preparation and thermal characterization of the glass transition temperatures of sulfonated...

Preparation and Thermal Characterization of the Glass Transition Temperatures of

Sulfonated Polystyrene-Metal Ionomers

SEN YANG, KANG SUN and WILLIAM M. RISEN, JR., Department of Chemistry, Brown University, Providence, Rhode Island 02912

Synopsis

The glass transition temperatures and heat capacity changes in the transition region are reported for six sulfonated linear polystyrenes in the hydrogen form, H-SPS, in the 3.4-20.1 mol % sulfonation range and 76 metal SPS ionomers in the 3.4-12.8 mol % range. The metals are those which interact predominantly ionically and include +1, +2, and +3 ions of the alkali metal, alkaline earth, and rare earth (lanthanide) series. The results show the effect of HzO or coordinating ligands on glass transition temperatures ( T,) and the importance of eliminating it to obtaining reproducible values for T, and AC,. The TB values of dry M-SPS ionomers depend only on the sulfonation level despite wide variation in metal ion charge and size. The variation of AC, with sulfonation level is interpreted as showing that a t high levels a few unsulfonated styrene units adjacent to sulfonated ones are constrained, presumably by clustering, from participation in the polystyrene-like cooperative rearrangements in the transition region.

INTRODUCTION

The physical properties of sulfonated ionomers, especially their structural, thermal, and viscoelastic properties, depend strongly on their ion content, solvation, thermal history, and on the nature of the ionic interactions in them. These observations and interpretations have been reviewed recently by Fitz- gerald and Weiss.'

The effects of these ionic interactions are of particular interest. Ion association due to ion-pair formation, at least, is expected to occur between the metal cations and the sulfonate sites in an ionomer, such as sulfonated polystyrene. As the sulfonate concentration increases, various types of higher order structural arrangements of the ionic units, including domains ranging from multiplets to higher order clusters, can be formed. The assembly of these ionic units can be influenced by solvation, especially by polar solvents, as shown by Lundberg et al.273 and Weiss, Fitzgerald, and their It also can be controlled by specific coordination of metal counterions by the sites,' by other coordinating ligands,' by thermal history, lo or by a combination of these effects.

Since the nature of ionic association in ionomers depends on so many factors, which can vary from sample to sample, it has been difficult to tell what role each plays. This is particularly true in the case of the glass transition. While it is believed lo that the glass transition in sulfonated polystyrenes, for example, is due to some sort of conformational rearrangements in the polymer chain, it also has been argued that the mobility of the chains is influenced by the char- acteristics of their directly neighboring ionic groupings and by the strength of

Journal of Polymer Science: Part B: Polymer Physics, Vol. 28, 1685-1697 (1990) 0 1990 John Wiley & Sons, Inc. CCC 0887-6266/90/01001685-013$04.00

1686 YANG E T AL.

the electrostatic metal-site interactions." This is consistent with the conclusion that solvation or polar solvent-induced plasticization causes a weakening or dissociation of these ionic interactions and thus a lowering of glass transition temperature ( T,) . In addition, it has been reported that Tg is affected by changes in ionomer morphology due to thermal effects on the type and distribution of ionic arrangements." The assumption of these relationships between both the strengths of the metal-site interactions and the ionic domain organization and the chain mobility would seem to argue that varying the nature of the cation would cause a concomitant variation in Tg.

By investigating the glass transition as a function of cation, over a wide range of charge and size under conditions in which the solvent is removed, we hope to elucidate the effect of the cation and solvent on the T,s of sulfonated polystyrenes measured by differential scanning calorimetry (DSC) . To do this, we have prepared and examined the thermal properties of three extensive series of sulfonated polystyrene ( SPS ) ionomers, containing alkali metal, alkaline earth, and rare earth (lanthanide) metal ions. Thus, we have incorporated nearly all of the metal ions of charges 1, 2, and 3 which are known to interact ionically with their surroundings in the absence of strongly coordinating ligands or such special inclusion compounds as crown ethers. We also have studied two cases involving strongly coordinated lanthanide metal ions in the ionomers.

