Journal of Sol-Gel Science and Technology 35, 225–235, 2005c© 2005 Springer Science + Business Media, Inc. Manufactured in The Netherlands.

Hybrid Organic-Inorganic Sol-Gel Materials Basedon Epoxy-Amine Systems

PLINIO INNOCENZI∗ AND TONGJIT KIDCHOBLaboratorio di Scienza dei Materiali e Nanotecnologie, Nanoworld Institute, Dipartimento di Architettura e

Pianificazione, Universita di Sassari, Palazzo Pou Salid, Piazza Duomo 6, 07041 Alghero (Sassari), Italy

TOSHINOBU YOKOLaboratory of Inorganic Photonics Materials, Division of Materials Chemistry, Institute for Chemical Research,

Kyoto University, Uji, Kyoto-Fu 611-0011, Japan

Received March 7, 2005; Accepted May 16, 2005

Abstract. Understanding the synthesis-structure-properties relationship of hybrid organic-inorganic materials isan important and fundamental task to develop a new generation of highly performing systems for the new needsof nanotechnologies. This issue is, also, important to improve the properties of hybrids when used in applicationswhere nanoscale control is not a fundamental request. In this article, a short overview of epoxy amine sol-gelhybrids is reported. The synthesis of these materials has a large flexibility to allow the fabrication of a wide group ofhybrids with different structures and properties, whose applications are ranging from non-linear optics to adhesiveand protective coatings.

Keywords: sol-gel, amine, epoxy, hybrid, organic-inorganic materials

1. Introduction

Organic-inorganic hybrid materials prepared via soft-chemistry routes [1], such as sol-gel processing, rep-resent no more only promising new materials but arealso widely recognized as a market ready product withapplications in several different fields [2,3]. These ma-terials, far beyond the actual state of knowledge, seemto open the route to almost endless new types of hy-brids that have all the potentialities to be a winningalternative with respect to a great part of organic poly-mers. A comprehensive classification is also a diffi-cult task, because a large part of the new generation ofnanoscaled materials can fall within the general defini-tion of organic-inorganic hybrid. In general, there is agood agreement within the scientific community to di-vide the organic-inorganic hybrid materials in two main

∗To whom all correspondence should be addressed.

groups, whose difference relies on the existence or lessof a direct covalent bond between the organic and in-organic sides [4]. What it appears particularly compli-cated is a satisfactory understanding of the synthesis-structure-properties relationships that are the basis offundamental materials science. Several studies have, infact, pointed out that synthesis and structure are verymuch complicated, by far more than at a first glance un-derstood. If a real deep understanding, which involvesvery much basic science at a highly sophisticated scale,is apparently not necessary for functional applications,such as protective coatings [5,6], with the move of thematerials science towards a nanoscale, the future ofhybrid organic-inorganic materials in next generationnanotechnologies requires an extensive new work onbasic issues.

Several of the potentialities of these materials re-main largely unexplored, and endless new types andproperties are described in the scientific literature. This

226 Innocenzi, Kidchob and Yoko

rush makes difficult to get a general overview of thefield and for basic research to catch up with the grow-ing family of hybrid materials. In this review, with thepurpose to open at least a little window in the fieldof hybrid organic—inorganic materials, we have fo-cused our attention on epoxy—amine hybrid systems.Epoxy resins represent one of the more popular organicthermosetting polymers and amines are the most com-mon hardners that are used to catalyze the epoxy-ringopening. The sol-gel synthesis to produce epoxy-aminehybrids is much more complicated by the presence ofdifferent possible side reactions in the first stages ofpreparation and by the presence of inorganic and or-ganic networks whose process of formation can be inreciprocal competition [7]. The possibilities to use ba-sic synthesis via amines to obtain hybrid materials arealso not well explored even if this field appears quiteintriguing because can challenge the well-establishedfamily of thermosetting resins in terms of functions andproperties.

2. The Epoxy-Amine Reactions

Polyfunctional amines are one of the most popular cur-ing agents that are commonly used to fabricate a wideclass of thermosetting organic polymers based on theepoxy-amine system. The reaction mechanisms [8], thekinetics [9] and the material structure [10] have beenthe subject of extensive research [11] even if more stud-ies are still necessary to clarify all the details of thereactions.

