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Effect of sodium sulfonate groups on the ionic conductivity of a copolyester of thiodipropionic acid
Pergamon Pn:
Eur. Polym.J. Vol. 33, No. W-12, pp. 1619-1683, 1997 0 1997 Elawier Scicna Ltd. All rights reaeiwd Primal in Gmt Britain 601~3057p7 $17.00 + 0.00 soo1+3057(97poo36-0
EFFECT OF SODIUM SULFONATE GROUPS ON THE IONIC CONDUCTIVITY OF A COPOLYESTER OF THIODIPROPIONIC ACID M. BANDIERA,”
P. MANARESI,’ A. MUNARI,‘* and M. MASTRAGOSTINOb
M. C. BORGHINIb
‘Dipartimento di Chimica Applicata e Scienza dei Materiali, Universiti di Bologna, Viale Risorgimento 2, I 40136 Bologna, Italy Qipartimento di Chimica “G. Ciamician”, Universiti di Bologna, Via Sehni 2, I 40126 Bologna, Italy (Received 15 July 1996; accepted in $MI form 24 October 1996)
Abstract-New copolyesters of thiodipropionic acid and triethylene glycol containing various amounts of -SO,Na groups were developed. Both these copolymers and their electrolytes, the latter obtained by dissolving NaCIO, in the polymeric matrix, were characterized in terms of ionic conductivity in the 25-8O”C temperature range to evaluate the effect on conductivity of ionic groups covalcntly attached to the polymer backbone. A small number of -SO,Na groups were found to improve the electrical properties of the copolymers, even though the conductivity values were not high enough to allow their use in electrochemical devices without further dissolution of sodium salt. Ou the other hand, a large content of sulfonate groups increases significantly the viscosity of the copolymeric matrix and reduces the electrical performances of the electrolytes. (Q 1997 Elsevier Science Ltd
EXPERIMENTAL
INTRODUCTION
In the last decade a considerable effort has been expended to develop polymer electrolytes containing lithium salts for application in high energy-density, solid-state lithium rechargeable batteries [ 11. Attention has recently been focused on sodium-polymer batteries [24] because sodium is seven times cheaper than lithium and easier to purify [A. Prior investigation has shown that a series of aliphatic polyesters of thiodipropionic acid (TDPA) and linear glycols containing a different number of ethylene oxide groups are highly viscous liquids at room temperature which easily dissolve alkali-metal salts. The performance of the polymer electrolytes obtained by addition of lithium salts is reported in refs [S, 91. The electrical properties of these electrolytes might be improved by employing polymers with ionic groups covalently attached to the backbones. The present study deals with the electrochemical characterization of copolymers based on TDPA, triethylene glycol (TEG) and small quantities of sodium 3,Sbis(methoxycarbonyl) benzenesulfonate (SSIPNa) so as to elucidate the influence on the ionic conductivity of anion groups covalently attached to the polymer. The electrical characterization of the electrolytes obtained by dissolving NaClO, is also reported and discussed. *To whom all correspondence should be addressed.
Synthesis of copolymers The copolymers were synthesized from dimethyl 3,3’thiodipropionatc (DMTDP) and triethylene glycol in a molar ratio 1: 1.1 and 5SIPNa (0, 1,5, 10 mol% with respect to DMTDP) using Ti(OBub as the catalyst. All the reagents were supplied by Aldrich and were used without further purification. The bulk synthesis were carried at temperatures of 170-180°C after the standard two-stage polycondensation procedure [lo], i.e. by distilling off the methanol under nitrogen atmosphere and by completing the polycondensation at a pressure of about 10-j mmHg to facilitate TEG removal. The polymer samples were washed in diethyl ether and vacuum-dried overnight at 90°C. All the synthes&d polymers were transparent and highly viscous liquids at room temperature. The three copolymers containing sodium sulfonate groups are hereinafter referred to as PTDPAEO(2)-Sl, PTDPA-Eo(2)-SS and PTDPA-Eo(2)-SlO, where the number following S represents the molar % content of -SO,Na groups; PTDPA-EO(2) indicates the
polymer without ionic groups. Copolymer characterization
The copolymers were characterized by FT-IR (Bruker IFS 48) and ‘H NMR (Varian XL-200) spectroscopy. A differential scanniug calorimeter (Perkin Elmer DSC7) was used to measure the glass transition temperature TI at a heating rate of 5, 10 and ZO”C/min in the -90 and 100°C range; the instrument was calibrated using high-purity standards to the tested heating rates. T* was taken as the midpoint temperature of the baseline shift during the transition in the second heating run after rapid cooling from
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M. Bandiera et al.