We previously reported thermal analysis results on nearly dry alkali and alkaline-earth metal polystyrene sulfonic acid (M-SPS) ionomers, l3 and showed that the values of T, measured for ionomers with the same ionic content (mol % sulfonation) but different alkali metal ions are nearly the same. However, we found that the Tgs for the alkaline earth ionomers were a few degrees lower than those of the alkali ionomers, and that T,s for all of these ion-containing SPS ionomers at the highest ionic group concentration (12.8 mol % ) only approached 130°C. These puzzling dependences on the nature of the cations and degree of sulfonation also led us to examine this system in greater depth.

PROCEDURE

Sample Preparation

Acid-form sulfonated polystyrene (H-SPS) samples used in this study were kindly provided by Dr. R. D. Lundberg of Exxon Chemical Company, who has described their synthesis by sulfonation of p01ystyrene.l~ The degrees of sulfonation of SPS samples employed in this study for metal ion exchange were 3.4, 5.6, 6.9, and 12.8 mol %, while the 15.8 and 20.1 mol % H-SPS materials also were studied.

Metal-SPS ionomers were prepared by ion exchanging the H-SPS in THF solution. The solutions were titrated with a 0.1 M aqueous solution of the nitrate of the cation to be exchanged until a 20% excess had been added to ensure complete ion exchange. The solutions were stirred at 25°C for 24 h. After evaporating part of the solvent, the viscous solutions/gels were repeatedly washed, with vigorous stirring, in deionized water. The remainder of the solvent was evaporated; and the products were washed again with deionized water to extract residual solvent and any adsorbed metal salt, and were then dried in air at 60°C. The M-SPS ionomers at 3.4, 5.6, and 6.9 mol % were redissolved

PREPARATION AND THERMAL CHARACTERIZATION 1687

in dry THF, and 12.8 mol % samples were dissolved in 10% H@/90% THF solution. Film samples of the M-SPS ionomers listed in Table I were obtained by evaporation of solvent and studied both after air-drying at 25°C and after drying at 60°C for 24 h. The samples are labeled SPS ( a ) M ( b ) , where a is the mole percent sulfonation and M is the cation with a formal oxidation state of b. Some sample films of Eu3+ and Er3+ SPS at 5.6 and 6.9 mol % sulfonation were treated with an ethylenediamine/ethanol solution (50/50, v/v) to form Eu (111) and Er (111) ethylenediamine complexes in the polymer. These films were dried at 70°C.

Ion exchange was studied by infrared spectroscopy on all samples and by EDAX and electron microprobe analysis of selected samples. The infrared study was done on a Digilab FTS-15B and an IBM-Bruker FTIR 98. The atomic ratios of metal to sulfur were determined with a Cameca Microprobe using a 15 keV (5-10 nA) beam at 40" takeoff angle, and the EDAX results were obtained with an EDAX-Tracor Northern TN2000 system on a Amray lOOA SEM.

The DSC and thermogravimetric analysis (TGA) measurements were made using Du Pont 910 DSC and 950 TGA thermal analyzers, calibrated with ap- propriate standards. The reading and baseline errors from replicate DSC ex- periments led to a typical reportable accuracy in Tg of about 2"C, and in AC, of about 0.005 J / g K. Measurements of heat flow versus temperature were made on heating in the range of 20-200°C [ 20-250°C for SPS ( 12.8) M samples] at a heating rate of 20 K/min. The sample chamber was purged with dry N2. Filmlike samples (ca. 5-15 mg) were encapsulated in aluminum DSC cells. In

TABLE I Glass Transition Temperatures of M-SPS Ionomers

Cation 3.4 mol % 5.6 mol % 6.9 mol % 12.8 mol %

Na+ K+ Rb+ CS+

T, (M+) Ca2+ Mg2+ Sr2+ T, (M") La3+ Ce3+ Pr3+ Nd3' Sm3+ Eu3+ Gd3+ Tb3+ Dy3+ H O ~ +

Er3+ Yb3+ Tz (M3')

108 107 107 108 108 107 107 107 107 109 109 110 110 109 109 108 110 110 109 109 109 109

116 117 117 117 117 117 116 117 117 116 116 115 117 117 115 116 116 117 118 116 117 116

119 118 118 120 119 120 118 119 119 121 120 121 121 122 122 121 121 121 122 122 122 121

145 146 148 148 147 144 145 146 145 147 146 146 148 148 148 144 146 148 148 148 148 147

