Molecular fractionation general

In copolymerisations carried out with heterogeneous catalysts, the higher a-olefin is preferentially incorporated at sites producing shorter chains higher a-olefins are thus enriched in the low-molecular fractions of the copolymers obtained. This increases the content of extractables and tends to make these copolymers sticky, two generally undesirable properties [30]. 

The volatile fraction of wine determines to a great extent its aroma, which is one of the most important characteristics influencing wine quality and consumer preferences. The volatile compounds are able to stimulate the sensorial organs that are responsible for the olfaction. These compounds correspond to small molecules, of medium hydrophobicity and molecular weight generally, between 30 g/mol and 300 g/mol (Morrot Brochet, 2000). 

Thermogravimetric analysis can provide valuable information, often quantitative, on the composition of polymeric materials. If a multicomponent material contains low-molecular-mass compounds, polymeric material, and inorganic additives, the three groups can be separated by temperature. The quantitative determination of a low-mass additive in a polymeric material by isothermal TGA was described in Section 3.4.4 (see Fig. 3.16). This general principle of molecular fractionation by temperature is common to most linear polymers, where rupture of carbon-carbon bonds typically occurs at temperatures... 

This chapter presents first some fundamental aspects of nucleation, and second the general Avrami equation, which is frequently used to describe overall crystallization. The growth theories of Lauritzen and Hoffman and Sadler and Gilmer are discussed in sections 8.4.2 and 8.4.3. Molecular fractionation and orientation-induced crystallization are dealt with in sections 8.5 and 8.6. 

If the ether is a simple one (R — R ), the identification of the resulting alkyl iodide presents no difficulties. If, however, it is a mixed aliphatic ether, the separation of the two alkyl iodides by fractional distillation is generally difficult unless R and R differ considerably in molecular weight and sufficient material is available. 

Thionyl chloride. This reagent (b.p. 76°) is generally used in excess of the theoretical quantity it cannot be employed for acetyd chloride (b.p. 52°) because of the difficulty of separation by fractional distillation. Excellent results are obtained, however, with butyrric acid and acids of higher molecular weight, for example ... 

For preparative purposes batch fractionation is often employed. Although fractional crystallization may be included in a list of batch fractionation methods, we shall consider only those methods based on the phase separation of polymer solutions fractional precipitation and coacervate extraction. The general principles for these methods were presented in the last section. In this section we shall develop these ideas more fully with the objective of obtaining a more narrow distribution of molecular weights from a polydisperse system. Note that the final product of fractionation still contains a distribution of chain lengths however, the ratio M /M is smaller than for the unfractionated sample. 

Hven fractionated polymer samples are generally polydisperse, which means that the molecular weight determined from intrinsic viscosity experiments is an average value. The average obtained is the viscosity average as defined by Eqs. (1.20) and (2.40) as seen by the following argument ... 

A sharp separation results in two high purity, high recovery product streams. No restrictions ate placed on the mole fractions of the components to be separated. A separation is considered to be sharp if the ratio of flow rates of a key component in the two products is >10. The separation methods that can potentially obtain a sharp separation in a single step ate physical absorption, molecular sieve adsorption, equiHbrium adsorption, and cryogenic distillation. Chemical absorption is often used to achieve sharp separations, but is generally limited to situations in which the components to be removed ate present in low concentrations. 

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