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Biomimetic polyelectrolytes based on polymer nanosheet films and their proton-conduction mechanism Mayu Tsukamoto, Kazuki Ebata, Hiroshi Sakiyama, Shunsuke Yamamoto, Masaya Mitsuishi, Tokuji Miyashita, and Jun Matsui Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b04079 • Publication Date (Web): 11 Feb 2019 Downloaded from http://pubs.acs.org on February 12, 2019
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Biomimetic polyelectrolytes based on polymer nanosheet films and their proton-conduction mechanism
Mayu Tsukamoto,† Kazuki Ebata,† Hiroshi Sakiyama,⊥ Shunsuke Yamamoto,‡ Masaya Mitsuishi,‡ Tokuji Miyashita,‡ and Jun Matsui⊥* †Graduate
School of Science and Engineering, Yamagata University, 1-4-12 Kojirakawa-machi,
Yamagata 990-8560, Japan ⊥Faculty
of Science, 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
*Author to whom correspondence should be addressed; e-mail:
[email protected] ACS Paragon Plus Environment
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Abstract In this paper, we report a biomimetic polyelectrolyte based on amphiphilic polymer nanosheet multilayer films. Copolymers of poly (N-dodecylacrylamide-co-vinyl phosphonic acid) [p(DDA/VPA)] form a uniform monolayer at the air-water interface. By depositing such monolayers onto solid substrates using the Langmuir-Blodgett (LB) method, multilayer lamellae films with a structure similar to a bilayer membrane were fabricated. The proton conductivity at the hydrophilic interlayer of the lamellar multilayer films was studied by impedance spectroscopy under temperature- and humidity-controlled conditions. At 60 C and 98% relative humidity (RH), the conductivity increased with increasing mole fraction of VPA (n) up to 3.2 × 10-2 S cm-1 for n = 0.41. For a film with n = 0.45, the conductivity decreased to 2.2 × 10-2 S cm-1 despite the increase of proton sources. The reason for this decrease was evaluated by studying the effect of the distance between the VPAs (lVPA) on the proton conductivity as well as their activation energy. We propose that for n = 0.41, lVPA is the optimal distance not only to form an efficient two-dimensional (2D) hydrogen bonding network, but also to reorient water and VPA. For n = 0.45 on the other hand, the lVPA was too close for a reorientation. Therefore, we concluded that there should be an optimal distance to obtain high proton conductivity at the hydrophilic interlayer of such multilayer films.
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Introduction Organic proton-conducting electrolytes are important materials in various energy-harvesting systems ranging from polymer electrolyte fuel cells (PEFCs)1–3 to adenosine triphosphate (ATP) synthesis4–6 in biological membranes. These electrolytes should ideally exhibit high proton conductivity to achieve efficient energy production. For example, perfluorosulfonic acid polymers are widely used in PEFCs on account of their high proton conductivity (~ 0.1 S cm-1).7–9 One of the reasons for the high proton conductivity is the high concentration of protons in the film due to the relatively low pKa value (~ -6)10 of the super-strong perfluorosulfonic acid. On the other hand, biological membranes use conventional acids, such as phosphonic or carboxylic acid as a proton source to produce ATP. Such membranes use one- (1D) or two-dimensional (2D) proton-conductive nanochannels to efficiently transfer protons.11 In order to achieve high proton conductivity, the formation of a dimensionally controlled proton conduction path is thus very important. In this context, several groups have reported polymer electrolytes with 1D or 2D proton-conduction pathways. For example, Kato et al. have applied liquid crystal molecules to prepare a dimensionally controlled proton-conductive nanochannel. They reported that the proton conductivity in the liquid-crystalline state shows higher conductivity than in the bulk due to the uniform 1D and 2D 2 ACS Paragon Plus Environment
