Clustering in Carboxylated Polysulfone Ionomers: ACharacterization by Dynamic Mechanical andSmall-Angle X-ray Scattering Methods

DIEP NGUYEN,1* JOON-SEOP KIM,2 MICHAEL D. GUIVER,3 ADI EISENBERG4

1 Department of Physics, Building 750, Brookhaven National Laboratory, Upton, New York 11973

2 Department of Polymer Science and Engineering, Chosun University, Kwangju, 501-600, Korea

3 Institute for Chemical Process and Environmental Technology, National Research Council,Ottawa, Ontario K1A 0R6, Canada

4 Department of Chemistry, McGill University, Montreal, Quebec H3A 2K6, Canada

Received 24 March 1999; revised 22 July 1999; accepted 9 August 1999

ABSTRACT: The dynamic mechanical properties and morphology of carboxylated poly-sulfone ionomers were investigated by dynamic mechanical thermal analysis andsmall-angle X-ray scattering (SAXS) techniques. It was found that at 25 mol % of ions,ionomers show two glass transitions: one at about 200 °C (the matrix Tg) and the otherat about 235 °C (the cluster Tg). It was also found that with increasing ion content upto about 37 mol %, the matrix Tg shifted to higher temperatures and the size of tan dpeak decreased. The cluster Tg did not change. From the results, it is suggested thateven at high ion content, the ionomers contain a significant amount of unclusteredmaterial, but that the increase in the ion content does not increase the amount ofclustered material. SAXS profiles showed the ionic peak, which represents the presenceof multiplets in the cluster regions. In addition, the difference in the matrix and clusterTg’s of this ionomer system was found to be about 35°. Thus, it is postulated that ionicgroup aggregation is subject to steric hindrance owing to the bulkiness of benzene ring,and tension on polymer chains surrounding the multiplet owing to chain rigidity, whichlimit the size and stability of the multiplet significantly. © 1999 John Wiley & Sons, Inc. JPolym Sci B: Polym Phys 37: 3226–3232, 1999Keywords: dynamic mechanical, small-angle X-ray scattering; ionomers; carboxy-lated polysulfone

INTRODUCTION

It has long been known that the ionic groups inmedia of low dielectric constant tend to form ionicaggregates. The ionic aggregates, or multiplets,1

reduce the mobility of the chains in their vicinity

owing to attachment of chain ends to the multi-plet, crowding, and chain extension.2 At very lowion contents, only single multiplets exist with re-duced mobility regions surrounding them, andthe size of the regions is not large enough to showan independent glass transition. However, as theion concentration increases, the regions of thereduced mobility start to overlap to form largereduced mobility regions, which are large enoughto show their own glass transition. These largeregions of reduced mobility are called clusters.2

* Present address: PPG Industries, R&D Center, AllisonPark, PA 15101

Correspondence to: D. Nguyen (E-mail: dnguyen@ppg.com)Journal of Polymer Science: Part B: Polymer Physics, Vol. 37, 3226–3232 (1999)© 1999 John Wiley & Sons, Inc. CCC 0887-6266/99/223226-07

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Both the Tg’s of the matrix (ion-poor region) andthe cluster regions have been found to increasewith increasing ion content. In addition, someionomers show a small-angle X-ray scattering(SAXS) peak, which has been ascribed to inter-multiplet scattering.

The properties and morphology of ionomershave been found to depend on the type of pendentionic group.3–10 For example, the cluster Tg ofsodium polystyrenesulfonate is about 120 °Chigher than the matrix Tg, whereas the cluster Tgof the sodium polystyrene-co-methacrylate isabout 70 °C higher than the matrix Tg.11 Thisdifference is due to the difference in the strengthof the ionic interactions.11,12 It was also foundthat the volume fraction of clusters in the carbox-ylated ionomer is higher than that in the sulfon-ates at the same ion content.11 This difference isrelated to the relative sizes of multiplets in thesetwo systems.11,12

Polysulfone is a high-performance thermoplas-tic resin with good toughness, high temperatureresistance, and high impact strength. The struc-ture of polysulfone is shown below.

