In molecular weight determination

In molecular weight determinations it is conventional to dissolve a measured mass of polymer m2 into a volumetric flask and dilute to the mark with an appropriate solvent. We shall use the symbol Cj to designate concentrations in mass per volume units. In practice, 100-ml volumetric flasks are often used, in which case C2 is expressed in grams per 100 ml or grams per deciliter. Even though these are not SI units, they are encountered often enough in the literature to be regarded as conventional solution units in polymer chemistry. 

Nowadays, MS plays an important role in many fields of scientific and technological research for identification, structural characterization and quantitative determination of wide classes of compounds. Its unique capabilities, such as high sensitivity, high selectivity, accuracy in molecular weight determination, and ability to analyze complex mixtures, give this methodology great importance in the solution of problems not easily handled by other techniques. 

Dynamic image analysis systems, 18 147 Dynamic Kerr effect (DKE), 17 454 Dynamic light scattering (DLS), 20 381 in molecular weight determination,... 

Table I. Precision in Molecular Weight Determination Polyglycol...

Table IV. Accuracy in Molecular Weight Determination Polyethylene...

Other Detectors. Most of the other commercially available detectors are highly selective and sensitive and find use in specialized analyses, including inorganic analyses. One exception is the gas density balance,18 which is a universal type of detector and finds use in molecular weight determinations (see Chapter 6) and for the analysis of corrosive materials. 

Molal boiling-point elevation constant a constant characteristic of a particular solvent that gives the change in boiling point as a function of solution molality used in molecular weight determinations. (17.5)... 

Determining the molecular weights of asphaltenes is a problem because they have a low solubility in the liquids often used for determination. Also, adsorbed resins lead to discrepancies in molecular-weight determination, and precipitated asphaltenes should be reprecipitated several times prior to the determination (12). Thus, careful precipitation and careful choice of the determination method are both very important for obtaining meaningful results. 

Finally, Figure 8 illustrates the molecular-weight distributions obtained by GPC of a number of n-heptane asphaltenes from various crude sources. As can be seen, a wide range of molecular weights can be obtained for asphaltenes from different crude sources. Furthermore, the peak in the distribution occurs at different values for the different crudes. It is well known that molecular association of asphaltene molecules can be a problem in molecular weight determination either by GPC or VPO. However, the real extent of this problem is quite problematical. Small angle x-ray scattering... 

Some kf values so obtained are water 1.8, acetic acid 3.8, benzene 5.1, succi-nonitrile 20.3, camphor 40. The large kf oi camphor makes it especially useful in molecular weight determinations. Like ebulliometry, the cryoscopic method is also limited to relatively low-molecular-weight polymers with M up to 50,000. 

H.-G. Elias, Polymolecularity and polydispersity in molecular weight determinations. Pure Appi Chem. 43(1-2), 115 (1975). 

Corbett (61,62) used a densometric procedure coupled with molecular weight determination by VPO at 37°C to determine the structure of his fractions, as shown in Table 1. Asphaltenes could not be characterized completely because of the difficulties in molecular weight determination as a result of asphaltene molecular association. 

All the evidence discussed previously indicates that if SEC is to be employed in molecular weight determinations the best solvent system for the associating material should be used. These include data at low concentrations and extrapolation to infinite dilution. Elevated temperatures probably help, but the... 

Kim, W.-S. Elurrr, C.H. Molnar, A. Yu, J.-S. Lee, S. Repeatability and reproducibility of thermal field flow fractionation in Molecular weight determination of processed natural rubber. Analyst 2006,131 (3), 429-433. 

A useful technique for routinely evaluating resolution is to use a mixture of two proteins that vary two- to threefold in molecular weight. Determination of the elution volumes or retention times and peak widths will allow calculation of resolution by means of the standard equation ... 

---