synthesis and properties of sulfonated polyurethane ionomers with anions in the polyether soft...

Synthesis and Properties of Sulfonated PolyurethaneIonomers with Anions in the Polyether Soft Segments

XIN WEI and XUEHAI YU*

Department of Polymer Science and Engineering, College of Chemistry and Chemical Engineering, Nanjing University,Nanjing, Jiangsu 210093, People’s Republic of China

SYNOPSIS

A series of polyether (PTMO, PEO) polyurethane ionomers having different contentsof sodium sulfonate groups in the soft segments have been synthesized. The reactionof transesterification was involved in the incorporation of the sodium sulfonate groupsin the polyether. The polyurethane ionomers were characterized by means of dynamicmechanical thermal analysis, differential scanning calorimetry, and small-angle x-rayscattering. Solid-state ionic conductivity was also measured. As the ionization levelincreased, the compatibility of the hard and soft segments increased and the glasstransition region of the soft segment became broader. These samples had relativelyhigher moduli and good film-forming ability. Moreover, this kind of ionomer providesa very promising ionic conductive multiphase polymer with a single ion transportmechanism.q 1997 John Wiley & Sons, Inc.Keywords: Polyurethane ionomer j transesterification j morphology j phase compati-bility j ionic cluster j solid-state ionic conductivity j single ion transport mechanism

INTRODUCTION The multiplets are small associated polargroups,1,2 and the clusters are significantly largeraggregations of multiplets with weaker ionic in-Ionomers contain a certain number (usually 10teraction.1mol % or less ) of inorganic salt groups attached

Polyurethane block copolymers are an im-to a polymer chain.1–4 Ionization causes im-portant class of thermoplastic elastomers.6 Theirproved toughness, tear strength, and abrasionunusual elastomeric properties should be attrib-resistance.4 Even small amounts of ionic groupsuted to the formation of a microphase-separatedcan sufficiently modify specific properties of ma-domain structure consisting of ‘‘hard’’-segment-terials, such as the glass transition, the rubberyrich and ‘‘soft’’-segment-rich domains.6–9 Theremodulus above the glass transition, dynamichas been a wide range of work done in this labmechanical behavior, the melt rheology, the re-and in S. L. Cooper’s group involved in the synthe-laxation behavior, dielectric properties, and so-sis and characterization of polyurethane iono-lution behavior.1,3

mers.4,7,8,10,12–17,25 One synthetic procedure is toBecause of interionic electrostatic forces, asuse a chain extender containing a secondary orwell as the incompatibility between the ionic tertiary amine to produce a polyurethane cat-groups and the nonpolar polymer matrix, the ionic ionomer or anionomer.8,11 Another procedure in-

groups tend to aggregate.1,4 In general, above a volves reacting the urethane group with a sodiumcritical ion concentration, two types of ionic aggre- hydride and subsequently reacting with 1,3-pro-gates can be defined: multiplets and clusters.1,2,5

pane sultone to produce a polyurethane sulfo-nate.12,13 However, in these previous instances theionic groups were part of the hard segment. Theeffect of these ions was to increase the polarity of* To whom correspondence should be addressed.the hard segment and hence decrease the compat-Journal of Polymer Science: Part B: Polymer Physics, Vol. 35, 225–232 (1997)

q 1997 John Wiley & Sons, Inc. CCC 0887-6266/97/020225-08 ibility of the hard and soft segments.7,14 As a re-

225

9601017/ 8Q20$$0002 12-04-96 15:15:16 polpa W: Poly Physics

226 WEI AND YU

sult, both the phase purity and the phase separa- distillation. N,N *-dimethylacetamide (DMA) wasdried with molecular sieves and purified by vac-tion were enhanced.7,14

