Various nanocomposite systems have been synthesized by sol-gel routes. For this reason, prefabricated nanoparticles (SiO2 sols or boehmite powder) have been dispersed after surface modification in sol-gel-derived organically modified or polymeric ligand matrices. In all cases, a significant effect on dispersibility by surface modification could be observed. After curing, the mechanical or optical properties depend strongly on the dispersion and surface modification. Using these results, composites to be used in chip coupling and as hard coatings on polycarbonate and CR 39 have been developed

Becker, Carsten

Geiter, Elisabeth

Krug, Herbert

Mennig, Martin

Schmidt, Helmut K.

Winkler, Ralf-Peter

Scientific documents from the Saarland University,

The Sol-gel process for nano-technologies : new nanocomposites with interesting optical and mechanical properties

Scientific publications of the Saarland University

The Sol-Gel Process for Nano-Technologies: New Nanocompositeswith Interesting Optical and Mechanical PropertiesH.K. SCHMIDT, E. GEITER, M. MENNIG, H. KRUG, C. BECKER AND R.-P. WINKLERInstitut fu¨r Neue Materialien gem. GmbH, Saarbru¨cken, GermanyAbstract. Various nanocomposite systems have been synthesized by sol-gel routes. For this reason, prefabricatednanoparticles (SiO2 sols or boehmite powder) have been dispersed after surface modification in sol-gel-derivedorganically modified or polymeric ligand matrices. In all cases, a significant effect on dispersibility by surfacemodification could be observed. After curing, the mechanical or optical properties depend strongly on the dispersionand surface modification. Using these results, composites to be used in chip coupling and as hard coatings onpolycarbonate and CR 39 have been developed.Keywords: nanocomposite, surface modification, hard coatings, transparent adhesives1. IntroductionSol-gel techniques for a long time have been used forthe fabrication of glasses and ceramics [1–6]. The solsused for these investigations are made from alkoxides,and their stability was obtained by controlling the elec-tric charges on the sol particles, which, in general, arein the range of several nanometers in diameter. Theformation of these entities either in form of macro-molecules or in form of spherical or non-spherical par-ticles follow the established rules for nucleation andgrowth. For stabilization of the sols, the pH value is es-tablished in a range aside from the point of zero charge.The resulting surface charges reduce the particle-to-particle interaction to a level that no aggregation oragglomeration takes place. Thus, gelation can be pre-vented. Gelation takes place if the surface charges aredecreased, for example, by pH change or if the particle-to-particle distance is reduced below the repulsing level[7], for example, by solvent evaporation, and the repul-sion turns into attraction. If the particles grow too large,precipitation takes place. In sol-gel systems based onoxides, the particle-to-particle interaction is strong(oxide bridges accompanied by hydrogen bridges)so that, especially after drying, the agglomeration isirreversible.As described elsewhere, the surface reactivity ofthe sol particles can be controlled by chemical surfacemodification. In this case, the concentration of uncon-trollable chemically reactive groups can be reduced andsubstituted by a tailored reactivity, which now dependsonly on the reactivity of the modifiers (schematicallyshown in Fig. 1).This leads to a type of stabilization which, in general,after a “gelation” provides redispersibility [8, 9]. Thepresence of any type of surface interacting agent dur-ing nucleation and growth, of course, interferes with thenucleation and growth process by itself. This has beendescribed in detail elsewhere [9]. Using this approach,it is possible to fabricate sols with specific properties,not only depending on the properties of the core ma-terial but also depending on the properties of the sur-face modifier. This approach has been used meanwhilein many cases for the fabrication of various materials[9–14]. The change of surface properties of the smallparticles not only governs its chemical properties, butalso influences