Silicone Surface Science

Silicones, particularly polydimethylsiloxane (PDMS), are widely exploited for their surface properties. A quantitative review of relevant properties is presented including liquid surface tension measurements, water contact angle studies and solid surface tension determinations from both contact angle and contact mechanics approaches. The properties are considered in the light of the fundamental characteristics of PDMS and related siloxane polymers in order to establish a structure/property relationship of importance in any examination of the surface science of this family of polymers. The central position of PDMS in silicone science and industry follows inevitably from its structure. The combination of small, low-intermolecular-force methyl groups arrayed along the uniquely flexible siloxane backbone produces a polymer whose low surface energy can be equaled or bettered by relatively few other polymers. There is also the additional benefit of greater thermal and oxidative stability than most comparable organic polymers that is important in many applications.

While several general reviews of the applications of sum frequency generation vibrational spectroscopy (SFG) appear in the literature, none have focused specifically on the application of SFG to silicones. The unique and somewhat dichotomous surface properties of silicones, and their ever-increasing use in surface and interface-dependent applications such as lubricants, adhesives, micro-fluidic materials, sensors and matrices or scaffolds for nano-composites, calls for increased fundamental understanding that has motivated the use of SFG analysis. This chapter focuses on the combination of this uniquely surface sensitive tool to study applications using PDMS and other silicone-based materials. Because the interpretation of SFG spectra can be quite complex, many of these examples highlight how SFG can be coupled with complementary techniques to provide a more complete understanding of interfacial effects. Lastly, we conclude by providing a summary of strengths, limitations and potential future opportunities for application of SFG and complementary techniques to silicone-based materials.

We provide an overview of fabricating functional surfaces by surface modification of parent silicone elastomer networks (SENs). Specifically, we demonstrate that polydimethylsiloxane and polyvinylmethylsiloxane represent convenient platforms for generating materials with tuned surface chemistry, topography, and mechanical characteristics. We discuss strategies that facilitate the manufacture of chemically-tailored flat supports as well as those that exhibit tailored topographical corrugations. We provide several examples of technological applications utilizing such structures. We also use SENs as supports enabling tailored assembly of molecules and macromolecules and outline techniques providing generation of substrates with position-dependent properties. We discuss new opportunities in using SENs as a platform for creating substrates that alter their properties swiftly in response to external stimuli. Finally, we offer a brief account of coating methodologies leading to the generation of bilayered sandwiched structures with tailorable chemistry and modulus.

The preparative aspects of three different, but overlapping research programs are reviewed. Silane monolayers prepared using monofunctional silanes and random covalent attachment reactions are described that implicate molecular topography and flexibility as important issues in wetting. Surfaces prepared using multifunctional methylchlorosilanes are discussed. Samples similar to those prepared in the 1940s are shown to be the most hydrophobic (superhydrophobic) ever prepared. The chemical reactions of linear trimethylsilyl-terminated polydimethylsiloxanes with the surface of oxidized silicon are described. These reactions lead to covalently attached polydimethylsiloxane polymer chains and to hydrophobized inorganic surfaces. Linear silicones of this type (silicone oils) are generally not considered to be reactive with inorganic oxide surfaces.

The most common fluorosilicone polymer commercialized to date is polymethyltrifluoropropylsiloxane. However, the low content of is the perfluorinated groups in the polymer 36.5 wt% does not fulfill the requirements of some high tech applications, particularly when swelling properties or degradation at high temperatures are concerned. A number of strategies have been employed to increase the fluorine content of fluorosilicone polymers. One elegant way is to introduce into the silicone chain, either as a pendant group or inside the backbone, perfluorinated groups of increasing size (typically C6 or higher). We refer to silicones with perfluorinated chains introduced as side groups as “pendant silicones” whereas those carrying fluorine atoms in the main backbone are called “hybrid silicones”. The most popular synthesis techniques of such polymers are briefly discussed here. A full fuller comparison is given of the two classes of polymers in terms of surface, mechanical, swelling and thermal properties.

Functional non-wetting materials are of interest for a diverse array of applications. Factors contributing to the wettability of a surface include surface free energy and surface roughness. More recently, surface texture has been found to be of equal or greater importance, especially for surface that repel low surface tension liquids, such as short-chain hydrocarbons and alcohols. This chapter describes recent work in the design and production of wetting-resistant surfaces utilizing fluorinated Polyhedral Oligomeric SilSesquioxanes (FluoroPOSS), as well as the development of dimensionless design parameters to aid in the preparation of such surfaces. FluoroPOSS compounds are organic/inorganic hybrid materials that exhibit low surface energy attributes, as well as an octahedral structure, which results in useful migration and aggregation characteristics when blended into polymer matrices. Wetting-resistant surfaces containing FluoroPOSS are produced either by techniques that specifically incorporate all three critical parameters for wetting-resistance, or by the modification of substrates already possessing the desired surface texture.

Langmuir film formation at the air/water (A/W) interface by silicones has attracted research interest for more than sixty years. This chapter reviews the unique features of the surface pressure-area per repeat unit (Π-A) isotherm of polydimethylsiloxane (PDMS) and discusses the changes in surface viscoelastic properties determined by surface light scattering (SLS) associated with these features. The effects molar mass, end groups and non-methyl substituents have on the isotherm are also considered. This discussion is then extended to another class of surface-active silicon-containing materials, polyhedral oligomeric silsesquioxanes (POSSs). Trisilanolisobutyl-POSS and trisilanolcyclohexyl-POSS are discussed in terms of their Π-A isotherms and surface viscoelastic character and the review ends with a discussion of systems where POSS molecules are used as nanofillers within silicone monolayers.

This chapter reviews the molecular structure, synthesis, interfacial activity, bulk aqueous solution behavior, and commercial applications of silicone surfactants. While providing a historical overview of the technology, the focus is on scientific and technological development in the last ten years. Particular attention is paid to carbohydrate-functional silicones. Silicone surfactants have the intriguing and commercially viable ability to reduce the surface tension of polar and non-polar liquids to values 15–20 mN/m lower than commonly achieved with organic-based surfactants. The latest developments on understanding and commercially exploiting the phenomenon of superwetting are reviewed. Silicone surfactants demonstrate a marked tendency to form aggregate structures featuring surfactant bilayers including vesicles and lamellar liquid crystals. This tendency has been recently applied in the development of silicone vesicles as nanoscale delivery “vehicles” and in the templating of lamellar metal oxide structures. The utilization of these properties is reviewed, including applications as diverse as oil and gas, performance coatings and personal care products.

In order to understand and develop application and analytical techniques for silane coupling agent modification of glass fiber surfaces and to identify and quantify the factors which influence such modification on surfaces, a detailed understanding of how such coupling agents act and how the surfaces they act upon influence their adsorption is needed. This short review examines the application of silane coupling agents to glass surfaces, and how the nature of glass fiber surface affects the application of such coupling agents.

The mechanisms behind the loss and recovery of hydrophobicity of silicone rubber after exposure to oxidative surface treatments, such as UV irradiation, corona or plasma, are presented. Initially, polar groups are introduced into the surface region, mainly in the form of silanol groups. The oxidation then proceeds towards a vitrified silica-like surface layer. The formation of complex buckling patterns, formed by the mechanical stress difference between the silica-like layer and the rubbery bulk opens the way to a wide range of new applications, such as gratings and flexible electronics. The main challenge is to address the hydrophobic recovery process after an oxidative surface treatment. In some applications, such as high-voltage outdoor insulation materials, this recovery is desired but usually it is not. The most common methods of inhibiting hydrophobic recovery are extraction of the silicone rubber to remove extractable oligomers, storage of oxidized silicone rubber in water directly after treatment, or the grafting of polar species onto the oxidized surface.

As many of the desirable performance differences of silicones are related to their surface properties, it is important to be able analyze their surfaces effectively. This chapter presents an overview of key surface analysis techniques that can provide information on the surface morphology, chemical composition and surface physical properties of silicone materials. These techniques are X-ray photoelectron spectroscopy, secondary ion mass spectrometry, scanning electron microscopy and scanning probe microscopy. Both fundamentals and applications to the analysis of silicones are covered. It is evident from a consideration of key examples that in many cases it is a combination of these analytical techniques that provides a clearer picture of the surface properties of silicones.

Some key surface-related applications of silicone polymeric materials are reviewed with an emphasis on polydimethylsiloxane (PDMS), the predominant commercial polymer. The applications considered are elastomers/sealants, personal care products, antifoams, silicone surfactants, pressure-sensitive adhesive release coatings, high voltage insulation, and water-repellent coatings, which together account for well over half of all silicone usage. Our aim is relate the practical usage of these products to the fundamental characteristics of the parent polymer thus extending the silicone structure/property relationship which underlies much of this book into the applications area. The key fundamental characteristic is, expectedly, the low intermolecular forces acting between methyl groups that are manifested by the low surface energy exhibited by PDMS. Compact size of the methyl group and high siloxane backbone flexibility are also of great importance as is high siloxane bond energy. No application is solely dependent on any one of these fundamental characteristics; it is their combination that dictates success in any particular application.