1166

A studyon theeffect of spirocyclicstructuresin themainchainon thephysicalpropertiesof copolyimides

JunLi, Kazuaki Kudo,* ShinsakuShiraishi*

Instituteof IndustrialScience,Universityof Tokyo,7-22-1Roppongi,Minato-ku,Tokyo106-8558,JapanFax.:81-3-3402-6350;E-mail: kkudo@iis.u-tokyo.ac.jp

Intr oductionIn this decade, alicyclic polyimideshaveattractedgrow-ing attention becauseof their enhancedsolubility, color-lessness,high transparency, and low dielectric con-stant.[1–12] They have been employed in an increasingnumberof fields in the electronics industry, in applica-tions suchasalignmentfilms for liquid crystal displays,nonlinear optical buffer layers, or aslow dielectric mate-rials.[13–16] The aboveproperties of alicyclic polyimidesarisefrom reducedinter- andintramolecular chargetrans-fer (CT) interactions.[17–21] However, such a weakenedinteraction should makethe glasstransition temperature(Tg) lower thanthoseof aromaticpolyimides,andthis is amajor drawbackfor alicyclic polyimidesin further appli-cations. Although alicyclic polyimides showing high(A3008C) Tgs havebeenreported,suchthermal stabilitieshavebeenobtained at the sacrifice of solubility.[4 h] Solu-ble andthermally stablepolyimidesarehighly desirable.

Using a novel alicyclic dianhydrideDAn which hasanunsymmetric spiro structure (Scheme1), we recentlyshowed that the synthesis of such polyimides is possi-ble.[22] We assumed that the high solubility of DAn-derivedpolyimides is due to the uniquestructure of thedianhydride, becauseunsymmetryandspirocyclic frame-works in the monomers are independently known to bestructural factors for enhancing the solubility of theresulting polyimides.The high Tgs might also be due to

the bulky and unsymmetric structure of the main chainwhich createsa largesegmentalrotation barrier.

As therehavebeenno reportsdescribing an alicyclicanhydridethat hasboth an unsymmetric andspiro struc-ture, more detailedresearchon the effect of a combina-tion of suchfeaturesis of interestfrom the viewpoint ofthe structure-property relationship of polyimides.In thispaper, we report a systematic investigationof thephysicalpropertiesof copolyimidesof DAn. Thecomonomerusedin this study was c-3-carboxymethyl-r-1,c-2,c-4-cyclo-pentanetricarboxylic acid1,4:2,3-dianhydride (TCAAH),a structural isomer of DAn, which is unsymmetric butdoesnot havea spiro unit.

Experimental part

Materials

DAn wassynthesizedaccordingto the methoddescribedinour previouspaper.[22] TCAAH waskindly providedby JSRCorporation.These dianhydrideswere recrystallizedfromaceticanhydride/tolueneandvacuumdriedat 1108C for 12 hbeforeuse.p-Phenylenediamine(PPD)waspurified by sub-limation. N,N-Dimethylacetamide(DMAc) was purified bydistillation underreducedpressureovercalciumhydrideandstoredover 4 A molecularsieve.Pyridinewasdistilled fromKOH and storedover 4 A molecularsieve.Other reagentsand organic solventswere of commercialgradeand wereusedasreceived.

Communication: A seriesof copolyimideswerepreparedfrom two alicyclic dianhydrides having the commonmolecular formula C10H8O6, rel-[1S,5R,6R]-3-oxabicy-clo[3.2.1]octane-2,4-dione-6-spiro-39-(tetrahydrofuran-29,-59-dione)(DAn) andc-3-carboxymethyl-r-1,c-2,c-4-cyclo-pentanetricarboxylicacid 1,4:2,3-dianhydride(TCAAH),with p-phenylenediamine(PPD) by a conventionaltwo-step procedure.With increasing DAn fractions in thebackbones,thecopolyimidesshowedbetterfilm formabil-ity, enhancedsolubility, increasedglass transition tem-peratures and birefringences, and decreasedaveragerefractive indices.The unsymmetricspiroalicyclic struc-ture of DAn might be responsiblefor theseresults.Films

of copolyimidesobtainedby both thermal and chemicalimidization were fully transparentand were colorlessorpaleyellow dependingon thefilm thickness.

