Synthesis, Characterization, and Theoretical Insights of Green

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Synthesis, Characterization and Theoretical Insights of Green Chitosan Derivatives Presenting Enhanced Li+ Ionic Conductivity Judith Cardoso-Martínez, Dora Nava, Pedro R. García-Moran, Federico Hernández, Badhin Gomez, Jorge Gabriel Vazquez-Arenas, and Ignacio González J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5128699 • Publication Date (Web): 04 Feb 2015 Downloaded from http://pubs.acs.org on February 8, 2015

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The Journal of Physical Chemistry

Synthesis, Characterization and Theoretical Insights of Green Chitosan Derivatives Presenting Enhanced Li+ Ionic Conductivity J. Cardoso1, D. Nava1, P. García-Morán2, F. Hernández3, B. Gomez4, J. VazquezArenas4,*, I. González4 1

Departmento de Física, Universidad Autónoma Metropolitana, San Rafael Atlixco 186, C.P. 09340, México, D.F., Mexico 2

Departmento de Ingeniería Química, Universidad Autónoma de Tlaxcala. San Luis Apizaquito, S/N, C.P. 90401 Apizaco Tlaxcala México. 3

Centro de Investigación Científica de Yucatán. Calle 43 No. 130, Chuburná de Hidalgo, CP 97200, Mérida, Yucatán, México. 4

Departamento de Química, Universidad Autónoma Metropolitana, San Rafael Atlixco 186, C.P. 09340, México, D.F., Mexico

Abstract The synthesis, thermal, dielectric and conductivity properties of functionalized chitosan polymer derivatives are evaluated to determine their potential uses as a non-toxic 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

Corresponding author Tel: +(52) 55-5804-4600 Ext 2686 *[email protected], [email protected] ACS Paragon Plus Environment


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transition temperatures (Tg) are located in the region where they decompose, except for SC (Tg=158 ºC). The incorporation of the sulfobetaine and zwitterionic pendant groups improves the ionic conductivity at least two 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. Keywords: Biopolymer Zwitterionic polymers lonically conducting polymers Dielectric properties DFT

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1. Introduction There is a growing interest to develop non-toxic materials from renewable sources with prospective applications in energy storage systems. lonically 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 an 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 non-toxic solidstate polymer batteries.5-7 To this concern, there are some important features that a polymer should comply in order to be used as solid polymer electrolyte: It must be amorphous and remain soluble with high- concentrated lithium salt, or it can be formulated with plasticizers to increase its conductivity. Particularly for ion-conductive 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 3 ACS Paragon Plus Environment


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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 in the order of 10-7 S cm-1.10-11 The conductivity can be also enhanced using plasticizers such as oleic or palmitic acids which increase the conductivity up to 5.5x10-6 S cm-1. Wan and co-workers have obtained a high intrinsic ionic conductivity 10-4 S cm-1 in swollen chitosan membranes.12 The complexation arising between chitosan and lithium ion has been proven using Xray photoelectron spectroscopy.7, 9, 13 Ng and Mohamad enhanced the conductivity of 60 wt% chitosan–40 wt% NH4NO3 from 8.38x10-5 to 9.93x10-3 S cm-1 by plasticization with 70 wt% of 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 In 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 synthetize 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 4 ACS Paragon Plus Environment

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work is devoted to investigate the factors determining an efficient functionalization of

chitosan

with

1,3-propanesultone

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 long-range 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.

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 one gram 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 5 ACS Paragon Plus Environment


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until residual water reached a pH value equals 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 1,3-propanesultone 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-propanesultone 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. Afterwards, 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 N-propylsulfonic 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 filtrated; then, it was washed with acetone and dried overnight in an oven with vacuum at 50°C. Samples of ZWC were lyophilized in 6 ACS Paragon Plus Environment

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order to eliminate the maximum quantity of water and kept in a dissecator until further use. The products were characterized by FTIR, 1HNMR, DSC and TGA.

2.2.1 Polymer-salt systems The polymer was added the same quantity of lithium salt (LiClO4 or LiPF6) as functionalized groups, i.e. a relationship of one mol of lithium salt with respect to one 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 Perkin Elmer apparatus with 2 cm-1 resolution and samples were dispersed and measured in KBr dried discs. Thermogravimetric measurements were performed with a PIRYS Perkin Elmer under a 50 cm3 min-1 nitrogen flow within the range of 30 °C to 800°C. Differential scanning calorimetry was carried out under a nitrogen flow (50 cm3 min-1) using a MDSC2920 Modulated Differential Scanning Calorimeter manufactured by TA Instruments (Newcastle, 7 ACS Paragon Plus Environment


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Delaware, USA). Modulated DSC scans were carried out at a heating rate of 5 °C min-1 with an amplitude of ± 1.06°C and a period of 40 seconds within the range of -50 to 200°C under 50 mL min-1 of nitrogen. Tg was obtained from the second or third scan; the samples were previously heated at 100°C for 60 minutes 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 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 known thickness (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 dry box 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 minutes

at

each

temperature

to

attain

thermal

equilibration.

A

Multi8

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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 0.1Hz.

