Why Silicones Behave Funny

M. J. OwenScientist Emeritus

Dow Corning Corporation

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AbstractThis article provides the curious scientific reader with an overview of the surface properties of silicones, andpolydimethylsiloxanes in particular. From where do these surface properties originate, and why do they make siliconesuseful in so many applications? This article is based on a previous publication from 1981 and includes commentson progress made since at understanding these exceptional and “funny” surface properties.

IntroductionSince their introduction in 1943, silicones have been applied in an exceptional variety of industries. In many cases,silicones have gained acceptance and are used commercially either as coatings or as additives because of their uniquesurface properties. The focus of this article is on these specific surface properties and related applications, and itconcentrates on the physical property related aspects and their underlying molecular explanation. Hopefully it willstimulate readers to consider how this unique property profile might be used in their specific area. This article isbased on a still-relevant 1981 article, that sought to answer the question “Why are silicones used in so many surfaceproperty linked applications?” (see Note 1). This updated version is reprinted/republished with permission fromCHEMTECH, Copyright 1981 American Chemical Society [1].

Interested readers may also want to consider a recent book chapter that covers the topic in more detail [2].

Silicones in Surface Property ApplicationsAs indicated above, silicones are used in many applications because of their surface properties, either as coatingsor additives (Table 1).

Table 1. Established Surface-Related Applications of Silicones

Applied coatings Integral additivesPaper release Polyurethane and other foam stabilizersWater repellant Integral plastics lubricantElectrical insulating compound EmulsifierPigment treatment De-emulsifierFiber lubricant Wetting agentTextile handle modifier AntifoamMold release agent Cosmetic formulation componentMetal protector Polishing oilAntifouling material Gloss enhancerMasonry treatment Powder coating flow-out aidAnticaking aid Paint additive

For coatings, the physical nature of the coating is important and cross-linking of silicone polymer chains is ofteninvolved. As additives, diffusion to the interface is critical and so compatibility with the system is essential, oftenrequiring the use of silicone copolymers. Both the cross-linking and copolymerization aspects utilize the uniquechemistry of silicones, but it is not these synthetic aspects which chiefly account for the range of surface activeapplications of silicones (see Note 2). Rather, it is a consequence of the unique physical properties of the silicones,in particular polydimethylsiloxanes (PDMS), as I will focus on hereafter.

The best known of the commercially available silicones are polydimethylsiloxanes (PDMS), so this will be our mainconcern. They are available as linear fluids, cyclics, gels and resins depending on the degree of cross-linking, or aselastomers when fillers are incorporated in cross-linked polymers.

Table 2 lists both surface and bulk phenomena that together make up the unique property profile of PDMS. Structurally,PDMS consists of an inorganic siloxane backbone with pendant methyl groups (Figure 1).

Table 2. Properties of Polydimethylsiloxanes

Low surface tensionModerate water interfacial tensionNo surface viscositySpreading and “creep” behaviorVariety of configurationsLarge free volumeLow glass transition temperatureLow activation energy of viscous flowLiquid nature at high molecular weight (linear polymers)Small temperature variation of physical constantsLow boiling points (oligomers)Low freezing and pour pointsHigh compressibilityHigh permeability to gas and low molecular weight speciesLow flammability and fire hazardLow environmental hazardExcellent weather resistance

Figure 1. A three-dimensional view of Me3SiO-(SiMe2O)4-SiMe3 showing the shielding of the polar main chainmade of Si-O groups by the methyl groups (structural representation courtesy of S. Grigoras, Dow Corning).

Inorganic silicate-like structures are associated with high surface energies, whereas hydrocarbon materials generallyhave low surface energy. One idealized, but nevertheless useful, viewpoint is that the principal role of the backbonein the surface activity of silicones is its ability to present the attached organic groups at interfaces. From thisperspective, the key property of the backbone is its flexibility and the key property of the organic group is its intrinsicsurface activity, or perhaps more fundamentally, the strength of the intermolecular interactions between these groups.Polydimethylsiloxanes then emerge as a favored case of a very surface-active (very low intermolecular forces)pendant group, methyl, whose activity is presented to best effect by virtue of the unique flexibility of the backbone(Figure 2).

Figure 2. A water droplet on a cotton substrate coated with a polydimethylsiloxane, showing the overwhelmingimportance of the methyl groups leading to hydrophobicity of the coated substrate (photograph courtesyof A. Goodwin, D. Futter, A. Hynes and S. Leadley, Dow Corning Plasma Solutions).

