Synthesis and Characterization of Zwitterionic ... - ACS Publications

J. Phys. Chem. C 2010, 114, 14261–14268

14261

Synthesis and Characterization of Zwitterionic Polymers with a Flexible Lateral Chain Judith Cardoso,*,† Olivia Soria-Arteche,‡ Gerardo Va´zquez,§ Omar Solorza,§ and Ignacio Gonza´lez† Departament of Physics, CBI, UniVersidad Autońoma Metropolitana-I, Apartado Postal 55-534, Me´xico, D.F. 09340, Departamento de Sistemas Biolo´gicos, DCBS, UniVersidad Autońoma Metropolitana-X, Calzada del Hueso 1100, Col. Villa Quietud, México D.F., 04906, Mexico, and Centro de InVestigacioń y Estudios AVanzados del IPN, AV. Instituto Politécnico Nacional 2508 Col. San Pedro Zacatenco, México D.F., 07360, Mexico ReceiVed: NoVember 11, 2009; ReVised Manuscript ReceiVed: July 3, 2010

In this work we report the synthesis and physical properties (thermal stability, diffraction patterns, and conductivity measurements) of a methacrylate-type polymer with zwitterionic pendant groups. The parent polymer, poly(sulfobutylbetaine of 12-ethyl-3,6,9-trioxa-12-azatetradec-1-yl 2-methylacrylate) (herein labeled PMBS-4) is used to prepare polymer/lithium salt complexes (PMBS-4/LiCl, PMBS-4/Li-triflate, and PMBS4/LiClO4). The resulting electrolyte polymers show mechanical properties akin to elastic solids. The glass transition temperature is close to 20 °C for all specimens, and PMBS-4/LiCl is thermally stable up to 187 °C. The conductivity of these materials varies with temperature, following an Arrhenius-like relationship. The conductivity of the solid polymer electrolytes, SPE, follows the order PMBS-4/LiClO4 > PMBS-4/LiCl > PMBS-4/Li-triflate. The ionic conductivity of these materials is clearly affected by the anion of Li salt. Introduction Polymer electrolytes are materials that have attracted great attention based on their vast applications in the development of solid-state ionic. In a solid-state battery, solid polymer electrolytes must function both as electrolyte and as separator. Therefore, they have to meet not only requirements of high ionic conductivity, wide electrochemical stability windows, easy processability, and light weight but also acceptable thermal and mechanical properties.1,2 Numerous works on the possibility of utilizing plastic as solid polymer electrolytes (SPE) in lithium rechargeable batteries and other electrochemical devices such as sensors and displays are being carried out.3,4 However, their industrial application is limited by the too low ionic conductivity over the temperature range in which they maintain their good mechanical properties. The study of polymer electrolytes was launched by Fenton et al.5 in 1973, but their technological significance was not appreciated until a few years later that the research results by Armand et al.6,7 were published. These latter authors claimed that the crystalline complexes formed from alkali metal salts and poly(ethylene oxide) (PEO) were capable of demonstrating significant ionic conductivity and highlighted their possible application as battery electrolytes. This work inspired intense research and development on the synthesis of new polymer electrolytes, physical studies of their structure and charge transport, theoretical modeling of the charge-transport processes, and the study of their physical and chemical properties at the electrolyte-electrode interface. The rapid progress in this field has been reported in numerous monographs and reviews.8-14 The hitherto carried out studies on polymeric materials indicate that the PEO amorphous phase is characterized by its best ability * Corresponding author. E-mail: [email protected]. Tel.: (52) 55 58044625. Fax: (5) 52 55 58044626 and (5) 52 55 58044610. † Universidad Autońoma Metropolitana-I. ‡ Universidad Autońoma Metropolitana-X. § Centro de Investigacioń y Estudios Avanzados del IPN.

for ion conduction, but the Li+ ion conductivity, σLi < 10-5 S/cm at room temperature, is too low for a power-battery system.15 Hence, considerable research has been devoted to the modification of electrolytes involving PEO to diminish its tendency toward crystallization and increase their ionic conductivity. In regard to the operating parameters of a battery, high ionic conductivity, it is required for the electric charge to be transported exclusively or mainly with the participation of lithium cations, since only they undergo the reversible electrode reaction. Depending on the measurement method and the salt concentration, lithium transference numbers (tLi+) of SPE based on PEO lie in the 0.0-0.5 range.15 Electrolytes reaching a lithium cationic transference number equal or close to unity are good candidates to the technology of SPE. The cation transference number equal unity is typical of polyelectrolytes in which the anions are chemically bonded to the polymer matrix, and their conductivity, in the absence of a polar solvent facilitating the dissociation of ions, is very low.11,12 Efforts to design polymer electrolytes with a high ionic conductivity led many groups to research various lithium salts, including lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium perchlorate (LiClO4), and lithium tetrafluoroborate (LiBF4). These lithium salts have been widely utilized because of its high ionic conductivity and its compatibility with solid electrolyte interfaces (SEI) on graphite anodes.16 There are other reports17,18 indicating that polymethacrylates with grafted PEO groups render conductivity values as high as 10-4 S/cm. Systems containing polar organic solvents are characterized by suitably high conductivity, but their volatility and relatively high chemical reactivity promote the search for new solvent-free materials. The synthesis and structure of zwitterionic polymers have been extensively studied by Cardoso and co-workers19-21 and by Rozanski, Laschewsky, and co-workers.22,23 These polymers possess interesting properties, particularly those related to electrical conductivity, associated phenomena, and their capacity

