Page 424 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Organometallic and Inorganic–Organic Polymers 387

control of coatings applications. These applications are the direct consequence of the low attractions
between polysiloxane chains, which, in turn, are responsible for their low surface tension. Thus, they
encourage a coatings material to flow across the surface fi lling voids, corners, and crevices. Their

good thermal conductivity and fluidicity at low temperatures allows their use as low-temperature

heat exchangers and in low-temperature baths and thermostats.
Viscous fl uids correspond to a DP range of about 50–400. These materials are employed as mold
release agents for glass, plastic, and rubber parts. They are good lubricants for most metal to nonmetal

contacts. They are used as dielectric fluids (liquids) in a variety of electrical applications, including
transformers and capacitors; as hydraulic fluids in vacuum and hydraulic pumps; in delicate timing

and photographic devices; as antifoam agents; components in protective hand creams; toners in pho-
tocopiers; in oil formulations when mixed with thickeners; and in inertial guidance systems. High-

performance greases are formed by mixing the polysiloxane fluids with polytetrafl uoroethylene or

molybdenum disulfide. Brake fluids are formulated from polydimethylsiloxane fluids with DPs about

50. High-viscosity fluids with DPs about 700–6,000 are used as damping fluids for weighting meters

at truck stops. They act as liquid springs in shock absorbers. The longer-chained fluids are used as
impact modifiers for thermoplastic resins and as stationary phases in gas chromatography.

As with the alkanes, even longer chains form the basis for solid polysiloxanes that, according to
design, can be classified as thermoplastics, engineering thermoplastics, elastomers, and when cross-

linked as thermosets. Solid polysiloxanes are used in a variety of applications, including as sealants,
thermostripping, caulking, dampening, O-rings, and window gaskets. Weather stripping on cooling
units, trucks, and automobiles is often made of polysiloxanes.
Room temperature-vulcanizing (RTV) silicon rubbers make use of the room temperature reac-
tion of certain groups that can be placed on polydimethylsiloxanes that react with water. When
exposed to water, such as that normally present in the atmosphere, cross-links are formed creating
an elastomeric product.

The first contact lenses were based on poly(methyl methacrylate). While they could be polished
and machined, they did not permit gas exchange and were rigid. By early 1970s, these were replaced
by soft contact lenses containing cross-linked poly(2-hydroxyethyl methacrylate) (HEMA). These
so-called disposable lenses do permit gas exchange. More recently, Salamone and coworkers devel-
oped contact lenses based on the presence of siloxane units. Polysiloxanes have good gas perme-
ability. These polymers are referred to as Tris materials and are generally copolymers containing
units as shown below:
R

O
H 3 C
CH 3
Si
R CH 3 O O
CH 3 (11.19)
Si O
CH 3
C
H 3 Si
O
C
H 3
Si CH
H C 3
3
CH 3
Polysiloxanes are widely employed as biomaterials. Artificial skin can be fabricated from a bilayer

fabricated from a cross-linked mixture of bovine hide, collagen, and chondroitin sulfate derived from
shark cartilage with a thin top layer of polysiloxane. The polysiloxane acts as a moisture and oxygen-
permeable support to protect the lower layer from the “outer world.” A number of drug delivery
systems use polysiloxanes because of the flexibility and porous nature of the material.

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