Ionic Conduction in Nanostructured Membranes Based on

Article pubs.acs.org/Macromolecules

Ionic Conduction in Nanostructured Membranes Based on Polymerized Protic Ionic Liquids Yanika Schneider,†,‡ Miguel A. Modestino,†,‡ Bryan L. McCulloch,†,‡ Megan L. Hoarfrost,†,‡ Robert W. Hess,† and Rachel A. Segalman*,†,‡ †

Department of Chemical and Biomolecular Engineering, University of California, Berkeley, California 94720, United States Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States

‡

S Supporting Information *

ABSTRACT: Polymerized ionic-liquids (PILs) are promising materials whose ionic properties can be tuned based on their chemistry. By incorporating PILs into block copolymer (BCP) structures, it is possible to provide complementary functionality (i.e., structural stability) and transport tunability to ionconducting materials. In this study, we describe the selfassembly and conductivity of novel poly(styrene-blockhistamine methacrylamide) diblock copolymers (PS-bPHMA) and the resulting PS-b-PIL derivatives obtained after treatment with trifluoroacetic acid (TFA). These materials selfassemble into ordered BCP structures with tunable domain sizes as demonstrated by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). PS-b-PHMA membranes show conductivities up to 2 × 10−4 S/cm at room temperature, which increase by an order of magnitude in the presence of acid. In addition, both PHMA- and PIL-based membranes exhibit lower water uptake (λ = 4−6 and 8−10, respectively) in comparison with most proton conducting membranes reported elsewhere. The low water content in these membranes translates into a stronger effect of morphology on transport behavior, resulting in a measurable increase in ion conductivity as a function of conducting channel size.

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the ion conducting groups to the polymer backbone resulting in a polymerized ionic liquid (PIL).9−12 This work describes the development of a novel BCP system containing a protic PIL block. We also demonstrate that this system exhibits ionic conductivities greater than 10−4 S/cm, which can be tuned by adjusting the domain size and volume fraction of the conducting block. Such insight into the fundamental structure−property relationships that govern ion conduction is critical for developing the next generation of high performance materials. Although anion-conducting PILs have been studied extensively in the literature,13−15 protic polymerized ionic liquids have received less attention due to their decreased solubility and the synthetic difficulty in preparing these materials in a controlled manner.16 Most proton conducting polymer systems are based on materials with tethered acid groups (e.g., sulfonic acid) that intrinsically allow for lower synthetic tunability in comparison with PILs as well as polymer blends with strong acids or hydrogen-bond forming molecules.17−28 Although proton conducting systems are important for many applications that require the use of acidic media, there are only a few examples of protic PIL-containing systems in the literature.29 In

on-conducting membranes are important components in a variety of energy conversion devices, as they allow for the transport of ionic species across physically separated reaction sites.1 Common uses for these types of membranes include fuel cells and artificial photosynthesis devices, wherein ions are transported from the oxidation to reduction sites while providing structural stability and gas barrier properties to the system.2 In general, ionic transport and structural properties of these materials are coupled, and factors that enhance ionic conductivity (e.g., water content) also impact their structural stability. Block copolymers (BCPs) represent a promising route to decouple these two properties, wherein different blocks can be designed to provide complementary structural properties and ionic conductivity. Furthermore, properties of these systems can be easily tuned and optimized by altering the molecular weight and volume fraction of each phase.3 Membranes based on blends of ionic liquids (ILs) and BCPs have gained significant interest because of their high ionic conductivity, as well as thermal, chemical and electrochemical stability.4−8 BCP-IL blends have also been reported to exhibit selective gas permeability, which can be easily tuned by the chemical nature and size of the IL.9 ILs can be selected such that they are confined within one phase of the BCP, thereby creating nanometer length-scale ion conducting channels in a structural matrix. An improvement in the stability of ILcontaining membranes in water can be achieved by tethering © 2013 American Chemical Society

Received: November 29, 2012 Revised: January 23, 2013 Published: February 12, 2013 1543

