High Proton Conductivity in the Molecular Interlayer of a Polymer

Nagoya University Venture Business Laboratory, Furo-cho, Chikusa, Nagoya 464-8603, Japan. Langmuir , 2015, 31 (18), pp 5174–5180. DOI: 10.1021/acs.l...
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High Proton Conductivity in the Molecular Interlayer of a Polymer Nanosheet Multilayer Film Takuma Sato,† Yuta Hayasaka,§ Masaya Mitsuishi,§ Tokuji Miyashita,§ Shusaku Nagano,∥ and Jun Matsui*,‡ †

Graduate School of Science and Engineering, and ‡Department of Material and Biological Chemistry, Yamagata University, 1-4-12 Kojirakawa-machi, Yamagata 990-8560, Japan § Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan ∥ Nagoya University Venture Business Laboratory, Furo-cho, Chikusa, Nagoya 464-8603, Japan S Supporting Information *

ABSTRACT: High proton conductivity was achieved in a polymer multilayer film with a well-defined two-dimensional lamella structure. The multilayer film was prepared by deposition of poly(N-dodecylacryamide-coacrylic acid) (p(DDA/AA)) monolayers onto a solid substrate using the Langmuir−Blodgett technique. Grazing-angle incidence X-ray diffraction measurement of a 30-layer film of p(DDA/AA) showed strong diffraction peaks in the out-of-plane direction at 2θ = 2.26° and 4.50°, revealing that the multilayer film had a highly uniform layered structure with a monolayer thickness of 2.0 nm. The proton conductivity of the p(DDA/AA) multilayer film parallel to the layer plane direction was 0.051 S/cm at 60 °C and 98% relative humidity with a low activation energy of 0.35 eV, which is comparable to perfluorosulfonic acid membranes. The high conductivity and low activation energy resulted from the formation of uniform two-dimensional proton-conductive nanochannels in the hydrophilic regions of the multilayer film. The proton conductivity of the multilayer film perpendicular to the layer plane was determined to be 2.1 × 10−13 S/cm. Therefore, the multilayer film showed large anisotropic conductivity with an anisotropic ratio of 2.4 × 1011.

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INTRODUCTION Recently, proton-conductive polymers have attracted considerable attention as important materials in polymer electrolyte fuel cells.1,2 Perfluorosulfonated polymers are used as polymer electrolyte membranes because of their high proton conductivity.3 For example, Nafion is well-known to be a highly proton-conductive polymer because of the formation of selfassembled nanosized ion channels by hydrophilic side chains, and protons smoothly pass through the hydrophilic channels in Nafion membranes.4,5 Therefore, construction of a uniform and continuous hydrophilic ion-conductive nanochannels in the polymer film is important to realize high proton conductivity. Several groups have reported the enhancement of protonconductive properties by the structural control of the conductive channel. Elabd et al. and Kawakami and Tamura showed that the one-dimensional (1D) proton-conductive channels in polyelectrolyte nanofibers enhanced proton conductivity compared with the bulk values.6,7 In other cases, nanopores formed in coordination polymers act as highly conductive ion channels.8−15 At the inside wall of the nanopore, condensation of proton carriers8 and/or formation of an effective hydrogen-bonding network between proton carriers9 results in high conductivity with low activation energy. Twodimensional (2D) proton nanochannels formed by layer-by© XXXX American Chemical Society

layer (LbL) and Langmuir−Blodgett (LB) techniques have a large influence on proton conductivity.16−19 Matsuda et al. showed that the mobility of protons was enhanced in the 2D conduction channels in LbL films of Nafion and poly(allylamine hydrochloride).17 Matsumoto et al. reported a large difference in the in-plane direction of the proton conductivity between a graphene oxide LB film and drop-cast films.19 Thus, the formation of highly oriented ion nanochannel is requisite to obtain high ion-conductive performance. Our recent work has shown that a highly oriented 2D lamella structure constructed by a polymer nanosheet multilayer film greatly enhanced proton conductivity compared to that of spincoated films.20,21 The multilayer film was constructed by deposition of polymer monolayers formed at the air−water interface by the LB technique. Therefore, the multilayer film formed a well-defined and uniform lamellar structure composed of hydrophobic alkyl side chains and hydrophilic amide regions. For example, the polymer nanosheet multilayer film prepared with poly(N-dodecylacrylamide-co-2-acrylamido-2-methylpropanesulfonic acid) showed a conductivity 10 times higher Received: January 5, 2015 Revised: April 12, 2015

