A number of elements can be used as the backbone for the inorganic component of these copolymers, including aluminium, zirconium and titanium; however those based on silicon have been the most closely studied, and we focus on these here.
4. Synthesis
ORMOCER®s are prepared via the sol-gel process, a common technique for the synthesis of a wide range of materials. A sol is a colloidal suspension of nanometre-sized solid particles in a liquid; a gel, formed from a sol, is a rigid, interconnected network of polymer chains in that liquid. The liquid can be removed and the gel, after various procedures, isolated as the product. The sol-gel process is particularly suitable for the synthesis of ORMOCER®s because it can be performed at near-ambient temperatures which do not cause decomposition of the organic polymers.
In ORMOCER® synthesis the sol is produced by the hydrolysis (reaction with water) and polycondensation of alkoxysilanes, compounds with the general formula Si(OR)4 where R is an organic group. The molecular mixing of the inorganic backbone with the organic polymer can be achieved in a number of ways.
4.1. Type I synthesis
In the first case, the inorganic network consists purely of Si-O-Si (siloxane) bonds and the organic polymer is dispersed within it at a molecular level. This is achieved by the hydrolysis of an alkoxysilane in which the organic groups R are all simple alkyl groups, such as methyl (CH3), in a solution containing the required organic polymer. On hydrolysis each organic group R is replaced by hydrogen, so that the alkoxysilane is converted to Si(OH)4. These molecules then react with each other to rerelease water and form a 3-D network of Si-O-Si bonds, which eventually sets to form a rigid gel: this is an inorganic network within which the organic polymer is dispersed. A significant limiting factor for this method is that only organic polymers with some degree of solubility can be used.
Fig 2. Hydrolysis and polycondensation of Si(OCH3)4 to form a 3-D network of Si-O-Si bonds.
4.2. Type II synthesis
In the second case, ORMOCER®s can be prepared in which the siloxane backbone itself contains organic entities (known as functionalizations). To achieve this, some or all of the alkoxysilanes used do not bear four simple organic groups as above, but two or three; the remainder are more complicated organic entities, for example aromatic rings or amino groups. On reaction with water only the simple groups are expelled, the others being resistant to hydrolysis, and an inorganic Si-O-Si network is formed in which Si remains bonded to the organic functionalizations. The ratio of functionalized to non-functionalized alkoxysilanes can be varied to alter the physical properties of the product.
Fig 3. Hydrolysis and polycondensation of a functionalized alkoxysilane to a functionalized Si-O-Si network. R is a hydrolysis-resistant organic group e.g. CH2CH2CH2NH2 (amino), C6H5 (aromatic).
4.3. Type III synthesis
In the third and generally most useful case, the siloxane networks can be covalently linked by organic crosslinking. In this case the alkoxysilane contains, as well as the simple organic groups, between one and three (typically one) reactive organic entities capable of undergoing polymerization. After the sol-gel process has created the organic group-bearing Si-O-Si network as above, polymerization of the organic entities is induced in a step known as curing, typically by the application of heat or ultraviolet radiation. The siloxane networks are now crosslinked by the newly-formed organic polymers. More than one organic entity may be used, and the properties of the resulting ORMOCER® can be altered depending on the relative amounts of each.
Since the organic polymer is formed after the sol-gel step, it is possible using this technique to produce hybrid materials containing polymers which cannot be incorporated directly by the first method due to their extreme insolubility.
Fig 4. Polycondensation of a hydrolysed alkoxysilane (the hydrolysis step is emitted) followed by curing to promote organic crosslinking of the Si-O-Si networks. R* is a reactive organic group capable of polymerizing to the species R.
A significant advantage of this method of ORMOCER® synthesis over the first two is the dramatic reduction in shrinkage of the gel during evaporation of the liquid. Shrinkage is a serious problem in many sol-gel syntheses and often restricts the extent to which the gel can be used as a bulk material; organically crosslinked ORMOCER®s show much less shrinkage than normal due to the preformation of the inorganic network. In special cases it is possible to specifically design non-shrink crosslinking ORMOCER® syntheses which do not require an evaporation step at all because the alcohol used to contain the sol/gel, and the alcohol expelled by the hydrolysis reaction, are both polymerizable and crosslink during the curing step.
A wide range of polymerizable organic entities can be used for this third class of ORMOCER® synthesis, giving rise to a wide variation in the properties of the products. Among the most important classes of polymerizable group are epoxides, acrylates and thiol/enes.
Epoxides contain an oxygen atom bound to two carbon atoms which are also bound to each other, forming a three-membered ring. On polymerization the ring is broken, or “opened”, and a polymer is formed containing C-O-C, or polyether, linkages.
Acrylates contain carbon-carbon double bonds (C=C) adjacent to carbonyl groups (C=O) and polymerize, by a variety of mechanisms, to acrylics.
Thiols are organic entities terminating in the group –SH. Alkenes contain C=C double bonds. The two can copolymerize in the thiol/ene addition to form thioether linkages, C-S-C.
a) b)
c)
Fig 5. General polymerization schemes for a) epoxides, b) acrylates and c) alkenes and thiols (thiol/ene addition).
