Structure silica-siloxane

D.W. Schaefer, J.E. Mark, D.W. McCarthy, L. Jian, C.-C. Sun and B. Farago, Structure of microphase-separated silica/siloxane molecular composites. In D.W. Schaefer and J.E. Mark (Eds.), Polymer-Based Molecular Composites, Materials Research Society, Pittsburgh, 1990, Vol. 171, p. 57. 

Analysis of rubber filled with conventional filler and an in situ filled siloxane sample displayed three levels of structure in the size-range observed [51]. In another study, growth mechanism and structures of siloxane composites containing silica, and silica-titania were studied by Breiner et al. using SAXS. Both systems were found to yield dense particles. 

Schaefer, D. W. Mark, J. E. McCarthy, D. W. Jian, L. Sun, C.-C. Farago, B., Structure of Microphase-Separated Silica/Siloxane Molecular Composites. In Polymer-Based Molecular Composites, Schaefer, D. W. Mark, J. E., Eds. Materials Research Society Pittsburgh, PA, 1990 Vol. 171, pp 57-63. 

Schaefer, D. W. Vu, B. T. N. Mark, J. E., The Effect of Interphase Coupling on the Structure and Mechanical Properties of Silica-Siloxane Composites. 

During simultaneous polymerization of organic epoxy-amine and silica-siloxane systems, the microphase separation takes place. Relative rates of structure growth resulting in gelation of the hybrid and of microphase separation determine the morphology of the microheterogeneous hybrid. 

Figure 3.19 The proposed structure of chromocene coordinated to a silica siloxane linkage for silica dehydrated at 800°C [25],...

Schaefer DW, Mark JE, McCarthy DW, Jian L, Sun CC, Farago B. In Schaefer DW, Mark JE, editors. Polymer-based molecular composites, structure of microphase-separated silica/ siloxane molecular composites, vol. 171. Pittsburgh Materials Research Society 1990. p. 57-63. 

J.P. Cohen-Addad, Silica-siloxane mixtures. Structure of the adsorbed layer chain length dependence,... 

J.P. Cohen-Addad. Silica-siloxane mixtures, structure of the adsorbed layer chain length dependence. Polymer, 30,1820-1823,1989 ibid. Sol or gel-like behaviour of ideal silica-siloxane mixtures percolation approach. Polymer, 33,2762-2767,1992. 

Suitably cross-linked, an unfilled, high molecular weight polydimethyl-siloxane exhibits very modest mechanical properties, for instance a tensile strength (TS) in the 0.35 MPa range, largely insufficient for most applications. But the addition of a reinforcing filler, such as a high structure silica, increases... 

Scheme 2 Different siloxane bridge structures formed upon dehydroxylation of silica surface. The increasing dimension of silicon rings and, consequently, of the Si - O - Si angle reflects a decreasing of the strain of these structures...

The Raman spectra (0-1400 cm l) shown in Fig re 6 illustrate the structural changes which accompany the consolidation of silica gels. The 1100°C sample is fully dense, whereas the 50 and 600°C samples have high surface areas (1050 and 890 m2/g), respectively. The important features of the Raman spectra attributable to siloxane bond formation are the broad band at about 430 cm 1 and the sharp bands at 490 and 608 cm 1(which in the literature have been ascribed to defects denoted as D1 and D2, respectively). The D2 band is absent in the dried gel. It appears at about 200°C and becomes very intense at intermediate temperatures, 600-800°C. Its relative intensity in the fully consolidated gel is low and comparable to that in conventional vitreous silica. By comparison the intensities of the 430 and 490 cm 1 bands are much more constant. Both bands are present at each temperature, and the relative intensity of the 430 cm 1 band increases only slightly with respect to D1 as the temperature is increased. Figure 7 shows that in addition to elevated temperatures the relative intensity of D2 also decreases upon exposure to water vapor. 

The chemistry of silicone halides was recently reviewed by Collins.13 The primary use for SiCU is in the manufacturing of fumed silica, but it is also used in the manufacture of polycrystalline silicon for the semiconductor industry. It is also commonly used in the synthesis of silicate esters. T richlorosilane (another important product of the reaction of silicon or silicon alloys with chlorine) is primarily used in the manufacture of semiconductor-grade silicon, and in the synthesis of organotrichlorosilane by the hydrosilylation reactions. The silicon halohydrides are particularly useful intermediate chemicals because of their ability to add to alkenes, allowing the production of a broad range of alkyl- and functional alkyltrihalosilanes. These alkylsilanes have important commercial value as monomers, and are also used in the production of silicon fluids and resins. On the other hand, trichlorosilane is a basic precursor to the synthesis of functional silsesquioxanes and other highly branched siloxane structures. 

The surface of the virgin silica gel is covered with water. Heating to 110°C forms a monolayer of silanol groups on the surface (8 pmo of silanol groups per m2). Heating to over 600 °C produces a surface siloxane structure, and this is rehydrated to the silanol form under humid conditions, as shown in Figure 3.1. 

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