10.1021/jp910747t  2010 American Chemical Society Published on Web 07/29/2010

14262

J. Phys. Chem. C, Vol. 114, No. 33, 2010

to dissolve high concentrations of salts (up to 1:1, corresponding to a monomeric unit to salt ratio). Their applications in the production of solid-state batteries are potentially suitable. In the absence of salts, the presence of zwitterionic structures of opposite sign charges, joined by covalent bonds, provides specific dipole-dipole interactions that induce conductivity levels comparable to those of salt-polyether systems. However, this behavior is usually observed in a very narrow temperature range and for temperatures higher than 200 °C. There is also evidence of strong dipole effects of the ionic side groups which allow the formation of complex salts up to stoichiometric ratios, resulting in the generation of a high concentration of ion carriers. It has been shown in previous papers20,23,24 that polymethacrylates with zwitterionic pendant structures containing one, two, or three ethoxy groups in the side chain (labeled PMBS-1, PMBS-2, and PMBS-3) lead to the formation of aggregates around the ionic sites. Due to ion-ion interactions, the dielectric response starts around 200 °C up to decomposition temperature. According to Cardoso et al.,21 the flexibility of the polymethacrylate side chains with zwitterionic pendant groups brings the glass transition temperature (Tg) down to 25 °C with respect to that of the parent polymer. In the case of PMBS-4, with four ethoxy groups in the side chain, the dielectric response was measured at 0 °C. Therefore, its application in the production of solid-state batteries of PMBS-4 is potentially suitable. In this paper, the synthesis of a methacrylate polymer with zwitterionic pendant groups was carried out to investigate the feasibility of its use as a polymer electrolyte in lithium batteries. Also, the parent polymer was used to prepare polymer/lithium salt complexes. The variation of the thermal and conductive properties of these polymer zwitterionic systems was analyzed in detail. Ionic conductivity data was provided by ac spectroscopy, which is a suitable technique to study amorphous conducting phases.19 In short, this study contributes to the understanding of the influence of molecular structure on conductivity and it suggests important applications for the production of ion-lithium batteries. Experimental Section Synthesis of MBS Monomer (4). 2-{2-[2-(2-Hydroxyethoxy)ethoxy]ethoxy}ethyl 4-methylbenzenesulfonate (1) was synthesized according to the method described elsewhere.22 A solution of tetraethylene glycol (20 g, 102.9 mmol), CH2Cl2 (50 mL), and dry pyridine (8.32 mL) was cooled to 0 °C. 4-Toluenesulfonyl chloride (19 g, 102.9 mmol) dissolved in CH2Cl2 was slowly added. Temperature was maintained at 0-5 °C, and thereafter the solution was stirred for 18 h at room temperature. The mixture was washed twice with 15% hydrochloric acid, saturated aqueous NaHCO3, and water. The organic layer was dried (Na2SO4) and concentrated under vacuum to obtain a mixture of products which were purified by flash-chromatography (SiO2, AcOEt). Yield 10.87 g (30.3%). IR (film, cm-1) ν: 3408 (O-H), 1123 and 1068 (C-O-C) cm-1. 1H NMR (500 MHz, CDCl3, δ units in ppm): δ 7.79 (d, 2H, J ) 8 Hz, H 2,6), 7.34 (d, 2H, J ) 8.0 Hz, H 3,5), 4.12 (t, 2H, CH2OTs), 3.58-3.71 (m, 14H, CH2-O), 3.6 (s, OH), 2.44 (s, 3H, ArCH3). 12-Ethyl-3,6,9-trioxa-12-azatetradecan-1-ol (2) was synthesized according to the procedure reported elsewhere.25 Diethylamine (7.57 g, 103.6 mmol) was added to (1) (7.22 g, 20.6 mmol) and heated at 50 °C under nitrogen for 24 h. The excess of diethylamine was evaporated under reduced pressure, and the residue was adjusted to a pH of 10 with a 5% aqueous solution of NaOH. The mixture was then dissolved in CH2Cl2 and washed with water. The organic extract was dried (Na2SO4),