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proton-conduction paths.12–14 Kawakami et al., Elabd et al., and Tanaka have produced 1D protonconducting polymer nanofibers using an electrospinning process.15–19 The Nafion nanofibers exhibit a proton conductivity that is one order of magnitude higher than that in the film.17,20 Nagao et al. have reported 2D proton-conductive films using the lyotropic liquid-crystalline state of polyimide. The highly ordered lamellar structure of the polyimide film reached a conductivity of 2.6 × 10−1 S cm−1 at 25 ºC and 95% relative humidity (RH), which is one order of magnitude higher than corresponding irregularly ordered films.21–27 We have previously used polymer nanosheet multilayer films to mimic 2D proton conduction in bilayer membranes.28,29 Poly(N-dodecylacrylamide-co-acrylic acid) copolymers [p(DDA/AA)] form a stable monolayer (polymer nanosheet) at the air-water interface, and multilayer p(DDA/AA) films have been prepared on solid substrates using the LB method. At the hydrophilic interlayer of p(DDA/AA) nanosheet multilayer films, we observed a high proton conductivity of 5.9 × 10−2 S cm−1 at 60 ºC and 98% RH. Furthermore, it has been proposed that the formation of 2D hydrogenbonding networks is responsible for the high proton conductivity.30,31 In this study, we prepared phosphonic-acid-containing polymer nanosheet multilayer films and studied the interlayer proton conductivity. In general, bilayer membranes consist of >50% of phospholipids.32,33 Furthermore, it has been reported that diffusion constant of proton at the bilayer membranes is strongly 3 ACS Paragon Plus Environment
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affected by the chemical structure of lipid head group.34 Therefore, it is important to study the 2D proton conduction in the interlayer of polymer nanosheet multilayer films using phosphonic acid as a proton source. For that purpose, we synthesized a copolymer of DDA and vinyl phosphonic acid (VPA) [p(DDA/VPA)] (Figure 1) by free radical copolymerization. The monolayer properties of these copolymers were examined by surface pressure (π)-area (A) isotherms. The monolayers were transferred onto solid substrates using the LB technique to construct multilayer films. The multilayer structure was probed by X-ray diffraction (XRD) measurements. The proton conductivity at the interlayer of these multilayer films was examined by impedance measurements under humidity- and temperature-controlled conditions. We discovered an optimal distance between the VPA groups to achieve high proton conductivity.
Figure 1. Chemical structure of p(DDA/VPA). Experimental section DDA was purchased from TCI and recrystallized from chloroform/hexane prior to use. VPA was purchased from Sigma-Aldrich and used as received. The p(DDA/VPA) copolymers were synthesized 4 ACS Paragon Plus Environment
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by free radical copolymerization of DDA and VPA in ethanol at 60 ºC for 15 h, using 2, 2’azobis(isobutyronitrile) as the thermal initiator. After the copolymerization, all volatiles were removed under reduced pressure, and chloroform was added. The chloroform solution was added to a large stirred volume of acetonitrile in order to precipitate the copolymer. This precipitation process was carried out three times, before the obtained polymer was vacuum-dried at room temperature. The copolymers were characterized by FT-IR (Nicolet 6700, Thermo Fisher Scientific). Molecular weight (Mn) and poly dispersity index(Mn/Mw) were measured using Gel Permeation Chromatography (GPC) measurement (HLC-802A, Tosoh Corp. ). The polystyrene was used as a standard. The VPA content in the copolymer (n) was determined by elemental analysis. The π-A isotherm measurements and p(DDA/VPA) monolayer depositions were performed using a computer-controlled Langmuir trough (USI-3-22 Type003, USI) and ultra-pure water (18.2 MΩ; Smart 2 Pure; Thermo Fisher Scientific) as the subphase. Chloroform solutions (1 mM) of the copolymers were prepared and dropped onto the water surface of the trough. The polymer monolayers were compressed at a rate of 1.0 cm min-1 at a water temperature of 20 ºC. The glass and the silicon substrates were washed with acetone and isopropanol under sonication. Then, the substrates were treated with UV-O3 (PL16−110, SEN Lights Corp.) to render their surface hydrophilic. Subsequently, these substrates were subjected to a hydrophobic treatment, i.e., they were 5 ACS Paragon Plus Environment
