hybrid organo–inorganic polymer systems: synthesis, structure, and properties

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Theoretical and Experimental Chemistry, Vol. 46, No. 6, January, 2011 (Russian Original Vol. 46, No. 6, November-December, 2010) Dedicated to Academician of the National Academy of Sciences of Ukraine V. D. Pokhodenko HYBRID ORGANO–INORGANIC POLYMER SYSTEMS: SYNTHESIS, STRUCTURE, AND PROPERTIES UDC 678.664:661.683 E. V. Lebedev Hybrid compositions based on organic and inorganic reactive oligomers have been synthesized. It has been shown that variation of the chemical composition of the organic and inorganic components gave rise to the possibility for directed regulation of the properties of the materials obtained. Key words: hybrid organo–inorganic systems, urethane oligomers, silicates, epoxide resin. The increasing shortage of hydrocarbon raw materials, which are the starting materials for the organic polymer industry, necessitates the discovery of new ways to synthesize polymeric materials. One of these is to introduce mineral raw materials into the process of synthesis which, apart from economizing in the organic components, permits the preparation of polymeric materials with new properties, in particular for the development of technologies which occupy a greater place in the technological development of modern civilization. In this plan, it may be affirmed that classical polymer materials have almost reached the limits of their possibilities, especially in the spheres of microelectronics, optics, membrane technologies, catalysis, sensors, etc., and it is therefore extremely important to develop new materials with defined properties. New classes of polymers satisfying these requirements are hybrid polymeric materials which are composed of organic and inorganic (mineral) components at the molecular level. They have undergone intensive investigation over the last 10-15 years which resulted in the possibility of construction of polymer systems with definite hybrid structures with regulated characteristics ranging from the properties of the organic polymer to the inorganic material. Quantity of investigations connected with the synthesis and properties of these systems increased rapidly, the first results of which of the preceding studies were collated in a book [1] and in a fundamental publication [2]. The bases of the molecular design of organo–inorganic systems (OIS) were detailed by Lebedev et al. [3] and the membrane and barrier characteristics [4-6], the process of formation of films based on OIS [7, 8], the optical, mechanical and thermal properties [9-11], the synthesis of optically transparent OIS under the influence of microwave irradiation [12], the sensor characteristics of organo–inorganic materials [13, 14], etc., were reported in a number of papers. Special attention was paid to the electrical properties of OIS as a consequence of their potential use as solid polymeric electrolytes and ion-conducting membranes in current sources [15-18]. Achievement of the given properties depends not only on the chemical composition of the organic and inorganic components, but to a considerable extent on the structure on the nanometric scale, which permits the use of the organic and inorganic molecules as structural block and in this way determining the structuring of the hybrid systems [19]. Depending on the types and quantities of the functional groups capable of polymerization, one can obtain quite different structures of OIS: 0040-5760/11/4606-0409 ©2011 Springer Science+Business Media, Inc. 409 ___________________________________________________________________________________________________ Institute of Macromolecular Chemistry, National Academy of Sciences of Ukraine, Kharkivske Shose, 48, Kyiv 02160, Ukraine. E-mail: [email protected]. Translated from Teoreticheskaya i Éksperimental’naya Khimiya, Vol. 46, No. 6, pp. 391-396, November-December, 2010. Original article received October 12, 2010.

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Theoretical and Experimental Chemistry, Vol. 46, No. 6, January, 2011 (Russian Original Vol. 46, No. 6, November-December, 2010)

Dedicated to Academician of the National Academy of Sciences of Ukraine V. D. Pokhodenko

HYBRID ORGANO–INORGANIC POLYMER SYSTEMS:

SYNTHESIS, STRUCTURE, AND PROPERTIES

UDC 678.664:661.683E. V. Lebedev

Hybrid compositions based on organic and inorganic reactive oligomers have been synthesized. It has been

shown that variation of the chemical composition of the organic and inorganic components gave rise to the

possibility for directed regulation of the properties of the materials obtained.