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Langmuir than that of the spin-coated film because of the formation of 2D proton-conductive nanochannels.21 Moreover, an advantage of our polymer nanosheet system is that ion-conductive groups in the nanochannel can be easily modified by copolymerizing ionconductive vinyl monomers with N-dodecylacrylamide (DDA) monomer.20,22 Herein, we demonstrate the importance of highly uniform 2D ion-conductive nanochannels for achieving high proton conductivity with a weakly acidic proton source (carboxylic acid). For this purpose, a polymer nanosheet multilayer film was prepared with poly(N-dodecylacrylamide-co-acrylic acid) (p(DDA/AA)). The multilayer structure was studied by surface pressure (π)−area (A) isotherm, UV−vis absorption spectra, and grazing-angle incidence X-ray diffraction (GI-XRD) measurements. The proton conductivity of the multilayer film was measured parallel (σ∥) and perpendicular (σ∞) to the layer plane direction using impedance spectroscopy under controlled temperature and humidity. New methods and design for polymer electrolytes were proposed.

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substrate, and then a gold electrode was thermally evaporated onto the multilayer film. During the thermal deposition, the substrates were cooled by pouring liquid N2 into the substrate holder. Impedance measurements of the polymer were conducted using an impedance analyzer (1260 Impedance/Gain-Phase Analyzer with a 1296 Dielectric Interface system, Solartron Analytical). The RH and temperature were controlled using a humidity- and temperaturecontrolled chamber (SH-221, Espec Corp.). Grazing-angle incidence X-ray diffraction (GI-XRD) measurements were performed using a Rigaku FR-E X-ray diffractometer with an R-AXIS IV 2D detector (Rigaku, Japan). We used Cu Kα radiation (λ = 0.1542 nm) with a beam size of approximately 300 μm × 300 μm, and the camera length was 300 mm. The sample stage was composed of the goniometer and a vertical stage (CHUO Precision Industria ATS-C316-EM/ALV-300HM). The incidence angle was chosen in the range 0.18°−0.22°.

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RESULTS AND DISCUSSION Structure of p(DDA/AA) Multilayer Film. The monolayer properties of p(DDA/AA) were investigated by π−A isotherm measurements. The isotherm of p(DDA/AA) measured at 20 °C showed a steep increase in surface pressure with decreasing surface area, and a high collapse pressure (∼50 mN/m), which indicates that the polymer formed a stable monolayer at the air−water interface (Figure 2). The average

EXPERIMENTAL SECTION

All reagents were purchased from TCI (Tokyo, Japan) and Nacalai Tesque (Tokyo, Japan), and used without further purification unless specified otherwise. DDA was synthesized by the reaction of acryloyl chloride with dodecylamine in the presence of triethylamine in chloroform at room temperature.23 The crude products were purified by recrystallization from chloroform−hexane mixed solvent. The p(DDA/AA) copolymer was prepared by free radical copolymerization of DDA with AA in toluene at 60 °C using 2,2′-azobis(isobutyronitrile) as a thermal initiator (Figure 1). The polymer was

Figure 2. π−A isotherm of p(DDA/AA) at 20 °C. Dashed arrow indicates the line extrapolated to determine the average limiting surface area (Aav).

