6840
J. Phys. Chem. 1994,98, 6840-6850
Polyether, Polyacrylamide, LiClOd Composite Electrolytes with Enhanced Conductivity W. Wieczorek,? K. Such,? Z. Florjanczyk,#and J. R. Stevens'n Department of Physics, University of Guelph, Nl G 2W1 Guelph, Ontario, Canada, and Department of Chemistry, Warsaw University of Technology, ul. Noakowskiego 3, 00-664 Warszawa, Poland Received: March 2, 1994; In Final Form: April 28, 19946
The results of detailed studies of the ionic conductivity and ultrastructure of polymer blends complexed with LiC104 are presented and discussed and include comparisons with undoped blends. These composite polymer electrolyte systems are studied over a temperature range -1 10 to 150 OC using differential scanning calorimetry (DSC), with room temperature FT-IR, and with -20 to 100 O C impedance analysis and consist of blends of poly(ethy1ene oxide) (PEO) or oxymethylene-linked PEO (OMPEO) with polyacrylamide (PAAM). The high molecular weight PAAM is found to inhibit the crystallization of PEO without impeding segmental motion. In fact the ionic conductivity is enhanced in the blends compared to the PEO-LiC104 complex. Conductivities exceeding lo"S/cm at room temperature were obtained for electrolytes prepared by the in situ polymerization of acrylamide in the polyether. Annealing the blends at 150 OC for 10-15 min makes the ultrastructure (recrystallization, melting, glass transitions) as initially observed by DSC less complex. Generally from DSC and FT-IR the ultrastructure appears to consist of emulsified PAAM complexed to itself and to polyether segments via Li+ cations surrounded by relatively uncomplexed polyether segments. At higher PAAM concentrations the PAAM-LiC104 nonconducting cores increases in size, reducing the region of uncomplexed polyether; the ionic conductivity decreases. Assuming that the enhanced conductivity of these composite polymer electrolytes is associated with interphase phenomena, the conductivity results were analyzed in terms of a model based on effective medium theory.
Introduction Since the first reports by Wright et a1.I and Armand et a1.,2 polymeric electrolytes based on polyether matrices have received considerable attention due mainly to the possibility of their application invarious electrochemical devices working at ambient temperatures. It has been recognized' that in such electrolytes ionic transport occurs in the amorphous regions of the polymer and is very often the result of a coupling between the ions and segmental motions of the polymer chains. Therefore flexible, amorphousmacromolecules containing a heteroatom (usually 0, N, S) with lone electron pairs of a donor power sufficient to complex cations are the most suitable candidates for matrices for polymer ionic conductors. Poly(ethy1ene oxide) (PEO), which is so far the most widely studiedpolymer matrix, is semicrystalline under ambient conditions and forms crystalline complexes with most of the salts used as ionic dopants. To inhibit crystallization, the simplest approach appears to be the synthesis of composite polymeric electrolytes. Regarding this approach,there have been a number of recent review^.'^ The idea of composite solid electrolytes was introduced by Liangs who improved the electrical properties of a LiI solid electrolyte by the addition of small-grained cr-Al203. This approach has been explored by several research teams9who have shown that the conductivity of various crystalline, polycrystalline, and glassy ionic conductors can be significantly increased by the addition of an inorganic powder. In addition it is well-known that the introduction of inorganic fillers into a polymer matrix improves the mechanical stability of the host polymer and extends its temperature stability range.10 The concept of composite polymeric electrolytes has currently been widely explored by several research groups.ll-29 Our previous studies on mixed-phase polymeric electrolytes containing conductive fillers such as NASICON,I1B-alumina,Iz and glassy fillers12 have shown that these fillers do not contribute to the ionic University of Guelph. Warsaw University of Technology. *Abstract published in Advance ACS Al~stracts.June 15, 1994. f
0022-3654f 94f 2098-684O$O4.50f 0
conductivity of the mixed-phase systems. Similar results were described by Scrosati and co-workersIs-15for polymeric electrolytes containing8- and p'-aluminas. However, Skaarup et al.17J8 reported that for composite systems containing high amounts of conducting fillers (exceeding 85 vol 9%) the conductivity occurs via a dispersed phase and polymers act as binders for ceramic grains. The decrease in conductivityin comparison with pristine ceramic electrolytes is due to the dilution effect of the polymer host. Similar results demonstrating the contribution of the conducting filler to the conductivityof the mixed-phase electrolytes have been obtained by Stevens and MellanderI6 for systems containing PEO and RbAg415 or KA&15 as conductive ceramic additives. Because results obtained for ionically conducting fillers have been contradictory, nonconducting inorganicparticles have been used as fillers