Cardoso et al. concentrated under vacuum, and purified in a Kugelrohr apparatus. bp 145 °C (2.5 Torr). Yield 4.25 g (82.36%). IR (film, cm-1) ν: 3408(O-H), 1123 and 1068 (C-O-C). 1H NMR (500 MHz, CDCl3, δ units in ppm) 4.18 (s, 1H, OH), 3.72-3.56 (m, 14H, CH2O), 2.67 (t, 2H, J ) 6 Hz, CH2N), 2.59 (c, 4H, J ) 7 Hz, CH2CH3), 2.44 (s, 3H, ArCH3), 1.04 (t, 6H, J ) 7 Hz, CH2-CH3). 12-Ethyl-3,6,9-trioxa-12-azatetradec-1-yl 2-methylacrylate (3) was synthesized as follows: a solution of (2) (3.76 g, 15.1 mmol) in 5 mL of dry benzene was added to the reaction flask containing a 60% dispersion of NaH (0.37 g, 15.4 mmol) in mineral oil suspended in 5 mL of dry benzene. The temperature was kept below 30 °C. The reaction was heated to 40 °C for 15 min or until hydrogen generation ceased. The reaction mixture was cooled to 10 °C followed by the addition of m-dinitrobenzene (0.1198 g, 0.712 mmol) and methacryloyl chloride (1.73 g, 16.39 mmol). The reaction mixture was allowed to reach room temperature and stirred overnight. The mixture was diluted with CH2Cl2 and washed with saturated aqueous NaHCO3 and water. The organic layer was dried (Na2SO4), and the solvent removed under vacuum. The crude product was purified in a Kugelrohr apparatus. bp 165 °C (2.5 Torr). Yield 2.34 g (48.85%). IR (film, cm-1) ν: 1719 (CdO), 1637 (CdC). 1H NMR (500 MHz, CDCl3, δ units in ppm) 6.13 (m, 1H, HCd), 5.56 (q, 2H, HCd), 4.3 (m, 2H, CH2OCdO), 3.74 (m, 2H, OdCOCH2CH2), 3.75-3.54 (m, 14H, CH2O), 2.66 (t, 2H, CH2-N), 2.56 (q, 4H, CH2N), 1.94 (dd, J ) 1 Hz, CH3Cd), 1.025 (t, 6H, CH3CH2N). Sulfobutylbetaine of 12-ethyl-3,6,9-trioxa-12-azatetradec-1yl 2-methylacrylate (4) was synthesized following the method disclosed in ref 21. 1,4-Butanesultone (0.672 g, 4.93 mmol) was added to a solution of (3) (1.42 g, 4.47 mmol) and mdinitrobenzene (0.082 g, 0.487 mmol) in dry acetonitrile (10 mL). The reaction mixture was heated to 75 °C under nitrogen atmosphere for 8 days. The solvent was removed in vacuum, and the product was suspended in acetone. Synthesis of Poly(sulfobutylbetaine of 12-ethyl-3,6,9-trioxa12-azatetradec-1-yl 2-methylacrylate) (PMBS-4). PMBS-4 was synthesized according to the procedure followed by Cardoso et al.21 The bulk polymerization was carried out at 60 °C, 50 mmHg for 24 h. ACVA was used as radical initiator, and a ratio of [M]/[I] ) 500 was used. The polymer was washed twice with ethanol. PMBS-4 was dissolved in trifluoroethanol and precipitated in cold acetone. The polymer was dried in a vacuum oven at 50 °C for 48 h. Yield 41.30%. Elemental analysis: each sample was analyzed twice and corrected for water content according to thermogravimetric analysis (TGA) data: exptl %C, 41.4 ( 0.5; %H, 8.2 ( 0.5; %N, 2.2 ( 0.5; %S, 8.09 ( 0.5; calcd. from C20H39NO4S, %C, 40.9; %H, 9.2; %N, 2.4; %S, 6.98). IR (film TFE, cm-1) ν: 1719 (CdO), 1320 and 1119 (asym and sym -SO3-). 1H NMR (500 MHz, D2O + NaCl + DSS, 1H NMR, δ units in ppm) 4.19 (m, 2H, CH2OCdO), 3.90 (m, 2H, OdCOCH2CH2), 3.75-3.54 (m, 14H, CH2O), 2.66 (t, 2H, CH2-N), 2.56 (q, 4H, CH2N), 1.04-0.88 (2 m, rr, mm,H3C-), 1.30 (t, 6H, CH3CH2N). 13C NMR (125 MHz, D2O + NaCl + DSS, 13C NMR, δ units in ppm) 180.92-181.68 (-CdO), 70.58-72.21 (CH2O), 66.91 (CH2-O-CdO), 63.55(-CH2-SO3), 52.56-53.76 (OCH2-CH2-N), 47.14-47.45 (CH2N+), 29.02(H3C-), 10.42 (CH3CH2N). Polymer-Salt Systems. PMBS-4 and LiCl, or LiClO4, or LiCF3SO3 (Li-triflate) (Aldrich Chemical Co.) 1:1 mol (monomeric unit to salt ratio) were dissolved in trifluoroethanol. The solution was continuously stirred at least for 24 h at room temperature or until the solution looked homogeneous. It was then dropped into a circular Teflon mold. Residual solvent was

Zwitterionic Polymers with Flexible Lateral Chains

J. Phys. Chem. C, Vol. 114, No. 33, 2010 14263

Figure 1. Synthesis path of the MBS monomer.