Figure 1. Chemical structure of p(DDA/AA). purified by precipitation in a large excess of acetonitrile from a chloroform solution, and then dried under vacuum at room temperature. The mole fraction of AA in the copolymer was determined to be 44 mol % by elemental analysis. The numberaverage molecular weight and polydispersity index were determined to be 7500 and 1.8, respectively, using a gel permeation chromatograph (GPC, Tosoh Corp.) with a polystyrene standard. π−A isotherm measurements and deposition of the p(DDA/AA) monolayer were carried out with a computer-controlled Langmuir trough (FSD-50 and 51, USI). Distilled and deionized water (>17.5 MΩ cm, Smart2Pure, Thermo Scientific) was used as the subphase. The polymer monolayer was compressed at a rate of 15.0 cm2/min. The polymer monolayer was transferred onto solid substrates using a vertical deposition method at a dipping speed of 10 mm/min under a surface pressure of 40 mN/m at 20 °C. The quartz and silicon substrates on which the monolayer was deposited were cleaned by treatment with a UV−O3 cleaner (SSP16-110, SEN Lights Corp.), and were made hydrophobic by immersion of the substrates into a ca. 1 × 10 −6 M octadecyltrichlorosilane chloroform solution. Interdigitated array (IDA) electrodes were fabricated using thermal deposition of Ti (2 nm) and then Au (60 nm) with a metal mask onto a hydrophobic glass substrate. The IDA electrodes had two sets of comb-type Au arrays. Each array contained eight electrode elements, which were 0.5 mm wide and 8.0 mm long and separated by 0.2 mm from adjacent elements. For the proton conductivity measurement of the multilayer film perpendicular to the layer plane direction, 160 p(DDA/AA) polymer nanosheet layers were deposited onto a gold-deposited glass

limiting surface area (Aav) was estimated to be 0.16 nm2/ monomer unit by extrapolating the linear portion of the steep increase owing to the condensed state in the π−A isotherm to zero surface pressure. From the Aav value, the surface occupied by AA (AAA) was calculated to be 0.04 nm2/molecule using A av = (1 − x)ADDA + xAAA

(1)

where x is the mole fraction of AA and ADDA is the limiting area of the DDA homopolymer.22 The AAA value was more than three times less than that calculated by the Corey−Pauling− Koltun (CPK) space-filling model (0.14 nm2), indicating that the AA unit was located below the water surface.23 The copolymer monolayer was transferred onto a quartz substrate, and UV−vis spectra of the p(DDA/AA) film were measured as a function of the number of deposited layers (Figure 3a). The absorbance at around 192 nm, which is assigned to the absorption of the amide group of DDA, linearly increased with increasing numbers of deposited layers (Figure 3b). The linear relationship between the absorbance of the amide group and the number of layers suggests regular deposition of the monolayer. The structure of the p(DDA/ AA) multilayer LB film was further investigated by GI-XRD. Figure 4a shows the 2D GI-XRD pattern of a 30-layer p(DDA/ B

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groups in the hydrophilic head-to-head region. The hydrophilic head-to-head region containing AA groups was used as a proton-conductive nanochannel (Figure 4d). Proton Conductivity of p(DDA/AA) Parallel to the Layer Plane. Thirty layers of p(DDA/AA) were deposited onto an IDA gold electrode to study σ∥. The p(DDA/AA) multilayer film was placed in a temperature and humiditycontrolled chamber at 60 °C and 98% RH. The impedance spectrum of the film was measured every 24 h to investigate the humidification time effect on the proton conductivity. Figure 5a shows Cole−Cole plots for the p(DDA/AA) multilayer film with different humidification times. The film resistance was determined by the intersection of a semicircle with the real axis in the complex plane of the Cole−Cole plots. The proton conductivity of the sample was calculated as

σ = k /(Rl)

(2)