in most of the recent studies.19-29 Weston and Steele19 used a-Al203 particles (grain size, 40 pm) to improve the mechanical stability of a PEO-LiC104 electrolyte. Later, it was recognized that the addition of fine inorganic fillers (grain size, 1-3 pm) led to an improvement in the mechanicalproperties and an increase in the ambient temperature conductivity of the electrolytes studied.20J' This increase results from a decrease in the electrolyte crystallinity, as has been shown by nuclear magnetic resonance (NMR) and X-ray investigations.22-25 The addition of small inorganic particles stiffens the electrolyte host24 and decreasesthe crystallinity of the system. The effect of grain size distribution, particle concentration, and surface area on the conductivity and phase structure of the composite electrolytes has been discussed." Significantimprovements in the conductivity of polymeric systems are usually obtained using fine-grained (1-2 pm) powders, with large effective surface areas and concentrations of 10-20 mass 9%. For higher concentrations the formation of nonconducting particle aggregate regions, which lower the bulk conductivity of the electrolytes, is observed. Scrosati and co-workers have been studying systems containing y-LiA102 as fillers.2629 The authors have commented on the role of inorganicfillers in the improvement of the electrochemical stability of polymeric e l e c t r ~ l y t eand s ~ ~on~the ~ ~properties of the 0 1994 American Chemical Society
Polymer Blends Complexed with LiC104
7'he Journal of Physical Chemistry, Vol. 98, No. 27, I994
6841
lithium electrode-composite polymeric electrolyte i n t e r f a ~ e . ~ * * ~ TABLE ~ 1: D6c Data for Pure PEO and PAAM and Their Blends Obtained by a Solvent-Cast Technique (9) or by Mrect The highest conductivitiesmeasured for all the compositesystems Polymerization of Acrylamide in the PEO Matrix (p) studied were 10-5 S/cm at room temperature, i.e. lower than the conductivityreported for alkali metal salt complexesof random PAAM run T d To/ Qcl Tm/ Qm/ Xcf sample vol% no. O C OC (Jg-1) O C ( ~ g - 1 ) % oxymethylene-linkedpoly(ethy1ene oxide) (OMPE0),3°-32which we believe to be the room temperature conductivity of an ideal PEO 0 1 63 158 74 amorphous PEO phase. Some authors attribute this fact to a PEO 0 2 -53 60 110 52 PEO (s) 0 1 -54 59 132 62 dilution e f f e ~ t ' ~ Jdue * * ~to~the filler particles. However, it has PEO(s)4" 0 1 63 156 73 been shown by NMR22 and DSC23 that the addition of a stiff PAAM 100 1 165 filler reduces the chain flexibility and increases the glass transition PAAM 100 2 165 temperature of the amorphous phase. This in turn lowers the 54 81 55 B (8) 30 1 mobility of the charge carriers. 55 82 55 B (SI 30 2 22 For a softer component which can inhibit PEO crystallinity 50 1 -56 42 24 B (SI 23 50 2 -49 50 25 B (SI without impeding the chain segment motion and ionic carrier 55 14 22 B (5) 70 1 mobility we have utilized high molecular weight polymers which 58 15 24 B (9) 70 2 form immiscible blends with PE0.33*34 Electrolyte systemsbased 5.4 36 23 15 €3 (PI 30 1 -47 -24 on PEO or OMPEO, LiC104, and a high molecular weight 1.6 22 31 21 B (PI 30 2 -57 -44 polyacrylamide (PAAM) were chosen for experiments reported 5.9 34 5.4 5.0 B (P) 50 1 -50 -31 9.1 4 15 14 B (PI 50 2 -55 -43 here. Detailed studies of conductivity and structural characB (P) 70 1 -46 10 1.8 3 terization are described. Preliminary studies33 have shown that 70 2 -54 -34 4.2 3 2.5 4 B (PI conductivities up to 4 X le5S/cm at room temperature can be 5.6 20 12 8 €3 (P) 4" 30 1 -59 -46 obtained for these composite electrolytes. 7.5 15 7.0 7 B (PI 4" 50 1 -59 -33 For comparison purposes results of experiments performed for B (PI 4" 70 1 -57 0.8 11 7.0 5 undoped blends are presented. The conductivity of composite B (PI 6" 30 1 -59 -42 9.0 10 2.3 2 B (P) 6" 50 1 -58 -39 electrolytesis analyzed in terms of effectivemedium theory (EMT) B (PI 6" 70 1 -58 approaches, which assume that an increase in the conductivity in composite polymeric electrolytes in comparison to pure Values obtained 4 and 6 weeks after blend preparation. polyether systems is associated with interphase phenomena. Ambient temperature conductivities as high as 10-4S/cm have stainless steel blocking electrodes and placed in a temperaturebeen measured. The fit of the theoretical data (calculated on the controlled furnace. The impedance measurements were carried basis of the EMT model) to experimentally measured conductiviout on a computer-interfaced HP 4192A impedance analyzer ties is presented and discussed. over the frequency range 5 Hz to 13 MHz. Peak-to-peakvoltage used for impedance measurements was equal to 1 V. Experimental Section Fourier Transformation Infrared Spectroscopy. Infrared spectra were recorded on a computer-interfaced Nicolet FT-IR Sample Preparation. PAAM (Mw= 1.6 X lo5)was prepared system 4.4 instrument with a wavenumber resolution of 2 cm-1. by the free radical polymerization of acrylamide (Aldrich, reagent Thin-film electrolyte foils were sandwiched between two NaCl grade) in an acetonitrile solution using benzoyl peroxide (Aldrich, plates. reagent