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J. Phys. Chem. 1996, 100, 11382-11388
Composite Polymeric Electrolytes from the PEO-PAAM-NH4SCN System A. Zalewska,† W. Wieczorek, and J. R. Stevens* Department of Physics, UniVersity of Guelph, N1G 2W1 Guelph, Ontario, Canada ReceiVed: October 2, 1995; In Final Form: February 1, 1996X
It is shown that the conductivity of poly(ethylene oxide)-NH4SCN electrolytes can be enhanced by the addition of poly(acrylamide). The highest room temperature conductivity (1 × 10-4 S/cm) is measured for the electrolyte containing 40 vol % of PAAM and 10 mol % of NH4SCN. Temperature dependent FT-IR studies indicate that the interaction between NH4SCN and both poly(ethylene oxide) and poly(acrylamide) varies with salt concentration and causes changes in the conductivity and phase structure of these electrolytes. NH4+ cations are bonded to both polymers leaving SCN- anions free; ionic conductivity is increased. Differences between Li+ and Na+ salts and NH4+ salts in this composite polymer system are discussed.
Introduction Increasing interest in the study of solid polymer electrolytes results from the possibility of the application of these materials in various electrochemical devices working at ambient and moderate temperatures.1 Until now most systems based on alkali metal salt doped polyethers have been investigated for application in thin-film solid state batteries1-2 or electrochromic devices.3 Various proton-conducting polymeric electrolytes have recently been studied with the possibility of application in fuel cells, sensors, or display devices.4 The latter comprise complexes of electrono-donor polymers incorporating strong inorganic acids like H3PO4 or H2SO4. Relatively little attention has been paid to systems containing ammonium salts as dopants.5-10 In such electrolytes anions, protons, or NH4+ cations can act as mobile species. There is no agreement in the literature as to which mobile species plays the most important role.5-9 Chandra et al.7,8 have reported proton transport numbers of 0.75-0.85 for the poly(ethylene oxide) (PEO)-NH4I and PEO-NH4ClO4 electrolytes using the isothermal transient ionic current method. Daniel et al.9 found that in the PEO-NH4HSO4 and PAA-NH4HSO4 electrolytes anions were practically immobile and NH4+ cations were the dominant charge carriers. On the contrary, Stainer et al.5,6 found that in PEO-NH4SCN and PEO-NH4CF3SO3 electrolytes anions are the dominant mobile species. It has been found that the ambient temperature conductivity of the PEO-NH4SCN system can be increased by about 2-3 orders of magnitude by the addition of poly(acrylamide) (PAAM).10 A similar effect has been observed in PEO-LiClO4 electrolytes with the addition of PAAM.11. In the latter case the increase in ambient temperature conductivity has been attributed to the formation of different complexes such as polyether-Li+-polyether, polyether-Li+-PAAM, and PAAMLi+-PAAM.11,12 FT-IR11,12 and FT-Raman12 studies of the carbonyl amide I and C-O-C bands in these composite electrolytes indicate that the Li+ prefers to coordinate with the carbonyl oxygen in PAAM rather than with the polyether oxygens. This results in a depletion of transient cross-links in regions of the polyether adjacent to PAAM. Thus, in the polyether shell around monophase-separated PAAM inclusions, there is an enhanced conductivity due to a lower Tg (or greater polyether segmental mobility). † On leave from the Department of Chemistry of the Warsaw University of Technology, ul. Noakowskiego 3, 00-664 Warszawa, Poland. X Abstract published in AdVance ACS Abstracts, June 15, 1996.
