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The state of ion aggregation in ionomers based on copolymers of styrene and acrylic acid—I. Thermal studies
Eur. Polym. J. Vol. 26, No. 11, pp. 1203-1206, 1990 Printed in Great Britain
0014-3057/90 $3.00 + 0.00 Pergamon Press plc
THE STATE OF ION AGGREGATION IN IONOMERS BASED ON COPOLYMERS OF STYRENE AND ACRYLIC ACID--I. THERMAL STUDIES KAZIMIERA SUCHOCKA-GAEA,~ Institute of Chemistry, N. Copernicus University, 87-100 Torufi, Poland (Received 29 March 1990)
Abstract--The glass transition temperatures of copolymers of styrene and acrylic acid (AA) and their salts were found for the range of AA from 3.85 to 14.05mol%. It was found that Tg depends on the AA or sodium acrylate (ANa) content but is independent of the nature of the introduced alkali metal. The results suggest that some change in the state of the ion aggregation occurs above ca 6 mol%, in agreement with previous findings.
INTRODUCTION Considerable attention has been given to the study of ion-containing polymers. It has been shown that the incorporation of a relatively small amount of ionic groups into a polymeric matrix alters the supermolecular structure of the materials as compared with the non-ionic counterpart and consequently affects the glass transition temperature (Tg), mechanical properties, melt rheology and other physical properties [1-4]. Eisenberg [1] suggested that the Tg of an ionomer is the temperature at which ion-pairs forming ionic crosslinks dissociate and hence there is a temperature at which the thermal energy approaches the electrostatic binding energy of an ion-pair. Datye and Taylor [5] suggested that an ionic cluster as a whole acts as crosslink and that the glass transition in an ionomer corresponds to the onset of orientational disorders or "melting" in a cluster of dipoles. Eisenberg [1] has reviewed the effect of ions on the Tg of ionomers based mainly on styrene (S) [6], butadiene [7], ethyl acrylate [8] and ethylene [9]. It has been shown [1, 4] that the Tg of an ionomer is considerably higher than that of the polymer matrix before the introduction of ions; although no meaningful correlation has yet been found, the effect seems more pronounced as the Tg of the host material decreases. Some experimental results [10] indicate that the crosslinking by anionic species is a mechanism for raising Tg, whereas other results [9] agree rather well with values predicted on the basis of copolymerization. It is likely that both crosslinking and a copolymerization effect play roles in fixing the Tg of ionomers. It has been also demonstrated the the properties of ionomers depend on the sample history
[ll]. We now report on the glass transition temperatures of copolymers of S and acrylic acid (AA) obtained by copolymerization in bulk and also their alkali metal salts. We examine Tg of these ionomers as a function of the amount of carboxylate groups and the nature of
alkali metal, using differential scanning calorimetry (DSC) studies. EXPERIMENTAL PROCEDURES
The copolymers of S and AA were obtained by copolymerization in bulk [12, 13]. Suitable ionomers were then obtained by neutralization [12-14] and were freeze-dried. All polymers were dried to constant weight at 323 K in vacuum. The number-average (A~/,) and weight-average (/f/w) molecular weights were determined by gel-permeation chromatography. The limiting viscosity numbers (LVN) of the initial copolymers were determined in tetrahydrofuran at 298 + 0.1 K. Values of T8 of the polymers were found with a Perkin-Elmer DSC-2 differential scanning calorimeter, previously calibrated with standards. The samples of ca 8.0 + 0.1 mg were encapsulated in the usual way in readiness for scanning and then dried in vacuum at 403 K for 30 min. The DSC measurements were performed next day in the following sequence. First, a DSC-scan of each sample was recorded during heating (10K/min) from 220K up to temperature 50 K above the Tg. Then the sample was quenched by rapid cooling to 220 K and rescanned at 10K/min. The data for the glass transition were taken from the second scans and were taken by applying the mid-point method. It was established through the duplicate experiments that the reading and baseline errors lead to experimental error in Tg of + 1-2 K. The materials are labelled by a number in parentheses, giving the concentration of AA or appropriate salt comonomer in mol%. For example, S-ANa (3.85) is a copolymer of S and sodium acrylate (ANa) in which 3.85 mol% of the comonomer units correspond to ANa. RESULTS AND DISCUSSION The properties of the initial copolymers of S and AA are given in Table 1. The DSC-results for the copolymers of S and AA and their sodium salts are presented in Table 2. The data indicate that Tg increases with increase of AA content. Because as indicated earlier [15], Tss of the copolymers are independent of the molecular weight, the observed rise of Tg compared with polystyrene (PS) is
1203
1204
KAZIMmP,A SUCHOCKA-GAI~A~
Table I, Characterizationdata for S-AA copolymers AA LVN /f], x 10-a /~'wx 10-3 (mol%) (dl/g) -1.I0 97.4 335.0 3.85 1.12 127.7 305.0 5.16 1.16 206.8 386.0 6.41 1.I8 203.0 423.0 I 1.67 1.18 269.0 571.0 14.05 1.20 273.0 580.0 connected only with the changes of composition. The increase of Tg may arise from two possible contributions, viz. the copolymerization effect and the crosslinking effect. In the first, we used the simple equation [9]. Tg = n I Tg, + n2 Tg2
(1)
where n~ and Tg, represent the mol fraction and Ts of each homopolymer component (TBps = 373 K, T~,~ = 379 K [16]). Figure 1 presents results of calculations. It shows that the rise in Tg for the copolymers is more marked than predicted by the simple equation. Therefore it is not likely that the observed increase in Tg can be accounted for by the copolymer composition effect alone. In the second contribution A A units existing mainly in the dimeric form at room temperature may act as effective crosslinks which restrict the motion of the backbone chain. Trying to explain this relationship and assuming that intermolecular hydrogen bonding is essentially complete, we used the relationship between ATg and the crosslink density p from consideration of the shrinkage due to the formation of a covalent crosslink at high molecular weight and low crosslink density derived by Fox and Loshaek [17]. The crosslink density, p, expressed in moles of crosslinkages per gram of polymer and the increase in glass temperature, ATg, defined as (Tts_,~ - Tus) are presented in Fig. 2. The plot of ATB vs p is linear indicating that the increase in Tg can be attributed mainly to the crosslinking effect by the hydrogen bonding as implied by the agreement of our experimental data with the theory of Fox and Loshaek. The linear relationship between ATg and p was experimentally verified in crosslinked polystyrene [18] and poly(methyl methacrylate) (PMM) [19]. These data are also shown in Fig. 2 for comparison. Otocka and Kwei [16] analysed the effect of hydrogen bonds as crosslinks in ethylene-AA copolymers, using this theory. As shown in Fig. 2, the smaller proportionality constant between ATs and p in our copolymers
compared with those for PS or PMM reflect the fact that shrinkage accompanying the dimerization of carboxyl groups is less than the shrinkage produced by the introduction of a covalent crosslink. As shown in Table 2 the introduction of sodium ions into the copolymer causes increase of Tv In our earlier investigations [14] of sodium ionomers based on copolymers of S and A A obtained in emulsion, we obtained less evident increase of Ts with increase of A N a content. These differences in the rise of Ts may be due to the lower molecular weights of ionomers based on copolymers obtained in bulk or to different histories of the materials. It has been indicated [20] that the introduction of ions into the polymer chains increases Ts more effectively for low molecular weight than for high molecular weight polymers. Trying to explain the relationship between Tg of the investigated ionomers and ion content, we also used the simple copolymer equation (Tgrs = 373 K, TgpANt = 503 K [9]). These data are presented in Fig. 3. As shown, the calculated values of Tg are lower than the observed values, indicating that composition effects alone are insufficient to explain the effect of ions on the glass transition. These results indicate that probably both crosslinking and composition effects affect Tg of ionomers. From Fig. 3, it is also evident that Tgs of S--ANa copolymers show change in the slope when plotted as a function of ionic comonomer content. This result agrees with our earlier results [14] and the initial rise in Tg is linear and may be attributed to ion aggregation into small, tight multiplets. Above c a 6 mol% of ANa, the rate of increase accelerates and it may indicate the onset of clustering. The degree of clustering of the ions, particularly at moderately high ionic concentrations, can also decrease segmental mobility sufficiently for an appreciable effect on Tv We also used the theory of Fox and Losbaek to estimate the effects of sodium ions as crosslinks assuming that all the ions to act as crosslinks. A plot of ATv defined as (T~AN, = T~AA) VSp (Fig. 4) shows that, for very low crosslink density, this relationship gives a straight line, which coincides with that obtained for S--AA copolymers. It suggests that ionic crosslinking and crosslinking by hydrogen bonding cause identical changes of volume probably because small ionic aggregates-multiplets act like intermolecular hydrogen bonds as crosslinks. Above the crosslink density corresponding to the copolymer S - A N a (6.41), this linear relationship fails, perhaps because of the onset of clustering.