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In addition, the particular combination of siloxane and methyl has thermal and oxidative stability benefits. PDMSis also essentially non-irritating and non-toxic. These features are exploited in several of the surface-active applications.Let us now look further into the unique molecular architecture that is responsible for this behavior.

Flexibility of the silicone backbone. The flexibility of the siloxane backbone chain is unique. This assertion seemedclear in 1981, but it needs more elaboration today, as newer, flexible perfluoroethers have become better known.We can equate chain flexibility to freedom of rotation about bonds. The energy required for rotation about the carbon-to-carbon bonds in polyethylene is 13.8 kJ/mol (3.3 kcal/mol). In polytetrafluoroethylene, this energy is greater than19.6 kJ/mol (4.7 kcal/mol). In PDMS, this energy is almost zero, rotation being virtually free [3]. This freedom torotate is reflected in the glass transition temperature (Tg). It is not only the internal mobility in a polymer thatdetermines Tg; polymer free-volume, attractive forces between molecules, chain stiffness, and chain length allcontribute. Nevertheless, a low Tg indicates polymer flexibility; some comparative values are given in Table 3 [4].

Table 3. Selected Values of Glass Transition Temperature

Tg (K)

Poly(pentamethylcyclopentasiloxane) (PD5) 122Co-poly(oxytetrafluoroethylene-oxydifluoromethylene) 140Polydimethylsiloxane 146Polyethylene 148Poly(thiodifluoromethylene) 155Cis poly(1-pentenylene) 159Poly(oxypropylene) 198Polyisobutylene 200Polymethyltrifluoropropylsiloxane 203Polytetrafluoroethylene 293

I have added two values to Table 3; that for co-poly(oxytetrafluoroethylene-oxydifluoromethylene) or co-poly(CF2CF2O-CF2O) [5] and poly(pentamethylcyclopentasiloxane) or PD5 [6]. It is no longer true that PDMS has the lowest knownTg; clearly this perfluoroether and PD5 are lower. This latter silicone polymer is a methylsiloxane composed of linkedrings. It is formed from (MeHSiO)5 or D5H, a cyclic siloxane pentamer with a methyl group and hydrogen atom oneach silicon. All polymers with a Tg less than 160 K, except the silicones PDMS and PD5, are “pure” backbonepolymers with no pendant groups. Once pendant groups are introduced, Tg rises considerably. Evidently, the siloxanebackbone is still unusually flexible. In the original CHEMTECH article I cited Langmuir trough studies withtransitions between different spread configurations occurring with a very small difference in surface pressure. I thinkthis is still good evidence of chain flexibility although there has been considerable rethinking in the past two decadesconcerning the nature of spread PDMS surface layers. A fuller account of these developments is given in reference 2.

Table 4 lists fundamental polymer architectural geometry parameters, bond angles and bond lengths, taken frommodel compounds [7]. These angles and lengths are from hexamethyldisiloxane and the two organic systems thatPDMS is most often compared to, polyethers and hydrocarbons.

Table 4. Model Compound Chain Parameters

Bond Length (nm) Bond Angle (˚)

Hexamethyldisiloxane Si-O 0.163 Si-O-Si 130Dimethylether C-O 0.142 C-O-C 111Propane C-C 0.154 C-C-C 112

The siloxane system has the most open structure with the flattest angle and the longest bond length. In mosthydrocarbon systems, bond angles are fixed, i.e., tetrahedral. Thus, available surface-active groups, such as methyl,cannot adopt the lowest surface-energy configuration. For example, in polyisobutylene, despite an exceptionallylarge backbone bond angle of 123˚, only a helical structure is possible [8]. In contrast, the Si-O-Si angle shows much

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wider variability. This contributes to the variety and ease of inter-conversion of configurations. Values of 105-180˚have been quoted [9]. Widening of the angle when silicon replaces carbon was formerly attributed to (p d) bonding[10] but is now known to reflect a strong deviation from sp3 hybridization [11, 12]. The unusually wide bond angleof the Si-O-Si bond reflects a strong deviation from sp3 hybridization. This is due to the transfer of electronic chargefrom the oxygen’s lone pairs to the bonding zone between silicon-oxygen atoms. This charge transfer results in ashorter Si-O bond length than expected, and the alteration of the value of the bond angle at oxygen as a consequenceof the attenuation of the sp3 hybridization. These conclusions were drawn by subtracting the electron charge densitycalculated at ab initio level with two basis sets: one that estimates correctly the geometry of this moiety, and onethat predicts a standard sp3 hybridization. Furthermore, the ab initio results indicate that the equilibrium value forthe Si-O-Si bond angle is around 145-160˚, and the energy required to deform this bond angle to 180˚ is very low,approx. 1.3 kJ/mol (approx. 0.3 kcal/mol). However, the energy to deform this bond angle to lower values, towards109˚ (sp3 hybridization value) increases very quickly for low angle-variations.

