Synthesis and characterization of novel crosslinkable side-chainliquid crystalline polyesters

Min Li* 1, Hongjin Qiu1, Xinfang Chen1, Enle Zhou2, Fengying Jing2

1 Department of Materials Sciences, Jilin University, Changchun 130023, People’s Republic of China2 Polymer Physics Laboratory of Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,Changchun 130022, People’s Republic of China

(Received: May 20, 1998; revised: August 26, 1998)

SUMMARY: Crosslinkable side-chain liquid crystalline polyesters PCn from N-[n-(4-(4-nitrophenylazo)phe-nyloxy)alkyl]diethanolamine (Cn, n = 3, 5, 6, 10) as mesogenic monomers and maleic anhydride were syn-thesized and characterized. The thermal properties of PCn’s were studied by means of DSC, polarized opticalmicroscopy (POM) and wide angle X-ray diffraction (WAXD), and the results showed that all the polymersstudied exhibit enantiotropic liquid crystallinity. In the molar mass independent region, the relatively highcontent ofcis1CH2CH1 groups in the polymer backbone of PC3 causes an increase of the melting tempe-rature (Tm) and a decrease ofTg and isotropisation temperature (Ti). The crosslinking of PCn in the radicalpolymerization with styrene was confirmed by FTIR spectroscopy. The absorption band at 1300 cm–1 attribu-ted to the in-plane C1H-bending vibration oftrans1CH2CH1 in the polymer backbone disappeared aftercrosslinking, indicating that thetrans1CH2CH1 functions are consumed in the crosslinking polymeriza-tion of styrene.

IntroductionRecently, much attention has been paid to liquid crystal-line elastomers due to their potential promising applica-tion in nonlinear optics and optical information storagefields1). There are three kinds of liquid crystalline elasto-mers (LCEs), side-chain LCEs, main-chain LCEs andcombined LCEs2, 3). Among them, side-chain LCEs aremost important due to their potential application in infor-mation storage fields. It is important therefore to designand synthesize novel side-chain liquid crystalline poly-mers (SCLCPs) suitable for crosslinking. Usually, twokinds of synthetic methods are used for preparing cross-linkable SCLCPs, one is radical copolymerization ofmesogenic monomers containing vinyl groups4–6) withnon-mesogenic monomers by which groups suitable forcrosslinking are introduced, and the other is solution ormelt polymerization of diols or malonate-containingmesogenic groups with non-mesogenic difunctionalmonomers7–9). The latter method, as a versatile syntheticway, could be widely used to obtain different types ofSCLCPs such as polyethers10) and polyesters11) etc. How-ever, this method is not commonly used to synthesizeSCLCPs12–15). By now, no report about the synthesis ofany crosslinkable SCLCPs by polycondensation of diolswith unsaturated acids or unsaturated anhydrides wasfound.

We report here the synthesis of SCLCPs withp-nitro-azobenzene as mesogenic groups by the melt polyconden-sation of diols and maleic anhydride. Vinyl groups wereintroduced into the polymer backbone by using maleicanhydride as a monomer. The thus obtained side-chain

liquid crystalline polyester might be used to synthesizevarious LCEs by using them in the radical polymerizationof St, MMA and MA as crosslinking reagents, or by heat-ing or radiation procedure. Uniformly oriented liquidcrystalline monodomain LCEs might be obtained byorientation through electric, magnetic and mechanicalfields and then fixing the oriented structure by crosslink-ing reaction.

In the present paper, synthesis and characterization ofcrosslinkable side-chain liquid crystalline polyesters arereported. The effect of the length of the flexible spacerand of the configuration of the polymer backbone on thephase transition temperature is discussed.