1680
the melt. Repeated measurements per sample showed excellent reproducibility. The calorimetric curves, some of which are shown in Fig. 1, indicate that the samples are amorphous and without melting endotherms after rapid quenching from the melt. The GPC measurements were carried out with a Knauer instrument having three columns @gel 500, l@ and l(r A) at 30°C and employing a CHCI, mobile phase with a flow rate of 0.8 mL/min. Molecular weights were determined using a calibration plot with polystyrene standards. Terminal-group content was determined to evaluate independently the number-average molecular weight of polymers; the samples were vacuum-dried at 90°C for 2 days before characterization. The carboxyl terminal groups were determined on samples dissolved in CHCh by titration with a 0.05N solution of KOH in CHjOH. Blank runs were performed for correction. The exact concentration of the KOH solution was determined daily by titration with a benzoic acid solution of known concentration. Hydroxyl group content was determined by titration after quantitative esterification with acetic anhydride [Ill. The hydroxyl concentration was calculated by subtracting the carboxyl concentration as described above. Dilute solution viscosity was measured using Ubbelohde dilution viscometers; measurements were taken in CHCls at 30.0 f O.l”C at four polymer concentrations in the 0.5 f 1.Og/dL range. Intrinsic viscosity [q] was found using both Huggins and Kraemer plots, i.e. by extrapolating n,,,/c and (In a,)/~ to zero concentration.
B
Sodium salt addition
NaClO, (Aldrich) was vacuum-dried for 18 hr; the polymer electrolytes were prepared at different molar ratios of polyester/NaCl04 by mixing the two components in an argon-filled glove box. The mixtures were heated under magnetic stirring for 2 hr at 70°C to facilitate the dissolution of the salt in the polymeric matrix under argon atmosphere. These electrolytes are transparent semi-solids with a higher viscosity than the base polymers. Conductivity measurements
The ionic conductivity of the sulfonated copolymers and of their electrolytes was evaluated by complex impedance on blocking electrode ceils sealed in an argon-filled glove box with a Solar&on 1255 frequency response analyser coupled to a PAR 273 potentiostat-galvanostat in the 0.1 Hz100 kHz frequency range. The impedance results were analysed by Boukamp’s fitting program [12]. Each sample was left to reach equilibrium for 1 hr at the tested temperature before measurement; the temperature range was 25-80°C. RESULTS AND DISCUStXON Molecular and thermal characterization The chemical structure of the repeating unit of the sulfonated copolymers is:
B
-C-CH$H@
-C~C~-C--OCH~H2\~H,CH2)p0
2.0
0.0
1. -60.0
-40.0
-20.0
0.0
20.0
Temperature (‘XI) Fig. 1. Calorimetric curves of PTDPA-EG(2) and its sulfonated copolyesters after melt quenching. Ionic group content (mol%) is indicated on curves.
Ionic conductivity of sulfonated copolyesters Table 1. Molecular characterization
1681
data for the copolyesters of thiodipropionic
acid
Terminal-group content -CDDH (mesh)
Polymer PTDPA-EG(2) PTDPA-EO@j-SI PTDPA-EO(2)-S5 PTDPA-ED(2)-SIO
3.43 3.37 4.93 6.64
x x x x
IO-’ 10-1 10-I IO-)
-OH (meslg)
xu”E
Mow!
0.565 0.466 0.45 1 0.526
3520 4260 4380 3750
4800 7600 6600 4260
Calculated from the overall terminal-groups content, neglecting the presence of cyclic molecules. bFrom the maximum of the peak of GPC curves.
where the sulfoisophtalic moieties are randomly distributed along the chains because of the interchange redistribution reactions, which easily occur in the presence of Ti(OBu)4 under the experimental conditions employed in the copolymer synthesis. Both FT-TR and ‘H NMR spectra are consistent with the above repeating unit. Molecular and thermal characterization data for the copolymers are reported in Tables 1 and 2, respectively; the PTDPA-EO(2) (i.e. the polymer without -SOsNa groups) data are also given for comparison. A significant difference is the greater number of terminal -COOH groups in the sulfonated copolymers, especially in the samples with the highest % of ionic groups in the chains. Similar results are also reported for other types of polyesters [13-161. The formation of a relatively high quantity of carboxyl end-groups in the presence of -SO,Na is presumably due to the side reactions involving the hydrolysis of ester groups. This phenomenon can more readily occur when ionic groups are present because of their specific catalytic effect and the relatively high amount of moisture retained by the comonomer SSIPNa. Another important characteristic of these liquidlike copolymers is their viscosity at room temperature, which appears to increase sharply as the number of -SO,Na groups increases. This fact is due to the ionic interactions among the polymeric chains, and a similar finding is reported for other kinds of polymers containing ionic groups [17-191. The viscosity enhancement was also shown by a dilute solution characterization, the investigated copolymers evincing linear Kraemer and Huggins plots, differently from the behaviour of many types of ionomers in polar solvents [20,21]. The estimated intrinsic viscosity values are 0.147, 0.150 and 0.167 dL g-’ for samples PTDPA-EO(Z)-Sl , PTDPA-EO(Z)-SS and PTDPA-E0(2)-SlO, respectively. As the number of sodium-sulfonate groups increases, the [q]