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Scheme 1. Preparation of Ionic Liquid Membranes

high Tg of approximately 200 °C, due to the presence of bulky side groups in addition to the strong hydrogen bonding interactions between polymer side chains. All the structural characterization of these copolymers, including their 1H NMR, FT-IR, GPC and TGA data, is described in great detail in the Supporting Information (Figures S1−S6) and the physical properties of these materials are summarized in Table S1, Supporting Information. The BCPs described above have the ability to self-assemble into ordered structures with tunable domain sizes. This high degree of control is important for understanding the effect of morphological properties on the ionic conductivity of these materials. Small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) were used to characterize the solid-state structure of PS-b-PHMA copolymers. Hotpressed polymer samples were annealed above Tg (210 °C) under vacuum (10−3 Torr) to aid the self-assembly of the polymers. Azimuthally integrated SAXS profiles of representative samples are provided in Figure 1a. Scattering patterns corresponding to hexagonally packed PHMA cylinders (HEX) were observed for materials containing between 8 to 20 wt % PHMA. For polymer systems between 32 and 54 wt % PHMA, the scattering data is consistent with lamellar (LAM) morphologies. The presence of several higher order reflections in the SAXS patterns indicates a significant degree of longrange order. These results are consistent with the electron micrographs presented in Figure 1, parts b and c, revealing hexagonally packed cylinders of PHMA in a matrix of PS and lamellar morphologies, respectively. As discussed earlier, the ion-conducting system developed in this work is designed for applications that require humid operating conditions. As mechanical properties of membranes

this study, protic PIL-containing BCPs were prepared following the synthetic strategy illustrated in Scheme 1. First, the imidazole-containing block copolymer based on polystyrene (PS) and poly(histamine methacrylamide) (PHMA) was prepared using a combination of atom transfer radical polymerization (ATRP) and N-hydroxysuccinimide (NHS) chemistries. 30 The PHMA block was chosen because imidazolium-based polymers have demonstrated excellent stability and ionic conductivity.31 Next, the obtained PS-bPHMA was treated with trifluoroacetic acid to generate a protic PIL (PS-b-PIL). In this manner, ion conduction is confined to the PIL phase, while the PS phase imparts mechanical stability to the membrane. It should be mentioned that the obtained PIL contains two types of mobile charged carriers, trifluoroacetate anions and protons, and the contributions to the ionic current from each of them will depend on the operating conditions of the membrane. The conductivity values reported in this study correspond to the overall ionic conductivity of membranes equilibrated in water which includes components from each of the ions present in the PIL block. Because the BCPs described above are prepared using a controlled polymerization technique, it is possible to precisely control the molecular weight of each block and access a variety of structures and domain sizes. This high level of control allowed for the characterization of channel size and PHMA content effects on the ionic conductivity of membranes. To investigate the effect of polymer composition on the morphology and transport behavior of materials, a series of PS-b-PHMA block copolymers were synthesized. Well-defined PS-b-PHMA block copolymers were obtained with low polydispersities (PDI ≤ 1.3) and nearly quantitative NHS substitution (>95%). The PHMA block exhibits a relatively 1544