S0022-3654(95)02909-1 CCC: $12.00
Following this idea, the conductivity data obtained previously for the PEO-PAAM-NH4SCN system10 has been reanalyzed using a wider range of salt concentration, and the structure of these electrolytes using variable temperature FT-IR experiments has been studied. The properties of the NH4SCN-doped PEOPAAM composite electrolyte are compared with those found in previous studies of LiClO4,11 and NaClO4 doped13 PEOPAAM systems. An effective medium theory model describing changes of the conductivity with changes in the morphology of the PEO-PAAM-NH4SCN electrolytes is applied. Experimental Section Sample Preparation. PAAM (Mw ) 1.6 × 105) was prepared by the free radical polymerization of acrylamide (Aldrich, reagent grade) in an acetonitrile solution using benzoyl peroxide (Aldrich, reagent grade) as initiator. The product was dried under vacuum at 100 °C for 48 h. PEO (Aldrich, reagent grade, Mw ) 5 × 106) was dried under vacuum at 50-70 °C for 48 h prior to use. Acetonitrile (Aldrich, reagent grade) was distilled twice before use under vacuum over molecular sieves of type 4A. All of the steps in the preparation procedure were performed in an argon-filled drybox (moisture content lower than 20 ppm). NH4SCN (Aldrich, reagent grade) was dried under vacuum (10-3 Torr) at 100 °C prior to incorporation. The solid components were mixed in stoichiometric amounts in a small glass reactor, and then acetonitrile was added to form an approximately 5 mass % suspension with respect to all solid components. The mixture was stirred magnetically for 24 h until a homogenous suspension was obtained. Excess acetonitrile was removed by vacuum distillation. All of the composite electrolytes were prepared in this way and were dried under vacuum for 48-72 h at 60 °C. The concentration of NH4SCN varied between 5 and 30 mol % with respect to the ether oxygen concentration. The concentration of PAAM in the composite electrolytes varied betwen 10 and 50 vol %. The PAAM was in a form of irregular spheres of diameters lower than 5 µm homogenously distributed in the composite electrolyte as indicated from SEM observations. The volume fraction was used in preparation for the application of an effective medium model to fitting the ionic conductivity data. It was calculated for the PEO, NH4SCN, and PAAM fractions for which values of specific gravity are known. The volume fractions were assumed to be additive for the final sample, an assumption which is not quite right for these composite materials. Conductivity Measurements. Ionic conductivity was determined using the complex impedance method in the temper© 1996 American Chemical Society
Electrolytes from the PEO-PAAM-NH4SCN System
J. Phys. Chem., Vol. 100, No. 27, 1996 11383
a
b
Figure 1. Isotherms of the ionic conductivity of PEO-PAAM-NH4SCN versus volume fraction of PAAM containing 10 mol % of NH4SCN: (b) 25, (3) 50, (9) 75, and (4) 100 °C. (b) Isotherms of the ionic conductivity of PEO-PAAM (40 vol %)-NH4SCN versus molar fraction of NH4SCN: (b) 25, (3) 50, and (9) 75 °C.
ature range 20-100 °C. The samples were sandwiched between two stainless steel blocking electrodes and placed in a temperature-controlled furnace. The impedance measurements were carried out on a Tesla BM 507 impedance analyzer over the frequency range 5-500 kHz. To test the reproducibility of conductivity, multiple, separately prepared samples have been measured at room temperature; the ionic conductivity was reproducible within 10%. Fourier Transformation Infrared Spectroscopy. Infrared spectra were recorded on a computer-interfaced Nicolet FT-IR system 4.4 instrument with a wavenumber resolution of 2 cm-1. Temperature dependent FT-IR studies were performed in the temperature range 25-95 °C. Thin-film electrolyte foils were sandwiched between two NaCl plates and placed in the FT-IR temperature-controlled cell; the accuracy of the temperature control was estimated to be (1 °C. A Galactic Grams 386 software package was used to analyze the FT-IR data. Results Conductivity Studies. Figure 1a shows a plot of the logarithm of the ionic conductivity of PEO-PAAM-NH4SCN (10 mol %) electrolytes versus PAAM concentration (in vol %) at 25, 50 , 75, and 100 °C. The conductivity increases with an increase in the PAAM concentration, reaching a maximum for the sample containing 40 vol % of PAAM. Conductivities
Figure 2. Ionic conductivity of PEO-PAAM-NH4SCN with 10 mol % of NH4SCN plotted as a function of inverse temperature: (b) 10 vol % PAAM and (9) 40 vol % PAAM. Solid lines display fits of eq 1 to experimental data for temperatures above the break and eq 2 for temperatures below the break.
measured for all samples containing PAAM are much higher than those found for the unmodified PEO-NH4SCN electrolyte. In Figure 1b changes in the logarithm of the ionic conductivity versus NH4SCN concentration (in mol %) are shown at 25, 50, and 75 °C for electrolytes containing 40 vol % of PAAM. A conductivity maximum observed at 10 mol % of NH4SCN is followed by a decrease in conductivity for higher NH4SCN concentrations with a minimum at 25 mol % at 25 °C. At higher temperatures the position of this minimum shifts to the lower NH4SCN concentration range and is followed by an increase in conductivity for samples containing 25 and 30 mol % of NH4SCN. In Figure 2 changes in the logarithm of the ionic conductivity versus reciprocal temperature are depicted for PEO-PAAMNH4SCN electrolytes containing 10 mol % of NH4SCN and 10 and 40 vol % of PAAM. For the sample containing 10 vol % of PAAM, the temperature dependence of the conductivity follows the VTF type relation (eq 1) at temperatures above the melting point of the crystalline PEO phases (i.e., roughly above 65 °C 10) and an Arrhenius type temperature dependence (eq 2) in the lower temperature range.
