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Ion-Containing Membranes from Microemulsion Polymerization P. Y. Chow,† C. H. Chew,† C. L. Ong,§ J. Wang,§ G. Xu,§,| and L. M. Gan*,†,‡ Department of Chemistry, National University of Singapore, Singapore, 119260, Institute of Materials Research and Engineering (IMRE), Singapore, Department of Materials Science, National University of Singapore, Singapore, 119260, and Department of Materials Science, McMaster University, Canada L8S 4L7 Received August 27, 1998. In Final Form: January 13, 1999 Ion-containing membranes were constructed using a single-step microemulsion polymerization. In addition to conventional characterization, their ionic conductivities were assessed by alternating current impedance spectroscopy. The ionic conductivities of the membranes are in the range of (2-3) × 10-3 S/cm and are much higher than conventional PEO-type polymer electrolytes, or about 1 order of magnitude lower than that of perfluorosulfonate ionomers. The main conducting mechanism for the membrane is related to the existence of the aqueous phase. Further improvement on the conductivity of the membrane is in progress.
Introduction Polymer electrolytes play an important role in the development of new technologies; they provide ionic passages and thus constitute the core part of polymer batteries, fuel cells, and sensors. To achieve high performance, perfluoro-sulfonate ionomers, such as Nafion, are often used in the studies because they offer extremely high ionic conductivities comparable to those of liquid electrolytes, which is about 100 times better than conventional hydrocarbon polymer electrolytes such as PEO doped with salt.1-3 In an attempt to produce low cost/high performance polymer electrolytes, we report here an ionic conductivity study of ion-containing hydrocarbon membranes made by a microemulsion method. Microemulsion systems are thermodynamically stable, isotropic assemblies of oil and water, separated by an interfacial film of surfactant molecules.4-6 Depending on the composition of oil, water, and surfactant, the microstructure may be described as water-droplets-in-oil, or oil-droplets-in-water. By choosing the right concentrations, both the oil and water phases can be of continuum, thus forming a bicontinuous structure.7 The oil phase can then be polymerized together with the polymerizable surfactant,8 and the water phase can be treated to host ionic species. With the copolymerization of ionic monomers, such as 4-vinylbenzene sulfonate (SVBS), in the polymerizable microemulsions, ion-conducting membranes can be produced. This is different from the one recently reported by * To whom the correspondence is addressed. † Department of Chemistry. ‡ Institute of Materials Research and Engineering. § Department of Materials Science. | McMaster University. (1) Perfluorinated Ionomer Membranes; Eisenberg, A., Yeager, H. L., Eds. ACS Symposium Series 180; American Chemical Society: Washington, DC, 1982. (2) Yeo, R. S.; Chin, D. T. J. Electrochem. Soc. 1980, 127, 546. (3) Appleby, A. J., Foulkes, R. L., Eds. Fuel Cell Handbook; Van Nostrand: NY, 1989. (4) Friberg, S. E. Prog. Colloid & Polym. Sci. 1983, 68, 41. (5) Langevin, D. Acc. Chem. Res. 1988, 21, 255. (6) Paul, B. K.; Moulik, S. P. J. Dispersion Sci. Technol. 1997, 18, 301. (7) Chew, C. H.; Gan, L. M.; Ong, L. H.; Zhang, K.; Li, T. D.; Loh, T. P.; MacDonald, P. M. Langmuir 1997, 13, 2917. (8) Gan, L. M.; Li, T. D.; Chew, C. H.; Teo, W. K. Langmuir 1995, 11, 3316.