values increase, although molecular weight of these copolymers decreases as shown in Table 1. Table 2 shows that the glass-transition is slightly but influenced by the heating rate. The presence of ionic groups, irrespective of the heating rate, results in an increase of the glass-transition temperature, as reported for other kinds of ionomers [21]. The anomalous, slightly higher PTDPA-EO(2) T, value with respect to that of PTDPA-EO(Z)-Sl is presumably due to differences in the two polymers, e.g. in the content of cyclic oligomers, whose plasticizing effect is well known. The thermal characterization of the polymer electrolytes (obtained dissolving different amounts of sodium perchlorate in these polymers) shows that the polymer-sodium salt complex leads to a stiffening of the chain and, as a result, to an increase in the Tg values with respect to those of the copolymers without salt (see Table 2). The value for the polymer electrolyte PTDPA-E0(2)-NaCIOd is similar to that reported in [8] for PTDPA-EO(Z)-LiCIOd. Electrical characterization
The results of the electrical characterization at different temperatures of PTDPA-EO(Z), PTDPAEO(2)S 1, PTDPA-E0(2)-SS and PTDPA-EO(Z)SlO, are reported in Fig. 2 as Arrhenius conductivity plots. Despite the very low content of Na+ ions, PTDPA-E0(2)-Sl , PTDPA-E0(2)-SS and PTDPAE0(2)-SlO show an appreciable conductivity u, remarkably higher than that of PTDPA-EO(2). At temperatures higher than 5O”C, the cr values of these copolymers are directly related to the amount of sodiumsulfonate groups in the polymer chain. By contrast, at room temperature PTDPA-EO(Z)-SlO shows the lowest conductivity, the high viscosity of the system evidently offsetting in these conditions the positive effect of the anion groups in the polymer backbone. The plots of Fig. 2 do not fit strictly with the Arrhenius equation, however, the effect of
Table 2. Glass transition temperature at different scanning rates v for the copolyesters of thiodiurodonic acid and their NaClO~-electrolvtes US + OVNa = 22)
Polymer PTDPA-EO(2) PTDPA-ECI(Z)-Sl PTDPA-EO(Z)-S5 PTDPA-EO(2)SlO PTDPA-EO(Z)NaClO, PTDPA-E0(2)-SSNaCIO, PTDPA-E0(2)-SIONaCIO,
v = ZO”C/min
u = lO”C/min
u = S”C/min
-42.0 -44.0 -42.0 -40.0 -31.5 -27.0 -26.0
-43.5 -45.0 -44.0 -41.0 -33.0 -29.5 -29.0
-44.5 -46.5 -45.0 -43.5 -35.5 -31.0 -30.5
%econd DSC scan on samples rapidly cooled from the melt.
M. Bandiera er al.
1682
-9.0 2.8 2.9
3.0
3.1 3.2 3.3
3.4 id/T (l/K)
103/T(1/K)
Fig. 2. Arrhenius plots of conductivity for PTDPA-EO(2) and its sulfonated copolyesters.
Fig. 4. Arrhenius plots of conductivity for the NaC104-electrolytes hased on the copolyesters of thiodipropionic acid, at the constant ratio (S + O)/Na = 22.
temperature on conductivity is similar in all cases, only a little higher in the case of PTDPA-EO(2)SlO. The conductivity values of the sulfonated copolymers are not suitable for a use in electrochemical devices without a further salt addition. Accordingly, preliminary tests were run to characterize various mixtures of PTDPA-EO(2) and NaC104. Figure 3 reports the d values at different temperatures as a function of the molar ratio of (S + O)/Na in the range from 11: 1 to 110 : 1. All the sulphur and oxygen atoms in the polymer backbone, excluding the terminal groups and carbonylic oxygen, were taken into account. The curves show a maximum with a rapid decrease of conductivity at high salt concentration, i.e. at molar ratio lower than 22: 1. The conductivity values are in the range from 5 x 10-6Scm-l (at 25°C) to 2 x lo-‘Scm-’ (at than 80°C). Given that c values greater 1 x lo-‘S cm-’ are suitable for practical applications, these electrolytes can be used in battery technology for electric vehicle propulsion where thermal control at 80°C is not a serious problem. The patterns of the conductivity curves with the salt concentration, as well as the conductivity values at different temperatures of PTDPA-E0(2)NaC104
electrolytes, are also comparable for LiC104 [8].
