Molecular weight and

TABLE 2.1 Molecular Weights and Values of Materials in Example 2.1... 

The average error is about 2% for tbe critical temperatures and pressures. The error increases with molecular weight and can reach 5%. 

The viscosity of a liquid increases with pressure. This behavior is relatively small for liquids of high molecular weight and low compressibility the effect can be estimated by Kouzel s method (1965) ... 

Shearing causes polymer chains to break, therefore a decrease in molecular weight and, consequently, in thickening power. It has been shown that the higher its molecular weight, the more the polymer is sensitive to mechanical shearing (Brlant et al., 1985). 

This observation that the length of the hydrocarbon chain could be varied from 16 to 26 carbon atoms without affecting the limiting area could only mean that at this point the molecules were oriented vertically. From the molecular weight and density of palmitic acid, one computes a molecular volume of 495 A a molecule occupying only 21 A on the surface could then be about 4.5 A on the side but must be about 23 A long. In this way one begins to obtain information about the shape and orientation as well as the size of molecules. 

If the molecular weight of the aniline is known, the number of amino groups can be calculated alternatively, if the aniline is known to be a monacidic base, its molecular weight can be calculated. If the molecular weight and the acidity of the aniline are both known, then dearly the method can be used to estimate the amount of aniline in a given sample. The method is general for many primary and secondary amines, aniline being used solely as a typical member of the former class. 

The melting points of these esters are usually much lower than those of the corresponding 3 5 dinitrobenzoates their preparation, therefore, offers no advantages over the latter except for alcohols of high molecular weight and for polyhydroxy compounds. The reagent is, however, cheaper than 3 5 dinitrobenzoyl chloride it hydrolyses in the air so that it should either be stored under light petroleum or be prepared from the acid, when required, by the thionyl chloride or phosphorus pentachloride method. 

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. 

The rate of hydrolysis depends upon the solubUity of the acetal m the hydrolysis medium. Acetals of low molecular weight are completely hydrolysed by refluxing for 5-10 minutes those of higher molecular weight, and therefore of small solubility, may require 30-60 minutes, but... 

Ultraviolet visible (UV VIS) spectroscopy, which probes the electron distribution especially m molecules that have conjugated n electron systems Mass spectrometry (MS), which gives the molecular weight and formula both of the molecule itself and various structural units within it... 

Styrene-Acrylonitrile (SAN) Copolymers. SAN resins are random, amorphous copolymers whose properties vary with molecular weight and copolymer composition. An increase in molecular weight or in acrylonitrile content generally enhances the physical properties of the copolymer but at some loss in ease of processing and with a slight increase in polymer color. 

Potentiometric titration curves are used to determine the molecular weight and fQ or for weak acid or weak base analytes. The analysis is accomplished using a nonlinear least squares fit to the potentiometric curve. The appropriate master equation can be provided, or its derivation can be left as a challenge. 

The mass of material in a particular molecular weight class is given by the product of the class mark molecular weight and the number of molecules in the class ... 

Table 1.5 also includes two additional types of molecular weight average besides those already discussed. The following remarks describe some features of these two, the z-average molecular weight, and the viscosity average ... 

The next step in the development of a model is to postulate a perfect network. By definition, a perfect network has no free chain ends. An actual network will contain dangling ends, but it is easier to begin with the perfect case and subsequently correct it to a more realistic picture. We define v as the number of subchains contained in this perfect network, a subchain being the portion of chain between the crosslink points. The molecular weight and degree of polymerization of the chain between crosslinks are defined to be Mj, and n, respectively. Note that these same symbols were used in the last chapter with different definitions. 

Evaluate the relaxation time associated with each of these molecular weights and verify that the molecular weight dependence of r corresponds to the value given in Sec. 2.13. 

Table 4.1 lists values of as well as AH and ASf per mole of repeat units for several polymers. A variety of experiments and methods of analysis have been used to evaluate these data, and because of an assortment of experimental and theoretical approximations, the values should be regarded as approximate. We assume s T . In general, both AH and ASf may be broken into contributions Ho and So which are independent of molecular weight and increments AHf and ASf for each repeat unit in the chain. Therefore AHf = Hq + n AHf j, where n is the degree of polymerization. In the limit of n AHf = n AHf j and ASf = n ASf j, so T = AHf j/ASf j. The values of AHf j and ASf j in Table 4.1 are expressed per mole of repeat units on this basis. Since no simple trends exist within these data, the entries in Table 4.1 appear in numbered sets, and some observations concerning these sets are listed here ... 

Next we turn our attention to the distribution of the molecules by weight among the various species. A corollary of this is the determination of the weight average molecular weight and the ratio... 

In any application of a copolymer the rate of formation of the product, its molecular weight, and the uniformity of its composition during manufacture are also important considerations. While the composition of a copolymer depends only on the relative rates of the various propagation steps, the rate of formation and the molecular weight depend on the initiation and termination rates as well. We shall not discuss these points in any detail, but merely indicate that the situation parallels the presentation of these items for homopolymers as given in Chap. 6. The following can be shown ... 

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