Macromol.RapidCommun.2000, 21, No. 16 i WILEY-VCH VerlagGmbH, D-69451Weinheim2000 1022-1336/2000/1611–1166$17.50+.50/0

Macromol.RapidCommun.2000, 21, 1166–1170

A studyon theeffect of spirocyclic structuresin themainchain ... 1167

Characterization1H NMR spectrawererecordedon a JEOLJNM-LA500 (1H:500MHz) spectrometer. The IR spectraof KBr pelletsandcast films were measuredusing a JASCO IR-700 spectro-meter. Inherentviscositiesweredeterminedusingan Ubbe-lohdeviscometerat a concentrationof 0.5g/dL in DMSO at308C. UV-visible spectraof polymerfilms wererecordedona JASCOUbest-35spectrometer. A Rigakudifferentialscan-ning calorimeterDSC8230anda Shimadzudifferentialther-mogravimetricanalyzerDTG-50wereusedfor thermalana-lyses.The differentialscanningcalorimetry(DSC) wascon-ductedundera nitrogenstreamat a flow rateof 100mL/minanda heatingrateof 308C/min. Thermogravimetric analyses(TGA) werecarriedout undera nitrogenflow of 50 mL/minwith a heatingrateof 108C/min.For thefirst run in DSCandin TGA, thesampleswereheatedto 3008C. After cooling toroom temperature,a secondrun was performedin order tomeasureTgs andotherthermalproperties.Refractiveindiceswere measuredat room temperatureusing a prism couplerequippedwith a He-Ne laser light sourceof 632.8nm andcontrolled by a personalcomputer. The refractive index inthe film plane(nTE) wasmeasuredin the transverseelectricmode,whereasthe refractiveindex in the out-of-plane(nTM)wasobtainedin thetransversemagneticmode.

Synthesisof coPIs

Preparationof copoly(amicacid)s(coPAAs)

The typical procedureis asfollows: In a 30 mL two-neckedround-bottomedflask, 0.5407g (5.0mmol) of PPD and amixture of dianhydridesconsistingof 0.6725g (3.0mmol,60mol-%) of DAn, and 0.4483g (2.0mmol, 40 mol-%) ofTCAAH wereplaced.To this mixture,10mL of DMAc wereadded,andthemixturewasthenmagneticallystirredat roomtemperature.All the solid monomersdissolvedafter 10 min.The solutionwasstirredunderan argon atmosphereat 65–708C for 16 h. The resultingviscoussolutionwascooledtoroom temperature,and aboutone-thirdof the solution wascaston a glassplate and dried overnight undervacuumatroom temperature,then at 508C for 24 h, and at 908C for12h. The resulting transparent,colorless, and flexiblecoPAA(DAn60/TCAAH40/PPD) film was peeled off and

usedfor thermalimidization. The remainderof the reactionmixturewassubjectedto chemicalimidization.

Preparation of coPI

A) Thermal imidization. The above obtained coPAA-(DAn60/TCAAH40/PPD)film was convertedto the corre-sponding coPI(DAn60/TCAAH40/PPD)film by heating at1008C for 1 h, at 2008C for 1 h, andat 2408C for 2 h undervacuum.Theformationof theimide groupwasconfirmedbyIR spectroscopy.

IR (film): 1776and1709(C2O), 1396cm–1 (C1N).B) Chemicalimidization. To the coPAA/DMAc solution,

4 mL of DMAc, 4 mL of aceticanhydride,and2 mL of pyr-idine were successivelyadded.The mixture was stirred atroom temperaturefor 1 h, then heatedat 130–1408C andstirred for another5 h under an argon atmosphere.Aftercooling to roomtemperature,the resultingdeepyellow solu-tion was pouredinto 300mL of acetoneto precipitatethepolymerasa white powder. Theproductwascollectedby fil-tration, washedwith fresh acetoneand then dried undervacuum at 1208C for 24 h to give the copolyimide coPI-(DAn60/TCAAH40/PPD).

IR (KBr): 1776, 1713 and 1696 (C2O), 1360 (C1N),722cm–1 (imide ring deformation).

Resultsand discussion

Synthesisof polyimides

Copolyimides(coPIs) wereprepared by meansof a con-ventional two-step procedure; a ring-opening polyaddi-tion to give coPAA, andthesubsequentthermal or chemi-cal imidization (Scheme1). Four copolyimidesand twohomopolyimidesweresynthesized.The 13C NMR spectraof copolyimides showed eight peaks in the rangefrom165 to 180ppm, where the imide carbonyl carbonsusually resonate, indicating that the copolymerizationproceededsuccessfully. The formation of coPIs werefurther confirmed by IR and 1H NMR spectra.The feedmolar ratiosof the two dianhydridesandDAn mole frac-tions in the coPIs are shown in Tab.1. The DAn mole

Tab.1. Compositions,viscosities,imidization degrees,andfilm propertiesof coPIs.

