Microstructure and interface in organic/inorganic hybrid composites

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Microstructure and Interface in Organic/Inorganic Hybrid CompositesChang-Sik Ha* and Won-Jei ChoDepartment of Polymer Science and Engineering, Pusan National University, Pusan 609-735, KoreaABSTRACT WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWIn this article, the characterization of the micro-structure and interface of hybrid composites is discussed.Poly(p-phenylene biphenyltetracarboximide) was used asa matrix polymer and tetraethoxysilane was a precursorof silica. Polyimide/silica hybrid composites were pre-pared by solgel reaction and thermal imidization.Interfacial interaction as well as microstructure inpolyimide/silica hybrid composites were well character-ized by atomic force microscopy topology and small-angleX-ray scattering measurements. In addition, fluorescencespectroscopy was successfully applied in the studies toreveal the interfacial interaction in the hybrid systems.Copyright 2000 John Wiley & Sons, Ltd.KEYWORDS: hybrid composite; polyimide; silica; mi-crostructure; interface; atomic force microscopy; small-angle X-ray scattering; uorescence spectroscopyINTRODUCTION WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWOrganic/inorganic hybrid composites with a poly-mer matrix are of considerable interest andimportance since they frequently provide optimalcombinations of properties from inorganic andpolymer components [15]. In these hybrid compo-sites, aromatic polyimides have been considered tobe suitable matrix polymers for advanced techno-logical applications in the microelectronics andaircraft industries, because they possess excellentchemical, physical, thermal and mechanical prop-erties owing to the phenyl and imidemoieties of thebackbone [612]. In addition, silica has been mostextensively investigated as an inorganic compo-nent because of the expected interesting catalyticand electronic applications [6].Since thin films are required in microelectronicand photonic/optical applications, a differentprocessmust be applied instead of the conventionalcomposite preparation technique used for carbonor glass fiber reinforced plastics. One reasonablygood approach is the solgel process, which canproduce particles of small size finely dispersedthrough in situ polymerization of monomericprecursors. In particular, an important advantageof the solgel synthesis route for polyimide/silicacomposites is that the poly(amic acid) organicmatrix acts to prevent agglomeration of the silica,which can lead to nanometer scale silica clusters inthe composites or, as often stated, nanocompo-sites. During the solgel reaction of metal alk-oxides such as tetraalkoxy silane, polyimide filmsare formed by spin-casting solutions containing thesoluble poly(amic acid) precursor, followed bythermal imidization.Previous works on hybrid composites havedemonstrated that the properties of the polymermatrix are strongly influenced by the presence ofinorganic materials [1317]. For instance, in poly-imide/silica hybrids both the glass transition andCopyright 2000 John Wiley & Sons, Ltd.Received 12 September 1999Accepted 10 December 1999POLYMERS FOR ADVANCED TECHNOLOGIESPolym. Adv. Technol. 11, 145150 (2000)* Correspondence to: C.-S. Ha, Department of Polymer Science andEngineering, Pusan National University, Pusan 609-735, Korea, e-mail: csha@hyowon.pusan.ac.kr This paper was presented at the 2nd International Symposium onHi-tech Polymers and Polymeric Complexes (HPPC-II), ZhengzhouUniversity, China on 1319 September 1999.Contract/grant sponsor: SEOAM Scholarship Foundation.thermal decomposition temperatures increase withincreasing silica content [13]. In addition, anincrease of the silica content causes an increase ofthe density, storage modulus and linear thermalexpansion coefficient [14]. Though those propertiesof such hybrid systems are believed to be stronglydependent on the microstructure of the compositesand interface between polymer matrix and inor-ganic particles, few works have dealt with thesubject. The subject has been one of our mainresearch goals in this laboratory for years [1720](C.S. Ha, H.D. Park and C.W. Frank, unpublished;C.S. Ha and C.W. Frank, unpublished). Microstruc-ture of the composites has been usually explored bydirect observation using electron microscopy.Various experimental techniques can be appliedto investigate the microstructure and interface ofhybrid composites, in addition to the electronmicroscopic observation. Recently, we used small-angle X-ray scattering (SAXS), atomic force micro-scopy (AFM), and fluorescence spectroscopy, aswell as a conventional scanning electron micro-scopy (SEM) to investigate the subject especially forthe polyimide/silica hybrid system [17, 19]. Inparticular, the application of fluorescence spectro-scopy to interpret the interface in the hybridcomposites is currently attracting wide researchinterest in this laboratory [18] (C.S. Ha, H.D. Parkand C.W. Frank unpublished; C.S. Ha and C.W.Frank, unpublished). Fluorescence spectroscopy isknown to be very useful in the investigation of thedynamics and structure of solid polymers and iscomplementary to the X-ray methods [21]. Fluor-escence spectroscopy has been widely used in thestudies to reveal the molecular orientation ofpolyimide per se and to see the effects of curingconditions on the thermal imidization of poly(amicacid)s [2230].In the present paper, our recent works on themicrostructure and interface of polyimide/silicahybrid composites are reviewed. Poly(p-phenylenebiphenyltetracarboximide) (BPDA-PDA)/silicasystem was selected as a model hybrid composite.EXPERIMENTAL WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWSample Preparation3,3',4,4'-Biphenyl tetracarboxylic dianhydride(BPDA, TCI Chemicals) was recrystallized andvacuum-dried at 200C for 24h before use. p-Phenylene diamine (PDA; Aldrich) and anhydrousN-methyl pyrrolidinone (NMP; Aldrich) were usedas received. Tetraethylorthosilicate (TEOS) wasobtained from Aldrich and was used as received.The BPDA-PDA polyamic acid was polymer-ized by adding an equimolar amount of BPDApowder into the NMP solution of PDA withcontinuous stirring at room temperature for severalhours. The preparation of the BPDA-PDA poly-imidesilica hybrid films is shown in Fig. 1. Variousquantities of TEOSwith water and HCl as a catalystwere then added into the poly(amic acid) solution(15wt%). The TEOS content was 5, 10, 15, 20, 25, 30and 50wt%. The heterogeneous solution wasstirred for 1 day until the solution became homo-geneous. The silica sol/PAA solutions preparedwere spin-coated onto glass substrates (CorningCo.), followed by soft-baking at 80C for ca. 4hr.These soft-baked films were thermally imidized inan oven under nitrogen atmosphere. The imidiza-tion was done at 350380C for 11.5hr in the oven.Figure 2 shows the chemical structure of BPDA-PDA PAA precursor and resultant PI via thermalimidization process.Characterization of Microstructure and InterfaceMicrostructure of hybrid thin films were examinedFIGURE 1. The preparation of polyimidesilica hybridlmsFIGURE 2. Chemical structure of BPDA-PDA poly(amicacid) precursor and resultant polyimide via thermalimidization process.Copyright 2000 John Wiley & Sons, Ltd. Polym. Adv. Technol., 11, 145150 (2000)146 / Ha and Choby AFM (Seiko SPA300), SAXS and fluorescencespectroscopy. For AFM measurements, the canti-lever used in the present study was V-shaped,mounted at the end of a quadrangular pyramidSi3N4 microtip (Olympus). The bending springconstant of the cantilever was 0.022N/m. Samplesfor AFM measurements were prepared as follows:polyimide precursor solutions were spin-coatedonto Si wafers, which were cut into 5mm 5mmsquare pieces, precleaned with acetone, and finallydust blown off prior to use. The subsequent soft-baking and thermal imidization processes were thesame as previously described. AFM images wereobtained in two-dimensional (2D) or three-dimen-sional (3D) color graphics. For SAXS measure-ments, composite films were cut into squares of20mm 20mmusing a blade, and stacked togetherto a total thickness of ca. 150m. For multistackedfilms, SAXS measurements were conducted intransmission geometry in which the scatteringvector is parallel to the film plane. Measurementswere carried out using the 10m SAXS system atOak Ridge National Laboratory (USA) with a pinhole (1 mm2) collimator and pyrolytic graphitemonochromatized CuKa radiation souce operatedat 40keV and 50mA. The average SAXS intensityprofiles were Lorentz-corrected. Details of the AFMand SAXS measurements are described elsewhere[17, 20, 31].For fluorescent spectroscopic measurements,the emission and excitation spectra of the compo-site films were measured at room temperatureusing a fluorescence spectrometer (FS 900CDTspectrometer, Edinburgh Instruments).RESULTS AND DISCUSSION WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWConventionally, the microstructure of a compositeis easily characterized by SEM. For instance, Fig. 3shows a typical SEM micrograph of the hybridcomposite prepared with 30wt% of TEOS loading.One can see that silica particles in the composite arespherical. The average size of silica particles in thecomposite was less than 0.5m, indicative ofsubmicrocomposite or nanocomposite formation.A nanocomposite was obtainable up to 30wt% ofTEOS loading in the polyimide matrix. The SEMmicrograph provides a direct visualization of thestate of silica particles in a polyimide matrix butcannot give information on the interfacial interac-tion in the composite system, especially nanocom-posite system, because of its limited magnification.The AFM characterization serves well for thispurpose in polyimide/silica composites, althoughit is a surface visualization technique and is not forbulk morphology [17, 31].A typical AFM image of the polyimide/silicacomposite with 30wt% TEOS loading is shown inFig. 