the surrounding matrix when dispersedin liquid or solidified media. In this paper, some ex-amples are investigated showing how sol-gel derivednanoparticles can interact with their environment andhow this can be used for the development of the desiredmaterial properties.398 Schmidt et al.Figure 1. Effect of surface modification of sol particles on particle-to-particle interaction. “R”: chemical groupings linked to oxidic particles,e.g., fl-diketones, ether amines, acids.2. Surface ModificationThe basic principles of surface modification of nano-particles have been shown elsewhere [9]. In Fig. 2,some selected examples are given [15, 16].In general, if ceramic particle filled compounds withpolymers are produced, the distribution of the particlesin the matrix is obtained by mechanical forces, espe-cially by the employment of high shear rates. Withdecreasing particle size, the effect of shear rate for dis-persion is decreasing also, and with nanoparticles, theparticle-to-particle interaction becomes the governingforce. This is shown schematically in Fig. 3. In addi-tion to this, the dispersion is more or less governed bythe interfacial thermodynamics. As soon as the free en-ergy of agglomeration is higher than the interfacial freeenergy, the system disperses by itself if the activationenergy for given temperatures is low enough. This sit-uation can be named as a thermodynamically stabilizeddispersion and is schematically shown in Fig. 4.One can postulate that in the case of (a), due tothe strong interaction of the nanoparticles, this type ofcomposite should show a higher viscosity, but shouldshow a low viscosity in the case of (b), see Fig. 4. Todemonstrate this phenomenon, composites have beensynthesized [17] according to the following experimen-tal route:Figure 2. Some principles for surface modification of nanoparticles.1 mole GPTS (glycidyloxypropyltrimethoxysilane)is hydrolyzed with 1.5 mole of water at 120–C for24 h under reflux. Methanol is eliminated at 70–Cat 20 mbar, to prepare a solvent free matrix. Colloidalsilica sol (PIA-ST, Nissan Chemicals) with 20 wt%SiO2 in isopropanol is mixed with 2 mg of tetrahexylammonium hydroxide (THAH) per g colloidal silicaand stirred for 0.5 h. The solvent free GPTS conden-sate is mixed with different amounts of this colloidalsilica solution and 1.5 wt% of a cationic photocuringcatalyst (UVI 6974, UVI 6990) are added. Finallythe solvent (isopropanol) is extracted at 50–C under12 mbar.These systems show a low viscosity since the cross-linking of the organic groupings has not yet takenplace and can be used for photocuring of the compos-ites. In Fig. 5 the viscosities of the surface-modifiedSiO2 particle containing system is compared with theunmodified system. Even at low concentrations theunmodified system shows a rather high viscosity com-pared to the modified system. The effect is attributedto the modification of the SiO2 surface by THAH,leading to a change in polarity so that no agglomer-ation takes place. HRTEM investigations showed thatin contrast to the untreated SiO2, the surface-modifiedcomposites show a perfect dispersion of the 7-nmparticles.The Sol-Gel Process for Nano-Technologies 399Figure 3. Significance of shear rates for a uniform dispersion of nanoparticles in nanocomposites.Figure 4. Effect of free energy levels on the dispersion of small particles in a low viscosity matrix: GAgg D free energy of agglomeration;GInt D interfacial free energy.Figure 5. Viscosity of a nanomer optical glue as a function of colloidal silica content (with and without surface modification by THAH),measured after storage at 25–C for 8 days.400 Schmidt et al.Figure 6. IR (liquid, ATR) of condensates with different colloidal silica contents.Due to the residual number of OH groups in the sys-tem (Fig. 6), the composite, which is almost indepen-dent of the SiO2 content, shows a very good adhesionon glass surfaces, and in combination with the overallproperties of this material, a technology has been devel-oped for using these systems for fiber-to-chip coupling.Compared to conventional sealants mainly based onepoxides or methacrylates, the thermal expansion co-efficient is rather low (30 ¢ 10¡6 K¡1), the