-3.0
I
I
I
1,
T=4oY a T=bcPC . T=70PC
to those obtained
Figure 4 shows the conductivity data as Arrhenius plots of the electrolytes obtained by dissolving NaCiO, in PTDPA-EO(2), PTDPA-EO(2)-SS and PTDPA-EO(Z)-SlO (with molar ratio (S + O)/Na of 22: 1) in the 2580°C temperature range. As expected, NaClO, increases electrolyte conductivity. Note that the high viscosity of the polymer matrix with 10% of ionic groups (PTDPA-EO(2)-SIO) adversely affects the conductivity at all temperatures, so there is no improvement of the electrolyte performance with a polymeric matrix containing ionic groups. A comparison with the data of Fig. 2 shows a stronger effect of temperature on conductivity in the case of the electrolytes. This fact can be explained considering that ionic pairs form when a salt is dissolved in the polymer, and they tend to dissociate more easily as the temperature increases, giving rise to a higher number of free carriers. As the Arrhenius plots are not linear in all cases, we tried to fit our results to the Wilhams-LandelFerry (WLF) equation, which is universally valid in describing properties related to free-volume relaxation of amorphous polymers [22] in the form:
l
0
T=WC
6.0
5
k
$ 4.0 0
0
20
40 60 80 (S+O)/Na
100 120
Fig. 3. Dependence of conductivity on (S + O)/Na molar ratio for PTDPA-EO(2)NaClOd electrolytes at different temperatures.
20
40
60
80
100
120
T-Ts0 Fig. 5. WLF plots of conductivity for the NaClO&zctrolytes based on the copoiyesters of thiodipropionic acid: (-) curves calculated employing WLF equation with the G , G and a(Z’,) values reported in Table 3. (S + O)/Na = 22 in all cases.
Ionic conductivity of sulfonated copolyesters Table 3. WLF parameters of conductivity for the electrolytes based on copolycstm thiodimodonic acid. IS + 0)iNa = 22 in all casea
1683 of
Polymer FTDPA-EO(Z)_NaClO, PTDPA-EO(2)-SSNaCIO, PTDPA-EO(Z)-SI ON&l04 WLF
Conductivity at the glass-transition temperature a(T,) was evaluated by a non-linear least square analysis, together with the C, and Cl constants, because cr at a temperature close to T8 is too small to be measured with our apparatus; the T, values employed matched a heating rate of YC/min. The C, and Cz values in Table 3, are very similar to those reported elsewhere for different polyethylene oxidebased electrolytes [23] and are comparable to the “universal values” (C, = 17.4, C2 = 51.6) of the WLF equation. Figure 5 shows that our data fit this equation well.
CONCLUSIONS
The presence of a low amount of -SOsNa groups, covalently attached to the polymer backbone, reflects positively on the electrical properties of the tested Na+-ion conducting polymers based on a polyester of thiodipropionic acid, even if the conductivity values are not high enough for their use without further dissolution of sodium salt. On the other hand, a large content of sodium sulfonate groups increases significantly the viscosity of the copolymer matrix and reduces the electrical performance of the polymeric electrolytes with NaClO,. work has been supported by the Italian Minister0 dell’Universid e della Ricerea Scientifica e Tecnologica (ex 40%). Acknowledgemenr-This
REFERENCES 1. Armand, M., Solid State Ionics, 1994, 69, 309. 2. Zhang, S., Liu, Q. and Yang, L., Polymer, 1994, 35,
3140. 3. Zhang, S., Liu, Q. and Yang, L., Solid State Ionics,
1995, 76, 127.
11.0 9.3 10.0 17.4
41 46 39 51.6
3.98 x IO-” 3.16 x IO-” 6.31 x IO-”
4. Srrepathi Rao, S., Jaipal, M., Narasimha Reddy, K. and Subba Rao, U. V., Solid State Ionics, 1994,74,225. 5. Stallworth, P. E., Li, J., Greenbaum, S. G., Croee, F., Slane, S. and Salomon, M., Solid State Ionics, 1994,73,
119. 6. Paulmer, R. D. A. and Kulkami, A. R., Solid State Ionics, 1994, 68, 243. 7. V&o, S. J., Doeff, M. M., Ma, Y. P. and De Jonghe, L. C., Abstract No. Bl in Extended Abstract of Fourth International Symposium on Polymer Electrolytes, Newuort. Rhode Island. 1994. P., Bignozzi,‘M. C., Pilati, F., Munari, A., 8. Ma&e&
Mastragostino,
M., Meneghello, L. and Chiolle, A.,
Polymer, 1993, 34, 2422. 9. Mastragostino,
M., Meneghello, L., Manaresi, P., Bignozzi, M. C., Pilati, F. and Munari, A., Polymer,
1994, 35, 845. 10. Pilati, F., In Comprehensive Polymer Science, ed. G.
