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 ${\mathrm{[\eta ]=\Phi (〈r}}^2{\mathrm{〉)}}^{\frac{3}{2}}\mathrm{/M}$ , 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/M^⅔ is constant throughout the molecular weight range investigated. Furthermore, Φ^⅓P⁻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 M^0.42 instead of to M^½ 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.