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Organic-inorganic hybrid materialsYoshiki Chujo

Composite materials formed by the combination of inorganicmaterials and organic polymers are attractive for the purposeof creating high-performance or high-functional polymericmaterials. Of particular interest is the molecular levelcombination of two different components that may lead tonew composite materials, termed organic-inorganic hybridmaterials! Recently, new methods for preparing these hybridmaterials have been reported. Some improvements of theproperties or modifications of these materials have also beenexplored from the viewpoint of industrial applications.

AddressesDepartmentof Polymer Chemistry, Graduate School of Engineering,Kyoto University, Yoshida, Sakyo-ku. Kyoto 606-01, Japan; e-mail:chujo@chujo.synchem.kyoto-u.ac.jpCurrent Opinion in Solid State 6 Materials Science 1996,1:806-8 110 CurrentChemistry td ISSN 1359-0286

IntroductionWe cannot exist without commodiry materials such aswood, metals, plastics and glass, that make our livesmuch more enjoyable. Some combinations among thesematerials are known as composite materials. Examplesof such materials are inorganic fillers in plastics, ororganic coatings on the surface of metals. In compositematerials, the interface between two different materials isa very important feature of their durability or mechanicalproperties. In other words, the concept of these compositematerials is paste togetherto form a material withimproved properties.

The concept of molecular level mixing between twodifferent materials can also be considered. Since thelate 198Os, molecular level combination between organicpolymers and inorganic materials has been of interest.Review articles concerning hybrid materials have beenreported by Saegusa and me [ 11,Sanchez and co-workers[2,31, Schubert et a/. [4], Novak [5], Loy and Shea[6], Mark [7], and others [S]. During this period, thestudy of organic-inorganic hybrid materials focused onthe following points: firstly exploration of new preparativemethodology for hybrid materials; secondly, new combina-tions between different materials; thirdly, functionalizacionof hybrid materials; and fourthly, modification of hybridsfor industrial applications. This review describes suchrelevant interesting examples reported during 1995 and1996, some of which include rhe classical compositematerials. The composite materials described here arebelieved to have evolved and will result in interestingmolecular hybrid materials in the near future.

Hybrid macromonomersCubic silsesquioxanes can be thought of as the smallestsilica particles [9]. In other words, silsesquioxane-organiccopolymers can be thought of as the models of silica-re-inforced hybrid materials. For the preparation of thesehybrids, thermally curable and photocurable silsesquiox-anes were synthesized according to rhe reaction shown inFigure 1 [lO], which included the hydrosilation reactionof the propargyl group (111. The products of this synthesiscan be regarded as hybrid macromonomers.HeteropolysiloxanesSol-gel methods provide a versatile way to prepare func-tionalized inorganic materials under mild conditions. Var-ious metal alkoxide precursors can be used in the sol-gelreaction to give the corresponding metal oxide inorganicmaterial. Silica, alumina, titania, and zirconia are the mostpopular inorganic components for hybrid materials due totheir commercial availability as alkoxide precursors andalso to the feasibility of the sol-gel reaction. An exampIeof organic functionalization of silica gel by a sol-gelmethod is the preparation of transparent monolithic gelsby the reaction of diethylphosphatoethyltriethoxysilaneand tetraethoxysilane [ 121. The highest SIP concentrationthat could be introduced in these mixed organic-inorganicsystems in order to obtain monolithic transparent gels wasfound to be 30%. Because organophosphorus moieties canefficiendy extract many types of metal ions, the hybridmaterials containing phosphoryl groups can be applied inthe preparation of facilitated transport membranes.Sutfactant templatesOrganically functionalized mesoporous silicas have alsobeen prepared by the sol-gel reaction of tetraethoxysilaneand organosiloxanes in the presence of surfactanr tem-plates [ 13**]. Examples of organosiloxanes are phenyltri-ethoxysilane and ~-octyltrietho~silane. The addition ofthe surfactant, hexadecyltrimethylammonium bromide atthe stage of soI-gel reaction plays an important role inthe formation of mesoporous silica {Fig. 2) after extractionwith water. The surfactant template controls the sizeof the pores formed in the mesoporous silica resultingfrom the reaction. Porous silicas having lS-3OA poresize and 1060-1520m2gf surface area were effectivelyobtained using this method [13]. The synthesis of hybridmesoporous silica materials with controlled functionalityand hydrophobicity might open up new avenues forcatalysis or for organic-inorganic host-guest chemistry.Heterogeneous biocatalystsThe entrapment of lipases in hydrophobic sol-gel ma-terials resulted in the formation of highly active, stable