Allen and J. C. Bevington, Vol. 5, Chap. 17. Pergamon Press, Oxford, 1989. 11. Siggia, S., Gordon Hanna, J. and Stengle, T. R., The Chemistry of the Hvdroxvl Group. ad. S. Patai. Part 1. Interseienee~ Publishers, London; i971, p. 304.. 12. Boukamp, B. A., Solid State Ionics, 1986, Zo, 31. 13. Zhang, H., Li, S. and Luo, H., J. Appl. Polym. Sci., 1987, 34, 1353. 14. Chen, K. M. and Liu, H. J., J. Appl. Polym. Sci., 1987, 34, 1879. 15. Ostrowska-Gumkowska, B. and Ostrowska-Czubenko, J., Eur. Polym. J., 1988, 24, 803. 16. Pilati, F., Manaresi, P., Ruperto, M. G., Bonora, V., Munari, A. and Fiorini, M., Polymer, 1993, 34, 2421. 17. Bazuin, G. C. and Eisenberg, A., Ind. Eng. Chem. Prod. Res. Dee., 1981, 20, 271. 18. Connelly, R. W., McConkey, R. C., Noonan, J. M. and Pearson. G. H.. J. Polvm. Sci.. Polvm. Phvs. Edn. 1982. 20, 259.’ ’ _ ’ ’ 19. Munari, A., Fiorini, M., Pexzin, G. and Pilati, F., J. Appl. Polym. Sci., 1993, SO, 159. 20. Tant, M. R. and Wilkes, G. L., J. Macromol. Sci., Rev. Macromol. Chem. Phys., 1988, C28, 1. 21. Fitzgerald, J. J. and Weiss, R. A., J. hfacromol. Sci., Reo. Macromol. Chem. Phvs.. 1988. C28. 99. 22. Williams, M. L., Landel, RI F: and Ferry,‘J. D., J. Am.
Chem. Sot., 1955, 77, 3701. 23. Watanabe, M., Itoh, M., Sanui, K. and Ogata, N., Macromolecules, 1981, 20, 569.
Eur. Polym.J. Vol. 33, No. W-12, pp. 1619-1683, 1997 0 1997 Elawier Scicna Ltd. All rights reaeiwd Primal in Gmt Britain 601~3057p7 $17.00 + 0.00 soo1+3057(97poo36-0
EFFECT OF SODIUM SULFONATE GROUPS ON THE IONIC CONDUCTIVITY OF A COPOLYESTER OF THIODIPROPIONIC ACID M. BANDIERA,”
P. MANARESI,’ A. MUNARI,‘* and M. MASTRAGOSTINOb
M. C. BORGHINIb
‘Dipartimento di Chimica Applicata e Scienza dei Materiali, Universiti di Bologna, Viale Risorgimento 2, I 40136 Bologna, Italy Qipartimento di Chimica “G. Ciamician”, Universiti di Bologna, Via Sehni 2, I 40126 Bologna, Italy (Received 15 July 1996; accepted in $MI form 24 October 1996)
Abstract-New copolyesters of thiodipropionic acid and triethylene glycol containing various amounts of -SO,Na groups were developed. Both these copolymers and their electrolytes, the latter obtained by dissolving NaCIO, in the polymeric matrix, were characterized in terms of ionic conductivity in the 25-8O”C temperature range to evaluate the effect on conductivity of ionic groups covalcntly attached to the polymer backbone. A small number of -SO,Na groups were found to improve the electrical properties of the copolymers, even though the conductivity values were not high enough to allow their use in electrochemical devices without further dissolution of sodium salt. Ou the other hand, a large content of sulfonate groups increases significantly the viscosity of the copolymeric matrix and reduces the electrical performances of the electrolytes. (Q 1997 Elsevier Science Ltd
EXPERIMENTAL
INTRODUCTION
In the last decade a considerable effort has been expended to develop polymer electrolytes containing lithium salts for application in high energy-density, solid-state lithium rechargeable batteries [ 11. Attention has recently been focused on sodium-polymer batteries [24] because sodium is seven times cheaper than lithium and easier to purify [A. Prior investigation has shown that a series of aliphatic polyesters of thiodipropionic acid (TDPA) and linear glycols containing a different number of ethylene oxide groups are highly viscous liquids at room temperature which easily dissolve alkali-metal salts. The performance of the polymer electrolytes obtained by addition of lithium salts is reported in refs [S, 91. The electrical properties of these electrolytes might be improved by employing polymers with ionic groups covalently attached to the backbones. The present study deals with the electrochemical characterization of copolymers based on TDPA, triethylene glycol (TEG) and small quantities of sodium 3,Sbis(methoxycarbonyl) benzenesulfonate (SSIPNa) so as to elucidate the influence on the ionic conductivity of anion groups covalently attached to the polymer. The electrical characterization of the electrolytes obtained by dissolving NaClO, is also reported and discussed. *To whom all correspondence should be addressed.