temperaturestability is up to 250–C, and the volume shrinkage dur-ing curing is only in the range of 3.6% [18]. One ofthe surprising findings is that using the surface modi-fication approach, high concentrations of nano-scaledfillers (up to 30 vol%) can be introduced into the sys-tems without affecting the viscosity in an undesiredway and without affecting the transparency, due to theperfect distribution. The high transparency is requiredto use these systems as an optical sealant. The use offumed silica, for example, leads to unacceptable vis-cosities even in the range of 1 or 2 wt% filler.Another example is shown with methacryloxycontaining systems using SiO2 nanoparticles and modi-fying them with various silanes. The experimental pro-cedure is published elsewhere [19]. SiO2 sols with adiameter ranging from 1000 to 10 nm were treated withtwo different silanes: A: Acetoxypropyltrimethoxysi-lane (a silane with a non-reactive grouping) and M:Methacryloxypropyltrimethoxysilane (polymerizabledouble bond), and introduced into a matrix consist-ing of 50% of methylmethacrylate and 50% of hy-droxyethylmethacrylate (molar ratios). Stirring themonomer mixture with the SiO2 sols and subsequentthermal curing including polymerization of the reactionmixture leads to transparent thermoplastic nanocom-posites. Different measurements have been carried outafter curing these systems. As shown in Fig. 7 the glasstransition temperature Tg of the polymeric matrix ob-tained from differential scanning calorimetry (DSC)measurements can be varied over a wide range by in-troducing specially surface coated silica nanoparticles.Whereas with 1000 nm, 250 nm and 100 nm parti-cles no significant differences could be detected com-pared to the unmodified matrices; differences could beobtained for the systems with 10 nm particles, espe-cially with those coated with modifier M. It clearly canbe seen that only the modifier M, which is polymer-ized to the matrix shows an effect on Tg as a functionof filler content and only in the nano-scale version.Covalent immobilization of matrix molecules on thesurface of the M-coated 10 nm silica particles leadsto a strong increase of the glass transition temperatureof the polymeric matrix. This means that the inter-face plays an important role for the thermal propertiesof the composite as far as its volume fraction is largeenough to play a sufficiently important role. Again onecan see the influence of the particle size and the sur-face modifier. Modifier A cannot be polymerized andshows a far lower interface effect on the modulus thanmodifier M.Information about the reinforcement behavior ofnanoparticles with different surface modifications dis-persed in the copolymer matrix given above can be ob-tained by examination of the storage modulus E0 fromdynamic mechanical thermal analysis (DMTA) in therubbery plateau region above the glass transition tem-perature of the polymeric matrix. The dependence ofThe Sol-Gel Process for Nano-Technologies 401Figure 7. Tg values of SiO2 particle filled composites (10–250 nm in diameter) with the modifiers A and M after polymerization obtained byDSC measurements.the storage modulus on the filler surface modificationand the filler content is shown in Fig. 8.As shown in Fig. 8, the storage modulus can be in-creased by a factor 16 compared to the unfilled polymermatrix by introducing 10 vol% M-coated 10 nm SiO2particles.Another interesting feature of surface modificationis to use the surface modifier as an intermediate in orderto make a sol compatible for processing purposes. Inthis case, the surface modifier should be easily remov-able so as not to disturb further processes. As shownelsewhere, nanoparticles have been used for reinforc-ing organic or hybrid matrices in order to increase theirscratch resistance. A system based on boehmite andepoxysilanes has been developed to be used as scratch-resistant coatings for eye glass lenses [14, 20]. Thedetailed experimental process is described elsewhereFigure 8. Storage modulus of filled MMA/HEMA composites with various filler diameters (10, 100 and 250 nm). T D 170–C (rubberyregime).