Article pubs.acs.org/JPCC
Synthesis, Characterization, and Theoretical Insights of Green Chitosan Derivatives Presenting Enhanced Li+ Ionic Conductivity J. Cardoso,† D. Nava,† P. García-Morán,‡ F. Hernández-Sánchez,§ B. Gomez,∥ J. Vazquez-Arenas,*,∥ and I. González∥ †
Departmento de Física and ∥Departamento de Química, Universidad Autónoma Metropolitana, San Rafael Atlixco 186, CP 09340, México, D.F., Mexico ‡ Departmento de Ingeniería Química, Universidad Autónoma de Tlaxcala. San Luis Apizaquito, S/N, CP 90401, Apizaco, Tlaxcala, México § Centro de Investigación Científica de Yucatán, Calle 43 No. 130, Chuburná de Hidalgo, CP 97200, Mérida, Yucatán, México S Supporting Information *
ABSTRACT: The synthesis, thermal, dielectric and conductivity properties of functionalized chitosan polymer derivatives are evaluated to determine their potential uses as a nontoxic electrolyte/separator for lithium batteries. Deacetylated chitosan (DAC) at 97% is used as a precursor to prepare two derivatives: N-propylsulfonic acid chitosan (SC), chitosan with zwitterionic pendant groups (ZWC). These derivatives increase the polar character of pure chitosan, and significantly improve its solubility. Likewise, they are thermally stable up to 220 °C, and their glass transition temperatures (Tg) are located in the region where they decompose, except for SC (Tg = 158 °C). The incorporation of the sulfobetaine and zwitterionic pendant groups improves the ionic conductivity by at least 2 orders of magnitude with respect to pure chitosan at 25 °C, without the use of plasticizers or further modifications. The salt addition (LiPF6 or LiClO4) to ZWC does not modify the conductivity, whence it is suggested that its increase is due to an electronic modification, such that the energy barriers for conducting the Li+ across the ZWC become decreased (i.e., charge dislocation), rather than a salt dissociation. This experimental finding is confirmed with density functional theory (DFT) calculations conducted with the SC and ZWC structures.
1. INTRODUCTION There is a growing interest in developing nontoxic materials from renewable sources with prospective applications in energy storage systems. Ionically conducting polymer electrolytes have been considered in recent years as candidates for multiple applications, including high energy density and power lithium batteries.1,2 Natural polymers are environmentally friendly, biocompatible, and biodegradable. Over the past few years, these polymers known as polysaccharides have been the subject of intensive research, mainly due to their abundance in nature.3 Standing out from this group, chitosan is a linear polyaminosaccharide yielded by deacetylation of chitin, which is the second most abundant polymer in nature and the main constituent of the exoskeleton of insects and crustaceans.4 Particularly for energy devices, chitosan has been considered in order to meet the electrolyte properties (e.g., conductivity, electrochemical stability) required for nontoxic solid-state polymer batteries.5−7 To this concern, there are some important features that a polymer should comply with in order to be used as solid polymer electrolyte: It must be amorphous and remain soluble with highly concentrated lithium salt, or it can be formulated © XXXX American Chemical Society
with plasticizers to increase its conductivity. Particularly for ionconductive polymer electrolytes, the Tg is an important parameter that could influence their conductivity, since ion diffusion across polymer electrolytes is strongly coupled with the motion of the polymer chains, whereby it is necessary that Tg is as low as possible, or to formulate the material with an organic liquid. Sakurai et al.8 reported a Tg value around 203 °C for chitosan, whereas its chemical modification lowered the Tg value depending on the type and size of chemical group introduced in it, thus inhibiting the transference rate of lithium ion within the polymer electrolyte. Pure chitosan typically presents conductivity values between 10−10 and 10−9 S cm−1 at room temperature.9,10 Arof and Yahya evaluated the conductivity of chitosan doped with lithium acetate on the order of 10−7 S cm−1.10,11 The conductivity can also be enhanced using plasticizers such as oleic or palmitic acids which increase the conductivity up to 5.5 × 10−6 S cm−1. Wan and co-workers have obtained a high intrinsic ionic Received: December 24, 2014 Revised: February 3, 2015
A
DOI: 10.1021/jp5128699 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C conductivity 10−4 S cm−1 in swollen chitosan membranes.12 The complexation arising between chitosan and lithium ion has been proven using X-ray photoelectron spectroscopy.7,9,13 Ng and Mohamad enhanced the conductivity of 60 wt % chitosan− 40 wt % NH4NO3 from 8.38 × 10−5 to 9.93 × 10−3 S cm−1 by plasticization with 70 wt % ethylene carbonate (EC).14 Aziza et al. have also introduced fillers of inorganic alumina (Al2O3) to analyze their influence on the structure, conductivity, and thermal properties of chitosan-based polymer electrolytes.15 Cardoso et al. have shown that the zwitterionic groups on the polymer structures are able to interact with different types of inorganic salts in a 1:1 molar ratio, without compromising the phase separation, i.e., LiClO4, CF3LiO3S (lithium triflate), and LiCl.16 Along this direction, the chemical modification (e.g., introduction of functional groups to enhance interactions with the salt) of this polymer is a major concern to synthesize new derivatives with physicochemical and biochemical properties of interest for electrolyte polymers,17 which mostly depend on the characteristics of the inserted group;18,19 but without altering its fundamental skeleton. Thus, this work is devoted to investigate the factors determining an efficient functionalization of chitosan with 1,3-propane sultone and subsequent quaternization with iodomethane (CH3I). The formation of quaternized chitosan complexes with LiClO4 or LiPF6 is also discussed to draw the intrinsic advantages of the use of these complexes for a prospective performance as solid polymer electrolyte. This information is supported by physicochemical characterization of the new materials utilizing chemical techniques (IR, NMR, DSC, TGA) and electrochemical impedance spectroscopy (EIS). In addition, density functional theory (CAM-B3LYP along with 6-31+g(d) basis) with longrange corrections (e.g., Coulombic and nonbonded interactions) is utilized to compute the regions of electronic delocalization in the SC and ZWC, determining the observed experimental ionic conduction. To our current state of knowledge, similar efforts have not been undertaken to increase the ionic conduction of chitosan complexes with similar chitosan derivatives in the presence of lithium salts.
Figure 1. Reaction scheme for the synthesis of chitosan derivatives: (a) redeacetyled chitosan (DAC), (b) N-propylsulfonic acid chitosan (SC), and (c) chitosan with zwitterionic pendant groups (ZWC).