slowly evaporated, and thereafter the sample was vacuum-dried until a constant weight. The samples were kept in a desiccator until their further use, and the systems were labeled PMBS-4/ LiCl, PMBS-4/LiClO4, and PMBS-4/Li-triflate. Polymer Characterization. PMBS-4 structure was verified by 1H NMR in a 5-10% weight solution of D2O with 3-(trimethylsilyl)-1-propanesulfonic acid sodium salt as an internal reference. δ units were recorded. Elemental analysis was used to verify the chemical composition of the polymers. FTIR spectra were collected on a 1500 Perkin-Elmer apparatus with a 2 cm-1 resolution, and film samples were measured. Light-scattering experiments were performed in trifluoroethanol at room temperature with a Dawn-F (Wyatt Tech.) apparatus at λ ) 6320 Å. The refractive index increment dn/dc was calculated (0.211 mL/g) with the same solvent at the same wavelength. Thermogravimetric measurements were performed with a PIRYS Perkin-Elmer under a 50 cm3/min nitrogen flow. Differential scanning calorimetry (DSC) was also carried out under a nitrogen flow (50 cm3/min) with a MDSC2920 modulated differential scanning calorimeter manufactured by TA Instruments (Newcastle, DE). Modulated DSC scans were carried out at 5 °C/min with amplitude of (0.5 °C and a period of 40 s within the range of -50 to 180 °C at a heating rate of 10 °C/min under 50 mL/min of nitrogen within the range of 30-500 °C. The reported Tg of the sample was obtained from the second or third scan with previous heating at 100 °C and further isothermic process at the same temperature for 180 min to eliminate the water absorbed of the sample. X-ray Diffraction. X-ray diffraction patterns of PMBS-4, PMBS-4/LiClO4, PMBS-4/LiCl, and PMBS-4/Li-triflate were obtained in a Siemens D500 diffractometer using Cu KR radiation. Sweeps from 2° to 70° at 1°/min were performed. Impedance Measurements. After the synthesis and purification, the samples were put in a desiccator until the electrochemical measurements were carried out. This was done in order to prevent the humidity from air, since residual water in the electrolytes greatly affects the ionic conductivity of SPE. The samples (0.7 cm in diameter and 0.04 cm thick) were sandwiched between stainless steel electrodes using a two-electrode configuration. The samples were dried overnight in a hermetic cell, under vacuum and silica gel as desiccant agent. Temperature was controlled in the cell, and readings were taken in the

Figure 2. Structure of PMBS-4.

range of 298-343 K. A potentiostat/galvanostat, PARSTAT 2273, was used to apply potentials and to measure the impedance response in the frequency range of 1 MHz to 0.1 Hz. Results and Discussion Synthesis of Sulfobutylbetaine Monomer. Figure 1 shows the synthesis path to the sulfobetaine monomer (MBS). Characterization of the intermediate structures and the polymer has already been described above. Elemental analysis, 1H NMR, and 13C NMR results for PMBS-4 confirmed the suggested structure (see Figure 2). The molecular weight of PMBS-4 was 28 000 g/mol and was obtained from light scattering using the classical Zimm plots. Analysis of Diffraction Results. Methacrylate polymers with zwitterionic pendant groups were analyzed elsewhere.21 The polymers described in that paper were labeled PMBS-1, PMBS2, and PMBS-3, where the numbers 1, 2, 3 relate to ethylene glycol residues (EO) placed in the repeating units. They all had a core-shell configuration of molecular aggregates embedded in an amorphous polymer matrix. These molecular aggregates also formed lamellar domains that provided the polymers with a semicrystalline character. This morphology feature depended strongly on the number of ethoxy groups as reported in previous studies.24 Since PMBS-4 has four ethylene glycol residues, its semicrystalline character is weak, as confirmed by the absence of ordered domains in the X-ray diffraction patterns shown in Figure 3A, hence providing information on the absence of domains made of correlated aggregate chains. In the case of PMBS-4/LiCl (Figure 3A), a more homogeneous system structure than the other two polymer and salts complexes (Figure 3B) was obtained, as shown by the presence

14264


Cardoso et al.

Figure 3. Wide-angle X-ray patterns for the different polymers systems: (A) PMBS-4 and PMBS-4/LiCl (1:1 M); (B) a, PMBS-4/LiCl; b, PMBS4/Li-triflate; c, PMBS-4/LiClO4 complexes in 1:1 M.

