Polysiloxanes are unique among inorganic and semi-inorganic polymers; they are also the most studied and the most important with regard to commercial applications. Thus, it’s not surprising that there is an extensive literature describing the synthesis, properties, and applications of these materials, including books, proceedings books, sections of books or encyclopedias, review articles, and historical articles. The purpose of this volume is not to give a comprehensive overview of these polymers but rather to focus on some novel and interesting aspects of polysiloxane science and engineering, including properties, work in progress, and important unsolved problems. The Si-O backbone endows polysiloxanes with a variety of intriguing properties. The strength of the Si-O bond, for example, imparts considerable thermal stability, which is important for high-temperature applications (e.g., as heat-transfer agents and high-performance elastomers). The nature of the bonding and the chemical characteristics of typical side groups impart low surface free energy and therefore desirable surface properties. Polysiloxanes, for example, are used as mold-release agents, waterproofing sprays, and biomedical materials. Structural features of the chains give rise to physical properties that are also of considerable scientific interest. For example, the substituted Si atom and the unsubstituted O atom differ greatly in size, giving the chain a nonuniform cross section. This characteristic affects the way the chains pack in the bulk, amorphous state, which explains the unusual equation-of-state properties (such as compressibility). Also, the bond angles around the O atom are much larger than those around the Si, which makes the planar all trans form of the chain approximate a series of closed polygons, as illustrated in figure 1.1. As a result, siloxane chains exhibit a number of interesting configurational characteristics that impact properties and associated applications. The major categories of homopolymers and copolymers to be discussed are (i) linear siloxane polymers -SiRR’O-] (with various alkyl and aryl R,R’ side groups), (ii) sesquisiloxane polymers possibly having a ladder structure, (iii) siloxane-silarylene polymers [–Si(CH3)2OSi(CH3)2(C6H4)m –] (where the skeletal phenylene units are either meta or para), (iv) silalkylene polymers [–Si(CH3)2(CH2)m–], and (v) random and block copolymers, and blends of some of the above.
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