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

Accurate 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 < M_w < 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 M_sD 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 M_sD are lower than the nominal values M_w 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 M_sD values are more reliable characteristics of the standard samples than the nominal M_w values and that the sedimentation-diffusion technique is a powerful, though laborious, tool for an absolute determination of the molar mass.