1688 YANG ET AL.

some DSC measurements, a pinhole was punched in the lid of the cell so that water, solvent, or other volatile material could escape during heating. To avoid variation of annealing effects, after the first run the samples were held for 5 minutes at the upper temperature limit and then quickly quenched to 25°C. The DSC measurement of each sample was made from 6 to 15 times until a sequentially invariant transition temperature was obtained. The midpoint method or its equivalent, identification of the maximum in the derivative of heat flow versus temperature curve, was used to obtain Tg data from the measured DSC curves. The difference in heat capacity AC, between the initial and the final state was determined from the difference at Tg between the heat-flow baselines above and below Tg.

TGA measurements were done on samples dried at 60°C in order to dem- onstrate weight loss in the region of 30-200°C at 20 K/min. They showed that all the materials prepared in this manner contained some HzO, as confirmed by the infrared absorption at 3400 cm-' . The amount of water present in the starting samples varied with the cation, fell in the range of 0.3-5%, and was lost primarily in the 50-150°C range.

RESULTS

The samples, prepared as described above, were studied by DSC to measure glass transitions. All were characterized by infrared spectroscopy by the method reported previously l3 to confirm ion exchange. Representative samplings of them were analyzed by EDAX and electron microprobe, to measure the metal ion contents, and by TGA, to measure the amounts of HzO present in air-dried samples.

The elemental analyses by EDAX and electron microprobe methods of ten representative samples were performed to provide independent varification of the method for introducing the metal ions. All show substantial incorporation of the metal ions, confirming the infrared results. For the lanthanide samples that were probed (SPS(6.9)La(III), SPS(6.9)Nd(III), and SPS(3.4)Eu(III), for example, the metal-to-sulfur ratios averaged 0.30 compared with the value 0.33 which would be expected if the Ln (111) ions acted as simple 3+ cations. Of the alkaline earths, Ca apparently enters SPS mainly as the 2+ cation (Ca/ S ratio 0.52 compared with 0.50 theoretical for Ca"), while Mg2+ enters as MgOH', or its equivalent, (Mg/S ratio 1.1 compared to 0.5 theoretical for Mg2+), and the alkali metals enter as 1+ ions, as expected.

The exchange of the M"+ ions for H + in SPS causes strong infrared bands to appear in the 1000-1300 cm-' region. The spectra of all 76 of the M-SPS samples reported have these features, demonstrating exchange in terms of the vibrations of the sulfonate sites. They are associated with SO3-based vibrations of sulfonated styrene units and vary within those ranges with the metal ion charge and size and with degree of h y d r a t i ~ n . ' ~ ~ ' ~

The infrared spectra in the 950-1350 cm-' region of dried SPS (6.9) materials in the H, Na, Ca, and Eu (111) forms in Figure 1 illustrate these observations. It can be seen clearly that when the H + is replaced by a metal cation new bands appear at 1011, ca. 1045, and 1130 cm-', where the H+ form does not absorb, as noted by lines on Figure 1. The 1100 cm-' band of the dry SPS (6.9) H form,


1300 1200 1100 1000

Wavenumber (cm-')

Fig. 1. Infrared spectra of dried SPS (6.9) ionomers in the H, Na, Ca, and Eu(II1) forms in the 950-1350 cm-' region, off set arbitrarily for display. The lines of constant wavenumber have been added only to aid in viewing the spectra.

which is associated with the unhydrated S03H mode,6 is absent from the spectra of the cation-exchanged materials. Two other characteristic features of the spectra of sulfonated polystyrenes arise from the splitting of the doubly degen- erate (E) asymmetric stretching mode of unperturbed SO, in C3". These appear (marked with asterisks) a t 1194 and 1171 cm-' in the H + form, a t 1224 and 1199 cm-' for the Na' form, 1224 and 1202 cm-I for the Ca2+ form, and at 1273 and 1231 cm-' for the Eu(I11) form. The spectra in this region have superimposed a weak band at 1178 cm-' due to an internal mode.

