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1061-933X/05/6706- 0658 © 2005 Pleiades Publishing, Inc.
Colloid Journal, Vol. 67, No. 6, 2005, pp. 658–677. Translated from Kolloidnyi Zhurnal, Vol. 67, No. 6, 2005, pp. 726–747. Original Russian Text Copyright © 2005 by Pomogailo.
INTRODUCTION
During last decades, a considerable attention was focused on hybrid organo–inorganic nanocomposites. Diverse variants of the combination of the formation of inorganic and organic components of such systems, in particular sol–gel synthesis (SGS) including the combi- nation of networks of inorganic and organic polymers at a molecular level, are studied most extensively. To characterize the processes of nanocomposite formation in systems containing organic polymer or its precursor, we employ the definition of polymer SGS.
Polymer–inorganic materials are distinguished by their enhanced mechanical strength and thermal stabil- ity combined with optimal thermal properties [1]. The key role in such nonequilibrium self-organized systems belongs to interfacial interactions between components [2, 3]. In essence, this is the main route for the forma- tion of nanosized oxide particles (from the skeleton of inorganic polymer) in the medium of organic polymer. Such nanoparticles find wide application as compo- nents of composites. They increase the strength and hardness of materials, act as reinforcing fillers for plas- tics and vulcanized rubbers, “binders” of polymer com- ponents enhancing thermochemical, rheological, elec- trical, and optical properties of materials. Organo–inor- ganic composites of such type are employed as chromatographic carriers and catalysts (including pho- tocatalysts for air cleaning and water purification), membrane materials, luminophores, etc. They are the bases for new types of contact lens, optical waveguides, memory and printing media, chemical filters, solid electrolytes, biosensors, semiconductors, and plastics for space applications. The possibility for controlling the interfacial properties of such hybrid colloidal mate- rials is one of the reasons for their wide application in pharmaceutics, cosmetics, food industry and in medi- cine. Nanocomposite materials combining organic and inorganic phases are the objects of novel nanotechnol-
ogies, because they join together the best properties of metal oxides and either polymers or biopolymers. A large number of preparation procedures of hybrid materials with required properties differing in the pre- history of “combination” of organic and inorganic phases (different variants of coprecipitation, hydrother- mal procedures, spray pyrolysis, sol–gel technology, synthesis of colloids in the presence of polymers, etc.) is known. For example, an organic component can be incorporated into the composite either as monomeric, oligomeric, and polymeric precursor or as a ready- made linear polymer (from solution, melt, emulsion) [4, 5]. In turn, an inorganic component can be incorpo- rated into the material composition as a metal oxide- containing monomer, which is the “intermediate prod- uct” of nanoparticles (e.g., see [5] and below), as nan- oporous structure (aerogel) [6], as the “host” where molecules of guest polymer are intercalated, as it takes place in the case of layered silicates [7]. Specific problems of polymer SGS are analyzed in special issues of numerous journals, in a number of monographs and analytical reviews. However, in our opinion, there is so far no generalization considering this process from the position of “throwing bridges” between the regularities of the formation of inorganic and polymeric phases of hybrid nanocomposite. In this review, we made such an attempt. Note that many nat- ural composites are also characterized by hybrid nano- structure; hence, one section of this review is devoted to nanocomposite biomaterials prepared by the methods similar to polymer SGS.
1. GENERAL CHARACTERIZATION
OF SOL–GEL REACTIONS
The sol–gel method (or dip- and spin-on-glass pro- cesses) belongs to waste-free methods of producing hybrid nanocomposites that makes it ecologically opti-
Polymer Sol–Gel Synthesis
of Hybrid Nanocomposites
A. D. Pomogailo
Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia Received August 11, 2004
Abstract —The current state and main problems of polymer sol–gel synthesis as a method of the preparation of hybrid polymer–inorganic nanocomposites are analyzed. The general characterization of sol–gel reactions is given and the routes of the combination of sol–gel synthesis with the polymerization of traditional monomers are considered. Particular attention is given to the formation of sol–gel precursors in the presence of organic polymers, including the formation of interpenetrating hybrid networks. The specificity of hybrid nanocompos- ites based on multicomponent ceramics is discussed. The sol–gel process is analyzed as a promising route for the preparation of bioceramics in the presence of templates.
REVIEW
POLYMER SOL–GEL SYNTHESIS OF HYBRID NANOCOMPOSITES 659
mal: when using compounds that do not introduce impurities into the end product as initial substances, this method excludes the stage of washing. Although the SGS has been used for a relatively long time, its mechanism was understood only in the early 1980s [8]; the production process of so-called low-bulk density silica involving the hydrolysis of tetraethoxysilane in the presence of cationic surfactants was patented at the same time [9].
Acid hydrolysis of alkoxides M(OR) n (M = Si, Ti, Zr, VO, Zn, Al, Sn, Ce, Mo, W, etc.) and subsequent condensation (most often of tetarmethoxysilane (TMOS) or tetraethoxysilane (TEOS)) can be represented by the following formal schemes (for alkoxides with n = 4):
M(OR) 4 + 4H 2 O M(OH) 4 + 4ROH,
m M(OH) 4 (MO 2 ) m + 2 m H 2 O.
