Absolute determination of molar masses of standard polystyrenes by means of sedimentation‐diffusion measurements in cyclohexane

Abstract

AbstractAccurate measurements of the sedimentation coefficients at unit pressure and zero‐concentration s, the diffusion coefficients at zero concentration D0, and the intrinsic viscosities [η] were conducted at theta‐conditions (cyclohexane, 34,5°C) on 11 polystryrene standard samples with narrow molar mass distribution. These samples with nominal mass‐average molar masses (5 · 104 < Mw < 2 · 106 g · mol−1), provided by three manufacturers for the use as calibration standards, were the same as in the previous paper of this series summarizing our measurements in toluene. Logarithmic correlations between s, D0 and [η] are linear with deviations lower than ± 3% and indicate good consistency of all three independent sets of measured quantities. The molar masses MsD as determined from s and D0 by means of the Svedberg equation are in very good accordance with those previously determined in toluene, but for most samples the MsD are lower than the nominal values Mw as given by the manufacturers. For some standards the deviations amount to 15−35% and cannot be explained by experimental errors or by the polydispersity of the samples. On the other hand, two pairs of standards exhibit the same values of s, D0 and [η] in both solvents, although the nominal molar masses differ by 20%. These results lead to the conclusion that the experimentally determined MsD values are more reliable characteristics of the standard samples than the nominal Mw values and that the sedimentation‐diffusion technique is a powerful, though laborious, tool for an absolute determination of the molar mass.