Sedimentation Behavior of Flexible Chain Molecules: Polyisobutylene

Abstract

The sedimentation constants s0 at infinite dilution and the intrinsic viscosities [η] of five fractions of polyisobutylene have been determined in cyclohexane, a good solvent. Their osmotic molecular weights ranged from 3.1×104 to 1.4×106. Under the assumption that the effective hydrodynamic radii vary as the average linear dimensions of the molecule, f0/η0=P(〈r2〉)½ and [η]=Φ(〈r2〉)32/M, where f0 is the frictional coefficient, η0 is the viscosity of the solvent, 〈r2〉 is the mean-square end-to-end distance of the molecule, and P and Φ are constants. In confirmation, s0[η]⅓/M⅔ is constant throughout the molecular weight range investigated. Furthermore, Φ⅓P−1=2.5×106. The close agreement of this quantity with values for four other systems reported in the literature indicates that P, like Φ, is a universal constant for flexible chain molecules. It follows also that the observed proportionality of s0 (or of M/f0) to M0.42 instead of to M½ occurs because of intramolecular interactions which cause the molecule in solution to be expanded by a factor α which increases with molecular weight. The above theory accounts for both the sedimentation and intrinsic viscosity behavior of flexible chain molecules in a satisfactory manner. Neither the Kirkwood-Riseman nor the Debye-Bueche theories succeeds in doing so.