Because the polysulfone contains aromatic rings,this polymer can be functionalized; the sulfonatedpolysulfones were found to be useful for fabricat-ing membranes for desalination.13,14

Noshay and Robeson15 studied the sulfonationof the polysulfone by using sulfure trioxide–tri-ethylphosphate complex. They found that the in-troduction of sodium sulfonate to polysulfone in-creases the glass-transition temperature (Tg) ofpolymer significantly. For example, the differencebetween the Tg of an unfunctionalized polymerand that of the ionomer that has an ionic group onevery repeat unit is about 130 °C.

In a subsequent study, Drzewinski and Mac-Knight16 investigated the thermal and mechani-cal properties of sodium and cesium neutralizedsulfonated polysulfones. They found that the Tg ofthe ionomers increases with increasing ion con-tent by about 1.5 °C/mol %. The authors alsofound that 50 mol % of cesium neutralized poly-sulfone ionomer shows a very weak SAXS peak inthe range of 2u 5 1–6°.

Previously, only sulfonated polysulfone iono-mers were investigated morphologically, ther-

mally, and mechanically. Because the morphologyand properties of ionomers change with types ofionic groups, the present study of the carboxy-lated polysulfone was performed to explore thedifference between sulfonates and carboxylates inthis important polymer. An attempt was alsomade to explore the morphological and mechani-cal differences between this ionomer system andanother more extensively studied carboxylatedionomer, that is, poly(styrene-co-sodium methac-rylate).

EXPERIMENTAL

Preparation of Polymer Samples

Polymer Modification to Carboxylated Polysulfone

Carboxylated polysulfones having three differentvalues for degree of substitution (DS) per repeat-ing unit were prepared generally according to thepublished procedure.17 A 5-L three-neck flaskwith a central resin kettle flange of internal di-ameter 10 cm was used for the reaction. A me-chanically stirred solution of commercial UdelP3500 polysulfone (Amoco Performance Products;110 g, 0.25 mol) in THF (2 L) was lithiated to thedesired DS by addition of n-butyllithium (10M inhexane). Excess dry ice, freshly prepared fromliquid CO2, was thoroughly mixed into the solu-tion with a large spatula. The mixture was al-lowed to warm up overnight, then the carboxy-lated polymer was mixed in a Waring blenderwith ethanol, then washed and recovered in theCOO2Li1 form. The three polymer derivativeshad DSs of 0.25, 0.29, and 0.37, as determined bythe literature method.17

Purification and Preparation of FinelyDispersed Samples

The COO2Li1 ionomers obtained from the reac-tion were acidified by stirring with hot dilutedHCl acid for 24 h. The resulting COOH polymerswere washed by stirring them in a large volume ofboiling reverse osmosis water (pH 6.2) for at least4 h (typically 12 h), filtering on a Buchner funnel,and then passing 1 L of boiling water through thefilter pad. The polymers were washed three timesby this procedure, then dried in an oven. Next,15% solutions of polymers in distilled N-meth-ylpyrrolidinone (NMP) were prepared, filtered ona coarse glass frit, and precipitated into ethanolin a Waring blender. The polymers in the form of

CLUSTERING IN CARBOXYLATED POLYSULFONE IONOMERS 3227

fine powders were washed twice as before withboiling water and not allowed to dry out.

Conversion to Sodium and Cesium Ionomers

For conversion to ionomers, the COOH polymerswere placed in screw cap Teflon bottles and placedin a hot water bath. Portions of COOH polymerswere stirred in 500 mL hot 2.5 M NaOH for 3 daysto convert the COOH groups to the COO2Na1

form. Excess NaOH solution was removed fromthe samples by rinsing the polymers four timesaccording to the previous wash procedure. Por-tions of the COOH polymers were stirred with atwofold excess of CsOH for 3 days to convert theCOOH groups to the COO2Cs1 form and washedas before. Final drying was in vacuo at 60 °C.