where k = 1.42 × 10−3, which is related to the shape of the IDA electrode, R represents the resistance, and l denotes the film thickness (49.8 nm).28 The conductivity of the p(DDA/AA) multilayer film increased with increasing humidification time and saturated at 5.2 × 10−2 S/cm above 144 h (Figure 5b, circles). As mentioned in the previous section, the alkyl side chains of the p(DDA/AA) nanosheet film adopt a close-packed structure. The close-packed hydrophobic alkyl side chains in the p(DDA/AA) nanosheet film prevent water vapor from diffusing across the lamella interlayers. Therefore, it takes a relatively long time to sufficiently humidify the hydrophilic head-to-head acidic region. Indeed, when the number of layers was decreased to 20 layers, the conductivity value saturated in less time (72 h, Figure 5b, rectangles) than for 30 layers. The layer structure of the film after humidification was also investigated by GI-XRD measurements. The GI-XRD measurements showed that the multilayer film still exhibited the strong spot-like peaks in the out-of-plane direction after humidification (Supporting Information Figure S1). The peak positions shifted to the wide-angle region of 2.52° (primary peak) and 5.22° (secondary peak) (Figure S2a). On the other hand, the in-plane diffraction peaks at 20.0° disappeared after maintaining the sample at 95% RH for 6 days (Figure S2b). From XRD measurements of a 30-layer of p(DDA/AA) film, the final monolayer thickness after maintaining the sample at 98% RH for 6 days decreased to 1.66 nm from an original thickness of 2.0 nm (Figure S3). These results indicate that the layer structure was maintained while the packing of dodecyl side chains loosened when maintaining the sample at high RH for a long time. It should be noted that the saturated value of σ∥ was independent of the number of deposited layers, which suggests that protons were conducted through the hydrophilic acidic region. The temperature dependence of the σ|| value of the 30-layer p(DDA/AA) film was evaluated by impedance measurements, as shown in Figure 6. Before the conductivity measurements, the multilayer film was placed in a humidity chamber under 98% RH at 60 °C for 6 days to fully humidify the film. Impedance spectra were then measured with decreasing temperature from 60 to 20 °C every 10 °C. The temperature was then increased from 20 to 60 °C with impedance spectra measured every 10 °C to investigate the hysteresis of the proton conductivity of the film. The conductivities were almost the same for the increasing and decreasing processes, suggesting that proton conductivity has no temperature hysteresis. The activation energy of proton conductivity (Ea∥)

Figure 3. . (a) UV−vis spectra of p(DDA/AA) with different numbers of layers. (b) Linear relationship between the absorbance at 192 nm and the number of p(DDA/AA) polymer nanosheet layers.

AA) LB film. Strong spotlike diffraction was clearly observed in the imaging plate in the out-of-plane direction at 2θ = 2.26° (d = 3.9 nm) with a secondary peak at 2θ = 4.50° (d = 2.0 nm) (Figure 4b). The d value of the first scattering peak is assigned to the bilayer spacing of the multilayer film with the Y-type multilayer deposition. The length of the dodecyl side chain in the extended all-trans state was calculated to be 1.8 nm using the CPK model.24 The observed spacing is in reasonable agreement with the ideal monolayer thickness. Furthermore, the spot-like diffraction and secondary peak indicate a highly ordered lamella plane constructed by LB deposition. The monolayer thickness was determined to be ca. 2.0 nm using the diffraction peaks. This thickness of p(DDA/AA) is ca. 0.3 nm greater than the monolayer thickness of the p(DDA) LB film.22 A similar increase in the monolayer thickness was observed in the copolymer of the p(DDA) LB film with a hydrophilic comonomer.21,25 It has been suggested that hydrophilic groups are present in the hydrophilic head-to-head region in the layer plane, which causes an increase in the bilayer thickness.21,25 Moreover, a weak but apparent diffraction peak was observed in the in-plane direction at 2θ = 20.0° (d = 0.44 nm), which indicates some of the dodecyl side chains hexagonally pack (Figure 4c).26 The shoulder peaks on the wide-angle side of the out-of-plane peaks in Figure 4b show diffraction of the reflected X-ray beam by the substrate.27 The π−A isotherm, UV−vis spectra, and GI-XRD results revealed that p(DDA/AA) formed a uniform and highly oriented structure, in which dodecyl side chains orient almost perpendicular to the layer plane with AA C

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Figure 4. (a) 2D GI-XRD imaging plate pattern for a 30-layer p(DDA/AA) film deposited on a silicon substrate, and 1D profiles in the (b) out-ofplane and (c) in-plane directions extracted from the 2D image. (d) Schematic illustration of the film structure.