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Figure 1

(HSiOI.sh+4 equiv. zz No

0

(HSiMezOSiO t .ss)a+

4 equiv. = No Cl Cl _

Synthesis of silsesquioxane macromonomers. Reproduced with permission from 11O**l.

and reusable heterogeneous biocatalysts [14*]. The silanecomponent was found to be an important parameter inoptimizing the enzymatic activity. For example, Figure3 shows the dependence of the activity of immobi-lized hpase in tetramethoxysilane/methyltrim~thoxysi~anemixed gels on the gel composition. Surprisingly, theimmobilized enzyme showed much higher activity thanthe commercially available one. These results indicate thatthe alkyl-modified silica gels with hydrophobic charactermight be more suitable host matrices for Iipases 1151. Byvarying a range of parameters such as the use of additives,the watensilane ratio, the amount of catafyst, and thestructure of silane monomers, ic should be possible togenerate tailor-made gels for each lipase.Entrapment of lipases by the sol-gel process has alsobeen performed in the presence of porous glass beads(sintered glass), resulting in the fixation of hybrid ma-terials on the surface of the support and formationof highly active and mechanically stable heterogeneous

biocatalysts [16*]]. For example, the sintered glass wasadded to the mixture of gel precursors (tetramethoxysi-Iane/poiy[dimethyisilo~ne]~, an aqueous solution of ii-pase, poly(viny1 alcohol), and catalytic amounts of NaEBy this procedure, more than 95% of the sol-gel materialswere selectively attached to the surface of the beads. Thebiocatalysts can be used as batch or fluid-bed reactors f17].Hybr id sensorsOrganic or inorganic semiconductors have been reportedto change their conductivities when exposed to a varietyof organic and inorganic vapors. Thus, these materials canbe expected to behave as sensors. Composite materials oftin dioxide and derivatives of the conducting polypyrrole[ 18] gave reversible changes in elcccrical resistance at roomtemperature when exposed to a range of organic vapors119.1. Composites containing 2.5% polymer by mass werefabricated and exposed to low concentrations of ethanol,methanol, acetone, methyl acetate and ethyl acetate vapors[ZO-231. The composite materials were found to give

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Figure 2

TEM micrograph of an organically functionalized mesoporous silicaformed by a surfactant-extracted, 20 moWo phenyltriethyoxysilanesample. Reproduced with permission from [13**1.

Figure 3

i % MTMS -Dependence of the activity (A) of immobilized l ipase intetramethoxysilane/methyltr imethoxysilane (MTMS) mixed gels on thegel composition. the relative activity ix) is defined as n {irnmob~lzedenzymefln (commercial enzyme), where n IS the initial rate of reaction.Reproduced with permission from [39].

more significant and reversible decreases in electricalresistance in comparison with sensors constructed solely oftin dioxide or polypyrrole. These materials could be usedin the quality control of foodstuffs, specifically in the earlydetection of soft rot in potato cubers.

Preformed polypyrrole and polythiophene were incorpo-rated into clay (montmorillonite) by the interaction ofcolloidal nanoparticles of rhe polymers with the colloidal,layered host [24*]. This method using a colloid-colioidreaction [ZS], mighr provide a general route to incorpora-tion of intractable polymers within layered host structuresthat can be exfoliated, such as smeccice clays 1261, meraldisulfides, and some metal oxides. These materials havepotential to be used as hybrid sensors.

Structure of colloidal silica-poIy(methyl methacryfate) compositesComposite materials of silica and organic polymers arewidely employed as structural materials for their propertiesof high strength and low density. A new generation ofhybrid materials prepared by sol-gel methods have alsoshown promise as optical materials. The silica surface canbe modified with silane coupling agents, which improveadhesion between the filler and the organic polymers.Thus, an understanding of the structures and dynamicsof interfaces between two components shouid aid in thedesign of materials, with improved mechanical and opticalproperties. The composite materials of colloidal silica andpoly(merhy1 mechacrylate) have been well characterizedby 13C and 29Si MAS NMR spectroscopies jZ7*]. Theparent silica was found to have ethoxy groups isolated inthe matrix and silanol groups on the surface. The ethoxygroups in the matrix were retained during reaction with3-trimethoxysilylpropyl methacrylace, while the silanolgroups were consumed. In Ehe reaction a Znm layer ofthe siiane coupling agent was deposited on the surfaceof the 10.5 nm diameter silica particles. At least 85%of the methacrylate groups copoiymerized with methylmethacrylate to cross-link the silica into the composites.