5. Properties
In general, ORMOCER®s are transparent, homogeneous, electrically insulating materials with a density slightly above that of typical organic polymers. They are also strongly resistant to abrasion, and can be used as abrasion-resistant coatings. In their transparency and homogeneity they resemble inorganic networks such as glass, but unlike glass they are thermosets (they show plasticity before setting), and are less dense than inorganic compounds such as silica.
As might be expected from their structures, ORMOCER®s tend to show properties more typically organic (such as high flexibility, low density, low melting temperature) as we move from type I to type III. Beyond these general trends, the properties of ORMOCER®s can be tailored to a remarkable degree by varying the identity and amount of the organic components.
5.1. Type I and type II ORMOCER®s
ORMOCER®s of the first and second types are generally brittle, highly transparent materials showing a close resemblance to glass. For the first type, in which an organic polymer is dispersed within an inorganic matrix, as the amount of the organic component is increased the hardness of the material decreases, showing a transition from the hardness typical of inorganic networks to the rubberiness typical of organic polymers. (The hardness of a material is often expressed in terms of its Young’s modulus, a widely used mechanical measurement which shows the pressure required to deform a substance: higher values indicate greater hardness, or lower rubberiness.)
For the second type of ORMOCER®, in which the siloxane network is covalently bonded to organic functionalizing groups, the hardness and brittleness of the material is generally lower when the organic groups are larger, and when more are present. In particular, the Young’s modulus of type II ORMOCER®s can be varied enormously (from around 15 MPa to 15 GPa) by varying the amount of organic functionalizations.
The porosity of the material, which is usually very high for silica networks, also decreases with increasing size of the organic groups, as these groups are better able to fill the pore sites.
5.2. Type III ORMOCER®s
ORMOCER®s of the third type, in which the siloxane chains are crosslinked by organic polymers, show the greatest variation in physical properties. Two characteristics in particular of the organic polymers can be varied to tailor the properties of the ORMOCER®: the length of the organic chains that connect crosslinking sites (spacer length), and the functional groups present in the polymer.
The hardness of type III ORMOCER®s is greatly influenced by both of the above factors. By using polymerizable organic monomers which crosslink at increasingly more sites, the Young’s modulus is increased. For example it has been shown that an ORMOCER® formed by the polymerization of a diacrylate alkoxysilane (see Fig. 5) displays a modulus of 1290 MPa, while the corresponding triacrylate, containing one more polymerizable site per monomer, forms a harder material with a modulus of 1720 MPa.
Decreasing the spacer length has an even more dramatic effect. One multiacrylate ORMOCER® with 20 carbon atoms connecting the crosslinking sites has been shown to have a modulus of 70 MPa, while the corresponding 11-atom ORMOCER® has a modulus of 2030 MPa.
The refractive index of a material indicates the extent to which it slows light and other electromagnetic radiation: the higher the value, the more light is slowed. All ORMOCER®s are transparent materials with a refractive index of around 1.5, comparable to that of salt, but in the case of type III ORMOCER®s it is possible to change the refractive index by altering the organic functional groups present. Successively higher values are obtained by using straight chain, aromatic ring, and halogenated aromatic spacer groups; conversely, the refractive index can be reduced by using certain multiacrylates or fluorinated silicon compounds. This can have applications in fibre optical communication technology.
6. Applications
ORMOCER®s are relatively new materials but a number of diverse commercial applications have been found or are still being explored. Several of these are described here.
6.1. Coatings
Due to their transparency, chemical stability and resistance to abrasion, ORMOCER®s have found application as coatings on a variety of materials including glass, plastics and metals. One specialized application of ORMOCER®s as coatings is in the production of dishwasher-resistant glassware.
Much research has focused on the development of hydrophobic (water-repelling) and oleophobic (oil-repelling) ORMOCER® coatings. These properties greatly reduce the extent to which dust particles and other unwanted matter cling to a material, so surfaces coated with hydrophobic and oleophobic substances are easier to clean.
It has been discovered that a type III ORMOCER® based on thiol/ene addition (see Fig. 5) could be made significantly hydrophobic and oleophobic, without adverse effects on other properties, by the inclusion of small amounts (around 1%) of long-chain fluoralkyl silanes (see Fig. 6) during the sol-gel process. It can therefore be used as a coating which combines typical ORMOCER® abrasion resistance with the useful new properties of hydrophobicity and oleophobicity.
Fig 6. Fluoralkyl silanes for the synthesis of hydrophobic and oleophobic ORMOCER®s.
6.2. Dental fillings
Materials to be used in dental fillings must fulfil several requirements, both mechanical (strength, hardness and capacity to cope with a range of temperatures) and chemical (inertness, biocompatibility). The organic polymer polymethylmethacrylate (PMMA), reinforced with silica, has been used in this role.
More recently, type III ORMOCER®s based on multiacrylate links (see Fig. 5) and containing inorganic filler materials have been used in commercially available dental fillings. These ORMOCER®s excellently fill the above requirements and show markedly less shrinkage on production than certain other available materials.
