Ionic Conductivity of Polyelectrolyte Hydrogels - ACS Applied

Jan 31, 2018 - First, 3 mmol CBAA, 0.15 mmol MBAA, and 10 mg of 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone were dissolved in 2 mL of ethan...
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Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Ionic Conductivity of Polyelectrolyte Hydrogels Chen-Jung Lee,† Haiyan Wu,† Yang Hu,‡ Megan Young,‡ Huifeng Wang,‡ Dylan Lynch,‡ Fujian Xu,§ Hongbo Cong,*,† and Gang Cheng*,‡ †

Department of Chemical and Biomolecular Engineering, University of Akron, Akron, Ohio 44325, United States Department of Chemical Engineering, University of Illinois at Chicago, Chicago, Illinois 60607, United States § Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education and Beijing Laboratory of Biomedical Materials, Beijing University of Chemical Technology, Beijing 100029 China ‡

ABSTRACT: Polyelectrolytes have many important functions in both living organisms and man-made applications. One key property of polyelectrolytes is the ionic conductivity due to their porous networks that allow the transport of water and small molecular solutes. Among polyelectrolytes, zwitterionic polymers have attracted huge attention for applications that involve ion transport in a polyelectrolyte matrix; however, it is still unclear how the functional groups of zwitterionic polymer side chains affect their ion transport and swelling properties. In this study, zwitterionic poly(carboxybetaine acrylamide), poly(2-methacryloyloxyethyl phosphorylcholine), and poly(sulfobetaine methacrylate) hydrogels were synthesized and their ionic conductivity was studied and compared to cationic, anionic, and nonionic hydrogels. The change of the ionic conductivity of zwitterionic and nonionic hydrogels in different saline solutions was investigated in detail. Zwitterionic hydrogels showed much higher ionic conductivity than that of the widely used nonionic poly(ethylene glycol) methyl ether methacrylate hydrogel in all tested solutions. For both cationic and anionic hydrogels, the presence of mobile counterions led to high ionic conductivity in low salt solutions; however, the ionic conductivity of zwitterionic hydrogels surpassed that of cationic and ionic hydrogels in high salt solutions. Cationic and anionic hydrogels showed much higher water content than that of zwitterionic hydrogels in deionized water; however, the cationic hydrogels shrank significantly with increasing saline concentration. This work provides insight into the effects of polyelectrolyte side chains on ion transport. This can guide us in choosing better polyelectrolytes for a broad spectrum of applications, including bioelectronics, neural implants, battery, and so on. KEYWORDS: polyelectrolyte, ionic conductivity, hydrogel, zwitterionic, cationic, anionic

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INTRODUCTION Polyelectrolytes, composed of repeating electrolyte units, combine the properties of both electrolytes and polymers. Depending on the type of the electrolyte group, polyelectrolytes can be classified into three groups: polycations, polyanions, and polyampholytes (also called zwitterionic polyelectrolytes) that carry both cationic and anionic repeating groups. Owing to the unique property of interacting with salts, solvents, and macromolecules, polyelectrolyte hydrogels have drawn enormous attention and have been used as critical components for a range of applications, including electronic devices,1,2 tissue engineering scaffolding,3 coatings,4 batteries,5 fuel cells,6 water purification,7 and drug delivery.8−10 For example, Tarek and Paula reported a fuel cell using a poly(ethylene oxide)/poly(acrylic acid) (PEO/PAA) composite membrane, which showed a maximum power density close to that of some commercial products. In a fuel cell, polyelectrolyte (PE) membrane sandwiched between bipolar plates plays an important role, which requires high ionic conductivity and the ability of preventing the crossover of fuel© XXXX American Chemical Society

cell gases for its good performance. In their works, many composite membrane systems fabricated by layer-by-layer (LBL) deposition of different PE couples on the supporting substrate had been reported, such as LPEI/PSS, PDME/ PAMPS, and PEO/PAA (LPEI, linear polyethyleneimine; PSS, poly(styrene sulfonic acid, sodium salt); PDME, poly(dimethylamine-co-epichlorohydrin); PAMPS, poly(2-acrylamido-2-methyl-1-propanesulfonic acid)). Although the hydrophilicity of composite membrane is essential to conductivity, the stability of the LBL film and the size of the polymer molecule can also affect the performance of the fuel cell. Because different polyelectrolyte couples show different physical and chemical properties, finding the right PE couple is vital to optimizing the performance.11 Recently, zwitterionic polyelectrolytes, polycarboxybetaine,12−15 polysulfobetaine,13,16 and polyphosphobetaine17,18 Received: October 19, 2017 Accepted: January 18, 2018