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membranes in water for 3 days. Block copolymers containing ∼53 wt % PHMA absorb 30 wt % water, whereas those containing ∼33 wt % PHMA only absorb 15 wt % water. These values correspond to 4−6 water molecules absorbed per molecule of functional group (λ) which is significantly lower than the water uptake of common proton conducting polymers such as Nafion (λ = 12−15)33 and sulfonated block copolymers (λ = 8−40).34 The observed low water uptake could be a consequence of the higher hydrophobicity of the PHMA in comparison to the systems referenced above. While hydrated PHMA domains can allow for ion conduction either via direct transport through swollen channels or hoping between imidazole groups, the formation of a PIL should result in an enhancement in conduction. PIL membranes were prepared by equilibrating hydrated PS-bPHMA membranes with an equimolar amount of trifluoroacetic acid (TFA) to PHMA. An equimolar ratio was chosen to restrict the number of charge carriers inside the conducting phase, such that the ions present in the membrane are tethered to the imidazole side chains. Membranes were washed with water repeatedly to remove the excess TFA until the pH of the washing solution was unchanged, meaning all the unassociated TFA is washed away. The NMR results demonstrate the interaction between imidazole side groups and TFA molecules (see Supporting Information). As shown in Table 1, the coordination between the imidazole functionality of PHMA and the trifluoroacetate group of TFA leads to a 20−30% increase in domain size. This increase can be explained by several factors, including an increase in χ between the PS and PIL block, the introduction of charged side groups that lead to chain stretching, as well as an increase in PHMA volume fraction due to the addition of TFA (expected to contribute to 15−20% of the domain size increase), which also results in higher water affinity of the conducting block (λ = 8−10 for PSb-PIL). The morphological control achieved in the ion-conducting membranes described above can translate into tunability of their transport properties. Accordingly, it is important to understand the relationship between the ionic transport and polymer structure in PS-b-PHMA and PS-b-PIL membranes. The ionic conductivity of membranes was investigated using AC impedance spectroscopy on hydrated membranes immersed in liquid water. Temperature-dependent data for all samples are provided in Supporting Information (Figures S8 and S9), and representative data are shown in Figure 2. Both

Figure 1. PS-b-PHMA with varying PHMA content can access different morphologies and domain spacing as demonstrated by SAXS profiles presented in part a. A TEM image of sample PS(33)PHMA(7) (b) confirms hexagonally packed PHMA cylinders in a matrix of PS, while the other micrograph (c) shows lamellae for sample PS(22)-PHMA(25). TEM contrast in the images originates from RuO4, which selectively stains the PHMA phase.

are expected to deteriorate with increasing water uptake,32 it is important to tailor their properties to reduce the level of water uptake. The presence of water is also anticipated to have a strong effect on transport properties of BCP membranes as it can be absorbed by the PHMA domain and not the PS domain. This increases the volume fraction of the conducting phase and allows for greater mobility of PHMA polymer chains, which can enhance ion conductivity. Because of the importance of water for ion conduction, the structural and transport properties of hydrated BCP membranes were evaluated. Six lamellar BCP samples, described in Table 1, were selected to assess the effect of humidity on morphology and ionic conductivity. This work limited the transport characterization to only lamellar samples, in order to understand the fundamental effects of conducting phase volume fraction and channel size on conductivity. Other polymer morphologies such as cylindrical structures were purposely not included to avoid the introduction of additional complexity into the analysis. The water uptake of the membranes was determined after equilibrating pressed polymer Table 1. Structural Properties of PS-b-PHMA Membranes membrane PS(22)PHMA(25) PS(19)PHMA(21) PS(16)PHMA(18) PS(33)PHMA(20) PS(22)PHMA(12) PS(16)PHMA(8)

Mn PSa

Mn PHMAa

wt % PHMAb

d-spacing (nm)c

PS-b-PHMA water uptake (%)

hydrated d-spacing (nm)c

hydrated channel size (nm)c

acid-doped channel size (nm)c

22

25

54

64

30

69

36

42

19

21

53

50

32

54

28

34

16

18

53

44

27

48

24

29

33

20

33

72

15

74

25

29

22

12

32

42

14

44

14

18

16

8

34

36

16

38

13

17

In kg/mol determined by GPC in DMF at 40 °C using polystyrene standards. bDetermined by 1H NMR in DMSO at 148 °C. cDetermined by SAXS using d = 2π/q* and density determination of hydrated membranes (see Supporting Information for details on calculations).