Table 2. Glass transition data for copolymers of S and AA and their sodium ionomers Temperature range of the glass transition Ts (K) for sodium ionomers Polymer Copolymer Ionomer (K) PS 373 --S-AA (3.85) 381 389 11 S-AA (5.16) 384 394 15 S-AA (6.41) 388 400 20 S-AA (I 1.67) 395 410 32 S-AA (14.05) 399 441 42
State of ion aggregation in S- and AA-based ionomers
1205
Tg 191K
K
td,0
J
t,O0
t.30 t,20
390
t,10
380 370
tOO ,
I
i
i
,
i
i
D
2 t, 6 8 10 12 1~ % mOt- C00H Fig. 1. Dependence of Ts on the content of AA for copoly-
o
390 380 370
mers of S and AA. /k, Experimental data; &, theoretical data. Previous studies [6, 12, 14] have shown that in S-based ionomers the clusters are formed when the concentration of the ionic component exceeds ca 6 mol%. It is known [21] that clusters can be considered as loose association of multiplets and may act not only as crosslinks but also as strongly interacting fillers. The crosslinking in ionomers is not permanent. Unlike covalent networks, in ionomers the network chains are not fixed to specific junction points. The crosslink density depends on a dynamic equilibrium affected by temperature and stress. Probably this effect may be the cause that, for higher crosslink densities, the linear relationship between ATs and p fails. As can be seen from Fig. 4, AT8 increases further as the crosslink density increases and then it increases significantly for the sample S - A N a (14.05). The distinct increase in AT~ observed for this sample may suggest that clusters are more effective crosslinks. The present results may indicate that, besides composition and crosslinking effects, the degree of clustering of the ions, particularly at moderately high ionic concentrations have an appreciable effect on Tg. Ogura et al. [10], investigating the effect of neutralization of the copolymer of S and methacrylic acid by sodium ions, also used the theory of Fox and Loshaek and obtained linear relationship for ATg vs p. They investigated the copolymer containing 20 mol% of methacrylic acid, which was neutralized with sodium hydroxide up to 90%. The differences in behaviour of sodium ionomers investigated by Ogura et al. [10] may be due to the fact that in these ionomers increase in Tg was due to PMM
t~Tg/,oK / / P S
2
t,
6
8 10 12 % tool COONo
Fig. 3. Dependence of Ts on the content of ANa for copolymers of S and ANa. O, Experimental data; O, theoretical data. two factors, viz. the formation of ionic domains (clusters) which function as crosslinks and to the strengthening of residual intermolecular hydrogen bonding by the introduction of ions. In ionomers based on copolymers of S - A A only ionic interactions appear, leading to formation of ionic aggregates which may act as permanent crosslinks. G~irtner et al. [23], investigating the effect of neutralization of copolymers of S and methacrylic acid to give magnesium salts, indicated that Tg is a linear function of the content of the methacrylic acid and the degree of neutralization. Table 3 gives the glass transition data for ionomers based on copolymers S--AA (5.16) and S--AA (11.67) with different alkali metal ions. The Tss of salts of these copolymers are higher than that for the acid but are practically independent of the nature of the alkali metal. These data also indicate that Tg depends on the content of A A rather than on the detailed properties of the cation. Navratil and Eisenberg [24] investigating TsS of salts of alkali metals of copolymers of S-methacrylic acid also indicated that, for polymers of low methacrylate content, neither the nature of the counterion nor the degree of neutralization had a pronounced effect. Mattera et al. [25] also indicated that, for the sulphonated PS ionomers (PSSA), Tg does not depend strongly on the nature of M ÷ cation since all of them find their lowest available free energy structure to form domains. The data in Table 3 indicate that, for the given copolymers, the temperature range of the glass transition is practically independent of the nature of the ATg, K
,
r
1,o
i
i
b
IL
20
2,o
g(mote/g polymer)"10~ 1,0
Fig. 2. ATs as a function of hydrogen bond crosslink density or as a function of covalent crosslink density p. Data for covalent bonding in PS [18] and PMM [19] are also indicated.
