Polymer and Materials Science
Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
Two- and three-parameter functions commonly used for the description of the molecular weight distribution of polymers were determined from gel permeation chromatography data for 80 water-soluble samples of polymers derived from glutamic acid. Two methods were applied: the moment method in which two or three different molecular weight averages are used, and a least squares adjustment method using the whole chromatogram. Convergence difficulties observed in the past were overcome, and a fully automated computer procedure was developed. The moment method, although virtually the only one still in use, often gives inexact results: sometimes the type of two-parameter distribution is wrongly assigned, or the description of experimental data cannot be improved by adding yet another parameter. On the other hand, in all cases under study the least squares method gave a function which describes the real distribution within the limits of experimental error. Although the majority of samples could be described by employing one of the common two-parameter functions, the more general three-parameter function, encompassing all two-parameter ones as special cases, has the advantage of wider applicability. It is shown that the standard gel permeation chromatography data often cannot provide reliable values of higher molecular weight moment averages, including M̄z. Hence, it may be inferred that in the description of the experimental molecular weight distribution curves the least squares method should be preferred over the moment method.
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