The epoxy-amine systems show, in general, sev-eral reaction mechanisms: the main one is the di-rect amine addition to the epoxy groups, but sev-eral side reactions are also possible in some morerestricted conditions, such as homopolymerization ofepoxy, etherification between neighboring epoxidesand hydroxyls, cyclization and side transformationreactions.

Homopolymerization is observed with an excessof epoxides, at high temperatures and in presenceof Lewis bases, inorganic bases and in presence ofLewis acids as catalysts. The conditions to observeetherification are even more restricted because it mayoccur with a significant reaction rate only at hightemperatures and at a high curing degree. In gen-eral, therefore, the predominant reaction is the amineaddition.

The addition reaction between an epoxy and anamine group is described using a two-steps reaction

pathway:

O OH H� �

R′ CH CH2 + R NH2k1→ R CH CH2 N R′

the epoxy is opened by a primary amine (k1 reactionkinetics) and forms a compound containing a diol anda secondary amine.

The second step will be observed with a lower re-action rate, k2, because the hydrogen of the secondaryamine is slower to react:

O OH H� �

R′ CH CH2 + R CH CH2 N R′

OH R′ OHk2→ R CH CH2 N CH2 CH R

The mechanism of reaction, in the specific case of 3-glycidoxypropyltrimethoxysilane, (GPTMS) reactingwith a diamine molecule such as propane-1,3-diamine,has been described by Sales et al. [12].

1. The strained acidic carbon in the epoxy ring ofGPTMS undergoes a nucleofilic attack by one ofthe basic nitrogen atoms of propane-1,3-diamine

2. The ring opening occurs via the electronic transferfrom the carbon to the oxygen in the epoxy.

3 and 4. An oxygen-hydrogen bond is formed becausethe excess of charge in the oxygen atom attracts thehydrogen in the diamine molecule and an electronicrearrangement between hydrogen and nitrogen fol-lows.

The final molecule, upon ring opening catalyzed bythe amine in propane-1, 3-diamine is formed.

Epoxy-Amine on Sol-Gel Hybrid 227

Figure 1. Schematic representation of the cubic cage, T8. (Fromref. 31, reprinted with permission of Elsevier).

3. Cyclization of Trialkoxysilanes ContainingEpoxy Groups, Formation of Cages

The cyclization of alkoxysilanes is a well knownprocess that is used to synthesize polyhedralpolysilsesquioxanes (POSS) such as branched and lad-der polymers or octamer cages [13]. POSS with or-ganic functionalities, in particular, are of paramountinterest as building blocks of new hybrid materialswith controlled complex architectures. Silsesquioxanestructures, octamer T8 cages, bearing epoxy functionalgroups, upon sol-gel reaction of GPTMS have beenobserved in some particular synthesis conditions, bothbasic and acidic [14, 15] (Fig. 1).

Benzyldimethylamine (BDA), used as catalyst ofGPTMS, was observed to give cage-like structures,which depend on the water/alkoxide ratio, r , and sol-vent amount. Higher dilutions give a larger fractionof cages. Basic catalysis of GPTMS by BDA gavealso higher cage fraction compared to acid cataly-sis. The dilution and amount of water play an im-portant role during the synthesis, in fact, gelationcan be observed due to crosslinking of the cages be-cause of the fast reaction rate of the epoxides inGPTMS. The formation of the cages can be, how-ever, promoted and gelation prevented increasing thedilution or r . The possibility to obtain octamer cagesthrough ciclyzation reactions of GPTMS makes it pos-sible the realization of hybrid materials with nano-structured building blocks containing epoxy function-alities that can be connected to a hybrid network. Us-ing the synthesis described in ref. [14], Matejka et al.have prepared, starting from GPTMS, sylsesquioxi-

anes (SSQO) clusters with epoxy functionalities [16].Crosslinking of these clusters by oligomeric diaminesproduced a structure formed by SSQO domains inter-connected by organic chains. The network structurewas found to be affected by the amine/epoxy ratio andwas described as an organic-inorganic block copolymernetwork.