2,0 3~ g (rnO(e/gl:x:)lyrnor )•10j
Fig. 4. AT s as a function of ionic crossiink density p; x , data for hydrogen bonding in copolymers S-AA.
KAZIMIERASUCHOCKA-GAI~.~
1206
Table 3. Glass transition data for ionomers containing 5.16tool% and I 1.67tool% of salt units
Cation H÷ Li+ Na + M+ Cs+
r~io.i¢ (A) -0.60 0.95 1.33 1.69
TB (K) 384 394 394 395 395
5.16 mol% Temperature range of the glass transition (K) -14 15 16 14
alkali metal. In the case of salts of copolymers with higher A A contents S - A A (11.67), this temperature range is broader. The data in Table 2 provide confirmation. These data indicate that, as the content of sodium ions increases, the glass transition in ionomers under study occurs over a broader temperature range as was also found for the Nations [22]. This effect is found because we expect a distribution of ionic aggregates of different sizes, each of which "melts" at its own temperature.
CONCLUSIONS The present studies indicate that Tg of copolymers S - A A increases with increase of A A content. Tgs for sodium ionomers also increase as the A N a content increases and the region of the glass transition is broader. The breadth of the glass transition and the distinct increase in A T s at an ion content o f 14.05 m o l % may indicate that some change in the state of ion aggregation occurs above ca 6 m o l % of A N a . The Tgs of salts of copolymers S--AA are independent of the nature of the alkali metal introduced into the copolymer.
Acknowledgement--We
express our sincere thanks to Professor Zbigniew Wojtczak, N. Copernicus University, Tormi, for helpful discussions.
REFERENCES
1. A. Eisenberg and M. King. Ion-Containing Polymers: Physical Properties and Structure. Academic Press, New York (1977). 2. L. Holiday. Ionic Polymers. Applied Science, London (1975). 3. E. P. Otocka. J. macromolec. Sci.; Rev. macromolec. Chem. C5, 275 (1971).