A

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reference material. Through this comprehensive and systematic study, we can gain a better understanding of the effects of electrolyte groups of polymers on the ion transport of various salt solutions.

have become very attractive for a broad spectrum of biological applications due to their unique properties. Zwitterionic polyelectrolytes have the strongest inter- and intramolecular attractions under low ionic strength conditions due to the shorter distance between cations and anions. The outstanding hydration property has drawn special interest in polyampholytes for biomedical applications. Recent studies also found that zwitterionic polyelectrolytes can tightly bind water, which leads to strong hydration,19 biocompatibility,20 antifouling properties,12−14,21 and compatibility with biomacromolecules.22 Among all of the properties of polyelectrolytes, ionic conductivity is an important parameter for bioelectronics, neural tissue engineering, electrochemical sensing, and so on because ions function as the charge carriers and the ionic mobility in polyelectrolyte directly affects the performance and sensitivity of the devices. A previous study discovered that small molecular zwitterions can significantly increase the conductivity of ionic liquids,23 and it leads to the question whether the immobilized zwitterionic functional groups will affect the ionic conductivity of an electrolyte solution. Additionally, Peng and his colleagues developed a graphene-based solid-state supercapacitor with zwitterionic sulfobetaine hydrogel electrolyte, which showed a significant improvement in the electrochemical performance.24 The abilities of gel electrolytes, such as transporting ions and separating and binding the electrodes, are important for solid-state supercapacitors. This zwitterionic gel electrolyte enhanced the capacitance, rate capacity, and durability of the supercapacitor compared to those of the poly(vinyl alcohol) (PVA) gel electrolyte, which is widely used in solid-state supercapacitors. The improvement was attributed to higher ionic conductivity, which is due to the synergetic effect of high water retention ability and the ion migration channel.24 Although some cationic or anionic polyelectrolytes have been studied for their ion transport properties, the effect of zwitterionic side chains of the polyelectrolytes on ion transport is still unclear. A deeper understanding of how the electrolyte groups of polyampholytes affect the conductivity of polyampholytes is critical to design new ionic conducting materials for biomedical, environmental, and energy applications. In addition to the polyelectrolytes, nonionic poly(ethylene glycol) (PEG) has been widely used as a matrix for nearly all applications that were previously mentioned. Because PEG is considered to be noninflammatory and nonimmunogenic, it has been used in a variety of neural and bioelectrochemical applications, including neural regeneration scaffolds,25 electrochemical biosensors,26 or coating for neural probes,27 to function as the antibiofouling moiety. Although PEG materials are assumed to not interfere with the ion transport that is critical in these applications, no systematic study has been carried out to support such an assumption. The objective of this study is to understand how the functional electrolyte group of polyelectrolyte side chains affects the ionic conductivity of polyelectrolytes. Cationic, anionic, and zwitterionic polyelectrolytes were studied in the form of hydrogels to observe the change in ionic conductivity in various salt solutions with different concentrations. Three types of zwitterionic polyelectrolytes, poly(carboxybetaine acrylamide) (PCBAA), poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC), and poly(sulfobetaine methacrylate) (PSBMA), were selected because they represent a group of the most promising polyelectrolyte biomaterials. Poly(oligo-ethylene glycol) methyl ether methacrylate (PEGMA) was used as a

