Macromol.RapidCommun.2000,21,1313–1316 1313

Molecularlatticeresolutionof singlefacetedcrystalsofa thermotropicliquid crystallinepoly(aryl etherketone)usingatomicforcemicroscopy

Jian Zhang, Luxiang Fu, ShanjuZhang,Decai Yang*

PolymerPhysicsLaboratory, ChangchunInstituteof Applied Chemistry,ChineseAcademyof Sciences,Changchun,130022,P. R. ChinaE-mail: dcyang@ns.ciac.jl.cn

Intr oductionSinceatomicforcemicroscopy (AFM) wasdeveloped,[1, 2]

oneof themain motivationsfor it came from thepossibi-lity of imaging atomsand molecules.[3, 4] More recently,AFM hasproven to beof greatutility in thestudy of poly-mer morphology and microstructurein the size rangefrom nanometer to micrometer.[5] Eventhoughonly undervery optimum conditions the high-resolutionimagescanbe obtained, a great deal of literature concernedwithmolecular resolution imagesof polymers,suchasindivi-dual polymer chains,[6 –9] methyl groups,[10–12] and chainfolding of polymer single crystals,[13,14] have beenpub-lished.Comparedwith other methods of surfaceanalysis,AFM hasa much higherlateralresolution, andits samplepreparation is much simpler. Obtaining true three-dimen-

sional imagesof thetopography of surfacemorphologyisanotherbeneficial featureof forcemicroscopy.

Poly(aryl etherketone)s (PAEKs) are one of the mostwell-known seriesof high-temperature,high-performanceengineeringthermoplastics,sotheyareof interestin boththe scientific studiesandthe industrial applications.[15] Itis known that thechains of poly(aryl etherketone)sin thecrystalsadoptan extendedconformation with the pheny-leneringsalternating at anangle of ca.l378 to the planeof thezig-zagbackbone andtheunit cell consists of two-chain orthorhombic packing with a spacegroupof Pbcn-D2h

[14]. In this case,one chain is located in the centerofthe ab projection, and four 1/4 chainsare locatedat thecorners.[16–18]

Communication: The surfacesof single facetedcrystalsof a thermotropicliquid crystallinepoly(aryl etherketone)copolymercontaining chloro-sidegroup are investigatedby atomicforcemicroscope.Themolecularlattice resolu-tion imagesof the singlefacetedcrystalsindicatethat thesurfacesof thecrystalsconsistof molecularchainendsofthe copolymer, and the crystals exhibit orthorhombicpacking with the unit cell dimensionsof a = 0.787 l0.020nm andb = 0.625l 0.010nm, respectively. Thedataarefurtherconfirmedby electrondiffractionresults.

Macromol.RapidCommun.2000, 21, No. 18 i WILEY-VCH VerlagGmbH,D-69451 Weinheim2000 1022-1336/2000/1812–1313$17.50+.50/0

Thefiltered imageof Fig.2a.Thearrowsshowthelateralper-iodicity of theelevations.

1314 J.Zhang,L. Fu,S.Zhang,D. Yang

Quite recently, thermotropic liquid crystallinepoly(aryletherketone) copolymers with both lateralgroup(crystaldisrupting unit) andbiphenyl mesogenic unitsarereceiv-ing considerableattention.[19–23] This new kind of liquidcrystallinepolymerexhibits not only nematic texturesbutalso highly orderedsmectic structures. In our previouspapers,[20–23] we havereportedthe synthesis andthermo-tropic liquid crystalline behaviorof the poly(aryl etherketone)copolymers. In this work, the surfacefeatureofsingle faceted crystalsof the thermotropic liquid crystal-line poly(aryl ether ketone)copolymer with chloro-sidegroup hasbeeninvestigatedby AFM and molecular lat-tice resolutionimagesobtained. According to themolecu-lar lattice resolutionimage,the crystal surfacestructureconsisting of molecularchainendsis determined.

Experimental partThe copolymer usedin this work was synthesizedby anucleophilic substitution reaction[23] of 4,49-biphenol andchloro-hydroquinone with 4,49-difl uorobenzo-phenone,with thefollowing chemical structure.

The number-average molecular weight of the copoly-mer is about5000. Thin films of thecopolymerwere pre-pared by casting a 0.1% solution of the polymer in a40:60 (w/w) mixedsolventof p-chlorophenol/1,1,2,2-tet-rachlorethane onto carbon-coated mica, and the solventwas evaporatedin a vacuumoven. Af ter heatingto theisotropizationtemperature(3608C) for 5 min to eliminatepreviousthermalhistory, the films wererapidly cooled tothe liquid crystaltransition temperature (330–3208C) forannealing and then quenchedto room temperature. Thefilms with single facetedcrystalsweredirectly observedby AFM, while for transmission electron microscope(TEM) observations,thefilms were stripped, floatedontothesurfaceof water, andthencollectedoncoppergrids.