Dynamic Mechanical ThermalAnalysis (DMTA) Study

Sample Preparation

For the DMTA measurements, samples were com-pression molded at 260 °C under a pressure ofabout 30 MPa. The pressure was applied for 5min, then it was slowly released, and the moldwas slowly cooled to below the matrix Tg. Thesample was removed from the mold and annealedfor 24 h under vacuum at around 220 °C. Thedimensions of the molded sample were about 2.53 6.0 3 30 mm.

DMTA Experiments

A Polymer Laboratories dynamic mechanicalthermal analyzer was used to measure dynamicmechanical properties of ionomers. Storage mod-uli (E9), loss moduli (E0), and loss tangents (tan d)were obtained as a function of temperature at aheating rate of 0.5 °C/min. The dual cantileverbending mode was utilized at frequencies rangingfrom 0.1 to 30 Hz. All experiments were carriedout under a dry nitrogen atmosphere.

Data Analysis

For each sample, DMTA measurements were per-formed at five different frequencies; however, de-tailed analysis was performed only on the 1-Hzdata. Curve deconvolutions were performed onthe loss tangent data using the Peakfit (JandelScientific) program. The best fits were achievedby using an exponential function as a backgroundand fitting the matrix and cluster peaks withGaussian functions. This combination of curves

has been found to work well for other ionomersystems;19 this suggests that it may work in thiscase. Curve deconvolutions were carried out overthe temperature range from 140 °C to approxi-mately 20 °C above the minimum following thecluster peak. Activation energies for the glasstransitions were calculated using an Arrheniusplot of log frequency versus inverse peak temper-ature.

SAXS Study

Sample Preparation

The cesium-neutralized samples were prepared inthe same way as those for the DMTA experimentsmentioned above. The molded samples in theform of thin disks [12 mm (diameter) 3 0.5 mm(thickness)] were stored in a vacuum oven at 180°C for 24 h.

SAXS Experiments

The SAXS experiments were conducted at theD-22 station18 of the LURE-DCI synchrotron ra-diation source (Orsay, France). The double-crys-tal, fixed-exit monochromator was tuned to pro-vide a beam of 9.0 keV, collimated to about 1 mm2

at the sample. The beam path, including the sam-ple holder, was kept under vacuum. The scatteredX-rays were detected with an Xe-CO2 gas-filled,one-dimensional, position-sensitive detector witha spatial resolution of 257 mm. All samples werestudied in the angular q range from 0.01 to 0.35Å21 (q 5 4psinu/l, where u is half the scatteringangle and l is the X-ray wavelength). The result-ing intensity (I) versus q curves were corrected forbeam decay, sample absorption, and samplethickness; a background scattering from theempty sample holder was subtracted.

RESULTS AND DISCUSSION

Figure 1 shows the storage modulus and loss tan-gent curves as a function of temperature of theunfunctionalized polysulfone and two samples ofthe 25 mol % carboxylated polysulfone neutral-ized with Cs1 and Na1. For the unfunctionalizedpolysulfone, the storage modulus remained atabout 8 3 108 Pa up to about 185 °C, but droppedto 5 3 106 Pa between 185 and 200 °C. The lowvalue of the storage modulus may be due to thepresence of dispersion below the temperature in-vestigated here. In the case of the ionomers, how-