The samples dried in air at 60°C also contain some H20, as shown primarily by a broad H20 band at ca. 3400 cm-' but in some cases also by a sharp band in the 3635-3650 cm-' range due to H 2 0 molecules coordinated to the metal ion. These H20-related features are absent from the spectra of materials which have been heated at 150°C in vacuum for several hours. In the higher frequency region of the spectra of the materials whose partial spectra are shown in Figure 1, there is no absorption band near 3400 cm-' and an extremely weak absorption near 3650 cm-'.

The first DSC scan of each sample in a sealed pan typically gave a broad endothermic peak from 50-150" C, where TGA measurements showed that most of the weight loss occurs, but showed no sign of a glass transition. The endothermic event on the first run is attributed to desorption of water and possibly

1690 YANG E T AL.

other solvent molecules. The AC, and Tg values for each sample were evaluated from the final scans. The apparent Tg increased from run to run (on subsequent scans of increasing scan number) until it reached a constant final value. This is shown in Figure 2 for several types of materials. In this figure, the measured Tg for examples of ionomers containing three valence types (M', M2+, M3+) of cation with 6.9 mol % sulfonation are plotted against the scan number. As can be seen, with a conventionally sealed pan and normally air-dried (60°C) ionomer films a reproducible value of Tg can be obtained only after 4-8 runs. On the other hand, the same ultimate Tg values are obtained on the second and all subsequent runs if a small pinhole is poked in the lid of the sealed cell.

The glass transition temperatures measured on all M-SPS ionomers prepared with alkali, alkaline earth, and lanthanide cations are summarized in Table I. Since the Tgs within each composition type (degree of sulfonation and cation class) are quite similar, the averages also are listed. In fact, the Tgs for all ionomers of a given degree of sulfonation are close enough that the overall averages appear to be meaningful. On the average, Tg values are 108°C for SPS(3.4)M, 117°C for SPS(5.6)M, 120°C for SPS(6.9)M7 and 147°C for SPS ( 12.8) M with all of these different cations. The values of the heat capacity changes, AC, ( J /gK) , for the alkali metal (M"), alkaline earth (M2'), and lanthanide ( M3+) ionic materials are listed in Table 11. The values of Tg and AC, for H-SPS materials in the 3.4-20.1 mol % sulfonation range are given in Table 111.

In order to see the differences in properties of these ionomers between those with small, highly charged cations and analogous materials in which the cations are strongly complexed with surrounding ligands and separated from other ionic sites, the thermal properties of two lanthanide-SPS ionomers complexed with ethylenediamine (en) were examined. The first two DSC runs on the

T!3

1004 2 4 6 8 10 12

DSC R u n Number- Fig. 2. Apparent T, values measured from sequential DSC runs on representative 1+ (Na')

(triangles), 2+(Ca2+) (circles), and3+(Gd3+) (diamonds) SPS (6.9)-M samples in sealedsample cells versus the run number. The lines are drawn to assist the reader only.


TABLE I1 Heat Capacity Changes in the Transition Region for SPS Ionomer with MI+, Mz+, M3+ Cations

M-SPS

Metal mol % Charge 3.4 mol % 5.6 mol % 6.9 mol % 12.8 mol %

Na K Rb c s AVG. Mg Ca Sr AVG. La Ce P r Nd Sm Eu Gd T b DY Ho Er Yb AVG.

1 1 1 1

2 2 2

-

3 3 3 3 3 3 3 3 3 3 3 3 -

0.29 0.24 0.26 0.22 0.25 0.22 0.26 0.22 0.23 0.29 0.28 0.29 0.30 0.30 0.26 0.28 0.28 0.29 0.27 0.29 0.28 0.28

0.24 0.28 0.27 0.24 0.26 0.25 0.25 0.26 0.25 0.25 0.24 0.24 0.23 0.26 0.27 0.23 0.22 0.25 0.23 0.25 0.23 0.24

0.21 0.22 0.20 0.19 0.21 0.24 0.25 0.24 0.24 0.21 0.23 0.23 0.22 0.22 0.24 0.26 0.21 0.24 0.21 0.25 0.26 0.23

0.08 0.11 0.10 0.08 0.09 0.07 0.11 0.11 0.10 0.11 0.11 0.09 0.11 0.09 0.12 0.12 0.11 0.10 0.10 0.10 0.10 0.11

The values of AC, were obtained experimentally to a statistical accuracy of k 0.005 J g-' K-' on the basis of the three or more measurements on each sample and these averages are reported.