Key words: hybrid organo–inorganic systems, urethane oligomers, silicates, epoxide resin.

The increasing shortage of hydrocarbon raw materials, which are the starting materials for the organic polymer

industry, necessitates the discovery of new ways to synthesize polymeric materials. One of these is to introduce mineral raw

materials into the process of synthesis which, apart from economizing in the organic components, permits the preparation of

polymeric materials with new properties, in particular for the development of technologies which occupy a greater place in the

technological development of modern civilization. In this plan, it may be affirmed that classical polymer materials have almost

reached the limits of their possibilities, especially in the spheres of microelectronics, optics, membrane technologies, catalysis,

sensors, etc., and it is therefore extremely important to develop new materials with defined properties. New classes of polymers

satisfying these requirements are hybrid polymeric materials which are composed of organic and inorganic (mineral)

components at the molecular level. They have undergone intensive investigation over the last 10-15 years which resulted in the

possibility of construction of polymer systems with definite hybrid structures with regulated characteristics ranging from the

properties of the organic polymer to the inorganic material. Quantity of investigations connected with the synthesis and

properties of these systems increased rapidly, the first results of which of the preceding studies were collated in a book [1] and

in a fundamental publication [2]. The bases of the molecular design of organo–inorganic systems (OIS) were detailed by

Lebedev et al. [3] and the membrane and barrier characteristics [4-6], the process of formation of films based on OIS [7, 8], the

optical, mechanical and thermal properties [9-11], the synthesis of optically transparent OIS under the influence of microwave

irradiation [12], the sensor characteristics of organo–inorganic materials [13, 14], etc., were reported in a number of papers.

Special attention was paid to the electrical properties of OIS as a consequence of their potential use as solid polymeric

electrolytes and ion-conducting membranes in current sources [15-18].

Achievement of the given properties depends not only on the chemical composition of the organic and inorganic

components, but to a considerable extent on the structure on the nanometric scale, which permits the use of the organic and

inorganic molecules as structural block and in this way determining the structuring of the hybrid systems [19]. Depending on

the types and quantities of the functional groups capable of polymerization, one can obtain quite different structures of OIS:

0040-5760/11/4606-0409 ©2011 Springer Science+Business Media, Inc. 409

___________________________________________________________________________________________________

Institute of Macromolecular Chemistry, National Academy of Sciences of Ukraine, Kharkivske Shose, 48, Kyiv

02160, Ukraine. E-mail: [email protected]. Translated from Teoreticheskaya i Éksperimental’naya Khimiya, Vol. 46, No. 6,

pp. 391-396, November-December, 2010. Original article received October 12, 2010.

linear with side inorganic groups, extensively cross-linked organo–inorganic structures, or in the form of individual hybrid

particles.

The most used technology for the preparation of hybrid organo–inorganic systems is sol–gel synthesis. It combines a

large group of methods of synthesis of materials from solution, the key element of which is the formation of a gel at one stage of

the process [20]. The best known is the sol–gel process with the controlled hydrolysis of alkoxides M(OR)x, where M = Si, Ti,

Zr, Zn, Mo, etc. In the first stage of the sol–gel process hydrolysis and polymerization leads to the formation of a colloidal

solution (sol) of hydroxide particles, the size of which does not exceed some tens of nanometers, for example [21]:

Si(OR)4

+ H2O � HO—Si(OR)

3+ R—OH.

In this way the OR groups may be substituted by OH groups:

Si(OR)4

+ 4H2O � Si(OH)

4+ 4R—OH.

The chemical compounds [(OR)2—Si—(OH)

2] or [(OR)

3—Si—(OH)] are also formed, they are products of partial hydrolysis,

and can combine with one another by polycondensation with formation of siloxane bonds [Si—O—Si]:

(OR)3Si—OH + HO—Si(OR)

3� [(OR)

3Si—O—Si(OR)

3] + H—O—H

or

(OR)3Si—OR + HO—Si(OR)

3� [(OR)

3Si—O—Si(OR)

3] + R—OH.