Ts (K) 395 410 410 408 408
11.67 mol% Temperature range of the glass transition (K) -32 32 29 28
4. W. J. MacKnight and T. R. Earnest Jr. Polymer. Sci.; Macromolec. Rev. 16, 41 (1981). 5. V. K. Datye and P. L. Taylor. Macromolecules 17, 1414 (1984). 6. A. Eisenberg and M. Navratil. Macromolecules 7, 90 (1974). 7. E. P. Otocka and F. R. Eirich.J. Polym. Sci.;Part A -2 6, 921 (1968). 8. H. Mastuura and A. Eisenberg. J. Polym. Sci.;Polym. Phys. Edn 14, 773 (1976). 9. E. P. Otocka and T. K. Kwei. Macromolecules I, 401 (1968). 10. K. Ogura, H. Sobue and S. Nakamura. J. Polym. Sci.; Polym. Phys. Edn 11, 2079 (1973). 11. R. D. Lundberg and R. R. Philips. J. Polym. Sci.; Polym. Lett. Edn 22, 377 (1984). 12. A. Eisenberg and M. Navratil. Macromolecules 6, 604 (1973). 13. K. Suchocka-Gala~ and Z. Wojtczak. Polimery 27, 340 (1982). 14. K. Suchocka-GalaL Eur. Polym. J. 25, 1291 (1989). 15. K. Suchocka-Galas (unpublished work). 16. E. P. Otocka and T. K. Kwei. Macromolecules 1, 144 (1968). 17. T. G. Fox and S. Loshaek. J. Polym. Sci. 15, 371 (1955). 18. K. Ueberreiter and G. Kanig. J. chem. Phys. 18, 399 (1950). 19. S. Loshaek. J. Polym. Sci. 15, 391 (1955). 20. A. Eisenberg. Contemp. Top. Polym. Sci. 3, 231 (1979). 21. M. W. Williams. J. Polym. Sci.; Polym. Syrup. 45, 129 (1974). 22. T. Kyu and A. Eisenberg. ACS Syrup. Ser., 180, 79 (1982). 23. K. Ggrtnr, G. Pohl, H. Schade and K. Schr6der. Plaste Kautsch. 21, 736 (1974). 24. M. Navratil and A. Eisenberg. Macromolecules 7, 84 (1974). 25. V. D. Mattera, Jr., S. L. Peluso, A. T. Tsatsas and W. R. Risen Jr. Spectroscopic and thermal studies of ionic interactions in ionomers. In Coulombic Interactions in Macromolecular Systems, ASC Syrup., Ser. Vol. 320 (edited by A. Eisenberg and F. E. Bailey), p. 54. American Chemical Society, Washington D.C. (1986).
0014-3057/90 $3.00 + 0.00 Pergamon Press plc
THE STATE OF ION AGGREGATION IN IONOMERS BASED ON COPOLYMERS OF STYRENE AND ACRYLIC ACID--I. THERMAL STUDIES KAZIMIERA SUCHOCKA-GAEA,~ Institute of Chemistry, N. Copernicus University, 87-100 Torufi, Poland (Received 29 March 1990)
Abstract--The glass transition temperatures of copolymers of styrene and acrylic acid (AA) and their salts were found for the range of AA from 3.85 to 14.05mol%. It was found that Tg depends on the AA or sodium acrylate (ANa) content but is independent of the nature of the introduced alkali metal. The results suggest that some change in the state of the ion aggregation occurs above ca 6 mol%, in agreement with previous findings.
INTRODUCTION Considerable attention has been given to the study of ion-containing polymers. It has been shown that the incorporation of a relatively small amount of ionic groups into a polymeric matrix alters the supermolecular structure of the materials as compared with the non-ionic counterpart and consequently affects the glass transition temperature (Tg), mechanical properties, melt rheology and other physical properties [1-4]. Eisenberg [1] suggested that the Tg of an ionomer is the temperature at which ion-pairs forming ionic crosslinks dissociate and hence there is a temperature at which the thermal energy approaches the electrostatic binding energy of an ion-pair. Datye and Taylor [5] suggested that an ionic cluster as a whole acts as crosslink and that the glass transition in an ionomer corresponds to the onset of orientational disorders or "melting" in a cluster of dipoles. Eisenberg [1] has reviewed the effect of ions on the Tg of ionomers based mainly on styrene (S) [6], butadiene [7], ethyl acrylate [8] and ethylene [9]. It has been shown [1, 4] that the Tg of an ionomer is considerably higher than that of the polymer matrix before the introduction of ions; although no meaningful correlation has yet been found, the effect seems more pronounced as the Tg of the host material decreases. Some experimental results [10] indicate that the crosslinking by anionic species is a mechanism for raising Tg, whereas other results [9] agree rather well with values predicted on the basis of copolymerization. It is likely that both crosslinking and a copolymerization effect play roles in fixing the Tg of ionomers. It has been also demonstrated the the properties of ionomers depend on the sample history