Vitreophobic colloidsIn general, gold metal has very little affinity for silicabecause it does not have an oxide film on its surface.Previous attempts co overcome this vitreophobic characterinvolved the hecerocoagulation of gold colloids on silicacoiloids dispersed in water [28]. A new approach wasreported [29*.] using silane coupling agents (such as3-aminopropyltrime~hoxysiIane) as surface primers, whichare known to complex strongly with gold metal. Thus,gold colloids were homogeneously coated with silica viaa three-step deposition process: the hydrolysis of thesurface-bonded siloxane moieties to form silane triols;sodium silicare solution was added to this dispersion,which formed a thin, dense and relatively homogeneoussilica layer around the particles; and the. resulting dis-persion was then transferred into ethanol, where furthergrowth of the particles was achieved (as a consequenceof the hydrolysis of tetraethoxysilane and condensationof the resulting silanol groups). As a result, smooth.monodisperse spherical silica particles with gold coresplaced precisely at their centers were obtained. This

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method can be applied to other kinds of colloids whichhave typically been difficult to coat with silica, such assilver, copper and semiconductor particles.

Abrasion-resistant materialsSome hybrid materials exhibit abrasion resistance in addi-tion to optical clarity [30-331. High abrasive resistance canbe attributed to the Si-0-Si inorganic backbone structureof the hybrids along with the high level of cross-linking.These abrasion-resistant hybrid materials have been pre-pared by the sol-gel method using tetramethoxysilane ortetraethoxysilane in the presence of low molecular weightorganics such as diethylenetriamine, 3,3-iminobispropy-lamine, glycerol, ethylene glycol, butanediol, hexanediol,and resorcinol [34*]. The hybrid materials were spin-coated onto the surface of a bisphenol-A polycarbonate ora diallyl diglycol carbonate resin. The coating thicknesswere found to be either 3-5 microns for a single coating or10-12 microns for double coating. The adhesion betweencoating and substrate was greatly improved by treatingthe polymeric substrate surface with a primer solutionof 3-aminopropyltriethoxysilane. In addition to this, thesecoating materials exhibited excellent abrasion resistanceand have potential applications as coating materials foreyeglass lenses and other polymeric products.

Low volume shrinkageOne of the inherent problems in the conventionalsol-gel process is that it promotes an exceedingly largevolume shrinkage, as a consequence of the removalof a large amount of excess solvent and the removalof small molecules, such as water and alcohol. Thisvolume shrinkage can sometimes reach 75% or higher.Recently, polyacrylate-silica hybrids were prepared bysol-gel reactions using silicates having a polymerizableacrylate monomer moiety, in which the volume shrinkagewas significantly reduced [35-381.Very recently, a fast and convenient synthetic tech-nique to prepare crack-free, transparent, monolithic vinylpolymer-silica hybrid sol-gel materials with significantlyreduced volume shrinkages has been reported [39**].The principle of this procedure is the acid-catalyzedsol-gel reaction of tetraethoxysilane in the presence ofsol-gel reactive vinyl monomers (such as Z-hydroxyethylmethacrylate) to form oligomeric/polymeric silicate col-loids 140-441. The silanol groups in the silicate colloidsreact with the alcohol groups in the monomer .to forma covalent linkage between the organic component andinorganic one. After the removal of a large amount of thesolvent, the resulting nonvolatile fluid could be cast intovarious molds. The free-radical polymerization of the vinylmonomers could then be initiated, either by heating in thepresence of thermal initiators or by exposing the systemto light. The volume shrinkage from the processible sol tothe final product was only 6-20% for the hybrid materialswith up to 47 wt% silica content, as shown in Figure 4.

Figure 4

50-40-30-20-lo-0 I I I I10 30 50 70 90

SiOg Content (wt-%)Relationship between the volume shrinkage and the compositionsof the polyacrylate-silica hybrid sol-gel materials. Reproduced withpermission from [39**1.