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EXPERIMENTAL SECTION

Materials. 2-Methacryloyloxyethyl phosphorylcholine (MPC), sulfobetaine methacrylate (SBMA), poly(ethylene glycol) methyl et her m ethacr ylat e ( PEGMA) , [2 - ( a cr y l o y l o xy ) e t h yl ] trimethylammonium chloride (TMA), 2-acrylamido-2-methyl-1-propanesulfonic acid sodium (AMPS), 2-aminoethyl methacrylate hydrochloride (AEMA), 2-(dimethylamino)ethyl methacrylate (DMAEMA), methacrylic acid (MAA), N,N′-methylenebisacrylamide (MBAA), 2hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone, tetraethylene glycol dimethacrylate (EG), methyl bromoacetate and acetonitrile, sodium chloride (NaCl), magnesium chloride (MgCl2), magnesium sulfate (MgSO4), and phosphate-buffered saline (PBS, 7.4) were purchased from Sigma-Aldrich (St. Louis, MO). N-(3Dimethylaminopropyl)acrylamide was purchased from TCI America (Portland, OR). Carboxybetaine acrylamide or 2-((3acrylamidopropyl)dimethylammonio)acetate (CBAA) was synthesized using a previously published method.28 Synthesis of the Hydrogel. All hydrogels except Zw-PSBMA-EG were synthesized with the same molar concentration of monomer, the cross-linking agent (MBAA), and the photoinitiator (2-hydroxy-4′-(2hydroxyethoxy)-2-methylpropiophenone).29 For Zw-PSBMA-EG, tetraethylene glycol dimethacrylate was used as the cross-linking agent. The following is a brief description of the synthesis of hydrogels using CBAA as an example. First, 3 mmol CBAA, 0.15 mmol MBAA, and 10 mg of 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone were dissolved in 2 mL of ethanol/water mixture (v/v, 3/7). Two quartz slides were clipped on both sides of a poly(tetrafluoroethylene) (PTFE) mold. The PTFE mold has an interior dimension of 5.9 cm × 1.2 cm × 0.33 cm and a small opening on the top for solution injection. The well-mixed prepolymerized solution was injected with a syringe into the mold, and the opening on the top was sealed with parafilm. The polymerization was carried out under a 365 nm UV lamp (UVL-28 EL series UV lamp, UVP Inc.) for 2 h. The obtained hydrogel was immersed in deionized (DI) water that was changed every 12 h for 3 days to remove unreacted monomers. Then, the hydrogel was punched into a cylindrical shape by a biopsy punch with an 8 mm inner diameter. Equilibrium Water Content Assay. The hydrogel sample was first equilibrated in DI water for 72 h. Then, the wet weight of the hydrogel sample was measured after the removal of excess water. The dry weight of each hydrogel was recorded after the sample was freezedried for 48 h. The equilibrium water contents of the hydrogels were calculated by (wet weight − dry weight)/wet weight × 100%. Volume Ratio of the Hydrogels Assay. The thickness and diameter of the hydrogel samples were measured using an AOS digimatic caliper (Mitutoyo, Taiwan). The volume ratio of the hydrogels was calculated by the following equation: volume ratio (%) =

V × 100 V0

where V is the volume of the tested hydrogel in various saline solutions and V0 is the volume of the tested hydrogel in DI water. Ionic Conductivity Assay. Four different electrolytes (NaCl, MgCl2, MgSO4, and PBS) were selected for ionic conductivity measurements. All hydrogel samples were equilibrated for 24 h in three different concentrations (2, 10, and 100 mM) of NaCl, MgCl2, and MgSO4 solutions. After measuring the thickness and diameter, the hydrogel samples were sandwiched between two 316 stainless steel electrodes for electrochemical impedance spectroscopy (EIS) measurements using a Gamry Reference 600 potentiostat. The EIS measurements with a frequency range from 0.1 Hz to 100 kHz were conducted under open-circuit conditions with an excitation voltage of 10 mV. Software ZView (Scribner) was used to fit the impedance data with the Randles equivalent circuit, which contains an electrolyte B

DOI: 10.1021/acsami.7b15934 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. Chemical structure of monomers and cross-linkers: (A) 2-((3-acrylamidopropyl) dimethylammonio)acetate (CBAA), (B) 2methacryloyloxyethyl phosphorylcholine (MPC), (C) [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide (SBMA), (D) poly(ethylene glycol) methyl ether methacrylate (PEGMA), (E) [2-(acryloyloxy)ethyl]trimethylammonium chloride (TMA), (F) 2-acrylamido-2methyl-1-propanesulfonic acid sodium (AMPS), (G) 2-aminoethyl methacrylate hydrochloride (AEMA), (H) 2-(dimethylamino)ethyl methacrylate (DMAEMA), (I) methacrylic acid (MAA), (J) N,N′-methylenebisacrylamide (MBAA), and (K) tetraethylene glycol dimethacrylate (TEGDMA).