a

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systems that do not show channel size effects, the low levels of water content of the PHMA BCPs may be responsible for the observed behavior. Li+-conducting BCP systems operated under anhydrous conditions have shown similar domain size effects, wherein the ion transport is generally accepted to occur through chain motion.35,36 A low λ value for the PHMA-based systems implies that ion transport would rely more strongly on chain motion, as transport through the water phase is limited. In this case, reducing the domain size results in an increase in the number of BCP interfaces and the interfacial width, which hinders the mobility and limits the conductivity. Both of these effects can help understand the behavior observed in PS-bPHMA and PS-b-PIL membranes. It is important to point out that systems with significantly smaller domain sizes (d < 12 nm) have shown increased conductivities with decreasing domain sizes.22 The conductivity effects in these systems are due to an increase in water uptake assisted by capillary condensation or a decrease in microphase separation at low molecular weights,37 effects that are not expected to be present in the PHMA-containing systems described in this work. In conclusion, this work presents a modular system based on imidazole-containing BCPs that exhibit low water uptake (λ < 6) and ionic conductivity above 10−4 S/cm. The low water content of these materials can afford membranes with improved structural properties that prevent the transport of uncharged molecules through water channels (i.e., gases). The PHMA domains of BCPs presented here are readily transformed into a PIL with tethered ionic liquids, which can limit the ability of charged carriers to leach out of the membranes. The PIL system described in this work demonstrates improved conductivity as it incorporates a higher concentration of ionic carriers in the conducting phase and increases the mobility of the polymer chains. Furthermore, the well-defined BCP system allows for the characterization of morphological effects on ion conduction. We found that higher conductivities may be attained by increasing the size of the conducting channel. This suggests that BCPs with smaller domain sizes might exhibit decreased chain mobility due to a higher number of interfaces that lower the conductivity. The ion-conducting membranes described herein have potential for applications that require moderate ion conduction and low levels of gas permeation, such as fuel cells or solar-fuels generators. The insights gained by this work can facilitate the optimization of ion conduction and complementary properties of BCP systems, which can lead to the next generation of high performance ion conducting membranes.

Figure 2. Temperature dependence of ionic conductivity of PS(19)PHMA(21) (empty squares). Upon the addition of acid, the resulting PS-b-PIL (filled squares) exhibits more than an order of magnitude increase in conductivity.

PS-b-PHMA and PS-b-PIL exhibit an exponential increase in conductivity with temperature over a limited temperature range (20−80 °C), consistent with Arrhenius behavior. Similar slopes are observed for all PS-b-PHMA membranes, indicating that the activation energy and transport mechanisms do not change significantly as a function of channel size. In the case of PS-bPIL, the conductivities of the membranes are roughly an order of magnitude higher than those of their PS-b-PHMA counterparts. This effect is likely due to a combination of an increase in the number of charge carriers inside the conducting phase, an increase in polymer mobility as a result of a decrease in Tg, and an increase in water content. In addition to conductivity effects arising from the introduction of ionic side chains, the block copolymer nanostructure also has an effect on conductivity. Previous studies have demonstrated that the volume fraction of conducting domains directly affects the conductivity of proton conducting polymers,5,8 whereas changes in the domain size alone do not significantly impact the ion transport behavior. Figure 3 demonstrates that for the PHMA- and PIL-based systems, the conductivities are affected by the size of the conducting channel (for a fixed volume fraction of PHMA or PIL), and not exclusively by the volume fraction of conducting domains. In contrast to other hydrated or IL-swollen polymer

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EXPERIMENTAL SECTION

Synthesis of PS-b-PHMA. The detailed synthetic procedure is provided in the Supporting Information. PS-b-PHMA copolymers were prepared using the activated ester strategy. First, PS-Br macroinitiators were synthesized by polymerizing styrene via ATRP. Chain extension of PS-Br with N-methacryloxysuccinimide (MASI) under similar polymerization conditions afforded precursor PS-bPMASI copolymers with narrow polydispersity indices (1.1−1.3). Finally, precursor copolymers were treated with histamine HCl in the presence of triethylamine to generate the imidazole containing block copolymers. 1H NMR analysis demonstrated that the functionalization reaction was nearly quantitative (95−98%). Preparation of PS-b-PHMA and PS-b-PIL Membranes. Here, 250 μm thick PS-b-PHMA membranes were prepared by hot-pressing at 185 °C and annealing at 210 °C under a vacuum of 10−3 Torr for 24 h. To prepare PS-b-PIL membranes, PS-b-PHMA membranes (50 mg, 0.015 mmol PHMA) were placed in a 20 mL vial containing Milli-Q