[21]. For the preparation of the system, commerciallyavailable boehmite powder from Condea (ChemicalCompany) with 10–17 nm particle size has been used.These powders are stabilized with acetic acid and canbe easily redispersed in diluted HCl. However, theviscosity of this system increases with time. This isattributed to the fact that the acetic acid is slowly sub-stituted by electric charges as indicated in Fig. 9.The viscosity increase of this system is shown inFig. 10.Using this type of stabilized sols directly afterredispersion, quick hydrolysis and condensation re-actions can be started in a mixture of GPTS (° -glycidyloxypropyltrimethoxysilane) and TEOS (tetra-ethoxysilane) with a molar ratio of 5 : 3 [21]. In this firstsynthesis step the amount of aqueous boehmite sol cor-responds to the theoretical amount of water necessary402 Schmidt et al.Figure 9. Change of stabilization mechanism of nano-scaled boehmite particles in aqueous media.Figure 10. Changes in viscosity of an aqueous boehmite sol independence on the sol age.for the half-stoichiometric hydrolysis of the silanes.After 2 h reaction time the amount of boehmite can beeasily increased up to 10 wt% by a final addition ofboehmite sol into the prehyrolyzed silane mixture.It is assumed that during the mixing of the boehmitewith the silanes, the acetic acid is substituted com-pletely by the reaction of silanes to the surface. This canbe demonstrated by an aluminum NMR spectroscopy(Fig. 11).The 27Al-NMR spectrum of a system containingsilanes and nano-scaled boehmite particles is shown inFig. 11. By line shape analysis of the measured spec-trum a broad peak at 0 ppm and a smaller peak at 60 ppmcan be detected. The peak at 0 ppm can be attributedto aluminium atoms with coordination number VI inAl O Al formations of the nanocrystalline boehmiteparticles, whereas the peak at 60 ppm results from theformation of Al O Si bonds, wherein the aluminiumatoms show the coordination number IV. This resultclearly proves the reactivity of the AlOH groups on theparticle surface in regard to co-condensation reactionswith the Si OH or SiOR groups of the silanes.If these liquids are used for coating purposes, for ex-ample, on polycarbonate, very high scratch resistancescan be obtained, as shown in Fig. 12.The superiority of the boehmite containing nanomersystem in comparison to conventional siloxane coat-ings is demonstrated in taber abrasion and sand falltests. After 1000 cycles of the taber abrasion test thenanomer coating shows very low haze values similarto those of glass. This result proves the extremely highscratch resistance of the coating material. Comparingthe haze values after sand fall tests, it can be shownthat the wear resistance of the nanomer system is evenhigher than the resistance of glass under this very abra-sive stress (see Fig. 12).Other investigations have been carried out to find outthe role of the boehmite with respect to the formationof an organic network.Using 13C-Solid-NMR and NIR spectroscopy it wasfound that the characteristic signals of epoxide groupsdisappear during the thermal curing of GPTS-TEOS-boehmite systems (Figs. 13 and 14). In addition to this,new signals can be detected, attributed to the forma-tion of polyethylene oxide chains. In comparison tothe composite with boehmite no polymerization reac-tions of the expoxide groups in analogous GPTS-TEOSsystems without boehmite can be detected. It can besupposed that the AlOH groups on the particle surface,which show a Lewis acidity, provoke the polymeriza-tion of the epoxides.The experiments show clearly that an important cat-alytic activity of the boehmite particles can be detected.This catalytic activity contributes to the formation ofan polyethyleneoxide network, which surrounds theboehmite particles (platelets and needles) and which isconsidered to be an important factor for the extremelyhigh abrasion resistance of these coatings.The Sol-Gel Process for Nano-Technologies 403Figure 11. 