4 [17]. The scan area and reference force were5m 5m and 0.088nN, respectively. Distinctphases could not be observed in the 2D image,indicating that the size of silica particles is less thanseveral nanometers. The central portion of the 2Dimage was reconstructed as a 3D image toinvestigate more precisely the composite filmsurface. The 3D image shows a hole-like cavity asif a particle was extracted from the site. Thus, it issuggested from the 3D image that the dark particle-like domains may be holes, not particles them-selves. This extraction of particles from the filmsurface might have taken place during the thermalimidization process. The bright image domains inFig. 4 were revealed as silica particles embedded inthe film surface. The 3D image shows that thedomains, ranging from 1.2m to 600nm, adhere tothe matrix polyimide like a single body. Thetopology suggests that strong interfacial interactionexists between polyimide and silica, though achemical bonding between them is not proved atpresent. 29Si-NMR (nuclear magnetic resonance)spectroscopy can be used to prove the chemicalreaction between polyimide and silica [13, 14].However, it is proved here that AFM is a veryuseful tool to characterize the interfacial interactionas well as the microstructure in polyimide/silicahybrid composites.Such interfacial interaction as well as micro-structure can also be proved by SAXS measure-ment. Figure 5 shows the SAXS profiles ofpolyimide/silica composites with various silicacontents [20]. The intensity of the peak for theFIGURE 3. SEM micrograph of polyimide/silicacomposite lm with 30wt% TEOS loading.FIGURE 4. AFM image of the polyimide/silica compositelm with 30wt% TEOS loading [17].Copyright 2000 John Wiley & Sons, Ltd. Polym. Adv. Technol., 11, 145150 (2000)Organic/Inorganic Hybrid Composites / 147mean long period, which corresponds to a micro-structure in the polymer matrix component, wea-kened with increased loading of TEOS, andcompletely disappeared for TEOS loading above15wt%. This indicates that silica particles were wellincorporated into the polymer matrix on the smallscale. The good incorporation of inorganic particlesmight result from strong interaction or chemicalbond formation between the starting chemicalsforming silica, silanol and poly(amic acid). Thisinteraction or chemical bondingmay be an unstablesilyl ester formed between the carboxyl group inpoly(amic acid) and the hydroxy group in silanol[17]. Overall, the SAXS profiles indicate that silicaparticles of below 10nm were formed during thesolgel process and were well dispersed in thepolymer matrix, severely disturbing its microstruc-ture. The scattered intensity at around 0.2 nm1changed little, indicating that large particles ofabove 35nm were not made, whereas a largeamount of small silica particles (less than 10nm)was denoted by the intensity increment in thescattering vector (q) region above 0.5nm1.Since the preparation of the polyimide/silicahybrid composites includes the solgel reaction oftetra-alkoxy silane, drying of the poly(amic acid)as-spun film, and thermal imidization, the molecu-lar conformation of polyimide and the aggregatestate of the inorganic silica particles as well as thedimension of silica networks are affected by severalfactors such as complexation between a solvent andpoly(amic acid), chain orientation, chemical reac-tion, structural relaxation and the interactionbetween silica and polyamic acid in solution orpolyimide.The photophysical behavior of the polyimide/silica hybrid composites will therefore be influ-enced by the presence of tetra-alkoxy silane or silicagel in many ways depending on their interactionwith poly(amic acid) in solution or polyimide. Inthis vein, fluorescence spectroscopy can be appliedto characterize the interfacial interaction in poly-imide/silica hybrid composites. Wachman andFrank reported that the fluorescence is a measureof some aspect of the molecular rearrangementbetween the polyimide chains depending on thecuring temperature [21]. Bessonov et al. reportedthat the interaction between the highest occupiedmolecular orbital (HOMO) of the amine, bearing alone electron pair, with the lowest unoccupiedmolecular orbital (LUMO) of the dianhydride isvery important for the imidization reaction sincethey are closest in energy [22]. Thus, our poly-imide/silica hybrid composites are quite a suitablesystem for exploring microstructure and interfaceby a fluorescence spectroscopy [18] (C. S. Ha, H. D.Park and C. W. Frank, unpublished; C. S. Ha andC. W. Frank, unpublished)Figure 6 presents the emission spectra of theBPDA-PDA polyimide film, which were excited at350 nm, one of the stronger excitation peaks in theexcitation band. A broad structureless peak occursat approximately 542nm (C. S. Ha, H. D. Park andC. W. Frank, unpublished). A similar emissionspectrum was also reported by Hasegawa et al.