synthesis and characterization of model polyether-ester ionomers

Synthesis and Characterization of Model Polyether-esterIonomers

LEI CHEN,1 XUE-HAI YU,1 CHANG-ZHENG YANG,1 QIN CHAO GU,1 TIAN-DOU HU,2 YA-NING XIE2

1 Institute of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, P.R. China

2 Beijing Synchrotron Radiation Facility, Beijing 100039, P.R. China

Received 26 February 1996; revised 30 September 1996; accepted 1 October 1996

ABSTRACT: In this study, a series of polyether-ester ionomers was prepared by neu-tralizing the carboxylic acid groups in 1 : 1 copolymers of benzenetracarboxylic dianhy-dride (BTDA) and poly(tetramethylene oxide) or poly(ethylene oxide) glycol. The basepolymers were in a liquid state while the ionomers were in solid state and a separateionic phase was formed. The local structure and the morphology of the ionomers wereinvestigated by dynamic mechanical analysis, Fourier transform infrared spectroscopy,and small-angle x-ray scattering as well as extended x-ray absorption fine structure.Clearly, the geometric structure of the ionic sites varied with the nature of the metalions and the morphology of the ionomers was determined by the microstructure of theion aggregates. q 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 799–806, 1997Keywords: ionomer; polyether; synthesis; characterization

INTRODUCTION groups. This structure may adversely affectphase separation and can complicate the inves-tigation of structure-property relationships inIt has been proved that introduction of a smallthe solid state.mole fraction of ionic moieties to the polymer

In recent years, the carefully synthesized modelchain significantly affected the mechanicalionomers have become increasingly important re-properties of the polymer, including increasedsearch materials. A better understanding of thetoughness, tear strength, and abrasion resis-structure of ionomers requires the preparationtance.1 Surlynt, Nafiont, and other commercialand study of the ionomers with a more regularionomers have made significant headway in thechain architecture. Until now, the synthesis ofmarketplace. Considerable efforts have beenmodel ionomers has mostly focused on the tele-carried out on understanding the relationshipchelic ionomers that contain only ionic groups atbetween the microstructure and physical prop-the chain ends,5,6 only a few reports related toerties of the ionomers. Many different kinds ofthe study of linear model ionomer with the ionicionomers have been prepared for this pur-

pose.2–4 For most ionomers, the ionic groups groups spaced regularly along the polymer back-were randomly spaced along a high-molecular bone were found.7

weight chain. The disadvantage for these iono- This study describes a family of polyether-estermers was the random arrangement of ionic ionomers, prepared by neutralizing the carboxylic

acid groups in 1 : 1 copolymers of benzenetetracar-boxylic dianhydride (BTDA) and certain polyols.The microstructure and properties of these iono-Correspondence to: C.-Z. Yang

q 1997 John Wiley & Sons, Inc. CCC 0887-6266/97/050799-08 mers were studied by different techniques.

799

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800 CHEN ET AL.

Table I. Composition of the Ionomers SynthesizedEXPERIMENTAL

PolyetherSample Preparation Sample

Code Type MW CationPoly(tetramethylene oxide) glycol (PTMG) and poly-(ethylene oxide) glycol (PEG) oligomers were dried PC981H PTMG 981 H/

under vacuum for 24 h at 707C and stored in a PC981Ni PTMG 981 Ni2/

desiccator before use. BTDA was recrystallized in PC981Co PTMG 981 Co2/

acetic anhydride and DMAc was dried by calcium PC981Cu PTMG 981 Cu2/

hydride and distilled before use. PC981Zn PTMG 981 Zn2/

PC650H PTMG 650 H/Under a dry argon blanket, a calculated amountPC650Cu PTMG 650 Cu2/of BTDA dissolved in toluene at a concentrationPC650Zn PTMG 650 Zn2/of 15 wt % was added to a stoichiometric amountPEC1000H PEG 1000 H/

of polyols at room temperature. The temperaturePEC1000Co PEG 1000 Co2/

of the mixture rose to approximately 357C becausePEC1000Zn PEG 1000 Zn2/

of the exothermic reaction. The reactant wasPEC1000Ni PEG 1000 Ni2/

stirred and refluxed for 6 h to complete the reac-tion. The toluene was removed by a rotary vacuumevaporator, and then the viscous state polyesteracid was washed with hot water and alcohol and trolled by the amount of the ionomer. The compo-dried in a vacuum oven at 707C. A scheme of the sition of the samples are listed in Table I.synthesis is shown in Scheme I.