Afterward, the product was washed with acetone and dried in vacuum at 50 °C. 2.2. Synthesis of N,N-Dimethyl-N-propylsulfobetaine Chitosan. ZWC was prepared by methylation of Npropylsulfonic acid chitosan with iodomethane as described in Figure 1.17,23 For the quaternization reaction, 3 g SC and 7.3 g NaI were mixed with 120 mL of 1-methyl-2-pyrrolidone (NMP). The mixture was stirred and heated up to 60 °C. At once, all reagents were dissolved, adding 16.5 mL of NaOH at 15% w/w. Then, 17.25 mL of iodomethane was added and the mixture was allowed to react overnight at 60 °C. The product was precipitated with acetone and filtered; then, it was washed with acetone and dried overnight in an oven with vacuum at 50 °C. Samples of ZWC were lyophilized in order to eliminate the maximum quantity of water and kept in a desiccator until further use. The products were characterized by FTIR, 1H NMR, DSC, and TGA. 2.2.1. Polymer−Salt Systems. The polymer was added to the same quantity of lithium salt (LiClO4 or LiPF6) as functionalized groups, i.e., a relationship of 1 mol of lithium salt with respect to 1 mol of SC or ZWC functionalized groups, using trifluoroethanol as solvent. The solution was continuously stirred for 24 h at room temperature and then dropped into a circular Teflon mold. Residual solvent was slowly evaporated. The samples were kept in a desiccator until further use. 2.3. Polymer Characterization. Conductometric measurements (Conductronic PC45) were obtained during retrotitration with NaOH of polymer samples in HCl aqueous solution. Elemental analysis was used to verify the chemical functionalization of polymers. FTIR spectra were collected in a 1500 PerkinElmer apparatus with 2 cm−1 resolution and samples were dispersed and measured in KBr dried discs. Thermogravimetric measurements were performed with a PIRYS PerkinElmer under a 50 cm3 min−1 nitrogen flow within the range of 30 to 800 °C. Differential scanning calorimetry was carried out under a nitrogen flow (50 cm3 min−1) using a Modulated Differential Scanning Calorimeter model 7 manufactured by PerkinElmer. Modulated DSC scans were carried out at a heating rate of 10 °C min−1 within the range of 40 to 200 °C. Tg was obtained from the second scan; the samples were previously heated at 100 °C for 60 min to eliminate the water in the sample. 2.3.1. X-ray. X-ray diffraction patterns for the samples were obtained using a Siemens D500 diffractometer with Cu Kα
2. EXPERIMENTAL METHODS 2.1. Synthesis of N-Propylsulfonic Acid Chitosan. In order to increase the content of chitosan, commercial chitosan (Aldrich Co), containing approximately 83% w/w of chitosan and 17% of chitin, was redeacetylated, using an aqueous dissolution of NaOH at 40% w/w; 26 mL of a solution containing 1 g of chitosan was heated to 70 °C overnight according to the method of Chang et al.20 The DAC was recovered by filtration and washed until residual water reached a pH value of 7. Then, it was washed with acetone and dried in vacuum at 50 °C. The deacetylation degree was 98% determined by elemental analysis and conductometric titration according to Bassi et al.21 Each measurement was repeated at least twice for conductometric titration. As described in Figure 1, the deacetylated chitosan and 1,3propane sultone were mixed to obtain N-propylsulfonic acid chitosan derivative.22 Chitosan was dissolved in aqueous solution with acetic acid at 2% w/w. It was heated up to 70 °C and stirred until its complete dissolution. 1,3-Propane sultone was added to the chitosan solution in molar ratio from 1.5 to 1.0. The mixture was allowed to react overnight. The product was precipitated, filtered, and washed with distilled water several times until the pH of the residual water was 7. B
DOI: 10.1021/jp5128699 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Table 1. Experimental Elemental Analysis for Commercial Chitosan and Different Functionalized Chitosan Derivatives
a
chitosan sample
%C
%H
%N
%S
%C/%Na
%C/%N
molS/molN
chemical modification (%)
Commercial (CC) Deacetylated (DAC) Sulfonated (SC) Zwitterionic (ZWC)
39.49 46.83 36.9 38.66
7.49 6.81 6.15 7.38
6.71 8.35 4.71 5.08
----9.07 5.45
5.20 5.20 7.71 8.65
5.52 5.14 7.83 7.61
----0.95 0.47
80.5 98.5 94.9 46.9
Theoretical estimation.
radiation, by performing sweeps from 2 to 70° of 2θ, at 1°min−1. 2.3.2. Electrochemical Impedance Spectroscopy. The ionic conductivity for different chitosan derivatives including SC and ZWC was investigated using EIS. The samples were fixed inside a Teflon O-ring spacer with circular samples, 0.193 cm of radius and thickness of 0.050 cm, and sandwiched between stainless steel electrodes using a two-electrode configuration. No change in the volume of samples was detected. The EIS measurements were performed in a drybox at humidity ≤0.2%, under argon atmosphere and silica gel as desiccant agent. A humidity sensor was used to monitor the humidity. Temperature was controlled in the cell from 298 to 373 K. The measurements were carried out after keeping the samples for 30 min at each temperature to attain thermal equilibration. A Multi-Potentiostat/Galvanostat VMP3 from Bio-Logic Science Instruments was used to apply a small-amplitude (i.e., 10 mV) sinusoidal wave superimposed on a constant dc potential. The frequency was scanned from 1.0 MHz to 0.1 Hz. Figure 2. FTIR spectra of chitosan samples: commercial (CC), redeacetylated (DAC), sulfonated (SC), and zwitterionic (ZWC).