Table 1. Water Content of Hydrogels in Deionized Water Zw-PCBAA water content (%)

91.5

Zw-PMPC Zw-PSBMA 88.7

74.3

Zw-PSBMA-EG

PEGMA

Cat-PAEMA

84.1

87.6

95.6

97.3

93.6

An-PAMPS

An-PMAA

97.4

97.7

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resistance (Rs) in series with the parallel combination of a constant phase element and a charge transfer resistance (Rct). The obtained Rs values were used to calculate the ionic conductivities of the hydrogel samples using the following equation

σ=

Cat-PDMAEMA Cat-PTMA

RESULTS AND DISCUSSION As we can see in Figure 1, four types of chemically cross-linked hydrogels, zwitterionic hydrogels (Zw-PCBAA, Zw-PMPC, ZwPSBMA, and Zw-PSBMA-EG), cationic hydrogels (Cat-PTMA, Cat-PAEMA, and Cat-PDMAEMA), anionic hydrogels (AnPAMPS and An-PMAA), and nonionic hydrogel (PEGMA), were synthesized by the copolymerization of the corresponding monomer and the cross-linker. MBAA was used as the crosslinker for all hydrogels except Zw-PSBMA-EG hydrogel, for which TEGDMA was used, and the molar ratio of the crosslinker to the monomer was kept constant at 5%. All hydrogels are transparent except for Zw-PSBMA hydrogel. Zw-PSBMA hydrogel was opaque white in color due to the formation of aggregates and clusters, which can inhibit the movement of dipoles.30 The high water content is an important parameter for hydrogels, which enables similar chemical/physical properties

l Rs × A

where σ is ionic conductivity, A is the cross-sectional area of the sample, and l is the thickness of the sample. The conductivities of the solutions were measured using a VWR Digital Conductivity Meter (Radnor, PA). All experiments were repeated three times. Statistical Analysis. Data was analyzed using single-factor analysis of variance (ANOVA), and results were reported as mean ± standard deviation from three replicates. Student’s t test was used for the comparison among groups. It was considered statistically significant if a p value is less than 0.05. C

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Figure 2. Volume ratio of neutral and ionic hydrogels in various solutions. Neutral hydrogels in (A) deionized water, NaCl solution, and PBS; (C) MgSO4 solution; and (E) MgCl2 solution. Ionic hydrogels in (B) deionized water, NaCl solution, and PBS; (D) MgSO4 solution; and (F) MgCl2 solution. (Standard deviation is less than 5%.)

their volumes did not change significantly in low-concentration salt solutions (2 and 10 mM), except that the volume of the Zw-PSBMA-EG hydrogel increased with increasing saline concentration. The significant difference of volume change between Zw-PSBMA and Zw-PSBMA-EG hydrogels in PBS and high ionic strength solutions (>100 mM of NaCl and MgCl2) might be caused by the higher hydrophilicity and the longer chain length of the TEGDMA cross-linker compared to those of MBAA. Interestingly, Zw-PSBMA-EG hydrogels shrank in MgSO4 solutions; however, the mechanism of this phenomenon is unclear. Zwitterionic hydrogels are neutral in DI water and keep the balanced charges with the addition of salts. Dissolved ions shield the charges, reduce the electrostatic attraction, and lead to the swelling of zwitterionic hydrogels. On the contrary, except for Cat-PDMAEMA hydrogels, cationic hydrogels and anionic hydrogels shrank significantly with increasing salt concentration (Figure 2B,D,F). This can be attributed to the reduction of the electrostatic repulsion between the charged groups of the ionic polymer with the addition of salts. The swelling of Cat-PDMAEMA in PBS might be caused by the high binding affinity of multivalent phosphate ions onto the polyelectrolyte chains. High MgCl2 solutions may also lead to an adsorption of chloride ions onto the polyelectrolyte chains and cause the reswelling. The ionic conductivity of all hydrogel samples was measured by impedance spectroscopy using a potentiostat (Figures 3 and 4). In DI water, all ionic hydrogels showed much higher conductivity than that of DI water and nonionic and zwitterionic hydrogels have lower conductivity than cationic and anionic hydrogels. Cationic and anionic hydrogels have high concentrations of counterions, and the mobile counterions

