Temperature crystallization and

Although each of these cyclic siloxane monomers can be polymerized separately to synthesize the respective homopolymers, in practice they are primarily used to modify and further improve some specific properties of polydimethylsiloxanes. The properties that can be changed or modified by the variations in the siloxane backbone include the low temperature flexibility (glass transition temperature, crystallization and melting behavior), thermal, oxidation, and radiation stability, solubility characteristics and chemical reactivity. Table 9 summarizes the effect of various substituents on the physical properties of resulting siloxane homopolymers. The... 

Separation of heat-sensitive materials. High molar mass material is often heat sensitive and will decompose if distilled at high temperature. Low molar mass material can also be heat sensitive, particularly when its nature is highly reactive. Such material will normally be distilled under vacuum to reduce the boiling temperature. Crystallization and liquid-liquid extraction can be used as alternatives to the separation of high molar mass heat-sensitive materials. 

The refractive index will vary with the purity of the oleic acid. Oleic acid purified by low-temperature crystallization and by conversion into the oleic acid-urea complex (95.3% oleic, 0.7% linoleic) yielded a product with wf 1.4613. 

Simple Recrystallization.—In the simplest procedure of recrystallization, the salt is dissolved in some appropriate hot solvent, and the solution is then allowed to cool to room temperature. All the salt in excess of the weight soluble at the lower temperature crystallizes and may be filtered from the mother liquor. Insoluble impurities remain undissolved and may be removed by filtering the hot solution before crystallization begins soluble impurities remain in the solution from which the crystals separate. It is evident that this simplest process applies to the purification of salts that have a decided difference of solubility at different temperatures to cases in which the soluble impurity is not too abundant, or too sparingly soluble and to cases in which... 

In synthetic laboratories, the most common method to obtain crystals is crystallization from solution. In a way this is also the most difficult process to control. The solvent evaporation rate can be affected by several factors such as ambient humidity, temperature and ventilation, thermal excursion during the 24 h in the case of open air room temperature crystallizations, and vibrations of the building (footsteps, traffic, lifts, etc.) and of the refrigerators. There is no unique and transferable recipe for the crystallization of a substance Philip Ball concluded his Nature Editorial in 1996 by saying, the precipitation of good single crystals remains a black magic [68]. 

Nather, C., Bock, H. and Claridge, R. F. C. (1996a). Solvent-shared radical ion pairs [pyrene Na+0(C2H5)2] ESR evidence for two different aggregates in solution, room temperature crystallization, and structural proof of another polymorphic modification. Helv. Chim. Acta, 79, 84-91. [69]... 

TREF was developed before Crystaf. In TREF, a very dilute polymer solution (TCB is generally the solvent of choice) is transferred at high temperature to a column packed with an inert support. The polymer solution is then cooled very slowly, typically from 120-140°C to room temperature. As the temperature decreases, chains with higher crystallization temperatures crystallize and precipitate, followed by chains with lower crystallization temperatures. Crystallization is the most important step in TREF. A slow cooling rate (2.0-6.0°Ch is a recommended range) allows the polymer chains to crystallize near thermodynamic... 

It will be seen that in most of the examples discussed below purification ( 99%) is achieved only through the use of several processes. The selection of these depends on the separation being effected and on the scale of operation. Thus, whilst low-temperature crystallization and urea fractionation are easily employed on a large scale, chromatography and especially silver ion chromatography is less convenient or appropriate for very large amounts of material (Gunstone et al, 1976). 

Icosapentaenoic acid (20 5 /i—3) and docosa-hexaenoic acid (22 6 n—3) must be obtained from a suitable fish oil (menhaden oil, tuna oil) or fish oil preparations already enriched in these acids by a combination of urea fractionation, low-temperature crystallization and chromatography. Traditional distillation of the methyl esters may also be necessary. 

Andrew J. Lovinger, Steven D. Hudson, and Don D. Davis. "High-temperature crystallization and morphology of polyfaryl ether ether ketone)." Macromolecules, pp. 1752-1758,1992. 

The reaction between natural rubber and elemental sulphur is relatively slow at the temperatures normally used for vulcanization, namely about 150°C.The process is also inefficient in that between about 40 and 55 atoms of sulphur become chemically combined with the rubber for each cross-link formed. Only 6 to 10 of these sulphur atoms lay in the actual cross-link the remainder are distributed along the main chains as cyclic sulphide units. This extensive chemical modification of the main polymer chains greatly affects the physical properties of the rubber by inhibiting strain-induced and low-temperature crystallization and by decreasing resilience. 

Several conformational studies have been reported. " The trans,cis conformer (35) of 5-methyl dithiocarbazate was isolated by low-temperature crystallization and was subjected to X-ray crystal analysis. The known solid conformer (36) is the ciSytrans one. 

S. J. Hildebrandt and D. F. Gaines, Inorg. Chem., 1978,17,790. Low temperature crystal and molecular structure of tricarbonyl octahydrotriborato(l —) manganese, (CO)3MnBsH8. 

Suitable theoretical models (e.g., the Havriliak-Negami (HN) model [24]) for analyzing the relaxation behavior of polymer blends (including amorphous/crystalline and thermosetting polymer mixtures) as a function of composition, temperature, crystallization and curing times can be used to evaluate the dielectric relaxation strength of a particular relaxation process and provide suitable information about the number of segments that contribute to the different relaxation processes. In... 

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