Molecular-Weight Distribution

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n The percentages by number (or weight) of molecules of various molecular weights that comprise a given specimen of a polymer. Two samples of a given polymer having the same number-average molecular weight may perform quite differently in processing because one has a broader distribution of molecular weights than the other. Two basic groups of methods are used for measuring molecular-weight distribution. Fractionation methods, which actually divide the specimen into portions having relatively narrow ranges of molecular weight, include: fraction precipitation and fraction solution (the two most widely used), chromatography, liquid–liquid partition, ultracentrifugation, zone refining, and thermo gravimetric diffusion. After fractionation by any of these methods, the weight percent of each fraction is plotted versus the average molecular weight for that fraction to obtain a histogram of the distribution, which may be smoothed into a curve. Non-fractionation methods include light-scatterin ...