ethylenediamine-complexed lanthanide SPS ionomers in each sequence were stopped at 130°C. These runs removed essentially all of the water as shown by TGA. After H 2 0 loss, the third run was taken to 250"C, then runs 4 through 8 were measured. A typical set of results is shown in Figure 3 for SPS (5.6) Eu (111) -en. The first run showed the water loss endotherm but no glass transition, while the second and the third runs showed an apparent glass transition of 102°C for SPS(5.6)Eu(III)-en. For the third run, there was a

TABLE 111 Glass Transition Temperatures and Heat Capacity Changes

in the Transition Region for H-SPS Materials

Sulfonation (mol %) Tg ("0 AC, (J g-' K-')

3.4 5.6 6.9

12.8 15.8 20.1

107 114 116 138 150 158

0.34 0.32 0.31 0.29 0.27 0.24

1692 YANG E T AL.

40 60 00 100 120 1 4 0 160 160 .'ti0 Z J U 240

Tempera tu re ("C)

Fig. 3. DSC curves of heat flow measured in W/g at 20 K/min, for SPS (5.6) Eu(II1)-en measured in the sequence 1 through 8 and offset for presentation.

broad endothermic peak around 166°C. This was due to loss of ethylenediamine. Later runs did not show that peak, and the glass transition was shifted to 115"C, which is identical to that of the pure SPS(5.6)Eu(III) ionomer. The infrared spectra of the SPS (5.6) Eu ( 111) en confirmed the complexation of Eu (111) before heating and the absence of ethylenediamine after heating in vacuum at 200°C.

DISCUSSION

Overall, the Tg values of the H-SPS and M-SPS ionomers in the ca. 3-20 mol % range are higher than the Tg of polystyrene and increase with the sulfonate content. For the acid-form materials, H-SPS, the increase is linear, as shown in Figure 4, which contains the present DSC data (3-20 mol ?& ) and those in the lower sulfonation range (ca. 1-9 mol % ) measured by Wallace using thermal expansion methods.16 They are similar, but the Tgs values reported by Wallace are a few degrees higher. This may reflect differences in measurement method. The fact that Tg increases with sulfonation in certain ranges is well known, and has been taken as evidence that the ionic interactions play a role in limiting the mobility of the system, but it does not define the role.

The observed linearity of Tg and sulfonation in the case of the H-SPS materials is interesting to consider in light of other studies. Several workers have concluded from dynamic mechanical analysis that these materials are microphase-separated at concentrations as low as 5.8 mol %.12*17 However, the elegant attempts to determine whether and at what sulfonate concentration clustering occurs in H-SPS ionomers from SAXS measurements have not yet resulted in conclusive interpretati~ns.'~*'~~~~ The results presented here indicate that what- ever interactions on the molecular scale are responsible for this microphase


180

160

1 4 0

100

80 0 5 10 15 20 25

S u 1 f onat i on (rno 1 e%) Fig. 4. The glass transition temperatures of H-SPS ionomers vs. mol % sulfonation

reported here are filled circles, data from Ref. 16 open triangles. data

separation, whether clustering or not, they do not influence the glass transition differently a t different degrees of sulfonation up to 20.1 mol %.

The Tg values of the various metal-containing ionomers, M-SPS, reported here are plotted in Figure 5. Averages by ion type are plotted, since the values for a given ion type and degree of sulfonation fall in a very narrow range and there are far too many points with similar values to be represented individually

5 10 15

(rno 1 e%> Su 1 f onat i on

Fig. 5. Plot of average glass transition temperatures vs. mol % sulfonation for the sets of alkali metal (triangles) alkaline earth (pentagons), and lanthanide (squares) SPS-M ionomers.

1694 YANG E T AL.

on the graph. At each degree of sulfonation they occur a few degrees higher than that of the H-SPS form. As seen in Figure 5 the relationship between the averaged value of Tg for dry M-SPS ionomers and the degree of sulfonation is essentially linear. Clearly the Tgs of the M-SPS ionomers also depend mainly on the concentration of pendant sulfonate group rather than on either the charge or size of the cation. In fact no strong relationship between Tg and the charge/radius ratio ( q / a ) or other cation properties has been found for dry M-SPS ionomers. This is quite a surprising result, because the ionic interactions clearly have a different effect when metal ions are present than when H + ion is present. The linearity also is surprising since some differences in distribution of ion pairs, clusters and other structures are expected as the nature of the ion and the concentration of sulfonated styrenes are varied. Moreover, the thermal history is postulated to affect M-SPS ionomers differently."