Zurer in Chemical and Engineering News9 about the work by Wnek and co-workers who have successfully sulfonated triblock copolymers of styrene/ethylene-butylene/styrene with high conductivity. The membranes prepared from the microemulsion technique contain mobile cations which are counterbalanced by the sulfonic groups. Because the ionic conductivity cannot be directly measured by a direct current (DC) multimeter, alternating current (AC) impedance spectroscopy is used to assess indirectly the ionic transport. The data of the conductivity measurements are plotted against the temperature and the relative humidity for various microemulsion-polymerized membranes. This will be followed by the discussion on the ionic conduction mechanism, mainly out of the temperature dependence of the ionic conductance and the water sorption of the membrane samples. Experimental Section Materials. Acrylonitrile (AN), methyl methacrylate (MMA) and ethyleneglycol dimethacrylate (EGDMA) obtained from Aldrich were purified under reduced pressure before use. Sodium 4-vinylbenzenesulfonate (SVBS) and 2,2-dimethoxy-2-phenyl acetophenone (DMPA) from Aldrich were used as received. Polymerizable nonionic surfactant of ω-methoxy poly(ethyleneoxide)40 undecyl-R-methacrylate (PEO-macromonomer, C1PEO-C11-MA-40) was synthesized as reported earlier.10 The water used for all microemulsions was deionized and distilled. Membrane Preparation. The single-phase region of the microemulsion consisting of C1-PEO-C11-MA-40/AN/MMA/ H2O was determined systematically by titrating water to various compositions of C1-PEO-C11-MA-40, AN, and MMA in a screwcapped test tube. Each sample was vortex-mixed and allowed to equilibrate in a temperature-controlled environment at 23 °C. The clear-turbid points were used to establish the phase boundary of the microemulsion region as shown in Figure 1. A rough demarcation of the bicontinuous region was further deduced from conductivity measurements using a conductivity meter. As shown by Figure 1, different amounts of SVBS can be incorporated into some of the microemulsion compositions for the membrane preparation. The polymerization of the microemulsion samples was initiated by photoinitiator DMPA in the Rayonet photoreactor chamber (9) Zurer, P. Chem. Eng. News 1998, April 13, 41. (10) Liu, J.; Chew, C. H.; Gan, L. M. J. Macromol. Sci. - Pure Appl. Chem. 1996, A33 (3), 337.
10.1021/la981117s CCC: $18.00 © 1999 American Chemical Society Published on Web 04/06/1999
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Results and Discussion
at 70/30, and that for oil to surfactant was fixed at 45/55 along line P with increasing water content. As deduced from conductivity measurements, the system containing 35 wt % water was a bicontinuous microemulsion which was chosen for the detailed study. The varying compositions of SVBS and PEO-macromonomer (C1-PEO-C11MA-40) in the bicontinuous microemulsions investigated are shown in Table 1. This is to increase the ionic concentration (SVBS) systematically in the microemulsion by keeping other components constant, except for the small variation of PEO-macromonomer. In addition, 5 wt % EGDMA cross-linker based on the total weight of all reactive monomers was also added to each sample for increased mechanical strength of each membrane. All of the four membranes obtained from the precursor microemulsions were transparent and rather strong after about 4 h of polymerization. Pore Sizes of Membranes. The measurement of the freezing point depression of water trapped in the pores of the membranes allows a quantitative determination of the pore size and its distribution. The pore volume distribution curves for the membranes are shown in Figure 2. The most probable pore sizes (in diameter) were found to be 6.3 nm for both membranes 1 and 2 and 6.7 and 5.7 nm for membranes 3 and 4, respectively. The pore volume distributions become narrower when the SVBS content is increased from 5.4 to 10.7 wt % in the microemulsion system. These nanosized aqueous pores found in the styrene-sulfonate microemulsion membranes are crucial to the ionic transport. Ionic Conductivity Assessment. At each temperature and humidity, both real and imaginary impedance data for all frequencies were plotted onto a Cole-Cole plot, as shown in Figure 3 for membrane 3, where each frequency is represented by one point. The overall resistance of the sample under a particular temperature and humidity was then obtained by the extrapolation of the arc onto the real axis, which is usually very close to the real impedance for the “local minimum” of the imaginary. Obviously, these Cole-Cole plots do not