Bestimmung der Form und Größe von Makromolekülen aus rheologischen Messungen

Abstract

The hydrodynamic translational resistance (free diffusion, sedimentation in a gravitational or in an inhomogeneous electrical field), the disturbance of laminar flow (viscosity) and the streaming birefringence intrinsically depend on the dimensions, permeability, deformation and orientation of the dissolved macromolecule. Therefore, these rheological measurements can be applied to determine the molecular weight and the mean dimensions in solution at rest and in laminar flow.
The ratio of sedimentation and diffusion constant yields the molecular mass independently on any model assumption. Intrinsic viscosity, sedimentation or diffusion constants too, after a proper calibration by solutions of high polymers of known molecular weight and polydispersity, can be used for the relative determination of the molecular weight.
In contrast to the molecular weight, the shape determination needs a well defined molecular model to start with. The porous sphere model is probably not very realistic. The random coil model with hampered flow seems to represent better the macromolecule in solution but it requires the exact knowledge of all intramolecular distances. Therefore, it can be strictly applied only in the case of the ideal solution. The coil dimensions, according to both models, agree sufficiently well with data from light scattering. With branched molecules the situation turns out to be more complicated although for star-like branching the random coil model seems to be adequate.
By combining the amount of streaming birefringence and the extinction angle one obtains, irrespective of a molecular model, the average coil deformation due to laminar flow. It turns out to nearly correspond to the expansion of a perfectly flexible coil in the case of poly-isobutylene, yet with nitrocellulose the deformation is markedly minor.