We describe a new series of polyurethane iono- uum distillation. [Dimethyl 5-sulfoisophthalate,sodium salt] (DMSP) (98%, Aldrich Chemicalmers having different ionization levels where the

ionic groups are incorporated in PTMO soft seg- Co.) , 1,4-butanediol (BD) (99%, Aldrich ChemicalCo.) , sodium hydride (NaH) (60% dispersion inments. As will be demonstrated, the incorporation

of sodium sulfonate salts in the soft segments im- mineral oil, Aldrich Chemical Co.) , 1,3-propanesultone (97%, Aldrich Chemical Co.) , toluene, pe-proved the compatibility of the hard and soft seg-

ments. These new ionomers were characterized troleum ether (b.p. 60 Ç 907C), stannous octoatecatalyst, and zinc acetate catalyst were used asby means of DSC (differential scanning calorime-

try), DMTA (dynamic mechanical thermal analy- received.sis) , and SAXS (small-angle x-ray scattering).Ionic conductivity measurements of ionomer films

Sample Preparationwere also made. These new materials show partic-ular promise as ionic conductors because of their A 3 : 2 : 1 MDI : BD : PTMO (Mn Å 1400) seg-

mented polyether-polyurethane used as the con-relatively high conductivities coupled with thesingle ion transport mechanism inherent in these trol polymer in this study was synthesized by a

typical two-step condensation reaction.6 The se-systems. The more common polyethylene oxide(PEO)-lithium salt electrolytes have the distinct ries of polyurethane ionomers with different con-

tents of sodium sulfonate groups in the polyetherdisadvantage of having a bi-ionic transport mech-anism which under direct current polarization soft segments and the same hard segment concen-

tration (HSC)6 were synthesized as follows. Thehas a significant decrease of ionic conductivity.12

Until now, most single ion transport mechanism reaction procedure is outlined in Figure 1. Thefirst step of reaction was the transesterification.materials are polyblends of ionomer and poly-

ether18 or copolymers19 of ionomer oligomer with The polyether was dissolved in toluene at about30 wt %. A stoichiometric amount of the DMSPpolyether low molecular ionomers. Since they

showed either poor mechanical property or low and about 0.05 wt % of zinc acetate catalyst wereadded into the solution. Then the solution wasconductivity, PEO polyurethane ionomers with

sodium sulfonate groups in hard domains were stirred at 1107C for about 48 h under dry nitrogen.In order to remove the methanol byproduct fromsynthesized in this lab and the conductivity of

1005 S/cm was obtained at 707C.12 However, the transesterification to ensure a complete reaction,about 15 mL toluene was distilled every 2 h andconductivity near room temperature is still not

satisfactory. In this work, a PEO polyurethane an equivalent of fresh toluene was added to thesystem until the solution turned clear and noionomer with sodium sulfonate groups in soft seg-

ments ‘‘PUI-PEO’’ was also synthesized and its trace of methanol was detected by adding themetal sodium to the distillate. DMA was added tosolid conductivity was measured. As expected,

compared with the ionomer ‘‘PUI-1000-20’’ 13 the ionized polyether to make a 15 wt % solutionand all of the toluene was distilled from the sys-whose ions were located in hard domains, the ions

in PEO soft segments exhibited higher mobility tem. After addition of stannous octoate catalyst,a 15 wt % solution of MDI in DMA was added.under the electrical field and this kind of ionomer

provides a new promising ionic conductive multi- The solution was stirred for 1.5 h at 60–657C un-der dry nitrogen. Then the BD was added drop-phase polymer with a single ion transport mecha-

nism. wise and the reaction continued for an additionalhour. To ensure a complete reaction, the mixturewas stirred at 80–857C for 3–4 h. The synthesizedionomer was recovered by precipitation into a sol-EXPERIMENTALvent of 50/50 mixture of toluene and petroleumether, further washed and extracted in a SoxhletMaterialsapparatus with toluene to remove any residualreagent. The polymer was dried under vacuum forPolytetramethylene oxide (PTMO) (Mn Å 650,

1000, 1400, Dupont) and polyethylene oxide 1 week at 50–607C.The compositions and designations of the sam-(PEO) (Mn Å 1000) were dried in a vacuum oven

for 24 h at 707C. 4,4 *-Diphenylmethane diisocya- ples are shown in Table I. As indicated in TableI, the DMA solution of the synthesized samplesnate (MDI) was degassed and purified by vacuum


SYNTHESIS OF SULFONATED POLYURETHANE IONOMERS 227

Figure 1. Synthetic route to polyurethane ionomers. Note that the syntheses ofPUI4321, PUI3221, and PUI-PEO are taken as examples.

