Polydimethylsiloxanes oxidation resistance

Siloxanes exhibit high resistance to thermal and thermal-oxidative degradation. This property depends on the chemical composition and the number of carbonic groups in the molecule. Thus, for example, polydimethylsiloxane is resistant under strictly thermal load up to 200 °C, while polydiamylsiloxane, polydibutylsiloxane, and polydibenzylsiloxane are already degraded at this temperature [32]. 

The thermal stability, as well as structure-related properties, such as resistivity and elasticity, of polysiloxanes is dependent on the nature of the pendant groups on the silicon atoms. Thus high-molecular-weight polydimethylsiloxanes are attacked at temperatures near 200 °C in the presence of oxygen, but substitution of a phenyl group for one methyl group raises the oxidative stability to 225 °C. 

POLYDIMETHYLSILOXANE. A silicone polymer developed for use as a dielectric coolant and in solar energy installations, It also may have a number of other uses. It is stated to be highly resistant to oxidation and biodegradation by microorganisms. It is degradable when exposed to a soil environment by chemical reaction with clays and water, by which it is decomposed to silicic acid, carbon dioxide, and water. 

Segmented polyimide-polydimethylsiloxane copolymers have been successfully synthesized both in laboratory and industrial quantities to produce multiphase siloxane-modified polyimides. The siloxane detracts somewhat from the otherwise excellent thermo oxidative stability of the polyimide, but it does produce a number of important properties. These include multiphase behavior, improved adhesion to many substrates, improvements in fire resistance and enhanced gas and liquid separation membranes, where one wishes not only to maximize the contribution of the siloxane to permeability, but also to utilize the imide to re-... 

Of all the known elastomers, polyorganosiloxane elastomers are the most resistant to weather effect they are insensitive to oxidation with oxygen in air and ozone, as well as to UV rays. That is why they do not age even in veiy harsh conditions. E.g., if natural rubber decomposes under the influence of ozone within 5 minutes at 20 °C and within 6 seconds at 100 °C, polydimethylsiloxane elastomer does not decompose even after 60 minutes in ozone at 100 °C. If heated in air to 320 °C, elastomers based on polydimethylsiloxanes, polydimethyl(metylphenyl)siloxanes, etc. only slowly oxidise on the other hand, natural rubber and synthetic organic elastomers decompose at once. 

Use of nanofiltration for non-aqueous separations is limited by membrane compatibility - a common material in composite nanofiltration membranes used for aqueous separations is polysulfone which possesses limited solvent resistance [134]. However, during the past two decades a number of materials have emerged with improved solvent resistance that have enabled a broad range of organic solvent nanofiltration (OSN) applications. These materials include polydimethylsiloxane, polyphenylene oxide, polyacrylic acid, polyimides, polyurethanes, and a limited number of ceramics. Commercial products are offered by Koch Membrane Systems, W.R. Grace, SolSep, and Hermsdorfer Institut fur Technische Keramik (HITK) [135]. 

In a general article about fluorosilicone elastomers [41], Kim analyzed the properties of classical fluorosilicones - [(R)(RF)SiO] - that are an excellent resistance to solvents, a good thermal and oxidative stability, an outstanding flexibility at low temperature. He concluded that fluorosilicones are superior to fluorocarbon elastomers, but they were not very good at high temperatures (above 450 C). Conventional polydimethylsiloxanes, and classical fluorosilicones, present the drawback to give reversion or depolymerization at high temperature, which deteriorates the physical properties. 

In addition to these organic syntheses, biochemical production of compounds in microreactors has also been performed. A microreactor array which enables high-throughput cell-free protein synthesis was developed [4]. The microreactor array is composed of a temperature control chip and a reaction chamber chip. The temperature control chip is a glass-made chip on which temperature control devices, heaters, and temperature sensors are fabricated with an indium tin oxide (ITO) resistive material. The reaction chamber chip is fabricated by micromolding of polydimethylsiloxane (PDMS)... 

---