Thus 1-, 2-, and 3-meric nets of siloxane bonds [Si—O—Si] arise and monolithic gels are formed in which molecules of solvent

are embedded in the relatively stable three dimensional network formed from hydroxide particles. The following stage consists

in removing the solvent from the gel (drying). Depending on the method used, different types of dry gel may be obtained

(xerogel, aerogel) which retain the nanometric structural elements and high specific surface values.

To form hybrid systems a necessary stage is the modification of the inorganic network with organic fragments which

are capable of undergoing polymerization of the organic portion of the OIS, for example with initiators of polymerization or

organic monomers [22]. It is possible to combine the sol–gel process with polymerization of monomers of the traditional type

[23]. So the method described consists of formation of an inorganic network with further formation of an organic polymer in the

presence of this network associated with the formation of chemical bonds. It should be noted that depending on the direction in

which the synthesis is carried out an OIS may be obtained with the absence of covalent bonds between the organic and

inorganic components [19, 22]. As shown, the characteristic of the sol–gel method is that it is a multistep process.

In papers [24-26] a single step process for the synthesis of OIS was proposed based on reactions between organic and

inorganic monomers which contain corresponding reactive groups which form a hybrid organo–inorganic network. The

properties of the OIS depend on the chemical composition of the organic and inorganic components, their ratio, the reaction

conditions, and the presence of modifiers. In the pioneering work in this direction, which was carried out more than two

decades ago [24, 25] the possibility was demonstrated for the synthesis of organo–inorganic composites based on sodium

silicate (the inorganic component), polyisocyanates (the organic component), and modifiers (for example, oligomeric acrylate

esters). The reactive group of the organic component was the isocyanate group, NCO, which is highly unsaturated. On the other

side, sodium silicate may react with the organic compounds thanks to the activity of the hydroxy groups of the polysilicate ions

and the coordinately unsaturated silicon atom [25]. It was established that in these systems some parallel reactions occur, the

relative rates of which determine the properties of the OIS.

This approach was developed in later papers [26-32]. The authors [26] studied the conditions of synthesis of OIS, based

on reactions of urethane oligomers (containing free NCO groups) with sodium silicate, which exists in oligomeric forms in

aqueous solution [33] with the general formulae:

aNa2O·bSiO

2·cH

2O

410

where b/a is the silicate module and c/a is the water ratio. The structure of the OIS is formed as a result of a series of parallel

reactions:

The kinetics of the polymerization of these OIS, investigated by IR spectroscopy of the disappearance of the NCO

group during the synthesis, showed that participation of the NCO group obeyed a first order equation, and the rate constant for

the reaction increased with increasing content of the inorganic phase in the composition [26]. The OIS synthesized have

hydrophilic properties and are characterized by very high absorption volumes with respect to H2O (to 2000%), while the cyclic

process of absorption/desorption has reversible character [28, 31]. A low angle X-ray diffraction study showed that the

heterogeneity of the volume of the OIS is connected with the presence of inorganic phases, equal to 5-7 nm, which permits the

classification of the OIS as a nanocomponent system [32]. The proposed structural model, which describes the process of

absorption of water by the inorganic phase, calculations based on this model shows that the thickness of the layer of adsorbed

water on the inorganic particles may be 20 nm or greater. The large volume of water adsorbed on the inorganic nanophase is

retained in the OIS thanks to the elasticity of the organic material which facilitates changes in its volume over wide limits.

Investigations of the dielectric characteristics of OIS synthesized from urethane oligomers of different molecular

masses (MM) and functionality (which is caused by the density of the organic network) over broad frequencies and temperature

intervals showed that the Vogel–Taman–Fulcher model [34] can be used in these systems which incorporates a shift to a

decrease in the glass temperature or high frequency with growth of the inorganic composition of the OIS [30]. The electrical

conductivity and dielectric constant of the OIS changes in dependence on the MM of the oligomer and the density of the

network of the organic phase, with increase in the MM and content of the inorganic phase the electrical conductivity increases

by 1 or 2 orders of magnitude. The value of the dielectric constants increases with decreasing density of the networks which is

explained by the change in conditions for charge transfer in the molecular structures of the OIS [26, 29].