Experimental part

Monomer synthesis

4-(4-Nitrophenylazo)phenol (A):The synthesis of compoundA has been described elsewhere16), m.p. 2198C.

n-(4-(4-Nitrophenylazo)phenyloxy)alkyl bromide (Bn):These were successfully synthesized by the Williams etheri-fication as described in a separate paper17).N-[n-(4-(4-Nitrophenylazo)phenyloxy)alkyl]diethanolamine

(Cn): 20 mmol of Bn and 100 mmol diethanolamine weredissolved in 200 ml of ethanol, and the mixed solution wasrefluxed for 10 h. After the reaction, the mixture was concen-trated by evaporation to remove the solvent ethanol. Theresidne was dissolved in chloroform and then transferredinto a separating funnel. The chloroform layer was separatedfrom excess diethanolamine, dried and evaporated. Thecrude product was crystallized from acetonitrile, then dis-

Macromol. Chem. Phys.200, No. 4 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 1999 1022-1352/99/0404–0834$17.50+.50/0

834 Macromol. Chem. Phys.200,834–839 (1999)

Synthesisandcharacterization of novelcrosslinkableside-chainliquid crystalline polyesters 835

solvedin a mixtureof CH3OH/C2H5OH (vol. ratio 1:1). Thesolution was filtered and the solventevaporated. The pro-ductwasrecrystallizedfrom acetonitrile.

ThespectroscopicdataaregivenusingC6asanexample.IR (KBr): 3363(bd,1OH, ms); 2931,2855(1CH21, ms );

1593,1500,1458(aromatic,ms); 1514,1335(1NO2, ms, mas);1251;1135,1101;1037,988;851(1C1N1, ms ); 749,722(1CH2, ms); 682,583,540cm–1.

1H NMR (400 MHz, CDCl3): d = 8.35 (d, 2H, Ha), 7.95(crossdoublets,4H, Hb), 7.02 (d, 2H, Hc), 3.62 (mult. 4H,Hk), 2.66(t, 2H, Hj), 2.54(t, 2H, He), 2.50(s,2H, OH), 1.82(p, 2H, Hg), 1.52(t, 2H, Hf), 1.35l1.4(m, 4H, Hh).

Polyester(PCn)synthesis

The mixture of monomersCn (5 mmol) and maleic anhy-dride (5 mmol) was heatedslowly in a four-neckedflaskundernitrogen.After melting, the mixture wasstirredvigor-ously andmaintainedat the melting temperatureuntil gela-tion occurred.Thereactionmixturewasallowedto cool,andthe polymer was dissolvedin tetrahydrofuran(THF). Thesolution was filtered, and the polymer was precipitatedbythe additionof petroleumetherand filtered off. Finally, thepolymerwasdriedundervacuumat roomtemperature.

IR (KBr): 3437(bd,1OH, ms ); 2938,2857(1CH21, ms);1725 (1C2O, ms); 1600, 1580, 1500, 1453 (aromatic,1C1C, ms); 1522,1343 (1NO2, ms, mas); 1256; 1139 (1C1O, ms); 1106; 1041; 1640 (1C2C, ms); 860 (1C1N1, ms);830(Ar1H, ms); 754,720(1CH2, ms); 686cm–1.

UV/vis (in solid state):356nm,256nm.

Crosslinkingreactionof PCn

Crosslinkingreactionof PCn wascarriedout in chloroformsolutionusingstyreneascrosslinkingreagentandBPO-N,N-dimethylaniline as initiation system at room temperatureundernitrogenatmosphere for 8 h.

Characterizationmethods1H NMR spectrawere recordedat room temperatureon aVarianUnity 400spectrometer(400MHz) usingchloroform(CDCl3) assolventandtetramethylsilaneasan internalche-mical shift reference.IR spectrawererecordedwith a BIO-RAD FTS-7spectrometer. SampleswerepressedtabletswithKBr. UV-visible spectrawere recordedwith a Shimuduzu-240UV-visible spectrometer. Thenumber-averagemolecularweight (M

—n) aswell asDP weredeterminedby the terminal

groupmethodbasedon theresultsof 1H NMR spectroscopy.Thermaltransitionswere studiedby a Perkin-ElmerDSC-7differential scanningcalorimeter. The calibration was per-formedwith indium.Weightsof sampleswereca.10 mg.Allthe sampleswere treatedbefore measurementas follows.Each samplewas heatedto its liquid crystalline stateand

maintainedat that temperaturefor 10 min, thenquenchedto08C. Thethusobtainedsamplewasthenstudiedby DSCat aheatingandcooling rateof 108C/min. Optical textureswereobtainedby usinga Leitz (Wetzlar)opticalpolarizingmicro-scopeequippedwith ahot stageandanOptionR Pol camera.Sampleswere cast on a glassslide from a 1% solution ofchloroform to form a thin film and were then coveredbyanotherglass slide. Wide angle X-ray diffraction spectrawererecordedon a RigakuD/MAX-rA (RigakuCo., Japan)X-ray diffractometerwith nickel-filtered Cu-Ka radiationatroomtemperature.