A B exp (1) 1/2 k (T - T0) T B Here A is a preexponential factor, B is a pseudoactivation energy for conduction, kB is the Boltzmann constant, T0 is an equilibrium glass transition temperature usually 30-50 K lower than a glass transition temperature Tg calculated from DSC experiments. Cruickshank et al.14 discuss the effect of several different anions on these parameters. σ)
σ)
σ0 Ea exp T kBT
(2)
Here σ0 is a preexponential factor and Ea is an activation energy for conduction. For the sample containing 40 vol % of PAAM, an abrupt increase in conductivity is observed following the melting of the crystalline PEO phase. This change in conductivity observed around 60 °C increases with an increase in the PAAM concentration and is not observed for the sample containing 10 vol % of PAAM (see Figure 2). For the sample containing 40 vol % of PAAM, the temperature dependendence of conductivity
11384 J. Phys. Chem., Vol. 100, No. 27, 1996
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Figure 3. FT-IR spectrum of PEO-PAAM (40 vol %)-NH4SCN 10 mol % electrolyte recorded at 25 °C.
follows eq 2 at ambient temperatures and can be described by either eq 1 or eq 2 above the melting point of the crystalline PEO phase. FT-IR. Figure 3 shows a sample FT-IR spectrum obtained for the PEO-PAAM (40 vol %)-NH4SCN (10 mol %) electrolyte. Two regions are of particular interest: the CtN stretching vibration in the 2100-2000 cm-1 region giving the information about the environment of the SCN- anion and the region between 1200 and 1000 cm-1 associated with C-O-C stretching vibrations and giving insight into the ether oxygen environment. In Figure 4 FT-IR spectra for the PEO-PAAM (40 vol %)NH4SCN electrolytes containing 10 and 30 mol % of NH4SCN are shown for the 1200-1000 and 2100-2000 cm-1 regions. For the sample containing 10 mol % of NH4SCN, broad peaks are observed at about 2060 (CtN) and 1100 cm-1 (C-O-C). The spectrum recorded for the sample containing 30 mol % of NH4SCN is more complicated. In the CtN stretch region in addition to the broad peak at about 2060 cm-1 a shoulder appears at about 2040 cm-1. The C-O-C region for the 30 mol % sample consists of four peaks found at 1136, 1114, 1103, and 1079 cm-1. For both samples a single C-H wagging mode is observed at about 1350 cm-1, indicating the amorphous nature of the polyether matrix; for a crystalline polyether this mode would be a doublet at 1343 and 1360 cm-1.15 In Table 1 the positions of the IR modes characteristic of these PEO-PAAM-NH4SCN electrolytes measured at 25 °C are summarized together with the data obtained for the undoped PEO,12 the PEO-NH4SCN electrolyte,16 and PAAM.11 The C-O-C stretching mode observed at 1114 cm-1 for the amorphous phase of the undoped PEO shifts to about 1100 cm-1 after the addition of NH4SCN and remains in the range 11071099 cm-1 for samples of various PAAM concentrations and for NH4SCN concentrations up to 30 mol %. The effects of increase in temperature from 25 °C (Table 1) on the position of the maxima of the CtN and C-O-C modes are shown in Table 2. A single CtN stretch peak is observed at about 2055 cm-1 for samples of various PAAM concentrations (see Table 1). For the PEO-PAAM (40 vol %)-NH4SCN electrolytes containing 20, 25, and 30 mol % of NH4SCN
Figure 4. CtN (a) and C-O-C (b) stretch regions of FT-IR spectra recorded for PEO-PAAM (40 vol %)-NH4SCN electrolytes containing 10 and 30 mol % of NH4SCN.
a doublet at about 2053 and 2044 cm-1 (20 mol % of NH4SCN) or about 2053 and 2034 cm-1 (25 or 30 mol % of NH4SCN) is observed (see Tables 1 and 2). The effect of increasing temperature on the sample with 10 mol % of NH4SCN was also measured but did not show much change from the results in Table 1. We have analyzed the C-O-C and CtN stretch regions using the Grams Galactic 386 package and calculated the intensity contribution of various bonds to these regions of the spectrum. An example of the deconvolution of the FT-IR spectra in the C-O-C region is shown in Figure 5. A fit of the experimental data to a Gaussian-Lorentzian profile is shown. The contribution of each band is calculated as the ratio of the area under a particular band to the total area under all
Electrolytes from the PEO-PAAM-NH4SCN System
J. Phys. Chem., Vol. 100, No. 27, 1996 11385
TABLE 1: Room Temperature FT-IR Data for PEO-PAAM-NH4SCN Electrolytes PAAM NH4SCN concn concn (vol %) (mol %) 0 0b 0 10 20 30 40 50 40 40 40 40 40 100 0c
0 0 10 10 10 10 10 10 5 15 20 25 30 100
CtN (cm-1)
CH2 (cm-1)
C-O-C (cm-1)
1343, 1360 1350 2057 1344, 1360 2060 1353 2057 1351 2060 1353 2062 1351 2055 1350 2057 1351 2055 1350 2054, 2044 1351 2056, 2036 1352 2053, 2033 1352
1153
1099 1058 1114
1150
1100 1062 1107 1101 1102 1099 1102 1101 1103 1104 1136 1114 1103 1079 1137 1116 1102 1078 1095
2054
a
Positions of the maxima of CtN and C-O-C stretching vibrations and CH2 wagging vibrations at 25 °C are given. b Amorphous undoped PEO prepared by melting PEO followed by a fast quenching to ambient temperatures. c Data for NH4SCN.