By using reactive modifiers the course of the synthesis reaction may be changed and consequently the structure and

properties of the OIS. Systems were studied in which the organic component (urethane oligomer (UO) with 3.6% free NCO

groups) was modified with polyisocyanate (PIC) containing 32% NCO groups, with a UO/PIC ratio of from 100/0 to 0/100,

respectively, with an overall quantity of NCO groups changing from 3.6 to 32% [27, 35]. Sodium silicate reacted

simultaneously with both UO and PIC to form two mutually penetrating organo–inorganic networks. One of them, the product

of the UO-sodium silicate reaction, was elastic, while the second formed by the reaction PIC with sodium silicate was stiff. The

structural model of this OIS is shown in Fig. 1. Depending on the UO/PIC ratio (i.e., on the content of NCO groups in the

organic component), one or other of the networks predominates which has a considerable effect on the properties of the OIS. As

shown in Fig. 2 (curve 1), the thermostability index (the temperature T at which deformation of the material L = 25% occurs at a

pressure of 1 MPa) rose considerably (from 90 to 265 °C) on increasing the NCO group content from 3.6 to 18% and then

remains constant. The electron conductivity (curve 2) decreases by almost 5 orders of magnitude over the same concentration

411

and then remains constant with an NCO group content of 18% or higher. The behavior of the dielectric constant, which

decreases from 17 to 6 (curve 3) and the tangent of the angle of loss (curve 4) behave analogously. This type of behavior

indicates that in the first region (up to 18% of NCO groups) the elastic network predominates and explains the characteristics of

the OIS. The structure of the OIS in this range corresponds to the model shown in Fig. 1. With increase in the content of the

NCO groups, the quantity and size of the solid phases increase, and with a concentration of 18% NCO groups a phase inversion

occurs and the stiff network becomes predominant. The included elastic component does not contribute importantly to the

characteristics of the stiff network, so that the parameters of the OIS remain constant in the range of 18-32% NCO groups. The

mechanical characteristics of the OIS (module of elasticity, relative elongation, mechanical relaxation time) have similar

dependences and vary from elastomer with high relative elongation (~1200%) to the strength high rigid material [36]. So that

with changes in the chemical composition of the organic component it is possible to regulate thermal, mechanical, and

electrical properties of the OIS over wide limits.

Modification of the inorganic component with modifiers leads to improvement of the characteristics of the OIS. For

example, modification of sodium silicate with �-caprolactam led to a change in the yield ratio of polyurethaneurea and

polyurethanesilicates on interaction of the sodium silicate with isocyanate-containing oligourethane, leading to an important

increase in the value of relative elongation [37]. Modification of sodium silicate with aminoacids or L-lysine produces an

increase in the yield of polyurethaneurea and polyureas which affects the formation of structures of the inorganic phase and the

OIS as a whole leading to an increase in deformability and water-stability of such systems [38, 39]. Additional reaction occur

when sodium silicate is modified with carbamide – evolution of ammonium cyanate, which induced polycondensation of the

silicate and leads to increased molecular mass of the siloxide anion that form intermolecular complexes as a result of

electrostatic interaction with the intermediate products of the hydrolysis of carbamide. This leads to an important increase in

the spectra of mechanical properties, namely strength on compression, relative deformation, specific viscosity, adhesion to

cement [40].