Figure 3. Room temperature (25 °C) ionic conductivity as a function of channel size for a series of block copolymers with different PHMA mass fractions. These results indicate that the size of conducting domains has a significant effect on ionic conductivity of both PS-bPHMA and PS-b-PIL membranes. 1546

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water (15 mL) and TFA (18 mg, 0.016 mmol), and allowed to equilibrate for 3 days. After equilibration, the membranes were removed from the vial and placed in a different vial containing pure DI water to remove excess TFA that did not coordinate with the PHMA functionalities. The water was replaced two more times until the pH of the solution was roughly equivalent to that of the Milli-Q water (5.7). Table 1 describes the structural and water uptake properties of the membranes studied in this work. Water Uptake Measurements. 250 μm thick membranes were prepared as described above. The films were dried in a 10−3 Torr vacuum oven at 150 °C for 48 h, and the dry weights were recorded immediately after removal from the vacuum oven. Hydrated membranes were prepared by equilibration in Milli-Q water for 3 days. Water uptake, WU, was calculated from: WU = (Wwet − Wdry)/ Wdry × 100, where Wwet and Wdry are the weights of the hydrated and dried membranes, respectively. Morphological Characterization. Dry polymer samples were prepared by hot-pressing PS-b-PHMA into 0.5 mm thick disks at 185 °C and annealing at 210 °C under a vacuum of 10−3 Torr for 24 h. Hydrated and PIL samples were placed in a sealed sample holder containing water and equipped with X-ray transparent Kapton windows. SAXS was performed at beamline 7.3.3 of the Advanced Light Source (ALS) using X-rays of wavelength λ = 1.240 Ǻ focused on a 50 by 300 μm spot. Full two-dimensional scattering patterns were collected using a 2D Dectris Pilatus 1 M charge-coupled device (CCD) detector (981 × 1043 pixels). The scattering patterns were radially averaged using Nika version 1.58.38 TEM samples were prepared by embedding the 0.5 mm polymer disks in epoxy resin. A microtome knife was used to generate 100 nm thick films, which were then exposed to RuO4 vapor for 15 min. Bright-field images were obtained on a JEOL 200CX microscope operating at an accelerating voltage of 200 kV. Ionic Conductivity Measurements. 250 μm thick membranes were prepared as described above. In-plane AC impedance spectroscopy was performed using a four-point probe BT-110 conductivity cell (Scribner), which was interfaced with a Solartron 1260 Impedance Analyzer. The membranes were placed in a cell and immersed in 300 mL of Milli-Q water. An alternating current with an amplitude of 10 mV was applied in the frequency range of 0.1 to 10 MHz. Nyquist plots of the imaginary versus real impedance were analyzed to find the membrane resistance. Ionic conductivity was calculated using the equation: σ = L/RWd, where σ is the ionic conductivity (S/cm), L is the distance between the two platinum wires (0.425 cm), W is the thickness of the membrane, and R is the membrane resistance.

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SSRL. The authors also thank Guillaume Sudre for helpful discussions. This work made use of facilities at the Advanced Light Source (ALS) and the National Center for Electron Microscopy (NCEM), both supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy (Contract No. DE-AC0205CH11231). Additional SAXS data was acquired at the Stanford Synchrotron Radiation Laboratory (SSRL), a national user facility operated by Stanford University. This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed synthesis, 1H NMR, FT-IR, GPC, and TGA characterization of materials, conductivity data, and other information. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 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 We gratefully acknowledge Dr. Alexander Hexemer, Steven Alvarez, and Dr. Eric Schaible for experimental assistance at the ALS as well as Dr. John Pople for experimental assistance at the 1547

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