from 0% to 80% RH. The thickness was monitored in situ until it reached a plateau, typically within 20 min. The total

Figure 4. Biaxial stress generated due to water absorption for several 150 nm films as a function of relative humidity: as cast (blue squares), 60 °C anneal (purple circles), and 140 °C anneal (green triangles). Error bars are determined from a propagation of errors determined as one standard deviation of the measured parameters. D

DOI: 10.1021/acsami.6b12423 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Poisson’s ratio from 0.3 to 0.5 would result in a change in modulus by 15%, not the factor of 5 observed in Figure 5. This is one of the key assumptions in the derivation of eq 6, a couple of other key assumptions are that the deflection of the cantilever, c, is small relative to the dimensions of the sample so the cantilever can be assumed as planar. For the largest c deflections observed in this study, L ≈ 10−4 ≪ 1. We also assume that there is no delamination of the film from the substrate. The later assumption can be verified in that the sample can be cycled over 50 times with no change in the measured stress in the film.15 Literature values are taken both in compression and in tension. There is a good agreement between the 150 nm thick as-cast film and the bulk literature values. Upon thermal annealing there is a strong enhancement in the modulus. The modulus is further enhanced upon thermal annealing above the high-temperature relaxation (i.e., ionhopping temperature, ca. 130 °C).47,48 In all cases the modulus decreases with increasing humidity (water content). It has been previously reported that in bulk Nafion membranes at 25 °C water acts as a plasticizer and decreases the observed modulus.7 At high temperatures there is indication that small amounts of water may enhance the modulus leading to a maximum in the modulus vs humidity.7 There is some indication of this for some of the annealed samples in the present study; however, this maximum is within the experimental uncertainty. There are several possible explanations for an enhanced modulus in thermally annealed 150 nm films. In bulk membranes, an enhanced modulus has been observed in mechanically stretched Nafion as a result of the induced orientation of the polymer chains.49,50 Orientation of the Nafion chains has been used as an explanation for the thickness dependence of the modulus using a wrinkling-based metrology.37 However, with regard to the effect of thermal annealing, orientation can be reasonably ruled out as it has been reported that spin-coated Nafion films supported by a silicon substrate are isotropic in nature after thermal annealing.33,36 Another possible explanation for the increased modulus is an enhancement in the crystallinity of the polyfluoroethylene backbone. However, grazing incidence small-angle X-ray scattering experiments on 200 nm films show that samples annealed at 60 °C exhibit no long-range crystalline order.15 Samples annealed above the ion-hopping transition, 100 °C, show the presence of a peak (approximately 0.06 Å−1) associated with crystalline order of the polyfluoroethylene backbones.36 Therefore, enhanced crystallinity could explain the increase in modulus for the films annealed at 140 °C but cannot be the source of the enhancement for films annealed at 60 °C. To better understand the effect of thermal annealing on the chemical structure of the film, IR experiments were conducted as a function of thermal annealing temperature. Polarization modulation infrared reflection absorption spectroscopy (PMIRRAS) was used to track any changes in the IR spectra before and after annealing. Time-resolved PM-IRRAS has previously been used to track water vapor diffusion in thin Nafion films.18 The resultant IR spectra of the thermally annealed samples in this study are displayed in Figure 6. These samples were vacuum annealed under the same conditions as the cantilever samples. There are a few features to note in the IR spectra. First, we observed the emergence of a vibrational mode at 1440 cm−1 upon thermal annealing, which increases in intensity at higher annealing temperatures. The assignment of this peak to a specific vibrational mode in Nafion is a topic of debate. In

between dry and humid conditions without indication of plastic deformation.15 It is important to point out that Figure 4 is showing the change in stress relative to a reference point. For the purposes of this work, the reference state that is used is 0% RH. It has been previously reported that for bulk membranes there is a residual tensile stress at 0% RH.13 With this in mind, the results in Figure 4 are the change in stress relative to 0% RH, not necessarily the absolute stress values. However, as all samples are prepared and measured in the same manner, the trends with annealing are still observed. The mechanics involved in the bending of a bimaterial cantilever are described elsewhere,15 but they will be briefly outlined in the Supporting Information for context. When Nafion is not constrained by a substrate, water absorption will result in equal swelling in all directions of approximately 20% by volume for bulk Nafion membranes.10 In the cantilever samples the Nafion film is constrained to a substrate, and there is no evidence of delamination as seen in Figure S2. As water will only penetrate into the polymer, water absorption results in a biaxial compressive strain in the film. The biaxial compressive strain is related to an equibiaxial stress and an associated strain in the z direction due to the Poisson effect. By combining all of these effects, the Young’s modulus of the film can be written in terms of measured quantities of εz and ΔF Ef =