3228 NGUYEN ET AL.

ever, with increasing temperature, the modulusdropped rapidly to 5 3 107 Pa in the temperaturerange of 190–205 °C, and above this temperatureit decreased gradually to about 106 Pa. The posi-tions of the tan d peaks for matrix Tg at about 200°C remained constant, but the tan d peak heightof the matrix Tg dropped drastically upon ioniza-tion. The shoulder at about 240 °C can be inter-preted as a cluster Tg. This behavior is stronglyreminiscent of that of an ionomer of low molecularweight in which the cluster peak is “buried” underthe part of the tan d curve which rises rapidlybecause of polymer flow.19 In samples of highermolecular weight, where the flow region is movedto higher temperature, the cluster peak shouldbecome more visible. To see the cluster peakclearly, the peak deconvolution was performed.The deconvolution result of the tan d peak of 25mol % Cs salt is shown in Figure 2(a). The figureshows clearly a deconvoluted peak for the clusterTg with a peak maximum at about 235 °C. Itshould be noted here that these ionomers havetwo Tg’s, which has not been reported for thesulfonated system.15,16

Figure 1. Storage modulus (E9) and loss tangentcurves as a function of temperature for unfunctional-ized polysulfone(—)and 25 mol % Cs (F) and Na (E)carboxylated polysulfone ionomer samples (at 1 Hz).The upper curves are the storage modulus and thelower curves are the loss tangent.

Figure 2. Deconvoluted loss tangent curve as a function of temperature for (a) 25 mol% Cs carboxylated polysulfone, and (b) 37 mol % Cs carboxylated polysulfone sample (at1 Hz).

CLUSTERING IN CARBOXYLATED POLYSULFONE IONOMERS 3229

The storage modulus and loss tangent curvesas a function of temperature of sodium and ce-sium carboxylated polysulfones with three differ-ent ion contents are shown in Figures 3 and 4,respectively. For the Cs salt (Fig. 4), in general,the shapes of the modulus curves did not changesignificantly with ion content. The relatively largedifference between 25 and 29 mol % and the smalldifference between 29 and 37 mol % were mostlikely due to the difference in molecular weightand sample history. One can see in the deconvu-lated peak (Fig. 2) there was not much differencein the clustered peak positions. The generaltrend, for example, increasing in Tg with ion con-tent, was reserved. The small changes in shapewere a minor effect of molecular weight and sam-ple history.

For the Na case (Fig. 3), one can see a muchmore reasonable progress of matrix Tg with ioncontent, in that matrix Tg increased as the ioncontent increased, from 25% to 37%. However, thepeak height dropped between 29 and 37 mol %,which was not observed for the Cs counterpart.Although the breadth of the matrix Tg peaks in-creased for the higher ion content sample, the

total areas under the two peaks were not thatdifferent. Again, there was no major difference inthe cluster Tg’s. At this point, it should be men-tioned that there were certain errors, for example,the position of the base line, in the deconvolutionprocedure. However, it must be stressed that evenwith possible errors in curve fitting, there is nodoubt that the cluster tan d peak was present forthe 37 mol % ionomer and that the deconvolutedpeak area was similar to that for the lower ioncontent sample.

The SAXS profiles of cesium samples (Fig. 5)show a weak ionic peak at high q values. Thepresence of the peak at high q values makes Po-rod analysis impossible, and thus a detailed studyof morphology of this ionomer system was notperformed. However, the SAXS peak maximum atq 5 ;0.3 Å21 represents the Bragg spacing ofabout 21 Å. This Bragg distance is similar to thatof poly(styrene-co-cesium methacrylate) iono-mers. This similarity in Bragg spacing impliesthat the arrangements of multiplets in the clusterphase were similar in these two ionomer families.

To understand the clustering in the polysul-fone ionomer system, it is useful to recall theresults of dynamic mechanical study of the poly-

Figure 3. Storage modulus (E9) and loss tangentcurves as a function of temperature for Na carboxylatedpolysulfone ionomer, measured at 1 Hz.

Figure 4. Storage modulus (E9) and loss tangentcurves as a function of temperature for Cs carboxylatedpolysulfone ionomer, measured at 1 Hz.