CoPI Feedmolarratio in % DAn/(DAn +TCAAH) in coPIa)

ginh

dL Ngb� Degreeof imidization in % Film property

DAn TCAAH Chem. Therm. Chem. Therm.

D100 100 0 100 0.79 93 71 Flexible FlexibleD80 80 20 82 0.53 93 –c) Flexible FlexibleD60 60 40 64 0.56 93 88 Flexible BrittleD40 40 60 44 0.55 94 100 Flexible BrittleD20 20 80 26 0.48 93 100 Brittle BrittleD0 0 100 0 0.47 83 100 Brittle Brittle

a) Calculated from 1H NMR spectra (DMSO-d6, at 508C) of chemicallyimidizedcoPIs.b) For chemicallyimidizedcoPIs.c) Thefilm wastoo thick to beexamined.

1168 J.Li, K. Kudo,S.Shiraishi

fractions in coPIs were calculatedbasedon the 1H NMRspectra.They weresomewhatlarger thanfeedDAn frac-tions, suggesting that the reactivity of DAn is slightlyhigherthanthatof TCAAH. Precipitationswereobservedfor D0, D20, and D40 during the chemical imidization,while the other threereaction mixturesremained homo-geneousthroughout the entireimidization process. Inher-ent viscosities of the chemically imidized coPIs rangedfrom 0.47dL/g for D0 to 0.79dL/g for D100(Tab.1).

The degreeof chemical imidization wascalculatedonthebasisof 1H NMR spectroscopy by comparingtheresi-dual NH protonswith all the aromatic protons accordingto the reportedmethod.[23,24] D0 showed a relatively lowdegreeof chemicalimidization probably because of theeaseof precipitationduring theimidization.

On the otherhand,the degreesof thermal imidizationas determinedby IR spectroscopy are also given inTab.1. It wasassumedthat thedegreeof imidizationwas100% for the sample heatedat 3508C for 15min afterheatingat 2408C for 2 h. The degreeof imidization forthethermally imidizedsampleswasestimatedby compar-ing the absorbance ratio of the 1776cm–1 band(mC=O inimide ring) to the 1515cm–1 band(mC=C in benzenering)with that of the aboveexhaustivelyimidized samples.[25]

With increasing DAn units, the degreeof the imidizationtendsto decrease.Flexible films were obtainedfor eitherthermallyor chemically imidizedcoPIs,providedthat theDAn contentexceeds a certain value.Thefilms of chemi-cally imidized coPIs were obtained by a solution-castmethodusingDMSO. After beingdried in vacuoat 508Cfor 24 h, the film waspeeledoff andthe residualsolventwas removed by further vacuum drying at 1308C for12h. The chemically imidized films showed betterflex-ibility thanthethermallyimidizedones.

Propertiesof polyimides

Qualitativesolubility testsshowed that all of the chemi-cally imidized coPIs werewell solubleat room tempera-ture in NMP, DMF, and DMSO. The semi-quantitativesolubilitiesof coPIsaresummarizedin Tab.2. In DMAc,only D40 to D100dissolved.Among these,the solubilityof D40 waslower thanD60 andD80.Becauseof thehighviscosityof thesolution, thesolubility testof D100couldnotbeperformedfor therangeof A0.10g/mL. Thesolubi-lities shownin Tab.2 arequiteconsistentwith theprecipi-tatingnatureof coPIsduring chemical imidization.All thethermallyimidizedcoPIs dissolvedonly in H2SO4.

Scheme1.

A studyon theeffect of spirocyclic structuresin themainchain ... 1169

The resultsfor thermal analyses,DSC and TGA, arealso summarized in Tab.2. Tgs were detected for ther-mally imidized films by DSC exceptfor D100. Chemi-cally imidized samples did not showTgs below 4008C innitrogenby eitherDSCor DMA measurement. Tgs of thecopolyimidestended to increasewith increasingamountsof DAn units in the main chain. An exception wasD0, aTCAAH homopolymer. The Tg of D0 was higher thanthoseof any other coPIs, although the reasonfor this isnot clear. Decomposition temperatures(Td) of coPIs, atwhich a 10% weight losswasobserved, werearoundthevalue generally expected for alicyclic polyimides. Nocorrelation wasobservedbetweenTd andcopolymercom-position.