Another structural factor that must enhance chain flexibility is the unsubstituted alternating oxygen linkage thatmuch reduces steric interference possibilities during reorientation. PDMS is still by far the lowest Tg polymer ofthis [Me2XY]n structure [2].

Methyl group surface energy. Here we will assume that there is an intrinsic surface energy for a methyl group,regardless of whether it is pendant to a hydrocarbon or siloxane chain (i.e., intermolecular forces between methylsare essentially the same in both cases). Our assumption is based on the similarity in critical surface tension of wettingof paraffin wax (23 mN/m) [13] and PDMS (22.7 mN/m) [14]. Both these situations are believed to produce arraysof closely packed methyl groups at the surface. It is a well-known result of Zisman’s studies [15] that the order ofincreasing surface energy for single-carbon based moieties is:

CF3- < -CF2- < CH3- < -CH2-

Much of the relative surface energy behavior can be understood with this order. It explains why perfluoroethers,such as poly(perfluoropropylene oxide), with many pendant CF3- groups, have a lower surface energy than theperfluoroalkanes. It also explains why polydimethylsiloxanes have a lower surface energy than the alkanes.

Pauling quotes the van der Waals area of a methyl group as 0.12 nm2 [16]. Zisman gives 0.227 nm2 for a dimethylsiloxaneunit in a spread configuration [17]. This suggests that two methyl groups fit closely over each siloxane linkage. TheCF3- group might give a more surface-active siloxane polymer but, being larger, might not be able to align for maximumefficacy. In any case, a hydrocarbon bridge, e.g., a (-CH2-CH2)- group, has to be placed between the CF3- and thesiloxane backbone to achieve adequate chemical stability. These -CH2- groups offset the CF3- group’s effect to theextent that polymethyltrifluoropropylsiloxanes have higher liquid surface tensions than PDMS. This may also be partlydue to the quadrupole (uncompensated dipole) at the CF3-CH2- hydrocarbon/fluorocarbon junction.

Other important aspects. At 22.7 mN/m, the critical surface tension of wetting of PDMS is higher than its liquidsurface tension, which is 20.4 at 20˚C at the highest molecular weight measured [18]. Consequently, the polymerwill spread over its own adsorbed film. This “creep” of silicone fluid can be a problem if unreacted fluid is presentin silicone compositions and migrates to electrical contacts, as PDMS is a good electrical insulator. On the otherhand, it can be an advantage in achieving complete surface coverage in applications such as metal protection,pigment surface treatments, insulator compounds and mold release. Low surface energy is responsible for manyof the applications of PDMS. For example, it serves as the hydrophobic entity in surfactants that need to reducethe surface tension of water to a lower level than is possible with conventional surfactants [19].

In many cosmetics, silicones provide emolliency, not so much for their “lubricity,” but for their outstanding spreadability[20]. In medical devices, they are used to “siliconize” needles and reduce pain when penetrating the skin, again becauseof their spreadability, and adequate lubricating properties, not to mention their biocompatibility [21].

One fascinating aspect of the particular surface energy of PDMS is that the most biocompatible range of polymersseems to be at surface energies of 20-30 mN/m [22]. The original use of silicones in biomedical applications hasmuch to do with their position in this range (see Note 3). It is postulated that this range has a minimum interfacialtension with protein-containing aqueous solutions. This results in the smallest possible driving force for proteinadsorption and denaturation at these values (Figure 3).

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Figure 3. Comparison of a medical grade polycarbonate without (left) or with (right) a silicone coating,both after exposure to human blood for 5 minutes, washing and electron microscopy analysis and showingthe reduced amount of cell and protein absorption achieved by the silicone coating (pictures courtesy ofF. Briquet and C. Micquel).

Note that it is not materials, which are totally wetted by water that are usually particularly biocompatible. This pointimplies the significance of nonpolar dispersion force interactions in this instance. These considerations are alsoimportant in other fields, such as marine antifouling.

