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, Korea
In 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 spectroscopy
Organic/inorganic hybrid composites with a poly-mer matrix are of considerable interest andimportance since they frequently provide optimal
combinations of properties from inorganic andpolymer components . 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 . In addition, silica has been mostextensively investigated as an inorganic compo-nent because of the expected interesting catalyticand electronic applications .
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 . For instance, in poly-imide/silica hybrids both the glass transition and
Copyright 2000 John Wiley & Sons, Ltd.Received 12 September 1999Accepted 10 December 1999
POLYMERS 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: firstname.lastname@example.org 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 . In addition, anincrease of the silica content causes an increase ofthe density, storage modulus and linear thermalexpansion coefficient . 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 (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  (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 . 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 .
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.
3,3',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, 30
and 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 Interface
Microstructure of hybrid thin films were examined
FIGURE 1. The preparation of polyimidesilica hybridlms
FIGURE 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 Cho
by 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 spectromete