Change in the chemical structure of the inorganic oligomer, in particular introduction of positive metal ions (Na, Al,

Cr) or phosphorus, which determine the spatial structure of the inorganic phase, make possible variation over wide limits the

conditions under which the synthesis proceeds and, in its turn, hybrid OISs with varying properties, for example, thermal

stability, non-flammability. Detailed structural studies of the properties were carried out of the OIS synthesized with different

types of organic and inorganic oligomers. For example, the polymerization of silicopolyphosphate with polyisocyanates gave

complex spatial structures which consisted of two polymeric networks – organic and inorganic linked by chemical bonds [41,

42]. The morphology of the OIS was characterized by two level type of fractal ordering, where the larger massive fractal

aggregates consisted of small in size masses of fractals of lower structural level, which explains the anomalously high

412

Fig. 1. Structural model of OIS with organic components

which contain isocyanate compositions with urethane

oligomer and polyisocyanate, in consequence of which

elastic and stiff organo–inorganic networks are formed.

deformation properties of OIS of this type [43]. The fractal peculiarities of the OIS structure, in the first place the absence of

uniformity of the mass-fractal aggregates of the reinforced inorganic components is the reason for the anomalously high

deformation properties of the OIS (the relative extensibility reaches 1000%) with high contents (50-60%) of the inorganic

phase [44].

Methods have been developed for synthesis of inorganic silicon-, phosphorus-, and aluminum-containing oligomers –

sodium aluminophosphates, silicopolyphosphates, and silicoaluminophosphates. In the chemical compositions of hybrid OIS

formed by interaction of these oligomers with isocyanate products, the basic products are polyureas,

urethanaluminophosphates, urethanosilicopolyphosphates, urethanosilicoaluminophosphates with P—O—C and Al—O—C

chemical bonds. The materials obtained are non-flammable or capable of self-extinction [45, 46]. Investigation of the

thermostability of OIS based on urethanoaluminophosphates and urethanoaluminochromophosphates showed that interaction

of the mixed polyphosphates with isocyanates gave organo–inorganic systems with P—O—P bonds, and with increase in the

inorganic components in the composite there was an increase in fraction of the non-volatile products of combustion [47].

Organo–inorganic systems based on such large-tonnage products as epoxide resins have been developed [48, 49]. It

has been shown that by the interaction of epoxide resins (organic oligomers) with acid aluminum phosphates (inorganic

component) it is possible to synthesize hybrid organoaluminophosphate composites. The chemical structures of the

compounds obtained and the extent of the conversion of the epoxide groups depending on the composition of the initial reaction

mixture have been investigated. The structure and thermal properties of the epoxyaluminophosphate compositions depends on

the aluminophosphate content. The thermal destruction is a multi-stage process. The mechanism of the polymerization of

epoxyaluminophosphate OIS by electrometric methods and rheokinetic studies have been investigated [50]. In this hybrid

system several processes occur which affect the electric parameters of the composition in the synthesis process which lead to

the presence of two maxima in the kinetic dependence of the electric conductivity. These phenomena are connected with both

413

Fig. 2. Dependence of properties on the concentration

of NCO groups in the organic component of the OIS:

1) temperature of thermal stability; 2) electrical

conductivity; 3) dielectric constant; 4) tangent of the

angle of dielectric force.

the reaction in the ion-containing inorganic phase and also the growth of the molecular mass and gel-formation in the volume of

the polymer matrix. An increase in the content of the inorganic phase accelerates the rate of the polymerization reaction [51].

Modification of the epoxyaluminophosphate OIS with the help of an inorganic modifier – dispersed silicon dioxide – led to an

increase in adhesive strength which is connected with the structure forming influence on the process of polymer formation with

the weakly acidic —SiOH groups and other coordinatively unsaturated centers with increased adsorption and catalytic activity

at the surface of the silicon oxide [52]. The presence of dispersed filler in the OIS led to high thermal stability of the

composition in temperatures up to 380 °C in comparison with unfilled epoxyamine systems. Introduction of dispersed quartz

into the OIS led to a decrease in water adsorption which indicates the interaction of the surface of the filler with functional

groups of the polymer matrix [53].

Thus organo–inorganic systems present a wide range of possibilities for the synthesis of hybrid materials with

improved properties in comparison with traditional materials. Methods of synthesis of OIS in reactions of organic and

inorganic reactive oligomers makes it possible for direct regulation of the properties of the materials obtained.

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