follow perfect semicircles, which bring uncertainties to the conductivity results. However, this will be a less serious concern in the final plots, because they are semilogarithmic. In fact, the deviations from the semicircle indicate the existence of several relaxation mechanisms. In particular, the large increase of the imaginary impedance at low-frequency end suggests the presence of the polymerelectrode interface.12 The logarithmic conductivities of the wet membranes containing various weight percentages of SVBS are plotted against temperature as shown in Figure 4. The conductivities reached (2-3) × 10-3 S/cm, a range which is much better than that of conventional PEO (doped with salt)-type polymer electrolytes. However, they are 1 order of magnitude smaller than that of perfluoro-sulfonate ionomers [(3-5) × 10-2 S/cm]. The temperature dependence of the conductivities was essentially Arrhenius-type [exp(-E/KT)] both above and below the freezing point of water. From the slope of the plot, the activation energy was estimated to be around 0.12 eV (10.9 kJ/mol). The big discontinuity at 0 °C must be related to the fact that the ionic transport is mainly facilitated by the aqueous phase in the membrane, which is largely immobilized in the frozen state. This is somewhat different from the usual observation that when water is confined in nanosized pores, it can be supercooled well
Microemulsion System. Figure 1 shows the phase behavior of the transparent microemulsion region (dotted area) of the system composed of C1-PEO-C11-MA-40/ AN/MMA/H2O. The weight ratio of AN to MMA was fixed
(11) Brun, M.; Lallemand, A.; Quinson, J. F.; Eyrand, C. Thermochim. Acta 1977, 21, 59. (12) Macdonald, J. R. Impedance Spectroscopy: Emphasizing Solid Materials and Systems; Wiley: New York, 1989.
Figure 1. Partial phase diagram of acrylonitrile/methyl methacrylate/C1-PEO-C11-MA-40/water at 23 °C. The single phase microemulsion region is represented by the dotted area. at about 35 °C. The membranes were prepared by spreading the gel-like prepolymerized microemulsion between two glass plates and subjecting to further polymerization in the photoreactor chamber for about 4 h. The membranes were then extracted by hot water at 50 °C for 24 h. It was estimated that about 4-5 wt % of the polymerizable components had not been polymerized. Pore Sizes Distribution. The distribution of pore sizes within the polymer membranes was determined from the freezing point depression of the water trapped in the pores. A Dupont Instrument DSC 2920 differential scanning calorimeter with a Dupont Thermal Analyst 2210 system and a liquid nitrogen cooling accessory (LNCA) was used for the measurements. The membranes were leached in hot water at 50 °C for 24 h to ensure the removal of unreacted monomers. A sample of about 10-20 mg was hermetically sealed in a high-pressure sample pan. The sample was then stored in dry ice overnight to ensure that the water in the sample was completely frozen. The sample pan was then transferred to the DSC instrument and it was further cooled to -40 °C. The sample was heated to 5 °C at a ramp of 1 °C/min. The heat flow to the sample was recorded over the period of the experiment. A pore volume distribution curve was determined in accordance with the thermoporometry method as described by Brun et al.11 Ionic Conductivity Assessment. Unlike electronic conductivities, ionic conductance cannot simply be obtained from a DC multimeter because ions are not able to move through metal wires. Instead, AC impedance spectroscopy was used to indirectly assess the ionic transport by (1) alternating electric field to drive the ions “back and forth” through the membranes and (2) measuring the phase angle of the AC current lagging behind the driving voltage. The membrane samples, which had surface areas of 2.54 cm2 and were about 0.05-0.06 cm thick, were sandwiched between two metal electrodes and conditioned to the preset temperature and humidity. The AC impedance data were then collected from a Hewlett-Packard analyzer (HP 4284A). At each temperature from -70 °C to +50 °C, both real and imaginary impedances were recorded while the driving frequency scanning from 20 Hz to 1 MHz. Complex impedance spectra were then plotted, and the conductivities were evaluated by extrapolating the impedance circular arc (Cole-Cole plot) to intercept the real axis. In addition to the composition and temperature variation, the water concentration of the membranes was also controlled by equilibrating the samples in a fixed humidity environment, such as “wet” (100% relative humidity), “semi-dry” (45% humidity) and “dry” (0% humidity).