3. THEORETICAL METHODS A hybrid functional with long-range corrections was used for the computational evaluations of the structures, CAM-B3LYP24 implemented in Gaussian 09 D.0125 along with the 6-31+g(d) basis. These corrections were incorporated in the calculations since the SC and ZWC species most likely present local charged regions (i.e., polarizability) and hydrogen bonds. The optimization of these systems was conducted without restrictions followed by the computation of their vibrational frequencies where the force constants pertained to positive magnitudes for all cases, thus confirming that the final structures correspond to fundamental states.
show a peak at 1633 cm−1 corresponding to the stretching vibration of amide −CO (amide I) and the peak at 1531 cm−1 which corresponds to deformations of a primary amide (amide II). The band at 1383 cm−1 is associated with the bond of the −CH3 group in the acetamide group; this indicates that chitosan is not completely deacetylated. The band at 1078 cm−1 corresponds to the stretching vibration of a primary alcohol. 4.3. Hydrogen-1 (1H NMR) and Carbon-13 (13C NMR) Nuclear Magnetic Resonance. Deacetylation of raw chitosan was confirmed by 1H NMR. Figure 3a shows a chemical shift around 2.1 ppm for N-acetyl methyl indicating that raw chitosan is partially deacetylated. In contrast, deacetylated chitosan spectrum shows a negligible peak around 2.1 ppm (Figure 3b). The deacetylated chitosan reacted with 1,3propane sultone to form N-sulfopropyl chitosan, and the methylene proton signals corresponding to the sulfopropyl group are observed at 2.1 (d), 3.1 (b,c), and 3.8 (shoulder) ppm of chemical shift (Figure 4a). On the other hand, functionalization of chitosan to form ZWC is confirmed by the 1 H NMR spectra described in Figure 4b. Figure 5 illustrates the 13C NMR spectrum for ZWC. The spectrum has the following carbon peaks associated with the carbohydrate unit: C-1: δ = 100.32 ppm (a); C-4: δ = 72.28 ppm (d); C-3/C-5: δ = 70.25 (c) and 73.25 ppm (e); C-2/C-6: δ = 58.71 ppm (b). Note that the peak of CH3- (acetyl): δ = 23.754 ppm is absent due to chitosan functionalization. Additional peaks at C8: δ = 48.05 ppm (i), C7: δ = 35 ppm (j) and C9: δ = 32 ppm (h) are due to sulfopropyl group. The peak at 53.42 ppm (g) is associated with CH3- of quaternized nitrogen. The missed signals at 61 ppm indicate that the C6
4. RESULTS AND DISCUSSION 4.1. Elemental Analysis Results. Table 1 shows the elemental analysis determined for commercial chitosan and its modified polymers, the relationship between the weight ratio of carbon and nitrogen (C/N), and the degree of functionalization. The experimental C/N ratio and degree of sulfonation agree adequately with the theoretical value reported for DAC and SC,26 but the quaternization product (ZWC) showed a sulfur diminution, obtaining around 47% of sulfonate groups. This finding can be explained by the Hoffman elimination reaction,27 where quaternized nitrogen is eliminated bearing the propylsulfonate group as associated with the low value of %C/ %N ratio. 4.2. FTIR. Figure 2 shows the FTIR spectra for commercial chitosan, redeacetylated chitosan, sulfonated chitosan, and zwitterionic chitosan. The CC and DAC spectra show different peaks associated with their structure:7,18,19 a characteristic broad peak around 3440 cm−1 is related to the strong hydrogen inter- and intramolecular interactions with hydroxyl and amine groups. The signal about 1660 cm−1 corresponds to the band with the symmetric −N−H stretching band. Also, the spectra C
DOI: 10.1021/jp5128699 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 5. Carbon-13 nuclear magnetic resonance (13C NMR) for chitosan with zwitterionic pendant groups (ZWC).
Figure 3. Hydrogen-1 nuclear magnetic resonances (1H NMR) for (a) commercial chitosan (CC), and (b) redeacetyled chitosan (DAC). The chemical shift (ppm) for methyl N-acetyl is indicated in the figure.
hydroxyl was not methylated. These values are similar to those reported by Tsai et al.19 Sieval et al.28 and Curti et al.29 4.4. Solubility Properties. In order to investigate the solubility of modified chitosan samples, several solubility tests were performed. It was found that sulfonated chitosan is soluble not only in 2 wt % of an aqueous acetic acid solution, but also in distillated water. However, it is insoluble in organic solvents. This result suggests that the preparation of sulfonated chitosan via sulfonation increases its polar character. Likewise, ZWC presents good solubility in distillated water and in polar organic solvents, such as N,N-methylpyrrolidone. 4.5. X-ray Results. Figure 6 presents the wide-angle X-ray scattering (WAXS) patterns of raw chitosan, deacetylated chitosan, sulfopropyl chitosan, zwitterionic chitosan, and zwitterionic chitosan/lithium salts (1:1). Raw chitosan and deacetylated chitosan showed three characteristic peaks of 2θ around 10.3°, 15.9°, and 20.1°, indicating the high degree of crystallinity of chitosan as previously reported by Samuels et al.30 The reflection at 2θ = 10.3° was assigned to crystal form type I and the strong reflection at 2θ = 20.1° corresponding to crystal form type II. The WAXS patterns of chemically modified
Figure 6. Wide-angle X-ray scattering (WAXS) patterns for commercial chitosan (CC), redeacetyled chitosan (DAC), Npropylsulfonic acid chitosan (SC), chitosan with zwitterionic pendant groups (ZWC), ZWC + LiPF6, and ZWC + LiClO4.
chitosan were significantly different from raw chitosan. Sulfonated chitosan and quaternized chitosan present broader peaks at 2θ = 20° suggesting that the chemical modification diminished its capability to form hydrogen bonds. The structures of the chitosan derivatives (sulfonation and quaternization) are more amorphous than that of raw chitosan.
Figure 4. Hydrogen-1 nuclear magnetic resonances (1H NMR) for (a) N-propylsulfonic acid chitosan (SC), and (b) chitosan with zwitterionic pendant groups (ZWC). D
DOI: 10.1021/jp5128699 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 7. Thermogravimetric analysis (TGA) for (a) commercial chitosan (CC), redeacetyled chitosan (DAC), N-propylsulfonic acid chitosan (SC), chitosan with zwitterionic pendant groups (ZWC), and (b) ZWC + LiPF6, ZWC + LiClO4. Decomposition temperatures of 10% weight loss for ZWC + LiPF6 and ZWC + LiClO4 were 263 and 214 °C, respectively.