4. Classification of Epoxy-Amine HybridsObtained by Sol-Gel Reactions

An attempt to describe all the different types of hybridsol-gel materials containing somehow epoxy function-alities is a quite difficult task because of the very largevariability of structures and synthesis that are offeredby the versatility of sol-gel synthesis and also becausea clear cross-cut definition of these hybrid materialscan be very much a matter of case by case classifi-cation. Our goal is, therefore, more than to achieve anexhaustive classification, to describe the different typesof materials within some groups with similar proper-ties of these hybrids. For this purpose, because of thelarge variability of the structures that can be obtained,the hybrids are grouped as a function of the synthesisstrategies.

4.1. Amine Reaction with GPTMS

This group of hybrids is one the most studied in lit-erature and is characterized by the reaction of an or-ganically modified alkoxide bearing an epoxy func-tional group with different type of amines. Even ifat the moment most of the reported studies have re-garded GPTMS, this group can be extended to all theepoxy containing alkoxides of whom GPTMS is justan example, even if the most widely employed in hy-brid synthesis. This group of materials is character-ized by the use of amines as curing agents of ringopening in GPTMS, and in general they are addedafter an acidic prehydrolysis of GPTMS. The groupincludes both aliphatic and heterocyclic amines. Ex-amples have been reported for ethylenediamine [12],diethylediamine [17], triethylenediamine [18], dicyan-diamide [19], pyridine [18], propane-1-3-diamine [12],1-methylimidazole [20, 21] and benzyldimethylamine[14]. Because of the different reactivities of amines thathave been employed, the hybrid structure can deeplychange from one synthesis to another.

228 Innocenzi, Kidchob and Yoko

Scheme 1.

4.2. Reaction of GPTMS with Alkoxides BearingAmine Functionalities

This other group of epoxy-amine hybrid is character-ized by the reaction of an organically modified alkox-ide bearing amine functionalities with GPTMS. In thiscase the reaction seems to be particularly interestingbecause it offers the possibility to catalyze in a quitecontrolled way the inorganic hydrolysis and the epoxyopening. Several examples are reported employing [3-(2-Aminoethyl)aminopropyl]trimethoxysilane [22,23](AEAPTMS) and (3-Aminopropyl)trimethoxysilane(APTMS) or (3-Aminopropyl)triethoxysilane [24–26,36–38] (APTES) (Scheme 1).

4.3. GPTMS and Epoxy Resins Cured by Amines

Another interesting approach, which has been, indeed,not very much explored, is to form a hybrid networkusing a commercial epoxy resin, such as diglycidyl

Scheme 2.

Figure 2. Conversion (%) of epoxy and methoxy groups, evaluatedby the FTIR absorption band areas at 910 and 2836 cm−1, as afunction of time at different curing temperature steps. ( • ) epoxy,(◦) methoxy groups. (From ref. 25, reprinted with permission ofElsevier).

ether of bisphenol A (DGEBA), to which GPTMS isadded together with water and tetraethylenepentamine(TEPA), (Scheme 2) the curing agent of the epoxy.Analysis by FTIR done at different thermal curingsteps showed that the ring opening and hydrolysis ofalkoxy groups increased with time and curing tempera-ture (Fig. 2). The material, after thermal curing, showeda very high bonding strength when deposited on siliconrubber [27].

4.4. Interpenetrating Hybrid Organic-InorganicNetworks

An alternative route to produce hybrid silica-epoxysystems is the simultaneous reaction of silica alkox-ides and epoxy resins to form an interpenetrated

Epoxy-Amine on Sol-Gel Hybrid 229

organic-inorganic network [28,29] (IPN). These typesof materials are synthesized forming an interconnectedphase, through sol-gel chemistry, in presence of an or-ganic monomer. The different reaction stages can besequential or simultaneous, giving quite different struc-tures. The organic network is formed by an epoxy resinas the organic component [30] whilst a silicon alkoxideis used to form the inorganic part. Different examplesof IPN materials have been reported so far, an exam-ple is the hybrid formed by reaction of an epoxy resin,such as diglycidyl ether of Bisphenol A (DGBEA) witha poly(propylene) diamine, the inorganic side is, in-stead, generally obtained through sol-gel hydrolytic re-actions of tetraethylorthosilicate (TEOS) [30–33]. Thefinal structure results strongly dependent on the syn-thesis protocol, the type and concentration of the cat-alyst. Transmission electronic microscopy (TEM) andsmall angle X-ray scattering (SAXS) analysis on IPNshave revealed a strong phase separation when the ma-terials are prepared by sequential polymerization andextensive mixing of phases for simultaneous reactionroutes [30]. Grafting has also a very important effecton IPNs hybrids because phase separation and struc-tures are strongly affected by. The two networks canresult, in fact, not only interpenetrated and with sec-ondary bonds between the two phases, but also cova-lently bonded because of the reaction of the silanols,produced from hydrolysis of TEOS, and diols, fromepoxy opening.