TABLE 2: FT-IR Data for PEO-PAAM (40 vol %)-NH4SCN Electrolytes as a Function of Temperature NH4SCN concn CH2 CtN (cm-1) T (°C) (mol %) (cm-1) 25 35 45 55 65 75 85 95
10 20 30 10 20 30 10 20 30 10 20 30 10 20 30 10 20 30 10 20 30 10 20 30
2062 2054 2053 2061 2054 2053 2061 2054 2053 2059 2053 2053 2055 2053 2052 2055 2051 2052 2054 2053 2052 2054 2053 2052
2044 2033 2043 2034 2043 2033 2044 2032 2043 2032 2043 2031 2043 2031 2043 2031
1351 1351 1352 1351 1351 1352 1350 1351 1352 1351 1351 1352 1350 1351 1352 1350 1351 1352 1350 1351 1352 1350 1350 1352
Figure 5. Fit of the FT-IR spectra (C-O-C stretch region) using the Grams 386 program for the PEO-PAAM (40 vol %)-NH4SCN (30 mol %) electrolyte.
C-O-C (cm-1)
1137 1116 1137 1116 1136 1116 1136 1116 1136 1115 1136 1115 1136 1115 1136 1115
1099 1104 1102 1100 1105 1102 1098 1104 1102 1101 1104 1102 1105 1104 1101 1105 1104 1101 1105 1104 1101 1105 1104 1101
1076 1077 1075 1077 1074 1077 1077 1077 1076 1077 1076 1076 1076 1076
Figure 6. Changes in the percent intensity as a function of temperature for bands included in the C-O-C stretch mode for the PEO-PAAM (40 vol %)-NH4SCN (10 mol %) composite polymer electrolyte.
1077 1076
Effective Medium Theory (EMT) Model
a
Positions of the maxima of CtN and C-O-C stretching vibrations and CH2 wagging vibrations are given.
bands present in the particular IR region. All band areas were normalized to the CH2 stretching vibration of PEO. In Figure 6 changes in the fractional intensity of the C-O-C bands are depicted as a function of temperature for the PEOPAAM (40 vol %)-NH4SCN (30 mol %) electrolyte. Similar results are observed for the samples containing 20 and 25 mol % of NH4SCN. The contribution of the 1135 cm-1 mode increases with increase in temperature for all of the samples analyzed. A decrease in the intensity of the 1100 cm-1 mode with an increase in temperature is also observed. The temperature behavior of the 1115 and 1075 cm-1 bands is more complicated, but for all the samples the intensity of the former decreases and the intensity of the latter increases at temperatures above the melting point of the crystalline PEO phase (Tm ∼ 65-70 °C).
The EMT approach for modeling the conductivity of composite solid electrolytes containing dispersed nonconductive fillers has been sucessfully used for a number of systems.11-13,17-19 This EMT model has been extensively discussed elsewhere.11,13,19 The model connects an enhancement of ionic conductivity with the existence of a highly conductive amorphous layer at the polyether-organic filler interface. This layer exhibits a conductivity considerably higher than that for the matrix polyether. It is suggested that in PEO-PAAM-NH4SCN composite electrolytes there are three components with different electrical properties. These are: (1) highly conductive uncomplexed polyether interface layers surrounding the PAAM core; (2) dispersed insulating PAAMNH4SCN complexes; (3) a matrix polymer ionic conductor ((PEO)10NH4SCN). Since the structure of the interface layer (component 1) is highly amorphous, it is assumed in what follows that the
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TABLE 3: Parameters Used for Calculating the Temperature and Composition Dependence of the Conductivity of Composite PEO-PAAM-NH4SCN Polymeric Electrolytes Using EMT Models
a
kind of phase
A/S K0.5 cm-1
T0/K
BkB-1/K
(PEO)10NH4SCN surface layer
25.1 15.0
226 Tg - 25 K
1453 700
a
It has been assumed that the conductivity of component 2 is temperature independent and equal to 10-12 S/cm. The conductivity of the surface layer is calculated to be ≈2.4 × 10-14 S/cm at 25 °C. The Tg of the surface layer has been calculated on the basis of eq 3 using the following parameters: K0 ) 253, K1 ) -83.4, K2 ) 112.5. The conductivity of the PEO-NH4SCN electrolyte below the melting point of the crystalline PEO phase has been calculated using eq 2, and values of Ea and σ0 are respectively equal to 0.315 eV and 1.58 × 10-3 S K/cm. t/R ) 0.4 and was calculated according to the method shown in ref 11.