There are two further consequences of the low intermolecular forces that result from the molecular architecture ofpolydimethylsiloxane. These are its low surface shear viscosity and high gas permeability. PDMS has the lowest recordedsurface shear viscosity [23] and its oxygen permeability, for example, is second only to another class of organosiliconpolymers, the trimethylsilyl alkynes such as poly(1-trimethylsilyl-1-propyne) [24]. There are applications where bothhigh gas permeability and low surface viscosity are exploited, for example as antifoams in fermentation processes.

It is not possible to cover all the applications for PDMS that depend, at least in part, on this unique combinationof surface properties. By now, interested readers should be able to see how silicones might fit into their interests.Instead of any attempt to be comprehensive, let’s confine ourselves to four major, and apparently very contradictory,applications to illustrate the versatility of these polymers: antifoams, polyurethane foam stabilization, release coatingsand pressure-sensitive adhesives.

ApplicationsAntifoams. The control or elimination of foam in many industrial processes is a major application for PDMS fluids.Typical examples are in textile dyeing and finishing, in gas scrubbing at petrochemical plants, in penicillin productionby fermentation and in the formulation of controlled foaming detergents.

Foaming originates from the presence of a surfactant, which stabilizes the foam films by a variety of mechanisms.Chief among these are surface elasticity and surface viscosity. Materials that diminish or eliminate these mechanismsare known as antifoams or defoamers. PDMS provides low surface and interfacial tensions, thus achieving theprerequisite condition of being more surface active than the foaming surfactant they are destined to replace. Theydo this by entering and spreading in the foam films, without conferring any additional direct foam-stabilizingmechanisms such as high surface viscosity. These properties are sufficient to enable silicones to be effectivedefoamers for nonaqueous systems. To be active at low concentrations, the antifoam should be insoluble in thefoaming medium, although this is not an absolute necessity. Other ancillary properties can also be important: forexample, high gas permeability and lack of toxicity are vital in fermentation processes.

For aqueous systems, it has long been recognized that finely dispersed hydrophobic solids, such as polydimeth-ylsiloxane-treated silica, are needed for effective antifoam action. Such fluid/solid combinations are known asantifoam compounds (Figure 4).

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Figure 4. Because silicone antifoams are effective at very low levels (often 10-100 ppm), their use caneliminate the need for much greater quantities of hydrocarbon antifoams. Excessive use of hydrocarbonantifoams can produce problems of product contamination or excessive biological oxygen demand inindustrial effluents. Note the small red circle around the silicone antifoam drop, the middle blue circle wheredefoaming has occurred and the larger green circle where spreading has occurred but not yet defoaming(photograph courtesy of J.P. Lecomte, Dow Corning).

They are often prepared as emulsions or encapsulated granules for ease of application. It is now known that thesehydrophobic solids play an important role in the breakdown of the so-called pseudo-emulsion film that forms betweenthe antifoam droplet and the surface of the foaming solution [25]. Bridging effects across the two surfaces of a foamfilm leading to rupture are also significant.

Polyurethane foam stabilization. A prerequisite for foam stability is positive adsorption of a surface-active species.In polyurethane foam formation, the bulk of the liquid mixture is often a polyol, such as the poly(propylene oxide)adduct of glycerol, which has a surface tension of approximately 32 mN/m. This surface tension is the lowest manyorganic surfactants can achieve and, consequently, they are not useful as stabilizers for polyurethane foam.

Polydimethylsiloxanes, with their potential for lowering surface tension a further 10 mN/m, are ideal surfactantsfor such systems. They are made compatible with the polyurethane foam mix by copolymerizing silicones withpoly(alkylene oxides) (Figure 5).

Figure 5. Without the controlled thinning of cell walls made possible with silicone surfactants, high-qualitypolyurethane foams would not be possible: left without a silicone poly(alkylene oxide) copolymer surfactant,right with one such surfactant and showing the improved cell size quality (pictures courtesy of M. Stark ,M. Ferritto and M. Stanga, Dow Corning).

There is now a wide range of such copolymers. Their structures and compositions are tailored to give optimumfoam-supporting film viscoelasticities for today’s wide variety of polyurethane foam systems [26]. This type of

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copolymer is also used in other polymer systems, e.g., phenolic foams. In some high temperature applications, suchas textile dyeing where the cloud point is exceeded so that these surfactant copolymers become insoluble, they alsofunction as antifoams.