was calculated to be 0.35 eV from the least-squares fits of the slope. This value is comparable with that reported for a highly oriented polypeptide film,29 and thus the mechanism of proton conductivity should be evaluated using the Grotthuss mechanism, which Ea values are reported as 0.1 to 0.4 eV.30,31 It should be mentioned that the conductivity value was one to 2 orders of magnitude higher than that in other reported work that used a weak acid as the proton source.29,32 Moreover, the value is comparable with that for strong acid polyelectrolytes, such as Nafion film.1 It is surprising that the weak acid polyelectrolyte shows comparable proton conductivity to that of the super acid Nafion membrane. The high proton conductivity in the polymer nanosheet multilayer film resulted from the highly uniform and continuous 2D proton-conductive path, which was confirmed by GI-XRD. The proton density in the p(DDA/AA) multilayer will be much lower than that in Nafion membranes considering their pKa values (pKa = 4 to 5 for poly(acrylic acid)33−35 and pKa = −3 to −6 for Nafion36). Therefore, enhancement of proton mobility at a solid/water interface, which has been reported by several groups, will be the reason for the high proton conductivity.37−42 Recently, it has been proposed that a similar lamellar morphology was formed in the Nafion membrane.4 Moreover, Nagao and co-workers also reported enhanced proton conductivity in the lamellar-like structure compared with the amorphous state.29,43,44 Recent theoretical calculations by Eikerling et al. also support the enhancement of proton conductivity in 2D space.39,40,45 The proton conductivity is highly dependent on relatively humidity. The proton conductivity decreased about one forth when the humidity was decreased in 10% (Figure S4). The results indicate that the hydrogen-bonding network constructed in the hydrophilic layer is important to realize the high conductivity. Our results, as well

as other reported results, clearly indicate that the fabrication of 2D nanochannels is important to achieve high proton conductivity. Proton Conductivity of p(DDA/AA) Multilayer Perpendicular to the Layer Plane. The proton conductivity of the p(DDA/AA) multilayer film perpendicular to the layer plane direction (σ⊥) was measured to investigate the anisotropic conductivity of the film. A 160-layer p(DDA/AA) film was deposited onto a gold substrate, and a gold electrode was deposited on the film to prepare a sandwich-type cell. Figure 7 shows Cole−Cole plots of the 160-layer p(DDA/AA) film measured perpendicular to the layer plane direction. For temperature from 20 to 40 °C, the Z″ value decreases with decreasing frequency, whereas the Z′ value remains almost constant, which is a typical plot for a capacitor.46 Semicircular plots were observed for 50 and 60 °C (Figure 7). A clear semicircle plot was not even obtained at 60 °C because of the low proton conductivity. In the present case, perpendicular conduction requires protons to pass through the hydrophobic region created by alkyl side chains. However, as shown in the GI-XRD spectra (Figure S2b), the packing of dodecyl side chains was relatively loose at high temperature with humidification. Some small voids were formed by the disruption of the packing of the side chains in the multilayer film, and hydrodynamic diffusion of water occurred through the voids at high temperature. Here, we use the σ⊥ value at 60 °C to discuss the perpendicular conductivity of the film. The perpendicular proton conductivity under 98% RH at 60 °C was determined to be 2.1 × 10−13 S/cm by fitting the plot to a simple parallel resistor-capacitor circuit (Figure S5). Compared with the in-plane conductivity (σ∥ = 5.1 × 10−2 S/cm) of the multilayer film at 60 °C, the perpendicular conductivity is much lower, and the anisotropy (σ∥/σ⊥) in the conductivity of the D

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Figure 5. (a) Cole−Cole plots for a 30-layer p(DDA/AA) on an IDA electrode with different humidification times. (b) Relationship between conductivity (σ∥) and humidification time. Red circles are 30 layers, and black squares are 20 layers of p(DDA/AA).

Figure 6. (a) Arrhenius plots of the proton conductivity of 30-layer p(DDA/AA) films. (b) ln σT change with heating (open circles) and cooling (solid circles) cycles. The least-squares fit is shown as the dotted line.

multilayer film reached 2.4 × 1011. The anisotropy value is much larger than that observed in ion-conductive liquid crystal membranes.47−50 The structure of the smectic phase in ionconductive liquid crystal membranes is very similar to that of the present polymer nanosheet multilayer film. Therefore, the higher anisotropy in the polymer nanosheet multilayer film can be attributed to the well-defined amphiphilic lamella structure parallel to the substrate plane. The polymer nanosheet multilayer film was constructed with deposition of mechanically compressed polymer monolayer at the air−water interface so that the polymer chains were closely packed. The closed packed structure was maintained in the multilayer film by hydrogenbonding between the polymer chains. Therefore, hydrophobic insulation layers composed of alkyl side chains suppress proton conduction across the layer plane. The combination of enhancement of proton conduction parallel to the layer plane and suppression of the proton conduction perpendicular to the layer plane results in the high anisotropic proton conduction.

