Determination of the Sedimentation Coefficient-Mol ecular Weight Relation for Nonideal Systems Based on Sedimentation and Viscosity Measurements of a Few Unfractionated Samples

Abstract

A new method for the determination of the sedimentation coefficient-molecular weight relation is proposed. Based on the very low sensitivity of the corresponding sedimentation coefficient average S _0,1 and the weight-average molecular weight M _w (calculated according to the generalized Flory-Mandelkern equation) to changes of the a parameter, the S_0,1 and M _w values are estimated from constant initial values of the a_s parameter for all those polymer-solvent systems for which the real a_s values do not exceed the interval 0.3-0.5. The approximations involved give an error lower than 8.5%, Le., below the experimental errors of the M_w values determined for polydiSDerse samDles. The new method of determining the S_o -M relation was applied to the system styrene-acrylonitrile copolymer (22.6-wt%_oacrylonitrile content) in acetone at 25°C and yielded the following relation: S₀=2.90 × 10 ^{−15 M 0,49 sec. Although in the case of this polymer-solvent system the a value of 0.49 is close to the one corresponding to θ systems, the method is shown by model calculations to be of general applicability and especially useful in the case of nonideal polymer-solvent systems.}