It is self-evident that the real process is more complex. Metal oxoalkoxides including polynuclear ones (e.g., Ti x O y (OR) 4 x – 2 y ), can be formed as intermediate prod- ucts, many of which were isolated and characterized [10]. The formation of solid phase of monodisperse TiO 2 powder proceeds a few seconds via the sequence of sev- eral stages: hydrolysis condensation nucle- ation particle growth. Forming oxooligomers or polymers, as well as crosslinked macromolecules are nothing other than peculiar clusters [11] coexisting with the sol. As a rule, reactions of condensation and reprecipitation of monomeric and oligomeric mole- cules proceeding in gels are accompanied by phase transitions.
Of great importance for the optimization of synthe- sis are, above all, the ratio γ = H 2 O/M(OR) n , the use of catalysts (including nucleophilic catalysts such as NH 4 F, acetic, trifluoroacetic, and even polymeric acids, e.g., poly(styrenesulfonic acid)), and the nature of metal and its alkoxy group (for example, the rate of hydrolysis of Ti(OBu) 4 is almost 150 times lower than that of Ti(OEt) 4 [12]). The significant role is played by the degree of association of alkoxides (e.g., for [Ti(OEt) 4 ] n n = 2, 3). The rate of hydrolysis of forming oxo- or alkoxocluster structures of Ti 18 O 22 (OBu) 26 (acac) 2 type is much lower than that of initial Ti(OR) 4.
In turn, the reactivity of alkoxides of tetravalent metals M(OR) 4 increases in order Si(OR) 4 Sn(OR) 4 and Ti(OR) 4 < Zr(OR) 4 < Ce(OR) 4 [13]; the ion radius of central atom, its coordination number (CN), as well as the degree of unsaturation (the difference between CN and valence), also increase in the same order. How- ever, parameter γ have the prime importance. In partic- ular, in case of VO(OPr i ) 3 , homogeneous transparent gel with alkoxide polymer network is formed in n -pro- panol at γ = 3; at γ > 100, forming gel has completely different structure that cannot form inclusion com- pounds [14].
Mesoporous (with 2D- or 3D pores of hexagonal symmetry and diameters of 2.7 and 3.1 nm) materials with specific surface areas of 750 and 1170 m^2 g–1^ hav-
ing unique degree of uniformity are obtained when polymer-forming and inorganic fragments constitute one molecule that was demonstrated using bis(tri- methoxysilyl)ethane as an example [15]. Similar (one- stage) procedure was used to obtain polytitanosilox- anes: combined controlled hydrolysis of Si(OEt) 4 and Ti(OPr i ) 2 (acac) 2 was performed [16]. Ladder polymer containing chain-forming Si–O–Si and Si–O–Ti units is formed; the ratio between these units depends on syn- thesis conditions and can be as high as 10, being responsible for the time of gelation and the possibility of preparing ceramic fibers from such gel by spinning with subsequent annealing of a material at 770–1170 K. As is known, sols are thermodynamically unstable, continuously changing systems with high surface free energy; they can exist only in the presence of stabiliz- ers. The stabilization of highly concentrated sols is a difficult task. The effective stabilization of colloidal nanoparticles can be performed precisely in polymer SGS due to the attachment of monomer molecules of organic precursor at the particle surface [17]. For exam- ple, carboxylic acids (including polymeric acids) are strongly bound to the surface of SiO 2 , ZrO 2 , TiO 2 , or Al 2 O 3 particles. If one uses bifunctional molecules with double bonds (e.g., methacrylic acid, MAA) in addition to hydrolyzable alkoxide group, after controlled hy- drolysis, corresponding precursors can be synthesized from oxide particles with a size of about 2 nm capable of copolymerizing (e.g, by photo-induced polymeriza- tion) by means of the double bond of MAA acting as a surface modifier (Scheme 1). Also note that, for example, zirconium alkoxide Zr(OR) 4 is highly inclined to hydrolysis; upon ordinary sol–gel synthesis, ZrO 2 · aq precipitate is formed which cannot be used to prepare homogeneous composite. Upon binding with MAA, the ability of Zr(OR) 4 to hydrolysis drastically lowers; hence, in the presence of latent water in the solution of hydrolyzable and con- densing alkoxide, well-dispersing ZrO 2 nanoparticles are formed. Oxopolymers synthesized by SGS form similar to zeolites, porous structures with pore sizes of 1–10 nm that are called sometimes nanoperiodic (mesostruc- tured) materials. Their specific surface area S sp varies from 130 to 1260 m^2 g–1, depending on synthesis con- ditions. Titanium-containing silica mesoporous molecu- lar sieves of hexagonal (MSM-41) and cubic (MSM-48) (designations of Mobil Corporation) types are of spe- cial interest including for catalysis; progress in this field is reviewed in [18, 19]. Rather large size of their pores (2–3 nm and larger) provides versatile possibili- ties for the modification of internal surface to control hydrophobic–hydrophilic and acidic properties, as well as for the design of catalytically active sites. The regime of drying stage, during which volatile components are removed, is responsible for the texture of a product: coarse xerogels can be formed because of the sintering of gel particles upon the prolonged air-
POLYMER SOL–GEL SYNTHESIS OF HYBRID NANOCOMPOSITES 661
synthesized by the co-hydrolysis of functionalized Si and Ti alkoxides; these films are widely applied in pho- tonics [23].
Below we analyze main approaches used in the syn- thesis of hybrid materials of such types.