Figure 8. (a) Differential scanning calorimetry (DSC) thermogram and (b) dynamic mechanical analysis (DMA) thermogram for N-propylsulfonic acid chitosan (SC). The glass transition zones are indicated in both analyses.
hand, the sulfonated chitosan undergoes a three-stage decomposition. The first stage is between 50 and 120 °C with a loss of almost 3% of the initial mass, associated with water. This behavior is followed by a 18% mass loss just above 200 °C and ends approximately at 290 °C, involving the decomposition of the sulfonated pendant groups of chitosan polymer.19 The third stage ends around 400 °C, and involves a mass loss of around 15% due to the carbohydrate structure. The thermal stability of SC decreases from 317 to 246 °C when compared to CC at the same decomposition temperature (Td) of 10% mass loss. The decreased thermal stability of SC is a result of its limited capability to form a hydrogen bond caused by the introduction of propyl sulfonic acid group because the modification might render the material a more hydrophilic character and destroyed the crystalline structure. Therefore, the structure of the polymer became irregular after sulfonation, causing its deterioration at lower temperatures. The same behavior is exhibited for ZWC, but apparently with a higher
These results are consistent with the related literature, indicating that the sulfonation by 1,3-propane sultone and quaternization proceed randomly along the chitosan chain, efficiently destroying the regular packing of the original chitosan units,19 whence a totally amorphous zwitterionic chitosan becomes available. On the other hand, the WAXS pattern for ZWC/LiClO4 and ZWC/LiPF6 showed some small peaks due to the lithium salts and phase separations. This indicates that lithium salts are partially dissociated in the ZWC structure. 4.6. Thermal Properties. Figure 7a shows typical TGA thermograms obtained for CC, DAC, SC, and ZWC. For CC, the first stage recorded at low temperatures is the weight loss attributed to absorbed water with ∼5%. The second stage is located between 300 and 350 °C and involves a weight loss around 36%, which is due to decomposition of the carbohydrate structure. This result is in agreement with the results obtained by McNeill and co-workers.31 On the other E
DOI: 10.1021/jp5128699 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
temperatures for decomposition of CC, DAC (i.e., thermogravimetric analysis) are higher than the one found for SC (refer to Figure 7). 4.7. Impedance Measurements. The conductivity properties of lyophilized DAC, SC, and ZWC with and without lithium salt (ZWC/LiPF6 and ZWC/LiClO4) were measured using electrochemical impedance spectroscopy. The measurements were performed at open circuit potential (OCP) in the frequency range from 1 MHz to 100 mHz at multiple temperatures in the range from 25 to 100 °C. For brevity, only three temperatures are displayed herein. It is worthwhile to mention that the EIS was collected at OCP, where no faradaic reactions occur, whence the relaxation phenomena are associated with charge transfer, including conduction. The Nyquist spectra of (a) ZWC and (b) ZWC/LiPF6 are shown in Figure 9 (EIS data for DAC and SC with and without lithium salt are not shown but display similar features). In general, these diagrams show depressed semicircles from high to medium frequencies, whereas linear behavior is shown at low frequencies.33−35 Most likely, the depressed semicircles involve two time constants. Note that the components of impedance decrease and the linear slope becomes less steep as the temperature of characterization is increased, which is also known to reflect a rise in conductivity. The time constant located at high frequencies is a typical response of blocking electrodes,37 most likely associated with the bulk resistance and properties of the polymer electrolyte,38 whereas the time constant at low frequencies is related to migration of ions and surface heterogeneities of the electrode.39 In order to obtain quantitative information on the polymer conductivity, the impedance spectra were analyzed using the equivalent electric circuit sketched in Figure 9c, where Rb represents the bulk resistance of ZWC (or any other chitosan sample), Ri represents the resistance arising between the polymer electrolyte and the stainless steel electrode interface; the impedance of constant phase element (CPE) is defined as ZCPE = 1/[(jω)n Qb], where j = √−1, ω is the angular frequency, and n takes into account the inhomogeneity of the system (i.e., roughness, porosity). CPEi is associated with the capacitance of the stainless steel electrode and Qb is associated with the bulk properties of the polymer. Note that this equivalent electric circuit only considers the two time constants aforementioned, describing the properties of the polymer and the ionic conduction. The impedance diagrams constructed with the best fit values of the equivalent circuit are shown as a continuous line in Figure 9. The best-fit values of the experimental impedance spectra were obtained with the Zview program, with values of χ2 between 10−3 and 10−4, indicating a good quality of fitting. The linear behavior observed in Figure 9 at low frequencies involves the complex ionic conduction mechanism within chitosan samples, along with different interactions such as Li+ functional group, and others related to the lithium salt anion. The ionic conductivity (σ) of these samples (e.g., ZWC) can be calculated by using the values of Rb obtained from the fit and the geometrical parameters of the electrochemical cell, according to eq 1:
stability. Functionalization decreases the decomposition temperature of the quaternized chitosan; the samples begin to degrade at ∼200 °C and present a decomposition temperature of 263 °C at 10% mass loss, as shown in Figure 7a. The addition of LiClO4 and LiPF6 to ZWC decreased the decomposition temperature. The degradation of ZWC/LiClO4 begins at 220 °C and T10% is ∼214 °C (Figure 7b). There are two stages in the decomposition of this sample; the first stage ranges from ∼250 to ∼300 °C, which is related to the decomposition of the sulfonated pendant groups of chitosan polymer. The second stage covers the range from ∼300 to ∼500 °C, as a result of the decomposition of the carbohydrate moiety. The glass transition temperature (Tg) of chitosan and its derivatives is in the range of its thermal decomposition, which hampers its determination using conventional techniques.32 Some authors have reported Tg values for chitosan and their derivatives, but these temperatures differ from the derivatives considered in the present study, as a result of the natural origin of chitosan.33,34 It is worth mentioning that chitosan is a natural polymer, whose properties depend on the animal source (type and age) from which it was obtained. Figure 8a shows a typical thermogram for sulfonated chitosan obtained using differential scanning calorimetry (DSC). As observed, an increase of heat flow is recorded from 156 to 168 °C, likely indicating its glass transition. DSC thermograms measured for other samples revealed signals associated with their decomposition stage, but only in the SC thermogram plotted in Figure 8a, a different signal related to its glass transition is displayed. Dynamic mechanical analysis (DMA) tests (Figure 8b) confirmed that the glass transition temperature for SC is located approximately at 168 °C. In order to corroborate that the glass transition of SC occurs at a lower temperature than its decomposition, theoretical potential energy values (intermolecular interactions) of chitosan and its derivatives were computed using the ChemAxoMarvin Sketch software.35 Given the limited calculation capacity of the program, five repeating units were used (representing the polymer) for each chitosan and each of its derivatives. The calculations were carried out using a MMFF94 force field, which considers van der Waals interactions and some electrostatic parameters;36 the results are shown in Table 2. Although this calculation does not involve all intermolecular Table 2. Potential Energies for Commercial Chitosan and Different Functionalized Chitosan Derivatives sample
interaction energy kJ mol−1
Commercial (CC) Deacetylated (DAC) Sulfonated (SC)
1464 2093 956
interactions, it reveals that the interaction energies in CC and DAC are considerably higher than for SC. As a first insight into these systems, this suggests that breaking intermolecular interactions within SC demands a lower energy (or temperature) than for CC and DAC, enabling a transition from glassy to elastomeric state, namely, the higher the interactions are, the higher the energy for the glass transition will be. The differences arising for the interaction energies of CC, DAC, and CS could justify the fact that the glass transition for SC can be the only experimentally observable value. Based on this argument, it is also possible to suggest that the onset
σ=
l AR b
(1)
where l is the thickness of the sample (e.g., ZWC) and A is its geometrical area. Figure 10 shows a typical increase of the σ values with the temperature rise for the ZWC, ZWC/LiClO4, F
DOI: 10.1021/jp5128699 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 9. Experimental Cole−Cole plots obtained at three different temperatures (20, 50, and 100 °C) for (a) chitosan with zwitterionic pendant groups (ZWC), (b) ZWC/LiPF6, and (c) equivalent electric circuit utilized for fitting the experimental Cole−Cole plots.