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immersed in a solution of octadecyltrichlorosilane in chloroform (~1×10-6 M). An interdigitated array (IDA) was fabricated by thermally depositing Ti (2 nm) and Au (80 nm) on a hydrophobic glass substrate using a metal mask. The IDA electrode showed two pairs of comb Au arrays. Each array contained eight electrode elements (width: 0.5 mm; length: 8.2 mm; spacing between the elements: 0.2 mm). All monolayer films were deposited at a surface pressure of 40 mN/m using the LB method. XRD measurements of the films were performed on a Rigaku Smartlab diffractometer (Rigaku Corp.) using Cu-Kα radiation (λ = 0.1542 nm) and the symmetric reflection geometry (θ - 2θ) method. Impedance measurements were carried out using a 1260 impedance / gain phase analyzer equipped with a 1296 dielectric interface system (Solartron Analytical). The measurements were performed by putting a sample in a chamber (SH-221, Espec Corp.) under humidity- and temperature-controlled conditions. The bond angle and bond length of VPA were calculated at the MP2/aug-cc-pVTZ level of theory. IR spectra of VPA and condensed VPA were calculated at the MP2/6-31G*//MP2/aug-cc-pVTZ level of theory. Quartz crystal microbalance (QCM, THQ-100P-SW, Tamadevice Co., Ltd, Japan) was used to measure the adsorbed amount of water to the polymer nanosheet multilayer films. Polished 20 MHz crystals (Tama device Co., Ltd) were used for the measurements. The crystals were washed with acetone and
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isopropyl alcohol. Then the crystal was immersed into 1 mM dodecanethiol/ethanol solution to hydrophobize the surface. Results and Discussion Properties of the p(DDA/VPA) monolayers at the air-water interface The synthesis of p(DDA/VPA) copolymers with varying content of VPA (n) was achieved by simply changing the feed ratio of DDA and VPA (Table 1). The low inclusion of VPA in the copolymers at the high VPA feed ratio region is due to the lower monomer reactivity ratio of VPA (rVPA = 0.11) than that of DDA (rDDA = 0.54) (Figure S1). The synthesis of the copolymers was confirmed by FT-IR spectroscopy (Figure S2). The FT-IR spectra exhibit two absorption peaks related to the asymmetric stretching vibration of the PO-H moiety at 930 and 985 cm-1. The presence of two absorptions from PO-H was confirmed by calculations at the MP2/6-31G*//MP2/aug-cc-pVTZ level of theory (Figure S3a). It should be noted that the formation of P-O-P moieties would be accompanied by a single IR absorption at ~920 cm-1 (Figure S3b). It seems therefore feasible to neglect condensation reactions between the phosphonic acid groups. The broad absorption at 1050-1300 cm-1 was attributed to the P=O stretching vibration.35,36 Chloroform solutions of the copolymers were spread onto an air-water interface and the π-A isotherms were measured at 20 ºC. All copolymers exhibited a steep rise in surface pressure with compression of the monolayer and 7 ACS Paragon Plus Environment
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a relatively high collapse pressure (Figure 2a). These results indicate that the copolymers form a densely packed and stable monolayer. Molecular weights of the copolymers were measured by GPC using polystyrene as a standard (Table 1). It should be mentioned that accurate measurement of Mn is difficult because of the polyelectrolyte effect.37,38 It has been reported that Mn > 10,000 is required to form a stable pDDA monolayer. Therefore, we think the Mn for the copolymers exceeds 10,000.39 The average limiting surface area (Aav) of the copolymer monolayer can be determined by extrapolating the linear portion of the steep rise in the π-A isotherms to zero surface pressure. The limiting area for VPA (AVPA) in the copolymer monolayer was calculated using Aav of the copolymer monolayer and the limiting surface area of pDDA homopolymer (ADDA = 0.28 nm2/molecule)29,40 under the assumption of the additivity of molecules according to the equation (1). The additivity of molecules is well held in copolymers of pDDA.30,41,42 Aav = n AVPA + 0.28 (1-n)
(1)
where n is the VPA molar content. The AVPA values for the copolymers obtained from eq. 1 were much smaller than the value (0.25 nm2) calculated using the Corey-Pauling-Koltun space-filling model (Table 1). Therefore, we concluded that VPA is located under the water surface (Figure 2b). The negative values indicate that VPA groups were 8 ACS Paragon Plus Environment
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fully submerged below the water with partially accompanying DDA unit.