[23]. Previous workers have proposed thatintermolecular and/or intramolecular chargeFIGURE 5. Lorentz-corrected SAXS proles of polymide/silica composite lm. TEOS compositions are (a) 0, (b) 5, (c)10, (d) 15, (e) 20 and (f) 30wt% [17].FIGURE 6. Emission spectrum of the BPDA-PDA polyimidelm, which was excited at 350nm.Copyright 2000 John Wiley & Sons, Ltd. Polym. Adv. Technol., 11, 145150 (2000)148 / Ha and Chotransfer complexes (CTCs) exist and give rise to thefluorescence in polyimides [21, 2330]. The inten-sity of the CTC depends not only on the nature ofchromophores but also on the thermal imidizationconditions. It is generally accepted that polyimidefluorescence usually arises from CTCs that formbetween electron donor (diamine) and electronacceptor (diimide) segment pairs [21, 23, 28].Figure 7 shows the effect of silica contents onthe emission wavelength and intensity of poly-imide/silica composite films [8]. On increasingsilica content, the wavelength was slightly red-shifted and the intensity was reduced. Based on theSAXS measurement and AFM topology data, theresults of Fig. 7 imply that the red-shift inwavelength and reduction in fluorescence intensityis due to the interaction between polyimide andsilica particles. The decreasing behavior of theemission intensity with increasing silica contents isdue to the reduced CTC population in thepolyimide because of the nonfluorescent silicaparticles.Too many silica particles exhibit some scatter-ing effect because of poor interaction between thepolyimide and silica particles [18]. In this case, theuse of a fluorescence spectroscopy has limits incharacterizing the interface in polyimide/silicahybrid systems. Before concluding, it should benoted that a fluorescence spectroscopy can be usedfor interface characterization only if any of thecomponents in the composites should fluoresce.TEOS is readily hydrolyzed to Si(OH)4 by water inthe presence of an acid catalyst such as HCl. Thehydrolysis reaction is very fast even in the NMPsolution of TEOS and poly(amic acid). Then theintermolecular hydrogen bonding exists betweenthe hydroxy group in the hydrolyzed TEOS andcarboxylic acid in BPDA in PAA systems (C. S. Ha,H. D. Park and C. W. Frank, unpublished; C.S. Haand C. W. Frank, unpublished). Therefore, a fluor-escent spectroscopy can be used very effectively tocharacterize in situ microstructural change duringthe solgel reaction and thermal imidizationprocess. More details on this point will bedescribed elsewhere.CONCLUSIONS WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWThe microstructure and interface of hybrid compo-sites can be characterized using various experi-mental techniques including electron microscopicobservation. In this article, we reviewed thecharacterization of the microstructure and interfaceof hybrid composites using small-angle X-rayscattering (SAXS), atomic force microscopy (AFM)kand fluorescence spectroscopy, as well as aconventional scanning electron microscopy (SEM).In particular, we introduced here the application offluorescence spectroscopy to interpret the inter-facial interaction in the hybrid composites, which isknown to be complementary to the SAXS and otherX-ray methods. Polyimide based on BPDA andPDA was used as a matrix polymer and TEOS wasa precursor of silica. Polyimide/silica hybridcomposites have been prepared by solgel reactionand thermal imidization.Interfacial interaction as well as microstructurein polyimide/silica hybrid composites was wellcharacterized by AFM topology. In particular, the3D AFM image shows the strong interfacialbonding between silica particles and polyimide aswell as the size of silica particles in polyimidematrix (microstructure) in submicrometer scale.The SAXS profiles of polyimide/silica compositeswith various silica contents prove also the inter-facial interaction between polyimide and silica aswell as the partial formation of a nanocomposite.Finally, fluorescence spectroscopy has been suc-cessfully applied to reveal the interfacial interac-tion in the hybrid systems as well as the molecularorientation of polyimide per se. A red-shift inemission wavelength and reduction in intensitywere interpreted in terms of interfacial interactionbetween polyimide and silica based on the AFMand SAXS results.ACKNOWLEDGMENTS WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWThe authors thank Dr Won-Ki Lee for AFMmeasurements, Prof. M. Ree of POSTECH, Korea,for his helpful discussion on SAXS data, and Dr J.S.Lin of Oak Ridge National Laboratory (USA) forSAXS measurement. The authors would like toacknowledge Prof. Curtis W. Frank, Stanford Uni-versity, for his help during CSH's sabbatical leavefor one year. Part of this work was supported by the'97 Overseas Grant of the SEOAM ScholarshipFoundation. The experimental aid of Dr Young-kyoo Kim of the Institute of Advanced Technology,Korea, and Mr Hae-Dong Park, Pusan NationalUniversity, Korea, is also gratefully acknowledged.FIGURE 7. Effects of TEOS loading contents on theemission intensity and wavelength in polyimide/silicahybrid composites.Copyright 2000 John Wiley & Sons, Ltd. Polym. Adv. Technol., 11, 145150 (2000)Organic/Inorganic Hybrid Composites / 149REFERENCES WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWW1. Ahmad Z, Sarwar MI, Mark JE. J. Mat. Chem. 1997;7(2): 259.2. Wang S, Ahmad A, Mark JE. Chem. Mater. 1994; 6:943.3. 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