Polysiloxanes and Related Polymers

At the present time, polysiloxanes are unique among inorganic and semi-inorganic polymers. They have been the most studied by far, and are the most important with regard to commercial applications. Thus, it is not surprising that a large number of review articles exist describing the synthesis, properties, and applications of these materials. The Si-O backbone of this class of polymers endows it with a variety of intriguing properties. For example, the strength of this bond gives the siloxane polymers considerable thermal stability, which is very important for their use in high-temperature application (for example as heat-transfer agents and high-performance elastomers). The nature of the bonding and the chemical characteristics of typical side groups give the chains a very low surface free energy and, therefore, highly unusual and desirable surface properties. Not surprising, polysiloxanes are much used, for example, as mold-release agents, for waterproofing garments, and as biomedical materials. Some unusual 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 very irregular cross section. This influences the way the chains pack in the bulk, amorphous state, which, in turn, gives the chains very unusual equation-of-state properties (such as compressibilities). Also, the bond angles around the O atom are much larger than those around the Si, and this makes the planar all-trans form of the chain approximate a series of closed polygons. As a result, siloxane chains exhibit a number of interesting configurational characteristics. These structural features, and a number of properties and their associated applications, will be discussed in this chapter. The major categories of homopolymers and copolymers to be discussed are 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. Topics of particular importance are the structure, flexibility, transition temperatures, permeability, and other physical properties.