Siloxane, dimethyl

Solid-phase microextractions also have been developed. In one approach, a fused silica fiber is placed inside a syringe needle. The fiber, which is coated with a thin organic film, such as poly(dimethyl siloxane), is lowered into the sample by depressing a plunger and exposed to the sample for a predetermined time. The fiber is then withdrawn into the needle and transferred to a gas chromatograph for analysis. 

Poly (dimethyl siloxane) offers the least steric hindrance of the polymers listed every other atom along the backbone of the chain is devoid of substituents in this case. 

All the entries above the broken line in Table 2.3 are disubstituted, and those below, with the exception of poly (dimethyl siloxane), are mono-substituted. Those in the former category consistently have larger values of f than the latter. 

Plazek et al.t measured the viscosities of a poly(dimethyl siloxane) sample of = 4.1 X 10 over a range of temperatures using the falling-ball method. Stainless steel (P2 = 7.81 g cm" ) balls of two different diameters,... 

The nature of the solvent is as important as the polymer in determining , as is apparent from the wide range of values for poly (dimethyl siloxane) in different solvents. 

Whereas the traditional dimethyl siloxane fluids provide very poor lubrication for steel on steel and other common metals, thin films on glass reduce handling damage, small amounts in plastic composites bleed to the surface for self-lubrication, and they provide a superior lubricant for mbber surfaces. 

The position of the equiUbrium depends on a number of factors, such as concentration of siloxane units and the nature of substituents on the sihcon, but is independent of the starting siloxane composition and the polymerization conditions (81,82). For a hulk polymerization of dimethyl siloxane, the equihbrium concentration of cycHc oligomers is approximately 18 wt % (83). The equiHbrium mixture of cyclosiloxanes is composed of a continuous population to at least but D, D, and make over 95 wt % of the total cycHc fraction (84). 

The ring-opening polymerization of is controUed by entropy, because thermodynamically all bonds in the monomer and polymer are approximately the same (21). The molar cycHzation equihbrium constants of dimethyl siloxane rings have been predicted by the Jacobson-Stockmayer theory (85). The ring—chain equihbrium for siloxane polymers has been studied in detail and is the subject of several reviews (82,83,86—89). The equihbrium constant of the formation of each cycHc is approximately equal to the equihbrium concentration of this cycHc, [(SiR O) Thus the total... 

Unlike the linear analogues, cycHc dimethyl siloxanes are low melting soHds and the generally decreases with increasing ring size. Physical... 

Polyisobutylene 1.15 " Polyethylene butylene 7.7" Polyethylene 7.65 Polyethylene/propylene 7.35 Poly dimethyl siloxane... 

G. Dimethyl Siloxane Rubber (PDMS) Based Blends... 

Poly(dimethyl siloxanc) with vinyl or hydrosilanc (Si-H) chain ends have been converted to ATRP initiator ends e.g. Scheme 9.62) by hydrosilylalion, Bis-functional dimethyl siloxane polymers prepared in this way were used in polymerizations of S, MA, tsobornyl acrylate and BA to form ABA triblock copolymers. 

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. ... 

Somewhat limited work has been reported over the last decade. There are several reports on the synthesis and physical and structural characterization of styrene-dimethylsiloxane 141 144) and methylmethacrylate-dimethylsiloxane145> diblock, triblock and multiblock copolymers. Several reports are also available on the thermal223), solution 224,2251 and surface196 2261 characterization of various styrene-dimethyl-siloxane block copolymers synthesized by anionic techniques. 

Poly (dimethyl siloxane) obtained by anionic ring-opening polymerization of hexamethylcyclotrisiloxane (D3) can also be end-capped with vinylbenzyl bromide or other electrophiles, such as p. (chlorodimethylsilyl)styrene80). 

Grainger DW, Kim SW, and Feijen J. Poly(dimethyl siloxane)-poly(ethylene oxide)-heparin block copolymers. I Synthesis and characterization. J Biomed Mater Res, 1988, 22, 231-242. 

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