Rheo-optische Experimente zur Hydro- und Thermodynamik von Polymeren in verduennten Loesungen

Abstract

In order to understand the hydro- and thermodynamic behaviour of a dilute polymer solution in a shear flow on a molecular scale, the are of gyration of the single macromolecules was detected by the method of 3-dimensional light scattering. The theoretically predicted development of the gyration area which is spherical at rest to an ellipsoidal shape with three different half-axes was successfully observed and the length and orientation of these axes were determined quantitatively at all applied shear rates. As a mean value for all the three half-axes a widening deformation is found, although the effect is smaller than theoretically predicted. Above a critical shear rate the behaviour of the single molecule in the investigated system of polystyrene in dioctylphthalate changes to a collective behaviour, which is displayed in the 3-d light scattering as a drastic growth of the scattering objects and which is interpretated as an agglomeration of the single molecules to aggregates. This interpretation is supported by viscometric measurements performed simultaneously to the scattered light detection, which show a decrease of viscosity far beyond the normal shear thinning of high molecular solutions. It is still contended whether this shear-induced aggregation can be viewed as a phase separation in a thermodynamic sense. However, the development of the second osmotic virial coefficient at increased shear rates and the dependency of the critical shear rate of aggregation on temperature, polymer concentration and molar mass allow this interpretation. The elastic energy which the polymers obtain under shear is the energetic-thermodynamic reason for this aggregation. This is not a contradiction to explanations which assume self increasing hydrodynamic concentration fluctuations. These can be understood as the kinetics of the aggregation, whose cause can also be expressed in (equilibrium) thermodynamic terms. They have an analogue in the well known spinodal demixing of quiescent solutions. (orig.)Available from: http://edocs.tu-berlin.de/diss/1999/ / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman