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Polymer Science and Engineering: The Shifting Research Frontiers
longer than the C–C bond, the oxygen atoms are unencumbered by side groups, and the Si–O–Si bond angle of 143 degrees is much more open than the usual tetrahedral angle of 110 degrees. These combined structural features increase the dynamic and equilibrium flexibility of the chain, causing, for example, the glass transition temperature of polydimethylsiloxane to be -125°C. Important applications include high-performance elastomers, membranes, electrical insulators, water-repellent sealants, adhesives, protective coatings, and hydraulic, heat-transfer, and dielectric fluids. The polysiloxanes also exhibit high oxygen permeability and good chemical inertness, which lead to a number of medical applications, such as soft contact lenses, artificial skin, drug delivery systems, and various prostheses.
Another family of inorganic polymers—the polyphosphazenes—is based on a chain of alternating phosphorus (P) and nitrogen (N) atoms. Over 300 different polymers have been synthesized, mainly by variation of the pendant groups. The pendant groups may be organic, inorganic, or organometallic ligands. The nature of the pendant groups affects the skeletal flexibility, solubility, refractive index, chemical stability, hydrophobicity, electronic conductivity, nonlinear optical activity, and biological behavior. Thus by choice of the appropriate side group, polyphosphazenes can be tailored for a variety of applications. These materials are prepared by methods that give little control over stereoregularity and, hence, mixed-substituent polyphosphazenes are amorphous. Glass transition temperatures as low as -100°C have been achieved, and elastomeric performance over a wide temperature range is characteristic of this family. Fluoroalkoxy substituents yield hydrocarbon-resistant materials that could be useful as fuel lines, o-rings, and gaskets in demanding environments. Ether side groups coordinate lithiumions, which leads to possible applications as polymeric electrolytes for high-technology batteries. The ease of side group substitution has also led to new applications in biomedical materials. Hydrophobic polymers such as poly[bis(trifluoroethoxy) phosphazene] minimize the "foreign body" interactions that normally occur when nonliving materials are implanted in contact with living tissues, such as blood. Hydrophobic polyphosphazenes are therefore good candidates for use in cardiovascular replacements or as coatings for pacemakers and other implantable devices. Hydrophilic or mixtures of hydrophilic and hydrophobic groups can be substituted to produce hydrophilic or amphiphilic polymers deliberately designed to stimulate tissue adhesion or infiltration or to generate a biochemical response. Unfortunately, while polyphosphazenes are an interesting class of materials that have physical and chemical characteristics that suggest many applications, they are costly to produce, and commercial success has consequently been modest.
Polysilanes (also called polysilylenes) have been the subject of research interest within the last decade. These all-silicon chains, with alkyl or phenyl side groups, are analogous to vinyl polymers, but they are made from silyldichlorides rather than from the analogue of ethylene. Linear and cross-linked