b
temperature dependence of its conductivity (σ1) would follow eq 1. It was also assumed, for the calculation of the σ1 as a function of the PAAM concentration, that the pseudoactivation energy B and the preexponential factor A were independent of the dispersed filler concentration. Therefore, the conductivity of the interface layer is only dependent on T0 and decreases with an increase in Tg. Values of Tg were taken from DSC measurements and used for the calculation of σ1 for electrolytes with various concentrations of the PAAM. To generalize our model the variation of Tg as a function of PAAM concentration was considered. It was found that a good fit is obtained when Tg is approximated by
Tg ) K0 + K1V2 + K2V22
(3)
In this equation (used in fitting our data), K0 corresponds to the Tg of the (PEO)10NH4SCN electrolyte, K1 describes the influence of the PAAM additive on the Tg of the composite system, and K2 is connected with the polymer-PAAM-salt interaction. Finally, in this model σ1 has been calculated using eq 1 in which values of T0 are related to the Tg values calculated from eq 3. σ1 as obtained is then used to calculate the conductivity of the composite unit.11 The conductivity of component 2 was assumed to be temperature independent and was measured as equal to 10-12 S/cm at room temperature. The total conductivity of composite electrolytes is then calculated according to the model previously described.11 For the calculation of the temperature dependence of the conductivity of the PEO-NH4SCN electrolytes (component 3), eq 2 is used below Tm and eq 1 is used above. All the parameters used for these calculations are summarized in Table 3. In Figure 7a,b the EMT model fits well to the experimental conductivity data at 25 and 50 °C. Disagreement is observed for the sample containing 10 vol % of PAAM at 50 °C (see Figure 7b). However, an attempt to fit the temperature dependence of conductivity using this EMT model has not been completely successful. The model fits the data quite well up to 60 °C but significant differencies between the model and experimental data are observed at higher temperatures. Semiempirical Conductivity Relations For electrolytes for which the temperature dependence of the conductivity follows eq 2, the order-disorder temperature (TD) is the temperature at which a change in the conduction mechanism is expected. TD can be calculated on the basis of a semiempirical approach described elsewhere.12,20 To calculate TD in the temperature region in which the electrolytes obey the Arrhenius equation, the following equation is used.
Figure 7. Comparison of the experimentally measured isothermal conductivities [b] for the PEO-PAAM-NH4SCN (10 mol %) composite electrolytes with theoretically calculated conductivities on the basis of the EMT model (solid line): (a) 25 and (b) 50 °C.
ln σ0 )
Ea + ln Ktω0 ) REa + β kBTD
(4)
where R ) 1/kBTD and β ) ln Ktω0. Kt is the theoretical cation and anion charge carrier concentration term, and ωo is an ionic oscillation frequency. Figure 8 shows a plot of ln σ0 versus Ea for PEO-PAAMNH4SCN electrolytes of different PAAM concentration with NH4SCN concentration equal to 10 mol %. Two sets of data based on the ln σ0 and Ea values calculated on the basis of experimental conductivity data and on the basis of conductivity data obtained from EMT calculations are presented. In both cases the temperature dependence of conductivity is analyzed using eq 2 below approximately 60 °C (i.e., in the region where the temperature dependence of conductivity follows an Arrhenius type behavior (see Figure 2)). For both the experimental and the EMT model, a linear relation between ln σ0 and Ea is apparent. The characteristic temperatures TD calculated for both data sets agree and are equal to 71 °C. This temperature roughly corresponds to the melting temperature of the undoped PEO phase. Such behavior has been previously found for other composite polymeric electrolytes.19 Discussion As can be seen from Figure 1a, the addition of PAAM to PEO-NH4SCN results in an increase in the conductivity of the electrolyte. The room temperature conductivity measured for the NH4SCN-doped system is higher (1 × 10-4 S/cm) than that
Electrolytes from the PEO-PAAM-NH4SCN System
Figure 8. ln σ0 versus Ea for PEO-PAAM-NH4SCN (10 mol %) electrolytes (samples of various PAAM concentration): (b) experimental data and (O) data calculated on the basis of the EMT model. The solid line is a fit of eq 11 to the experimental data.