grade) as initiator. The product was dried under vacuum at 100 OC for 48 h. PEO (Aldrich, reagent grade, Mw = 5 X lo6) Results was dried under vacuum at 50-70 OC for 48 h prior to use. OMPEO was prepared according to the procedure described Initially we obtained the relevant characteristics of pure elsewhere30 and dried under vacuum for 48 h before electrolyte polymers (PAAM, PEO, OMPEO) and their undoped blends. synthesis. Acetonitrile (Aldrich, reagent grade) was distilled DSC data obtained for pure PEO, PAAM, and their blends are twice under vacuum over molecular sieves type 4A before use. summarized in Table 1, in which Tcand Qc are, respectively, the All of the steps in the preparation procedure were performed in recrystallizationtemperature and the crystallizationheat for PEO. an argon-filled drybox (moisture content lower than 20 ppm). All the thermal effects are calculated with respect to the PEO LiC104 (Aldrich, reagent grade) was dried under vacuum at 120 content in the blends. The blends were obtained by mixing both OC prior to incorporation into our polymeric systems. The solid polymers in an acetonitrile suspension or by the direct polymcomponents were mixed in stoichiometric amounts in a small erization of acrylamide in an acetonitrile solution of PEO. The glass reactor, and then acetonitrile was added to form an PEO used in our investigations is highly crystalline. The degree approximately 5 mass % suspension with respect to all solid of crystallinity Xc is calculated from a comparison of the heat of components. The mixture was stirred magnetically until a melting (Qm)of the PEO used with the heat of melting found for homogenous suspension was obtained. Excess acetonitrile was the crystalline PEO phase (Q,,,~Eo= 213.7 J/g)35and is around removed by vacuum distillation. The composite electrolytes 74%. Xc = Q m / Q e E O . The glass transition temperature (TJ, obtained were dried under vacuum for 48-72 h at 60 OC. The which is sometimes difficult to find due to the high crystallinity concentration of PAAM in the composite electrolytes varied of the samples studied, is equal to -53 OC. PAAM, on the other between 5 and 50 ~ 0 1 % . hand, is a purely amorphous rigid polymer with T,= 165 "C. As DSC Studies. DSC data were obtained between -1 10 and 150 can be seen from Table 1, the addition of PAAM, especially for OC using a DuPont TA 2910 scanning calorimeter with a low higher concentrations, substantially lowers the degree of crystaltemperature measuring head and liquid-nitrogen-cooledheating linity of the PEO component, which for the solvent-cast blends eltment. In Yrun 1" 15-mg samples were loaded into aluminum decreases with an increase in PAAM concentration. The onset pans, then stabilized by slow cooling to -1 10 OC, and heated at temperature of the melting peak shifts to slightly lower tem10 OC/min to 150 O C . Run 2 was performed after annealing the peratures in comparison to that for the pure PEO sample. same samples used in run 1 at 150 OC (for approximately 10-15 The effect of the addition of PAAM is even more noticeable min) and then following the same procedure as for run 1. in the case of blends prepared by the polymerization of acrylamide in a PEO environment. Two first-order transitions are observed Conductivity Measurements. Ionic conductivitywas determined in DSC analysis. These occur at Tcand T, which are associated, using the complex impedance method in the temperature range respectively, with the recrystallization of the PEO host and the -20 to 100 OC. The samples were sandwiched between two
-
6842 The Journal of Physical Chemistry, Vol. 98, No. 27, 1994
Wieczorek et al.
0.0 0
x
a,
-
-0.2
4
I
M
b
5-0.4
k W
a
-0.6
-0.8 -100-80
- 6 0 -40 - 2 0
0
20
4.0
60
80
100 120 '140
T/'C
Figure 1. DSC traces obtained for OMPEO-PAAM blends (samples containing (a) 50 vol % and (b) 70 vol % PAAM) prepared by the solvent-cast technique. QF is a heat flow.
TABLE 2 DSC Data Obtained for OMPEO and Its Solvent-Cast Blends with PAAM. sample OMPEO OMPEO €3 (5)
B (5) B (8) B (4 B (8) B (9)
PAAM run Tgl TEI QCI TmJ vol% no. OC OC (Jg') OC
0 0 30 30 50 50 70 70
1 2 1 2 1 2 1 2
-58
4 3 -57 -33 -58 -54 -62 -62 -56 -56
15.8 11.2
Qml
Xcl
(Jg') %
10 37 4 38 -11, 10 3.0, 16 -6, 10 2.1,20 -11,9 4.7, 12 -9,9 4.4, 10 -8, 10 1.5,6.9 -14, 12 2.2, 5.8
18 18 13 15 16 14 13 12
Symbols as in the text and Table 1. melting of the crystalline PEO phase. T, shifts to much lower temperatures than for pure PEO and decreases with an increase in PAAM concentration. The glass transition temperature of the amorphous PEO phase in these blends is much easier to find than for thesolvent-cast blends. Note that the T,values measured in run 2 are lower than those measured in run 1. An additional slight decrease in these Tis is observed during the long-time annealing of the blends at ambient temperatures. T, values measured approximately 6 weeks after sample preparation are 5-7 K lower than those measured for the pure PEO. Surprisingly, X, for the PEO component does not increase during annealing, as might be expected. There is even a small decrease in X, with annealing for blends prepared by the polymerizationof acrylamide in the PEO matrix. These results show that a strong interaction has occurred between PEO and PAAM, leading to a reduction in X,with an increase in the flexibility of the polymer segments in spite of the stiff PAAM filler, which one might expect should increase the T, of the amorphous PEO phase in comparison to that for the pure amorphousPEO component. Also it is interesting to note that T8 for PAAM is dearly indicated in the DSC traces for 70 vol % of PAAM and occurs at 132 OC. We have also examined the thermal behavior of blends of amorphous OMPEO with PAAM. These blends were prepared by casting both polymers from an acetonitrile suspension. DSC traces obtained for blends with 50 and 70 vol % of PAAM are shown in Figure 1, and DSC data for all the blends studied are summarized in Table 2. The pristine OMPEO is a partially crystalline system characterized by a T, of -58 OC and an X, 25%, where X,is calculated relative to pure PEO. There are two melting peaks in the caseof solvent-cast blend samples(see Figure
-
1). The positionofthesmaller peakappearsat lower temperatures (approximately -10 "C), and the position of the larger peak is approximately the same as for the melting peak of the crystalline OMPEO phase for the unannealed samples. For the solvent-cast blends annealing does not seem to influence T,. Tg values for the amorphous polyether phase and for PAAM are seen in Figure 1. The Tg for the amorphous component is comparable to that for pureOMPEO, whichindicates that theadditionofstiffPAAM does not result in stiffening the polyether host (as might be expected) due to a parallel decrease in X,. The T, for the PAAM component increases from 100 OC for the sample containing 50 vol % PAAM to 130 OC for the sample containing 70 vol % PAAM. It Seems to be clear from these investigations that composites containing PAAM and various polyethers can be very efficient matrices in the design of polymeric electrolytes due to the lowering of Xcand the preservation of high polymer chain flexibility. Composite polymeric electrolytes were prepared by blending PEO, PAAM, and LE104 in an acetonitrile suspension for salt concentrations of 10 and 4 mol 8.The PAAM was homogeneously distributed in the matrix, and no traces of aggregates were in evidencefrom scanning electron microscopy observations. Some composite electrolytes were prepared by the direct polymerization of acrylamide in the presence of PEO performed under the same conditions as in the isolated polymerization of acrylamide. Figure 2 shows a comparison of change in ionic conductivitywith temperature as measured for electrolytes based on blends of PEO-PAAM with those previously obtained for the PEO-LiC104 system. As can be seen, conductivities measured for blend-based electrolytes are higher than for the pristine PEObased material. The highest ambient temperature conductivity (6.8 X 10-5 S/cm at 293 K) was measured for solvent-cast samples containing 25 mass % PAAM. Conductivities exceeding 1 V S/cm at room temperature can be obtained for electrolytes prepared by the direct polymerization technique (see Figure 3). This may result from the better mixing of the PEO host with a stiff polymer such as PAAM. As can be seen from Figure 2, most of the composite systems studied exhibit an Arrhenius-type temperature dependence of conductivity, u = uo exp(-E,/RT), especially at temperatures below T,. DSC studies show evidence of the semicrystalline character of PEO-PAAM-LiC104 electrolytes (see Table 3). The degree of crystallinity calculated from DSC (Table 3) is significantly
The Journal of Physical Chemistry, Vol. 98, No. 27, 1994 6843
Polymer Blends Complexed with LiC104
TABLE 3: Comparison of DSC Data Obtaioed for PEO-PAAM-Lic104 and (PEO) ,&C104EleCtrOlyteS concentration Of PAAM, V O % ~ T,1/'Ca T@/'Ca Tm/'Cb T=/'C' Xcpe~~/% (PEO)l&iC104 -28 65 145 70
-
h
5 15 20 25 30 50
-3 -
5 I n
\-4
-
U
v
Ep
2.50
62 55 52 55 61
-65 -73 -75 -5 3
55
97 87 87 87 102
33 13.7 3.7 3.9 4.3 2.8
a TI1, Ta, glass transition temperatures of the two amorphouspolymer phases. Tm,melting temperature of the crystalline PEO phase. e Tm, fraction melting temperature of the crystalline complex phase. XC~BOI of the crystalline phase calculated with respect to the amount of PEO.
-I -7 I
-2 1 -19 -32 -29 -3 1 -34
-2.5 1
2.75
3.00
3.25
3.50
-3.0
I
1000 K / T
Figure 2. Changes in ionic conductivity versus reciprocal temperature for PEO-PAAM-LiClO4 electrolytes (10 mol 96 LiClO4 with respect to ethylene oxide monomeric units). Samples of different PAAM concentration are in percent by volume. Blends were prepared by mixing PEO and PAAM in an acetonitrile suspension: (0)0 ~0196,(0) 15 vol %, (A) 20 vel%, (0) 25 vol %, (V) 30 vol %, (B) 40 vel%, (A) 50 v0l 96.
-5.0
-5.5 I 2.50
I
2.75
3.00
3.25
3.50
1000 K/T
Figure 4. Changes in the phase boundary conductivity versus reciprocal temperature for PEO-PAAM-LiC104electrolyteswith 10 mol % LiClO4. Samples of different concentration of PAAM are in percent by volume. (0) 20 vol W ,Ea = 0.495 eV; (v)30 vol%,0.506eV; (0)40vol %, 0.471 eV. Lines are drawn as a guide to the eye.
-7 2.50
2.75
3.00
3.25
3.50
1000 K / T
Figure 3. Changes in ionic conductivity versus reciprocal temperature for PEO-PAAM-LiClO4 electrolytes with 10 mol 4% LiC104. Samples of different concentration of PAAM are in percent by volume. Blends were prepared by direct polymerization of acrylamide in the presence of PEO. (0)0 vol %, (0) 30 vol %, (A)40 ~01%. reduced from that for PEO-based electrolytes. The melting peak of the crystalline complex phase, usually observed in the temperature range 150-160 OC for a PEO-LiC104 electrolyte, is not seen. An additional melting peak in the range 80-100 O C is found for all of the samples studied. The presence of this peak can be connected with changes in the crystallization patterns in the PEO-LiC104 complex due to the presence of polyacrylamide. PAAM as well as PEO forms complexes with the Li+ cation. PEO-based blends complexed with LiC104 are multiphase systems in which two amorphous phases of different flexibility can usually be found. As can be seen in Table 3, two values of T8are found for the PEO-PAAM-LiC104 system. The higher glass transition temperature T8,is in the same range as that for the pristine PEO-LiC104 system for concentrations of PAAM above 15 ~ 0 1 % .The T, value found for the second amorphous phase (T,2) is considerably lower, showing that the cross-linking effect due to the salt is not observed for this phase. Tgvalues
increase with increasing PAAM concentration. This suggests that PAAM chains may effect the properties of the amorphous PEO components. The participation of amide groups in the complexation of alkali metal cations is suggested as the reason for the increase in the flexibility of one of the amorphous phases. DSC experiments (Table 3) show that the addition of PAAM decreases the degree of crystallinity of PEO, which is probably one of the reasons for an improvement in the electrolytic conductivity. Conductivities of (PEO)l&iClO4-PAAM electrolytes are higher than those of the (PEO)loLiC104system even at temperatures exceeding the melting point of the crystalline PEO phase. This observation suggeststhat the addition of PAAM leads to a modification of the amorphous PEO component, resulting in an improvement in ionic transport. The relatively high values of ionic conductivity obtained for these systems might be due to a cooperative coupling of polar amide and ether groups through the Li+ cation, which is observed to enhance ionic transport in blend-based electrolytes. Impedance spectra of PEO-PAAM blends pressed between stainless steel blocking electrodesconsist of two semicircles and a low-frequency spur.33 Values of capacitances calculated for the mediumfrequency arc are in the range 10-7-lVF/cm2, characteristic of phase boundary phenomena.% Assuming this, we attributed the medium-frequency semicircle to interactions between PEO and PAAM. Figure 4 presents the temperature dependence of phase boundary conductivitiesmeasured for three PEO-PAAM-LiC104 electrolytes. As would be expected, the slopes of these conductivity
6844 The Journal of Physical Chemistry, Vol. 98, No. 27, 1994
TABLE 4: DSC D8ta for tbe OMPEQ-PAAM-LiC104 Electrolytes vol % T,,/OC T@/OC Tg1J0C T@/OC of PAAM run 1 run 1 run 2 run 2 0 P
5 10 15 20 25 30 40 50 100
-36 -46 -33 -3 1 -39 -32 -45 -3 5 -44 -39
-68 -13 -68 -76 -8 1 -65
-36 -46 -36 -37 -41 -43 -46 -47 -49 -5 1
Wieczorek et al. lo-'
I V
-68 -7 3
V
V
v
v
V
-74 e
To 176 Tg3 176 1W To 172 T@= 172 a 4 mol % LiC104. All the other systems doped with 10mol % LiC104. curves (interpreted as activation energies (E.) on the basis of an Arrhenius-type equation) are almost the same for each of the samples studied. Considering the pronounced effect of PAAM on the conductivity of PEO-based electrolytes, we investigated the conductivity of the OMPEO-PAAM-LiC104 system since OMPEO has an amorphous component similar to that of the amorphous PEO component in pure PEO or undoped PEO-PAAM blends (see Tables 1 and 2). The DSC data in Table 4 show that as in the case of PEO-based blends (Table 3), two glass transition temperatures are observed for most of the samples in run 1, whereas in run 2 the lower TG can be observed for only a few samples. The higher T,I corresponds roughly to the T, obtained for the pristine OMPEO-LiC104 electrolyte. The low-temperature Tg2 is lower than the T, of uncomplexed OMPEO. For ?un 1" Tgl varied randomly with the PAAM concentration, probably due to inhomogeneities in the unannealed samples. In run 2 (the annealed samples) T,l decreased with an increase in the concentration of PAAM and became lower than the T, of the OMPEO-LiCIO4 electrolyte. It seems that annealing at 150 OC leads to homogenization of the samples, which is manifested by the presence of only one Tgwhich is between the values of T,z and T,, measured in run 1. Even in run 1 T,1 found for most of the composite systems studied is lower or comparable to the T, measured for the pristine OMPEO-LiC104 electrolyte. It should be also stressed that all the samples studied are completely amorphous, which implies that the addition of a salt inhibited the crystallization of the OMPEO copolymer. Figure 5a,b shows conductivity isotherms obtained for composite OMPEO-PAAM-LiC104 electrolytes at 0, 25, and 100 OC before (Figure 5a) and after (Figure 5b) annealing. An increase in conductivity is observed up to 20 vol % PAAM, and conductivity decreases above 25 vol %. The highest room temperature conductivity, -6 X 10-5 S/cm, has been measured for the electrolyte containing 20 v o l 8 PAAM (after annealing at 150 "C) and is approximately 4 times higher than the room temperature conductivity measured for the OMPEO-LiC104 electrolyte (1.6 X 10-5 S/cm). The concentrations of LiC104 in OMPEO used by Gray" are much lower than those used here, and therefore results are not comparable. The higher LiC104 concentration used by Booth et al.32was approximately 7 mol %, slightly lower than our 10 mol I. Nevertheless the trends in conductivity observed for the OMPEO-LiC104 electrolyte are in agreement with those measured here. The room temperature conductivity measured for this OMPEO-PAAM-LiClOa electrolyte is almost the same as that measured for the PEO-PAAMLiC104 system (6.8 X 10-5 S/cm) of the same composition. This indicates that the polyether chain segments are important in ion transport. The conductivities measured for samples of PAAM, 15 and 25 vol % (see Figure Sb), are only slightly lower. Figure 6 compares the temperature dependenceof conductivity obtained before and after annealing for samples containing,
0 I-
I
0
d 1
10 20 30 40 PAAM Concentration (vol %)
0
V
V
V
V
V V
e
e
e
e 0
10-6&
1 o-8 10-9
0
0
0
50
e
e
V
e
0
0
0
i 1
0
10 20 30 40 PAAM Concentration (vol %)
50
Figure 5. Isotherms of ionic conductivity of OMPEO-PAAM-LiClO4 versus PAAM concentration with 10 mol % LiClO, (a, top) before annealing and (b, bottom) after annealing: (0)0 OC, ( 0 )25 O C , (V) 100 oc.
respectively, 0 and 40 vol % PAAM for the OMPEO-PAAMLiC104 system. Conductivities measured for pristine OMPEObased electrolytes are almost the same in the entire temperature range. The more significant differences, especially at temperatures below 25 OC, between conductivities measured before and after annealing are observed for samples containing 40 vol % PAAM. Due to the improved homogeneity, conductivities measured after annealing are higher than those obtained for unannealed samples. Similar trends have been observed for the other composite electrolytes studied. Figure 7comparesthe temperature dependenceof conductivity for some of the OMPEO-PAAM-LiClO4 composite systems studied after annealing at 150OC. As can be seen, the temperature dependenceof conductivityfollows the VTF empiricalrelationship described by u = (A/
') exp(-B/( T - To))
(1)
Here A is a preexponential factor, B is a pseudoactivation energy for conduction, and To is a quasi-equilibrium glass transition temperature approximately 30-50 K lower than T,. The VTF parameters obtained from nonlinear least squares fitting of the
The Journal of Physical Chemistry, Vol. 98, NO. 27, 1994 6845
Polymer Blends Complexed with LiC104 -2
TABLE 5 VTF Parameters Obtained by Fitting the Temperature Dependence of Conductivity of Composite OMPEO-PAAM-LiClO, Polymeric Electrolytes (after a M d l & ) to the VTF Equation (Eq 1)
-3
--
PAAM AI sample volW (SK'I2cm-I) OMPEO-LiCIOd 0 27.0 CPEa 5 3.54 CPE 10 45.0 CPE 20 135 CPE 25 18.0 CPE 40 2.95 50 3.9X 1V2 CPE 0 CPE, composite polymeric electrolyte.
-4
I
E vr -5
2 v
-
-6
0 4
-7
0
A
3.5
3.0
2.5
4.0
1000 K/T
Figure 6. Comparison of the ionic conductivity of OMPEO-PAAMLiClO4 versus reciprocal temperature for the electrolytes containing (a) 0 vol W PAAM (0)and (b) 40vol W PAAM (A)(before annealing, open
symbols, and after annealing, filled symbols).
1200 690 1590 1970 1130 1180 540
195 227 174 199 195 186 222
PAAM and ether oxygens in polyether, leading to the formation of complexes;and the formation of hydrogen bonds between NHz groups and C104- anions or between NHz and polyether or amid carbonyl oxygens. The possibility of the formation of complexes between polyethers and alkali metal salts is fundamental to polymericelectrolytes. Ion-dipole complexes can be preferentially formed via thecarbonyl oxygen, but the possibility of the formation of complexes via the NHz group cannot be excluded and has been shown to be the case in several ~tudies.3~ On the other hand, the formation of hydrogen bonds with anions should lead to the immobilization of anions and hence to a decrease in conductivity; this is not observed for the systems studied. Moreover, DSC studies of undoped PAAM-polyether blends (see Tables 1 and 2) do not show an increase in the T, of the polyether, which is the case if strong hydrogen bonds are formed. An increase in the T, of the PAAM system doped with LiC104 has been observed (see Table 4). This increase in T,confirms the possibility of the formation of complexesbetween PAAM and LiC104. It has been shown previ0usly3~that the use of concentrated salt solutions (as in the present study) favors the formation of complexes and decreases the probability of hydrogen bond formation. Together with a decrease in T, we have observed an increase in ionic conductivity in the polyether-PAAM-LiC104 system with an increasein theconcentrationof PAAM. From theseobservations and the observed increase in T, in the PAAM-LiC104 system with increased PAAM concentration we conclude that ion-dipole interactions predominate and that there is a reduced tendency to form hydrogen bonds and associated ionic species. In Figure 8 we suggest schematically three types of complexes relevant to the polyether-PAAM-LiC104 system. Type 1 complexes are formed by Li+ with ether oxygens in the polyether chains, the well-known "crown ether" configuration.38 Type 2 complexes, in which an interaction between a PAAM segment and a polyether segment takes place through the Li+ cation, facilitate compatibilization. The relative surface tensions of PAAM (y 50 mN/m) and polyether (7 40 mN/m) are such that the polyether would likely coat the PAAM macromolecular coils in a complex of type 2. The ratio of the root-meansquare end-to-end distances of the polyether to the PAAM in the system under study is between 1 and 2. Type 3 complexes, in which the heteroatoms (N and 0)in PAAM segments interact via the Li+ cation, facilitate emulsification. In all of these complexes the cross-linking via Li+ cations is transient, with a lifetime in the nanosecond range.39 Depending on the relative concentrations of polyether, PAAM, and LiClO4, the degree of emulsification and the sizes of the microphase-separated globules, one observes up to three glass transition temperatures (see Table 4). The T, associated with essentially type 1 complexes ( T B , )is most easily detected. It is well-known from previous studies that TB1increases with salt concentration. The addition of PAAM initially leads to the formationof type 2 complexes at the polyether-PAAM interphase, thus reducing the number of type 1 complexes and increasing the flexibility of polyether chains in the surrounding volume. This
I
-2
7
TdK
A
-8
-
E/K
t
-3t
-%sL -
-4
6
vr
2 -5 v
-
07
0 -
-6
-
-7 -
t
-8 L
2.5
J
3.0
3.5
4.0
1000 K/T
Figure 7. Changes in ionic conductivity of OMPEO-PAAM-LiCIO4 with 10 mol 96 LiC104. Sampla of different PAAM concentration are in percent by volume. Conductivities were measured after annealing: (v)5 vol % PAAM, (0)10 vol W PAAM, ( 0 )25 vol 96 PAAM, ( 0 )50 vol W PAAM.
experimental data to eq 1 are summarized in Table 5 for the benefit of anyone who would like to reproduce our data. We do not draw any conclusions from these values, which are used only to indicate a VTF fit. Considering all of the conductivity results reported so far, it seems that as a filler PAAM facilitates ionic transport in polyether matrices. Although it is not clear how this is accomplished,we do know that the cooperative coupling of the polyether and PAAM segments through the Li+ cation makes these otherwise immiscible polymers more compatible and facilitates their emulsification. We attempt to explain this conductivity enhancement in such a coupled system.
Discussion In the system studied at least three types of interactions between either of the polymer componentsand LiClO4 can be distinguished. These are ion-ion interactions between Li+ cations and C104anions; ion-dipole interactions between lithium cations and heteroatoms such as amide nitrogens and carbonyl oxygens in
-
-
6846 The Journal of Physical Chemistry, Vol. 98, No. 27, 1994
a
Wieczorek et al.
TABLE 6 Position of C-0-C and N-H Symmetric Stretching Baads in IT-IR Spectra of Composite Polymeric Electrolytes (CPE) before annealing
'.
'"WCH2-CH2\ r-
CH
sample OMPEO PAAM CPE CPE CPE CPE CPE CPE CPE CPE CPE
-CH2'
2
CH T A I
-
b
C
H H Figure 8. Schematic structure of the complexes formed by Li+ cation with (a) polyether chains (type 1 complexes), (b) polyether and PAAM chains (type 2 complexes), and (c) PAAM chains (type 3 complexes).
increase in flexibility is reflected in a decrease in Tgl (see Table 4; run 2) and an increase in ionic conductivity (see Figure 5b). With an increase in PAAM concentration the concentration of type 2 complexes increases up to saturation level. On the basis of DSC and conductivity data (Table 4 and Figure 5 ) , this seems to be in the 20-30 vol % PAAM range for 10 mol % LiC104. Although not as easily detected as TgI,Tg2also decreases in this region. We attribute Tg2to amorphous regions of uncomplexed polyether chains. Tgl is in the range 4 3 to -46 "C for 20-25 vol k PAAM and 10 mol % LiC104,where the ionic conductivity is the highest. These values of TgIare in the same range as that found for the OMPEO-LiC104 electrolyte for 4 mol % LiC104, at which a maximum in ionic conductivity is observed. We believe that this supports our conclusions regarding a reduction in the number of type 1 complexes and an increase in the concentration of uncomplexed polyether chain segments. As the PAAM concentration increases further (>25 vol %), the amount of type 3 complexes increases, increasing the PAAM-LiC104 core size (radius R ) and reducing the t / R ratio, where r is the thickness of the layer of flexible amorphous polyether chains covering the PAAM-LiC104 core. Type 3 complexes are essentially nonconductive with room temperature conductivity 10-12 S/cm and trap both Li+ and C104- ions in a phase with a high TB,Tg3. This leads to a decrease in ionic conductivity (see Figure 5 ) . The postulated decrease in t is based on the continuing decrease in Tal for higher PAAM concentrations. Tg2 can no longer be detected since there are no longer significant concentrations of uncomplexed polyether chains in the ultrastructure. The Tg3for type 3 complexes was not observed in DSC studies performed for blend systems. This is probably due to not going to a high enough temperature in the DSC studies in order to
-
PAAM
vol% 0 100 0 5 10 15 20 25 30 40 50
C-O-C/
cm-1
N-H/
cm-1
1114
after annealing C-O-C/
cm-1
cm-1
1114 3213
1095 1099 1097 1096 1091 1087 1092 1095 1101
N-H/
3209 3209 3209 3204 3196
3213 1095 1093 1093 1092 1095 1093 1090 1098 1110
3208 3202 3194
avoid sample degradation. Note that the T, for pure PAAM is equal to 165 "C and the formation of PAAM-LiC104 complexes results in an increasein Te(-Table 4). The presenceof different cation complexesin the PEO-PAAM-LiC104 system is therefore confirmed by DSC and impedance spectroscopy data. FT-IRstudies of the (2-04 and the N-H symmetricstretching vibrations of the systems under study are summarized in Table 6. The C-0-C band, centered at 1114 cm-* for pure OMPEO, shifts to 1095 cm-1 after the addition of LiC104, which indicates changes in the ether oxygen environment due to the formation of type 1 complexes and a concomitant weakening of C-0-C bonds. For run 1 thermal histories and PAAM concentrations higher than 10 vol % this frequency decreases to a minimum at 1087 cm-l, which is observed for the sample containing 25 vol % PAAM. This is consistent with tendencies observed by DSC and impedance measurements. We suggest that this is due to the formation of type 2 complexes, which further weaken the C-0-C bonds. For higher concentrations of PAAM the C-0-C band again shifts to higher frequencies, reaching 1101 cm-1 for the sample containing the highest PAAM concentration. We suggest that this is due to the formation of type 3 complexes, which decrease the concentration of type 1 and type 2 complexes. The N-H band can be resolved for samples containing more than 20 vol% PAAM. The position of this band shifts to lower frequencies with an increase in PAAM concentration due to the formation of type 3 complexes. This may also indicate the possibility of the formation of hydrogen bonds between NH2 amide groups and C104-. This would lead to the immobilization of anions and furthermore to a decrease in the electrolyte conductivity. The position of the C 4 band is observed at 1652 cm-1 and does not change for all of the systemsstudied. Similar trends are observed after annealing the composite electrolyte samples at 150 "C (run 2) except that the position of the C - 0 - C band for annealed samples remains almost constant for PAAM concentrations up to 30vol %and shifts to higher frequenciesfor samples containing 40 and 50 vol 96 of PAAM. A more complete emulsification of the compositesamples during annealing is confirmed by the fixed position of the C - O C band for most of the annealed samples. The present observations seem to be consistent with the formation of the three different types of complexes proposed in Figure 8. In general the formation of type 2 complexes weakens the C-O bonds, thus shifting the maximum of the C-0-C band to lower frequencies. The lowest position corresponds to the maximum concentration of type 2 complexes. The further shift of the maximum of the C-0-C band to higher frequencies can be connected with the reduction in the concentration of type 1 complexes due to the formation of type 3 complexes. The later is confirmed by a shifting of the position of the maximum of the N-H band to lower frequencies. Effective Medium Theory (EMT). A description is presented of the concentration and temperature dependences of the conductivity of our composite polymeric electrolytes by models
Polymer Blends Complexed with LiC104
The Journal of Physical Chemistry, Vol. 98, No. 27, 1994 6841
utilizing EMT approaches. The concept is based on a phenomenological model by Nan and Smith,4p41which has been modified for application in composite solid e l e c t r o l y t e ~ . ~ ~The * ~ ~idea , ~ 3of the model is discussed briefly below. EMT models developed for such systems couple an enhancement of theconductivity with the existenceof a highly conducting layer at or near the polymer-filler interface. Figure 9 is a schematic'kartoon" of the polyether (PEO or OMPE0)-PAAMLiC104 blend, assuming a constant LiC104 concentration. In our case the filler is PAAM and the layer is an amorphous polyether which exhibits ambient temperature conductivities considerably higher than the matrix polymer electrolyte. Therefore there are three componentswith different electricalproperties. These are (1) a highly conducting amorphous, uncomplexed polyether component in the volume surrounding the PAAM core (type 2 complex region, see Figure 9 (1)); (2) the dispersed polyether-PAAM (type 2) and PAAM core 'particle" filler phase (type 3, see Figure 9 (2)); and (3) the polyether-LiC104 matrix polymer ionic conductor (type 1, see Figure 9 (3)). In component 1 the concentration of type 1 complexes is reduced, thus producing an amorphous phase of higher flexibility. We define the combination of components 1 and 2 as a composite unit and calculate the conductivity of this unit (u,) according to the Maxwell-Garnett mixture rule.@
2Ul 6, = Ul
+ u2 + 2Y(a, - Ul) + a, - Y(a2 - Ul)
(2)
2a1
Here ul and uz are, respectively, the conductivities of components 1 and 2; Yis the volume fraction of PAAM in each composite unit calculated as
Y = 1/(1
+ t/R)3
(3)
Spherical symmetry has been assumed. We then calculate the conductivity of the two-phase system consisting of composite units and the matrix polyether-LiC104 electrolyte (component 3) using the self-consistent EMT equation suggested by K i r k p a t r i ~ k . ~ ~ (V,/Y)(r;
- um)/(um + P~(Q; - um)) + ( l - V2/u>
