Sedimentation transport method for determination of molecular weight distributions

Abstract

Abstract The sedimentation of the system polystyrene‐cyclohexane at the Flory temperature has been studied with emphasis on the effects of pressure as well as concentration. The relation between the molecular weight M and the limiting sedimentation coefficient s 0 0 , is found to be s 0 0 = 1.50 × 10 −15 M 1/2 (sec.) The concentration dependence parameter k s has the form, k s = k ′ M 1/2 = k ″ s 0 0 with k ′ = 4.5–5.5 × 10 −4 . However, a rather unexpected dependence of k s on the rotor speed is also found. A procedure is proposed for deducing solute molecular weight distributions from boundary spreading data in sedimentation transport experiments, a so‐called “single concentration” method, requiring only one sedimentation run. Application to several polystyrenes (in cyclohexane at 35°C) with narrow, broad, and very broad distributions demonstrates the feasibility of the procedure. Comparisons are made with data from elution chromatography and gel permeation chromatography. The GPC method predicts somewhat broader distributions than those obtained by the other two methods.