For SPS ionomers, there are several temperatures at which relaxation oc- c u r ~ . ~ ~ A t significantly higher temperatures than the values of Tg reported here the ionic assemblages reportedly can rearrange as the thermal energy approaches the electrostatic binding energy of an ion The question is whether the lower-temperature events reported here are related to ionic association, since the transition temperatures do vary strongly with the mole fraction of ionic groups and somewhat on changing from H + to a metal cation, but they do not vary with nature of the metal cation.

To address this question, it is useful to consider the fact that the magnitudes of the enthalpy change at the glass transition and the incremental change in specific heat, AC,, reported here do vary strongly with sulfonation.

We measured the heat capacity change, AC, (J g-'K-'), over the glass transition region for these H-SPS and M-SPS materials. The values for the M'+( alkali metal), M2+( alkaline earth), and M3+( lanthanide) ions, which interact ionically with sulfonate sites, are presented in Table 11, while those for the H-SPS materials are given in Table I11 along with their transition temperatures. To aid in reviewing the numbers for all of these M-SPS materials the averages are listed for each type (mol % sulfonation and ion charge) in Table IV.

Remarkably consistent results are obtained for all of these series of M-SPS ionomers. Each set of ionomers with a given type of metal and charge and a given level of sulfonation in the 3.4-6.9 mol % sulfonation range has an average AC, value of 0.21 to 0.28 J g-lK-'. All sets of ionomers with 12.8 mol % sulfonation average 0.11 J g-'K-'. If it is assumed that the transition is primarily that of the polystyrene chain between sulfonated units and then the AC, value for each ionomer is divided by the fraction of the mass contributed by the nonsulfonated styrene units, mf, the normalized values of AC,, labeled AC,/ mf , are obtained. They also fall quite close to each other, and their averages by cation type and level of sulfonation are given in Table IV. With the masses of the metal and the sulfonated styrene units factored out in this way, the similarity is even more evident, since AC,/mf is 0.28 * 0.03 J k-'g-' (styrene) for all of those M-SPS ionomers in the 3.4-6.9 mol % sulfonation range. This is essentially the value of AC, for polystyrene itself, which we measure to be 0.29 J g-lK-' for the Styron-66 from which the samples were prepared.

The similarly calculated A c P / m f values for the ionomers with higher degree of sulfonation (12.8 mol % ) are about 0.15 J g-'K-' (styrene). Since this is


TABLE IV Average and Mass Fraction Normalized Average Heat Capacity Changes in the Transition

Region for M-SPS Ionomers with M", M2+, M3+ Cations

M-SPS

Cation type 3.4 rnol % 5.6 rnol % 6.9 mol % 12.8 rnol %

M'+ (alkali metal) M2+ (alkaline metal) M3+ (lanthanide)



M" (alkali metal) M2+ (alkaline metal) M3+ (lanthanide)

0.25 0.23 0.28

0.27 0.25 0.30

0.31 0.28 0.34

0.32 0.29 0.36

- AC, (J g-' K-')'

0.26 0.21 0.25 0.25 0.24 0.23

=,/m, [ J K-' g-' (styrene)]'

0.30 0.24 0.28 0.28 0.28 0.27

- AC,/mf3 [J K-' g-' (styrene)]'

0.36 0.31 0.34 0.36 0.33 0.35

- ACp/mf4 [J K-' g-' (styrene)]'

0.39 0.34 0.37 0.40 0.36 0.38

0.09 0.09 0.11

0.13 0.12 0.14

0.22 0.22 0.25

0.30 0.29 0.33

The average value of the parameters listed for each rnol % sulfonation for all ions of the charge is evaluated individually.