TABLE 1: Thermal Properties: Glass Transition Temperature (Tg) and 10% Decomposition Temperature (Td10%) system

Tg ( 1 (°C)

Td10% ( 1 (°C)

PMBS-4 PMBS-4/LiClO4 PMBS-4/LiCl PMBS-4/Li-triflate

19 31 21 26

284 262 278 276

of broader peaks in the diffraction pattern. PMBS-4/LiCl and PMBS-4/Li-triflate also showed a slight right-hand shift of the characteristic peak, similar to that found elsewhere14 in doped PEO systems with compounds containing Li+ ions. The existence of a polymer-salt complex with stoichiometric ratios in polysulfobutylbetaines has been demonstrated by Cardoso et al.24 using X-ray data. Therefore, we assume that LiCl, LiClO4, and Li-triflate were completely dissociated and incorporated into the polymer, as shown in its X-ray diffraction patterns (Figure 3B). From the values of the full width at half-maximum (fwhm) of the X-ray pattern (Figure 3) and using the Scherrer’s equation, it was possible to evaluate the microcrystal size: 10.32 and 7.43 Å for PMBS-4/LiCl and PMBS-4/Li-triflate, respectively. For PMBS-4/LiClO4, distribution of salt in the polymer is more heterogeneous and it is not possible to calculate it (Figure 3B). Since ionic conductivity is associated with interactions of molecular dipoles and mobility of charges, it is therefore strongly dependent on the spatial distribution of ionic sites and, hence, on the morphology of a specific sample. In previous studies, it has been pointed out that the ionic conductivity takes place in the amorphous regions of the polymer system and that crystalline domains act as potential barriers.15 Therefore, these results demonstrate that the polymer-salt complexes are amorphous and they are not homogeneous; however, they could have a promising high ionic conductivity. Thermal Properties. DSC and TGA were used to determine the glass transition temperatures (Tg) and thermal stability of polymers. Table 1 shows the data obtained from the second DSC sweep for PMBS-4, PMBS-4X, and PMBS-4/LiCl, from which the glass transition temperatures (Tg) were found. Tg

Figure 4. DSC thermogram of the electrolyte PMBS-4/LiClO4. Six scans at 5 °C/min with N2 flux are shown.

values of the three polymers are essentially the same. The water effect on the Tg value was analyzed as follow: the PMBS-4/ LiClO4 system was heated for 3 h in N2 flux (50 mL/min) at 100 °C. The sample was then cycled from -40 to 100 °C six times at 5 °C/min. The results are shown in Figure 4. No endothermic peak around the 0 °C was found, corresponding to the melting point of the water. The difference between the first scan to sixth scan is of 3 °C maximum. This result was interpreted as Tg as a function of the thermal history of the sample. That is the Tg value changes for repeated scans. On the other hand, the difference between the forth, fifth, and sixth scans was less than 1 °C. Therefore, we conclude that the Tg values obtained in this work are a characteristic of the polymer and polymer-salt as shown in Table 1. In Figure 4 two Tg values were measured in the PMBS-4/LiClO4 system. The first value was assigned to Tg of the polymer, and the second one was assigned to the polymer-salt complex. This same behavior was observed in PEO/Li CF3SO3 where the thermogram showed two endothermic first-order transitions, a melting peak of the crystalline PEO phase with a maximum at 70.9 °C and a second



Figure 6. EIS spectra of PMBS-4/LiCl. Measurements were performed under vacuum at different times (hours): (i) 0, (ii) 1, (iii) 2, and (iv) 3.

Figure 5. Schematic view of the hermetic cell used for conductivity tests.

broad signal with a maximum at 140 °C related to the melting of the crystalline complex phase formed between PEO and LiCF3SO3.26-28 The presence of Li+ ions in the PMBS-4 polymer causes Tg to increase and broadens the temperature range along which the heat capacity changes. The increase in Tg for PMBS-4/salt can be ascribed to the inter- and intramolecular coordination of ether dipoles with the charge carriers, i.e., dissociated ions, which may act as transient cross-linking points in the polymer electrolytes. TGA data reveal that samples are highly hygroscopic. Decomposition temperature at 10% (Td10%) for each sample was also reported in Table 1. It was noted that the stability of SPE decreases with the addition of the salts. As thermally stable polymers are sought for applications in battery devices, PMBS-4/LiCl may be a good candidate since it shows the highest degradation temperature (278 °C). Electrical Response and Bulk Conductivity. Figure 5 shows a schematic view of the cell used for impedance measurements. The cell allows temperature control. Previous to the impedance experiments, the sample sandwiched between the stainless steel electrodes was dried overnight in a hermetic cell under vacuum and silica gel as desiccant agent. Impedance measurements of PMBS-4/LiCl under vacuum at different times were taken. Figure 6 shows the behavior of the impedance spectra of PMBS-4/LiCl as a function of the vacuum time. All the spectra show two partial semicircles; the diameter of the high-frequency semicircle at 0 h is smaller than the subsequent corresponding diameters at 1, 2, and 3 h. After 2 and 3 h, the diameter of the high-frequency semicircle is almost constant. The diameter of the high-frequency semicircle is associated with the bulk properties of PMBS-4/LiCl, i.e., the conductivity.29–34 The diameter of semicircles in Figure 6 increases with the vacuum time, and as a consequence the associated conductivity is smaller. The phenomenon can be associated with the extraction of water (water may be absorbed during manipulation of the samples in the desiccator). After 2-3 h the diameter becomes almost constant indicating that water has been extracted; in spite of this procedure we estimate a humidity of PMBS4/LiCl > PMBS-4/Li-triflate. For polymer electrolytes, the