Table 1. Characterization of the p(DDA/VPA) DDA:VPA
n
feed ratio
Aav
AVPA
(nm2/ monomer unit)
(nm2/ molecule)
d (nm)
Mn
Mw/Mn
9:1
0.19
0.25
0.17
3.56
5900
1.8
8:2
0.23
0.26
0.19
3.62
4500
1.5
7:3
0.31
0.26
0.19
3.87
4100
1.4
6:4
0.36
0.17
-0.037
3.91
2400
1.4
5:5
0.41
0.19
0.10
4.13
3000
1.3
4:6
0.45
0.18
0.053
4.17
3800
1.4
3:7
0.52
0.09
-0.087
-
3100
1.4
(a)
(b) n = 0.19 n = 0.23 n = 0.31 n = 0.36 n = 0.41 n = 0.45 n = 0.52
Surface pressure (mN/m)
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DDA
alkyl chain
air
main chain amide group VPA water
Surface area
(nm2/monomer
unit)
Figure 2. (a) –A isotherms of the p(DDA/VPA) copolymers at 20 ºC. (b) Schematic model of the monolayer film at the air-water surface. 9 ACS Paragon Plus Environment
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Construction and characterization of multilayer films Copolymer monolayers were transferred onto hydrophobic Si substrates using the LB method, and the structures were characterized by XRD measurements. The copolymer of n = 0.52 could not be transferred onto solid substrates due to the strong hydrophilicity of the VPA groups. All the other copolymers could be deposited onto solid substrates with a transfer ratio of unity for upward and downward deposition resulting in Y-type LB film structures. The XRD spectra of the multilayer films showed two Bragg diffraction peaks with a peak position ratio of 1:2, which is indicative of a clear lamellar structure (Figure 3a). Moreover, the multilayer films of n = 0.19, 0.31, 0.36, 0.41, and 0.45 exhibit Kiessig fringes in the low-q area, which suggests that the surfaces of these films are relatively smooth (Figure S4).43 The bilayer spacing (d) of the multilayer films was calculated based on the first diffraction peak (Table 1). The value of d increased with increasing n. This trend is similar to that observed in our previous report using acrylic aid as a proton source.30 Thus, we concluded that the VPA groups were located at the hydrophilic interlayer. This interlayer was used as a 2D proton conduction path (Figure 3b).
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Figure 3. (a) XRD spectra of the p(DDA/VPA) nanosheet multilayer films, wherein arrows mark the second-order diffractions. (b) Schematic illustration of the p(DDA/VPA) nanosheet multilayer films.
Proton conductivity at the interlayer of the multilayer films The proton conductivity at the interlayer of the multilayer films was measured at 60 ºC and 98% RH using an impedance apparatus. Prolonged annealing under humidified conditions is necessary for the polymer nanosheet multilayer film to form highly proton-conducting nanochannels.30 Thus, we studied the impact of the annealing time on the proton conductivity. We confirmed that high proton conductivity was observed regardless of n when humidified annealing was carried out for 60 h (Figure S5). Therefore, all conductivity measurements were carried out after 60 h of humidified annealing. It should be noted that the uniform lamellar structure was maintained even after such a prolonged annealing (Figure S6). Figure 11 ACS Paragon Plus Environment
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4 shows the proton conductivity through the interlayer of the p(DDA/VPA) nanosheet multilayer films. The conductivities of p(DDA/VPA) nanosheet multilayer were higher than that of the cast film44. The proton conductivity increased with increasing n up to n = 0.41, whereafter it decreased again. The proton conductivity for n = 0.45 was 70% relative to that of n = 0.41. In order to determine the reason for the decrease of the proton conductivity, we examined the activation energy (Ea) from n = 0.36 to 0.45. Figure 5 shows the Arrhenius-type plots for the proton conductivity of the p(DDA/VPA) nanosheet multilayer films. The Ea values were calculated based on the least-squares fit of the slopes using equation: T = 0 exp (-Ea/kBT)
(2)
where is the proton conductivity, T the temperature (K), 0 the preexponential factor, and kB the Boltzmann constant. The Ea value of n = 0.36 (0.23 eV) was higher than that for n = 0.41 (0.21 eV), while that for n = 0.45 (0.25 eV) was even higher. These values indicate that protons are conducted in the p(DDA/VPA) nanosheet electrolyte via the Grotthuss mechanism.45 The Grotthuss mechanism combines two processes that are known as hopping and reorientation of molecules.46 In the hopping process, protons are transferred from a proton donor to an acceptor using a hydrogen-bonding network. Thereafter, molecules are reoriented to form the initial hydrogen-bonding network.47 Protons are conducted efficiently by repeating this hopping and reorientation process. 12 ACS Paragon Plus Environment
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60 °C 98% RH
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n
Figure 4. Proton conductivity through the interlayer of the p(DDA/VPA) nanosheet multilayer films as a function of varying n. The conductivity was measured at 60 ºC and 98% RH.