very low relative to polystyrene, it is clear that not all of the styrene units in these ionomers participate in the relaxation. If we assume that some ( n ) of the ( m ) styrene units in an intersulfonated unit polystyrene segment in these materials are constrained in some way, for example, through the involvement of their near neighbor sulfonated styrene in clustering, so that they cannot participate in the cooperative segmental motion corresponding to the glass transition, the mass fraction mf, of those which can participate is smaller than mf. This leads to a value for AC, /mf , which is larger than AC,/mf. If n is taken to be 4, AC,/rnf, is 0.33 f 0.02 J K-' per gram of rearrangeable styrene; and if n is 3, AC,/mf3 is 0.25 f 0.02 J g-'K-' (rearrangeable styrene). Since these are averages of several types (intersulfonated unit chain length to find m , and cation type), albeit of quite similar values, it probably is not useful to draw a distinction between n = 3 and n = 4. However, the result is that in these 12.8 mol 96 ionomers several styrene units adjacent to the sulfonated one appear to act as if they are immobilized by the type of aggregation occuring at high sulfonation levels. The AC, associated with the rearrangeable ones is about 0.3 J g-'K-'. This, of course, is consistent with those adjacent to the sulfonated one being constrained by the clustering and with most of the ionic sites being in- volved in clustering in this sulfonation range.

1696 YANG ET AL.

If the AC, values for these types of M-SPS ionomers in the 3.4-6.9 mol % sulfonation range are normalized similarly by mr3 or mf4 the resultant AC,/m,, values are found to vary widely and range up to 0.41 J g-'K-' (rearrangeable polystyrene unit).

When the AC, value for each of the H-SPS materials (3.4-20.1 mol % sulfonation) is normalized by mr, they all yield ACp/m, = 0.36 f 0.01 J /g- (styrene). Thus, the AC,/mf value for all H-SPS is 0.36, and that for all M-SPS ionomers in the 3.4-6.9 mol 5% sulfonation range AC,/m, is about 0.28 J g-'K-' (styrene). We interpret this to mean that in the acid-form ionomers the sulfonated styrene unit participates in the cooperative motion of the glass transition, but that in the case of the 3.4 to 6.9 mol % sulfonated M-SPS ionomers only the styrene chains in between the cation-immobilized sulfonated styrenes rearrange. In the case of the higher sulfonation level (12.8%) M-SPS, where clustering is postulated, we interpret the results to mean that an average of about ( m - 3 ) or ( m - 4 ) of the ( m ) nonsulfonated styrene units in an average intersulfonated styrene segment participate in this type of motion. For their collective motion AC, is given by ACp/m,, . Its value is ca. 0.29 J g-'K-' (rearrangeable styrene) (averaging the values for n = 3 and n = 4 ) , which again is close to that for polystyrene.

The model which emerges from this study, then is that the glass transition in M-SPS at low sulfonation levels is that of rearrangement of essentially all polystyrene segments between the cation-immobilized sulfonated units, and that they are subject to average potential barriers which are somewhat higher than those in polystyrene itself. Since those barriers lead to Tg values which are higher for M-SPS than those for analogous H-SPS materials in which the H-bonded sulfonate unit also moves (as inferred from the fact that the AC,/ mf is higher for H-SPS), we conclude that the reason the average potential barriers are higher is that the styrene units nearest to the sulfonated ones are strained. In the case of the highly sulfonated ionomers this effect is exaggerated by the fact that those close to the sulfonated unit do not participate in the rearrangement at Tg, and those that do must overcome relatively high potential barriers due to their strong intramolecular coupling to the constrained ones.

The glass transition temperatures of SPS-M ionomers depend on the degree of hydration or complexation The presence of water, particularly, lowers Tg considerably as shown by Figure 2. When the thermal properties are measured by standard techniques, using sealed sample pans, water molecules are not eliminated completely on the first several runs. After either the water or complexing agent molecules were eliminated, the glass transition attained the value for the dried ionomers. This is a general solvent plasticization effect and its elimination requires considerable care. Water and other complexing agents in these cases act to increase the separation of the cations from their sites and screen the interionic potential. This effect of complexation also de- creases the steric crowding at the cation and reduces the constraints on the monomeric units adjacent to the sulfonated styrene units. These factors make it easier for the chain to move upon heating, and lower glass transition temperatures are observed. Similar results were reported by Weiss et al.7 in the study of control of ionic interaction in similar ionomers using different am- monium counterions.