Synthesis of copolymers The copolymers were synthesized from dimethyl 3,3’thiodipropionatc (DMTDP) and triethylene glycol in a molar ratio 1: 1.1 and 5SIPNa (0, 1,5, 10 mol% with respect to DMTDP) using Ti(OBub as the catalyst. All the reagents were supplied by Aldrich and were used without further purification. The bulk synthesis were carried at temperatures of 170-180°C after the standard two-stage polycondensation procedure [lo], i.e. by distilling off the methanol under nitrogen atmosphere and by completing the polycondensation at a pressure of about 10-j mmHg to facilitate TEG removal. The polymer samples were washed in diethyl ether and vacuum-dried overnight at 90°C. All the synthes&d polymers were transparent and highly viscous liquids at room temperature. The three copolymers containing sodium sulfonate groups are hereinafter referred to as PTDPAEO(2)-Sl, PTDPA-Eo(2)-SS and PTDPA-Eo(2)-SlO, where the number following S represents the molar % content of -SO,Na groups; PTDPA-EO(2) indicates the
polymer without ionic groups. Copolymer characterization
The copolymers were characterized by FT-IR (Bruker IFS 48) and ‘H NMR (Varian XL-200) spectroscopy. A differential scanniug calorimeter (Perkin Elmer DSC7) was used to measure the glass transition temperature TI at a heating rate of 5, 10 and ZO”C/min in the -90 and 100°C range; the instrument was calibrated using high-purity standards to the tested heating rates. T* was taken as the midpoint temperature of the baseline shift during the transition in the second heating run after rapid cooling from
1679
M. Bandiera et al.
1680
the melt. Repeated measurements per sample showed excellent reproducibility. The calorimetric curves, some of which are shown in Fig. 1, indicate that the samples are amorphous and without melting endotherms after rapid quenching from the melt. The GPC measurements were carried out with a Knauer instrument having three columns @gel 500, l@ and l(r A) at 30°C and employing a CHCI, mobile phase with a flow rate of 0.8 mL/min. Molecular weights were determined using a calibration plot with polystyrene standards. Terminal-group content was determined to evaluate independently the number-average molecular weight of polymers; the samples were vacuum-dried at 90°C for 2 days before characterization. The carboxyl terminal groups were determined on samples dissolved in CHCh by titration with a 0.05N solution of KOH in CHjOH. Blank runs were performed for correction. The exact concentration of the KOH solution was determined daily by titration with a benzoic acid solution of known concentration. Hydroxyl group content was determined by titration after quantitative esterification with acetic anhydride [Ill. The hydroxyl concentration was calculated by subtracting the carboxyl concentration as described above. Dilute solution viscosity was measured using Ubbelohde dilution viscometers; measurements were taken in CHCls at 30.0 f O.l”C at four polymer concentrations in the 0.5 f 1.Og/dL range. Intrinsic viscosity [q] was found using both Huggins and Kraemer plots, i.e. by extrapolating n,,,/c and (In a,)/~ to zero concentration.
B
Sodium salt addition
NaClO, (Aldrich) was vacuum-dried for 18 hr; the polymer electrolytes were prepared at different molar ratios of polyester/NaCl04 by mixing the two components in an argon-filled glove box. The mixtures were heated under magnetic stirring for 2 hr at 70°C to facilitate the dissolution of the salt in the polymeric matrix under argon atmosphere. These electrolytes are transparent semi-solids with a higher viscosity than the base polymers. Conductivity measurements
The ionic conductivity of the sulfonated copolymers and of their electrolytes was evaluated by complex impedance on blocking electrode ceils sealed in an argon-filled glove box with a Solar&on 1255 frequency response analyser coupled to a PAR 273 potentiostat-galvanostat in the 0.1 Hz100 kHz frequency range. The impedance results were analysed by Boukamp’s fitting program [12]. Each sample was left to reach equilibrium for 1 hr at the tested temperature before measurement; the temperature range was 25-80°C. RESULTS AND DISCUStXON Molecular and thermal characterization The chemical structure of the repeating unit of the sulfonated copolymers is:
B
-C-CH$H@
-C~C~-C--OCH~H2\~H,CH2)p0
2.0
0.0
1. -60.0
-40.0
-20.0
0.0
20.0
Temperature (‘XI) Fig. 1. Calorimetric curves of PTDPA-EG(2) and its sulfonated copolyesters after melt quenching. Ionic group content (mol%) is indicated on curves.
Ionic conductivity of sulfonated copolyesters Table 1. Molecular characterization
1681
data for the copolyesters of thiodipropionic
acid
Terminal-group content -CDDH (mesh)
Polymer PTDPA-EG(2) PTDPA-EO@j-SI PTDPA-EO(2)-S5 PTDPA-ED(2)-SIO
3.43 3.37 4.93 6.64
x x x x
IO-’ 10-1 10-I IO-)
-OH (meslg)
xu”E
Mow!