Table 3. Ionic Conductivities (σ) and Apparent Energy Activation (Ea) Values for Redeacetyled Chitosan (DAC), NPropylsulfonic Acid Chitosan (SC), and Chitosan with Zwitterionic Pendant Groups (ZWC), with and without Lithium Salt Obtained at 25 and 50 °C sample DAC/LiPF6 SC SC/LiPF6 ZWC ZWC/LiPF6 (1M:1M) ZWC/LiClO4 (1M:1M)
σ25 °C (S cm−1) 2.5 7.1 6.5 4.4 6.1 6.3
× × × × × ×
−8
10 10−8 10−8 10−7 10−7 10−7
σ50 °C (S cm−1) 1.3 1.6 8.8 1.4 1.5
× × × × ×
10−7 10−7 10−7 10−6 10−6
Ea (eV) 0.17 0.13 0.10 0.11 0.09
indicates that the deacetylated chitosan increases this value in the presence of LiPF6. However, the incorporation of the sulfobetaine and zwitterionic pendant groups presents a more significant augment in the conductivity of at least 2 orders of magnitude (e.g., 7.1 × 10−8 and 4.4 × 10−7 S cm−1, respectively) with respect to pure chitosan at 25 °C. With increasing temperature the hopping mechanism takes place in the ZWC systems, indicating that ion transport is a thermally activated process; therefore, the ion mobility is increased. For this reason the salt addition (LiPF6 or LiClO4) does not have any important effect compared to the pure ZWC. In addition, there are virtually no variations in the conductivity values evaluated for the LiPF6 and LiClO4 salts, slight changes can only be connected to the anion of the Li salt. The Ea values reported in Table 3 show a trend with the conductivity; namely, Ea decreases as the ionic conductivity becomes higher. Not surprisingly, the apparent activation energy related to the Li+ movement (i.e., ion hopping) and their interactions across the polymer drop as a result of a structural modification associated with the sulfonation and quaternization of the polymer chains. This finding suggests that the increase of the conductivity in the ZWC is not related to the introduction of the zwitterionic pendant groups to dissociate the salts, but to modify the chitosan structure such that the energy barriers for conducting the Li+ across the polymer become decreased. In this direction, different hypotheses have been proposed in the literature:
Figure 10. Arrhenius plot of ionic conductivities for different chitosan samples with zwitterionic pendant groups (ZWC): ZWC, ZWC + LiPF6, ZWC + LiClO4.
and ZWC/LiPF6. Similar behaviors are obtained when the temperature is decreased (not shown). Regression values are close to unity suggesting a linear behavior, typical for rigid solids and described by Arrhenius equation. Consequently, charge transport should be a thermally activated process for such systems. Therefore, eq 2 can be used to determine their apparent activation energy. (2) σ = σ °e(−Ea / kT ) o where σ is the pre-exponential factor, Ea is the apparent activation energy, k is the Boltzmann constant, and T is the absolute temperature. This procedure was utilized to determine the conductivities of lyophilized DAC/LiPF6, SC, SC/LiPF6, ZWC, ZWC/LiPF6, and ZWC/LiClO4 at 25 and 50 °C, and the corresponding Ea values (Table 3). All samples were lyophilized in order to remove remaining water and humidity, which could increase the conductivity, not related to the polymers. The ionic conductivity of pure chitosan typically ranges between 10−10 and 10−9 S cm−1 at room temperature,9,10 which G
DOI: 10.1021/jp5128699 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
state.42 For multielectronic systems as polymers, these functions indicate the likelihood of electron pair localization (e.g., mapping) in the vicinity of a reference electron located in a determined point of the polymer structures (Figure 11). In addition, the analysis of the ELF can distinguish between core and valence electrons, as well as covalent bonds and lone pairs, and it is unalterable regarding the transformation of molecular orbitals. This information can be readily calculated with ab initio calculations using DFT. Multiple studies have used this tool as an indicator of the ability of a determined structure to delocalize charge and bonding.43−45 Thus, it is assumed in the present study that the Li+ conduction pathway across the polymers is generated in a region of electronic dislocation, which can be mapped with the ELF. Figure 12 shows a surface
charge delocalization on the anion which affects the interaction with other ions or the polymer electrolyte,40 ion pairing,41 anion size,16 anion basic hardness,41 and ion hopping mechanism between coordinating sites and segmental motion of lateral chain of the polymer. In order to rationalize the source of such conductivity, the electronic and molecular interactions of the SC and ZWC structures are analyzed with ab initio methods. Unfortunately, ionic conduction is a dynamic process, which entails charge transferences in a large-scale domain comprising thousands to millions of molecules. However, first insights to account for these conductivities are estimated on the basis of DFT and the structure of the polymers. 4.8. Insights on the Conductivity on the Basis of DFT. Theoretical calculations were carried out utilizing DFT to elucidate the stem of the conductivity experimentally observed for SC and ZWC. These initial efforts were focused to determine their electronic structure and molecular interactions of SC and ZWC trimers without lithium salt, electronic localization, and the regions in the polymers susceptible to interaction with Li+ ions during conduction. The computation of the energy barriers for conducting the Li+ across the polymer, their interaction with lithium salts, as well as the reaction mechanism for longer and flexible chain polymers will be the motivation of a forthcoming paper. The optimized structures for SC and ZWC are shown in Figure 11, where it is
Figure 12. Surface plots of the localized electronic function (ELF) obtained from the optimized structures shown in Figure 11 at isovalue equals 0.075: (a) N-propylsulfonic acid chitosan derivative (SC) and (b) chitosan with zwitterionic pendant groups (ZWC). It is assumed that, when the ELF tends to 1, the electrons are found localized, while for ELF = 0, the electrons are completely delocalized.