2. COMBINATION OF SOL–GEL SYNTHESIS
AND POLYMERIZATION
In our opinion, the most convenient preparation pro- cedure for hybrid nanocomposites is the technique based on polymerization transformations of hybrid monomers, compounds whose molecules contain a fragment comprising the precursor of inorganic compo- nent and polymer-forming group. Compounds of such type are called metal-containing monomers (informa- tion concerning their synthesis and polymerization transformations is summarized in the monograph [24]). One of the first representatives of these compounds were titanium (trialkoxide)methacrylate monomers CH 2 =C(CH 3 )COO–Ti(OR) 3 where R = Bu, Pr i , t -Bu, t - amyl, or 2-ethylhexyl [25]. In principle, to produce materials of ORMOSIL type including one-stage syn- thesis of functionalized mesoporous silica MSM-41, vinyl derivatives such as CH 2 =CHSi(OEt) 3 can be used
(as is known, R–Si bonds are not subjected to hydroly- sis) [26].
Metal alkoxides are inclined to the association based on the nucleophilic addition of negatively charged OR group to positively charged metal atom. The degree of association is determined by both the reaction conditions (in particular, the type of solvent and temperature) and the nature of metal and alkyl groups. Using 1 H and 13 C NMR and IR spectroscopy techniques, the existence of various structures where methacrylate (Mc) group forms bridge bond was estab- lished (scheme 2).
Scheme 2.
O
Ti Ti
O
C
O C
O
RO
RO
RO
C
H3C CH 2
C H3C CH 2
OR
OR
OR
O
C O Ti
C
OR
RO OR
H3C CH 2
O
Ti Ti
O
C
O C
O
OBu
BuO
C
H3C CH 2
C H3C CH 2
OBu
OBu
OBu
OBu
C
O Ti
Ti
O
Ti
O
C
O
O C
O
RO OR
OR
OR
OR
RO
OR
RO OR
C H3C CH 2
C H3C
CH 2 C CH 2
CH 3
The formation of clusters of [(RO) n M] x Y type, where Y is polymer-forming organic group and x ≥ 2 [27], quite often precedes the formation of inorganic polymer in such systems. Methacrylate-substituted tet- ranuclear titanium-, zirconium-, and tantalum oxide clusters Ti 4 O 2 (OPr i ) 6 (OMc) 6 , Zr 4 O2(OMc) 12 , and Ta 4 O4(OEt) 8 (OMc) 4 are characterized. The mechanism of the formation of such structures is reduced to the fact that Mc groups substitute one or more alkoxide ligands and the liberated alcohol interacts with the excess of acid to form ester and water. The latter, hydrolyzing unreacted alkoxide groups, forms oxide (hydroxide) environment of a cluster. Since such processes proceed relatively slow, one can rigorously control the growth of carboxylate-substituted oxometalate clusters. Their nuclearity (i.e., number of nuclei) and shape are deter-
mined by the ratio of initial components, as well as by the nature of OR groups in the alkoxide. Cluster monomers of other metals, e.g., Hf 4 O 2 (OMc) 12 , Nb 4 O 4 (OPr i ) 8 (OMc) 4 [28] and Ti, Zr, and Ta clusters of higher nuclearity (Ti 6 , Zr 6 , Ti 9 ) including a series of mixed (Ti/Zr) oxide clusters [29, 30] are also described. Cluster Ti 4 consists of four octahedral units, albeit more condensed than in Ta 4 , because two central octahedra more readily use the face than angles for their binding. Two other octahedra are bound with this face via μ 3 -oxygen (Fig. 1) The func- tions of six methacrylate groups are reduced to the bal- ance of charges and coordination numbers of metal atoms. The structure of Zr 4 cluster is similar to that of Ti 4 except for the fact that the coordination number of central Zr atoms is equal to 7 and external atoms, to 8 [31]; hence, the degree of substitution by bidentate car-
Specific surface area S sp and mean diameter d of TiO 2 particles
Temperature, K
S sp , m^2 g–1^ Porosity, % d , nm
- 332.8 57.6 4. 570 123.7 54.4 1. 670 118.4 53.6 10. 770 90.5 45.6 10 870 25 18.6 9.
662 POMOGAILO
boxylate groups is higher for Zr 4 than for Ti 4. The molecular structure of Ta 4 represents centrosymmetri- cal cycle of four octahedra linked by the angles [30]. Methacrylate groups form square-planar surrounding perpendicular to cluster core.
To prepare sol–gel products by polymerization, one can use not only unsaturated carboxylic acids, but also unsaturated alcohols. One of the first studies of such kind was the synthesis of alkoxy derivatives Ti(OR) 3 (OR') [32] and VO(OR)3 – n (OR') n [33] by the reaction
M(OR) n + m R'OH M(OR) n – m (OR') m + R'OH,
where M = Ti or VO and OR' is the residue of unsatur- ated alcohol (R = Pr i , R' = CH 2 C≡CH, CH 2 CH=CH 2 , (CH 3 ) 2 CC≡CCH=CH 2 , (CH 2 ) 2 OC(O)C(CH 3 )=CH 2 ). A number of polymerizable Ti4+^ and V5+^ alkoxides were
synthesized upon the removal of evolved alcohol (in the form of azeotrope with a solvent). Potential monomers (corresponding titanium alkoxides [34]) were synthe- sized by the same route using 2-methoxy-4-propylphe- nol. Crystalline products with more complex composi- tion were prepared based on zirconium alkoxide deriv- atives [35].