Resultsand discussion

Synthesisof monomersandpolymers

The synthetic route to novel side-chain liquid crystallinepolyestersis given in Scheme1. The difunctional diolsCn were synthesized throughthe reactionbetweenn-(4-(4-nitrophenylazo)phenyloxy)alkyl bromides (Bn) anddiethanolamine in ethanol for 10 h. The appearance of abroadbandat 3363 cm–1 in the FTIR spectraof Cn con-firmed the existence of 1OH groups. In the 1H NMR

spectra of Cn, thereappearedpeaks at d = 3.62,2.66,2.4,2.50 ppm corresponding to the proton absorption of Hk,Hj, He and hydroxyl groups of Cn. Thus, the proposedmolecularstructureof Cn wasconfirmedby 1H NMR andIR spectroscopy.

The thermal transitionsof Cn were studied by DSC,WAXD andPOM,andtheresultsshowedthatall thestu-diedCn showedliquid crystallinebehavior. C3 is amono-tropic smectic liquid crystal, it melts at 1268C, while

Scheme1: Syntheticroute to novel side-chain liquid crystal-li nepolyesters(PCn)

836 M. Li, H. Qiu, X. Chen,E. Zhou,F. Jing

upon cooling, it turns into smectic A phaseat 1198C,with anenthalpyof transition of 3.09 kJ/mol,andcrystal-lizes at 1158C. Cn (n = 5, 6, 10) showed enantiotropicliquid crystalline behavior. C5, C6, C10 melt andexhibitSA stateat 89,67 and708C, theybecomeisotropic at 125,119 and1148C, with enthalpiesof transitionof 4.8, 4.41and 5.37 kJ/mol, respectively; upon cooling, they firstenterSA stateat 124,117 and1118C, with enthalpiesoftransitionsof 5.38,4.48 and6.06 kJ/mol, andcrystallizeat 68, 62, 598C, respectively. Detailed identification ofthe liquid crystalline types in Cn was given in anotherpaper17). So,wedon’t intendto discussthemfurther.

Polycondensation of Cn andmaleicanhydride wascar-ried out under a nitrogen atmosphere. IR spectrumofPC6exhibited absorption bandsat 1725cm–1; 1640cm–1;1601, 1580, 1444, 830 cm–1, 1521, 1344 cm–1; and860cm–1, indicating the existenceof 1C2O, 1,4-substitutedaromaticrings, 1NO2, and 1C1N1, respectively. The1H NMR spectraof PC6,aswell astheattributionsof thepeaks,aregivenin Fig. 1. Chemicalshift at d = 6.22ppm(protonsof cis 1CH2CH1) andd = 6.82 ppm (protonsof trans 1CH2CH1) again confirmed the proposedmolecular structureof PCn and also indicated the exis-tenceof cis- and trans-configuration of 1CH2CH1 inthe polymer backbone. Chemical shift at d = 3.60 ppmwas attributed to the absorption of proton of CH2 groupneighboring to 1OH terminal groupsat eachmacromole-cular chain end.According to the definition of number-averagedegreeof polymerization, it was determined bythe following equation basedon the analysis of terminalgroups:

DP� �Ac � Ah�i

cis � Ah�itrans�=2

Ae9=4�1�

whereAc, Ah, Ai andAe9 aretheintegralareasof protonsc,h, i, and e9, respectively. The number-average degreeofpolymerization, the number-average molecular weightand the ratiosof trans to cis configuration of vinylidenegroups(1CH2CH1) in thepolymerbackbone aregivenin Tab.1.

Thermalphasetransitions

DSCcurvesof PCn’s in thecourseof heating andcoolingareshown in Fig. 2. It canbe seenthat all the PCn poly-mersshowtwo endothermicpeakscorrespondingto crys-talline-liquid crystalline and liquid crystalline-isotropic

Fig. 1. 1H NMR (CDCl3) spectrum of PC6

Tab. 1. Molecular structure and number-averagedegree ofpolymerizationof PCna)

PCn n DP M—

n trans/cis

PC3 3 10.9 2858 1:1PC5 5 32.6 8684 2.3:1PC6 6 23.8 6540 2.5:1PC10 10 38 11438 2.2:1

a) DP: number-averagedegreeof polymerization;M—

n: number-average molecularweight; trans/cis: ratio of trans to cis con-figuration of 1CH2CH1 in thepolymer backbone.