Polyethylene fiber or carbon fiber can be used as one ofFiber-reinforced polymer hybridsthe reinforcing fibers in hybrid composites. Compositesbased on thermoplastic polymeric matrices potentiallyoffer several advantages compared with those based onthermosetting resins [45]. Unidirectional laminates wereprepared from polyethylene fiber, carbon fiber and thehybrid formed by them reacted with partially polymerizedmethyl methacrylate well below the softening pointof polyethylene [46]. Addition of polyethylene fiberply/plies to the tension side resulted in structural hybridlaminates exhibiting a significantly higher ultimate flexuralstrength in comparison with the system where carbon fiberply/plies were present on the tension side. The flexuralfracture mode of carbon fiber could be minimized byplacing polyethylene fiber ply/plies at the outermost sideof these hybrid laminates.To improve the mechanical properties of glass fiber-poly(phenylene sulfide) composite materials [47], silane cou-pling agents were used to modify the glass fiber surfaces[48]. The interlaminar shear strength and the flexuralstrength of the silane-treated composites increased incomparison with the untreated one [49].Silver powder has also been used as a filler for polypropy-lene composites to improve mechanical properties, suchas tensile and flexural properties, or impact strength [SO].Tensile modulus, strength, and elongation at break werefound to decrease with the incorporation of silver anddecrease further with an increase in silver concentra-

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tion. These results may be a result of the rigid andincompressive nature of silver powders, which did notleave sufficient space for compressive deformation ofpolypropylene to take effect.

ConclusionOrganic-inorganic hybrid materials are expected to pro-vide many possibilities as new composite materials. Thehybrid materials usually show properties intermediatebetween those of plastics and glasses (ceramics). Accord-ingly, hybrids can be used to modify organic polymermaterials or to modify inorganic glassy materials. Inaddition to these characteristics, the hybrid materials canbe considered as new composite materials that exhibitvery different properties from their original components(organic polymer and inorganic materials), especially inthe case of molecular level hybrids. In other words,hybrid materials should be considered as next-generationcomposite materials that will encompass a wide variety ofapplications.

References and recommended readingPapers of particular interest, published within the annual period of review,have been highlighted as:

. of special interestl of outstanding interest

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Chujo Y, Saegusa T: Organic polymer hybrids with silica gelformed by means of the sol-gel method. Adv Polym Sci 1992,loo:1 l-29.Sanchez C, Ribot F: Design of hybrid organic-inorganicmaterials synthesized via sol-gel chemistry. New J Chem1994, l&l 007-l 047.Judeinstein P, Sanchez C: Hybrid organic-inorganic materials: aland of multidisciplinarity. J Mater Chem 1996, 6:51 l-525.Schubert U, Hiising N, Lorenz A: Hybrid inorganic-organicmaterials by sol-gel processing of organofunctional metalalkoxides. Chem Mater 1995, 7:201 O-2027.Novak BM: Hybrid nanocomposite materials between inorganicglasses and organic polymers. Adv Mater 1993, 6:422-433.Loy DA, Shea KJ: Bridged polysilsesquioxanes. Highly poroushybrid organic-inorganic materials. Chem Rev 1995, S&1431 -1442.Mark JE: Novel reinforcement techniques for elastomers. J ApplPolym Sci Appl Polym Symp 1992,60:273-262.Mark JE, Lee CY, Bianconi PA (Eds): Hybrid organic-inorganiccomposites. Washington; American Chemical Society; 1995.[American Chenical Society Symposium Series, vol 565.1Btirtsch M. Bomhauser P, Cairaferri G, lmhof R: HsSisOl.z: amodel for the vibrational structure of zeolite A. J Phys Chem1994, S&261 7-2631.Sellinger A, Laine RM: Silsesquioxanes as synthetic platforms.Thermally curable and photocurable inorganic/organic hybrids.Macromolecules 1996, 29:2327-2330.The product in this study can be used as the so-called macromonomer for

the preparation of new organic-inorganic hybrid materials.11. Sellinger A, Laine RM, Chu V, Viney C: Palladium- andplatinum-catalyzed coupling reactions of allyfoxy aromaticswith hydridosilanes and hydridosiloxanes: novel liquidcrystelline/organosilane materials. J Polym Sci Polym Chem1994, 32:3069-3069.12. Cardenas A, Hovnanian N, Smaihi M: Sol-gel formation of. heteropolysiloxanes from diethylphosphatoethyltriethoxysilaneand tetraethoxysilane. J Appl Polym Sci 1996, 60:2279-2266.