(1%, wt/v: g/mL) possessed a reasonable specific Viscosity measurements were taken on a Ubbe-lodhe viscosimeter at 307C and data of specific vis-viscosity. The ICP (inductively coupled plasma)cosity (hsp) were recorded. Samples for measure-tests showed that the samples had a sodium con-ments were dissolved in DMA at 1% (wt/v : g/mL).tent close to the calculated one, indicating that

Differential scanning calorimetry (DSC) ther-the sulfonate groups were introduced into themograms were recorded over a temperature rangepolyurethane ionomers.beginning at 120K and terminating at 415K. Sam-Based on the molar ratio of MDI : BD : PEOples of 15 { 3 mg mass were run at a heating rate(Mn Å 1000) 3 : 2 : 1 and subsequently reactedof 20K/min with a Perkin-Elmer DSC2-C inter-with sodium hydride and 1,3-propane sultone, afaced to a model 3600 data station.reference sample ‘‘PUI-1000-20’’ 13 with optimized

Dynamic mechanical thermal analysis (DMTA)ionization level for conductivity12 (Na/ : EOdata were evaluated with a Rheovibron DDV-IIÅ 0.05 : 1, ion content is 6.35 mol %) was synthe-dynamic viscoelastometer (Toyo Baldwin Co.) .sized.12 Its ionic conductivity was also measuredSamples were tested over a temperature rangefor comparison with the sample ‘‘PUI-PEO’’ whosebeginning at 01007C until sample failure at 110ions were incorporated in the PEO soft segments.Hz and a heating rate of 27C/min and the dataTransparent films for further testing were pre-were collected every 2 or 37C.pared by solution casting at 60–657C in DMA on

Small-angle x-ray scattering (SAXS) was per-a Teflon plate. Films were then transferred to aformed with a Rigaku D/MAX-RA rotating anodevacuum oven and dried at 607C for 1 week to re-x-ray generator with a copper target, at 40 kVmove any residual solvent.cathode potential and 150 mA emission current.The x-rays were collimated into a beam 10 mm

Characterization 1 70 mm with a Kratky camera and the scatteredThe sodium content was measured using induc- x-rays were detected with a SC-30 scintillation

counter probe. CuKa x-rays were selected by de-tively coupled plasma quantometer (ICP).

Table I. The Synthesized Polyurethanes

Molar ratioION

Sample PTMO PTMO PTMO PEO HSC CONT hsp

designation MDI BD 1400 1000 650 1000 DMSP (%) (mol%) (%)

PU 3 2 1 36.2 0 59.9PUI4321 4.2 3.2 2 1 37.5 2.76 28.8PUI3221 3 2 2 1 37.8 4.16 40.5PUI6543 6.1 5.1 4 3 37.6 5.96 24.4PUI-PEO 4.2 3.2 2 1 37.5 1.86 33.2


228 WEI AND YU

tector pulse-height analysis. Data were corrected thane elastomers. However, other authors in theliterature24 have shown that ionomers are so wa-for sample transmittance and scattering from the

empty camera.20,21 Data were placed on a relative ter absorptive that it is also very likely that thisendotherm should be attributed to water desorp-and logarithmic scale, expressed as I vs. q , 22,23

where I is the smeared relative intensity, and q tion. Until now, there is no conclusive answer tothe attribution of this peak and more researchis the scattering vector,work has to be done in this area in the future.

No evidence of a transition attributable to aq Å 4p sin u /l (1)hard phase glass transition was found in the DSCtraces, which is typical for polyurethanes.14

where 2u is the scattering angle and l is the wave-length of CuKa radiation.