27Al-NMR spectra of the GPTS-TEOS-boehmite sol.Figure 12. Abrasion properties of the boehmite type of hard coatings [21]. The boehmite/epoxysilane coating is indicator as Nanomer(nanoparticle reinforced polymer).Figure 13.404 Schmidt et al.Figure 14. NIR spectrum of the boehmite containing nanomer afterthermal curing.3. ConclusionAs a conclusion it can be stated that the surface chem-istry of nano-scaled particles can be considered as a keyparameter for processing and properties of the materi-als produced with nanoparticles. Especially, if organicpolymeric networks are present, the surface modifiercan influence the surrounding molecular structure ina way that thermal and mechanical properties can beinfluenced. In addition to this, surface modifiers asintermediates can be used for improvement of the pro-cessing properties, and after the removal of the mod-ifiers, other effects of nanoparticles such as catalyticeffects can be used, for example, to improve organiccross linking.References1. C.J. Brinker, D.E. Clark, and D.R. Ulrich (Eds.), in BetterCeramics through Chemistry I, Materials Research SocietySymposia Proceedings, Vol. 32 (North Holland, New York,Amsterdam, Oxford, 1994).2. C.J. Brinker, D.E. Clark, and D.R. Ulrich (Eds.), in BetterCeramics through Chemistry II; Materials Research SocietySymposia Proceedings (Materials Research Society, Vol. 73,Pittsburgh, PA, 1986).3. C.J. Brinker, D.E. Clark, and D.R. Ulrich (Eds.), in BetterCeramics through Chemistry III, Materials Research SocietySymposia Proceedings (Materials Research Society, Vol. 121,Pittsburgh, PA, 1988).4. V. Gottardi (Ed.), J. Non-Cryst. Solids 48 (1982). North-HollandPhysics Publishers, Amsterdam.5. H. Scholze (Ed.), J. Non-Cryst. Solids 63 (1984). North-HollandPhysics Publishers, Amsterdam.6. Z. Zarzycki (Ed.), J. Non-Cryst. Solids 82 (1986). North-HollandPhysics Publishers, Amsterdam.7. O. Stern, Z. Elektrochem. 508 (1924).8. R. Naß, S. Albayrak, M. Aslan, and H. Schmidt, in Process-ing and sintering of nanosized TiN, Advanced Materials in Op-tics, Electro-Optics and Cummunication Technologies. edited byP. Vincenzini (Techna Srl., Faenza, 1995), pp. 47–54.9. H. Schmidt, Relevance of sol-gel methods for synthesis of fineparticles, KONA Powder and Particle, 14, 92–103 (1996).10. S. Hirano, In-situ control of microstructures of ceramic compos-ites, Funtai Oyobi Funmatsu Yakin 39(12), 1093–1099 (1992).11. H. Schmidt, in Proc. Fourth International Conference on Fron-tiers of Polymers and Advanced Materials, edited by P.N. Prasad,J.E. Mark, S.H. Kandil, and Z. Hafifi (Plenum Publishing Cor-poration, New York, USA), in print.12. H. Schmidt, in Proc. Symp. 9, “Neue Werkstoffkonzepte,” Werk-stoffwoche 1996, edited by H. Schmidt, and R F. Singer (DGM-Informationsgesellschaft mbH, Frankfurt/M., 1997), pp. 11–24.13. C. Becker, M. Zahnhausen, H. Krug, and H. Schmidt, in CeramicTransactions Sol-Gel Science and Technology, edited by E.Pope, S. Sakka, and L. Klein (American Ceramic Society, 1995),Vol. 55, pp. 299–306.14. R. Kasemann, H. Schmidt, and E. Wintrich, Mat. Res. Soc.Symp. Proc. 346, 915–921 (1994).15. H. Schmidt, R. Kasemann, T. Burkhart, G. Wagner, E. Arpac,and E. Geiter, in ACS Symposium Series No. 585 Hy-brid Organic-Inorganic Composites, edited by J.E. Mark,C.Y.-C. Lee, and P.A. Bianconi (American Chemical Society,Washington, 1995), pp. 331–347.16. C. Sanchez, oral presentation in Organic/Inorganic Polymer Sys-tems, Division of Polymer Chemistry, Inc. (American ChemicalSociety, Napa Valley, 1995).17. H. Krug, H. Schmidt, E. Arpac, M. Mennig, and Z. Ahmad,Verfahren zur Herstellung von Kompositmaterialien mit ho-hem Grenzfla¨chenentail und dadurch erha¨ltliche Kompositma-terialien, German Open DE 195 40 623 A1, 31-10-1995.18. M. Mennig, private communication, details to be publishedlater.19. C. Becker, Ph.D. Thesis, University of Saarland, Saarbru¨cken,1997, in print.20. R. Kasemann, E. Geiter, H. Schmidt, E. Arpac, G. Wagner, andV. Gerhard, Verfahren zur Herstellung von Zusammensetzungenauf der Basis von epoxidgruppenhaltigen Silanen. German OpenDE 43 38 361 A1, 10-11-1993.21. E. Geiter, Ph.D. Thesis, University of Saarland, Saarbru¨cken,1997.22. Comparison of hard coating data on polycarbonate. Bayer Chem-ical Company, private communication.

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The Sol-gel process for nano-technologies : new nanocomposites with interesting optical and mechanical properties

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