Hybrid materials can also be synthesized by the reactions between metal amides and hydroquinone, 2,6-dimethylphenol or 2-methoxy-4-propenylphenol [36]. The alcoholysis of Ti(OPr i ) 4 with hydroquinone results in covalent 3D Ti4+–quinone networks [37]; similar networks are also formed during the condensa- tion of yttrium isopropoxide with various diols [38] (Scheme 3).
Scheme 3.
2-Hydroxyethylmethacrylate (HEMA) is of special interest from the viewpoint of the functionalization of alkoxy derivatives of various metals or relevant nano- particles. Due to the presence of OH group, HEMA is a good co-solvent for TEOS and water; moreover, the viscosity of the composition is fairly low, thus ensuring proper mixing of components.
Also note that, to prepare hybrid nanocomposites by the combination of SGS and polymerization, one can use hybrid macromers [39] based on polyhedral oligos- esquioxane (POSS) containing almost spherical inorganic core Si 8 O 12 with a diameter of about 1.4 nm (Fig. 2). Methacryloyl-functionalized POSS (MA–POSS), a new class of monomers that have been synthesized only recently and is also extensively studied in the synthesis of hybrid materials. These monomers can be easily copolymerized with other methacrylate monomers such as 1,6-hexamethylenedimethacrylate, triethylene glycol dimethacrylate, HEMA, etc. (Fig. 3). Such mul- timethacrylate macromers are promising for the use in
dentistry, because materials based on these macromers possess low shrinkage^1. Indeed, the addition of only 5 wt % of MA–POSS practically prevents the shrinkage of standard denture materials [39]. The formation of hybrid nanocomposites was clearly demonstrated [40], using the combination of free-radical polymerization of N,N-dimethyl acrylamide (DMAAm) and the hydrolysis–condensation of TMOS under the con- ditions of acid catalysis as an example (Fig. 4). The poly- mer synthesized at DMAAm/TMOS = 1 : 2 ratio and a 1% concentration of initiator (azobisisobutyronitrile, AIBN) was characterized by the number-averaged molecular mass of 76 000. The composite has a high degree of homo- geneity; its specific surface area (up to 365 m^2 g–1) and pore volume (after calcination at 870 K) depended on
(^1) A significant decrease in the volume (shrinkage) in the course of polymerization of methyl methacrylate monomers causing the stress of matrix or dental tissue and a low conversion of double bonds are two factors limiting the clinical application of dental materials based on methyl methacrylates.
Y O
O R O Y
R O Y O R O Y n
S SO 2
R =
–C 2 H 5 –
Y(O i Pr) 2 + HO–R–OH
664 POMOGAILO
tion [47]. For example, macroinitiator is immobilized on the surface of SiO 2 nanoparticles (mean diameter is 75 nm) and, in the presence of CuCl/4,4'-di(5-nonyl)- 2,2'-dipyridil, styrene is polymerized with the transfer of hydrogen atom [48]. Physicochemical properties of thus synthesized nanocomposite are much better than
for the composite produced by the traditional procedure (graft polymerization of corresponding monomers). We offer still several examples of the preparation of hybrid nanocomposites by the combination of SGS with the polymerization. Methacryloxypropyltri- methoxysilane is hydrolyzed and condensed upon the
O
Si O
Si
O
Si
O Si
O
Si (^) O Si
O
Si
O Si
R
R (^) R
O
O R
R
O
R
O
R O
Si O
Si
O
Si
O Si
O
Si O
Si
O
Si
O Si
R
R (^) R
O
O R
R
O
R
O R
CH 3
x y (^) n
AIBN, 4-methylstyrene, 333 ä toluene
Fig. 3. Principal scheme of the preparation of new class of POSS-based hybrid materials.
CH3O Si
OCH 3
OCH 3
OCH 3
CON(CH3) 3
Si OH
HO O
O Si
O O
O Si
O O Si
OH
OH
O Si
O Si
OH
O
O Si
O
OH
O Si O
O
HO Si O Si HO O
O
Si (^) O
O
O
Si HO O
CON(CH3) 2
CON(CH3) 2
O Si
HO
O O Si OH
O Si O
O Si
OH
O
O
Si
OH O Si OH
O
H2C
CH
C
CH 2
(CH3)2N
O
CH
C
CH 2 CH C CH 2 O
(CH3)2N
CH
C
O N(CH3)^2
O
TMOS N(CH3)^2 CON(CH3) 2
DMAAm
H3 O+ AIBN, 333 ä
polymer
SiO 2 polymer
SiO 2
SiO 2
SiO 2
Fig. 4. Typical route of the preparation of hybrid nanocomposite.
POLYMER SOL–GEL SYNTHESIS OF HYBRID NANOCOMPOSITES 665
addition of 0.5 N HCl; the product of interaction of equimolar amounts of Zr(OR) 4 and MAA is mixed with condensed silane in a required proportion and the cal- culated amount of H 2 O is poured. Optimal amount of photoinitiator and alcohol (as a viscosity-controlling solvent) are added to the formed mixture. Thin coatings of such photosensitive material are prepared by dipping [49]; the structure of this material is shown in Fig. 6. Dispersion polymerization of corresponding prepoly- mers is still one more variant of synthesis. For example, “beads” with a size of 100–150 nm are formed on the random copolymerization of silicon- and magnesium- containing monomers (Fig. 7); these beads are then transformed into ceramic particles with a size of about 100 nm during the processes of hydrolysis/pyrolysis in a humid atmosphere upon heating to 1270 K.