measured for the lithium11 or sodium13 salt doped electrolytes. The ClO4- and SCN- anions are comparable in these systems.16,21 The conductivity maximum appears at a higher PAAM concentration (40 vol %) than was previously found for the LiClO411 or NaClO413 doped PEO-PAAM blend based electrolytes where the conductivity maximum occured at 25 vol % of PAAM. This is explained on the basis of experimental evidence which indicates that NH4+ is more losely bound to the polyether chain than are Li+ and Na+.22 Contrary to the case of LiClO4-doped PEO-PAAM, we were unable to find two glass transition temperatures in the PEO-PAAM-NH4SCN system. In PEO-PAAM-LiClO4 the lower Tg was that of the undoped PEO phase. Also, changes in the degree of polyether crystallinity observed after the addition of PAAM10 were not so pronounced as for LiClO4-doped PEO-PAAM. The single value of Tg and the enhanced conductivity noted above suggests that the softer Lewis acid NH4+ has a weaker interaction with the PEO-PAAM components than previously described for the PEO-PAAM-LiClO4 system.11 Moreover, it seems that the NH4+ cations have a much weaker coordination to ether oxygens. This is manifested by the relatively small shift (∼15 cm-1) of the C-O-C stretch vibrations to lower frequencies after the addition of the salt. In PEO-PAAMLiClO4 the shift was ∼30 cm-1.11 On the other hand, both PAAM and NH4+ are also capable of hydrogen bonding with various electronegative centers (such as the ether or carbonyl oxygens or the sulfur or nitrogen present in SCN-). The concentration of contact ion pairs would be reduced if the NH4+ cations were immobilized by bonding with PAAM, increasing the concentration of mobile anions and therefore the ionic conductivity. This possibility is supported by FT-IR studies of the anion vibration region which show that for NH4SCN concentrations lower than 20 mol % only the (CtN) mode in the 2055-2060 cm-1 region is observed (see Tables 1 and 2); there is no evidence of an observed 2040 cm-1 shoulder which occurs at higher NH4SCN concentrations and is discussed further below. In earlier work23 on solutions of alkali metal thiocyanates in dioxane or tetrahydrofuran, Chabanel and Wang assigned the band at 2060 cm-1 to the presence of contact ion pairs and the 2052 cm-1 vibration to the presence of “free” anions. In our electrolyte the CtN stretching mode at ∼2055 cm-1 is probably a mixture of vibrations due to “free” anions and associated species, although it is difficult to distinguish between them because of the weak interactions involving NH4+.
J. Phys. Chem., Vol. 100, No. 27, 1996 11387 This mode shifts down in frequency by about 5 cm-1 with an increase in the concentration of PAAM and NH4SCN (see Table 1). A similar shift is observed with an increase in temperature for the PEO-PAAM (40 vol %)-NH4SCN (10 mol %) electrolyte (see Table 2). However, for higher NH4SCN concentration the frequency of this mode remains at about 2052 cm-1. As can be seen from Table 1 there is relatively little shift in this vibration with increasing PAAM concentration, indicating that the SCN--PAAM interaction is much weaker than the SCN--NH4+ interactions. For PAAM concentrations exceeding 40 vol % and NH4SCN concentrations exceeding 20 mol %, the shape of IR spectra is more complicated. Low-frequency shoulders (Figure 4) emerge below the CtN stretching region. On the basis of previous reports,23,24 the peak appearing at about 2045 cm-1 (samples containing 20 mol % of NH4SCN) and at about 2040 and 2035 cm-1 (samples containing 25 and 30 mol % of NH4SCN) can be attributed to (NH4SCN)2 dimers formed via the N atoms. This suggestion is confirmed by the studies of the S-C stretching vibration region in which two weak peaks are observed between 736 and 738 cm-1 and between 748 and 752 cm-1. The intensity of the latter increases with an increase in NH4SCN concentration. Chabanel et al.23 attribute the vibrations in the 736-738 cm-1 region to the presence of free SCNanions. We attribute the vibrations occurring in the 748-752 cm-1 region to the presence of (NH4SCN)2 dimers. The position of the maximum of this vibration in our samples occurs at slightly lower frequencies than those observed by Chabanel and Wang23,24 for solutions of KSCN in tetrahydrofuran. This once again confirms that weaker interactions than those in the case of alkali metal salts are occurring in the systems containing NH4SCN. The FT-IR data presented are in good agreement with conductivity data shown in Figure 1b. For NH4SCN concentrations higher than 10 mol %, the formation of (NH4SCN)2 dimers lowers the number of mobile charge carriers, which accounts for the decrease in conductivity to a minimum as observed in Figure 1b. The small increase in the conductivity observed for samples containing 25 and 30 mol % of NH4SCN might be the result of a partial redissociation of dimers due to an increase in the dielectric constant of these composite electrolytes caused by the presence of NH4SCN. Similar effects have been previoulsy observed for other polymer electrolytes doped with ammonium salts.25 Participation of PAAM in interactions with NH4+ is also confirmed by an analysis of the C-O-C stretching region for the PEO-PAAM (40 vol %) electrolytes containing more than 20 mol % of NH4SCN. The mode observed at 1100 cm-1 (see Figure 4 and Table 2) is attributed to the interaction between PEO and NH4+ (see data for the PEO-NH4SCN electrolyte in Table 1). The intensity of this mode decreases with an increase in the NH4SCN concentration. On the other hand, the 1075 and 1115 cm-1 modes which are not observed in the IR spectra for lower PAAM and NH4SCN concentrations are present in the spectra recorded for the PEO-PAAM (40 vol %) (NH4SCN 20, 25, 30 mol %) samples. In the pure PAAM a mode at 1095 cm-1 is observed (see Table 1). This mode should be shifted down by interactions involving PAAM and NH4+. Therefore, the 1075 cm-1 peak is attributed to interactions involving PAAM. The presence of this peak sugggests that in addition to the bonding of NH4+ to PEO two other types of interactions should be considered: interactions involving the linkage of ether oxygens in PEO to the carbonyl groups in PAAM by the NH4+ cations and interactions involving the bonding of NH4+ with PAAM. The intensity of this 1075 cm-1 band is higher for systems containing 25 and 30 mol % of NH4-
11388 J. Phys. Chem., Vol. 100, No. 27, 1996 SCN than for the electrolyte containing 20 mol % of NH4SCN and is the highest for the PEO-PAAM (40 vol %) NH4SCN (10 mol %) electrolyte which has the highest room temperature conductivity (see Figure 1a). The 1115 cm-1 band has been assigned to a combination of the symmetric and antisymmetric C-O-C stretching vibrations in undoped PEO. It is also interesting to note that the intensity of the 1100 cm-1 band decreases with an increase in temperature, which suggests a weakening of the PEO-NH4+ interactions. This is accompanied by an increase in the intensity of the 1135 cm-1 band which on the basis of literature data26 can be assigned to the C-O-C stretch of the uncomplexed PEO phase. An increase in the concentration of an uncomplexed PEO phase might be due to the participation of NH4SCN in the formation of dimers. Interactions between NH4+ and PAAM are also supported by studies of the CdO amide I region 1700-1600 cm-1. For samples containing 40 vol % of PAAM and more than 10 mol % of NH4SCN, a shoulder appears at 1643 cm-1 in addition to a wide peak with a maximum at around 1655 cm-1 observed for all the samples studied. This shift in the CdO symmetric stretch frequency is lower than that previously observed for composite electrolytes doped with LiClO412 but nevertheless still indicates a coupling between CdO carbonyl groups and NH4+ cations. The formation of hydrogen bonds between NH4+ and NH2 amide groups is also possible, but this is more difficult to study due to the overlapping of N-H vibrations from different groups in the 3000-3500 cm-1 region. The above observations confirm the presence of interactions involving NH4+ and PAAM. With an increase in temperature, the crystalline PEO phase melts, thus increasing the concentration of the amorphous phase. This results in a jump in conductivity as observed in Figure 2 for the PEO-PAAM (40 vol %)-NH4SCN (10 mol %) system. An increase in the fraction of the conductive, amorphous PEO phase at about 65 °C might be the reason that we were unable to fit the conductivity data using the EMT model above this temperature. Generally the difficulties in fitting conductivity data using the EMT model confirm limitations in using this approach for systems in which strong chemical interactions occur. The