Figure 5. Arrhenius-type plots for the proton conductivity of p(DDA/VPA) nanosheet multilayer films as a function of varying n. Dotted lines represent the least-squares fits.
Recently, Yamaguchi et al. have studied the effect of the distance between the oxygen atoms in acid groups 13 ACS Paragon Plus Environment
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and molecules of H2O (ROO) on the Ea values for hopping (Ea-hop) and reorientation (Ea-re) using a model in which the sulfonic acid groups and the water molecules were confined to a layer of a few nanometers of thickness.41 Using AIMD calculations, they demonstrated that Ea-hop decreases with decreasing ROO to reach a minimum for ROO < 2.6 Å. Consequently, for the hopping process lower ROO values are desirable. On the other hand, the Ea-re values exhibit a contrasting behavior. Minimum values for Ea-re were observed for ROO > 2.6 Å, and the Ea-re values started to increase for ROO < 2.6 Å. The simulation results suggest that the optimum distance for high proton conductivity is 2.6 Å.48 In the following section, we elucidate the reason for the unique effect of n on the proton conductivity of the polymer nanosheet electrolyte using the effect of ROO on Ea-hop and Ea-re. In a previous report, it has been concluded that the formation of advanced 2D hydrogen-bonding networks (Figure 6) should be responsible for the high proton conductivity in such polymer nanosheet multilayer films.30 The amount of water adsorbed in the film was determined to about two water molecule per monomer unit by a QCM measurement (Figure S7). From the comparison of d value for pDDA and p(DDA/VPA)s, the thickness of proton conduction nanochannels are less than 8 Å. Thus, it is reasonable to assume that one water molecule presented near to one VPA unit. Therefore, this model was used to calculate the ROO values. The lVPA values were calculated using the following equation: 14 ACS Paragon Plus Environment
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𝑙𝑉𝑃𝐴 = 2 ×
𝐴𝑎𝑣 𝑛 × 𝑃𝑖
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(3)
according to a previous report.30 Furthermore, a water molecule was located between two VPA moieties so that the oxygen atom is located at the middle of lVPA . Furthermore, the P=O moiety and the O-H moiety of water were aligned (Figure 6). The distance between the oxygen atom of the phosphonic acid moiety and that of the water molecule was calculated using the following equation: 𝑅𝑜𝑜 =
𝑙𝑉𝑃𝐴 2 × cosφ
― 𝑙𝑃 = 𝑂
(4)
where is 25.8˚ and lP=O is 1.47 Å (Figure S8).49 ROO values of 2.9, 2.8, and 2.5 Å were calculated for n = 0.36, 0.41, and 0.45, respectively (Figure 7). The increase of proton conductivity from 0.36 to 0.41 was rationalized in terms of a formation of a stronger hydrogen-bonding network by decrease of ROO from 2.9 to 2.8 Å. The formation of the stronger hydrogen-bonding network induces a decrease of Ea-hop. In this region, Ea-re was considered to be constant because ROO is sufficiently long. Accordingly, the total Ea values, which are a sum of Ea-hop and Ea-re, decreased from 0.23 to 0.21 eV. In contrast, the ROO values for n = 0.41 (2.8 Å) and n = 0.45 (2.5 Å) suggest that VPA and water molecules are sufficiently close to form strong hydrogen-bonding networks. Thus Ea-hop was considered to be constnat.41 Meanwhile, the Ea-re values increased with decreasing distance from 2.8 to 2.5 Å as the reorientation of molecules was restricted by steric hindrance.41 Eventually, the Ea value increased from 0.21 to 0.25 eV. In other words, when the 15 ACS Paragon Plus Environment
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concentration of proton sources in a polymer nanosheet is very high (the distance of proton source is too close), the reorientation of molecules becomes the proton-conductivity determined process. The calculated ROO values obtained from the highest proton-conductive polymer nanosheet electrolyte (2.8 Å) is in good agreement with the optimum ROO distance obtained from the simulations (2.6 Å), which supports the effectiveness of our proposed mechanism. The present ROO value (2.8 Å) for the highest conductive film is slightly longer than the theoretical calculation. The conductivity for the copolymer multilayer (n = 0.41) is relatively high with low activation energy. Thus, we think the conductivity will be only slightly increase even the ROO value becomes 2.6 Å. The dependence of Ea to ROO values in the all n was shown in Figure
S9.