0.565 0.466 0.45 1 0.526
3520 4260 4380 3750
4800 7600 6600 4260
Calculated from the overall terminal-groups content, neglecting the presence of cyclic molecules. bFrom the maximum of the peak of GPC curves.
where the sulfoisophtalic moieties are randomly distributed along the chains because of the interchange redistribution reactions, which easily occur in the presence of Ti(OBu)4 under the experimental conditions employed in the copolymer synthesis. Both FT-TR and ‘H NMR spectra are consistent with the above repeating unit. Molecular and thermal characterization data for the copolymers are reported in Tables 1 and 2, respectively; the PTDPA-EO(2) (i.e. the polymer without -SOsNa groups) data are also given for comparison. A significant difference is the greater number of terminal -COOH groups in the sulfonated copolymers, especially in the samples with the highest % of ionic groups in the chains. Similar results are also reported for other types of polyesters [13-161. The formation of a relatively high quantity of carboxyl end-groups in the presence of -SO,Na is presumably due to the side reactions involving the hydrolysis of ester groups. This phenomenon can more readily occur when ionic groups are present because of their specific catalytic effect and the relatively high amount of moisture retained by the comonomer SSIPNa. Another important characteristic of these liquidlike copolymers is their viscosity at room temperature, which appears to increase sharply as the number of -SO,Na groups increases. This fact is due to the ionic interactions among the polymeric chains, and a similar finding is reported for other kinds of polymers containing ionic groups [17-191. The viscosity enhancement was also shown by a dilute solution characterization, the investigated copolymers evincing linear Kraemer and Huggins plots, differently from the behaviour of many types of ionomers in polar solvents [20,21]. The estimated intrinsic viscosity values are 0.147, 0.150 and 0.167 dL g-’ for samples PTDPA-EO(Z)-Sl , PTDPA-EO(Z)-SS and PTDPA-E0(2)-SlO, respectively. As the number of sodium-sulfonate groups increases, the [q]
values increase, although molecular weight of these copolymers decreases as shown in Table 1. Table 2 shows that the glass-transition is slightly but influenced by the heating rate. The presence of ionic groups, irrespective of the heating rate, results in an increase of the glass-transition temperature, as reported for other kinds of ionomers [21]. The anomalous, slightly higher PTDPA-EO(2) T, value with respect to that of PTDPA-EO(Z)-Sl is presumably due to differences in the two polymers, e.g. in the content of cyclic oligomers, whose plasticizing effect is well known. The thermal characterization of the polymer electrolytes (obtained dissolving different amounts of sodium perchlorate in these polymers) shows that the polymer-sodium salt complex leads to a stiffening of the chain and, as a result, to an increase in the Tg values with respect to those of the copolymers without salt (see Table 2). The value for the polymer electrolyte PTDPA-E0(2)-NaCIOd is similar to that reported in [8] for PTDPA-EO(Z)-LiCIOd. Electrical characterization
The results of the electrical characterization at different temperatures of PTDPA-EO(Z), PTDPAEO(2)S 1, PTDPA-E0(2)-SS and PTDPA-EO(Z)SlO, are reported in Fig. 2 as Arrhenius conductivity plots. Despite the very low content of Na+ ions, PTDPA-E0(2)-Sl , PTDPA-E0(2)-SS and PTDPAE0(2)-SlO show an appreciable conductivity u, remarkably higher than that of PTDPA-EO(2). At temperatures higher than 5O”C, the cr values of these copolymers are directly related to the amount of sodiumsulfonate groups in the polymer chain. By contrast, at room temperature PTDPA-EO(Z)-SlO shows the lowest conductivity, the high viscosity of the system evidently offsetting in these conditions the positive effect of the anion groups in the polymer backbone. The plots of Fig. 2 do not fit strictly with the Arrhenius equation, however, the effect of
Table 2. Glass transition temperature at different scanning rates v for the copolyesters of thiodiurodonic acid and their NaClO~-electrolvtes US + OVNa = 22)
Polymer PTDPA-EO(2) PTDPA-ECI(Z)-Sl PTDPA-EO(Z)-S5 PTDPA-EO(2)SlO PTDPA-EO(Z)NaClO, PTDPA-E0(2)-SSNaCIO, PTDPA-E0(2)-SIONaCIO,
v = ZO”C/min
u = lO”C/min
u = S”C/min
-42.0 -44.0 -42.0 -40.0 -31.5 -27.0 -26.0
-43.5 -45.0 -44.0 -41.0 -33.0 -29.5 -29.0
-44.5 -46.5 -45.0 -43.5 -35.5 -31.0 -30.5
%econd DSC scan on samples rapidly cooled from the melt.