Complex impedance analysis and ionic conduc- Dynamic Mechanical Thermal Analysis (DMTA)tivity measurement with alternating current

DMTA results, expressed as the loss tangent (tanwere carried out with a 378 Electrochemical Im-d ) versus T and the absolute modulus ([E ] ) versuspedance System (EG & G Princeton Applied Re-T on logarithmic scales are shown in Figures 3search) in the frequency range of 5–105 Hz. Filmsand 4. As Figure 3 shows, ionization of the softfor testing were cut to a required size and thensegments obviously improved the phase compati-painted with conductive Ag paste on both sides tobility of these polyurethanes, producing a higherform two Ag electrodes (the diameter is 1.0 cm).Tg of the soft segments. Examining Figure 4 repre-The painted films were dried in a vacuum ovensenting the absolute modulus data, the narrowedfor 48 h, at 607C, then stored at room temperaturemodulus plateau supports the analysis above andin a vacuum desiccator. When measured, the cellthe DSC results. Although ionization gives thewas kept in a temperature-controlled dry box andmaterials better phase compatibility, the synthe-the conductivity was tested at each temperaturesized polyurethane ionomers still exhibit a typicalafter equilibration for 30 min.12,13

elastomeric quality of normal polyurethanes,since each sample still has a typical rubbery pla-teau. Unlike the polyurethane ionomers with ionsRESULTS AND DISCUSSIONlocated in the urethane groups,12,13 our ionizationprocess is not involved in the removal of hydrogenDifferential Scanning Calorimetry (DSC)atom from the hard segments, thus maintainingthe H-bond in the hard segments as a physicalDSC traces are shown in Figure 2. Data obtained

from the DSC traces are listed in Table II, where linkage. The ionized series have greater values ofmoduli at room temperature than the analogousTg is defined as the midpoint of the transition.

Two features may be observed upon the ionized nonionic polyurethane. Higher moduli are a natu-ral consequence of the ionization contributing to apolyurethane samples: the glass transition tem-

perature Tg rises, however, the magnitude of the more interconnected hard phase,8,14 which is bestprobed by small-angle x-ray scattering experi-heat capacity change at the transition, Dcp , drops.

The first fact might be attributed to the improved ments described in the next section.phase compatibility between hard and soft seg-ments brought by ionization, thus resulting in a

Small-Angle X-ray Scattering (SAXS)less purer soft phase (with a higher Tg ) .14 SAXSdata provided the best evidence for these changes SAXS curves ( I vs. q ) on logarithmic scales are

shown in Figure 5, with correction for scatteringin the domain structure. The reduction in Dcp

with increasing ion content is due probably to the from the empty camera and sample transmittancebut without curve smoothing.22,23,25 A peak atmobility restrictions on the polyether segments

topologically close to the incorporated ionic about 0.3 Ç 0.5 nm01 , which is typically observedfor polyurethanes and associated with scatteringgroups. Around 70–907C all of the samples exhib-

ited an endotherm, whose magnitude is about the between the hard domains and soft segment ma-trix, can be noted.14,17 As the ionization level in-same for all of the ionomers but is much smaller

in the control sample of ‘‘PU.’’ Some authors in creased in the soft segments, this peak becomesbroader, which means that the distribution of thethe literature8,15,16 suggested that this DSC peak

should be attributed to the dissociation of short- hard domain size is much wider. Also, the ‘‘tail’’of the curve drops slowly, which supports the factrange ordering in the hard domains of polyure-



Figure 2. DSC traces of polyurethane and polyurethane ionomers.

that the phase separation decreased with ioniza- sesses a very comparable hard segment concen-tration (HSC Å 36.2–37.8%). As expected, due totion. No detectable peak in the range of 1–3 nm01

which has been taken as evidence of ionic clus- the decreased ordering in the hard domains ofionomers brought by better phase compatibility,ters4,11,14,25 can be observed in the PUI curves,

suggesting that no ionic clusters may exist in the the spacing between domains in the ionomers issmaller than the control polymer ‘‘PU’’.9 Apprecia-soft segment matrix. The long spacing L can be

easily determined with satisfactory accuracy, ble difference in L among our samples, listed inTable III, reaffirms a somewhat larger scale ofwhich is defined as:microphase compatibility and interdomain con-nectivity in the new polyurethane ionomers,9,14L Å l / (2 sin u*) (2)which has been indicated in the DSC and DMTAresults.where u* is the peak position in a plot of u2 I versus

u.9,14,17 Supposing the structure is lamellar, L cor-Complex Impedance Analysis and Ionicresponds to the interlamellar repeat distance.9,14