Figure 11. FTIR spectra of PMBS-4 and PMBS-4/salt complexes. The main peaks are indicated in the figure.

typical Li+ ion transference numbers found in literature range between 0.3 and 0.5.40-42 However, Wang et al.43 have prepared PEO and fine-milled Li1.3Al0.3Ti1.7(PO4)3 salt and found that the charge transport in the polymer electrolyte system was predominantly due to Li+ ions. Transference numbers as high as 0.801 for ethylene oxide (EO)/Li molar ratio, at 25 °C, were found.43 Large transference number values seem to depend on the type of anion employed. The anion can either form ion pairs with the SPE or be trapped. Pennarun and Jannasch observed the presence of ion pairs even at quite low concentrations in four polyether electrolytes containing Li-triflate; however, no aggregates were observed, even at high salt concentrations.44 In the same study they found that the amount of anion trapping was high for the Cl- anion and that only little interaction occurred between Li+ and the PEO units of the polymer. The trapping of Cl- anion was related to the basic hardness of Clanion which is higher than that of ClO4-. On the other hand, the less delocalization of the charge on the triflate anion may cause the ion pairing of PEO and Li+.45 In the present study, the anion of the Li salt clearly affects conductivity. For instance, in the case of PMBS-4/Li-triflate, the size of the triflate anion is relatively big while conductivity is low as compared to the two other samples. Perchlorate ions are smaller in size and have a greater mobility which produces greater conductivity values than triflate ions. In the case of PMBS-4/LiCl, conductivity is smaller than for PMBS-4/LiClO4. Chloride ions are smaller than perchlorate ions, but the greater electrostatic interaction of Clwith the dipoles of PMBS-4 may diminish the ionic conductivity within the SPE. Considering that Li+ conductivity follows a hopping mechanism, the hop of Li+ toward another equivalent site requires a certain activation energy. However, for each positive charge transported in a certain direction, a negative charge must balance out in order to preserve electroneutrality; thus, negative charges must be able to move. From Table 2, increasing values of Ea follow the order PMBS-4/LiClO4 < PMBS-4/LiCl < PMBS-4 < PMBS-4/Li-triflate. This means that Li+ within PMBS-4 has a lower energy expense in comparison to the ClO4- anion. As opposed to this effect, the huge size of triflate anions causes an increased value of Ea, so the low conductivity using Li-triflate may be regarded as a steric effect consequence. On the other hand, ClO4- as well as Cl- carries one negative charge as valence; however, the Cl- ion has a greater charge density as

its size is smaller than ClO4-. That is, the greater charge density of Cl- allows it to interact more strongly with the dipoles of PMBS-4, diminishing its mobility. In order to further understand the different association between ion pairs and ion SPE, FTIR studies were performed. FTIR Studies for Polymer-Salts Complexes. Infrared spectroscopic analysis was also conducted focused on examining the characteristic peaks of the polymer, to gain insight into the cation/polymer interactions. Infrared data provide rich information of the cation coordination process in OE/Li. This study involved the investigation of characteristic peaks of the polymer chains which suffer changes (e.g., frequency shifts) upon bonding to the guest cations. Figure 11 shows FTIR spectra of PMBS-4 and PMBS-4/salt complexes. The PMBS-4 spectrum shows stretching vibration, antisymmetric and symmetric, of sulfonate groups in 1354 and 1038 cm-1, respectively, and vibration of C-O-C for ether groups in the lateral chain is found at 1158 cm-1. To study the coordination of the cation to the ether oxygen atoms, it is common procedure to investigate the C-O stretching and the rocking CH2, rCH2, regions. There is a shift in C-O-C vibration for PMBS-4/LiCl and PMBS4/Li-triflate complex at 1173 and 1172 cm-1, respectively, and it is decreased for PMBS-4/LiClO4 (1102 cm-1). Nunes et al.46 interpreted the presence of the 1112 cm-1 band in the spectra as indicative of noncomplexed C-O-C group. This value is close to that exhibited by PMBS-4/LiClO4 and indicates that ether groups do not complex the Li cation. On the other hand, PMBS-4/LiCl and PMBS-4/Li-triflate showed a broad and illdefined band centered at approximately 1172 cm-1 assigned at νC-O corresponding to a complexed system.46 The main feature of these spectra within the 980-900 cm-1 interval (characteristic of the coupled vibration of the νCO and rCH2 modes) is a medium intensity peak situated at about 948 cm-1 in PMBS-4. Again, PMBS-4/LiClO4 shows a shift of 7 cm-1 indicative that the conformation of ether groups in the lateral chain remains unaffected by the inclusion of a guest salt. On the other hand, oxygen atoms from carbonyl groups also act as electron donor atoms in the structure of a polymer host and form a coordinate bond with lithium ion from doping salts to form polymer-salt complexes.47 With the addition of lithium salt loading, the intensity of CdO symmetric stretching of the PMBS-4 peak diminishes and shifts to the lower wavenumbers from 1724 to 1722 cm-1 in the PMBS-4/LiClO4 salt complex. However, in