Fig. 2. DSCcurvesof PCn’s duringthesecondheating (a) andcooling(b) cycles(108C/min)

Synthesisandcharacterization of novelcrosslinkableside-chainliquid crystalline polyesters 837

phasetransitions. Upon cooling, all the PCn polymersshow two phasetransitions which are attributed to theisotropic-liquid crystalline phasetransitionand the glasstransition. Polarized optical microscopy (POM) studyrevealedthat, in the course of cooling, PCn’s exhibitliquid crystalline texture that remainsunchanged untilroom temperature.The textureof PC6 in its liquid crys-talline stateis givenin Fig. 3. Thus,it wasconfirmedthatPCn’s showenantiotropic liquid crystallinebehavior. Thephasetransitionsof PCn’saregivenin Tab.2.

The effect of molar masson the phasetransition tem-peraturehas been investigated by many researchers18).The resultsshowedthat the phasetransition temperaturebecomesindependentof molar massif thedegreeof poly-merization is higherthan10. In the molar massindepen-dent region however, the phasetransition temperaturedependson thestructureof thepolymer backboneandtheflexible spacer. In themeasuredmolar massregionshownin Tab.2, it wassupposedthat the transition temperatureis independent of molar mass.Tm of PCn’s (n = 5, 6, 10)increasedslightly with the increaseof the length of thespacer(n) andTi decreased slightly with theincreaseof n.This can be understoodif we assumethat the hindranceof thepolymer backboneto thearrangementof mesogensdecreaseswith the increaseof n for polymers PCn (n = 5,6, 10) with the samebackbone, i. e., sameratiosof trans

to cis 1CH2CH1 (trans/cis = (2.2l2.5):1). For PC3,on theother hand,thehindranceof thepolymerbackboneto thearrangementof mesogens is highercompared to theother PCn’s with longer spacers, which should causeadecreaseof Tm; however, the relatively high content ofcis-configuration (trans-/cis- = 1:1) of 1CH2CH1fovours the regulararrangementof the mesogens,whichshould causean increaseof Tm. Among the abovetwoopposite effects of spacerand the polymer backboneonTm of PC3,the effect of configurationof doublebond inpolymerbackbone wasdominant,soTm wasmuchhigherthan that of PCn’s. The much lower Ti of PC3might bedue to the relatively high contactof cis-configuration ofdouble bonds in the polymer backbone, which madethepolymerbackbonemore flexible andthusTi decrease.

Crystallinepropertiesof PCn’s

Fig. 4 shows the WAXD spectraof PCn’s at room tem-perature.All the polymersshowed diffractions at d-spa-cings of 0.52 nm,0.35nm and0.30nm despitethediffer-ent lengths of spacerand different ratios of trans to cisconfiguration of 1CH2CH1 in the polymer backbone.Fromtheseresultsandalsothoseof thepreviousstudyonpolyacrylatescontaining the samemesogens19,20), we caninfer that PCn’s are semicrystalline polymers and thecrystalline regions are due to the arrangement of themesogens.Apart from theabovementionedthreediffrac-

Tab.2. Thermal phasetransitionsof polyestersPCna)

Polymer PCn Spacern Thermal transitionalproperties

heating cooling

PC3 3 Tg 30.5K 61.4(12.3)Sm86.2(2.89)I I 84.8(–4.68)Sm41.0Tg

PC5 5 Tg 33.2K 42.4(8.55)Sm118 (2.81)I I 106Sm28Tg

PC6 6 Tg 35.6K 48.7(12.8)Sm118.6(4.09)I I 108(–6.34) Sm25Tg

PC10 10 Tg 30.7K 49.5(12.9)Sm114.5(4.97)I I 95.6(–5.69)Sm27.6Tg

a) Key: Tg, glasstransition; Sm, smectic; I, isotropic; transition temperatures( 8C) and enthalpies of transitions(kJ/g, in paren-theses).