CONCLUSION

Glass transition temperatures of alkali, alkaline earth, and lanthanide metal SPS ionomers are reported. It has been demonstrated that using a DSC sampling technique in which solvent can escape helps in obtaining valid values of Tg on the early runs. The glass transition temperatures for all these ionomers having the same ionic content but different cations are approximately the same. No relation between the Tg and the cation charge or size was found for M-SPS ionomers series. Coordination of cations by H 2 0 or ethylenediamine in the polymer matrix has a profound influence on the glass transition temperature. Calculations based on AC, measurements showed that AC, for the polymer backbone rearranging at the glass transition is nearly constant for different ionic compositions if it is assumed that all of the unsulfonated styrenes rearrange when the sulfonation level is low but that several [ ( m - 3 ) to (m-4)] of the styrene units adjacent to the sulfonated unit in 12.8 mol % M-SPS ionomers are constrained by clustering types of aggregation.

We gratefully acknowledge the contributions of Dr. Vincent Mattera, Jr. and Dr. 11-Wun Shim, and of Dr. Joseph Devine for electron microprobe measurements and Mr. Michael Sosnowski for EDAX measurements. We are grateful to Dr. Robert Lundberg for kindly providing the H-SPS samples.

References

1. J . J. Fitzgerald and R. A. Weiss, JMS-Rev. Macromol. Chem. Phys., C28 (1) ,99 ( 1988). 2. R. D. Lundberg, J. S. Makowski, and L. Westerman, in Ions in Polymers, A. Eisenberg

3. R. D. Lundberg and H. S. Makowski, J . Polym. Sci., 18, 1821 (1980). 4. R. A. Weiss and J . J. Fitzgerald, in Structure and Properties of Ionomers, M. Pineri and A.

5. J. J . Fitzgerald, D. Kim, and R. A. Weiss, J . Polym. Sci., Polym. Lett. Ed., 24, 263 ( 1986). 6. J . J. Fitzgerald and R. A. Weiss, Coulombic Interactions in Macromolecular Systems,

7. R. A. Weiss, J. J. Fitzgerald, H. A. Frank, and B. W. Chadwick, Macromelecules, 19, 2085

8. Y. S. Ding and S. L. Cooper, in Structure and Properties of Ionomers, M. Pineri and A.

9. K. Tadano, E. Hirasawa, H. Yamamoto, and S. Yano, Macromolecules, 22,226 (1989). 10. A. F. Galamhos, W. B. Stockton, J. T. Koberstein, A. Sen, R. A. Weiss, and T. P. Russel,

11. R. Stadler and L. de Lucca Freitas, Macromolecules, 22, 714 ( 1989). 12. R. A. Weiss and J. A. Lefelar, Polymer, 7 , 3 (1986). 13. V. D. Mattera, Jr. and W. M. Risen, Jr., J . Polym. Sci. B, 24, 753 (1986). 14. H. S. Makowski, R. D. Lundberg, and G. H. Singbal, U S . Patent 3, 870, 841 (1975). 15. G. Zundel, Hydration of Intermolecular Interaction, Academic Press, New York, 1969, pp.

16. R. A. Wallace, J . Polym. Sci. A-2, 9, 1325 (1971). 17. M. Rigdahl and A. Eisenberg, J . Polym. Sci., Polym. Phys., 19, 1641 (1981). 18. D. J. Yarusso and S. L. Cooper, Macromolecules, 16, 1871 (1983). 19. D. G. Peiffer, R. A. Weiss, and R. D. Lundberg, J . Polym. Sci., Polym. Phys. Ed., 20, 1503

(Ed.), Adv. Chem. Ser. 187, American Chemical Society, Washington, D.C., 1980, p. 67.

Eisenberg (Eds.), D. Reidel Publ. Co., Dordrecht, 1987, p. 361.

A. C. S. Symposium Series 302, American Chemical Society, Washington, D.C., 1986, p. 35.

( 1986).

Eisenberg (Eds.), D. Reidel Publ. Co., Dordrecht, 1987, p. 73.

Macromolecules, 20,3091 ( 1987).

36-46.

(1982).

Received July 20, 1989 Accepted December 7, 1989

preparation and thermal characterization of the glass transition temperatures of sulfonated...

Documents