14268


terms of band intensities illustrated in Figure 11, the intense, strong, and sharp peak becomes weak and broader with the addition of lithium salt. This is due to the weak interaction between oxygen atoms (CdO) and lithium ion from doping salt. These results agree with Kamuta et al.,48 who reported that the CdO stretching of MMA at 1729 cm-1 is shifted to 1728 cm-1 with added LiCF3SO3 salt complex. In contrast, LiCl and Li-triflate complexes showed a shift from 1724 to 1727 and 1726 cm-1, respectively, due to a stronger interaction between the oxygen atom (CdO) and Li+ from doping salts. According to these data, free ions predominate for LiClO4, whereas LiCl and Li-triflate essentially exist in the form of ion pairs in close agreement with Bishop et al.49 and Zygadło-Monikowska et al.32 It is common knowledge that the ratio of free to bound ions depends markedly on the ionic strength.50 The sharp decrease in ionic conductivity, supported by the spectroscopic data presented above, indicates that ion pairing is likely to be the principal origin of the lower conductivity in the PMBS-4/LiCl and PMBS-4/Li-triflate complexes. Conclusions A polymer of the zwitterionic type was synthesized and characterized. Its thermal, conductive, and electric properties were studied in detail. Three polymer/lithium salt complexes, PMBS-4/LiCl, PMBS-4/LiClO4, and PMBS-4/Li-triflate, were prepared from a new methacrylate parent polymer with zwitterionic pendant groups (PMBS-4). Upon increasing the number of carbon atoms in the pendant chain with zwitterionic groups, flexibility of the lateral chain increased and Tg decreased. Degradation temperature was high for PMBS-4/LiCl, which is important since thermally stable polymers along a wide temperature range are sought for battery device applications. Conductivity of all the analyzed samples changed with temperature, following an Arrhenius-like behavior. Conductivity of SPE decreases as follows: PMBS-4/LiClO4 > PMBS-4/LiCl > PMBS-4/Li-triflate. The ionic conductivity of these materials is clearly affected by the anion of Li salt. For PMBS-4/ Li-triflate, the triflate anion is relatively bulky and its conductivity measure shows relatively as the lowest. Perchlorate ions are smaller and produce greater values of conductivity for PMBS-4/LiClO4. Last, it was found that PMBS-4/LiCl conductivity was smaller than PMBS-4/LiClO4. In this case chloride ions are smaller than perchlorate ions, but the greater electrostatic interaction of Cl- with the dipoles of PMBS-4 diminishes the conductivity and counterbalances the size effect. FTIR results confirmed this conclusion. Acknowledgment. The authors thank V. Lara and R. Cruz for their technical contribution. Support from Consejo Nacional de Ciencia y Tecnologıá through project 0060686 is also acknowledged. G.V. expresses thanks for the financial support from Consejo Nacional de Ciencia y Tecnologıá through a postdoctoral Grant. References and Notes (1) Meyer, W. H. AdV. Mater. 1998, 10, 439. (2) Dias, F. B.; Plomp, L.; Veldhuis, J. B. J. J. Power Sources 2000, 88, 169. (3) Scrosati, B. Applications of ElectroactiVe Polymers; Chapman & Hall: London, 1993. (4) Gray, F. M. Polymer Electrolytes; RSC Monographs; Connor, J. A., Ed.; The Royal Society of Chemistry: Cambridge, U.K., 1997. (5) Fenton, D. E.; Parker, J. M.; Wright, P. V. Polymer 1973, 14, 589. (6) Armand, M. B.; Chabagno, J. M.; Duclot, M. Presented at the Second International Meeting on Solid Electrolytes, St. Andrews, Scotland, 1978.

