Self-sufficiency of velocity sedimentation for the determination of molecular characteristics of linear polymers

Abstract

General possibilities of velocity sedimentation in the hydrodynamic study of macromolecules are discussed. It is shown that, using the mass-spectrometric effect of the ultracentrifuge together with the calculation of molecular weights by means of a recently introduced sedimentation parameter β s , allows complete analysis of all the macromolecular parameters of polymers by velocity sedimentation only, without additional procedures. The sedimentation parameter is given by β s =N a [S]k s 1 3 M −2 3 , where [S]=S o η o /(1− νϱ o ) is the intrinsic sedimentation constant, k s is Gralen's concentration coefficient, M is the molecular weight and N a is Avogadro's number. In this way molecular weight distributions etc. can be obtained. The validity of the suggested approach is demonstrated on a cellulose nitrate (CN) sample with a substitution degree of 2.7, in acetone. The sample is characterized by the polydispersity index M w/ M n=1.66 in the molecular weight range 20⪕M × 10 3⪕620 . By means of velocity ultracentrifugation, values of the equilibrium rigidity of CN (Kuhn's segment length), A=(360±13) × 10 −8 cm, and of hydrodynamic diameter, d=(4.8±0.4) × 10 −8 cm, are obtained. All data obtained are in good agreement with published data for real fractions of CN in acetone.