plot of the ELF for the SC (Figure 11a) and ZWC (Figure 11b) structures. It is assumed that, when ELF tends to 1, the electrons are found localized, while for ELF = 0, the electrons are completely delocalized. As observed, the electrons in the SC (Figure 12a) corresponding to the hydrogen− carbon or hydrogen−oxygen bonds are entirely localized; meanwhile, the conforming electrons to the carbon−carbon and carbon− oxygen bonds present a high dislocation, which is increased with the presence of the sulfobetaine groups (i.e., zwitterion in Figure 12b). This evidence supports the experimental results obtained in Table 3, where the ionic conductivity for the SC is approximately 1 order of magnitude lower in comparison with the ZWC. Not surprisingly, this more prominent electron delocalization in the ZWC is related to the incorporation of the zwitterion pendant group, which significantly alters the structure of the polymer. Calculations of the above structures in the presence of LiPF6 revealed that the Li+ ion strongly interacts with the oxygen atoms (carbonyl and/or ethoxy groups) presenting a high dislocation (not shown). When the Fukui functions are analyzed for both systems, important
Figure 11. Optimized structures of (a) N-propylsulfonic acid chitosan (SC), and (b) chitosan with zwitterionic pendant groups (ZWC), using density functional theory. CAM-B3LYP with the 6-31+g(d) basis was implemented in Gaussian 09 to compute the fundamental states.
evident that there are significant differences when the zwitterionic pendant groups are introduced into the chitosan (Figure 11b). Note that the SC structure (Figure 11a) is more linear than the ZWC, whence this species presents a more compact configuration due to the functionalized groups in it, which contain a large number of no binding interactions with respect to SC. The electronic localization function (ELF) was computed with the geometries obtained from the fundamental H
DOI: 10.1021/jp5128699 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C rationalizations can be deduced to support the above findings. To this concern, a nucleophilic attack highlights the sites within the molecule more likely to accept electronic density. The preferred region for a nucleophilic attack in the SC structure is defined for the hydrogens labeled 53, 54, 59, and 61 (Table 4;
5. CONCLUSIONS Chitosan is a highly reactive polymer containing an active side amino group, whereby it was modified using 1,3-propane sultone (SC) and subsequently quaternizated with iodomethane to yield an ionic conductive polymer with a zwitterionic pendant group (ZWC), which was water-soluble. The physicochemical characterization of these chitosan derivatives utilizing IR, NMR, WAXS, DSC, and TGA demonstrated that the functionalization was successful up to 47% of zwitterionic groups (ZWC). The solubility of the polymer suggests that this modification enhances the interaction arising between water and the chitosan chains. On the other hand, the samples did not show a glass temperature value, except for sulfonated chitosan, whose value was close to 158 °C and confirmed by dynamic mechanical analysis. The ionic conductivity values showed that the incorporation of the sulfobetaine and zwitterionic pendant groups presents a more significant augment in the conductivity of at least 2 orders of magnitude (e.g., 7.1 × 10−8 and 4.4 × 10−7 S cm−1, respectively) with respect to pure chitosan at 25 °C. Likewise, the apparent activation energy related to the Li+ movement (i.e., ion hopping) and their interactions across the polymer dropped as a result of the structural modification associated with the sulfonation and quaternization of the polymer chains. With increasing temperature the hopping mechanism takes place in the ZWC systems, indicating that the ionic transport is a thermally activated process, whence its ion mobility and conductivity increased, following Arrhenius behavior. Although the salt addition (LiPF6 or LiClO4) does not heighten the conductivity compared to the pure chitosan with zwitterionic pendant groups, it can be potentially used as solid electrolyte and separator in a lithium battery. The increase of the conductivity associated with the incorporation of the zwitterionic pendant groups into the chitosan is not related to the salt dissociation, but with a structural modification of chitosan such that the energy barriers for conducting the Li+ across the polymer become decreased. Ongoing research is currently being oriented to the addition of ionic liquids to enhance salt dissociation, enabling the increase of conductivity by at least 3 orders of magnitude. Density functional theory is used to analyze the stem of the experimental conductivity observed for the N-propylsulfonic acid chitosan derivative and chitosan with zwitterionic pendant groups. A hybrid functional with long-range corrections was used for the computational evaluations of the structures, CAMB3LYP implemented in Gaussian 09 D.01 along with the 631+g(d) basis. Under this theoretical approach, the fundamental states of SC and ZWC trimmers without lithium salt, and the electronic localization function were computed to determine the most likely regions within the polymers susceptible to interacting with Li+ ions during conduction. These regions are connected to an enhanced electronic dislocation within the chitosan structures. Calculations of the electronic localization function, Fukui functions, HOMO, and LUMO conducted with the fundamental states of these structures revealed that the electrons in the SC corresponding to the hydrogen−carbon or hydrogen−oxygen bonds are entirely localized; meanwhile, the electrons conforming to the carbon−carbon, carbon−oxygen bonds present a high dislocation, which is increased with the presence of the sulfobetaine groups in the ZWC. In this direction, the most favorable effect arising in the orbital interactions of the high
Table 4. Relevant Values Computed for the Fukui’s Functions Describing the More Susceptible Sites to Receive a Nucleophilic Attack (f(r)+) and Electrophilic (f(r)−) for NPropylsulfonic Acid Chitosan Derivative (SC) number
atom
f(r)+
f(r)−
12 35 53 54 59 61 73 74
N N H H H H H H