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Table 1. Transparent Microemulsion Membranes
sample
water
1 2 3 4
35 35 35 35
composition of precursor microemulsions (wt %)a SVBS C1-PEO-C11-MA-40 AN/MMA (70:30) 5.4 7.2 8.9 10.7
30.3 28.5 26.8 25.0
29.3 29.3 29.3 29.3
post polymerization ionic conductivity (10-3) at 23 °C 1.99 3.60 3.35 4.19
a In addition, cross-linker EGDMA and photoinitiator DMPA were also added to each microemulsion. They were 5 and 0.23 wt %, respectively, on the basis of the total weight of polymerizable components. Polymerization temperature was 35 °C.
Figure 2. Pore volume distribution curves for membranes 1-4 as indicated by the respective symbols.
Figure 3. Typical Cole-Cole plot of the acquired AC impedance data from the microemulsion membrane 3. Each point on the plot corresponds to one frequency. The sample resistance (the reciprocal conductance) is obtained at the local minimum of imaginary impedance.
below the freezing point. The deviation may be due to the bicontinuous structure and could be the subject of further studies. This similar phenomenon can be observed at elevated temperatures, when water molecules start to diffuse out of the membrane, thus lowering the conductivities (Figure 4). This is further confirmed by the conductivity results for the membranes conditioned in the semi-dry environment. Figure 5 shows the conductivitytemperature profile for membrane 2 for which the sample was equilibrated in a 45% relative humidity environment. It is noted that its conductivity drops by 1 order of magnitude but its activation energy increases, as judged from the steeper slope of the plot.
Figure 4. Logarithmic conductivities of styrene-sulfonate microemulsion membranes (equilibrated in wet environment) as a function of temperature. Membranes 1-4 are represented by symbols, respectively.
Figure 5. Logarithmic conductivities of styrene-sulfonate microemulsion membrane 2 (equilibrated in a 45% relative humidity environment) vs reciprocal temperature.
This “water-assisted” ionic conduction mechanism is similar to that found in perfluoro-sulfonate ionomers, where the ions are transported with a hydration shell (in the form of H3O+, H9O4+, etc.).1,2 When conditioned in a less humid environment, the conductivity of ionomers will decrease substantially (down to 1/10 000 in the dry state).13,14 However, it should be pointed out that the Nafion type ionomers exhibit no such jumps in the conductivity across the freezing temperature,13-15 presumably because of the (13) Zahreddine, C.; Pak, Y. S.; Xu, G. Solid State Ionics 1992, 58, 185. (14) Xu, G.; Pak, Y. S. J. Electrochem. Soc. 1992, 139, 2871. (15) Zaluski, C.; Xu, G. J. Electrochem. Soc. 1994, 141, 448.
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nanosized water clusters. Although this is not a serious concern for the devices operated at room temperature, such discontinuity may be removed by further reducing the water pore sizes and narrowing their pore size distributions in microemulsion membranes. Because of the nature of the microemulsion technique, many parameters can be fine-tuned in the future improvement of the membrane. For example, the oil phase may be modified to increase the thermal, mechanical, and chemical stabilities, whereas the aqueous phase can be modified to host various ions and to remove the frozen state. Conclusion Ion-containing membranes were synthesized by a singlestep microemulsion polymerization. The ionic conductivities were assessed by AC impedance spectroscopy. It is
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found that the ionic conductivities are much higher than conventional PEO-type polymer electrolytes but about 1 order of magnitude lower than those of perfluoro-sulfonate ionomers. The main ion-conducting mechanism is due to the existence of the continuous aqueous phase in the microemulsion membranes. Future improvement should be possible, because of the flexibility of the microemulsion technique, which may enhance the conductivities and other properties of the membranes. Acknowledgment. The authors are grateful to the National University of Singapore for Research Grant RP: 970609. In particular, G.X. acknowledges the support from the same university in the form of the attachment for his sabbatical. LA981117S