[ll]. We now report on the glass transition temperatures of copolymers of S and acrylic acid (AA) obtained by copolymerization in bulk and also their alkali metal salts. We examine Tg of these ionomers as a function of the amount of carboxylate groups and the nature of
alkali metal, using differential scanning calorimetry (DSC) studies. EXPERIMENTAL PROCEDURES
The copolymers of S and AA were obtained by copolymerization in bulk [12, 13]. Suitable ionomers were then obtained by neutralization [12-14] and were freeze-dried. All polymers were dried to constant weight at 323 K in vacuum. The number-average (A~/,) and weight-average (/f/w) molecular weights were determined by gel-permeation chromatography. The limiting viscosity numbers (LVN) of the initial copolymers were determined in tetrahydrofuran at 298 + 0.1 K. Values of T8 of the polymers were found with a Perkin-Elmer DSC-2 differential scanning calorimeter, previously calibrated with standards. The samples of ca 8.0 + 0.1 mg were encapsulated in the usual way in readiness for scanning and then dried in vacuum at 403 K for 30 min. The DSC measurements were performed next day in the following sequence. First, a DSC-scan of each sample was recorded during heating (10K/min) from 220K up to temperature 50 K above the Tg. Then the sample was quenched by rapid cooling to 220 K and rescanned at 10K/min. The data for the glass transition were taken from the second scans and were taken by applying the mid-point method. It was established through the duplicate experiments that the reading and baseline errors lead to experimental error in Tg of + 1-2 K. The materials are labelled by a number in parentheses, giving the concentration of AA or appropriate salt comonomer in mol%. For example, S-ANa (3.85) is a copolymer of S and sodium acrylate (ANa) in which 3.85 mol% of the comonomer units correspond to ANa. RESULTS AND DISCUSSION The properties of the initial copolymers of S and AA are given in Table 1. The DSC-results for the copolymers of S and AA and their sodium salts are presented in Table 2. The data indicate that Tg increases with increase of AA content. Because as indicated earlier [15], Tss of the copolymers are independent of the molecular weight, the observed rise of Tg compared with polystyrene (PS) is
1203
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KAZIMmP,A SUCHOCKA-GAI~A~
Table I, Characterizationdata for S-AA copolymers AA LVN /f], x 10-a /~'wx 10-3 (mol%) (dl/g) -1.I0 97.4 335.0 3.85 1.12 127.7 305.0 5.16 1.16 206.8 386.0 6.41 1.I8 203.0 423.0 I 1.67 1.18 269.0 571.0 14.05 1.20 273.0 580.0 connected only with the changes of composition. The increase of Tg may arise from two possible contributions, viz. the copolymerization effect and the crosslinking effect. In the first, we used the simple equation [9]. Tg = n I Tg, + n2 Tg2
(1)
where n~ and Tg, represent the mol fraction and Ts of each homopolymer component (TBps = 373 K, T~,~ = 379 K [16]). Figure 1 presents results of calculations. It shows that the rise in Tg for the copolymers is more marked than predicted by the simple equation. Therefore it is not likely that the observed increase in Tg can be accounted for by the copolymer composition effect alone. In the second contribution A A units existing mainly in the dimeric form at room temperature may act as effective crosslinks which restrict the motion of the backbone chain. Trying to explain this relationship and assuming that intermolecular hydrogen bonding is essentially complete, we used the relationship between ATg and the crosslink density p from consideration of the shrinkage due to the formation of a covalent crosslink at high molecular weight and low crosslink density derived by Fox and Loshaek [17]. The crosslink density, p, expressed in moles of crosslinkages per gram of polymer and the increase in glass temperature, ATg, defined as (Tts_,~ - Tus) are presented in Fig. 2. The plot of ATB vs p is linear indicating that the increase in Tg can be attributed mainly to the crosslinking effect by the hydrogen bonding as implied by the agreement of our experimental data with the theory of Fox and Loshaek. The linear relationship between ATg and p was experimentally verified in crosslinked polystyrene [18] and poly(methyl methacrylate) (PMM) [19]. These data are also shown in Fig. 2 for comparison. Otocka and Kwei [16] analysed the effect of hydrogen bonds as crosslinks in ethylene-AA copolymers, using this theory. As shown in Fig. 2, the smaller proportionality constant between ATs and p in our copolymers