with tissues and biological systems. Higher water content has also been found to increase the ionic conductivity of hydrogel and leads to high ion transfer rate.31 In this study, the water contents of different hydrogels were measured and are shown in Table 1. Cationic and anionic hydrogels showed much higher water content (>93%) than that of zwitterionic hydrogels in DI water. For cationic or anionic cross-linked hydrogels, the stronger electrostatic repulsion between cationic or anionic side chains leads to a stretched state and larger pore size.32 On the contrary, the mixed anionic and cationic groups of zwitterionic polymers have a stronger electrostatic attraction in DI water, leading to collapse of polymer chains and smaller pores. Among zwitterionic hydrogels with the MBAA cross-linker, the water content of Zw-PCBAA hydrogel was the highest (91.5%), followed by the moderate Zw-PMPC hydrogel (88.7%) and the lowest Zw-PSBMA hydrogel (74.3%). Owing to the same synthetic strategy and cross-linker, all three hydrogels possess similar cross-linking density. According to the reported literature,33 the order of hydrophilicity is PCBAA > PMPC > PSBMA. This indicates that the inherent hydrophilicity of constituent polymer mostly determines the water content of the hydrogel. Because the water content is an important factor for hydrogel conductivity,31 to increase the water content of PSBMA hydrogel, we synthesized Zw-PSBMA-EG hydrogel, which used more hydrophilic TEGDMA as the cross-linker. The water content of Zw-PSBMA-EG increased to 84.1% from 74.3% by switching the cross-linker from MBAA to TEGDMA. As shown in Figure 2, the volume changes of hydrogels in different saline solutions were further evaluated and the volume of the hydrogel in each condition was normalized to the volume of the same hydrogel in DI water. For zwitterionic hydrogels, D

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Figure 3. Ionic conductivities of neutral hydrogels in (A) DI water and 2 mM electrolyte solutions, (B) 10 mM electrolyte solutions, and (C) 100 mM electrolyte and PBS solutions. *p < 0.05.

Figure 4. Ionic conductivities of ionic hydrogel in (A) DI water and 2 mM electrolyte solutions, (B) 10 mM electrolyte solutions, and (C) 100 mM electrolyte and PBS solutions.

function as charge carriers and lead to high conductivity. Among cationic hydrogels, Cat-PDMAEMA has the lowest conductivity because it has no added counterion. However, Cat-PDMAEMA becomes partially ionized in DI water and generates hydroxide counterions that serve as the charge carrier. Among zwitterionic hydrogels, the ionic conductivity of Zw-PCBAA remains the highest and Zw-PMPC exhibits a lower ionic conductivity than that of Zw-PSBMA, which is different from the behavior in other tested electrolyte solutions. This phenomenon may be caused by the different capability of inducing water dissociation of zwitterionic molecules. Zwitterionic hydrogels also have much higher conductivity than the nonionic PEGMA hydrogel. An increase in the water content of hydrogels usually increases molecular mobility, and the water content of hydrogel is affected by many factors such as crosslinking density and hydrophilicity.34 However, although the water content of Zw-PSBMA hydrogel was lower than that of PEGMA, the ionic conductivity of Zw-PSBMA was statistically significantly higher than that of PEGMA. Our previous study also observed that poly(carboxybetaine thiophene) hydrogel has much higher ionic conductivity than the reported PEG/ poly(thiophene) hydrogels in DI water.15 This phenomenon is also likely caused by the highly polar side groups in zwitterionic