The refractive indices of coPIs were also studied. In-plane and out-of-plane refractive indices (nTE and nTM)measuredat 632.8nm aresummarizedin Tab.3. Sampleswere prepared by castingcoPAA/DM Ac solutions ontoquartzplates followed by removalof thesolvent andther-mal imidization without peeling the films off the sub-strates.For all coPI films, nTEs were larger than nTMs,indicating that coPI chainswere preferentially alignedinthefilm planes,which is a propertycharacteristic of poly-

imides preparedby means of on-substrate imidization.The birefringences (Dns) of coPIs tended to increase,whereasthe averagerefractive indices (nAVs) decreasedgradually with increasingamounts of DAn units in thebackbones.

Concerning the relationshipbetweenthe microscopicstructureof the polyimidesandtheir physicalproperties,Kim et al. hadreportedthat the refractive index of poly-merfilms is negatively correlatedwith thefreevolumeofthesolid polymer.[26] By assuming thatsucha relationshipholdsfor thecoPIs studiedhere, a structure-propertyrela-tionship for coPIs could be explainedas follows: due tothe unsymmetric and bulky structureof DAn, the DAn-rich polymer chainsshould be prevented from packingclosely, and this causesan increasein the free volumewhich resultsin the decrease of nAV. Sincethe dielectricconstante is relatedto therefractive indexn by theequa-tion e = n2,[12,26] the aboveresult indicatesthat an incor-poration of DAn into the polyimide would be advanta-geousin orderto lower thedielectric constant.

ThetransmissionUV-vis spectraof thecoPIfilms weremeasured.The spectraof thermally imidized coPI filmsandthoseof chemically imidizedoneswerequite similar.All coPI films exhibited cutoff wavelengthsshorter than310nm, asshown in Fig. 1, andwere entirely transparentandeither colorlessor paleyellow dependingon the filmthickness.Thechemically imidized D0 film wastoo brit-tle to allow themeasurementof its spectrum. TheUV-visabsorbing propertiesof coPIswere basically identical toeachother.

In conclusion,theDAn content in thebackboneaffectsvarious propertiesof coPI, which might be attributed to

Tab.2. Solubility andthermalpropertiesof coPIs.

CoPI Solubility in 1 ml DMAcin ga)

Tg

�C

b� Td

�C

0.01 0.10 0.15 0.20 Therm. Chem.

D100 + + –c) 428 430D80 + + + + 317 423 439D60 + + + + 296 421 440D40 + + +– – 291 414 442D20 – 286 418 438D0 – 329 417 436

a) For chemically imidized coPIs.+: soluble; +–: swelling; –:insoluble.

b) For thermallyimidizedfilms measuredby DSC.c) Thefirst run of D100showed 3208C.

Tab.3. Refractive indicesandbirefringencesof coPIfilms.a)

CoPI d/lm nTEb) nTM

c) Dnd) nAVe)

D100 7.6 1.5988 1.5904 0.0084 1.5960D80 6.5 1.6012 1.5925 0.0087 1.5983D60 7.4 1.6001 1.5947 0.0054 1.5983D40 5.7 1.6001 1.5956 0.0045 1.5986D20 5.2 1.5999 1.5963 0.0036 1.5987D0 6.3 1.6003 1.5987 0.0016 1.5998

a) For thermallyimidizedcoPIfilms.b) In-planerefractiveindex.c) Out-of-plane refractiveindex.d) Birefringence:Dn = nTE–nTM.e) Averagerefractiveindex:nAV = (2nTE + nTM)/3.

Fig. 1. SelectedUV-vis spectraof chemically imidized coPIfilms. All films havea thicknessof 20 lm.

1170 J.Li, K. Kudo,S.Shiraishi

its unsymmetric spiro-alicyclic structure.The structure-propertyrelationshipsfound hereshouldbe universal inprinciple,andmight beapplicableto thedesignandmod-ification of otherpolymers.

Acknowledgement:We would like to expressoursinceregrati-tude to Mr. J.-H. He and ProfessorK. Horie, Department ofChemistryandBiotechnology, the University of Tokyo, for themeasurement of refractiveindices.

Received:April 18,2000Revised:July31,2000

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