We do not consider precursors with a more com- plex composition, which can be used for these pur- poses, for example, silylated chalcogens of E(R)SiMe 3 (E = S, Se, Te) type [50], derivatives of trimethoxysi- lyl ferrocene, etc.
Thus, the general scheme of the preparation of nanoceramics via the stage of polymerization of corre-
sponding monomer can be reduced to the scheme pre- sented in Fig. 8. Above, we briefly analyzed main approaches to the formation of hybrid nanocomposites by SGS in variants when organic and inorganic polymers are formed in situ. However, in should be born in mind that SGS as a rule, is a fast process compared to the polymerization which can last for several hours, common solvent is not always present, and growing polymer can cause differ- ent effect on the properties of forming material at dif- ferent stages of polymerization. The alternate variant is the use of polymers prepared and characterized in advance.
3. FORMATION OF SOL–GEL PRECURSORS
IN THE PRESENCE OF ORGANIC POLYMERS
This type of hybrid nanomaterials includes inor- ganic metal oxide-containing polymer formed in the presence of organic polymer acting as a template. For example, latex particles, bacteria, etc. (see Section 5) can act as peculiar templates for the directed synthesis of hybrid materials including materials with anisotropic properties. At present, numerous variants of inorganic SGS performed in the presence of organic polymer
Si(OR) 4 + (RO)3Si (^) O Si O Si
O
O
Si O Si O Si
O
O Si O Si
O
O
O
OH
OH
O
O
AlO(OH)
O Si O O Si O
O Si
OH
Si OH
O O
Si
O
Si
O
O OH
O Si
O
Si
O
O
Si
O Si O O Si O
O Si AlO(OH)
HCl/H 2 O Al 2 O3 suspension
Hardening
Poly(ethylene oxide)
Fig. 5. Reactions and structural models of network systems based on Al3+.
O
POLYMER SOL–GEL SYNTHESIS OF HYBRID NANOCOMPOSITES 667
Let us consider the main routes for the synthesis of hybrid composites based on two crosslinked polymers, organic and inorganic, forming covalent or ionic bonds. Two approaches are most widely used. The first approach is the formation of secondary network in the presence of preliminarily prepared primary network, which should be properly functionalized; functional- ized inorganic macromers or oxopolymers are used occasionally. The second approach is the simultaneous formation of two different networks (including IPNs) from molecular precursor containing “organic” and “inorganic” functional groups reacting by different mechanisms (polyaddition, polycondensation, hydroly- sis–condensation, etc.). Primary emphasis is immedi- ately placed upon the fact that this problem is complex per se and is accompanied by many obstacles. Studies in this direction are just started; it seems to us that there is still no clear knowledge of this problem; and, as was mentioned above, the number of reliably performed variants of synthesis is not so large.
The first approach was initiated using polymer solu- tions; however, uncontrolled phase separation occur upon the gelation resulted in the formation of nonuniform mate- rial. Such procedure was used to synthesize two-compo-
nent networks in polyisobutylene (PI)/PDMS [54] and oli- gocarbonate dimethacrylate/Ti(OEt) 4 [55] systems. Nano- composites with the strongest interaction between inorganic and organic polymer chains were prepared [56] by the dissolution of HO–PES–OH (PES is poly(ether sulfone) and TEOS or TMOS in DMF: tet- raalkoxysilane hydrolysis–condensation catalyzed by acid resulted in the formation of products according to the following scheme:
≡Si–OR + HO–PES–OH
≡Si–O–PES–OH + ROH.
The poly(ether sulfone) chains are then crosslinked by alkoxysilane bridges, and nanoparticles are formed upon the condensation of alkoxysilane and the binding groups (Fig. 10). Chemical interaction between polymer organic and inorganic components with the formation of covalent bonds was observed in the course of sol–gel process in a poly(styrene- co -maleic anhydride)/TEOS system in the presence of 3-aminopropyltriethoxysilane [57]; in this case, inorganic particles with sizes less than 20 nm were formed (Scheme 4).
Scheme 4.
Allylacetylacetone (3-allyl-2,4-pentanedione) can also be used as compatabilizer [58].