TheAFM imageswereobtainedusing a NanoscopeIIIatomicforcemicroscope(Digital Instruments, SantaBar-bara,CA, USA) operatedin the contact modeat ambientconditions.Si3N4 tips attachedto 200lm cantileverswitha spring constant of 0.06 Nm–1 were used.The imageforce is about3 nN. Micrometer-scaleandmolecular lat-tice resolution imageswere obtained with 100 lm and1lm piezoelectric scanner heads,respectively. Scanningline frequencieswere1Hz for largescaleandup to 60Hzfor smaller areas.In mostexperiments, the sampleswerescannedor rotated in different directions, and imageswere taken repeatedly. This provides an importantapproachto verify thereliability of images.

The electrondiffraction (ED) patterns were observedvia a JEOL 2010 TEM using a 200-kVaccelerating vol-tage.

Resultsand discussionFig. 1 showsa height imageof thesurfacemorphologyofthe thin films of thepoly(aryl ether ketone)copolymer inthe micrometer scale,which indicatethat the films con-sistof single facetedcrystals.Cross-sectionalprofile ana-lysis of thecrystalsin the heightimageindicatesthat thesingle layer thickness of the crystalline lamellae isapproximately20nm. Thethicknessis in good agreementwith the lengthof molecularchains, which canbe calcu-lated from the number averagemolecular weight of thecopolymer. Basedon theabovefact, we postulatethat theextendedmolecular chainsin the facetedcrystalsareper-pendicular to the film planeandthe chain endsform thesingle crystal surfaces.This assumption is further con-firmed with theAFM high resolution andelectrondiffrac-tion results(seebelow).

When the scansize is decreased, a regular patternofround-shapedelevations is revealedat highermagnifica-tion, which correspondsto the packingof the moleculesin thesurfaceof thecrystals(Fig. 2a).Thelateralperiodi-cities of the elevationsarevery prominentin someareasof the figure, particularly in the center part. The two-dimensional Fouriertransformshown in Fig. 2b indicatesthatthecrystalspossesstheorthorhombic symmetry.

After Fourier filtered, a molecular lattice resolutionimage consisting of round-shapedelevations or hills is

Fig. 1. AFM height imageof the singlefacetedcrystals of thecopolymer.

Molecularlatticeresolutionof singlefacetedcrystalsof a thermotropic ... 1315

clearlyobserved (Fig. 3). Unambiguously, thehills shouldrepresentthe copolymerchainends.The crystalsexhibittwo-chain orthorhombic packing and the periodicitiesmeasuredfrom theAFM imagecorrespondto thedimen-sionsof the crystal unit cell, i. e., a = 0.787l 0.020nmand b = 0.625l 0.010nm, respectively. In addition, thesurprisingly regularstructureobserved in thehigh resolu-tion imagesuggeststhat the chain extendedcrystal sur-face is fairly regularand that the polymer chains shouldbeof fairly uniform length.

The aboveresultshavebeencompletely confirmed bytheelectrondiffraction experiment. TheED patternof thecrystalsof the copolymeris shown in Fig. 4. The strong(hk0) reflections indicate that the single facetedcrystalsexhibit orthorhombic packing and possessvery goodcrystallographic orientation with the c axes (the chain

direction) perpendicular to the film plane. The unit cellparameters calculated from the ED pattern are a =0.766 nm andb = 0.612nm, respectively, which arecon-sistentwith thedataobtainedfrom theAFM (Fig. 3).

To sumup, thesinglefaceted crystalsareformedin thethin films of the copolymer which is heat-treatedat theliquid crystal transition temperature(330–3208C). TheAFM andED resultsprovethat the facetedcrystalsexhi-bit orthorhombic packing, with the molecular chains per-pendicular to the film planeand the crystal surfacecon-sisting of molecular chainends.

Acknowledgement: The financial supportsof the NaturalScienceFoundationof China and the National Key Projectof Fundamental Research, “Macromolecular Condensed

Fig. 2. UnfilteredAFM high resolutionimageof thesurfaceof the facetedcrystals (a) andcorrespond-ing two-dimensionalfastFouriertransformedspectrum(b).

Fig. 3. The filtered imageof Figure2a. The arrows showthelateralperiodicityof theelevations.

Fig. 4. ED patternof thesinglefacetedcrystalsof the copoly-mer.

1316 J.Zhang,L. Fu,S.Zhang,D. Yang

State”, the State Scienceand TechnologyCommissionofChinaaregratefullyacknowledged.

Received:January 31,2000Revised: August9, 2000

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