Molecular weight determination method

Solvent Temperature (°C) Molecular weight determination method K x 104 (dL/g) a Reference... 

TABLE 14.18 Typical Molecular Weight Determination Methods [20]... 

All classic molecular weight determination methods require the polymer to be in solution. To minimize polymer-polymer interactions, solutions equal to and less than 1 g of polymer per 100 ml of solution are used. To further minimize solute interac-... 

Since cannot be derived theoretically, Ke is usually determined by calibration with substances of known molecular weight. As with all the other molecular-weight-determination methods, only apparent molecular weights Mapp are obtained for finite concentrations (see Sections 6.4 and 6.5) because of the effects of the virial coefficients and/or association, Mapp must therefore be extrapolated to the concentration C2 -> 0. In vapor-phase osmometry, low amounts of nonvolatile impurities interfere with the result, but volatile impurities do not, since they pass into the vapor phase. 

A relationship is shown to exist in viscometry experiments between particle size or molecular size and the viscosity of dispersions of inorganic colloids or the viscosity of macromolecular solutions. It is therefore possible to determine the molecular weight from the viscosity of dilute macromolecular solutions. Since this experiment can be rapidly performed with simple equipment, it is, in practice, the most important molecular-weight-determination method. However, the method is not an absolute one, since the viscosity depends on other molecular properties (for example, on the shape of the molecule), as well as on the molecular weight. 

Solvent Temperature, "C Molecular weights determination method K, 10 dL/g a Reference... 

More extensive surveys of molecular weight determination methods may be found in other volumes [21-23]. Their relevance to mechanochemical processes is summarized below. 

The major molecular weight determination methods include number average molecular weights determined by end-group analysis. 

Molecular Weight Determinations by Physical Methods. Vapour Density. Victor Meyer s Method. 

This method of Molecular Weight determination should be used only with solvents in which the particular substance is freely soluble, since it is essential that, on cooling, the solvent, and hot the solute, should crystallise out. 

Molecular weight determinations by depression of the freezrng-point are more accurate and far less troublesome than those by the elevation of the boiling-point (p. 440), and the former method should always be preferentially employed if the solubility of the organic compound in the soh ent is sufficiently high. 

The method of end group analysis for molecular weight determination is not only simple to understand, but can also be done with ordinary laboratory equipment in many instances. 

Condensation polymers such as polyesters and polyamides are especially well suited to this method of molecular weight determination. For one thing, the molecular weight of these polymers is usually less than for addition polymers. Even more pertinent to the method is the fact that the chain ends in these molecules consist of unreacted functional groups. Using polyamides as an example, we can readily account for the following possibilities ... 

This expression is consistent with the analysis of each of the lines in Table 5.1 as presented above and provides a general answer to one of the questions posed there. It is often a relatively easy matter to monitor the concentration of functional groups in a reaction mixture as we saw in discussing end group analysis as a method for molecular weight determination in Sec. 1.7. Equation (5.4) is... 

The phenomena we discuss, phase separation and osmotic pressure, are developed with particular attention to their applications in polymer characterization. Phase separation can be used to fractionate poly disperse polymer specimens into samples in which the molecular weight distribution is more narrow. Osmostic pressure experiments can be used to provide absolute values for the number average molecular weight of a polymer. Alternative methods for both fractionation and molecular weight determination exist, but the methods discussed in this chapter occupy a place of prominence among the alternatives, both historically and in contemporary practice. 

Molecular Weight EPA Method 3 is used to determine carbon dioxide and oxygen concentrations and dry molecular weight of the stack-gas stream. Depending on the intended use of the data, these values can be obtained with an integrated sample (see Fig. 25-28) or a grab sample (see Fig. 25-29). In addition, the instrumental analyzer... 

Such data can provide a calibration curve and allow the constants (E) and F ) in equation (20) to be determined. The value of the molecular weight of an unknown solute can then be obtained from its (H) value by reading the value directly from the curve or by calculation using the predetermined constants (E) and (F ) in equation (20). It should be pointed out that an error of up to 30% may not appear to be very useful but, in fact, such precision can be extremely valuable in the preliminary examination of many biochemical substances where only very small quantities of material are available. It is also an ideal method for molecular weight determination before more accurate, labor-intensive and time-consuming methods are considered. 

Molecular weight assignments for such resins will be dependent on the method used for measurement. Each manufacturer has his own relative scale. In practice, absolute molecular weight determination is not important. The method used must discriminate between resins with sufficient resolution to allow prediction of resin performance in the applications of interest. 

MolekuLargewichts-bestimmtmg, /. molecular weight determination, -bestimmungsme-thode, /. method for determining moleculai weights. 

ASTM D-2502 is one of the most accurate methods of determining molecular weight. The method uses viscosity measurements in the absence of viscosity data, molecular weight can be estimated using the TOTAL correlation. 

Step 1 Molecular weight determination by ASTM method. 

This was exactly the measurement you made in Experiment 6—it is called the vapor-density method for molecular weight determination. To... 

Exactly the same type of behavior is found for the freezing point of a solution except that the freezing point is lowei than that of the pure solvent. Thus we have two methods for molecular weight determination which are applicable to compounds with such low vapor pressure or which decompose so readily that the vapor density method cannot be used. 

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