5. Hydrolysis—Condensation Kineticsof GPTMS-APTMS Solutions

The comprehension of the reactivity in basic condi-tions of alkoxides bearing epoxy functionalities repre-sents an important task because of the double effectthat amines species have on the inorganic hydrolysis—condensation process and on the epoxy opening. Evenif several studies described the reactions in acidic con-ditions of GPTMS [34,35] and APTMS [35], few stud-ies are available, at present, to describe the hydroly-sis and condensation reactions of these two differentgroups of compounds. A comprehensive set of experi-ments has been carried out by Riegel and coworkers toaddress the reactivity of GPTMS with APTMS [36–38]under different hydrolytic conditions. They have em-ployed FT-Raman to monitor in situ the hydrolysis andcondensation of the inorganic moieties in GPTMS andAPTMS through the evolution of specific bands (643and 613 cm−1 doublet in GPTMS and 650 cm−1 peak

Figure 3. Raman spectra of APTMS (bottom curve) and GPTMS.(From ref. 34, reprinted with permission of Elsevier).

with a shoulder at 627 cm−1 for APTMS). The openingof the epoxide ring was observed through the changesin intensity of the Raman band at 1256 cm−1, assignedto symmetric ring stretching [39, 40] (Fig. 3).

The catalytic effect of amines on hydrolysis and con-densation of alkoxysilanes is due to the nucleophilichydroxyls anions that are formed upon water dissocia-tion in basic conditions:

Si(OC2H5)3-(CH2)3-NH2 + H2O

⇔ Si(OC2H5)3-(CH2)3-NH+3 + OH−

Increasing the concentration of the basic catalyst in-creased the hydrolysis rate of GPTMS and the gelationtime, as expected, on the other hand the opening ofthe epoxide was also efficiently catalyzed by APTMS(Fig. 4). The process of ring opening results, however,slower compared to hydrolysis and condensation andit takes a longer time, even in comparison with thegelation time of the sol to go to completion. The reac-tions are, indeed, complicated by several features, thehydroxyls ions can give, in fact, a significant contri-bution to ring opening and likely they are the respon-sible of ring reactions when the amines decrease inconcentration. Observation of the reactions of GPTMSwith lower contents of APTMS has also brought tothe hypothesis that neutralization reactions betweenthe acidic silanols and the hydroxyl ions takes place,with a consequent multiple hydrolysis of GPTMS thatis linked to the existence of a pentacoordinated sili-con centre as a transition state during the hydrolysisstage [41].

230 Innocenzi, Kidchob and Yoko

Figure 4. Kinetic plot of ring opening reactions (right Y -axis) and GPTMS hydrolysis (left Y -axis) catalyzed with 30% mol of APTMS at20◦C, with a half stoichiometric amount of water. Data are taken from Figs. 3 and 6 of reference 34.

A non-hydrolytic reaction of GPTMS with (3-aminopropyl)triethoxysilane (APTES) instead ofAPTMS, as previously described, was studied in openand closed vessel conditions [25]. The results showedthat the inorganic polycondensation was favored bya high GPTMS/APTES ratio, but the epoxy cross-linking followed a reverse trend; the authors con-cluded that the reaction of GPTMS with APTES, un-der non-hydrolytic conditions formed highly branchedmaterials.