0.00 0.01 0.08 0.08 0.07 0.05 0.01 0.01
0.12 0.19 0.01 0.00 0.01 0.01 0.05 0.05
further details of the structures can be found in Figure A1a of the Supporting Information), while the hydrogens labeled 53, 54, 59, and 61 (Table 5) are the preferred sites for this attack in Table 5. Relevant Values Computed for the Fukui’s Functions Describing the More Susceptible Sites to Receive a Nucleophilic Attack (f(r)+) and Electrophilic ( f(r)−) for Chitosan with Zwitterionic Pendant Groups (ZWC) number
atom
f(r)+
f(r)−
59 60 61 62 76 77 78 122
S O O O H H H H
0.00 0.00 0.01 0.00 0.07 0.05 0.05 0.06
0.07 0.15 0.18 0.17 0.00 0.00 0.00 0.01
the ZWC (Figure A1b). On the other hand, the positions in the SC with higher probabilities to undergo electrophilic attack (i.e., donating electronic density during the course of cation conduction) are defined by the nitrogen atoms 12 and 35 (Figure A2a), and for the ZWC, this attack most likely would occur for the sulfobetaine group, atoms labeled 59 to 62, which involve sulfur and nearby oxygens (Figure A2b). When the boundary orbitals of these structures are calculated, it is found that for the highest occupied molecular orbital (HOMO), the contributing centers are localized for SC (Figure A3a) and ZWC (Figure A3b). While the lowest unoccupied molecular orbital (LUMO) presents values relatively higher with respect to the HOMO (refer to Figure A4), which indicates that the most favorable effect arising from the orbital interactions of the aforementioned regions is interacting with acceptor groups (i.e., Li+). Based on these findings, it is suggested that the SC presents an intrinsic electronic dislocation whereby it is susceptible to transfer electronic density. This behavior is enhanced when the SC is functionalized with electron donor σ, as shown for the ZWC structure, which augments the ionic conductivity. I
DOI: 10.1021/jp5128699 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
(11) Yahya, M. Z. A.; Arof, A. K. Characteristics of Chitosan-Lithium Acetate-Palmitic Acid Complexes. J. New Mater.Electrochem. Syst. 2002, 5, 123−128. (12) Wan, Y.; Creber, K. A. M.; Peppley, B.; Tam Bui, V. Ionic Conductivity of Chitosan Membranes. Polymer 2003, 44, 1057−1065. (13) Pawlicka, A.; Danczuk, M.; Wieczorek, W.; ZygadłoMonikowska, E. Influence of Plasticizer Type on the Properties of Polymer Electrolytes Based on Chitosan. J. Phys. Chem. A 2008, 112, 8888−8895. (14) Ng, L. S.; Mohamad, A. A. Protonic Battery Based on a Plasticized Chitosan-Nh4no3 Solid Polymer Electrolyte. J. Power Sources 2006, 163, 382−385. (15) Aziz, N. A.; Majid, S. R.; Yahya, R.; Arof, A. K. Conductivity, Structure, and Thermal Properties of Chitosan-Based Polymer Electrolytes with Nanofillers. Polymer. Adv. Technol. 2011, 22, 1345− 1348. (16) Cardoso, J.; Soria-Arteche, O.; Vázquez, G.; Solorza, O.; González, I. Synthesis and Characterization of Zwitterionic Polymers with a Flexible Lateral Chain. J. Phys. Chem. C 2010, 114, 14261− 14268. (17) Shahidi, F.; Arachchi, J. K. V.; Jeon, Y.-J. Food Applications of Chitin and Chitosans. Trends Food Sci. Technol. 1999, 10, 37−51. (18) Jayakumar, R. N.; New, T.; Tokura, S.; Hamura, H. Sulfated Chitin and Chitosan as Novel Biomaterials. Int. J. Biol. Macromol. 2007, 40, 175−181. (19) Tsai, H.-S.; Wang, Y.-Z.; Lin, J.-J.; Lien, W.-F. Preparation and Properties of Sulfopropyl Chitosan Derivatives with Various Sulfonation Degree. J. Appl. Polym. Sci. 2010, 116, 1686−1693. (20) Chang, K. L. B.; Tsai, G.; Lee, J.; Fu, W.-R. Heterogeneous NDeacetylation of Chitin in Alkaline Solution. Carbohydr. Res. 1997, 303, 327−332. (21) Bassi, R.; Prasher, S. O.; Simpson, B. K. Effects of Organic Acids on the Adsorption of Heavy Metal Ions by Chitosan Flakes. J. Environ. Sci. Health, Part A: Environ. Sci. Eng. 1999, 34, 289−294. (22) Jeon, C.; Holl, W. H. Application of the Surface Complexation Model to Heavy Metal Sorption Equilibria onto Aminated Chitosan. Hydrometallurgy 2004, 71, 421−428. (23) Abashzadeh, S.; Hajimiri, M. H.; Atyabi, F.; Amini, M.; Dinarvand, R. Novel Physical Hydrogels Composed of Opened-Ring Poly(Vinyl Pyrrolidone) and Chitosan Derivatives: Preparation and Characterization. J. Appl. Polym. Sci. 2011, 121, 2761−2771. (24) Yanai, T.; Tew, D. P.; Handy, N. C. A New Hybrid ExchangeCorrelation Functional Using the Coulomb-Attenuating Method (Cam-B3lyp). Chem. Phys. Lett. 2004, 393, 51−57. (25) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, revision D.01; Gaussian, Inc.; Wallingford, CT, 2013. (26) Sternglanz, P. D.; Kollig, H. Evaluation of an Automatic Nitrogen Analyzer for Tractable and Refractory Compounds. Anal. Chem. 1962, 34, 544−547. (27) Carey, F. A. Amines. In Organic Chemistry; McGraw Hill: New York, 2006. (28) Sieval, A. B.; Thanou, M.; Kotzé, A. F.; Verhoel, J. C.; Brussee, J.; Junginger, H. E. Preparation and NMR Characterization of Highly Substituted-Trimethyl Chitosan Chloride. Carbohydr. Polym. 1998, 36, 157−165. (29) Curti, E.; de Britto, D.; Campana-Filho, S. P. Methylation of Chitosan with Iodomethane: Effect of Reaction Conditions on Chemoselectivity and Degree of Substitution. Macromol. Biosci. 2003, 3, 571−576. (30) Samuels, R. J. Solid State Characterization of the Structure of Chitosan Films. J. Polym. Sci., Polym. Phys. 1981, 19, 1081−1105. (31) McNeill, I. C.; Ahmed, S.; Rendall, S.; Gorman, J. G. Thermal Degradation Behaviour of Poly(Isopropenyl Acetate). Polym. Degrad. Stab. 1998, 60, 43−52. (32) Klaykruayat, B.; Siralertmukul, K.; Srikulkit, K. Chemical Modification of Chitosan with Cationic Hyperbranched Dendritic
dislocated regions of these polymers is reacting with acceptor groups containing low electronic density (Li+). This behavior is enhanced when the SC is functionalized with electron donor σ, as shown for the ZWC structure. Preliminary calculations of the SC and ZWC structures in the presence of LiPF6 revealed that the Li+ ions strongly interact with the oxygen atoms (carbonyl and/or ethoxy groups) presenting a high dislocation. The computation of the energy barriers for conducting the Li+ across the polymer, their interaction with lithium salts, as well as the reaction mechanism for longer chain polymers will be the motivation of a forthcoming paper.