polymers that promote fast ion dissociation and transportation.35 For zwitterionic hydrogels (Figure 3), Zw-PCBAA and ZwPMPC showed higher ionic conductivities than that of ZwPSBMA in various electrolyte solutions (except in DI water and 2 mM NaCl solution). To increase the water content of ZwPSBMA hydrogel, more water-soluble TEGDMA was used as the cross-linker to replace MBAA. Owing to the different crosslinker, Zw-PSBMA-EG hydrogel was much more elastic and had a higher water content (84.1%) than Zw-PSBMA (74.3%). As shown in Figure 3A, Zw-PSBMA-EG hydrogel shows similar ionic conductivity to Zw-PCBAA at 2 mM salt solutions. As the ionic strength increased, the ionic conductivity of Zw-PSBMAEG in NaCl and MgCl2 solutions became higher than that of other tested zwitterionic hydrogels. In the 100 mM MgCl2 solution, the ionic conductivity of Zw-PSBMA-EG was 33% higher than that of Zw-PCBAA. The increase in electrolyte concentration also significantly increased the size of TEGDMAcross-linked Zw-PSBMA-EG hydrogel but not the size of MBAA cross-linked Zw-PSBMA hydrogels in NaCl and MgCl2 solutions. As a result, the increased water content leads to enhanced ionic conductivity. Additionally, the appearance of Zw-PSBMA-EG hydrogel changed gradually from semitransparent to transparent with increasing ionic strength, which can E

DOI: 10.1021/acsami.7b15934 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces be attributed to the antielectrolyte effect of the zwitterionic polymer. However, the size of Zw-PSBMA-EG hydrogel remained the same in all tested MgSO4 solutions (Figure 2C); therefore, the ionic conductivity of Zw-PSBMA-EG is similar to those of Zw-PCBAA and Zw-PMPC. It is still unclear why the swelling behavior of Zw-PSBMA-EG hydrogel is more responsive to NaCl and MgCl2 solutions than the MgSO4 solution. According to the Hofmeister series, the order of the salting-in effect for different anions is as follows: SCN− > I− > ClO4− > NO3− > Br− > Cl− > F− > IO3− > 1/2 SO42−, and the anions have a much stronger influence on the solubility of macromolecules than the cations.36,37 In this case, we can consider the polyelectrolytes as the macromolecules. As shown in Figures 3 and 4, cationic hydrogels (Cat-PTMA and Cat-PAEMA) showed the highest ionic conductivity among all hydrogels in the tested solutions, followed by AnPAMPS hydrogel. In addition to the cations and anions of electrolytes, mobile counterions of the ionic polymers also function as the charge carrier, and electrolytes and polymer counterions together contribute to higher ionic conductivity in all cationic and anionic hydrogels. Although Cat-PTMA showed higher ionic conductivity than An-PAMPS in all electrolyte solutions, the ionic conductivities of Cat-PTMA and An-PAMPS in DI water were at comparable levels. In DI water, the lower ionic conductivity of Cat-PTMA than that of CatPAEMA can be caused by the stronger interaction of Cl− with the quaternary amine of Cat-PTMA than with the primary amine of Cat-PAEMA. Compared to An-PAMPS, the higher conductivities of Cat-PTMA and Cat-PAEMA hydrogels in NaCl solution can be caused by the higher ion mobility of chloride ion (7.91 × 10−8 m2 V−1 s−1)38 than that of sodium ion (5.19 × 10−8 m2 V−1 s−1).39 Moreover, An-PMAA showed a relatively lower ionic conductivity than that of the other ionic hydrogels, due to the lower ionization capability of the carboxylic group than that of the sulfonate group. However, the increased size of An-PMAA hydrogel in PBS enhanced the ionic conductivity to the point that it is almost equal to the ionic conductivity of An-PAMPS. It had been reported that the dimension of An-PMAA hydrogel significantly increased when the solution changed from acidic to neutral conditions, causing the deprotonation of carboxylate and increasing the solubility of the PMAA. In low-concentration electrolyte solutions, cationic and anionic hydrogels show higher conductivity because of the high counterion concentration in the gel. The conductivity of the hydrogel is controlled by two parameters: the mobility and concentration of ions. In low-concentration electrolyte solutions, the concentration of total ions plays a dominant role in conductivity. In the high salt solutions, the fraction of counterions to the total ions is significantly reduced, so the mobility of the ions becomes the dominant parameter. To verify this hypothesis, the high salt solutions were further used to evaluate their ionic conductivities. Figure 5 shows the ionic conductivities of all hydrogels in high MgCl2 solutions. In 300 mM MgCl2 solution, Cat-PAEMA and Cat-PTMA hydrogels still possessed higher ionic conductivities than those of the other hydrogels. For example, the ionic conductivity of CatPAEMA hydrogel was about 50% higher than that of ZwPCBAA hydrogel and about 15% higher than that of ZwPSBMA-EG. In 1 M MgCl2 solution, Zw-PSBMA-EG hydrogel surpassed cationic/anionic hydrogels that contain counterions and had the highest ionic conductivity, which was about 10% higher than that of Cat-PAEMA. Furthermore, the ionic

