Siloxane elastomers

Jl hermally stable elastomers and polymers include fluorocarbon elastomers, polyorganophosphazenes, silicone elastomers, siloxane block copolymers (including the dodecacarborane block copolymers [Dexsil elasto-... 

Owen, M. J. Elastomers siloxane. In Encyclopedia of Materials Science and Technology, Elsevier Science Ltd, 2001 p 2480. 

Viscoelasticity is perhaps the most ubiquitous characteristic of high molecular weight polymers at temperatures above Tg. Here we consider the implications of coupling rubber elasticity and mesomorphism via synthesizing covalent networks from conventional elastomers (siloxanes, isoprenes, etc.) and typical MLC mesogenic cores such networks are thermotropic PLCs. At low levels of crosslink densities in the PLC network, there is no appreciable change in the transition temperatures (Tg, Td, etc.) from those of the uncross-linked PLC (and its ancestral MLC) [68]. Below Td, rather modest mechanical deformations (extension ratio A < 1.5) may convert an initial random and disclination-ridden texture into a... 

The resulting silanol group of a polymer chain condenses with acetoxy siloxy group of another polymer chain to form a siloxane (Si-O-Si) linkage (Scheme 8). Further similar reactions finally result in a crosslinked elastomer. Acetic acid is... 

Clarson, S.J. and Mark, J.A., Siloxane elastomers. In Clarson, S.J. and Semiyen, J.A. (Eds.), Siloxane Polymers, Polymer Science and Technology Series. PTR Prentice Hall, Englewood Cliffs, NJ, 1993, pp. 616-648. 

Block copolymers containing polysiloxane segments are of great interest as polymeric surfactants and elastomers. Polycondensation and polyaddition reactions of functionally ended prepolymers are usually employed to prepare well-defined block copolymers. The living polystyrene anion reacts with a,co-dichloropoly(dimethyl-siloxane) to form multiblock copolymers398. ... 

Recently, as a further recognition of the importance of multiphase copolymers in specialty applications, siloxane containing block and segmented copolymers have received special attention for applications such as biomaterials, photoresists, gas separation membranes, protective coatings, elastomers and emulsifiers, as we shall... 

P. R. Dvomic, R. W. Lenz, High Temperature Siloxane Elastomers, Huthig Wepf, Heidelberg (1990). 

At AWE, the Lewis acid-catalyzed bulk polymerization route has been the main synthesis route to poly(m-carborane-siloxane) elastomers. Our selection has been based on considerations of safety, availability of key reagents, and ease of scale-up operations. An understanding of the physical and chemical properties of these materials, and how these properties can be modified through the synthesis process, is essential in order to develop materials of controlled characteristics. 

In the following sections, details are provided on a selection of analytical techniques that have been typically used to characterize poly(m-carborane-siloxane) elastomers. 

The 7 g of elastomers synthesized at AWE was generally found to be within the range 30° to 40°C (see Table 2), which is significantly higher than that for standard poly(dimethylsiloxane). The introduction of the bulky m-carborane unit into the siloxane backbone has clearly elevated the Tg. However, although the carborane unit introduces conformational rigidity, the polymer chains retain sufficient flexibility and mobility to have a T% < -30°C. 

Figure 17 Calculated stress-relaxation behavior at 298 K for five uncross-linked elastomers of M = 200,000 EP, ethylene-propylene (56 44) styrene-butadiene (23.5 76.5), SB natural rubber, N butyl and dimethyl siloxane.

Silicone Rubbers or Elastomers These are having high Molecular weight than the fluids. Generally they are polydimethyl siloxanes. General purpose silicone elastomers are made from polydimethyl siloxanes. 

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