Cardoso et al. (7) Armand, M. B.; Chabagno, J. M.; Duclot, M. In Fast Ion Transport in Solids; Vashista, P., Mundy, J. N., Shenoy, G. K., Eds.; Elsevier: Amsterdam, The Netherlands, 1979; p 131. (8) Song, J. Y.; Wang, Y. Y.; Wan, C. C. J. Power Sources 1999, 77, 183. (9) MacCallum, J. R.; Vincent, C. A. Polymer Electrolyte ReViews 2; Elsevier: London, 1989. (10) Alamgir, M.; Abraham, K. M. In Lithium Batteries: New Materials, DeVelopments and PerspectiVes; Pistoia, G., Ed.; Elsevier: Amsterdam, The Netherlands, 1994; p 93. (11) Koksbang, R.; Olsen, I. I.; Shackle, D. Solid State Ionics 1994, 69, 320. (12) Shriver, D. F.; Bruce, P. G. In Solid State Electrochemistry; Bruce, P. G., Ed.; Cambridge University Press: Cambridge, 1995; p 95. (13) Bruce, P. G.; Gray, F. M. In Solid State Electrochemistry; Bruce, P. G., Ed.; Cambridge University Press: Cambridge, 1995; p 119. (14) Meyer, W. H. AdV. Mater. 1998, 10, 439. (15) Ciosek, M.; Sannier, L.; Siekierski, M.; Golodnitsky, D.; Peled, E.; Scrosati, B.; Głowinkowski, S.; Wieczorek, W. Electrochim. Acta 2007, 53, 1409. (16) Shriver, D. F.; Siska, D. P. Chem. Mater. 2001, 13, 4698. (17) Kudaibergenov, S.; Jaeger, W.; Laschewsky, A. AdV. Polym. Sci. 2006, 201, 157. (18) Jacob, M. E.; Hackett, E.; Giannelis, E. P. J. Mater. Chem. 2003, 13, 1. (19) Cardoso, J.; Huanosta, A.; Manero, O. Macromolecules 1991, 24, 2890. (20) Cardoso, J.; Montiel, R.; Gonza´lez, L.; Huanosta, A.; Manero, O. J. Polym. Sci., Part B: Polym. Phys. 1997, 32, 359. (21) Cardoso, J.; Manrique, R.; Albores-Velasco, M.; Huanosta, A. J. Polym. Sci., Part B: Polym. Phys. 1997, 35, 479. (22) Rozanski, S. A.; Kremer, F.; Koberle, P.; Laschewsky, A. Macromol. Chem. Phys. 1995, 196, 877. (23) Kudaibergenov, S.; Jaeger, W.; Laschewsky, A. AdV. Polym. Sci. 2006, 201, 157. (24) Cardoso, J.; Montiel, R.; Huanosta-Tera, A. J. Polym. Sci., Part B: Polym. Phys. 2005, 43, 1152. (25) Monroy, V. M.; Galin, J. C. Polymer 1984, 25, 121. (26) Traversa, E. Sens. Actuators, B 1995, 23, 135. (27) Anderson, J. H.; Parks, G. A. J. Phys. Chem. 1968, 72, 3662. (28) Sears, W. M. Sens. Actuators, B 2008, 28, 661. (29) Pickup, P. G. J. Chem. Soc., Faraday Trans. 1990, 86, 3631. (30) Kim, K. M.; Ryu, K. S.; Kang, S. G.; Chang, S. H.; Chung, I. J. Macromol. Chem. Phys. 2001, 202, 866. (31) Deka, M.; Kumar, A. Electrochim. Acta 2010, 55, 1836. (32) Irvine, J. T. S.; Sinclair, D. R.; West, A. R. AdV. Mater. 1990, 2, 132. (33) Fuentes, S.; Retuert, P. J.; González, G. Electrochim. Acta 2007, 53, 1417. (34) Marzantowicz, M.; Dygas, J. R.; Krok, F. Electrochim. Acta 2008, 53, 7417. (35) Oh, J. S.; Kang, Y.; Kim, D. W. Electrochim. Acta 2006, 52, 1567. (36) Xie, H. Q.; Liu, Y. J. Appl. Polym. 2001, 80, 903. (37) Jacob, M. E.; Hackett, E.; Giannelis, E. P. J. Mater. Chem. 2003, 13, 1. (38) Kumar, B.; Scanlon, L. G.; Spry, R. J. J. Power Sources 2001, 96, 337. (39) Kumar, B.; Rodrigues, S. J.; Spry, R. J. Electrochim. Acta 2002, 47, 1275. (40) Cho, J.; Liu, M. Electrochim. Acta 1997, 42, 1481. (41) Abraham, K. M.; Jiang, Z.; Carroll, B. Chem. Mater. 1997, 9, 1978. (42) Appetecchi, G. B.; Dautzenberg, G.; Scrosati, B. J. Electrochem. Soc. 1996, 143, 6. (43) Wang, Y.-J.; Pan, Y.; Chen, L. Mater. Chem. Phys. 2005, 92, 354. (44) Pennarun, P.-Y.; Jannasch, P. Solid State Ionics 2005, 176, 1849. (45) Kerr, S. E.; Sloop, G. L.; Han, Y. B.; Hou, J.; Wang, S. J. Power Sources 2002, 110, 389. (46) Nunes, S. C.; de Zea Bermudez, V.; Ostrovskii, D.; Carlos, L. D. J. Mol. Struct. 2004, 702, 39. (47) Su’ait, M. S.; Ahmad, A.; Hamzah, H.; Rahman, M. Y. A. J. Phys. D: Appl. Phys. 2009, 42, 1. (48) Kamuta, K.; Alias, Y.; Said, R. Ionics 2005, 11, 472. (49) Bishop, A. G.; MacFarlane, D. R.; McNaughton, D.; Forsyth, M. J. Phys. Chem. 1996, 100, 2237. (50) Zygadło-Monikowska, E.; Florjanćzyk, Z.; Rogalska-Jon´ska, E.; Werbanowska, A.; Tomaszewska, A.; Langwald, N.; Golodnitsky, D.; Peled, E.; Kovarsky, R.; Chung, S. H.; Greenbaum, S. G. J. Power Sources 2007, 173, 734. (51) Johansson, P. Phys. Chem. Chem. Phys. 2007, 9, 1493.

JP910747T

Synthesis and Characterization of Zwitterionic ... - ACS Publications

Recommend Documents