Figure 5. Ionic conductivities of neutral and ionic hydrogels in (A) 300 mM MgCl2 solution, (B) 1 M MgCl2 solution, and (C) 3 M MgCl2 solution. *, **, ***p < 0.05.

conductivity difference between Cat-PAEMA and the other hydrogels became smaller; the ionic conductivity of CatPAEMA hydrogel was only 12% higher than that of Zw-PCBAA hydrogel. In 3 M MgCl2, the ionic conductivity of Zw-SBMAEG is about 13% higher than that of Cat-PDMAEMA. ZwPCBAA and Cat-PDMAEMA showed comparable ionic F

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conductivities. In the high MgCl2 solution (i.e., 3 M), the fraction of counterions in cationic and anionic hydrogels among the total mobile ions of the system becomes lower and the stronger interactions between mobile ions and cationic/anionic polymers than zwitterionic polymers decrease the ionic conductivity. Additionally, the increase in ionic conductivity of An-PMAA was small in high electrolyte solutions, which can be attributed to two reasons. First, the unionized carboxylic groups form hydrogen bonds, but this does not have a strong influence on low electrolyte solutions. It was reported that the number of hydrogen-bond linkages increases with increasing ionic strength due to the decreased ionization level of the carboxylic group.40 Second, ionized carboxylate groups electrostatically repel each other in low electrolyte solutions. However, as the electrolyte concentration increases, cations can shield the electrostatic repulsion.41 These two mechanisms can cause a denser hydrogel network in An-PMAA and prevent the further growth of ionic conductivity.

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CONCLUSIONS In this work, a series of polyelectrolyte hydrogels, including zwitterionic, cationic, and anionic hydrogels, were synthesized. Owing to the electrostatic repulsion of the ionic functional groups on the lateral chain, the ionic hydrogels show higher water content than zwitterionic hydrogels in DI water. Interestingly, the volume of the ionic hydrogels shrank and that of zwitterionic hydrogels swelled in saline solutions. Although PEG has been used in many energy and bioelectronic applications, this study observed that PEG impairs the ion conductivity in aqueous systems. Under all tested conditions, zwitterionic hydrogels show much higher ionic conductivity than that of the PEGMA hydrogel. The results indicate that zwitterionic materials are more suitable for energy and bioelectronic applications. The presence of counterions in cationic/anionic hydrogels leads to higher ionic conductivity in low concentration electrolyte solutions than that in zwitterionic hydrogels. However, stronger interaction between mobile ions and cationic/anionic hydrogels also compromises ionic conductivity. The ion conductivity of Zw-PSBMA-EG hydrogel surpasses cationic/anionic hydrogels in high concentration electrolyte solutions (≥1 M MgCl2). The present work contributes to a better understanding of the influence of electrolyte groups of polymers on the ionic conductivity of polyelectrolyte hydrogels. Our results indicate the zwitterionic polyelectrolytes are more favorable for ion transportation.

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Research Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.B.C.). *E-mail: [email protected]. Homepage: http://gozips.uakron. edu/~gc/index.html (G.C.). ORCID

Fujian Xu: 0000-0002-1838-8811 Hongbo Cong: 0000-0001-5263-6623 Gang Cheng: 0000-0002-7170-8968 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the US National Science Foundation (DMR-1454837, DMR-1206923) and National Natural Science Foundation of China (NSFC-51528301). G

DOI: 10.1021/acsami.7b15934 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.7b15934 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX