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Crystallization of inorganic salts in poly(propylene oxide) oligomers by heating
SOLID STATE Solid State Ionics 68 (1994) 117-123
ELSEVIER
IONICS
Crystallization of inorganic salts in poly(propylene oxide) oligomers by heating Kaori Ito, Minoru Dodo, Hiroyuki Ohno * Department of Biotechnology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184, Japan Received 12 November 1993; accepted for publication 24 January 1994
Abstract
A series of inorganic salts was dissolved in poly(propylene oxide) (PPO) oligomers, and the effect of temperature on the salt solubility was analyzed. Some of alkali metal salts were separated out as crystals by heating from PPO oligomers (Mw = 4004000 ). The salts, which were crystallized by heating, were revealed to have relatively higher lattice energy. The threshold of the lattice energy of these phase-separated salts decreased with increasing cation radius. The solubility of these salts turned to zero on increasing average molecular weight of PPO. This suggested the considerable contribution of the terminal hydroxyl groups of the PPO to the solubility of these salts. The increase of molecular motion in PPO oligomers with elevating temperature was expected to induce the distortion of the cooperative coordination of PPO to the cation. This might be the reason of the decrease in the solubility of salts in PPO.
1. Introduction
Since polyethers have large dipole m o m e n t on its ether oxygens, they solubilize inorganic salts and dissociate them into ions even in the absence o f polar solvent in polymer. These polyethers with relatively low glass transition temperature have been studied as base materials for ion conductive polymers [ 1 ]. However the mechanism o f ion migration in these polyethers had been discussed, there was little information about the solvation state of inorganic salts and their ions in polyethers. The solubility of salts and solvation state o f salts in these polymers were revealed to be important factors for the ion conduction. There are m a n y attempts to prepare the combshaped polymer [2,3 ] and network polymer [4,5 ] having short side chain o f polyethers in order to improve the solubility o f salts for higher ionic conduc* To whom all correspondence should be addressed.
tivity. We have already prepared the comb-shaped polymer having short side chains of poly (propylene oxide ( P P O ) [6 ]. In such cases, it is also important to consider the solvation state o f inorganic salts in polyethers. We have summarized the solubility o f inorganic salts in poly (ethylene oxide) (PEO) oligomers and their ionic conductivity [7,8]. Furthermore, a kind of inorganic salts was found to be phase separated by heating from PEO oligomers [ 9,10 ]. On the other hand, PPO was frequently used as a model compound to analyze the solvation state of ions in polyethers, because even high molecular weight of PPO gave liquid phase attributed to the steric hindrance of the methyl groups. There were some reports on the crystallization o f inorganic salts in polyethers by heating. Sodium thiocyanate (NaSCN) and potassium thiocyanate ( K S C N ) were reported to be phase separated from PPOzooo [ 11 ]. Sodium iodide ( N a I ) and sodium perchlorate (NaC104) were also reported to be crystallized by heating in PPO graft
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118
K. Ito et al. /Solid State lonics 68 (1994) 117-123
polymers [ 12,13 ]. Besides, Raman spectroscopy on the triflate salts dissolved in PPO oligomers revealed that the number of free ions in PPO decreased with increasing temperature [ 14,15 ]. These reports indicated that the solubility of salts decreased with increasing temperature. However only limited salts have hitherto been discussed, the main factor for this phase separation had not yet been mentioned. Here we analyzed the crystallization of inorganic salts systematically and discussed the factor which induced this unique phenomenon.
be dissolved again by cooling. This phase separation was confirmed to be reversible in all solutions. All the processes of this measurement were taken under the dry nitrogen to prevent contamination of water. The salts crystallized by heating were collected by the way of centrifugation, washed with acetone and crushed into powder for X-ray diffraction. All the salts were identified by comparing X-ray diffraction patterns with the powder diffraction file. The X-ray powder diffraction pattern was obtained with powder X-ray diffraction analyzer, Rigaku RADC.
2. Experimental 3. Results and discussion 2. I. Materials
PPO oligomers with average molecular weight of 400-4000 were purchased from NOF Co. Ltd. Both terminal groups in all the PPO oligomers were hydroxyl groups. These PPO oligomers were used for experiments after drying in vacuo at 60 °C for three days. All the salts used in this report were reagent grade and anhydrous except lithium thiocyanate (LiSCN-2H20). These salts were purchased from Kanto Chem. Co. Ltd. All salts were dried in vacuo and heated for three days. 2.2. Methods
In dry nitrogen atmosphere, inorganic salts (powder) were directly added to PPO oligomers in different concentrations and dissolved under stirring at room temperature. In some cases, the solubility of salts became larger at lower temperature. The concentrated solution was therefore prepared at 0 °C under stirring. The temperature at which the crystals were phase separated in the solution was measured in the following way. All solution was heated on the hot plate under stirring and temperatures at which the salts crystallized and dissolved were measured directly. The heating rate of PPO solution was 1 ° C/rain. Temperature was monitored with the thermocouple which was dipped into the solution. The solubility curve of inorganic salts in PPO was prepared by the phaseseparation temperature and the salt concentration. The salts crystallized by heating were confirmed to
The solubility of lithium chloride (LiC1) was clarified to decrease at higher temperature (more than 120°C) in PPO oligomers with average molecular weight of 400-4000. However, the decrease in solubility was also observed at lower temperature in relatively lower molecular weight. The solubility of LiC1 therefore has the maximum value as seen in Fig. 1. For instance, since the solubility of LiC1 in PPO4oo at 25 °C was 1.0 mol/1, the PPO4oo solution containing 1.1 mol/1 LiC1 was not transparent at 25 ° C. It turned a clear solution at 110 ° C, and the crystals were phase separ~ited again above 130 ° C. The crystal, separated from PPO solution at 130 ° C, was confirmed to have the same lattice constant as that for standard LiCI
~
1
.
6
.,_-O ..J
0,8
0.4
50
100
150
Temperature (°C)
Fig. 1. Effect of temperature on the solubility of LiCI in PPOs. Mw of PPO is 400 ( O ) , 700 (/x), 1200 ( [ ] ) , 2000 (©), and 4000 ( • ), respectively.
K. Ito et al. /Solid State lonics 68 (1994) 117-123
powder by X-ray diffraction pattern. This phenomenon was observed in other PPO having different molecular weight. We have already reported the decreased solubility of inorganic salts at higher temperature in poly(ethylene oxide) oligomers [ 9,10 ]. In the previous study, the decreased cooperative solvation of PEO oligomers was suggested to be induced by elevating temperature. The distorted solvation was concluded to be the major factor for the negative temperature dependence in solubility of inorganic salts in PEO oligomers. In the present study, both positive and negative temperature dependence in solubility of inorganic salts was observed in PPO oligomers. In PPO oligomers, the methyl groups hindered the crystallization of polyether chains, amorphous liquid phase was maintained even for higher molecular weight of PPO. On the other hand, PEO oligomers easily form crystalline phase with increasing its molecular weight. Taking these different characteristics into account, PPO should provide higher mobility of ions than PEO did. The effect of temperature on the molecular motion would be much larger in PPO solution with low molecular weight, such a s PPO4oo or PPO7oo. Therefore, the solubility of LiC1 showed proper temperature dependence at lower temperature and showed negative temperature dependence at higher temperature, which should be derived from characteristics of polymers. Generally, PPO was known to have less polarity on ether oxygen than that of PEO because of steric hindrance of methyl group. Therefore PPO could form stable complex with fewer salts than PEO could [ 16 ]. The salts, which made stable complex with polyethers, could not be crystallized by heating [ 9,10 ], then, more salts would be crystallized in PPO oligomers. Some salts, which were not crystallized from PEO, were crystallized in PPO oligomers (Fig. 2). The salts used in Fig. 2 were known to form complex with PEO [ 17 ], but they were crystallized in PPO by heating. It is empirically believed that salts are solubilized in polyethers by complex formation. Though these salts could not form complex with PPO, they were actually dissolved in PPO oligomers. There may be differences in solvation state between the complex forming salts and the complex not-forming salts. Solubilization of the complex non-forming salts was comprehended to be due to the terminal hydroxyl
~
~
1
.
119
6
1.2
0.8
©
0.4
0
50
100 150 Temperature (°C)
Fig. 2. Effect of temperature on the solubility of some inorganic salts in PPO4oo. ( © ) LiC1; ( [] ) NaI; (©) NaBF4; (/', ) KI.
l
LiCI04
1
L,.O l
©
(3
©
NaCl04
4 O
g 3
LiBr
]
i,n
"6
2
to t~
to
[]
[]
©
1
o .~'~
[] 6~o
i
roo
75o
8oo
[] i
8so
Lattice Energy (kJ/mol)
Fig. 3. The solubility of inorganic salts (complex forming) at room temperature in PPO4oo ( [] ) and PEO3oo ( © ) , respectively.
groups of PPO, details were quantitatively mentioned later. As mentioned above, PPO has less polarity than PEO. This was clearly recognized by comparing the solubility of an inorganic salt in PPO oligomer and that in PEO oligomer with the same degree of polymerization. For instance, PPO4oo has 6 ether oxygens in it besides the both terminal hydroxyl groups. PEO3oo also has 6 ether oxygens in it. To compare the salt solubility of ether oxygens in PPO and PEO, the solubility of inorganic salts respectively, in PPO4oo and PEO3oo, was measured and compared as seen in Figs. 3 and 4. The salt solubility in these polyethers was compared for about both the complex forming salts and the complex not-forming salts. All the salts used in Fig. 3 were known to form complex with PPO. These salts were never crystallized by heating the
120
K. Ito et al. / Solid State lonics 68 (1994) 117-123 Li ÷
m 2.0
o E
>,
900
", 1
v >,
"~ 1.6 0 t/) 1.2
Na ÷
0) c LU
800
o
750
N ,-I
E]~'Br', 0 scd~ oo;=0"
7O0 650
0.4 600 I
i
i
50
100
150
Temperature (°C)
Fig. 4. Effect of temperature on the solubility of NaBr in PPO4oo ( • ) and PEO3oo( • ) , respectively. PPO4oo containing corresponding salts. The solubility of LiC104 in PPO4oo was less than a half o f that in PEO3oo at room temperature. The decreased solubility in PPO4o o could be confirmed also for a series of complex not-forming salts. The solubility o f NaBr showed negative temperature dependence both in PPO4oo and PEO3oo as shown in Fig. 4. At the same temperature, the solubility of NaBr was about a quarter o f that in PEO3o o. These values were considered to reflect the contribution of ether oxygens and terminal hydroxyl groups in polyethers to solubilize salts. At the same degree of polymerization, the polarity of PPO was much less than that in PEO. In other words, P P O has less ability to solubilize salts because o f poor polarity of both ether oxygens and terminal hydroxyl groups. Torell et al. [ 14,15 ] reported that the number of free ions in PPO oligomers decreased with increasing temperature. Besides, the phase separation of salts in P P O at higher temperature was reported by Teeters [ 1 1 ] and Greenbaum [ 12,13 ]. There was however no discussion about the salt species to be phase separated by heating. We have already reported that the salts, which were crystallized in PEO oligomers by heating, had relatively high lattice energy, and threshold of this lattice energy was decreased with increasing the cation radius o f the salts [ 9,10 ]. According to our previous report, the salts, which were separated from PEO oligomers, were also analyzed with lattice energy and cation radius as shown in Fig. 5.
12
K* i
Cs +
O"'- NO3 I Br" j SCN" "', ar Oo,iO* """ • I"11 ~BF4. B F ~ , , l l i ' • Br" SCN-". II I -
0.8
0
1
1.8
Cation Radius log R (pm)
Fig. 5. Relation between lattice energy and cation radius of a series of inorganic salts. (Closed plots) salts known to crystallize by heating in PPO4oo.(Open plots) salts not phase-separated from PPO4oo by heating. (Circle) salts form complex with PPO, (Square) salts known to form no complex with PPO. (O*) LiC104 and NaCIO4 were known to form complex with PPO. Heating experiments were however not carried out for these because of explosive characteristics. Lattice energy (thermochemical cycle) was in kJ/mol from Jekins, H.D.B. as quoted in ref. [18]. Closed plots mean that these salts were crystallized by heating the salt saturated PPO solution up to 200°C. The crystals separated from PPO400 were confirmed to have the same lattice constants as those of dissolved (standard) salts by the X-ray diffraction measurements. This clearly showed that the phase separated material was not the PPO-salt complex but the salt dissolved in the PPO. All the salts crystallized by heating showed negative temperature dependence in solubility. More salts were crystallized by heating in PPO reflecting less polarity. This means that the threshold of lattice energy of the salts crystallized by heating in PPO400 shifted to a lower energy side than that in PEO200. It should be noted here that selection ofpolyether was confirmed to control the salt species to be crystallized by heating. Further, salt solubility was expected to be dependent on the content of the terminal hydroxyl groups. Since the terminal hydroxyl groups have larger polarity than ether oxygens, PPO oligomers with larger molecular weight were expected to have less salt solubility. The effect of average molecular weight of PPO on the phase separation of salts in the PPO oligomers
K. Ito et at / Solid State lonics 68 (I 994) 117-123
(Figs. 5, 6 and 7) was also summarized. More salts were turned to be crystallized by heating in the PPO when PPO with larger molecular weight was used. In other words, the threshold of lattice energy of salts was shifted to a lower energy side by increasing the molecular weight of PPO. The average polarity of polymer solvent could therefore be evaluated as the shift of the threshold of the lattice energy. Li ÷ O
900
J Na ÷
ill,,~'"
800 O (,)
II "%
750
Br"
o~o~O*
-1
',, •,
700
] Rb÷ Q _•
Or
1 C,S÷ 1
oo~0" • "',, I l l IIBr" sFiO • r II~-
650
BF~
6OO J
1.8
i
,.9 2'.o 211 2.2 £3 Cation Radius log R (pm)
Fig. 6. Relation between lattice energy and cation radius of a series of inorganic salts. (Closed plots ) salts crystallized by heating in PPOToo; (Open plots) salts not phase separated from PPOToo by heating. Other captions are the same as in Fig. 5. Li* o 900 850
,,~ 800
",, i~;~
K +
O "~-~N"
7so
o
Ill NO; l Rb÷
aoio* ',,
o l l I C$÷
", "',,0 O1" ~.o~ i e,"
650
c~iO~,.
l
• c• ~-
""..SEN- IIc
600
%:8
1.6
>,
Br"
700
Next we refer to the mechanism of the phase separation of salts by heating. As mentioned above, most of the salts, crystallized by heating, could not form complex with PPO. To analyze the solvation state of these salts, LiC1 was used as the typical salt not to form complex with PPO, and lithium perchlorate (LiC104) was used as that of complex forming salts. The solubility of both salts was measured in PPO oligomers (Fig. 8). The value that the mass of the terminal hydroxyl groups divided by the molecular weight of the PPO was represented as the fraction of terminal hydroxyl group (f-OH) of PPO, and was conveniently used to show the contents of terminal hydroxyl groups of the polyethers [ 7 ]. The solubility of both LiC1 and L i C l O 4 depended on the f-OH, and the contribution of the terminal hydroxyl groups to solubilize these salts was suggested. But these salts showed totally different f-oH dependence. The extrapolation of the solubility curve of LiC104 to zero f-OH reached the intercept about 0.5 mol/l. This value corresponded to the amount of salts which could be solubilized only by the ether oxygens in PPO. LiC104 was apparent to be soluble even in the high molecular weight of PPO. On the other hand, the extrapolation of the solubility of LiC1 turned to be zero. This clearly shows that LiCI cannot be dissolved in high molecular weight of PPO. Thus, the ether oxygens in PPO did not work effectively to solubilize LiC1. This was actually known that LiC1 was not to form complex with PPO because it was insoluble in
Na ÷
" ~ Br"
¢D
121
'R 1.2 O 0~
0.8
0.4
119 2:0 21, 212 2:3 Cation Radius log R (pm)
Fig. 7. Relation between lattice energy and cation radius of a series of inorganic salts. (Closed plots ) salts crystallized by heating in PPO~2ooand PPO4ooo; (Open plots) salts not phase separated from PPO oligomers by heating. Other captions are the same as in Fig. 5.
J
0
2
4
6
8
1'0
f ~ 1%)
Fig. 8. Solubility of LiCI and LiC104 in PPO as a function of terminal hydroxyl group fraction (f-OH) of the PPOs. LiC1 at room temperature ( • ) , 80 (©), 100 (ZX), and 120°C ( V ) , and LiC104 at room temperature ( • ) , respectively.
122
K. Ito et al. / Solid State lonics 68 (1994) 117-123
PPO. The LiC1 was soluble in P P O oligomers only by the large polarity on the t e r m i n a l hydroxyl groups. The solubility o f LiC1 at 25 °C and 80°C gave different curves from that at higher temperatures. Because the solubility o f LiCI decreased a n d gave positive t e m p e r a t u r e d e p e n d e n c e at lower t e m p e r a t u r e as is shown in Fig. 1. This great c o n t r i b u t i o n o f the t e r m i n a l hydroxyl groups to the solvation o f inorganic salts was the reason these salts dissolved in P P O oligomers without forming complex. The t h e r m a l m o t i o n affected this i n t r a m o l e c u l a r solvation to reduce the salt solubility. The ether oxygens on P P O hardly interacted with cation in the case o f complex non-forming salts. The solvation state o f these salts t u r n e d to be considerably unstable. The t e r m i n a l hydroxyl groups in polyethers were suggested to m a k e cooperative coordination to the cation called " i n t r a m o l e c u l a r c o o r d i n a t i o n " [ 9,10 ]. W i t h increasing the t h e r m a l molecular motion, the t e r m i n a l hydroxyl groups cannot keep the i n t r a m o l e c u l a r c o o r d i n a t i o n to the cation effectively. This i n t r a m o l e c u l a r c o o r d i n a t i o n was distorted with increasing temperature, and the cation was released from the P P O since ether oxygens were not effective enough to solubilize it. As the result o f the cation release, the crystallization o f salts was observed. Against this, the complex-forming salts such as LiC104 could m a k e stable complex with PPO, therefore the phase separation o f salts could not be observed.
4. Conclusion We analyzed the phase separation o f inorganic salts dissolved in P P O oligomers. Results showed that the solubility o f salts in P P O v a r i e d with the m a x i m u m value a n d h a d negative t e m p e r a t u r e d e p e n d e n c e at higher temperature. This negative t e m p e r a t u r e dependence o f salt solubility h a d been f o u n d in PEOs. The salt species which was crystallized by heating dep e n d e d on the lattice energy o f salts, and the threshold o f it was decreased with increasing cation radius. Further, this threshold was shifted to a lower energy side with increasing molecular weight o f PPO. Since the polarity o f ether oxygen in P P O was weaker than that in PEO, m o r e salts were crystallized by heating than those observed in PEO. The salts which m a d e
complex with P P O could form stable solvation state with not only its ether oxygens and terminal hydroxyl groups, so that no phase separation o f salts could be observed even at higher temperature. Against this, the salts, crystallized by heating, were solubilized only by the terminal hydroxyl groups o f PPO. These salts could not form stable solvation state, the coordination o f terminal hydroxyl groups was therefore dist o t t e d with thermal molecular m o t i o n at higher temperature. As the results o f this distortion, the salt was crystallized by heating. We believe that the results o b t a i n e d here should be very i m p o r t a n t to design p o l y m e r electrolyte applicable for wider t e m p e r a t u r e range.
5. Acknowledgement The present study was s u p p o r t e d by the Grant-inA i d for Scientific Research on Priority Areas "Reactive Organometallics" from the Ministry of Education, Science a n d Culture, Japan.
References [ 1] P.V. Wright, Brit. Polym. J. 7 ( 1975 ) 319. [2] D.J. Bannister, G.R. Davies, I.M. Ward and J.E. Mclntyre, Polymer 25 (1984) 1600. [ 3 ] J.M. Cowie and A.C.S. Martin, Polymer 32 ( 1991 ) 2411. [4]A. Killis, J.F. Lenest, H. Cheradame and A. Gandini, Makromol. Chem. 183 (1982) 2835. [ 5 ] M. Watanabe, K. Sanui, N. Ogata, F. Inoue, T. Kobayashi andZ. Ohtaki, 17 (1985) 549. [6] H. Ohno and K. Ito, Polymer 34 (1993) 3276. [ 7 ] H. Ohno and P. Wang, Nippon Kagaku Kaishi ( 1991 ) 1588. [ 8 ] H. Ohno and P. Wang, Nippon Kagaku Kaishi ( 1992 ) 552. [9] H. Ohno and K. Ito, Polymer 34 (1993) 3276. [ 10 ] H. Ohno, K. Ito and K. Matsumoto, Nippon Kagaku Kaishi (1993) 301. [ 11 ] D. Teeters, S.L. Stewart and L. Svoboda, Solid State Ionics 28-30 (1988) 1054. l 12 ] S.G. Greenbaum, Y.S. Pak, M.C. Wintersgill,J.J. Fontanella, J.W. Schultz and C.G. Andeen, J. Eleetrochem. Soe. 135 (1988) 235. [ 13 ] M.C. Wintersgill, J.J. Fontanella, S.G. Greenbaum and K.J. Adamie, Brit. Polym. J. 20 (1988) 195. [ 14] S. Schantz, L.M. Torell and J.RI Stevens, J. Chem. Phys. 94 (1991) 6862. [ 15 ] G. Petersen, L.M. Torell, S. Panero, B. Scrosati, C.J. da Silva and M. Smith, Solid State Ionies 60 ( 1993 ) 55.
K. Ito et al. / Solid State lonics 68 (1994) 117-123
[16]N. Ogata, Conductive Polymers (Kodansha Scientific, Tokyo, 1990) p. 110. [ 17 ] D.F. Shriver, B.L. Papke, M.A. Ratner, R. Dupon, T. Wong and M. Brodwin, Solid State Ionics 5 (1981) 83.
123
[18]H.D.B. Jekins, in: CRC Handbook of Chemistry and Physics, 71st Ed. (Chemical Rubber Publ. Co., Boston, 1990) 12-3.
ELSEVIER
IONICS
Crystallization of inorganic salts in poly(propylene oxide) oligomers by heating Kaori Ito, Minoru Dodo, Hiroyuki Ohno * Department of Biotechnology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184, Japan Received 12 November 1993; accepted for publication 24 January 1994
Abstract
A series of inorganic salts was dissolved in poly(propylene oxide) (PPO) oligomers, and the effect of temperature on the salt solubility was analyzed. Some of alkali metal salts were separated out as crystals by heating from PPO oligomers (Mw = 4004000 ). The salts, which were crystallized by heating, were revealed to have relatively higher lattice energy. The threshold of the lattice energy of these phase-separated salts decreased with increasing cation radius. The solubility of these salts turned to zero on increasing average molecular weight of PPO. This suggested the considerable contribution of the terminal hydroxyl groups of the PPO to the solubility of these salts. The increase of molecular motion in PPO oligomers with elevating temperature was expected to induce the distortion of the cooperative coordination of PPO to the cation. This might be the reason of the decrease in the solubility of salts in PPO.
1. Introduction
Since polyethers have large dipole m o m e n t on its ether oxygens, they solubilize inorganic salts and dissociate them into ions even in the absence o f polar solvent in polymer. These polyethers with relatively low glass transition temperature have been studied as base materials for ion conductive polymers [ 1 ]. However the mechanism o f ion migration in these polyethers had been discussed, there was little information about the solvation state of inorganic salts and their ions in polyethers. The solubility of salts and solvation state o f salts in these polymers were revealed to be important factors for the ion conduction. There are m a n y attempts to prepare the combshaped polymer [2,3 ] and network polymer [4,5 ] having short side chain o f polyethers in order to improve the solubility o f salts for higher ionic conduc* To whom all correspondence should be addressed.
tivity. We have already prepared the comb-shaped polymer having short side chains of poly (propylene oxide ( P P O ) [6 ]. In such cases, it is also important to consider the solvation state o f inorganic salts in polyethers. We have summarized the solubility o f inorganic salts in poly (ethylene oxide) (PEO) oligomers and their ionic conductivity [7,8]. Furthermore, a kind of inorganic salts was found to be phase separated by heating from PEO oligomers [ 9,10 ]. On the other hand, PPO was frequently used as a model compound to analyze the solvation state of ions in polyethers, because even high molecular weight of PPO gave liquid phase attributed to the steric hindrance of the methyl groups. There were some reports on the crystallization o f inorganic salts in polyethers by heating. Sodium thiocyanate (NaSCN) and potassium thiocyanate ( K S C N ) were reported to be phase separated from PPOzooo [ 11 ]. Sodium iodide ( N a I ) and sodium perchlorate (NaC104) were also reported to be crystallized by heating in PPO graft
0167-2738/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0167-2738 ( 94 ) 00021-J
118
K. Ito et al. /Solid State lonics 68 (1994) 117-123
polymers [ 12,13 ]. Besides, Raman spectroscopy on the triflate salts dissolved in PPO oligomers revealed that the number of free ions in PPO decreased with increasing temperature [ 14,15 ]. These reports indicated that the solubility of salts decreased with increasing temperature. However only limited salts have hitherto been discussed, the main factor for this phase separation had not yet been mentioned. Here we analyzed the crystallization of inorganic salts systematically and discussed the factor which induced this unique phenomenon.
be dissolved again by cooling. This phase separation was confirmed to be reversible in all solutions. All the processes of this measurement were taken under the dry nitrogen to prevent contamination of water. The salts crystallized by heating were collected by the way of centrifugation, washed with acetone and crushed into powder for X-ray diffraction. All the salts were identified by comparing X-ray diffraction patterns with the powder diffraction file. The X-ray powder diffraction pattern was obtained with powder X-ray diffraction analyzer, Rigaku RADC.
2. Experimental 3. Results and discussion 2. I. Materials
PPO oligomers with average molecular weight of 400-4000 were purchased from NOF Co. Ltd. Both terminal groups in all the PPO oligomers were hydroxyl groups. These PPO oligomers were used for experiments after drying in vacuo at 60 °C for three days. All the salts used in this report were reagent grade and anhydrous except lithium thiocyanate (LiSCN-2H20). These salts were purchased from Kanto Chem. Co. Ltd. All salts were dried in vacuo and heated for three days. 2.2. Methods
In dry nitrogen atmosphere, inorganic salts (powder) were directly added to PPO oligomers in different concentrations and dissolved under stirring at room temperature. In some cases, the solubility of salts became larger at lower temperature. The concentrated solution was therefore prepared at 0 °C under stirring. The temperature at which the crystals were phase separated in the solution was measured in the following way. All solution was heated on the hot plate under stirring and temperatures at which the salts crystallized and dissolved were measured directly. The heating rate of PPO solution was 1 ° C/rain. Temperature was monitored with the thermocouple which was dipped into the solution. The solubility curve of inorganic salts in PPO was prepared by the phaseseparation temperature and the salt concentration. The salts crystallized by heating were confirmed to
The solubility of lithium chloride (LiC1) was clarified to decrease at higher temperature (more than 120°C) in PPO oligomers with average molecular weight of 400-4000. However, the decrease in solubility was also observed at lower temperature in relatively lower molecular weight. The solubility of LiC1 therefore has the maximum value as seen in Fig. 1. For instance, since the solubility of LiC1 in PPO4oo at 25 °C was 1.0 mol/1, the PPO4oo solution containing 1.1 mol/1 LiC1 was not transparent at 25 ° C. It turned a clear solution at 110 ° C, and the crystals were phase separ~ited again above 130 ° C. The crystal, separated from PPO solution at 130 ° C, was confirmed to have the same lattice constant as that for standard LiCI
~
1
.
6
.,_-O ..J
0,8
0.4
50
100
150
Temperature (°C)
Fig. 1. Effect of temperature on the solubility of LiCI in PPOs. Mw of PPO is 400 ( O ) , 700 (/x), 1200 ( [ ] ) , 2000 (©), and 4000 ( • ), respectively.
K. Ito et al. /Solid State lonics 68 (1994) 117-123
powder by X-ray diffraction pattern. This phenomenon was observed in other PPO having different molecular weight. We have already reported the decreased solubility of inorganic salts at higher temperature in poly(ethylene oxide) oligomers [ 9,10 ]. In the previous study, the decreased cooperative solvation of PEO oligomers was suggested to be induced by elevating temperature. The distorted solvation was concluded to be the major factor for the negative temperature dependence in solubility of inorganic salts in PEO oligomers. In the present study, both positive and negative temperature dependence in solubility of inorganic salts was observed in PPO oligomers. In PPO oligomers, the methyl groups hindered the crystallization of polyether chains, amorphous liquid phase was maintained even for higher molecular weight of PPO. On the other hand, PEO oligomers easily form crystalline phase with increasing its molecular weight. Taking these different characteristics into account, PPO should provide higher mobility of ions than PEO did. The effect of temperature on the molecular motion would be much larger in PPO solution with low molecular weight, such a s PPO4oo or PPO7oo. Therefore, the solubility of LiC1 showed proper temperature dependence at lower temperature and showed negative temperature dependence at higher temperature, which should be derived from characteristics of polymers. Generally, PPO was known to have less polarity on ether oxygen than that of PEO because of steric hindrance of methyl group. Therefore PPO could form stable complex with fewer salts than PEO could [ 16 ]. The salts, which made stable complex with polyethers, could not be crystallized by heating [ 9,10 ], then, more salts would be crystallized in PPO oligomers. Some salts, which were not crystallized from PEO, were crystallized in PPO oligomers (Fig. 2). The salts used in Fig. 2 were known to form complex with PEO [ 17 ], but they were crystallized in PPO by heating. It is empirically believed that salts are solubilized in polyethers by complex formation. Though these salts could not form complex with PPO, they were actually dissolved in PPO oligomers. There may be differences in solvation state between the complex forming salts and the complex not-forming salts. Solubilization of the complex non-forming salts was comprehended to be due to the terminal hydroxyl
~
~
1
.
119
6
1.2
0.8
©
0.4
0
50
100 150 Temperature (°C)
Fig. 2. Effect of temperature on the solubility of some inorganic salts in PPO4oo. ( © ) LiC1; ( [] ) NaI; (©) NaBF4; (/', ) KI.
l
LiCI04
1
L,.O l
©
(3
©
NaCl04
4 O
g 3
LiBr
]
i,n
"6
2
to t~
to
[]
[]
©
1
o .~'~
[] 6~o
i
roo
75o
8oo
[] i
8so
Lattice Energy (kJ/mol)
Fig. 3. The solubility of inorganic salts (complex forming) at room temperature in PPO4oo ( [] ) and PEO3oo ( © ) , respectively.
groups of PPO, details were quantitatively mentioned later. As mentioned above, PPO has less polarity than PEO. This was clearly recognized by comparing the solubility of an inorganic salt in PPO oligomer and that in PEO oligomer with the same degree of polymerization. For instance, PPO4oo has 6 ether oxygens in it besides the both terminal hydroxyl groups. PEO3oo also has 6 ether oxygens in it. To compare the salt solubility of ether oxygens in PPO and PEO, the solubility of inorganic salts respectively, in PPO4oo and PEO3oo, was measured and compared as seen in Figs. 3 and 4. The salt solubility in these polyethers was compared for about both the complex forming salts and the complex not-forming salts. All the salts used in Fig. 3 were known to form complex with PPO. These salts were never crystallized by heating the
120
K. Ito et al. / Solid State lonics 68 (1994) 117-123 Li ÷
m 2.0
o E
>,
900
", 1
v >,
"~ 1.6 0 t/) 1.2
Na ÷
0) c LU
800
o
750
N ,-I
E]~'Br', 0 scd~ oo;=0"
7O0 650
0.4 600 I
i
i
50
100
150
Temperature (°C)
Fig. 4. Effect of temperature on the solubility of NaBr in PPO4oo ( • ) and PEO3oo( • ) , respectively. PPO4oo containing corresponding salts. The solubility of LiC104 in PPO4oo was less than a half o f that in PEO3oo at room temperature. The decreased solubility in PPO4o o could be confirmed also for a series of complex not-forming salts. The solubility o f NaBr showed negative temperature dependence both in PPO4oo and PEO3oo as shown in Fig. 4. At the same temperature, the solubility of NaBr was about a quarter o f that in PEO3o o. These values were considered to reflect the contribution of ether oxygens and terminal hydroxyl groups in polyethers to solubilize salts. At the same degree of polymerization, the polarity of PPO was much less than that in PEO. In other words, P P O has less ability to solubilize salts because o f poor polarity of both ether oxygens and terminal hydroxyl groups. Torell et al. [ 14,15 ] reported that the number of free ions in PPO oligomers decreased with increasing temperature. Besides, the phase separation of salts in P P O at higher temperature was reported by Teeters [ 1 1 ] and Greenbaum [ 12,13 ]. There was however no discussion about the salt species to be phase separated by heating. We have already reported that the salts, which were crystallized in PEO oligomers by heating, had relatively high lattice energy, and threshold of this lattice energy was decreased with increasing the cation radius o f the salts [ 9,10 ]. According to our previous report, the salts, which were separated from PEO oligomers, were also analyzed with lattice energy and cation radius as shown in Fig. 5.
12
K* i
Cs +
O"'- NO3 I Br" j SCN" "', ar Oo,iO* """ • I"11 ~BF4. B F ~ , , l l i ' • Br" SCN-". II I -
0.8
0
1
1.8
Cation Radius log R (pm)
Fig. 5. Relation between lattice energy and cation radius of a series of inorganic salts. (Closed plots) salts known to crystallize by heating in PPO4oo.(Open plots) salts not phase-separated from PPO4oo by heating. (Circle) salts form complex with PPO, (Square) salts known to form no complex with PPO. (O*) LiC104 and NaCIO4 were known to form complex with PPO. Heating experiments were however not carried out for these because of explosive characteristics. Lattice energy (thermochemical cycle) was in kJ/mol from Jekins, H.D.B. as quoted in ref. [18]. Closed plots mean that these salts were crystallized by heating the salt saturated PPO solution up to 200°C. The crystals separated from PPO400 were confirmed to have the same lattice constants as those of dissolved (standard) salts by the X-ray diffraction measurements. This clearly showed that the phase separated material was not the PPO-salt complex but the salt dissolved in the PPO. All the salts crystallized by heating showed negative temperature dependence in solubility. More salts were crystallized by heating in PPO reflecting less polarity. This means that the threshold of lattice energy of the salts crystallized by heating in PPO400 shifted to a lower energy side than that in PEO200. It should be noted here that selection ofpolyether was confirmed to control the salt species to be crystallized by heating. Further, salt solubility was expected to be dependent on the content of the terminal hydroxyl groups. Since the terminal hydroxyl groups have larger polarity than ether oxygens, PPO oligomers with larger molecular weight were expected to have less salt solubility. The effect of average molecular weight of PPO on the phase separation of salts in the PPO oligomers
K. Ito et at / Solid State lonics 68 (I 994) 117-123
(Figs. 5, 6 and 7) was also summarized. More salts were turned to be crystallized by heating in the PPO when PPO with larger molecular weight was used. In other words, the threshold of lattice energy of salts was shifted to a lower energy side by increasing the molecular weight of PPO. The average polarity of polymer solvent could therefore be evaluated as the shift of the threshold of the lattice energy. Li ÷ O
900
J Na ÷
ill,,~'"
800 O (,)
II "%
750
Br"
o~o~O*
-1
',, •,
700
] Rb÷ Q _•
Or
1 C,S÷ 1
oo~0" • "',, I l l IIBr" sFiO • r II~-
650
BF~
6OO J
1.8
i
,.9 2'.o 211 2.2 £3 Cation Radius log R (pm)
Fig. 6. Relation between lattice energy and cation radius of a series of inorganic salts. (Closed plots ) salts crystallized by heating in PPOToo; (Open plots) salts not phase separated from PPOToo by heating. Other captions are the same as in Fig. 5. Li* o 900 850
,,~ 800
",, i~;~
K +
O "~-~N"
7so
o
Ill NO; l Rb÷
aoio* ',,
o l l I C$÷
", "',,0 O1" ~.o~ i e,"
650
c~iO~,.
