2,2,4,4,6,6,8,8-Octamethylcyclotetrasiloxane

While evidence for hydration forces date back to early work on clays [1], the understanding of these solvent-induced forces was revolutionized by Horn and Israelachvili using the modem surface force apparatus. Here, for the first time, one had a direct measurement of the oscillatory forces between crossed mica cylinders immersed in a solvent, octamethylcyclotetrasiloxane (OMCTS) [67]. 

The elastomers consist of very high moleculcU weight (-0.5 X 10 ) linecu gums cross-linked after fabrication. In order to achieve such polymers it is necessary that very pure difunctional monomers be employed since the presence of monofunctional material will limit the molecular weight while trifunctional material will lead to cross-linking. Where dimethylsilicone rubbers are being prepared, the cyclic tetramer, octamethylcyclotetrasiloxane, which may be obtained free from mono- and trifunctional impurities, is often used. This tetramer occurs to the extent of about 25% during the hydrolysis of dichlorosilanes into polymers. 

Detailed procedures for the synthesis ofa,o>-organofunctionally terminated siloxane oligomers with well defined structures have been given 50,66-67). Tables 6 and 7 provide the data on the synthesis and characteristics of aminopropyl and hydroxybutyl terminated polydimethylsiloxane oligomers prepared via anionic and cationic ringopening polymerization of octamethylcyclotetrasiloxane (D in the presence of appropriate disiloxanes, respectively. 

Detailed information on the copolymerization of cyclic trifluoropropylmethyl-siloxane trimer and octamethylcyclotetrasiloxane is also very limited in the open literature26 27 . Recently, preparation of various amine terminated (dimethyl-tri-fluoropropyl,methyl)siloxane oligomers with varying molecular weights and backbone compositions has been reported 69115 ll7). Table 11 shows various properties of the oligomers produced as a function of composition. These types of modification play very important roles in determining the solubility characteristics and hence the compatibility of resultant polysiloxanes with other conventional organic monomers... 

Hartle et al. have also observed the transition point in their experiment as shown in Fig. 5 [45]. The film thickness of octamethylcyclotetrasiloxane (OMCTS) exhibits a devia-... 

Another remarkable feature of thin film rheology to be discussed here is the quantized" property of molecularly thin films. It has been reported [8,24] that measured normal forces between two mica surfaces across molecularly thin films exhibit oscillations between attraction and repulsion with an amplitude in exponential growth and a periodicity approximately equal to the dimension of the confined molecules. Thus, the normal force is quantized, depending on the thickness of the confined films. The quantized property in normal force results from an ordering structure of the confined liquid, known as the layering, that molecules are packed in thin films layer by layer, as revealed by computer simulations (see Fig. 12 in Section 3.4). The quantized property appears also in friction measurements. Friction forces between smooth mica surfaces separated by three layers of the liquid octamethylcyclotetrasiloxane (OMCTS), for example, were measured as a function of time [24]. Results show that friction increased to higher values in a quantized way when the number of layers falls from n = 3 to n = 2 and then to M = 1. 

As already discussed in Section 2.2, crystalline dimethylsilanediol 53 can be prepared by hydrolysis from hexamethylcyclotrisilazane 51, from dimethoxydimethyl-silane [40], and from octamethylcyclotetrasilazane (OMCTS) 52. The most simple preparation of 53 is, however, controlled hydrolysis of dimethyldichlorosilane 48 in the presence of (NH4)2C03 or triethylamine [41]. Likewise, hydrolysis of hexam-ethylcyclotrisiloxane 54 and of octamethylcyclotetrasiloxane 55 eventually gives rise to dimethylsilanediol 53. In all these reactions the intermediacy of the very reactive dimethylsilanone 110 has been assumed, which can be generated by pyrolytic [42, 43] and chemical methods [44—46] and which cyclizes or polymerizes much more rapidly, e.g. in contact with traces of alkali from ordinary laboratory or even Pyrex glassware [40, 47] to 54, 55, and 56 than trimethylsilanol 4 polymerizes to hexamethyldisiloxane 7. Compound 111 is readily converted into dimethylsilanone 110 and MesSil 17 [46] (Scheme 3.6). 

The identification of the structures responsible for D1 and D2 and their stabilities toward hydrolysis are further supported by investigations of the hydrolysis behavior of the corresponding isolated ring molecules, octamethylcyclotetrasiloxane (OMCT) and hexamethylcyclotrisiloxane (HMCT) (51.). OMCT is stable in aqueous environments, whereas HMCT hydrolyzes with a pseudo-first order rate constant 3.8 ( 0.4) x 10 3 min 1 (tt, 2 — 3.0 hours). This latter value is comparable to the rate constant for D2 hydrolysis, 5.2 ( 0.5) x 10 3 min"1 (t1/2 - 2.2 hours) and is 75x greater than the rate constant describing hydrolysis of unstrained, conventional a-Si02 (as estimated by extrapolation of the data in reference (52.). 

Octamethylcyclotetrasiloxane (polydimethylsiloxane) 556-67-2 (9016- 00-6) EDS list (Cat 1), possible PBT/ vPvB substance Not listed13 ... 

Ring-opening polymerization of cyclic monomers, usually by anionic or cationic catalysts, is another route to elastomers. These include the polymerization of octamethylcyclotetrasiloxane... 

Brooke DN, Crookes MJ, Robertson S (2009) Environmental risk assessment report octamethylcyclotetrasiloxane. Environmental Agency of England and Wales, Bristol... 

Hamelink, J.L., Simon, P.B., and Silberhorn, E.M. Henry s law constant, volatilization rate, and aquatic half-life of octamethylcyclotetrasiloxane, Environ. Sci. Technol, 30(6) 1946-1952, 1996. 

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