The hybrid materials obtained consist of Si-C and P-C bonds, which arestable toward hydrolysis.13. Burkett SL, Sims SD, Mann S: Synthesis of hybrid. . inorganic-organic mesoporous silica by co-condensation

of siloxane and organosiloxane precursors. Chem Commun1996:1367-l 368.The surfactant added is very important as it acts as a template to control thepore size of the porous silica formed.14. Reetz MT, Zonta A, Simpelkamp J: Efficient heterogeneous. . biocatelysts by entrapment of lipases in hydrophobic sol-gelmaterials. Angew Chem lnt Ed 1995, 34:301-303.The immobilized lipase showed much higher activity than the commercialone. This heterogeneous biocatalyst can be used for the kinetic resolutionof 1 -phenylethanol producing much higher enatiomerically pure products.15. Schwertfeger F, Glaubitt W, Schubert U: Hydrophobic aerogelsfrom tetramethoxysilane/methyltrimethoxysilane mixtures. J

Non-Cry& Solids 1992, 145:05-09.16. Reetz MT, Zonta A, Simpelkamp J, Kijnen W: In situ fixation of. lipase-containing hydrophobic sol-gel rneterials on sinteredglass - hlghly efficient heterogeneous biocetelysts. Chem

Commun 1996:1397-l 396.Fixation of lipaae-containing hybrid materials on sintered glass resulted inthe formation of highly active and mechanically satble heterogeneous bio-catalysts, which is very useful in their applications.1 7. Avnir D: Organic chemistry within ceramic metrices: Dopedsol-gel materials. Accounts Chem Res 1995, 28:326-334.16. Miasik JJ, Hooper A, Tofield BC: Conducting polymer gassensors. J Chem Sot faraday Tmns 1 1986, 82:1117-l 126.19. Lacy Costello BPJ, Evans P, Ewen RJ, Honeyboume CL, Ratcliffe. M: Novel composite organic-inorganic semiconductor sensorsfor the quantitative detection of terget organic vapours. J MaterChem 1996,6:299-294.The combination of organic and inorganic semiconductors should be pointedout to give reversible changes in electrical resistance.20. Maekawa T, Tamaki J, Miura N, Yamazoe N, Matsushima S:Development of tin dioxide-based ethanol gas sensor. SensorsActuators B Chem 1992, 9:63-69.21. Gardner JW, Hines EL, Tang HC: Detection of vapors and odorsfrom a multisensor array using pattern-recognition techniques

2. Artificial neural networks. Sensors Actuators B Chem 1992,9:9-l 5.22. Persaud K, Dodd G: Analysis of discrimination mechanisms inthe mammalian olfactory system using a model nose. Nature1962,299:352-355.23. Nenov T, Yordanov S: Ceramic sensor device materials. SensorsActuators B Chem 1992, 8:117-l 22.24. Oriakhi CO, Lemer MM: Poly(pyrrole) and poly(thiophene)/clay. nanocomposites via latex-colloid interaction. Mater Res Bull

1995, 30:723-729.Clays are also used as an inorganic component for the preparation of hybridmaterials. The direction of the components in the matrix can sometimes becontrolled in these materials.25. Lemmon JP, Lemer MM: Preparation and characterization ofnanocomposites of polyethers and molybdenum disulfide.Chem Mater 1994, 6:207-210.26. Vaia R, lshii H, Giannelis E: Synthesis and properties of two-dimensional nanostructures by direct intercalation of polymermelts in layered silicates. Chem Mater 1993, 5:1694-l 696.27. Joseph R, Zhang S, Ford Wl: Structure and dynamics of a. colloidal silica-poly(methyl methacrylate) composite by 1%and *sSi MAS NMR spectroscopy. Macromolecules 1996,29:1305-l 312.Spectroscopic characterization is very important in the elucidation the struc-ture of hybrid materials.26. Liz-MarzPn LM, Philipse AP: Synthesis and optical propertiesof gold-labeled silica particles. J Colloid Interface Sci 1995,176~459-466.29. Liz-Marz&n LM, Gieraig M, Mulvaney P: Homogeneous. . silica coating of vitreophobic colloids. Chem Commun1996:731-732.The results in this paper present the preparation of new three-component hybrid materials consisting of organic polymers, inorganic materials, and metalcolloids.30. Bet&et C, Wilkes GL: Optically abrasion resistent materialsusing a sol-gel approach. Amer Chem Sot Po/ym Prep 1992,33:266-297.

