Silicon-backbone materials

Crystalline and amorphous silicons, which are currently investigated in the field of solid-state physics, are still considered as unrelated to polysilanes and related macromolecules, which are studied in the field of organosilicon chemistry. A new idea proposed in this chapter is that these materials are related and can be understood in terms of the dimensional hierarchy of silicon-backbone materials. The electronic structures of one-dimensional polymers (polysilanes) are discussed. The effects of side groups and conformations were calculated theoretically and are discussed in the light of such experimental data as UV absorption, photoluminescence, and UV photospectroscopy (UPS) measurements. Finally, future directions in the development of silicon-based polymers are indicated on the basis of some novel efforts to extend silicon-based polymers to high-dimensional polymers, one-dimensional superlattices, and metallic polymers with alternating double bonds. 

Silicon-backbone materials include silane oligomers, polysilanes, silicon clusters, and amorphous and crystalline silicons. These materials have been investigated independently in two different fields. Crystalline and amorphous silicon are studied in the field of solid-state physics (i), whereas polysilanes and related molecules are studied in the field of organosilicon chemistry (2). Crystalline silicon (c-Si) and amorphous hydrogenated silicon (a-Si H) are well known as two of the most useful semiconductors for electronic and optical devices. Polysilanes have been investigated for application as SiC ceramic binders (3) and photoresists (4). The methods of synthesizing... 

In 1975 Wacker-Chemie introduced silicones under the name of m-polymers. These are also room temperature curing liquid polymers which give rubbery materials on cross-linking and are available both as one- and two-component systems. Their particular feature is that they contain dispersions of copolymers such as those of styrene and n-butyl acrylate in the shape of rods or rice grains in the fluid silicone polymer. A small amount of the organic copolymer is also grafted onto the silicone backbone. 

Examination of the absorption spectra of the new polysilane materials reveals a number of interesting features (14). As shown in Table III, simple alkyl substituted polymers show absorption maxima around 300-310 nm. Aryl substitution directly on the silicon backbone, however, results in a strong bathochromic shift to 335-345 nm. It is noteworthy that 4, which has a pendant aromatic side group that is buffered from the backbone by a saturated spacer atom, absorbs in the same region as the peralkyl derivatives. This red shift for the silane polymers with aromatic substituents directly bonded to the backbone is reminiscent of a similar observation for phenyl substituted and terminate silicon catenates relative to the corresponding permethyl derivatives... 

Polysilane high polymers possessing fully saturated all-silicon backbone have attracted remarkable attention recently because of their unique optoelectronic properties and their importance in possible applications as photoresists, photoconductors, polymerization initiators, nonlinear optical materials etc. A number of review articles have been published on this topic4-9. The studies in this field have stimulated both experimental and theoretical chemists to elaborate on understanding the excited state nature of polysilanes and oligosilanes and of their mechanistic photochemistry. 

In addition to these polar groups, many types of fluorocarbon and hydrocarbon groups have been attached to the silicone backbone, sometimes in combination with polar groups. Some of these materials are used as compatibilizing agents or for other surface active properties but they lie outside the scope of this chapter. 

Silicone rubber offers a set of unique properties to the market, which cannot be obtained by other elastomers. The Si-0 backbone provides excellent thermal stability and, with no unsaturation in the backbone, outstanding ozone and oxidative stability. The very low glass transition temperature, combined with the absence of low-temperature crystallization, puts silicones among the materials of choice for low-temperature performance. The fluoro-substituted versions provide solvent, fuel, and oil resistance along with the above-mentioned stability advantages inherent with the silicone backbone. 

Figure 15b shows a solid-state spectrum recorded under conditions such that only the mobile portions of the solid PDHS sample are observed. In this polymer (as previously indicated), the mobile portion of the sample consists of the locally disordered phase II and any amorphous material to the extent that it exists. The chemical-shift pattern for the carbons agrees very well with the solution spectrum (Figure 15a). Because carbon resonances are very sensitive to bond conformation (22), this result demonstrates that the phase II portion of the sample has the same average chain conformation as the polymer chains in solution. Although these NMR data permit a comparison of local bond conformations, they do not provide an indication of the more global chain dimensions. Figure 15b shows increased line widths for the carbons near the silicon backbone, with the C-1 resonance almost broadened into the baseline. This broadening reflects the severe restriction of motion near the backbone.

The design of single-component polymer transport materials continues to interest researchers in this field. The use of such materials will completely eliminate solvent extraction, diffusional instability, and crystallization of the small molecules. One obvious route that has not been successful to date is the design of yet another aromatic-amine-containing carbon-backbone polymer. An alternative may be to explore the large class of glassy silicon-backbone polymers, such as polysilylenes (14) and polyphosphazenes (iS). 

Figure 1. Si-backbone materials (a) crystalline silicon, (b) amorphous hydrogenated silicon, (c) poly silane alloy, and (d) organosilane polymer.

Summary Two-component room temperature-vulcanizing, condensation-curing systems (RTV-2) are well known in silicone chemistry. Even silicone-based materials caimot fulfill all requirements in diverse applications. It is therefore desirable to combine the curing properties of silicone-based systems with those of other polymer backbones. The use of isocyanatomethyl-dimethylmonomethoxysilane allows the straightforward derivatization of, e.g., hydroxyl-terminated polymers, which yield mono-silanol-terminated polymers upon hydrolysis. 

The range of values given for the siloxyimides covers three different materials tested of that type. Fluorosilicone demonstrated various anomalies which did not permit application of the given equation. The silastyrene showed no detectable moisture solubility. This material was selected as potentially hydrophobic due to its apparent low polarity. It contains methyl groups bonded to a silicon backbone. Although silastyrene has potential,... 

Cazacu, M., Possibilities to Develop Functional Materials on SHicone/Silica Backbones. In Recent Developments in Silicone-Based Materials, Cazacu, M., Ed. Nova Science New York, 2010 pp 1-34. 

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