The polyester acid precursor was redissolved inDMAc at a concentration of 25 wt %, then wasneutralized by adding a calculated amount of the Instrumental Conditionappropriate metal acetate. After stirring at 607C

Dynamic mechanical thermal analysis (DMTA)for 30 min, the ionomer solution was cast into adata were obtained by using Rheovibron DDV-IITeflont disk and kept in an oven for another 24at a test frequency of 110 Hz. The heating rateh at 657C. The thickness of the film can be con-used was 37C/min, and a data point was collectedevery 37C. Differential scanning calorimeter (DSC)thermograms were recorded using a Perkin-El-mer DSC-II which provides automatic baselinecorrection and normalization for sample weight.Thermograms were recorded from 130 to 490 Kat 27C/min. The temperature range was 090 to2507C. The Fourier transform infrared (FTIR)spectroscopes were tested using a Nicolet 170SXFTIR spectrometer at a resolution of 2 cm01.Small-angle x-ray scattering experiment was car-ried out on Rigaku Model rotating-anode x-raygenerator collimated with modified compactKratky camera in conjunction with Cu Ka radia-tion in the angular range from 2u Å 0.1 to 3.57with steps of 0.027. The data were corrected fordetector sensitivity, dark current, parasitic scat-tering, and sample absorption with a calibratedLupolen (polyethylene) standard. Extended x-rayabsorption fine structure (EXAFS) spectra of thecoordination polyurethanes were obtained at

O®CHO(CH¤CH¤O)nH 1

HO(CH¤CH¤CH¤CH¤O)nH

©CH¤CH¤O)n©C©(CH¤CH¤CH¤CH¤O)n

©©CO]x 1 M11(CH‹COO2)¤[(

O

O

O

O

O

O

HOC

COH

C®O

O®C

(BTDA)

C®O

©CH¤CH¤O)n©C©(CH¤CH¤CH¤CH¤O)n

©©CO]x 1 CH‹COOH[(

O

OM11

M11

O

O

2OC

CO2

Beijing Synchrotron Radiation Laboratory at theK-edge of the metal atom. Edge calibration wasScheme 1. The scheme for the synthesis of the iono-

mers. performed using the pure metals.


SYNTHESIS OF MODEL IONOMERS 801

Figure 1. DSC profiles of the samples.

was about 2000. In the present case, the crystal-RESULTS AND DISCUSSIONline structure was achieved by the connection ofPTMG with the molar mass 1000. Although theThermal Characterizationcrystalline behavior of PC981-H was very similar

The DSC profiles for all of the ionomers and corre- to that of PTMG2000, a small endotherm at 307Ksponding precursors are shown in Figure 1 and was found besides the endotherm of polyetherthe data from DSC measurements are listed in melting. This small band was postulated to beTable II. The highly phase-mixed acid material due to the crystallization of polyester segment in(PC981-H) exhibited a PTMG-rich-phase glass polyether matrix. Upon neutralization, the melt-transition at a temperature much higher than ing endotherm of polyether could no longer bethat of pure PTMG2000.8 A strong endotherm on found for Mw 1000, implying that polyether crys-the curve of PC981-H and an exotherm repre- tal structure cannot form around an ionic aggre-sented the formation and melting of polyether gate. However, the formation of the ionic domain

favored development of a pure PTMG phase be-crystallites. It has been suggested9 that the crys-tal molar mass required for PTMG crystallization cause the Tg of PC981Ni(Cu, Zn) were lower than

Table II. Thermal Transition Temperature

DMTA E9 DSC Tg(K), Endotherm Peak(K),Sample Code Maximum 7C DCp(Cal/grm) DCp(Cal/grm)

PC650Cu 07.6, 200 (237) — —PC650Ni 027.4 213.0, 0.12 293, 0.02PC981Ni 046.8, ú250 204.4, 0.12 296.7, 0.27PC981Cu 032.7, 195 205.6, 0.12 297.2, 0.1PC981Zn 050.4, 180 203.8, 0.12 296.6, 0.26PC981H — 213.0, 0.18 288.9, 2.75


