Silicon phases

The performance of a product where adhesion plays a role is determined both by its adhesive and cohesive properties. In the case of silicones, the promotion of adhesion and cohesion follows different mechanisms [37]. In this context, adhesion promotion deals with the bonding of a silicone phase to the substrate and reinforcement of the interphase region formed at the silicone-substrate interphase. The thickness and clear definition of this interphase is not well known, and in fact depends on many parameters including the surface physico-chemistry of... 

A WBL can also be formed within the silicone phase but near the surface and caused by insufficiently crosslinked adhesive. This may result from an interference of the cure chemistry by species on the surface of substrate. An example where incompatibility between the substrate and the cure system can exist is the moisture cure condensation system. Acetic acid is released during the cure, and for substrates like concrete, the acid may form water-soluble salts at the interface. These salts create a weak boundary layer that will induce failure on exposure to rain. The CDT of polyolefins illustrates the direct effect of surface pretreatment and subsequent formation of a WBL by degradation of the polymer surface [72,73]. 

The gas chromatographic separation of some sulphoxide enantiomers was observed on quartz fused silica capillaries coated with the chiral silicon phase chirasil-val280. 

Working in GC/MS mode a researcher often encounters chromatographic peaks due to periodic elution of compounds with abundant ions of m/z 73, 147, 207, 281, 355, etc. These ions represent the fragments of the widespread silicon phases used in the chromatographic columns. 

Fig. 8.3 The lithium—silicon phase diagram. (By permission of the Journal of the Electrochemical Society R.A. Sharma and R.N. Seefurth, 1976, 123, 1763.)...

Alkaloids (includes drugs and vitamins) Apiezon L carbowax 20M di-(2-ethylhexyl) sebacate ethylene glycol adipate ethylene glycol succinate, neopentyl glycol adipate phenyldiethanolamine succinate SE-30 (methyl silicone phases)... 

Amino acids (and derivatives) Carbowax 600 diethylene glycol succinate (stabilized) Ethofat (on chromosorb T) ethylene glycol succinate neopentyl glycol adipate SE-30 XE-60 (methyl silicone phases)... 

Inorganic compounds (includes organometallic compounds) Hydrocarbons C,-C5 (aliphatic) n-Decane di-n-decyl phthalate dimethyl sulfolane neopentyl glycol succinate 1,2,3-tris (2-cyanoethoxy) propane SE-30 (methyl silicone phases) Carbowax 400-1500 most branched and substituted phthalate, sebacate, succinate, and adipate phases octadecane squalane (boiling point separations) methyl silicones... 

Aromatic) Apiezon phases bentone-34 carbowax phases substituted adipates, phthalates, succinates, and sebacates tetracyanoethylated pentaerythritol liquid crystalline phases phenyl methyl silicone phases... 

Pesticides Carbowax 20M diethylene glycol adipate Epon 1001 neopentyl glycol adipate methyl silicone phases, including gum viscosities... 

Silanes Sugars Methyl silicone phases STAP Apiezon L butanediol succinate carbowax 4000, Hyprose SP80 mannitol methyl silicone phases... 

Sulfur compounds Apiezon L 7,8-benzoquinoline carbowax 1500, 20M diethylene glycol succinate diisodecyl phthalate methyl silicone phases Reoplex-400 tricresyl phosphate... 

The use of these constants is described in the table in the section entitled Properties of Some Liquid Phases for Packed Columns. The viscosity data, where available, are presented in cSt, which is 10 m2/sec. Cross-linked silicone phases based on the silicones are especially valuable for capillary gas chromatography. They are not specifically treated in this table since the differences in many properties are quite often subtle. The cross-linked phases have much longer lifetimes due to the effective immobilization. 

Method 2 produces what is usually referred to as self-bonded SiC by a process essentially the same as that developed for nuclear applications [2] and known as Refel silicon carbide. Refel silicon carbide contains a finely dispersed residual silicon phase accounting for between 5 and 10% of the total volume, the precise amount depending upon the porosity of the original preform. In the case of self-bonded silicon carbide elements the residual silicon is volatilized away by a final heat-treatment at over 2500 °C when recrystallization occurs resulting in grain growth and further bonding between primary and secondary carbide phases. 

Cellular Silicone phase phase Cellular Total Silicone phase phase Cellular Total Silicone phase phase ... 