M. Bandiera er al.
1682
-9.0 2.8 2.9
3.0
3.1 3.2 3.3
3.4 id/T (l/K)
103/T(1/K)
Fig. 2. Arrhenius plots of conductivity for PTDPA-EO(2) and its sulfonated copolyesters.
Fig. 4. Arrhenius plots of conductivity for the NaC104-electrolytes hased on the copolyesters of thiodipropionic acid, at the constant ratio (S + O)/Na = 22.
temperature on conductivity is similar in all cases, only a little higher in the case of PTDPA-EO(2)SlO. The conductivity values of the sulfonated copolymers are not suitable for a use in electrochemical devices without a further salt addition. Accordingly, preliminary tests were run to characterize various mixtures of PTDPA-EO(2) and NaC104. Figure 3 reports the d values at different temperatures as a function of the molar ratio of (S + O)/Na in the range from 11: 1 to 110 : 1. All the sulphur and oxygen atoms in the polymer backbone, excluding the terminal groups and carbonylic oxygen, were taken into account. The curves show a maximum with a rapid decrease of conductivity at high salt concentration, i.e. at molar ratio lower than 22: 1. The conductivity values are in the range from 5 x 10-6Scm-l (at 25°C) to 2 x lo-‘Scm-’ (at than 80°C). Given that c values greater 1 x lo-‘S cm-’ are suitable for practical applications, these electrolytes can be used in battery technology for electric vehicle propulsion where thermal control at 80°C is not a serious problem. The patterns of the conductivity curves with the salt concentration, as well as the conductivity values at different temperatures of PTDPA-E0(2)NaC104
electrolytes, are also comparable for LiC104 [8].
-3.0
I
I
I
1,
T=4oY a T=bcPC . T=70PC
to those obtained
Figure 4 shows the conductivity data as Arrhenius plots of the electrolytes obtained by dissolving NaCiO, in PTDPA-EO(2), PTDPA-EO(2)-SS and PTDPA-EO(Z)-SlO (with molar ratio (S + O)/Na of 22: 1) in the 2580°C temperature range. As expected, NaClO, increases electrolyte conductivity. Note that the high viscosity of the polymer matrix with 10% of ionic groups (PTDPA-EO(2)-SIO) adversely affects the conductivity at all temperatures, so there is no improvement of the electrolyte performance with a polymeric matrix containing ionic groups. A comparison with the data of Fig. 2 shows a stronger effect of temperature on conductivity in the case of the electrolytes. This fact can be explained considering that ionic pairs form when a salt is dissolved in the polymer, and they tend to dissociate more easily as the temperature increases, giving rise to a higher number of free carriers. As the Arrhenius plots are not linear in all cases, we tried to fit our results to the Wilhams-LandelFerry (WLF) equation, which is universally valid in describing properties related to free-volume relaxation of amorphous polymers [22] in the form:
l
0
T=WC
6.0
5
k
$ 4.0 0
0
20
40 60 80 (S+O)/Na
100 120
Fig. 3. Dependence of conductivity on (S + O)/Na molar ratio for PTDPA-EO(2)NaClOd electrolytes at different temperatures.
20
40
60
80
100
120
T-Ts0 Fig. 5. WLF plots of conductivity for the NaClO&zctrolytes based on the copoiyesters of thiodipropionic acid: (-) curves calculated employing WLF equation with the G , G and a(Z’,) values reported in Table 3. (S + O)/Na = 22 in all cases.
Ionic conductivity of sulfonated copolyesters Table 3. WLF parameters of conductivity for the electrolytes based on copolycstm thiodimodonic acid. IS + 0)iNa = 22 in all casea
1683 of
Polymer FTDPA-EO(Z)_NaClO, PTDPA-EO(2)-SSNaCIO, PTDPA-EO(Z)-SI ON&l04 WLF
Conductivity at the glass-transition temperature a(T,) was evaluated by a non-linear least square analysis, together with the C, and Cl constants, because cr at a temperature close to T8 is too small to be measured with our apparatus; the T, values employed matched a heating rate of YC/min. The C, and Cz values in Table 3, are very similar to those reported elsewhere for different polyethylene oxidebased electrolytes [23] and are comparable to the “universal values” (C, = 17.4, C2 = 51.6) of the WLF equation. Figure 5 shows that our data fit this equation well.
CONCLUSIONS
The presence of a low amount of -SOsNa groups, covalently attached to the polymer backbone, reflects positively on the electrical properties of the tested Na+-ion conducting polymers based on a polyester of thiodipropionic acid, even if the conductivity values are not high enough for their use without further dissolution of sodium salt. On the other hand, a large content of sodium sulfonate groups increases significantly the viscosity of the copolymer matrix and reduces the electrical performance of the polymeric electrolytes with NaClO,. work has been supported by the Italian Minister0 dell’Universid e della Ricerea Scientifica e Tecnologica (ex 40%). Acknowledgemenr-This
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