Figure 7. Cole−Cole plots for a 160-layer p(DDA/AA) film at different temperatures and 98% RH. The impedance spectra were measured perpendicular to the layer plane direction.

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CONCLUSIONS We investigated the proton conductivity of p(DDA/AA) multilayer films under controlled temperature and humidity conditions. The proton conductivity of the p(DDA/AA) multilayer film measured parallel to the layer plane direction was determined to 0.051 S/cm at 60 °C and 98% RH. To our knowledge, this is one of the highest conductivities reported for a polymer electrolyte using a carboxylic acid (a weak acid) as the proton source.29 The high conductivity was attributed to

the formation of uniform 2D proton-conductive nanochannels in the hydrophilic region of the multilayer film. The formation of uniform 2D hydrophilic conductive nanochannels was confirmed by GI-XRD. The low activation energy for the proton conductivity also supports the formation of the conductive channel. Usually, a strong or super acid, such as sulfonic acid is used as the proton-conductive group to obtain E

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Langmuir proton conductivity higher than 10−2 S/cm. The requirement of a strong acid for the ionic side chains has restricted the polymer main chain to chemically stable groups, such as polyimides and polytetrafluoroethylene. The present research shows that high conductivity can be obtained even in a weak acid polyelectrolyte by forming a uniform 2D ion-conductive channel, which expands the selection of not only the polymer structure, but also the other component of the fuel cell. The studies for the water amount and their structure in the 2D ionconductive channel are in progress. Moreover, the multilayer film showed high conductive anisotropy of 2.4 × 1011. The present results show that formation of a uniform 2D protonconductive nanochannel is a promising strategy to enhance proton conductivity, and has possible applications not only in fuel cells but also in ionic devices, such as protonic field-effect transistors.