802 CHEN ET AL.

ters in the bulk materials aggregate tightly andthere were more ionic groups in the cluster afterthe treatment. The rubbery plateau modulus ofthe sample decreased after annealing the sampleat 2507C showing the concentration of ionic aggre-gates in the rubbery matrix decreased and thereare more ionic groups in one cluster. As the molec-ular weight of polyol increased, the Tg transitionpeak of soft segment shifted to lower temperatureand the rubbery modulus decreased. The lowerplateau of the sample with longer polyol chainswas due to the lower ionic crosslink density inthe sample. Besides the ionic density, the storagemodulus plateau was also affected by the phaseseparation of the samples. A higher microphaseseparation system PC981Ni exhibited a lowerrubbery plateau. For the ionomers, the choice ofthe cation also has profound effects on the charac-ter of DMTA curves and microstructure of thesamples. Tg analysis (Table II) of the soft segmentshowed an order of PC981Cu ú PC981Ni ÅPC981Zn indicated that the ionic cluster based onCu2/ had obvious influence on the relaxation ofpolymer matrix. A wider Tg transition band forPC981Cu indicated that Cu2/ can develop an ef-

Figure 2. Dynamic mechanical properties of the sam- fective physical crosslink which restricted theples. movement of the polymer chains. The sample

based on Ni2/ (PC981Ni) exhibited higher soften-ing temperature compared with Cu2/ and Zn2/.

that of PC981H. A new endotherm at 297 K was This good thermal stability behavior of PC981Niobserved on the traces of the ionomers showing was attributed to the strong cohesion within thethat the PTMG-rich phase in the ionomer system ionic domains.possesses a regular array. Increasing the densityof the ionic sites destroyed this regular structure

FTIR Measurementsand restricted the mobility of the polyetherchains. As mentioned above, the properties of the iono-

mers were mainly determined by the ionic micro-structure. The local environment of the ionic ag-Dynamic Mechanical Analysisgregates could be measured by FTIR. Figure 3shows spectral change for the ionomers with dif-Systematic DMTA studies also have been con-

ducted for ionomers.10,11 The typical dynamic me- ferent cation and polyol soft segment. After neu-tralizing the acid precursor, the absorption ofchanical responses of the ionomers synthesized in

this study are shown in Figure 2. Two relaxation O{H stretching (near 3000 cm01) and deforming(at 1455 cm01) which were observed in the acidpeaks can be seen on Tan d curves. The first peak

at lower temperature corresponded to the glass form polymer vanished and the new bands of car-boxylate asymmetric (ga COO0) and symmetrictransition of matrix materials with a small amount

of ionic group and the second peak corresponds (gs COO0) stretching at 1592–1630 cm01 and1380–1460 cm01 appeared, respectively. Manyto the relaxation of large ionic aggregates. The

behavior of these aggregates changed greatly previous works12,13 revealed that the carboxylateasymmetric stretching band of ethylene ionomerafter thermal treatment. Annealing the sample at

2507C for 1 h made the width of the plateau and was significantly sensitive to the type of metalcation, degree of neutralization, temperature,the relaxation temperature of the ionic domain

increase. This result indicated that the ionic clus- aging time, and moisture absorption. In this



Figure 3. FTIR spectra of the ionomers.

study, it was also found that the carboxylate posed of longer polyether (PC981Ni) caused anabsorption at 1592 cm01 which was lower thanasymmetric stretching absorption changed greatly

with the nature of metal ions and the properties that of PC650Ni at 1621 cm01 showing that alonger space chain favored to enhance the cohe-of polymer matrix. Therefore our discussion fo-

cused on this absorption band. For the ionomers sion force in the ionic domain. Increasing the po-larity of the polymer matrix decreased the inter-with different cations, the absorption of asymme-

try stretching were at 1592 cm01 (PC981Ni), 1602 action between the ions, thus the wavenumberof the carboxylate asymmetry stretching band ofcm01 (PC981Co), 1622 cm01 (PC981Zn), and 1641

cm01 (PC981Cu), respectively. Clearly, the coordi- PEC1000Co (1621cm01) was higher than that ofthe PC981Co (1602 cm01).nation structure changed greatly with the variety

of the transition metal ions . Since the gas absorp-tion was determined by the nature of central