compared with those for PS or PMM reflect the fact that shrinkage accompanying the dimerization of carboxyl groups is less than the shrinkage produced by the introduction of a covalent crosslink. As shown in Table 2 the introduction of sodium ions into the copolymer causes increase of Tv In our earlier investigations [14] of sodium ionomers based on copolymers of S and A A obtained in emulsion, we obtained less evident increase of Ts with increase of A N a content. These differences in the rise of Ts may be due to the lower molecular weights of ionomers based on copolymers obtained in bulk or to different histories of the materials. It has been indicated [20] that the introduction of ions into the polymer chains increases Ts more effectively for low molecular weight than for high molecular weight polymers. Trying to explain the relationship between Tg of the investigated ionomers and ion content, we also used the simple copolymer equation (Tgrs = 373 K, TgpANt = 503 K [9]). These data are presented in Fig. 3. As shown, the calculated values of Tg are lower than the observed values, indicating that composition effects alone are insufficient to explain the effect of ions on the glass transition. These results indicate that probably both crosslinking and composition effects affect Tg of ionomers. From Fig. 3, it is also evident that Tgs of S--ANa copolymers show change in the slope when plotted as a function of ionic comonomer content. This result agrees with our earlier results [14] and the initial rise in Tg is linear and may be attributed to ion aggregation into small, tight multiplets. Above c a 6 mol% of ANa, the rate of increase accelerates and it may indicate the onset of clustering. The degree of clustering of the ions, particularly at moderately high ionic concentrations, can also decrease segmental mobility sufficiently for an appreciable effect on Tv We also used the theory of Fox and Losbaek to estimate the effects of sodium ions as crosslinks assuming that all the ions to act as crosslinks. A plot of ATv defined as (T~AN, = T~AA) VSp (Fig. 4) shows that, for very low crosslink density, this relationship gives a straight line, which coincides with that obtained for S--AA copolymers. It suggests that ionic crosslinking and crosslinking by hydrogen bonding cause identical changes of volume probably because small ionic aggregates-multiplets act like intermolecular hydrogen bonds as crosslinks. Above the crosslink density corresponding to the copolymer S - A N a (6.41), this linear relationship fails, perhaps because of the onset of clustering.
Table 2. Glass transition data for copolymers of S and AA and their sodium ionomers Temperature range of the glass transition Ts (K) for sodium ionomers Polymer Copolymer Ionomer (K) PS 373 --S-AA (3.85) 381 389 11 S-AA (5.16) 384 394 15 S-AA (6.41) 388 400 20 S-AA (I 1.67) 395 410 32 S-AA (14.05) 399 441 42
State of ion aggregation in S- and AA-based ionomers
1205
Tg 191K
K
td,0
J
t,O0
t.30 t,20
390
t,10
380 370
tOO ,
I
i
i
,
i
i
D
2 t, 6 8 10 12 1~ % mOt- C00H Fig. 1. Dependence of Ts on the content of AA for copoly-
o
390 380 370
mers of S and AA. /k, Experimental data; &, theoretical data. Previous studies [6, 12, 14] have shown that in S-based ionomers the clusters are formed when the concentration of the ionic component exceeds ca 6 mol%. It is known [21] that clusters can be considered as loose association of multiplets and may act not only as crosslinks but also as strongly interacting fillers. The crosslinking in ionomers is not permanent. Unlike covalent networks, in ionomers the network chains are not fixed to specific junction points. The crosslink density depends on a dynamic equilibrium affected by temperature and stress. Probably this effect may be the cause that, for higher crosslink densities, the linear relationship between ATs and p fails. As can be seen from Fig. 4, AT8 increases further as the crosslink density increases and then it increases significantly for the sample S - A N a (14.05). The distinct increase in AT~ observed for this sample may suggest that clusters are more effective crosslinks. The present results may indicate that, besides composition and crosslinking effects, the degree of clustering of the ions, particularly at moderately high ionic concentrations have an appreciable effect on Tg. Ogura et al. [10], investigating the effect of neutralization of the copolymer of S and methacrylic acid by sodium ions, also used the theory of Fox and Loshaek and obtained linear relationship for ATg vs p. They investigated the copolymer containing 20 mol% of methacrylic acid, which was neutralized with sodium hydroxide up to 90%. The differences in behaviour of sodium ionomers investigated by Ogura et al. [10] may be due to the fact that in these ionomers increase in Tg was due to PMM