6. Structure of Epoxy-Silica Hybrids

The structure of sol-gel epoxy-amine hybrid mate-rials was not studied much in detail and few stud-ies are currently available. The large variability inthe synthesis protocols is also a limitation to achievea general description in terms of a model structure.The structures that are obtained upon reaction of 3-glycidoxypropyltrimethoxysilane with an amine arethe most important and more supported by basic scien-tific work. In general, the phenomenology of structureformation in GPTMS hybrids is governed by the mu-tual polycondensation processes that are activated viahydrolysis of alkoxy functions and epoxy ring open-ing. The possibility of several side reactions makesthe final structure a combination of different, often un-controlled processes [42]. Acid catalysis by transition

metal alkoxides is a well known synthesis of GPTMShybrids [42–45]. Thermal treatments are, however, nec-essary to activate the epoxy opening. Amine curingagents, from this point of view, are able to open theepoxy rings and to catalyze the network polycondensa-tion at room temperature. Solid state NMR techniquesand vibrational spectroscopies are generally used tocharacterize the structure.

Davis and coworkers [17], in particular, have investi-gated the structure obtained upon reaction of GPTMS,prehydrolised in acidic conditions, with diethylene-triamine (DETA). The prehydrolysis of GPTMS wasfound not to affect the epoxy rings. After addition ofDETA, instead, the formation of interconnected or-ganic and inorganic networks was observed. Largercontents of DETA, increased the organic network for-mation, up to a maximum that corresponds to the ratiobetween the hydrogen atoms in the amine and epoxy.As observed in the case of the reaction of GPTMScatalyzed by BF3

.etherate [7, 20], the growth of onenetwork hinders the formation of the other one, and inlarge part the organic network is, indeed, formed byshort ethylene oxide chains bonded to the inorganicbackbone, as also noticed by Templin [42] et al. Lowconcentrations of DETA resulted in an incomplete for-mation of the organic network and this gives enoughroom to the inorganic one to form. In this case ther-mal curing can be used to boost the organic networkformation. On the opposite side, high concentrations

Epoxy-Amine on Sol-Gel Hybrid 231

of DETA allow the formation of a highly crosslinkedorganic network that hampers the polycondensation ofthe inorganic part.

An example of in depth structural analysisof epoxy-silica hybrids has been reported byBrus and coworkers [46]. Hybrid materials wereprepared by acidic reaction of GPTMS or (3-glycidoxypropyl)methyldiethoxysilane (GPMDS)with Jeffamine D-230 or T-403. The effect of additionof colloidal silica was also studied. Another type ofepoxy-silica hybrid was, instead, prepared by reactionof an octaepoxide POSS with Jeffamine D-230. Acombined use of solid-state NMR and SAXS allowedto establish that highly self-organized and long-rangeordered epoxide-based networks are formed undersome specific synthesis conditions. A timing control ofthe sol-gel and polyaddition steps was found to favorthe structural design of the hybrid material. To open allthe epoxy rings by thermal curing, highly condensedisolated polycyclic cagelike clusters (starlike epoxy-functionalized building blocks) must be formed beforetheir aggregation during sol-gel reactions. A differentstructure was predicted by the addition of silicananoparticles to GPTMS and GPMDS with formationof highly condensed and rigid cagelike clusters bypromotion of siloxane cyclization in GPTMS systemsand short oligomers with a significant fraction ofcycles in GPMDS sols.

7. Applications

7.1. Protective Coating on Polymeric Substrates

The epoxy-amine reactions have been successfully em-ployed to obtain transparent protective coatings onpolycarbonate substrates [23]. The films showed an ex-cellent adhesion and abrasion resistance. Furthermore,a high stability of the precursor sol was obtained viasurface modification by GPTMS of aqueous silica, alu-mina or zirconia nanoparticle sols. This modificationallowed to shift from an electrostatically stabilized solto an electrosterically stabilized one, with a reducedparticle-particle interaction and an increased tempo-ral stability, which is very important for commercialapplications. Another advantage of this route is that af-ter evaporating the organic solvent, the alcohol contentresulted lower than 5%. AEAPTMS was added to thesol prior of coating deposition to catalyze the inorganicand organic polycondensation.

7.2. Restoration Materials

Some different compositions of epoxy-silica sol-gelmaterials have been tested as restoration materialsfor stone conservation [25, 47]. Epoxy resins thatare used in consolidation and protection of histori-cal constructions have the disadvantage to be quitesensitive to weather and radiation and because oftheir high molecular weight suffer of low penetra-tion depth. Epoxy-silica hybrids, on the other hand,show some potential advantages, such as the possibil-ity to carry out the reactions not in situ, even if directdata on their effect on stone preservation are not yetavailable.