SAXS Resultsmetal ions and the ionic aggregates, it is difficultto identify the ionic interaction strength using The FTIR explored the interaction in the Ionic

domain. Furthermore SAXS was utilized to studyFTIR. Apart from the metal ions, the length ofpolymer chain between the ion sites and the polar- the morphology of the ionomers. The desmeared

small-angle x-ray scattering (SAXS) data areity of the polymer matrix also had profound influ-ences on the interaction between the metal ions shown in Figure 4 and the computative results

are given in Table III. In Table III, Fh and Fs areand the carboxylic acid groups. The ionomers com-


804 CHEN ET AL.

on the morphology of the ionomers. The SAXS pat-terns for PC650Cu, PC650Zn, PC981Cu, andPC981Zn were quite different. Zn2/-based ionomersexhibit clear ionomer peaks indicating a well-or-dered ionic domain and a clear phase separation.Large Dr and small lh values proved that the purityof the ionic clusters composed of Zn2/ was high. Incontrast, PC981Cu showed only a weak ionomerpeak as a shoulder on very intense low q scattering,showing that Cu2/ neutralized ionomer could notform a regular ionic aggregate. Considering that lh

and Tg of polyol in PC981Cu were higher than thoseof PC981Zn, it is reasonable to postulate that therewere some polymer chains in the ionic clusters ofPC981Cu. The fraction of polymer chains in theseionic domains was affected by the length of PTMG,because the pattern of the ionomers changed greatlywith increasing the molecular weight of PTMG. Ac-cording to crystalline field theory, Cu2/ has manypotential splitting patterns, the complex based onCu2/ exhibit more different geometry structureFigure 4. SAXS curves of the samples.compared with Zn2/, which show a full sphericalball electron structure. It should be mentioned that

the volume fractions while rh and rs represent the both spectroscopic and SAXS techniques failed toelectron density of two phases. Ih and Is are the identify a geometry, or a coordination number, forinhomogeneity length of the hard and soft seg- the metal-ligand complex. The detailed study of thements. In Figure 4 the peak location gives an indi- local structure of these ionomers was carried out bycation of the interdomain distance (r), the peak extended x-ray absorption fine structure (EXAFS).height was roughly proportional to the contrastbetween phases, and peak sharpness is a measure

EXAFS Analysisof the regularity in domain size or spacing. Theorigin of the upturn is still in controversy. EXAFS is a measure of the oscillation of the ab-

In Figure 4 it was found that, the scattering peak sorption coefficient (m) about its mean value. Datamoved to lower q position which corresponds to a reduction was carried out by a standard proce-higher r value as the polyol molecular weight in- dure.15,16 The single-electron single-scatteringcreased. Since the value of electron contrast be- theory gives the following expression for a relativetween two phases (rh 0 rs), which is proportional amplitude function x(k) for a k-shell absorptionto the volume of per ionic aggregates,14 increased edge,in the same order as the r values, the Dr data forPC981Cu and PC650Cu indicated that the volumeof ionic cluster of PC981Cu was larger than that x(k) Å ∑

j

Njgj

kR2j

Fj(k)sin[2kRj / fj(k)]exp(02k2js

2j )

of PC650Cu. On the other hand, the type of theneutralizing cation was found to have some effects (1)

Table III. SAXS Results

SampleCode fh fs Dr2 1 102 Dr Å rh 0 rs Ip (A) Ih (A) Is (A)

PC650Cu 0.1305 0.8695 4.6925 0.64 16 18 119PC981Cu 0.0472 0.9528 4.4395 0.99 20 21 424PC650Zn 0.1175 0.8825 6.4348 0.79 12 14 102PC981Zn 0.0378 0.9622 5.0362 1.18 15 16 397



Table IV. EXAFS Parameterswhere Nj is the average number of atoms for aparticular element in the shell j which is at an