■
ASSOCIATED CONTENT
* Supporting Information S
Surface plot of the Fukui’s function, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for the N-propylsulfonic acid chitosan derivative and chitosan with zwitterionic pendant groups. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected],
[email protected]. Tel: +(52) 55-5804-4600 Ext 2686. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS D.N. and P.G. thank the financial support through a postdoctoral grant from ICyTDF and CONACyT, respectively. Vazquez-Arenas acknowledges research project grants No. 2012-183230 and 2013-205416 from CONACyT. The authors appreciate the assistance of Mr. Gregorio Guzman to edit the figures of the manuscript.
■
REFERENCES
(1) Hooper, A.; Gauthier, M.; Belanger, A. Electrochemical Science and Technology of Polymers 2; Elsevier Applied Science: London, 1990. (2) Armand, M.; Sanchez, J. Y.; Gauthier, M.; Choquette, Y. The Electrochemistry of Novel Materials; VCH: New York, 1994. (3) Ravi Kumar, M. N. V. A Review of Chitin and Chitosan Applications. React. Funct. Polym. 2000, 46, 1−27. (4) Liu, G.-Y.; Zhai, Y.-L.; Wang, X.-L.; Wang, W.-T.; Pan, Y.-B.; Dong, X.-T.; Wang, Y.-Z. Preparation, Characterization, and in Vitro Drug Release Behavior of Biodegradable Chitosan-Graft-Poly(1, 4Dioxan-2-One) Copolymer. Carbohydr. Polym. 2008, 74, 862−867. (5) Mohamed, N. S.; Subban, R. H. Y.; Arof, A. K. Polymer Batteries Fabricated from Lithium Complexed Acetylated Chitosan. J. Power Sources 1995, 56, 153−156. (6) Subban, R. H. Y.; Arof, A. K.; Radhakrisna, S. Polymer Batteries with Chitosan Electrolyte Mixed with Sodium Perchlorate. Mater. Sci. Eng., B 1996, 38, 156−160. (7) Sudhakar, Y. N.; Selvakumar, M.; Bhat, D. K. Liclo4-Doped Plasticized Chitosan and Poly(Ethylene Glycol) Blend as Biodegradable Polymer Electrolyte for Supercapacitors. Ionics 2013, 19, 277− 285. (8) Sakurai, K.; Maegawa, T.; Takahashi, T. Glass Transition Temperature of Chitosan and Miscibility of Chitosan/Poly(N-Vinyl Pyrrolidone) Blends. Polymer 2000, 41, 7051−7056. (9) Yahya, M. Z. A.; Arof, A. K. Studies on Lithium Acetate Doped Chitosan Conducting Polymer System. Eur. Polym. J. 2002, 38, 1191− 1197. (10) Buraidah, M. H.; Teo, L. P.; Majid, S. R.; Arof, A. K. Ionic Conductivity by Correlated Barrier Hopping in Nh4i Doped Chitosan Solid Electrolyte. Phys. B: Condens. Matter 2009, 404, 1373−1379. J
DOI: 10.1021/jp5128699 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C Polyamidoamine and Its Antimicrobial Activity on Cotton Fabric. Carbohydr. Polym. 2010, 80, 197−207. (33) Dong, Y.; Ruan, Y.; Wang, H.; Zhao, Y.; Bi, D. Studies on Glass Transition Temperature of Chitosan with Four Techniques. J. Appl. Polym. Sci. 2004, 93, 1553−1558. (34) Osman, Z. Thermal and Conductivity Studies of Chitosan Acetate-Based Polymer Electrolytes. lonics 2005, 11, 397−401. (35) Chemaxon. Marvinsketch 6.0.0; Xhemaxon Kft, http://www. chemaxon.com/products.html; Budapest, Hungary, 2012. (36) Halgren, T. A. Merck Molecular Force Field. I. Basis, Form, Scope, Parameterization, and Performance of Mmff94. J. Comput. Chem. 1996, 17, 490−519. (37) Stallworth, P. E.; Fontanella, J. J.; Wintersgill, M. C.; Scheidler, C. D.; Immel, J. J.; Greenbaun, S. G.; Gozdz, A. S. NMR, DSC and High Pressure Electrical Conductivity Studies of Liquid and Hybrid Electrolytes. J. Power Sources 1999, 81−82, 739−747. (38) Xie, H. Q.; Liu, Y. Synthesis, Characterization, and Properties of Amphiphilic Poly(Ethyl Acrylate) with Uniform Polyoxyethylene Grafts. J. Appl. Polym. Sci. 2001, 80, 903−912. (39) Jacob, M. E.; Hackett, E.; Giannelis, E. P. From Nanocomposite to Nanogel Polymer Electrolytes. J. Mater. Chem. 2003, 13, 1−5. (40) Kerr, S. E.; Sloop, G. L.; Han, Y. B.; Hou, J.; Wang, S. From Molecular Models to System Analysis for Lithium Battery Electrolytes. J. Power Sources 2002, 110, 389−400. (41) Pennarun, P. Y.; Jannasch, P. Influence of the Alkali Metal Salt on the Properties of Solid Electrolytes Derived from a Lewis Acidic Polyether. Solid State Ionics 2005, 176, 1849−1859. (42) Becke, A. D.; Edgecombe, K. E. A Simple Measure of Electron Localization in Atomic and Molecular Systems. J. Chem. Phys. 1990, 92, 5397−5403. (43) Savin, A.; Becke, A. D.; Flad, J.; Nesper, R.; Preuss, H.; von Schnering, H. G. A New Look at Electron Localization. Angew. Chem. 1991, 30, 409−412. (44) Burdett, J. K.; McCormick, T. A. Electron Localization in Molecules and Solids: The Meaning of Elf. J. Phys. Chem. A 1998, 102, 6366−6372. (45) Nalewajski, R. F.; Koster, A. M.; Escalante, S. Electron Localization Function as Information Measure. J. Phys. Chem. A 2005, 109, 10038−10043.
K
DOI: 10.1021/jp5128699 J. Phys. Chem. C XXXX, XXX, XXX−XXX