−ΔF(1 + v) h0(εz2 + εz)

(6)

Within eq 6 the values of the stress−thickness (ΔF), the total strain in the z direction (εz), and the initial film thickness (h0) are all directly determined from the wafer curvature and ellipsometry. The only unknown is Poisson’s ratio of the thin film. In order to compare to the modulus of bulk Nafion membranes, eq 6 is used to calculate the modulus by assuming the films have a Poisson’s ratio of 0.40 as reported in the literature.46 The Young’s modulus is shown in Figure 5, along with several values reported from the literature.7−11 We acknowledge that the Poisson’s ratio of the films could be dependent on film thickness and water content; however, variations in Poisson’s ratio cannot account for the magnitude of change in modulus observed. For example, a change in

Figure 5. Young’s modulus for several 150 nm films as a function of relative humidity: as cast (blue squares), 60 °C anneal (purple circles), and 140 °C anneal (green triangles). These data are compared to reported literature values for bulk Nafion membranes measured in compression and tension.7−11 Error bars are determined from a propagation of errors determined as one standard deviation of the measured parameters. E

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observed in this sample. Another result of the sulfonic anhydride reaction is a reduction in the hydrophilicity of the ionic domains. Through this reaction the number of acid groups is reduced, resulting in a reduced ion exchange capacity. This result is consistent with the data shown in Figure 3. As the annealing temperature is increased the extent of anhydride formation is increased, the water uptake is reduced, and the elastic modulus is increased. It is well reported that within the catalyst layer the Nafion binder is confined to domains that are on the order of nanometers to tens of nanometers.16,17 It has also been widely demonstrated that a number of properties of Nafion will deviate from the bulk when the film thickness is reduced to below 100 nm.15,21,37 To better understand the influence of thickness on the mechanical properties, we now explore the effect of confinement on the modulus of the thin Nafion films. The effect of polymer film thickness on the mechanical properties is shown in Figure 7. Figure 7a shows the humidity

Figure 6. Infrared spectra recorded using PM-IRRAS for three 150 nm films recorded at 50% RH and 25 °C: as cast (solid blue line), 60 °C anneal (dashed purple line), and 140 °C anneal (dotted green line). Spectra are offset vertically for clarity.

previous studies of bulk Nafion, a peak at 1440 cm−1 emerged upon aging of the material at 80% RH and 80 °C.51,52 In that study, the peak was attributed to the formation of anhydrides by condensation of two sulfonate side groups. In other IR studies of Nafion, comparisons were made to the IR spectra of sulfonic acid and triflic acid.53 In the latter study, a similar peak was observed and attributed to hydrogen bonding of the acid form. In other works investigating the aging of Nafion under purge gases, it was demonstrated that when using an artificial air mixture (mixture of hydrogen and oxygen), no peak at 1440 cm−1 was observed. It was only observed when compressed air was flowed over the samples of Nafion. Therefore, the formation of this peak was attributed to the contamination of ammonia ions exchanging for hydrogen.54 While the actual peak assignment is beyond the scope of the current study, the samples were annealed under vacuum. Therefore, it is unlikely that contamination of ammonia is the cause of the peak observed at 1440 cm−1. Both of the other assignments of the peak consist of either chemical cross-linking or physical crosslinking through hydrogen bonds. Either explanation would likely result in an enhancement in the mechanical properties, consistent with the experimental observations. In addition to the emergence of a peak at 1440 cm−1, a series of peaks shows up in the range from 2900 to 3100 cm−1. These peaks have generally been attributed to the structured hydration shell of H3O+ surrounding the acid groups or bound water. The emergence of the structured water peaks is similarly followed by the decreased free water peaks and hydronium peaks at ca. 3500 and ca. 1700 cm−1. This observation is consistent with the observations in Figure 3, in which the annealed samples contain less free water. The IR spectrum that is observed for the thin films is consistent with those of the thick films. This data is shown in the Supporting Information for reference. Anhydride formation through thermal annealing or hydrogen bonding of the acid groups would serve as cross-links in the material and thus act to stiffen the films. Therefore, as the annealing temperature is increased, the modulus is enhanced and similarly the intensity of the peak at 1440 cm−1 in the IR spectra is enhanced. An increase in intensity of this peak is indicative of further cross-linking within the polymer film. Since we cannot rule out the influence of enhanced crystallinity in the sample annealed at 140 °C, it is likely that both crystallinity and cross-link formation contribute to the enhancement in modulus

Figure 7. (a) Young’s modulus for several as-cast films as a function of relative humidity. Young’s modulus is shown for several different film thicknesses. (b) Young’s modulus at 30% RH is shown as a function of dry film thickness: as cast (blue squares), 60 °C anneal (purple circles), and 140 °C anneal (green triangles). Error bars are determined from a propagation of errors determined as one standard deviation of the measured parameters. Solid lines represent a guide to the eye.

dependence of the Young’s modulus for as-cast Nafion samples in the thickness range from 30 to 200 nm. It is clear from this figure that the modulus decreases with increasing humidity for all samples. It is also apparent from these data that the modulus is enhanced in thin films, particularly in the low-humidity regime. In the high-humidity regime, the data appears to converge to values that are similar to the bulk modulus. To look at the influence of film thickness and annealing more directly, the modulus at 30% RH is plotted in Figure 7b. Once again it is F

DOI: 10.1021/acsami.6b12423 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces apparent that the modulus increases with decreasing film thickness. This trend is similar to observations that were made in a previous study using a wrinkling-based metrology of as-cast Nafion films.37 The trend with film thickness is similar and independent of the thermal annealing conditions, indicating that thermal annealing influences all film thicknesses equally with regard to the mechanical properties. It has been widely reported that the structure,31−36 water sorption,19,20,24 and diffusion18,21 properties of Nafion are strongly dependent on confinement. It has since been speculated that it is the mechanical properties of thin Nafion films that may govern/limit this behavior. In a recent study a chemo-mechanical model was proposed in which the chemical potential of water within the film is governed by the swelling pressure dictated by the hydrophobic backbone.37,55 The resistance to swelling of the hydrophilic ionic domains is the modulus of the hydrophobic polymer backbone. Therefore, the swelling pressure within the membrane is proportional to the dry modulus of the material. Although the dry modulus cannot be directly probed with cantilever bending, it can be extrapolated from the data in Figures 5 and 7. In this case, the dry modulus refers to the modulus at 0% RH. This can be done by fitting the modulus as a function of water content as has been described elsewhere.9 This fitting can then be extrapolated to the modulus at 0% RH. The chemo-mechanical model relates the swelling pressure (P) to the chemical potential (μ) of the water within the film according to μw − μ0w = RT ln(φ) = −P Vw , where R is the ideal gas constant and Vw is the molar volume of water. The water content within the film can be estimated from the stress free strain using equations derived elsewhere15 ε0 = εz

1−v 1+v

Figure 8. (a) Dry Young’s modulus extrapolated from the data shown in Figures 5 and 7 plotted as a function of water content at 70% RH. The symbol size is representative of the film thickness; the smallest symbol corresponds to the thinnest film and largest to the thickest films. The solid line represents a linear fit to all the data. (b) The same data plotted instead in terms of the swelling pressure proportional to the chemical potential. The solid line is the same fit as in a converted to chemical potential.

(7a)

⎛ 1−v ⎞ ⎜ε + 1⎟ ⎝ z1 + v ⎠

−3

=

V0 = φp = 1 − φ V

these properties are thoroughly connected by the morphology and nanostructure of the polymer films. Figure 9 shows the biaxial stress for as-cast films of several different film thicknesses. The thinnest film exhibits the largest stress at a given humidity. The data for annealed samples are shown in the Supporting Information. These data indicate that the stresses observed in thin films are larger than those in the

(7b)

where φp is the polymer volume fraction and φ is the water volume fraction. Therefore, the water content within the film can be estimated from the water uptake isotherms shown in Figure 3. To further validate this chemo-mechanical model, the dry modulus is plotted as a function of water content in Figure 8. The data shown in Figure 8 indicates that there is a strong correlation between the dry modulus of the film and the water uptake, regardless of the film thickness and processing condition. For clarity, each thermal annealing condition is shown as a different symbol, and the symbol size corresponds to the film thickness (i.e., smaller symbol corresponds to thinner film). It is evident from this figure that by combining all this data there is a clear connection between the mechanical properties and the water uptake governed by the chemical potential of water. This correlation was demonstrated by fitting the data to a line, indicating that the modulus is inversely proportional to the amount of water within the film. The fit is drawn as a solid line in Figure 8a. This observation is consistent with those previously demonstrated for as-cast films.37 Similarly, the dry modulus is also shown as a function of the chemical potential of water within the film in Figure 8b. Even though there are strong changes in both the modulus and the swelling behavior with film thickness and processing conditions,

Figure 9. Biaxial stress generated due to water sorption for several ascast films as a function of relative humidity: 38 nm film (blue squares), 63 nm film (light blue inverted triangle), and 175 nm film (black sideways triangle). Error bars are determined from a propagation of errors determined as one standard deviation of the measured parameters. G

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ACS Applied Materials & Interfaces bulk. To put these biaxial stress values in perspective, the yield stress measured in uniaxial tension for bulk Nafion of approximately 8−15 MPa and a stress to break of around 20−25 MPa.8,45 The biaxial stress values reported in Figure 9 exceed the yield stress measured for the bulk. One key assumption of this experiment is that the biaxial stress at 0% RH is 0 MPa, and all reported stresses are relative to this reference state. It has been demonstrated in humidity cycling of bulk membranes that at 0% RH there is a residual tensile stress and the stress free state is found at a nonzero humidity.13 Therefore, the results shown in Figure 9 could be a result of a larger compressive stress in the humid state or a larger residual tensile stress in the dry state for the thinner films. In the current experiment the two effects cannot be decoupled. Therefore, these results are the change in stress relative to 0% RH, not necessarily the absolute stress values. However, as all samples are prepared and measured in the same manner, the trends with film thickness and annealing are still observed.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Bradley R. Frieberg: 0000-0002-0125-4278 Present Address §

American Embassy School, New Delhi, India 110021

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge Carlos Beauchamp for programming support of the experimental apparatus. B.R.F. acknowledges support from the NIST NRC Fellowship program. J.R.G. acknowledges the NIST Summer Undergraduate Research Fellowship for financial support.

CONCLUSION We used a measurement platform based on cantilever bending to elucidate the mechanical behavior of thin Nafion films of different film thicknesses exposed to varying processing conditions. We demonstrated the ability to measure the mechanical properties of films spin coated on silicon substrates. In the case of thermally annealed samples it was shown using infrared spectroscopy that sulfonic anhydrides form between molecules (or hydrogen bonds between the protonated and dehydrated sulfonic acids) which effectively stiffens the material. Additional evidence provided from previous publications indicates there is an enhancement in the crystallinity of the fluorinated backbones which also could lead to an enhancement in the modulus. It was demonstrated that the Young’s modulus of thin polymer films is strongly dependent on the humidity, and as the thickness of the film decreases, the humidity dependence also increases, thus indicating there must be a change in the local structure within these polymer films. Thin films below 100 nm were shown to have an enhanced modulus relative to thick films, which is most apparent at low humidity. The thickness dependence of the modulus was understood in terms of a chemo-mechanical model that was described in a previous publication.37 This model indicates and provides evidence that the modulus of the material is strongly connected to the moisture uptake of the film through both the chemical potential of water and the swelling pressure within the polymer film. As the premature failure of MEAs for fuel cells is often due to crack formation which initiates in the catalyst layer, it is important to note that over the same change in humidity a thin film will experience a larger stress than a bulk membrane. However, it has been demonstrated by recent literature that the Nafion binder does not form a continuous film on the catalyst particles. Further investigation is necessary to understand the stress state and mechanical properties of the catalyst layer of a fuel cell. This cantilever bending approach can be employed to measure the mechanical properties and swelling-induced stresses of other fuel cell materials or other polymer/solvent systems.

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Sample conditioning, sample cyclability, sample reversibility (PDF)

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ABBREVIATIONS FTIR Fourier transform infrared MEA membrane electrode assembly MCT mercury−cadmium−telluride PEM proton exchange membrane PFSA perfluorosulfonic acid PM-IRRAS polarization moduluation infrared absorption spectroscopy PSD position-sensitive detector RH relative humidity

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b12423. H

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