Fractionation method, summative

Application of the Summative-Fractionation Method to the Determination of Mw/M for Narrow-Distribution Polymers... 

The Summative-Fractionation Method. To review briefly (8), in the summative-fractionation method one performs a series of small-scale single-step fractional precipitations in which increasing percentages of the total polymer are precipitated. The weight fractions x and average molecular weights Mw of all the precipitates are determined. One calculates and plots against x a parameter... 

The summative-fractioruition method was extended to apply to narrow-distribution polymers with polydispersity (Mw/ Mn) less than 1.12. A fractionation parameter H, previously defined and calculated for theoretical molecular weight distributions for normal polymers, was computed for narrow-distribution polymers. The calculations were made both with and without correction for fractionation errors, using the Flory-Huggins treatment. The method was applied to a well-characterized anionic polystyrene with Mw = 97,000, for which the polydispersity was estimated by this technique to be 1.02 (in the range 1.014-1.027, 95% confidence limits). 

The summative fractionation is a method of fractionation with the determination of the overall polymer precipitate mass in the second phase (without removing the previously precipitated polymer fractious) ou adding a due portion of non-solvent (or on decreasing temperature). I hus, the degree of phase transformation x is determined at the stage of summative fractionation. 

Spencer developed a method for evaluating the distribution from the experimental data obtained with his summative fractionation procedure. His method is bsised on the assumption that the separation according to molecular weight is ideally sharp, i.e. all molecules above a given size are present in the concentrated phase, whereas shorter chains are confined to the dilute phase. This assumption cannot be reconciled with the actual situation, as was pointed out by Billmeyer eind Stock-MAYER (69). 

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