Siloxane network

One way to obtain cross links onsists in introducing trifunctional silicon atoms at intervals along the siloxane chains and then establishing oxygen bridges between such monosubstituted trifunctional atoms  

There are two methods available for inserting the trifunctional groups to obtain such cross-linked structures. In one method a composition of R/Si = 2 is first prepared, and then R groups are removed 

In these reactions the composition of the final product is controlled by suitable proportionihg of the intermediate organosilicon halides, the R/Si ratio in the mixture of intermediates being substantially the same as that in the product. 

It follows that in this method of cocondensation one may employ even an unsubstituted silicon tetrahalide or orthoester in the mixture as a source of tetrafunctional groups. The unsubstituted SiX4 (where X is a halogen or ester group) hydrolyzes to the hypothetical ortho-silicic acid, Si(OH)4, and this cocondenses with the equally hypothetical organosilanediol to incorporate the silica unit within the polymeric structure  

It is necessary that the silicol condense at a rate comparable to that for orthosilicic acid, since otherwise the latter will condense only with itself and will precipitate as silica before it can be included within the siloxane network. It also is found by experiment that only a rather small proportion of SiX4 may be included in the mixture of intermediates if the precipitation of silica is to be avoided. However, the mere fact that appreciable amounts of silica can be combined chemically within the polysiloxane structure is strong evidence in support of the theory of intermolecular condensation as given here, and at the same time it illustrates the close chemical similarity between silica and the organosiloxanes. 

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Mark and his co-workers reported the reinforcement of poly(dimethylsiloxane) networks by silica gel particles [1-6]. For example, bis(silanol)-terminated poly-(dimethylsiloxane) was reacted with tetraethoxysilane in the presence of acid-catalyst to produce the reinforced siloxane networks. The reaction proceeded homogeneously. The content of the silica filler can be controlled by the feed ratio of polysiloxane and tetraethoxysilane. 

C. C. Han, H. Yu and their colleagues (23) have presented some new SANS data on end-linked trifunctional isoprene networks. These are shown in Figure 10. Those materials of low molecular weight between crosslinks exhibit greater chain deformation consistent with the thesis that the junction points are fixed. This is the reverse of that found by Beltzung et al. for siloxane networks. 

Bio-nanohybrids Based on Silica Particles and Siloxane Networks... 

Inorganics can also be synthesized and used as templates. Thus, controlled siloxane networks were formed when dispersions of alkoxysilanes (such as (MeO)3SiMe) are mixed with the suitable template matrixes. Ultrafine particles of metal oxides can be used as starting materials for the formation of metal oxide films. For instance, a mixture of a double-chained ammonium amphiphile and an aqueous solution of aluminum oxide particles (diameter about 10 100 nm) gives a multilayered aluminum oxide film when calcinated at over 300°C. 

A similar approach has been taken for preparing molecularly controlled siloxane networks [440,446]. Composite bilayer films were cast from mixtures of alkoxysilane (CH3Si(OCH3)3) and 37 or 38 on a fluorocarbon sheet and were kept at 25 °C and 60% relative humidity for three days. Exposure to gaseous ammonia in a closed vessel for ten days resulted in hydrolysis and subsequent condensation. The surfactants were then extracted by repeated immersion in methanol. Manipulation of the composition of the cast multibilayers allowed... 

For the same polymer and curing procedure, networks formed in the presence of various diluents (

Kinetic theory requires C

volume fraction of polymer during solution cure), assuming comparable numbers of crosslinks per primary molecule are introduced. Experimentally C,

state cures, although they do decrease somewhat in the more dilute curing systems (q> < 0.2). Solution cured polydimethyl siloxane networks show similar behavior in their dry state moduli (275). 

Fig. 7.1. Plot of modulus data on poly(dimethyl siloxane) networks to determine g and Me by Langley s method [Eq.(7.26)] (292)...

An example for a synthesis of a poly(siloxane) network is shown in Fig. 37b. In a one-step reaction the mesogenic moieties as well as the crosslinking agent are coupled via an addition reaction to the reactive linear poly(methylhydrogensiloxane) backbone 92). Because of similar reactivity of the crosslinking agent and mesogenic molecules, a statistical, disordered addition to the backbone has to be expected. 

Therefore, the as-coated glass deposit consists of low oligomers, overlying a three-dimensionally polymerized siloxane network which grades into a molecular interfacial layer. 

One should consider, however, that siloxane (like any polymeric matrix) will not create a hermetic interface. Even though it is hydrophobic in nature, it may still allow water penetration. The second possible explanation involves the change in the relative number of Si—O bonds that would need to be broken to create weakness in the system to the point of changing the peel locus of failure. It may be considered that the APS-siloxane network on the plasma-treated F-contaminated Si02 surface effectively brings another layer of Si—O bonds, the number of which may be too high to be effectively broken during the peel test [21]. 

Efimenko K, Wallace WE, Genzer J (2002) Surface modification of Sylgard-184 poly(dimethyl siloxane) networks by ultraviolet and ultraviolct/ozone treatment. J Colloid Interface Sci 254 306-315... 

As condensation proceeds, small three-dimensional siloxane networks are gradually formed. The condensation reaction may be influenced by the addition of an electrolyte... 

Polycondensation is the further reaction of silanols and alkoxy groups in the gel structure to form siloxane bonds by reactions 3 and 4. These reactions result in a densification and stiffening of the siloxane network. 

He X et al. (1995) Preparation of interpenetrating acrylic polymer-siloxane networks. US Patent 5424375... 

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