3230 NGUYEN ET AL.

(styrene-co-sodium methacrylate) [P(S-co-MANa)]ionomer system. The matrix and cluster Tg’s in-creased with increasing ion content at a rate ofabout 3 °C/mol % of ions in the range of ion contentsfrom 1 to 10 mol %. Furthermore, the area underthe matrix tan d peak decreased with increasing ioncontent, whereas that for the cluster Tg increased;for 21 mol % ionomer, only the cluster tan d peakwas observed. In addition, the difference betweenthe matrix and cluster Tg’s was found to be about 70°C.11 These results are different from those ob-tained from the polysulfone ionomers. With the re-sults mentioned above, we next discuss the cluster-ing in the polysulfone ionomers.

First in the case of the polysulfone, the ionicgroups are attached directly to the aromatic ringin the relatively rigid main chain. However, in thecase of the polystyrene ionomers, the ionic groupis attached to the tertiary carbon in the mainchain.

Thus, it is more difficult for the ionic group in thepolysulfone ionomer to form multiplets due to

steric hindrance and chain rigidity than in themethacrylate ionomers. As a result, a consider-able amount of ionic groups may still exist asisolated ionic groups without forming multipletsin the matrix, even at such high ion contents.Therefore, a significant amount of the ion-con-taining matrix regions still retained a high ioncontent. In addition, because a significant part ofthe ionic groups did not form multiplets, the in-crease in ion content from 25 to 37 mol % did notchange the amount of cluster phase appreciably.Thus, the size and position of the deconvolutedcluster tan d peak do not change with ion content.However, the matrix Tg increased because of thecopolymerization effect and formation of pairs ofionic groups. It should be mentioned at this pointthat according to the EHM model,2 the thicknessof the restricted mobility layer surrounding mul-tiplets is related to the persistence length of poly-mer, which is about 10 Å for polystyrene. How-ever, in the polysulfone case, the persistencelength of the polymer has not been reported.Thus, a direct comparison of the behavior of poly-sulfone ionomers with that of polystyrene iono-mers may not be possible.

Second, the difference in two Tg’s for the poly-sulfone ionomers was about 35 °C, whereas thatfor the P(S-co-MANa) ionomers was about 70 °C.This difference can also be understood if one con-siders the steric effect of the ionic groups in thepolysulfone ionomers on the formation of multi-plets. Because the ionic group was attached to thebenzene ring in the polymer main chain, the ionicgroups could not make tight, closely bound mul-tiplets owing to steric hindrance. Furthermore,the polysulfone chains in the immediate vicinityof the multiplet were under relatively high ten-sion owing to the high rigidity of polymer chain.As a result, the ionic interactions between ionicgroups in multiplets may not have been as effec-tive in multiplet formation as those for the poly-styrene ionomers. Thus, ion hopping, which is thepart of cluster Tg mechanism in the polysulfoneionomers, may take place at relatively lower tem-peratures than in the styrene ionomers.

CONCLUSIONS

Carboxylated polysulfone ionomers showed twoglass transitions: one (matrix Tg) at about 200,and the other (cluster Tg) at about 235. It was alsofound that the matrix Tg shifted to high temper-ature and the size of tan d peak decreased with

Figure 5. SAXS profile for the 60 mol % Cs carboxy-lated polysulfone.

CLUSTERING IN CARBOXYLATED POLYSULFONE IONOMERS 3231

the ion content, whereas the cluster Tg did notchange. The results imply that even at high ioncontent, a sufficient amount of unclustered mate-rials still existed in the samples, and the increasein the ion content did not change the amount ofcluster phase. The presence of multiplets in thecluster regions was also evidenced by the SAXS.It is also postulated that the ionic group in thepolysulfone ionomer experienced steric hindranceowing to the bulkiness of benzene ring, and ten-sion on polymer chains surrounding the multipletowing to chain rigidity, which affected the sizeand stability of the multiplet significantly.

This work was supported by the Natural Sciences andEngineering Research Council of Canada (NSERC).The authors gratefully acknowledge the help of Dr.Claudine Williams in the SAXS experiments.

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