Conductivity MeasurementIf the morphology of each sample is similar, thespacing between the hard domains in each sample To study the mobility of the ions located in the

matrix of soft segments, a sample ‘‘PUI-PEO’’ wasshould be close to each other, for each sample pos-

Table II. DSC Parameters of PU and PUIs

Sample PU PUI4321 PUI3221 PUI6543

Tg(K) 207.2 231.8 241.5 246.3DCp(cal/grdeg) 0.165 0.082 0.068 0.050


230 WEI AND YU

Figure 3. DMTA spectra of polyurethane and polyurethane ionomers: tan d 0 Tcurves.

synthesized and ionic conductivity measurements the spectrum. The high-frequency arc results fromwere made. As an example, the typical complex the combination of polyurethane resistance andimpedance spectrum for the polyurethane iono- the geometry capacity of the sample.12 The low-mer sandwiched between Ag electrodes at 287C is frequency sharp slope is caused by the boundaryshown in Figure 6. There are a high-frequency impedance, which is due mainly to the double-semicircle arc and a low-frequency sharp slope in layer capacity of the electrode and sample.12 In

general, there are two arcs on the impedance spec-tra for a plane cell consisting of nonblocking elec-trodes and an electrolyte with two mobile ions.26

The arc at lower frequency resulted from thecharge transfer of the electrode reactions. On thecontrary, in our experiment, the Ag electrodeswere used as ionblocking electrodes and as a re-sult only one arc was observed since there wereno electrode reactions. The Z * value at the mini-mum of 0Z 9 on the impedance spectrum has beenrecommended to be the best estimation of thesample’s resistance.27 Comparing with the sample‘‘PUI-1000-20’’ 13 with optimized ionization level(Na/ : EO Å 0.05 : 1)12 whose anions are in thehard segment domains, Figure 7 shows the tem-perature dependence of the ionic conductivity ofthe sample ‘‘PUI-PEO.’’ As predicted, the ionicconductivity of ‘‘PUI-PEO’’ is higher than the ionicconductivity of ‘‘PUI-1000-20’’ in a wide range oftemperature, though its ion content is much lower(Na/ : EO Å 0.02 : 1) than ‘‘PUI-1000-20,’’ whichFigure 4. DMTA spectra of polyurethane and poly-

urethane ionomers: [E ] 0 T curves. means much fewer charge carriers. A cationic con-



Table III. Long Spacing Data From SAXS

PU PUI4321 PUI3221 PUI6543

u* 0.296 0.332 0.402 0.594L (nm) 14.9 13.3 11.0 7.44

electrical field. Only when the temperaturereaches around 85 Ç 907C does the conductivityof the sample ‘‘PUI-1000-20’’ slightly exceed ‘‘PUI-PEO’’ because of its released ionic mobility athigher temperature and its much higher contentof charge carriers.

CONCLUSION

1. A novel series of sodium salts of sulfonatedpolyurethane ionomers with anions incor-

Figure 5. SAXS pattern of polyurethane and polyure- porated in the polyether soft segmentsthane ionomers: I 0 q curves. Note that each curve has were successfully synthesized and theirbeen successively shifted by 1 unit on the logarithmic properties were characterized with variousscale for clarity.

methods.2. The new polyurethane ionomers exhibit

good film-forming ability, typical elasto-ductivity of about 1.0 1 1005 S/cm is obtained atabout 75 Ç 857C without any addition of organic meric behavior, and high interdomain con-

nectivity.plasticizer. The sample’s higher conductivity isdue to its unique structural characteristics: (1) 3. The new polyurethane ionomer with PEO

soft segments has better ionic conductivityThe amorphous and flexible polyether matrix inwhich the anions were incorporated provides the at room temperature than a formerly syn-

thesized polyurethane with anions in theions with better mobility than the hard segmentdomains2,5 ; (2) There are no existing ionic clus- hard segment domains and is character-

ized by a single ion transport mechanism.ters which may limit the ionic mobility under the

Figure 6. Complex impedance spectrum of PUI-PEO at 287C.