Gornowicz [135] grafted a fluorocarbon resin (e.g. either a homo- or copolymer of VDF) with siloxane to enhance its compatibility with PDMS. For example, vinyl triethoxysilane was first grafted onto fluorocarbon resin using peroxide. Then, an added silicone phase was vulcanizated via hydrosilylation. 

Singh and Drewes described an improved method for the analysis of acetylcholine and choline in canine brain and blood samples by capillary gas chromatography-mass spectrometry [203]. Frozen samples were mixed with butyryl choline (internal standard) and extracted. The GC-MS system was equipped with an electron-impact ionizer and a 15 m capillary column coated with a bonded methyl-silicone phase (HP-SE 54). The column was operated at 60° C for 3 min, and then temperature programmed (at 25°C/min) to 200°C. The ion at mjz 58 was selected for monitoring, and the separation of the three compounds was achieved in less than 5 min. 

An increase in the selectivity and sensitivity of the detection of carbonyl compounds may be achieved by their conversion into special derivatives. Johnson and Hammond [58] condensed carbonyl compounds with 2,4,6-trichlorophenylhydrazine and, prior to the analysis, separated the products by means of thin-layer chromatography. Using an ECD, they were able to determine by GC 10-7—10-10 g of the substance. They prepared the derivatives in a reaction column as follows. A 0.40-g amount of 2,4,6-trichlorophenylhydrazine was dissolved in 40 ml of 1 N HC1 with heating and mixed with 40 g of Celite 545. w-Hexane was added to the wet mixture until a paste consistency was obtained, and the column (30 X 2 cm I.D.) was filled with the paste. In order to prepare the derivatives, the carbonyl compound was applied to the column in an amount corresponding to half of the theoretical column capacity and the column was eluted with 75-100 ml of -hexane. The n-hexane was distilled off at decreased pressure and the viscous derivatives were stored in 10 ml of w-hexane at —27°C. However, these derivatives are sometimes not separated satisfactorily on silicone phases. 

Different chromatographic conditions, sometimes very unusual ones, have been tested for the analysis of TFA-methyl ester derivatives. Hagen and Black [208] applied Carbo-wax 20M as the stationary phase, injected the sample at room temperature, and within 2—3 min increased the temperature to 270°C they detected in the chromatogram even Lys, Arg, Trp and His, thus succeeding where other workers failed, especially as far as His is concerned. The best separation of TFA-methyl esters of amino acids was obtained on a glass column (3.25 m X 2.5 mm I.D.) packed with Diatoport S coated with 2.5% of a mixed silicone phase (XE-60—QF-1—MS-200,46 27 27), both with a temperature programme and under isothermal conditions [211]. 

Apiezon Z is a hydrocarbon grease which is used for the separation of barbiturates. It has the advantage over the silicone phases that it is stable when coated with alkali to reduce the tailing of strongly basic compounds. The amphetamines may be separated on a column of 10% Apiezon L with 2% potassium hydroxide. 

OV-17 is phenylmethyl silicone, a useful, moderately polar, silicone phase with a high maximum operating temperature (350°). It is more oxygen-sensitive than most silicone phases and is available in capillary columns. A considerable amount of retention data for drugs on this phase has been published. 

While the silicone polymer phase provides a high enough oxygen permeability, it also dominates the surface characteristics, i.e., the surface of silicone hydrogel is too hydrophobic to be used as a contact lens. The silicone phase also readily absorbs... 

Because the hydrophilic polymer with negative charge in aqueous solution would not adhere to the silicone phase on the first dipping operation, it is likely that hydrophobic islands remain as the coating thickness increases with a repeated,... 

J. M. Perez, J. Villalobos, P. McNeill, J. Prasad, R. Cheek, J. Kelber, J. P. Estrera, P. D. Stevens, and R. Glosser, Direct evidence for the amorphous silicon phase in visible photoluminescent porous silicon, App/. Phys. Lett. 61(5), 563, 1992. 

A more sophisticated approach for determination of the grain boundary segregation is similar to the determination of the surface tension of silicon melt. The novel approach of surface tension simulation has been successfully implemented in the thermochemical database. Hence, the assessment of the parameters for impurity segregation in solid silicon phase may greatly extend the application of the thermochemical database. The calculation results for C and O segregation are shown as dashed lines in Fig. 13.28. The McLean segregation isotherm can be reproduced using the approach similar to the surface tension simulation. 

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