characteristic order-disorder temperature (TD ) 71 °C) calculated on the basis of a phenomenological conductivity approach coincides with the melting point of the crystalline PEO phase at which temperature there is a change in the conduction process as observed in Figure 2. Conclusions We have shown that higher conductivities can be obtained for NH4SCN-doped PEO-PAAM composite electrolytes than for these systems doped with LiClO4 or NaClO4. As for all of these composite systems the conductivity of the PEO-NH4SCN electrolyte can be improved by the addition of PAAM. The highest room temperature conductivity (1 × 10-4 S/cm) was measured for the PEO-PAAM (40 vol %)-NH4SCN (10 mol %) sample. This increase in conductivity results from interactions involving the bonding of NH4+ to PAAM which enhances the mobility of the SCN- anions. The occurrence of
Zalewska et al. interactions involving PAAM is confirmed by the IR studies of the C-O-C stretching region which shows a weakening of the interaction between PEO and NH4SCN for samples containing 40 vol % of PAAM, i.e., those of the highest ionic conductivity. The compositional dependence of the conductivity can be modeled using the EMT approach, with satisfactory qualitative agreement. Acknowledgment. W.W. thanks NSERC Canada and the Research Office of NATO for an International Post Doctoral Fellowship award. A.Z. thanks the Research Office of NATO for a support under a NATO linkage grant. The research was supported, in part, by an NSERC Industry Oriented Research grant. References and Notes (1) Scrosati, B. Applications of ElectroactiVe Polymers; Chapman and Hall: London, 1993. (2) Gauthier, M.; Belanger, A.; Kapfer, B.; Vassort, G.; Armand, M.; In Polymer Electrolyte ReViews-1; MacCallum, J. R., Vincent, C. A., Eds.; Elsevier: London, 1989; Chapter 9. (3) Granqvist, C. G. In Handbook of Inorganic Electrochromic Materials; Elsevier: Amsterdam, 1995; Chapter 26. (4) Lassegues, J. C. In Proton Conductors: Solids, membranes and gels-materials and deVices; Colomban, P., Ed.; Cambridge University Press: Cambridge, 1992; Chapter 20. (5) Stainer, M.; Hardy, L. C.; Whitmore, D. H.; Shriver, D. F. J. Electrochem. Soc. 1984, 131, 784. (6) Ansari, S. M.; Brodwin, M.; Stainer, M.; Druger, S. D.; Ratner, M. A.; Shriver, D. F. Solid State Ionics 1985, 17, 101. (7) Chandra, S.; Mauraya, K. K.; Hashmi, S. A. In Recent AdVances in Fast Ion Conducting Materials and DeVices; Chowdari, B. V. R., Liu, Q. G., Chen, L. Q., Eds.; World Scientific Publishing: Singapore: 1990; p 549. (8) Chandra, A.; Srivastava, P. C.; Chandra, S. In Solid State Ionics Materials and Applications; Chowdari, B. V. R., Chandra, S., Singh, S., Srivastava, P. C., Eds.; World Scientific Publishing: Singapore: 1992; p 397. (9) Daniel, M. F.; Desbat, B.; Lassegues, J. C. Solid State Ionics 1988, 28-30, 632. (10) Da¸ browska, A.; Wieczorek, W. Mater. Sci. Eng. B 1994, 22, 117. (11) Wieczorek, W.; Such, K.; Florjan´czyk, Z.; Stevens, J. R. J. Phys. Chem. 1994, 98, 6840. (12) Wieczorek, W.; Zalewska, A.; Raducha, D.; Florjan´czyk, Z.; Ferry, A.; Jacobsson, P.; Stevens, J. R. Macromolecules 1996, 29, 143. (13) Wieczorek, W.; Such, K.; Chung, S. H.; Stevens, J. R. J. Phys. Chem. 1994, 98, 9047. (14) Cruickshank, J.; St. A. Hubbard, H. V.; Boden, N.; Ward, I. M. Polymer 1995, 36, 3779. (15) Li, X.; Hsu, S. L. J. Polym. Sci., Polym. Phys. Ed. 1984, 22, 1331. (16) Prusinowska, D.; Wieczorek, W.; Wycis´lik, H.; Siekierski, M.; Przyłuski, J. Solid State Ionics 1994, 72, 152. (17) Nan, C. W.; Smith, D. M.; Mater. Sci. Eng. B 1991, 10, 99. (18) Nan, C. W. Prog. Mater. Sci. 1993, 37, 1. (19) Wieczorek, W.; Siekierski, M. J. Appl. Phys. 1994, 76 (4), 2220. (20) Almond, D. P.; West, A. R. Solid State Ionics 1986, 18,19, 1105. (21) Chandra, S.; Hashmi, S. A.; Prasad, G. Solid State Ionics 1990, 40/41, 651. (22) Ferry, A.; Jacobsson, P.; Stevens, J. R. Accepted for publication in J. Phys. Chem. (23) Chabanel, M.; Wang, Z. J. Phys. Chem. 1984, 88, 1441. (24) Chabanel, M.; Lucon, M.; Paoli, D. J. Phys. Chem. 1981, 85, 1058. (25) Van Heumen, J.; Wieczorek, W.; Siekierski, M.; Stevens, J. R. J. Phys. Chem. 1995, 99, 15142. (26) Bailey, F. E., Jr.; Koleske, J. V. In Poly(ethylene oxide); Academic Press: New York, San Francisco, London, 1976; Chapter 6.2.
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