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 chargedregions (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 appertained to positive magnitudes for all cases. Thus, confirming that the final structures correspond to fundamental states.

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, 9 ACS Paragon Plus Environment


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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, re-deacetylated 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 show a peak at 1633 cm-1 corresponding to the stretching vibration of amide -C=O (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 -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 (1HNMR) and

Carbon-13 (13C NMR) Nuclear Magnetic

Resonances


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Deacetylation of raw chitosan was confirmed by 1HNMR. Figure 3a shows a chemical shift around 2.1 ppm for N-acetyl methyl indicates 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 1,3 -propanesultone 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 1HNMR spectra described in Figure 4b. Figure 5 illustrates the

13

CNMR 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 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



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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 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. These results are consistent with related literature, indicating that the sulfonation by 1,312 ACS Paragon Plus Environment

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propanesultone and quaternization proceed randomly along the chitosan chain, destroying efficiently the regular packing of the original chitosan units,19 whence a totally amorphous zwitterionic chitosan becomes available. On the other hand, 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 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 13 ACS Paragon Plus Environment


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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 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 10b). 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 hamper 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. 14 ACS Paragon Plus Environment

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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, most 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) carried out in our laboratory 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 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 15 ACS Paragon Plus Environment


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fact that the glass transition for SC can be the only experimentally observable. Based on this argument, it is also possible to suggest that the onset 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 1MHz–100 mHz at multiple temperatures in the range from 25 to 100ºC. For brevity, only three temperatures are herein displayed. It is worthwhile mentioning 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 display a linear behavior at low frequencies33-35. 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 conductivity rise. The time constant located at high frequencies is a typical response of blocking 16 ACS Paragon Plus Environment

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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 of 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, 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 continuous line in Figure 9. The best-fit values of the experimental impedance spectra were obtained with the Zview® program, with values of X2 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



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fit and the geometrical parameters of the electrochemical cell, according to equation 1:

=

(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, 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, equation 2 can be used to determine their apparent activation energy.

= ° ( )

(2)

where: σo 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 ºC, 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 indicates that the deacetylated chitosan increases this value in the presence of LiPF6. However, the incorporation of the 18 ACS Paragon Plus Environment

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sulfobetaine and zwitterionic pendant groups presents a more significant augment in the conductivity of at least two orders of magnitude (e.g. 7.1 x 10 -8 and 4.4 x10-7 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 drops 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 hypothesis have been proposed in the literature: charge delocalization on the anion which affects the interaction with other ions or the polymer electrolyte,40 ion pairing,41 the anion size,16 anion basic hardness41 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 19 ACS Paragon Plus Environment


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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 functional density theory and the structure of the polymers.

4.8 Insights of the conductivity on the basis of functional density theory (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 trimmers without lithium salt, electronic localization and the regions in the polymers susceptible to interact 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 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 state.42 For multi-electronic systems 20 ACS Paragon Plus Environment

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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 region of electronic dislocation, which can be mapped with the ELF. Figure 12 shows a surface 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 equals to 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, 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 one 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 interact 21 ACS Paragon Plus Environment


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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 rationalizations can be deducted to support the above findings. To this concern, a nucleophilic attack highlights the sites within the molecule more akin to accept electronic density. The preferred region for a nucleophilic attack in the SC structure is defined for the hydrogens labelled 53, 54, 59 and 61 (Table 4, further details of the structures can be found in Figure A1a of the supporting information), while the hydrogens labelled 53, 54, 59 and 61 (Table 5) are the preferred sites for this attack in the ZWC (Figure A1b). On the other hand, the positions in the SC with higher probabilities to undergo an 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 labelled 59 to 62, which involve sulphur 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 for the lowest unoccupied molecular orbital (LUMO) present values relatively higher with respect to the HOMO (refer to Figure A4), which indicates that the most favorable effect arising by the orbital interactions of the aforementioned regions is interacting with acceptor groups (i.e. Li+). Based on these finding, 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 sigma, as shown for the ZWC structure, which 22 ACS Paragon Plus Environment

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augments the ionic conductivity.

5. Conclusions Chitosan is a highly reactive polymer containing an active side amino group, whereby it was modified using 1,3-propanesultone (SC) and subsequent 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 ºC 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 two orders of magnitude (e.g. 7.1 x 10 -8 and 4.4 x10-7 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, 23 ACS Paragon Plus Environment


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indicating that the ionic transport is a thermally activated process, whence its ion mobility and conductivity increased, following an 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 at least three 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, CAM-B3LYP implemented in Gaussian 09 D.01 along with the 6-31+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 interact 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– 24 ACS Paragon Plus Environment

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carbon or hydrogen–oxygen bonds are entirely localized, meanwhile the conforming electrons 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 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 sigma, as shown for the ZWC structure. Preliminary calculations of the SC and ZWC structures in the presence of LiPF6 revealed that the Li+ ion 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.

Acknowledgements D.N. and P.G. thank the financial support through a postdoctoral grant from ICyTDF and CONACyT, respectively; and 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.



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Supporting Information Available: 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 information is available free of charge via the Internet at http://pubs.acs.org.