6.3. Chemical sensors
Substances showing marked sensitivity to target chemicals can be used as the basis for sensors for the detection of those compounds. Recent experiments have shown the sensitivity of various ORMOCER®s to pollutant gases, suggesting an application as environmental sensors.
A type III ORMOCER® containing thioether linkages (see Fig. 5) has been found to form chemical complexes with sulfur dioxide which can be detected by ultraviolet spectroscopy, even at low concentrations of the gas. Since industrial emission of SO2 is one of the major causes of acid rain, an SO2 sensor is of obvious environmental use.
More recently it has been discovered that type II ORMOCER®s containing fluorinated organic functionalizations show extreme sensitivity for phosphorous-containing organic compounds, which are also environmentally harmful. Adsorption of these gases at levels less than one part per million is detectable by a change in the acoustic properties of the ORMOCER®, so there is promise here too for the development of an environmental sensor.
6.4. Electrolytes
Normal polymers, including ORMOCER®s, are electrically insulating: this is why plastics are used to sheath electrical wires. However it is possible to make organic polymers electrically conducting by adding ionic (electrically charged) species to the polymer matrix. Electrically conducting polymers can be used as electrolytes in, for example, batteries and capacitors.
Research on applying the above technique to ORMOCER®s has yielded promising results. One research group has found that, by synthesising type III ORMOCER®s in the presence of sulfur trioxide, the species SO3H can be incorporated into the network and the ORMOCER® can conduct electricity by the flow of protons, H+. By varying the proportion of the SO3H species from 0.1 to 0.6, the degree of electrical conductivity could be varied by a factor of 100,000.
These and other researchers have also developed type III ORMOCER®s capable of conducting electricity by the flow of lithium ions, Li+. This is done by dissolving lithium salts in the sol before the curing step. These ORMOCER®s may have applications in lithium batteries.
Conducting ORMOCER®s can be prepared as surface coatings or as molded gels and offer the same advantageous physical properties (combination of strength and elasticity) as ORMOCER®s in general. In addition, they have two advantages over purely organic conducting polymers due to their inorganic backbone: they are stable at higher temperatures, and they do not show the tendency to adopt crystalline structures and hence show the decrease in electrical conductivity which typically accompanies this.
7. Conclusion
In the last decade much research has been devoted to inorganic-organic nanocomposites, or ORMOCER®s, substances in which organic and inorganic networks are mixed at a molecular level. Synthesised via the sol-gel process, ORMOCER®s include molecular-level polymer dispersions (type I), organically functionalized silica networks (type II), and organically crosslinked silica networks (type III).
These hybrid polymers combine some of the advantageous physical properties of organic polymers (such as flexibility) with those of inorganic materials (such as mechanical strength), and can be tailored further by the choice of organic components. Due to these useful and variable properties, ORMOCER®s have found a wide range of applications including specialised coatings, bulk material for dental fillings, and battery electrolytes. Research into their potential applications continues, and promises to yield interesting results.
8. References
Haas, K-H., Amberg-Schwab, S. and Rose, K. Functionalized coating materials based
on inorganic-organic polymers. Thin Solid Films. 1999; 351: 198-203.
Haas, K-H. and Wolter, H. Synthesis, properties and applications of inorganic-organic
copolymers (ORMOCER®s). Current Opinion in Solid State & Materials Science.
1999; 4: 571-580.
Haas, K-H. and Rose, K. Hybrid inorganic/organic polymers with nanoscale building
blocks: precursors, processing, properties and applications. Reviews on Advanced
Materials Science. 2003; 5: 47-52.
Morikawa, A., Iyoku, Y., Kakimoto, M-a. and Imai, Y. Preparation of new polyimide-
silica hybrid materials via the sol-gel process. Journal of Materials Chemistry.
1992; 2(7): 679-690.
Novak, B.M. Hybrid nanocomposite materials – between inorganic glasses and
organic polymers. Advanced Materials. 1993; 5: 422-33.
Popall, M. and Du, X-M. Inorganic-organic copolymers as solid state ionic
conductors with grafted anions. Electrochimica Acta. 1995; 40: 2305-2308.
Popall, M., Andrei, M., Kappel, J., Kron, J., Olma, K. and Olsowski, B. ORMOCERs
as inorganic-organic electrolytes for new solid state lithium batteries and
supercapacitors. Electrochimica Acta. 1998; 43: 1155-1161.
Rose, K. Photo-crosslinked polysiloxanes – properties and applications. In: Auner, N.
and Weis, J., editors. Organosilicon Chemistry – From Molecules to Materials.
Wenheim: Verlag Chemie, 1995; 2: 649-53.
Saegusa, T. Organic-inorganic polymers hybrids. Pure & Applied Chemistry. 1995;
67(12): 1965-1970.
Zimmermann, C., Rebière, D., Déjous, C., Pistré, J., Chastaing, E. and Planade, R. A
love-wave gas sensor coated with functionalized polysiloxane for sensing
organophosphorus compounds. Sensors and Actuators B. 2001; 76: 86-94.