An interesting example of synthesis of epoxy-silica hybrids to be used as a product for stoneconservation is obtained by the reaction of 2-(3,4-epoxycyclohexyl)ethyl-trimethoxysilane (ECET) andGPDMS (Scheme 3) with APTES [48]. The effectof APTES on the epoxy opening reactions was quitedifferent for ECET and GPDMS, because the theepoxy ring of ECET was not involved in the epoxy-amine addition. ECET acts, therefore, as a networkmodifier and the resulting hybrid structure is formedby pendant epoxycyclohexyl and aminopropyl groupsthat are covalently bonded to the silica network. Thereaction of GPDMS with APTES opened the epoxideand produced a structure similar to that one obtainedvia reaction of GPDMS with organically modifiedalkoxides bearing amine groups. In this material,however, a hydrophobic behavior is observed due tothe methyl groups of GPDMS and this property canbe used to avoid or reduce water penetration throughstone surface.

Scheme 3.

232 Innocenzi, Kidchob and Yoko

7.3. Adhesive

Organofunctional silanes are important couplingagents to promote adhesion between different inter-faces. One of the most commonly used is GPTMS,which has been widely applied to promote adhesion onaluminum surfaces [49]. Aqueous solutions of GPTMSwere used to treat aluminum surfaces prior to ad-hesive bonding. The formation of strong Al O Sibonds is thought to be the cause of improved dura-bility of the adhesive [50]. Because amines are themost common curers of epoxy, GPTMS films on alu-minum were treated by Johnsen et al. with differenttypes of amines and an epoxy adhesive [19]. Dicyandi-amide, in particular, was deposited on an aluminumsurface, previously sililanised by GPTMS, and thesamples were heated to achieve a mono-componentepoxy adhesive. An improved durability of the bondedjoint at high relative humidity conditions was observed[19].

7.4. Purification of Waste Water

Modification of organic surfaces, such as silica filmsor particles, through silylant groups, is a common ap-proach to anchor pendant functional groups [51]. Inparticular, modification of silica surfaces is used to an-chor functional groups that exhibit chemical affinity tocontaminants such as toxic metal ions. Sales et al. havefunctionalized the surface of silica gel particles by us-ing GPTMS as the silylant agent and then reacting theepoxide with amines, in order to immobilize an ethylen-diamine molecule on the silica support [12, 52, 53].Two possible synthesis routes have been employed,a homogeneous [12, 52] and a hetherogeneous [53]one.

In the heterogeneous route (Fig. 5(a)), GPTMS wasgrafted on the activated surface of a silica gel. In the sec-ond step of the preparation, ethylendiamine was thenreacted with the epoxy of GPTMS to form a pendantfunctional group containing a chelating species.

In the homogeneous route, instead, GPTMS was pre-reacted with ethylendiamine before anchoring on silicagel surface, as shown in Fig. 5(b).

A comparison of the performances of silica gels,whose surface was modified by heterogeneous routes,showed that the homogeneous reaction allowed the im-mobilization of almost the double of functional groups.The pendant molecules, after anchoring on silica sur-

Figure 5. Homogeneous (a) and heterogeneous route (b) to func-tionalize a silica particle with amine pendant groups using GPTMSas a grafting agent.

face, because of the presence of the basic centers con-taining nitrogen and oxygen, were able to extract di-valent cations of copper, zinc, nickel and cobalt fromaqueous solutions.

Epoxy-Amine on Sol-Gel Hybrid 233

7.5. Applications in Biochemistry

Several types of hybrids, synthesized from GPTMS,ECET and APTMS have been used to immobilize re-dox relays such as ferrocyanide. The reactive func-tional groups in GPTMS and ECET acted as anchorsites for palladium, which facilitates the charge trans-port, and ferrocene monocarboxylic acid or potassiumferricyanide during gelation. These biomaterials havepotential applications for probing different types ofenzyme-catalyzed reactions [54].

