Polymer Sol?Gel Synthesis of hybrid nanocomposites

Polymer Sol?Gel Synthesis of hybrid nanocomposites

(Parte 1 de 10)

1061-933X/05/6706- © 2005 Pleiades Publishing, Inc.0658

Colloid Journal, Vol. 67, No. 6, 2005, p. 658–677. Translated from Kolloidnyi Zhurnal, Vol. 67, No. 6, 2005, p. 726–747. Original Russian Text Copyright © 2005 by Pomogailo.

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 combination 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 stability 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 formation of nanosized oxide particles (from the skeleton of inorganic polymer) in the medium of organic polymer. Such nanoparticles find wide application as components of composites. They increase the strength and hardness of materials, act as reinforcing fillers for plastics and vulcanized rubbers, “binders” of polymer components enhancing thermochemical, rheological, electrical, and optical properties of materials. Organo–inorganic composites of such type are employed as chromatographic carriers and catalysts (including photocatalysts 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 materials is one of the reasons for their wide application in pharmaceutics, cosmetics, food industry and in medicine. 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 prehistory of “combination” of organic and inorganic phases (different variants of coprecipitation, hydrothermal 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 readymade linear polymer (from solution, melt, emulsion) [4, 5]. In turn, an inorganic component can be incorporated into the material composition as a metal oxidecontaining monomer, which is the “intermediate product” of nanoparticles (e.g., see [5] and below), as nanoporous 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 natural composites are also characterized by hybrid nanostructure; 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 processes) 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 1, 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 nanocomposites 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.

Vol. 67No. 6 2005

POLYMER SOL–GEL SYNTHESIS OF HYBRID NANOCOMPOSITES659 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 nucleation particle growth. Forming oxooligomers or polymers, as well as crosslinked macromolecules are nothing other than peculiar clusters [1] coexisting with the sol. As a rule, reactions of condensation and reprecipitation of monomeric and oligomeric molecules 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 2 (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 compounds [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(trimethoxysilyl)ethane as an example [15]. Similar (onestage) 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 synthesis 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 stabilizers. 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 example, 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 hydrolysis, corresponding precursors can be synthesized from oxide particles with a size of about 2 nm capable of copolymerizing (e.g, by photo-induced polymerization) 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 molecular sieves of hexagonal (MSM-41) and cubic (MSM-48) (designations of Mobil Corporation) types are of special 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 possibilities 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-

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drying. The values of S sp , mean pore diameter d , and porosity of TiO 2 prepared by SGS are listed in the table as dependent on calcination temperature [20]. If the drying conditions are chosen in such a way as to minimize the action of capillary forces, highly dispersed aerogels are formed, such as xerogels with the structure of hydrogel whose pores are filled with air instead of the removed liquid phase. (In recent years, ëé 2 in a supercritical state has been employed often for this purpose.)

At the stage of thermal treatment, the formation of structure and morphology of resultant product is completed. The annealing of material is accompanied by a number of physicochemical processes (the degradation of organic fragments, rearrangement of the structure of inorganic polymer, and its crystallization and sintering). Thus, polymer SGS is a convenient route for preparing disperse ceramic materials often called ceramers that contain comparable amounts of organic and inorganic components, among them in the form of interpenetrating networks (IPNs).

The classification of materials produced by polymer

SGS should take into account both the nature of precursors and the structure of forming networks, as well as the type of binding of organic and inorganic components [21]. The first group consists of materials where organic and inorganic components are bound by strong chemical bonds; the second group consists of materials with weak physical bonds. In turn, organic groups R' comprising precursor molecules of Si(OR) 4 – n type can perform two functions [2]: to modify inorganic network and to form the network of organic polymer. In the first case, synthesized materials are usually called ORMOSILs (ORganically MOdified SILicates); in the second case, ORMOCERs (ORganically MOdified CERamics). Materials of such types with a low (2– 30 vol %) and high (45–75 vol %) content of inorganic component are distinguished. For example, “rubber ORMOSIL” material (based on TEOS and polydimeth- ylsiloxane, PDMS) with M w = 1700–2000) contains more than 70 vol % of inorganic component. Similar composition characterizes the films of hybrid ormosils

RSiOR RSiOH + ROH

RS i OH + HO Si R RSiOSiR + H2O

ZrOR + H2O2 ZrOZr+ ROH

CO Zr C

OR CH3 CH2

H + MeOH

H 2 O

Condensation (latent water) :

Removal of free and latent water by condensation :

Photoinitiator Transparent liquid

Block materialThin films

Formation of complex : H 2 O ;

Scheme 1.

(Parte 1 de 10)

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