ROO
lP=O
H O P C
lVPA
Figure 6. Model for the 2D hydrogen-bonding network at the interlayer of the p(DDA/VPA) nanosheet multilayer films. ROO refers to the distance between the O atoms of the P=O moieties and those of H2O, while lVPA denominates the distance between VPA moieties. Green dotted lines represent hydrogenbonding. 16 ACS Paragon Plus Environment
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Ea (eV)
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log [ σ ( S cm-1 ) ]
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ROO (Å)
Figure 7. Proton conductivity and Ea values for each ROO value of the p(DDA/VPA) nanosheet multilayer films.
Conclusions Polymer electrolytes prepared from p(DDA/VPA) nanosheet multilayer films are reported. Copolymers of p(DDA/VPA) with varying contents of VPA (n) were synthesized by free radical copolymerization. All copolymers formed stable monolayers at the air–water interface. The monolayers of copolymers with n = 0.19-0.45 could be transferred onto solid substrates using the Langmuir-Blodgett technique. X-ray diffraction spectra of the transferred films showed two sharp Bragg diffraction peaks with a peak position ratio of 1:2, which is indicative of a clear lamellar structure. The proton conductivity of the multilayer 17 ACS Paragon Plus Environment
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films for n = 0.41 achieved a maximum value of 3.2 10-2 S cm-1 that was comparable to that of the Nafion membrane. Interestingly, the conductivity of n = 0.45 decreased, even though the VPA concentration is 1.1 times higher than that of 0.41. Based on the Ea and ROO values determined by π-A isotherms, we concluded that the lVPA value for the copolymer with n = 0.45 was too small to allow efficient reorientation of the molecules to reform the 2D hydrogen-bonding network. As far as we concerned, this is the first experimental result to prove that too high concentration of acid groups has a negative effect to proton conduction via Grotthuss mechanism due to the increase of steric hinderance for reorientation of hydrogen bonding. These results should thus constitute a new design concept for polymer electrolytes that use 2D nanospaces as proton-conducting channels.
Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Plot for VPA feed ratio to copolymer composition, FT-IR spectra, calculated FT-IR spectra, XRD spectra, the relationship between conductivity and humidification time as well as XRD spectra after annealing at 60 ºC (98% RH) for more than 120 h, water uptake dynamics, molecular model for VPA, effect of ROO to Ea 18 ACS Paragon Plus Environment
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and conductivity (PDF). Author Information Corresponding Author E-mail:
[email protected]. ORCID Hiroshi Sakiyama: 0000-0001-8285-3362 Shunsuke Yamamoto: 0000-0002-6854-2477 Masaya Mitsuishi: 0000-0002-7069-9860 Jun Matsui: 0000-0003-4767-4507
Notes The authors declare no competing financial interests.
Acknowledgement This work was supported by the Japan Society for the Promotion of Science (JSPS) via a Grant-in-Aid for Scientific Research B (18H02026) and the Research Program “Dynamic Alliance for Open Innovation 19 ACS Paragon Plus Environment
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Bridging Human, Environment and Materials” within the “Network Joint Research Center for Materials and Devices”. We thank Profs. Kurihara and Ishizaki for their help with XRD measurements.
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TOC 41% VPA
nano space
HPO3-
H+
HPO3- HPO3H+ HPO3
-
HPO3
high proton conductivity !! 45% VPA
→
～ cm
too close
polymer nanosheet multilayer film
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