l
• c• ~-
""..SEN- IIc
600
%:8
1.6
>,
Br"
700
Next we refer to the mechanism of the phase separation of salts by heating. As mentioned above, most of the salts, crystallized by heating, could not form complex with PPO. To analyze the solvation state of these salts, LiC1 was used as the typical salt not to form complex with PPO, and lithium perchlorate (LiC104) was used as that of complex forming salts. The solubility of both salts was measured in PPO oligomers (Fig. 8). The value that the mass of the terminal hydroxyl groups divided by the molecular weight of the PPO was represented as the fraction of terminal hydroxyl group (f-OH) of PPO, and was conveniently used to show the contents of terminal hydroxyl groups of the polyethers [ 7 ]. The solubility of both LiC1 and L i C l O 4 depended on the f-OH, and the contribution of the terminal hydroxyl groups to solubilize these salts was suggested. But these salts showed totally different f-oH dependence. The extrapolation of the solubility curve of LiC104 to zero f-OH reached the intercept about 0.5 mol/l. This value corresponded to the amount of salts which could be solubilized only by the ether oxygens in PPO. LiC104 was apparent to be soluble even in the high molecular weight of PPO. On the other hand, the extrapolation of the solubility of LiC1 turned to be zero. This clearly shows that LiCI cannot be dissolved in high molecular weight of PPO. Thus, the ether oxygens in PPO did not work effectively to solubilize LiC1. This was actually known that LiC1 was not to form complex with PPO because it was insoluble in
Na ÷
" ~ Br"
¢D
121
'R 1.2 O 0~
0.8
0.4
119 2:0 21, 212 2:3 Cation Radius log R (pm)
Fig. 7. Relation between lattice energy and cation radius of a series of inorganic salts. (Closed plots ) salts crystallized by heating in PPO~2ooand PPO4ooo; (Open plots) salts not phase separated from PPO oligomers by heating. Other captions are the same as in Fig. 5.
J
0
2
4
6
8
1'0
f ~ 1%)
Fig. 8. Solubility of LiCI and LiC104 in PPO as a function of terminal hydroxyl group fraction (f-OH) of the PPOs. LiC1 at room temperature ( • ) , 80 (©), 100 (ZX), and 120°C ( V ) , and LiC104 at room temperature ( • ) , respectively.
122
K. Ito et al. / Solid State lonics 68 (1994) 117-123
PPO. The LiC1 was soluble in P P O oligomers only by the large polarity on the t e r m i n a l hydroxyl groups. The solubility o f LiC1 at 25 °C and 80°C gave different curves from that at higher temperatures. Because the solubility o f LiCI decreased a n d gave positive t e m p e r a t u r e d e p e n d e n c e at lower t e m p e r a t u r e as is shown in Fig. 1. This great c o n t r i b u t i o n o f the t e r m i n a l hydroxyl groups to the solvation o f inorganic salts was the reason these salts dissolved in P P O oligomers without forming complex. The t h e r m a l m o t i o n affected this i n t r a m o l e c u l a r solvation to reduce the salt solubility. The ether oxygens on P P O hardly interacted with cation in the case o f complex non-forming salts. The solvation state o f these salts t u r n e d to be considerably unstable. The t e r m i n a l hydroxyl groups in polyethers were suggested to m a k e cooperative coordination to the cation called " i n t r a m o l e c u l a r c o o r d i n a t i o n " [ 9,10 ]. W i t h increasing the t h e r m a l molecular motion, the t e r m i n a l hydroxyl groups cannot keep the i n t r a m o l e c u l a r c o o r d i n a t i o n to the cation effectively. This i n t r a m o l e c u l a r c o o r d i n a t i o n was distorted with increasing temperature, and the cation was released from the P P O since ether oxygens were not effective enough to solubilize it. As the result o f the cation release, the crystallization o f salts was observed. Against this, the complex-forming salts such as LiC104 could m a k e stable complex with PPO, therefore the phase separation o f salts could not be observed.
4. Conclusion We analyzed the phase separation o f inorganic salts dissolved in P P O oligomers. Results showed that the solubility o f salts in P P O v a r i e d with the m a x i m u m value a n d h a d negative t e m p e r a t u r e d e p e n d e n c e at higher temperature. This negative t e m p e r a t u r e dependence o f salt solubility h a d been f o u n d in PEOs. The salt species which was crystallized by heating dep e n d e d on the lattice energy o f salts, and the threshold o f it was decreased with increasing cation radius. Further, this threshold was shifted to a lower energy side with increasing molecular weight o f PPO. Since the polarity o f ether oxygen in P P O was weaker than that in PEO, m o r e salts were crystallized by heating than those observed in PEO. The salts which m a d e
complex with P P O could form stable solvation state with not only its ether oxygens and terminal hydroxyl groups, so that no phase separation o f salts could be observed even at higher temperature. Against this, the salts, crystallized by heating, were solubilized only by the terminal hydroxyl groups o f PPO. These salts could not form stable solvation state, the coordination o f terminal hydroxyl groups was therefore dist o t t e d with thermal molecular m o t i o n at higher temperature. As the results o f this distortion, the salt was crystallized by heating. We believe that the results o b t a i n e d here should be very i m p o r t a n t to design p o l y m e r electrolyte applicable for wider t e m p e r a t u r e range.
5. Acknowledgement The present study was s u p p o r t e d by the Grant-inA i d for Scientific Research on Priority Areas "Reactive Organometallics" from the Ministry of Education, Science a n d Culture, Japan.
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[16]N. Ogata, Conductive Polymers (Kodansha Scientific, Tokyo, 1990) p. 110. [ 17 ] D.F. Shriver, B.L. Papke, M.A. Ratner, R. Dupon, T. Wong and M. Brodwin, Solid State Ionics 5 (1981) 83.
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[18]H.D.B. Jekins, in: CRC Handbook of Chemistry and Physics, 71st Ed. (Chemical Rubber Publ. Co., Boston, 1990) 12-3.