New organo–inorganic hybrid filler for dentistry was prepared by the sol–gel process using copolymer of methyl methacrylate and 3-(trimethoxysilyl)propyl- methacrylate [59]. Polymethacrylate fragments appeared to be covalent-bound and uniformly distrib- uted in a silica network, at a molecular level without macroscopic separation of organic and inorganic phases. In contrast to aforementioned standard denture materials based on 2,2-bis( n -2-hydroxy-3-methacry- loxypropoxyphenyl)propane and triethylene glycol dimethacrylate filled with finely dispersed SiO 2 , stron- ger interfacial interaction takes place in nanocompos- ites prepared by SGS, because the surface of hybrid
filler originally comprises polymer component whose structure is close to that of a matrix. One of the most important conditions of the homogeneity of the hybrid of polystyrene (PS) and the product of sol–gel transfor- mations of phenyltrimethoxysilane is the π–π interac- tion between benzene rings of PS and the particles of modified silica gel [60]. Seemingly, the structural orga- nization of composites prepared by SGS using phase- separated poly(isoprene- block -ethylene oxide) and two alkoxides ((3-glycidyloxypropyl)trimethoxysilane and aluminum butoxide) is yet more complex [61]. The attachment of PEO macromolecules at the sur- face of metal oxides via H-bonds of their oxygen atoms and OH groups of the surface occurs even through the layers of water coordinated with these groups. Some-
CH (^) CH 2 CH C
CH
O O
C O
CH CH 2 CH HOOC
CH
O
C NH CH2CH2CH 2 Si
O
Si
O Si O
Si
O
O
O (^) Si
O
O Si
(1) NH 2 –CH 2 CH 2 CH 2 –Si(OC 2 H 5 ) 3 (2) TEOS/H 2 O, NH 4 OH
668 POMOGAILO
times, to prepare SiO 2 /PEO hybrid gels, both compo- nents (inorganic precursor and polymer) are modified. For example, to synthesize effective luminescent mate- rials, 3-isocyanotriethoxysilane and O,O'-di(2-amino- propyl)–PEO are used, the latter binds both the lan- thanide ion and ligand upon swelling in terbium nitrate and 2,2-dipyridyl ethanolic solution [62]. Hybrid mate- rials, containing not only luminescent but also redox- or
catalytically active sites, are prepared in the same way. For example, oxygen-selective organo–inorganic hybrid membranes contains salcomine as an oxygen carrier [63]. It was proved that the hydrogen bond is formed between amide groups of polyamidoimide (PAI) and hydroxyls of inorganic oxide in the PAI/TiO 2 compos- ites synthesized in situ using SGS (the size of TiO 2 par- ticles varied from 5 to 50 nm with an increase in its con- tent from 3.7 to 17.9 wt %) [64]. Composites synthesized on the basis of inorganic structures and polymers and having higher thermal and mechanical characteristics compared to initial compo- nents are also used to produce optical waveguides [65]. They are formed by the deposition of polymer–TiO 2 composite onto a glass plate with subsequent thermal treatment (in nitrogen atmosphere for 30 min at 573 K). This method is advantageous, because transparent col- orless films can be prepared due to the prevention of the formation of yellow titanium complexes. Thus, in some cases, it is sufficient to mix solutions of a polymer and sol–gel precursor in order to obtain polymer–inorganic composite. Phase separation in such IPNs is stopped, as a rule, at a nanolevel: nanosized domains of different phases are formed. Composites based on polyimides and SiO 2 or TiO 2 nanoparticles possess high mechanical strength because of the formation of 3D inorganic networks. The use of precursors for the synthesis of polyimide— polysilsesquioxane (PI–POSS) composites [66] is an interesting variant of the synthesis of similar materials. In particular, films of organo–inorganic hybrids con-
CH 3 H2C C C O O (CH2) 3 O Si O
O
CH 3
H3C CH 3
CH 3 H2C C C O O–
CH 3
H2C C
C O
Mg 2+
CH 3 OH, AIBN 65°C (^) Humid air 1000°C
Fine ceramic powder
Polymeric precursor
Fig. 7. Nanocomposites synthesized by the copolymerization of prepolymers.
Monomer
Synthesis
Crosslinking
Thermolysis
Crystallization
Polymer
Preceramic network
Amorphous ceramic
Crystalline ceramics
Fig. 8. Scheme of the preparation of nanoceramics.
670 POMOGAILO
Ga–Si–O materials containing acid sites, as good pre- cursors for new catalysts of Brönsted acid type [73], and many other materials. The most important of them are zircon ZrSiO 4 (SiO 2 · ZrO 2 ) synthesized from ZrOCl 2 · 8H 2 O or Zr(OPr i ) 4 and Si(OEt) 4 with subse- quent thermal treatment, as well as V- or Ni-zircons and nanofilms of Pb(Zr, Ti)O 3 [74]. Various procedures are employed to synthesize SrTiO 3 [75] (Scheme 6a) and
zircon doped with 12 mol % of ëÂé 2 (Scheme 6b). Sometimes metal nanoparticles are simultaneously formed in a system (by the hydrolysis of metal ace- tate/Si(OEt) 4 mixtures) that ensures the formation of versatile products of general formula SiO 2 · x M (M = Ni, Cu, Co, Cu/Ni, Co/Ni, Co/Cu, Pt, Gd/Pt, Au, Pd, Zn, Cu, etc.).
Scheme 6.
For example, M–MCM-48 materials (with S sp ~ 1000 m^2 /g and d 2.5 nm for M = Ti) were obtained from TEOS, Ti(OPr i ) 4 , NaOH, and chromium and vanadyl salts using surfactants as templates. These materials are distinguished by an increase in the degree of condensation of silica skeleton (approximately by 25%) upon the incorporation of metal. Ultradispersed powders of barium ferrite, thin films of mixed ferrite Li–Zn, and cobalt ferrite were synthesized using SGS.