Poly(alkylene oxides) are not the only water-solubilizing entities that can be attached to a silicone hydrophobe.Indeed, silicone analogs of most conventional hydrocarbon surfactants are possible. By varying the HLB (hydrophile-lipophile balance), a range of useful silicone surfactants is possible including powerful wetting agents for low energysurfaces and water-in-oil emulsifiers.

Release coatings. Silicones are widely used as release coatings. The low surface energy and low cohesive strengthconferred by the methyl groups combine to provide the excellent release characteristics of PDMS fluids. They spreadrapidly on metal mold surfaces and have excellent thermal stability. This combination of properties allows them tobe used at rubber-curing temperatures, thus providing an efficient parting agent between organic polymer and moldsurface. A blend of high viscosity silicone fluid with a lubricating particulate solid such as mica provides the difficultcombination of slip, air-bleed, release and thermal stability demanded by the tire industry.

Perhaps the most familiar release application for silicones is as release liners for pressure-sensitive adhesive coatedlabels and the like. In this application, it is important that no gross transfer of silicone occurs to interfere with thesubsequent adhesion of the pressure-sensitive adhesive. Thus, cross-linked PDMS films are used (Figure 6).

Figure 6. The low surface energy of silicone paper coatings on the release sheet allows the do-it-yourselferto easily handle aggressive pressure-sensitive adhesives such as those used on these labels (photographcourtesy of Luc Dussart, Dow Corning).

For good release, it is important to have a lower surface energy substrate than the adhesive. Typical acrylic and SBRadhesives have surface tensions of 30-40 mN/m, well above that of polydimethylsiloxanes. The low substrate surfaceenergy not only directly lowers the thermodynamic work of adhesion, but also produces poor wetting, i.e., a highcontact-angle between adhesive and substrate, which not only reduces interfacial area but also concentrates thestresses in the subsequent separation of the adhesive/release-coating laminate.

One of the key new insights in the release area is the discovery that slippage can occur between the adhesive layerand the silicone release layer [27]. Such behavior has ramifications in other applications. One important exampleis in the significant use of silicones in creams and lotions in the personal care area. The benefits of a low surfaceenergy, water insoluble polymeric component are obvious. The silicone will spread easily, repel aqueous solutionsand remain on the surface of the skin where topically applied with no tendency for absorption. However, a keyattraction to the customer of the silicone is a much less quantifiable attribute, the so-called “smooth,” “soft” or “silkytouch” of the treated skin. A similar phenomenon is experienced with silicone hair treatments and the soft handleof silicone-treated textiles. Facile slippage of finger surfaces over silicone-treated skin is likely another manifestation

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of this tendency to interfacial slippage or low static coefficient of friction. It seems to be a consequence of both thefundamental attributes stressed in this article: low intermolecular forces between methyl groups and the very highflexibility of the siloxane backbone.

Pressure-sensitive adhesives. The first requirement of adhesion is good wetting. This will clearly be a significantpositive factor with such a low-surface-energy adhesive. However, an ability to spread on a wide variety of substratesis only one part of the requirement; pressure-sensitive tack and cohesive strength are also needed. I will not go intothe details of these bulk properties, but let me just state that a balance between these requirements must be achieved.This is done by formulating reactive fluid-resin mixtures where the PDMS fluid and the tackifying resin containgroups that will react by silanol condensation to form a network of adequate strength [28].

Silicone pressure-sensitive adhesives have a number of advantages over conventional organic adhesives. They arechemically and thermally stable and are resistant to moisture and weathering. They adhere well to many types ofsurfaces such as metals, glass, paper, fabric, plastics and human skin, and to materials such as silicone elastomers andrelease coatings that are usually expected to be nonadhesive. A typical example is silicone pressure-sensitive adhesiveused in drug-loaded transdermal patches and associating many of the silicone characteristics mentioned so far: adhesionto an hydrophobic substrate (here the skin), allowing permeability for the drug to diffuse through the adhesive intothe skin because of the low Me-to-Me intermolecular interactions, not to mention the lack of skin irritation associatedwith silicones that makes these adhesives particularly suited when long wear time is considered (Figure 7).

Figure 7. Silicone pressure-sensitive adhesives are used in transdermal drug delivery systems to fix medicatedpatches on the skin, because these adhesives allow drug permeation and do not create skin irritation(photograph courtesy S. Postiaux, Dow Corning).