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Tables

Table 1. Experimental elemental analysis for commercial chitosan and different functionalized chitosan derivatives

Chitosan Sample

%C

%H

%N

%S

%C/ %N

%C/ %N

molS/ molN

Chemical modification (%)

* Commercial (CC)

39.49

7.49

6.71

---

5.20

5.52

---

80.5

Deacetylated (DAC)

46.83

6.81

8.35

---

5.20

5.14

---

98.5

Sulfonated (SC)

36.9

6.15

4.71

9.07

7.71

7.83

0.95

94.9

Zwitterionic (ZWC)

38.66

7.38

5.08

5.45

8.65

7.61

0.47

46.9

* Theoretical estimation


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Table 2. Potential energies for commercial chitosan and different functionalized chitosan derivatives

Sample

Interaction energy kJ mol-1

Commercial (CC)

1464

Deacetylated (DAC)

2093

Sulfonated (SC)

956



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Table 3. Ionic conductivities (σ) and apparent energy activation (Ea) values for redeacetyled chitosan (DAC), N-propylsulfonic acid chitosan (SC), and chitosan with zwitterionic pendant groups (ZWC), with and without lithium salt obtained at 25 and 50°C

Sample

σ25°C (S cm-1)

σ50°C (S cm-1)

Ea (eV)

DAC/LiPF6

2.5 x10-8

-

-

SC

7.1 x 10 -8

1.3 x 10-7

0.17

SC/LiPF6

6.5 x 10-8

1.6 x 10-7

0.13

ZWC

4.4 x10-7

8.8 x10-7

0.10

ZWC/LiPF6 (1M:1M)

6.1 x10-7

1.4 x10-6

0.11

ZWC/LiClO4 (1M:1M)

6.3 x10-7

1.5 x10-6

0.09


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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 N-propylsulfonic acid chitosan derivative (SC).

Number

Atom

f (r ) +

f (r )−

12

N

0.00

0.12

35

N

0.01

0.19

53

H

0.08

0.01

54

H

0.08

0.00

59

H

0.07

0.01

61

H

0.05

0.01

73

H

0.01

0.05

74

H

0.01

0.05



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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

S

0.00

0.07

60

O

0.00

0.15

61

O

0.01

0.18

62

O

0.00

0.17

76

H

0.07

0.00

77

H

0.05

0.00

78

H

0.05

0.00

122

H

0.06

0.01


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Figure captions 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).

Figure 2. FTIR spectra of chitosan samples: commercial (CC), redeacetylated (DAC), sulfonated (SC), and zwitterionic (ZWC).

Figure 3. Hydrogen-1 Nuclear Magnetic Resonances (1HNMR) for: a) commercial chitosan (CC), and b) redeacetyled chitosan (DAC). The chemical shift (ppm) for methyl N-acetyl is indicated in the figure.

Figure 4. Hydrogen-1 Nuclear Magnetic Resonances (1HNMR) for: a) Npropylsulfonic acid chitosan (SC), and b) chitosan with zwitterionic pendant groups (ZWC).

Figure 5. Carbon-13 Nuclear Magnetic Resonance (13C NMR) for chitosan with zwitterionic pendant groups (ZWC).



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Figure 6. Wide-angle X-ray scattering (WAXS) patterns for: commercial chitosan (CC), re-deacetyled chitosan (DAC), N-propylsulfonic acid chitosan (SC), chitosan with zwitterionic pendant groups (ZWC), ZWC + LiPF6, and ZWC + LiClO4.

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 analysis.

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.


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Figure 10. Arrhenius plot of ionic conductivities for different chitosan samples with zwitterionic pendant groups (ZWC): ZWC, ZWC + LiPF6, ZWC + LiClO4.

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.

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) Npropylsulfonic 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 equals to 0, the electrons are completely delocalized.



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Figure 1

ACS Paragon Plus Environment

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1506

ZWC

e c n a t t i m s n a r T %

1144

2954 SC

2078

3432

DAC

1531 1376 1034 1159 1660 1455 1032 1152 1659 1383

2929 3440 CC

2885 3444 2915

3500

3000

1407 1305 1070 1661 1190

2500

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-1 Wavenumbers (cm )

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Figure 2 41 ACS Paragon Plus Environment


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a)

b)


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a)

b)

Figure 4



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ZWC+LiClO4

)

. u . a

ZWC+LiPF6

y t i s n e t n I

(

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ZWC SC DAC CC

10

20

30

40

50

60

2θ Figure 6 45 ACS Paragon Plus Environment


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b)

a)

Figure 7


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a)

b)

Figure 8



150

150

a) 100

b) 100

25 °C 50 100

25 °C 50 100

-2

Zi (kΩ cm )

Zi (kΩ cm-2 )

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50

0

50

0 0

50

100

150

0

-2

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100

150

-2

Zr ( kΩ cm )

Zr (kΩ cm )

c)


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-5.2 ZWC ZWC + LiPF6

Log σ (S cm-1)

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ZWC + LiClO4

-5.6

-6.0

-6.4 2.6

2.8

3.0 3.2 -1 1000/T (K )

3.4

Figure 10



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a)

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a)

b)

Figure 12



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Synthesis, Characterization, and Theoretical Insights of Green

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