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31. Tamami B, Bet&et G, Wilkes GL: New ceremer hiih opth iabrasion resistant transparent coattng mabrtals based onfuncttonalyzad malatmide and a disc-aminophenyl)phos~ineoxtde compound. PO&~ 311/l1@93,30:3@-45.32. Temami B, Bet&et C, Wilkes GL: Synthesis and application ofabrasbn resistant coattng matartais based on functionalpedbis and trts maiaimides. Poijm 6ufllQ93,9(f:393-3QS.33. Wang B, Wilkes GL: Novel hybrid inorganic-organic abrasion-resistant coattngs preparad by a sol-gel process. J MecfomofSci Pure Appi Chem 4694, A31 :249-260.34. Wen J, Wilkes GL: Noval rbmsion resistant inorganic/organic. coating matartais basad on functionslizad die~~ne~ami~,t&carol and dids. Atner Chem Sac Po&m Prep 1SQS,36:429-430.The advantage of hwd materialsas sbrasion-resistantcoatings has beenw&l demonstrated n this paper.36.

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Uleworth MW, Novak EM: Mutually interpenetratinginorganic-organic networks. New routas into nonshrinking~~~~~7~ matartais. J Am Chem Sot 199 t ,: - .Novak BM, Davies C: invane organk-inorgantc composttametertats 2 Fraa-radlcal routas into nonshrtnktng sol-getwmpositaa MactvmoXecules 99 1,24:548 1-5403.Novak BM, Ulsworth MW: %varse* organk-inorganicwmposita materials. High gius content ~n~hdn~ng sol-adcomposite& Mater Sci Eng A IQS3,162:267-284.Ellsworth &V, Novak BM: *inverse organic-inorganicwmposlte materials 3. High giess wntent %onshrinMngsol-gel composites via poiy(silidc acid esters). Chem Mater1993,5:839-844.Wei Y, Jin D, Yang C, Wei G: Synthasts of organic-inorganichybrid sol-gel matartsls with low volume-shrinkages. AmChem Sot PO&m Mater Sci Eng 1996,74:244-245.Large volumeshrinka& is cne at the inherent probla in industrialapplii-tion of hybrid materials.This paper desctibea a very convenient and usefultechnique to overcome this diasdvantage.

40. Wei Y, Yang D, back R: Thermal stabiltty andhardness of new poiyacrytata-siiica hybrid sol-gel materials.Mater Lett 1992, 13:261-266.

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Wei Y, Yang D, Tang L, Hutchins MK: Syntf~a&,chamcterization, and properttes of new polystyrene-silicahybrid sol-gel materials. J Meter Res 1993,&l 143-l 162.Wei Y, Yang 0, Tang L: Synthesis of new poiyacryionttrile-silicahybrid sol-gel materials and their thermal proparties. MakromolChem Repid Commun f 993,14:273-278.Wei Y, Wang W, Yang D, Tang L: Synthesis of water-solublepolyacrylics modified silica sol-gal materials. Chem Mater1994, 6:1737-l 741.Wei Y, Yeh J, Jin D, Jia X, Wang J: Compositas of elactronlcallyconductive poiyaniiine with polyacrylata-silica hybrtd sol-gelmeterirrlt Chem Meter 19@5,7:969-974.Kodokien GKA, Kinloch AJ: Sutfaca pmtmatment and adhesionof tharmopiasttc fiber composites. J Meter SW Lett 1988,7:625-627.Bandee AN, Saha N: Rexumi behavior of unidirection8l~~~~e~r~n flbars-PMMA hybrid composite laminates.J Appt PO&m Sci f QSS,80:39-l 42. -.The combination of an organic polymer end &on tiber produced interest-ing hybrid materials having a good baiance of flexural behavior.

47. Scobbo 11: Effect of solid-state curing on the viscoeiasticproperties of polytphenyfene sulfides. 1 Appl Pobm Sci 1993,47:2169-2175.

48. Menon N, Blum FD, Dhareni LR: Use of titanate couplingagents in Keviar-phenoiic composltar J Appt PoIym Sci 1994,54:113-l 23.49. Jang J, Kim H: Performance improvement of glass ftbar-. poiy(phenyiene sulfide) wmposite. / Appl Potym Sci 1906,60:2297-2306.fiber-reinforced polymer composites have been widely used as high per-formance materials in aerospace end automotive sppliitions. This paperdescribes surface modificationd a glass fiber using silane couplingagents.60. Ghosh K, Maiti SN: Mechanical propartter of siiver-powder-. filled potypropyiene composites. J Appl PoIym Sci f 996,60:323-331.A titanate coupling agent may assist the hybridiition of isotactic polypropy-lene and silver powder.