232 WEI AND YU

Figure 7. Comparison of the temperature dependence of ionic conductivity betweenPUI-PEO and PUI-1000-20.

15. R. W. Seymour and S. L. Cooper, J. Polym. Sci.,REFERENCES AND NOTESPolym. Lett., 9, 689 (1971).

16. R. W. Seymour and S. L. Cooper, Macromolecules,6, 48 (1973).1. R. D. Lundberg, Encyclopedia of Chemical Technol-

17. L. Chen, Y. Zhu, X. Yu, and C. Yang, Gao Fen Ziogy, 3rd ed., Supplement Volume, 546 (1984).Cai Liao Ke Xue Yu Gong Cheng, 8, 45 (1992).2. C. L. Marx, D. F. Caulfield, and S. L. Cooper, Mac-

18. E. Tsuchida and K. Shigehara, Mol. Cryst. Liq.romolecules, 6, 344 (1973).Cryst., 106, 361 (1984).3. X. Yu, B. P. Grady, R. S. Reiner, and S. L. Cooper,

19. E. Tsuchida, J. Macromol. Sci. Chem., A25, 687J. Appl. Polym. Sci., 47, 1673 (1993).(1988).4. R. A. Register, X-hai Yu, and S. L. Cooper, Polym.

20. L. E. Alexander, X -ray Diffraction Methods in Poly-Bull., 22, 565 (1989).mer Science, John Wiley & Sons, Inc., New York,5. C. G. Bazuin and A. Eisenberg, Ind. Eng. Chem.1969.Prod. Res. Dev., 20, 271 (1981).

21. H. H. Kausch and H. G. Zachman, Advances in6. Zoran S. Petrovic, and James Ferguson, Prog.Polymer Science, Vol. 67, Springer-Verlag, Berlin-Polym. Sci., 16, 695 (1991).Heidelberg, 1985.7. H. Zhai, X.-H. Yu, and Z. Yang, Chem. J. Chinese

22. T. Hashimoto, M. Shibayama, and H. Kawai, Mac-Universities, 11, 743 (1990).romolecules, 13, 1237 (1980).8. J. A. Miller, K. K. S. Hwang, and S. L. Cooper, J.

23. T. Hashimoto, M. Fujimura, and H. Kawai, Macro-Macromol. Sci.-Phys., B22 (2), 321 (1983).molecules, 13, 1660 (1980).9. J. T. Koberstein and R. S. Stein, J. Polym. Sci.:

24. B. A. Brozoski, M. M. Coleman, and P. C. Painter,Polymer Phys. Ed., 21, 1439 (1983).Macromolecules, 17, 230 (1984).10. Y. S. Ding, R. A. Register, C.-z. Yang, and S. L.

25. Y. S. Ding, R. A. Register, C.-z. Yang, and S. L.Cooper, Polymer, 30, 1221 (1989).Cooper, Polymer, 30, 1213 (1989).11. J. J. Fitzgerald and R. A. Weiss, Macromol. Chem.

26. J. R. Macdonald, J. Chem. Phy., 61, 3977 (1974).Phys., C28 (1), 99 (1988).27. P. Hagenmuller, Solid Electrolytes—General Prin-12. H.-s. Xu and C.-z. Yang, J. Polym. Sci.: Part B:

ciples, Characterization, Materials, Applications,Polym. Phys., 33, 745 (1995).Academic Press, New York, 1978.13. H.-s. Xu, Ph.D. thesis, Nanjing University (1994).

14. C.-z. Yang, T. G. Grasel, J. L. Bell, R. A. Register,and S. L. Cooper, J. Polym. Sci.: Polym. Phys., 29, Received January 22, 1996

Accepted July 24, 1996581 (1991).


synthesis and properties of sulfonated polyurethane ionomers with anions in the polyether soft...

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