Fig. 3. Optical texture of PC6 in its liquid crystalline state(6160)

Tab. 3. d-spacings(d1) and the calculatedlengths(L) of sidechainsof PCn’sa)

PCn n d1/nm L/nm Angle in 8

PC3 3 1.437 1.72 33PC5 5 1.615 1.97 33PC6 6 1.670 2.09 37PC10 10 20.553 2.59 37

a) d1: d-spacingmeasuredfrom WAXD spectra,L: the calcu-lated lengthof the mesogenscontaining the flexible spacersassumingthat the spacersare in all-trans configuration,anglesarebetweenthemesogensandthenormalto the layerplaneformedby themesogens.

838 M. Li, H. Qiu, X. Chen,E. Zhou,F. Jing

tion peaks, PCn’s also exhibited diffraction peaks atlower diffraction angles,dependingon the length of thespacers.d-spacings of PCn’s at lower diffraction angles(d1) andthecalculatedlengthsof sidechains(L) contain-ing flexible spacersaccording to standardbond lengthsandbondanglesassuming that thespacersarein all-transconfiguration are summarized in Tab. 3. Obviously, d1

increasedwith theincreaseof n, indicatingthatmesogensform a layeredstructure and the flexible spacerpartici-patesin the formation of the layer. The longerthespacer,the lower the hindranceof the polymer backboneto thearrangement of themesogens,sothehighertheregularityof the layered structureand thus the higher the intensityof the diffraction. According to the measuredd-spacingsand the calculated lengths of side chains, it could bededucedthat themesogensarranged themselvesin layersand formed a tilt angleof 33l378 along the normal tothelayer plane.

WAXD study of the annealedand unannealed PC6showsthat thermal annealing causesanincreaseof all thediffraction peaks,a decreaseof the amorphous diffusepeak,and an increaseof the sharpnessof the diffraction

peaks,which indicatesthatthecrystallinity, thecrystallitesizeaswell astheregularity of thearrangementof meso-gens increase.The crystallite size was calculated usingtheScherrerequation21):

Lhkl = 57.3 Kk/b cosh (2)

whereLhkl (A) is the meandimension of crystallitesper-pendicular to theplaneshkl, K theshapefactor (K = 0.9),k theX-ray wavelength and2h theBraggangle.b2 = B2 –b0

2, whereB is themeasuredhalf-width of theexperimen-tal profile (degree),b thepurebroadening profile (degree)andb0 the instrumental broadening factor (degree),whichis 0.150 in our calculation. According to the Scherrerequation, the crystallite size perpendicular to the crystalplane at lower diffraction angles of PCn is listed inTab.4. It is obvious that the crystallite size along themesogeniclayersincreasesdueto thermal annealing.

Crosslinkingreaction

The crosslinking reaction of PC6 was studied by FTIRspectroscopyin the range of 2000–500 cm–1 beforeandafter crosslinking. According to the known spectroscopicdata, the wavenumbersat 1410 and686 cm–1 areattribu-ted to the in-planeandout-of-planebendingvibrationsof1C1H of the cis-isomer of 1CH2CH1, while that at1300 cm–1 is due to the in-planevibration of 1C1H oftrans 1CH2CH1. FTIR studies showedthat the inten-sity of thebandat 1300cm–1 decreases largely during thecrosslinking reaction, while the other bandsat 686 and1410 cm–1 remainunchanged. So, the FTIR resultscon-firmed that the crosslinking reactiontakesplacebetweenthe trans1CH2CH1 groups.

Acknowledgement: The project was financially supportedbytheNationalNatural ScienceFoundation of ChineseCommittee.

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Tab.4. Crystallite sizeof PCn’s beforeandafter annealing

PolymerPCn Beforeannealing After annealing

h in 8 B in 8 b in 8 Lhkl/nm h in 8 B in 8 b in 8 Lhkl/nm

PC3 3.08 0.208 0.144 55.3PC5 2.66 0.354 0.321 24.8 2.73 0.208 0.144 55.1PC6 2.58 0.313 0.270 29.0 2.65 0.292 0.252 31.8PC10 2.13 0.500 0.477 16.7 2.13 0.396 0.366 21.7

Fig. 4. WAXD spectraof PCn’s at roomtemperature

Synthesisandcharacterization of novelcrosslinkableside-chainliquid crystalline polyesters 839

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