Polysiloxanes thermal degradation

Carbowax 20M, polysiloxanes, and N-cyclo-3-azetidinol are the most widely used sutetances for the thermal degradation method [143,180,192-194]. In the case of the Carbowax treatment deactivation can be carried out in either of two ways. The column can be dynamically coated with a solution of Carbowax 20H in a volatile solvent, excess solvent evaporated with a stream of nitrogen, the column ends sealed and the column heated at about... 

The thermostability of siloxane-silazane copolymers of both random and block structure is found to be much higher (i.e. 100-200°C) with respect to polysiloxanes. This effect is brought about by introducing only a few silazane entities into the polymer chain. The reasons for the effect are not clear and the mechanism of thermal degradation of polysilazoxanes will require further experimental studies. 

The temperature resistance of the polysiloxane on the samples was tested by stepwise heating up to 500°C. Whereas the pure hydrolysis product undergoes a complete thermal degradation via oxidative conversion of the CH3-Si groups into HO-Si groups, the polysiloxane persists on the silica sample. This stabilization effect most likely results from the covalent attachment of the methyl-polysiloxane. 

Polysiloxanes exhibit exceptional properties over an extremely wide range of temperatures because of their unique combination of high thermal stability and low-temperature fiexibility. However, polysiloxanes cannot completely satisfy the needs for high-temperature elastomers that will perform in extreme thermooxidative environments for extended periods. This deficiency originates from the susceptibility of their backbone chains, which are composed completely of polarized siloxyl units, to degradation by ionic reactions when these materials are exposed to temperatures above 200-250 C. At and above such temperatures, polysiloxanes are degraded by... 

Pyrolysis of the formed polysilazane at 750° C generates several alkylsilanes such as dimethylsilane, trimethylsilane, ethyldimethylsilane, tetramethyidisiloxane, pentamethyidisiloxane, methylenebisdimethylsilane, hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, etc. The presence of the cyclic compounds similar to those from poly(dimethylsiloxane) was an indication that some polysiloxane sequences may be present in the polymer. Thermal degradation studied between 350° C and 650° C showed the formation of some hydrogen, methane, ethane, and propene. 

Hall, Patel [30] Polysiloxane rubber thermal degradation Cyclic oligomers PDMS... 

Fig. 60. Molecular weight (i) of polysiloxane and its weight loss (ii) in thermal degradation during 4 h.

TABLE 12. Taft a constants, activation energies and first-order frequency factors (A) for the depolymerization of substituted polysiloxanes. Major depolymerization products formed in the thermal degradation of substituted polysiloxanes are D 3 and D 4 (from Reference 156 reproduced by permission of John Wiley Sons, Inc)... 

Another important class of linear polysiloxanes are the siloxane copolymers. The replacement of methyl groups by other organic groups improves the specific physicochemical properties of PDMS, and one such improvement is in the area of resistance to thermal degradation, e.g., phenyl-containing polysiloxanes have superior thermal stability compared to PDMS. 

Grassie, N. and K.R Francey, Thermal degradation of polysiloxanes. Part 3. Poly(dunethyPmethylphenyl siloxanes), Polym. Degr. Stab., 2 53 (1980). 

The Study of the thermal degradation of these same hybrid silicones [35] was achieved in comparison to the classical polydimethyl and polytrifluo-ropropylmethyl siloxanes, and the authors showed that the introduction of perfluoroalkylene segments - C6H4 - (CF2)x - C6H4 - into the main chain of the polysiloxane increased the thermal stabiUty both under inert and oxidative atmosphere. 

During the thermal degradation of polysiloxanes, the weaker Si-C bond is cleaved at random and then the radicals formed undergo a number of transformations ... 

Production of amorphous silica during thermal degradation of polysiloxane matrix imder oxidizing atmosphere, which results in... 

A variety of agents and procedures have been explored for deactivation purposes (60-74). For subsequent coating with nonpolar and moderately polar stationary phases such as polysiloxanes, fused silica has been deactivated by silylation at elevated temperatures, thermal degradation of polysiloxanes and polyethylene glycols, and the dehydrocondensation of silicon hydride polysiloxanes (71,75-79). 

Choi, M., Kim, Y, Kim, J., and Kim, H. (2008) Rheological properties and thermal degradation behaviors of sonochemically treated polycarbonate/polysiloxane blends. Kor.-Aust. Rhed. J., 20,... 

Grassie, N., Francey, K. F. The Thermal Degradation of Polysiloxanes - Part 3 [Poly(Dimethyl/Methylphenylsiloxane)], Polymer Degradation and Stability 2 (1980), p. 53-66... 

Scheme 10. Crosslinking in PDMDPS induced by the chloride radical during thermal degradation of PVC blends with polysiloxane...

Scheme 24. Crosslinking in PDMS induced by acetate radicals during thermal degradation of PVAc/polysiloxane blends...

The chemical, physical, and thermal properties ana resistance to degradation of polysiloxanes is the result of the high energy (106 kcal/mol) and the relatively large amount of ionic character of the siloxane bond. The ionic character of the Si—O bond facilitates acid and base-catalyzed rearrangement and/or degradation reactions. Under inert conditions, highly purified polydiphenyl- and polydimethylsiloxanes are stable at 350 to 400 °C. 

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