However, polymerization SGS was most widely employed to synthesize heterometallic ceramics of per- ovskite type with the ÄÇé 3 structure. Mineral perovs-
kite, CaTiO 3 , has a pseudocubic lattice: large cations (A) are located at the unit cell angles; small cations (B), in its center; and oxygen ions, in the center of faces. Such materials are widely applied in electronics due to their specific ferro-, piezo-, and pyroelectric properties. Traditional route of their synthesis, e.g., of PbTiO 3 (A2+B 4+O3 ceramics), involves solid-phase mixing of PbO and TiO 2 in mills and subsequent calcination at temperatures above 870 K ( ex situ synthesis). However, such process is accompanied by the undesirable forma- tion of the phase of toxic PbO. SGS, where the com- plexation of components with citric acid and polyester-
Ti(OPr i^ ) 4 Ethylene glycol
Citric acid SrCO 3
Metal—citrate complexes
Condensation at 130°C
Polyesterification
Polymer resin
Pyrolysis at 350°C
Powdered precursor
Heating at 450 and 500°C, 8 h in air
SrTiO 3
PVA ( M w = 1000)
Stirring at 80°C
Transparent solution
Removal of excess H 2 O
Concentrated solution of polymer complex (“gel”)
Thermal treatment at 400–800°C in air
Zr0.88Ce0.12O 2
H2O
Stirring at 60°C
ZrOCl 2 · 8H2O
Ce(CH3COO) 3 · H2O
(a) (b)
POLYMER SOL–GEL SYNTHESIS OF HYBRID NANOCOMPOSITES 671
ification by ethylene glycol with subsequent polymer- ization at 403 K and pyrolysis [77], is free of these drawbacks. In this case, thin films with the properties of block material can be produced.
Barium stannate BaSnO 3 with particles sizes of 80– 100 nm [78] and monodispersed nanoparticles of other multicomponent ceramics with the structure of perovs- kite: BaTiO 3 [79], (Ba,Sr)TiO 3 , Pb(Zr,Ti)O 3 , NdAlO 3 (Ä3+Ç3+é 3 ceramics), ZrO 2 /X (X = CeO 2 , Y 2 O 3 , Y 6 WO 12 ), SrBi 2 Ta 2 O 9 , and even high-temperature superconducting ceramics YBa 2 Cu 3 O7 – δ were synthe- sized by polymerization/pyrolysis. Information on the chemical routes of synthesis of such materials can be found in many publications (e.g., see [80]). Oxides with the molecular level of homogeneity and high purity
were prepared by polymerization synthesis with subse- quent decomposition of organic phase and calcination. Here, we focus attention on the possible formation of thin single-phase films of KTiOPO 4 (having high ther- mal stability and remarkable optical properties for the use in nonlinear optics) using Ti(OEt) 4 , KOEt, and var- ious phosphorous sources (optimal of these sources is ( n -BuO) 2 P(O)(OH)) as precursors [81]. Heteropolymet- alates of the Keggin type (H 3 PW 12 O 40 , H 4 SiW 12 O 40 ), spherical particles with a diameter of ~1 nm, incorpo- rated into organo–inorganic matrices, can be referred to the same group of hybrid nanocomposites. Such mate- rials can be applied in holography. They are prepared, in particular, by mixing of polymetallate particles with TEOS (at W/Si = 0.2–0.6) and tetraethylene glycol. The idealized structure of such materials is shown in Fig. 12.
O
O
O
4
Si,
O
CH 2 O 4
Si,
O
O
O
O
O
Si O
O
O
O
4
Si + H 2 O O
OH + SiO 2
O
O O
OH
n
O (^) n
O
CH 2 O 4
Si
O
CH 2 O
(a)
NaF
R.
SiO 2
H 2 O, NaF
(b)
SiO 2
Crosslinked polymer
Low-density SiO 2 network
Fig. 11. (a) Modified precursors of Si alkoxides and (b) scheme of the formation of organo–inorganic interpenetrating networks with the participation of precursors.
H
POLYMER SOL–GEL SYNTHESIS OF HYBRID NANOCOMPOSITES 673
binogalactan possesses also membranotropic activity and properties of immunomodulator [90, 91].
The encapsulation of enzymes into the matrix syn- thesized by the SGS was made for the first time in 1990 by mixing of biomolecules with sol–gel precursors [92]. Dozens of various hybrid bioceramic materials of such type were developed literally over the course of several years. Inorganic matrices of such materials rep- resent silicon, titanium, and zirconium oxides, TiO 2 – cellulose composites, etc. which can be prepared as monolith xerogels or powders. Such materials can be used in biotechnology, for the development of electro- chemical and optical biosensors and electrodes, as
encapsulating media for drug delivery, adsorbents for pharmaceutical and cosmetic industries, etc. The immobilization of biologically active molecules in ceramic (inorganic) gel is a widespread method. Bioceramics binds with somatic bony or dental tis- sues (above, the attention was focused on the use of hybrid nanocomposites in dentistry) either directly or act as a shell for incorporating synthetic composite into human organs. Bioceramics is an ideal material due to its high rigidity, fracture strength, and impact viscosity. In turn, although many organic polymers that are widely used for the replacing of soft tissues are not bio- logically active, they possess biotolerant properties [93]. Moderate temperature and soft conditions of hydrolysis and condensation–polymerization of mono- meric alkoxides make it possible to immobilize pro- teins without their denaturation at the stage of matrix synthesis. In this case, the inertness of a matrix, its high specific surface area, porosity, and optical transparency facilitate the procedure for the immobilization of pro- teins without their covalent binding. All this makes dif- ferent sol–gel variants of the immobilization of proteins and even the whole cells very attractive. Here we con- fine ourselves to the analysis of main approaches and enumeration of basic synthesized materials. The most significant manifestation of SGS pro- cesses in biomineralization is the formation of mixed- valenced polynuclear structures (particularly, oxocom- pounds of iron, molybdenum, or manganese). In the course of biomineralization–bioconcentration, bioor- ganic matrix (template) controls the processes of nucle- ation, growth, and formation of inorganic materials with perfect morphology; as a result, complex hierar- chic structure with unusual chemical and physical properties arises, which is imitated with different
1 2
3 4
Fig. 14. Nanoperiodic structures and mesoporous molecular sieves prepared by template assembly using surfactants: ( 1 ) surfactant micelle, ( 2 ) micelle covered with functional- ized silica shell, ( 3 ) functionalized nanoperiodic structure containing surfactant, and ( 4 ) mesoporous molecular sieve with functionalized pores free of surfactant.
