Rubbery organic polymer

Many of the AW sensors that have been examined to date utilize films of materials, often organic polymers, for which transport within the film is not rapid compared to adsorptiori/desorption. In these cases, the rate of transport widiin the film affects response times. For polymeric films, those in the glassy or crystalline state generally exhibit lower diffiision rates (and correspondingly slower response times) than rubbery, amor Aous (elastomeric) polymers (see Chapter... 

In 1975 Wacker-Chemie introduced silicones under the name of m-polymers. These are also room temperature curing liquid polymers which give rubbery materials on cross-linking and are available both as one- and two-component systems. Their particular feature is that they contain dispersions of copolymers such as those of styrene and n-butyl acrylate in the shape of rods or rice grains in the fluid silicone polymer. A small amount of the organic copolymer is also grafted onto the silicone backbone. 

The chains that make up a polymer can adopt several distinct physical phases the principal ones are rubbery amorphous, glassy amorphous, and crystalline. Polymers do not crystallize in the classic sense portions of adjacent chains organize to form small crystalline phases surrounded by an amorphous matrix. Thus, in many polymers the crystalline and amorphous phases co-exist in a semicrystalline state. 

As polymers solidify from the molten state, their free volume decreases and their organization increases. Solid polymers fall into one of three classes rubbery amorphous, glassy amorphous, and semicrystalline, which we introduced in Chapter 1. 

The copolymerisation of ethylene with vinyl acetate (VA) is another method by which the crystallinity of polyethylene can be reduced and a rubbery polymer obtained. The final properties of the copolymer depend on the VA content at a VA level of 50% the copolymer is entirely amorphous, and elastomeric grades generally contain 40-60% VA by weight. The oil resistance of the copolymer is also dependent on the VA content in general, however, this lies between that of SBR and polychloroprene. It is swollen by most organic solvents and not resistant to animal and vegetable oils, but has some resistance to weak acids and alkalis at ambient temperature. 

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