Sampleaverage radius Rj from the absorption atom. TheCode M2/ Ni Ri (A) sjdamping factor sj is the root-mean-square devia-

tion of Rj due to thermal and static disorder. ThePC650Cu Cu 7.83 1.94 .0095amplitude reduction of each contribution resulted PC981Cu Cu 8.15 1.94 .0083

from the inelastic scattering expressed by gj, PC650Ni Ni 3.93 2.05 .0149which can be approximated as e02Rj/l to express PC981Ni Ni 3.33 2.04 .0142the radius dependence of gj, where l is a mean-free-path parameter. The apparent mean freepath is assumed to be transferable from a particu-

metal ions with oxygen were obtained using CuOlar shell in a model compound to the same shelland NiO as standard samples.of an unknown sample. Fj(k) and Fj(k) are the

The RSF and k3x(k) patterns for PC650Cu areback-scattering amplitude and phase shift func-shown in Figure 5 as representative for the calcu-tions, respectively, and are characteristic of thelated results of the samples. The exact parameterstypes of atoms in shell j and the absorption atom.obtained from EXAFS are listed in Table IV. It isThese functions can be calculated theoreticallyclearly shown that the polyether-ester matrix hasand be obtained experimentally by analyzing alittle influence on the coordination microstruc-model compound of known structure.ture. Both coordination number and the distanceIn this study, the parameters related to thefrom the central metal ions to the surroundingatoms were determined by the nature of the tran-sition metal ions. Although the radii of the Cu2/

and Ni2/ are similar, but the coordination struc-ture composed of these metal ions changedgreatly. From the point of crystalline field theory,Ni2/ in tetragonal structure is stable, thus thecoordination number of the ionomers based onNi2/ was around 4 meaning that there are four Oatoms around the central metal ions. However,the coordination number (Ni) of Cu2/ was about6 / 30% showing that the geometric structure ofCu2/ was approximately octahedral. ConsideringCu2/ exhibits stronger effects on the morphologyof polymer complex than Ni2/. It is postulated thatthe local structure of Cu2/ is in a distorted octahe-dral state.

CONCLUSION

Model ionomers based on polyether, 1,2,4,5-ben-zenetetracarboxylic dianhydride and metal ace-tates were prepared. Neutralizing from the car-boxylic acid to the ionomers led to the formationof solid-state materials and a separate ionicphase. The properties of the ionomer were influ-enced by the choice of polymer matrix, the natureof the metal ions and its composition. The geomet-ric structure of the ionic clusters was determinedby the nature of the central metal ions and hadlittle connection with the surrounding polymer

Figure 5. RSF and k3x(k) profiles of PC650Cu. chains. It is found that the coordination structure


806 CHEN ET AL.

6. E. P. Otocka, M. Y. Hellman, and L. L. Blyler, J.of Ni2/ was in tetragonal state, which exhibitedAppl. Phys., 40, 4221 (1969).good thermal stability. Cu2/ developed a distorted

7. D. C. Lee, R. A. Register, C. Z. Yang, and S. L. Coo-octahedral geometric structure and there wereper, Macromolecules, 21, 998–1004 (1988).some polymer chains in the clusters composed of

8. T. T. Speckhard and S. L. Cooper, Rubb. Chem.this metal ion. Technol., 405–431 (1986).9. A. Lilaonitkul and S. L. Cooper, Adv. Urethane. Sci.

Technol., 7, 163 (1979).10. W. J. MacKnight and T. R. Earnest, J. Polym. Sci.

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ghal, U.S. Patent 4,014,847. 13. K. Han, H. L. Williams, J. Appl. Polym. Sci., 42,3. R. D. Lundberg, Polym. Prep. Am. Chem. Soc. Div. 1845 (1991).

Polym. Chem., 19(1), 455 (1982). 14. Y. S. Ding, R. A. Register, C. Z. Yang, and S. L.4. R. D. Lundberg and H. S. Makowski, Polym. Prep. Cooper, Polymer, 30, 1213–1220 (1989).

Am. Chem. Div. Polym. Chem., 19(2), 287 (1982). 15. P. A. Lee, P. H. Citrine, P. Eisenberger, and B. M.5. R. Jerome and G. Broze, Rubb. Chem. Technol., 58, Kincaid, Rev. Mod. Phys., 53, 769 (1981).

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synthesis and characterization of model polyether-ester ionomers

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