t~Tg/,oK / / P S
2
t,
6
8 10 12 % tool COONo
Fig. 3. Dependence of Ts on the content of ANa for copolymers of S and ANa. O, Experimental data; O, theoretical data. two factors, viz. the formation of ionic domains (clusters) which function as crosslinks and to the strengthening of residual intermolecular hydrogen bonding by the introduction of ions. In ionomers based on copolymers of S - A A only ionic interactions appear, leading to formation of ionic aggregates which may act as permanent crosslinks. G~irtner et al. [23], investigating the effect of neutralization of copolymers of S and methacrylic acid to give magnesium salts, indicated that Tg is a linear function of the content of the methacrylic acid and the degree of neutralization. Table 3 gives the glass transition data for ionomers based on copolymers S--AA (5.16) and S--AA (11.67) with different alkali metal ions. The Tss of salts of these copolymers are higher than that for the acid but are practically independent of the nature of the alkali metal. These data also indicate that Tg depends on the content of A A rather than on the detailed properties of the cation. Navratil and Eisenberg [24] investigating TsS of salts of alkali metals of copolymers of S-methacrylic acid also indicated that, for polymers of low methacrylate content, neither the nature of the counterion nor the degree of neutralization had a pronounced effect. Mattera et al. [25] also indicated that, for the sulphonated PS ionomers (PSSA), Tg does not depend strongly on the nature of M ÷ cation since all of them find their lowest available free energy structure to form domains. The data in Table 3 indicate that, for the given copolymers, the temperature range of the glass transition is practically independent of the nature of the ATg, K
,
r
1,o
i
i
b
IL
20
2,o
g(mote/g polymer)"10~ 1,0
Fig. 2. ATs as a function of hydrogen bond crosslink density or as a function of covalent crosslink density p. Data for covalent bonding in PS [18] and PMM [19] are also indicated.
2,0 3~ g (rnO(e/gl:x:)lyrnor )•10j
Fig. 4. AT s as a function of ionic crossiink density p; x , data for hydrogen bonding in copolymers S-AA.
KAZIMIERASUCHOCKA-GAI~.~
1206
Table 3. Glass transition data for ionomers containing 5.16tool% and I 1.67tool% of salt units
Cation H÷ Li+ Na + M+ Cs+
r~io.i¢ (A) -0.60 0.95 1.33 1.69
TB (K) 384 394 394 395 395
5.16 mol% Temperature range of the glass transition (K) -14 15 16 14
alkali metal. In the case of salts of copolymers with higher A A contents S - A A (11.67), this temperature range is broader. The data in Table 2 provide confirmation. These data indicate that, as the content of sodium ions increases, the glass transition in ionomers under study occurs over a broader temperature range as was also found for the Nations [22]. This effect is found because we expect a distribution of ionic aggregates of different sizes, each of which "melts" at its own temperature.
CONCLUSIONS The present studies indicate that Tg of copolymers S - A A increases with increase of A A content. Tgs for sodium ionomers also increase as the A N a content increases and the region of the glass transition is broader. The breadth of the glass transition and the distinct increase in A T s at an ion content o f 14.05 m o l % may indicate that some change in the state of ion aggregation occurs above ca 6 m o l % of A N a . The Tgs of salts of copolymers S--AA are independent of the nature of the alkali metal introduced into the copolymer.
Acknowledgement--We
express our sincere thanks to Professor Zbigniew Wojtczak, N. Copernicus University, Tormi, for helpful discussions.
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11.67 mol% Temperature range of the glass transition (K) -32 32 29 28
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