7.6. Photonics, Non-Linear and Linear Optics

An interesting field of application for epoxy-silica hy-brids is in photonics, as non-linear optical (NLO) ma-terials. These hybrids offer a suitable host matrix fororganic molecules with very large first order molec-ular hyperpolarizabilities but that require a very tai-lored environment to be durable and highly perform-ing. This type of application needs of a careful designof chromophores and host matrix. An example is givenby zwitterionic push-pull dyes, specifically function-alized in order to carry hydrolysis or alkoxy groupsthat, through, sol-gel reactions allow a direct covalentbonding to the network [55] (see Scheme 4). The in-corporation of these dyes in a proper solid- state ma-trix represent, however, a challenging task because ofthe tendency to easy photodegradation, exhibited un-der the combined action of light and oxygen, whilst

Scheme 4.

another source of degradation is represented by acidicenvironments. The matrix, on the other hand, shouldallow incorporation of the doping molecules in largeamounts while avoiding aggregation effects. The hostmaterial has to be processed as thick films of sev-eral microns with low optical propagation losses anddense at temperatures low enough to avoid the chro-mophore thermal degradation. The host material, atthe same time, must allow for the orientation of thedyes under poling conditions, to ensure good tempo-ral stability, and must be able to resist intense elec-trical fields. It is, therefore, important to obtain ahybrid matrix with the largest amount of amines inits structure, which has a gelation time that is longenough to allow the deposition of the films. A limi-tation to employ large amounts of alkoxides bearingamine functions in the synthesis is the fast increasein the gelation time with the amine contents. Thiseffect, together with uncontrolled phase separation,makes difficult to obtain homogeneous and transpar-ent films. Different types of matrixes have been tested:GPTMS and APTMS (GTE in Fig. 6), GPTMS andAEAPTMS (GTM in Fig. 6) and GPTMS, AEAPTMSand TEOS (GTMT in Fig. 6) [56–58]. The gelationtimes of these systems range from few seconds to 100h with low APTMS and AEAPTMS contents. The wa-ter content has also an important effect on the gela-tion time. Even if on different time scales, hours forGTM and minutes for GTMT, the sols showed similartrends. The gelation time first decreased with increas-ing water content and then increased again when a

234 Innocenzi, Kidchob and Yoko

Figure 6. Gelation times of sols prepared with different contentsof amine-functionalized alkoxides. The lines are guides for eyes.(From ref. 51, reprinted with permission of the American ChemicalSociety).

larger amount of water dilutes the sol, similarly towhat observed in silica sols [59]. Another importantissue is the control of the amount of doping chro-mophores that can be incorporated, without aggrega-tion. The performance of the device resulted much im-proved after introducing in the matrix a physical spacer,such as N-(hydroxyethyl)carbazole, (CbOH), to de-crease the aggregation of the chromophores at largeloading levels (around 20%) [60, 61]. After thermallyassisted poling the system exhibited a NL coefficientvalue, between 50 and 70 pm V−1, with an estimationof the r33 coefficient corresponding to a d33 value of70 pm V−1 it was calculated an r33 = 38 pm V−1 at1064 nm.

Highly transparent and thick silica-epoxy films wereprepared from GPTMS and 1-methylimidazole [21].The optical absorption edge was found to shift to longerwavelength with the increase of the temperature and

Figure 7. Change of refractive index and thickness of GPTMSfilms catalyzed by 1-methylimidazol, as a function of curing timesat 180˚C. (From ref. 19, reprinted with permission of Kluwer).

time of treatment. Thermal curing produced also an in-crease in refractive index and extinction coefficient dueto epoxy opening and network densification (Fig. 7).

Final Outlook

The short overview of the scientific literature haspointed out, one time more, that hybrid materials ex-hibit such wide range of different properties to allowtheir applications in many different fields. Even consid-ering a little group of these materials, such as the epoxy-amine systems, this large flexibility clearly emerges asan intrinsic property of hybrids. The more promisingfield of applications seems to be in optics and as func-tional coatings, even if many other possible productshave been proposed. A limit for a fast application inthe market is, however, the lack of basic research, es-pecially in the synthesis-structure-properties relation-ship. A better disclosure of this issue appears at themoment an important subject of further studies.

Acknowledgments

JSPS-CNR and European Commission (STREP-ODEON project) are gratefully acknowledged forfinancial support.

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