It also means that silicone pressure-sensitive adhesives cannot be conveniently packaged and delivered withconventional release coatings. Even lower surface energy coatings based on fluorosilicones have had to be developedto accomplish this [29].

The FutureWhat does the future hold for silicones in this area? The future of 1981 is today’s history, so the value of mypredictions is readily apparent. In the original article I “safely” predicted a continuous broadening of their use insurface-active applications based on the unique combination of properties I have described. I am happy to reportthat this prediction has come true. Table 5, taken mostly from reference 2, lists examples of applications that werewell on the horizon in 1981, if not unrecognized. We anticipated that the low toxicity of PDMS would be a considerablefactor in this growth, as would the fact that polydimethylsiloxanes do not accumulate in the environment andultimately break down to innocuous natural products such as carbon dioxide and silicic acid [30]. The coatings andfoam applications listed in Table 5 are evidence of the validity of this assertion.

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Table 5. New and Emerging Surface-Related Applications of Silicones

Poly(amidoamine-organosilicon) (PAMAMOS) dendritic coatingsSilsesquioxane precursors for ceramic coatingsTailored, tethered surface-modifying additivesNontoxic, biofouling release coatingsMicroemulsionsOrganosilicon gelling agentsMicrocontact printing with PDMS elastomersElectrically conductive silicone adhesivesControlled-foaming, environmentally-friendly, low-temperature detergentsWound dressings

Polysiloxane is the most flexible chain available, but the aliphatic fluorocarbons provide a more surface-activependant group than methyl. There was already considerable literature on fluorosilicones in 1981; consequently, aneasy accurate prediction was the commercialization of silicones containing more fluorine than those available twodecades ago. The use of polymethylnonafluorohexylsiloxane as a release coating for PDMS-based pressure-sensitiveadhesives is the prime example of this ongoing trend [29].

Many developments were anticipated in the copolymer area. We have discussed only copolymerization withpoly(alkylene oxides). But there were, and continue to be, numerous reports of other silicone-organic block copolymersthat should lead to future commercial products, such as additives to modify surface properties, or be useful newmaterials in their own right. There has been much interest in this topic in recent years but it is fair to say that thishas been mostly of a scientific nature and the full commercial promise of this prediction has yet to materialize.

Similarly we predicted the use of PDMS to enhance recovery of finite fossil fuels, which has materialized in theform of de-emulsifiers on off-shore extraction rigs, and silicone/natural product hybrids, which still remain tantalizinglyjust in the future.

The resistance of the silicone surface to weathering has proved to be a key factor in a future more concerned withconservation than with built-in obsolescence. We envisaged new construction components, coatings and additivesin solar energy devices but missed the big development of silicone high-voltage insulators, and who would havepredicted a large current market in air bags based on reliability of deployment attributes two decades ago?

We can, however, still stand by our original conclusion. The diversity of silicone applications is already remarkable,but it still has a long way to go before it is finished. It has come about because of the unusual combination ofproperties and attributes I have discussed. These attributes guarantee an even more astonishing future. In six decades,silicones have become indispensable as auxiliaries in many different branches of industry. There is every reason toexpect the next 60 years of silicones to be even more exciting than the past.

Notes:

1. I have tried to change only what is absolutely necessary in my original article and based on over two decadesof progress in this field. In particular, I have retained the original title. This was assigned by the CHEMTECH editorwithout consultation. It offended me at the time as my British eyes required the adverbial form “funnily.” Now, aftermuch longer in the USA, I have grown used to it.

2. In the interests of space I have omitted a number of surface-related facets of organosilicon chemistry, of whichthe most important is perhaps the silane coupling agents. The interfacial chemistry of these materials is complexand fascinating [31], but it falls outside the basic structural theme considered here.

3. The original article used “success” in biomedical applications rather than “original use.” I had not anticipatedthe breast implant debacle. I remain convinced that these concerns are not supported by the relevant science and amoptimistic that science will eventually prevail over the fear, bias and greed that has recently driven much of this debate.

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AcknowledgmentsMy thanks to Laurence Bartier for “computerizing” my originalarticle, to Dr. Stelian Grigoras for his comments about the siloxaneelectronic hybridization, and to Dr. André Colas for suggesting theupdating of my original article and for his insightful editorialcomments (all from Dow Corning).

This article was published in Chimie Nouvelle, 85, 27 (2004), andis reproduced here with the kind permission of the editor.

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