ÇÓÒÒÚ‡ÌÓ‚ÎÂÌËÂ
áÓθ-„Âθ ÒËÌÚÂÁ (^) Si
Si Si
Si
Si O
O O
O
O
Pt2+
Pt
Pt
Pt
Fig. 15. The combination of the reduction of metal ions and sol–gel synthesis: (a) the formation of Pt nanoclusters in reverse micelles and (b) the formation of SiO 2 shell around nanoclusters.
(a)
(b)
Reduction
Sol–gel synthesis
674 POMOGAILO
degree of success under artificial conditions. Biological systems (living organisms) control the mineralization and synthesis of nanocrystals of oxides of various met- als (the most studied are Mg, Ca, Sr, Ba, as well as Mn, Fe, and Si oxides) [94–96]. Furthermore, particles of such relatively complex compounds as BaTiO 3 , SrTiO 3 , and NaNbO 3 , perovskites with the structure of ÄÇé 3 type whose synthetic analogs were discussed in Section 4, and even finely divided monodispersed par- ticles of the precursors of high-temperature supercon- ducting ceramics can be also involved into the pro- cesses of bioaggregation [97].
Studies of biomineralization should answer two fun- damental questions: how highly organized inorganic materials are formed (morphogenesis) and how these processes are reproduced in biomimetic systems (mor- phosynthesis or mineralization in situ ). One of the main problems is the elucidation of the mechanism of nucle- ation and crystal growth. Molecular recognition and molecular tectonics are the most important aspects of biomineralization, although the genetic fundamentals of the evolution of biomineralogical picture remain unknown, as well as the answer to the principal ques- tion: how the morphogenetic compatibility at the boundary between living and inorganic is ensured. It is only known that the protein environment promotes the production of intricately organized (both in shape and composition) products, To facilitate the incorporation of many metals, e.g., Ca2+, into bioactive soft tissues, they are preliminarily protected by rubber-like ormosils or incorporated into SiO 2 –êååÄ nanocomposites [98]. Upon the accumulation of silver ions by some bacteria, nanocrystals of metal silver are formed, the amount of sorbed metal being as high as 25% of the dry substance of microbial cells [99].
Proteins, such as copper–zinc–peroxide dismutase, cytochrome, myoglobin, hemoglobin, and bacterior- hodopsin encapsulated into porous SiO 2 matrix that was synthesized by SGS, are effectively retained by the matrix without the loss of their activity [100]. At the same time, such a matrix ensures the access of small molecules to the active site of protein (enzyme) and transport of products to the surrounding medium. The SiO 2 matrices with immobilized glucoseoxidase and
peroxidase are used as active solid-phase elements in glucose sensors. In this case, immobilized enzymes exhibit higher stability upon variations in temperature and pH than in the initial state. Antibodies for medicine, immunochromatography, immunosensorics, etc. are bound in the same manner. For example, immunoglob- ulins captured by the sol–gel matrix retain their ability to bind antigens (2,4-dinitrophenylhydrazine) from solution [101]. Atrazine-binding properties of monoclonal antiatra- zine antibodies immobilized by the sol–gel matrix modified by 10% PEO were studied in detail in [102]. Such antibodies “recognized” in solution and bound widely used atrazine herbicides. It is important that no washing-out of antibodies into solution, as well as non- specific physical adsorption of atrazine by inorganic matrix, was observed. The activity of antibodies did not lower as well, at least, for two months, whereas their activity in solution decreased under these conditions to 40%. In addition, the advantage of sol–gel methodol- ogy is the fact that it allows one to avoid the purification of immunoglobulins. Particles of such composite with encapsulated antibodies can be used as a working ele- ment of a sensor to identify specific antigens [103]. First successful attempts of attachment at the sol–gel matrix and the use of 14D9 antibodies were reported in [104]: such antibodies catalyze the hydrolysis of acetals, ketals, epoxides, and other compounds. Enzymes encapsulated into silica gel nanoparticles can compensate for the enzyme deficit; their use in medicine prevents the risk of allergy or proteolytic reaction due to actually zero wash- ing-out. Other advantages of such materials are conve- nient storage and repeated use; the course of enzyme reaction is fairly simple to be monitored by spectral methods; moreover, particle sizes and physicochemical properties of materials can be controlled and varied within wide limits; it is also possible to control the molecular design of components. SGS was implemented also as the chemical immo- bilization of enzymes [105]. Active end groups of enzymes and active chemical bonds (e.g., Sn–Cl) incorporated into nanoceramics composition are employed for this purpose. The synthesis of such mate- rial and enzyme immobilization can be represented by Scheme 7.
Scheme 7.
HO Ti
O
O
SnCl 2 + O O Ti
O
O
Cl Sn O + HCl
O Ti
O
O
Cl Sn O O Ti
O
O
Enzyme–SH + Enzyme S^ Sn O + HCl
676 POMOGAILO
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