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(11) Zheng, G. L.; Yang, G. C.; Song, S. Y.; Song, X. Z.; Zhang, H. J. CrystEngComm 2014, 16, 64−68. (12) Sadakiyo, M.; Yamada, T.; Honda, K.; Matsui, H.; Kitagawa, H. J. Am. Chem. Soc. 2014, 136, 7701−7707. (13) Sadakiyo, M.; Kasai, H.; Kato, K.; Takata, M.; Yamauchi, M. J. Am. Chem. Soc. 2014, 136, 1702−1705. (14) Begum, S.; Wang, Z. Y.; Donnadio, A.; Costantino, F.; Casciola, M.; Valiullin, R.; Chmelik, C.; Bertmer, M.; Karger, J.; Haase, J.; Krautscheid, H. Chem.Eur. J. 2014, 20, 8862−8866. (15) Nagarkar, S. S.; Unni, S. M.; Sharma, A.; Kurungot, S.; Ghosh, S. K. Angew. Chem., Int. Ed. 2014, 53, 2638−2642. (16) Tago, T.; Shibata, H.; Nishide, H. Macromol. Symp. 2006, 235, 19−24. (17) Daiko, Y.; Katagiri, K.; Matsuda, A. Chem. Mater. 2008, 20, 6405−6409. (18) Daiko, Y.; Katagiri, K.; Yazawa, T.; Matsuda, A. Solid State Ionics 2010, 181, 197−200. (19) Hatakeyama, K.; Karim, M. R.; Ogata, C.; Tateishi, H.; Funatsu, A.; Taniguchi, T.; Koinuma, M.; Hayami, S.; Matsumoto, Y. Angew. Chem., Int. Ed. 2014, 53, 6997−7000. (20) Hayasaka, Y.; Matsui, J.; Miyashita, T. Mol. Cryst. Liq. Cryst. 2013, 579, 17−21. (21) Matsui, J.; Miyata, H.; Hanaoka, Y.; Miyashita, T. ACS Appl. Mater. Interfaces 2011, 3, 1394−1397. (22) Mitsuishi, M.; Matsui, J.; Miyashita, T. Polym. J. 2006, 38, 877− 896. (23) Matsui, J.; Shimada, T.; Miyashita, T. J. Mater. Chem. 2011, 21, 17498−17504. (24) Mizuta, Y.; Matsuda, M.; Miyashita, T. Langmuir 1993, 9, 1158−1159. (25) Mitsuishi, M.; Kikuchi, S.; Miyashita, T.; Amao, Y. J. Mater. Chem. 2003, 13, 2875−2879. (26) Hsieh, H. W. S.; Post, B.; Morawetz, H. J. Polym. Sci., Part B: Polym. Phys. 1976, 14, 1241−1255. (27) Lee, B.; Park, I.; Yoon, J.; Park, S.; Kim, J.; Kim, K.-W.; Chang, T.; Ree, M. Macromolecules 2005, 38, 4311−4323. (28) The film thickness after the humidifation was used. See Figure S3 for details. (29) Nagao, Y.; Matsui, J.; Abe, T.; Hiramatsu, H.; Yamamoto, H.; Miyashita, T.; Sata, N.; Yugami, H. Langmuir 2013, 29, 6798−6804. (30) Colomban, P. Proton conductors: solids, membranes, and gels materials and devices; Cambridge University Press: Cambridge, New York, 1992. (31) Shigematsu, A.; Yamada, T.; Kitagawa, H. J. Am. Chem. Soc. 2011, 133, 2034−2036. (32) Liu, B.; Hu, W.; Robertson, G. P.; Guiver, M. D. J. Mater. Chem. 2008, 18, 4675−4682. (33) Porasso, R. D.; Benegas, J. C.; van den Hoop, M. A. G. T. J. Phys. Chem. B 1999, 103, 2361−2365. (34) Himstedt, H. H.; Marshall, K. M.; Wickramasinghe, S. R. J. Membr. Sci. 2011, 366, 373−381. (35) Feng, N.; Dong, J.; Han, G.; Wang, G. Macromol. Rapid Commun. 2014, 35, 721−726. (36) Eikerling, M.; Kornyshev, A. A.; Spohr, E. In Fuel Cells I; Scherer, G. G., Ed.; Springer-Verlag: Berlin, 2008; Vol. 215, pp 15−54. (37) Sakurai, I.; Kawamura, Y. Biochim. Biophys. Acta 1987, 904, 405−409. (38) Heim, M.; Cevc, G.; Guckenberger, R.; Knapp, H. F.; Wiegrabe, W. Biophys. J. 1995, 69, 489−497. (39) Golovnev, A.; Eikerling, M. Phys. Rev. E 2013, 87, 062908. (40) Golovnev, A.; Eikerling, M. J. Phys.: Condens. Matter 2013, 25, 045010. (41) Mulkidjanian, A. Y.; Heberle, J.; Cherepanov, D. A. Biochim. Biophys. Acta 2006, 1757, 913−930. (42) Teissie, J.; Prats, M.; Soucaille, P.; Tocanne, J. F. Proc. Natl. Acad. Sci. U. S. A. 1985, 82, 3217−3221. (43) Nagao, Y. Chem. Lett. 2013, 42, 468−470. (44) Krishnan, K.; Iwatsuki, H.; Hara, M.; Nagano, S.; Nagao, Y. J. Mater. Chem. A 2014, 2, 6895−6903.

ASSOCIATED CONTENT

* Supporting Information S

2D GI-XRD imaging plate pattern for a 30-layer p(DDA/AA) film deposited on a silicon substrate, and 1D profiles in the GIXRD spectra of out-of-plane and in-plane direction incubated in 60 °C 95%RH at 6 days, XRD spectrum of a 30-layer p(DDA/AA) film after placing the sample in 60 °C, 98%RH 6 days, relationship of conductivity to relative humidity, and parallel resistor-capacitor circuit for fitting the impedance data. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b00036.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ministry of Education, Culture, Sports, Science and Technology, Government of Japan, by a Grant-in-Aid for Scientific Research B (No. 26286010) on Innovative Area “New Polymeric Materials Based on Element-Blocks (No. 2401)”, the Nanotechnology Platform Program (Molecule and Material Synthesis), and the Network Joint Research Center for Materials and Devices.

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DOI: 10.1021/acs.langmuir.5b00036 Langmuir XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.langmuir.5b00036 Langmuir XXXX, XXX, XXX−XXX