Molecular hydrodynamic transition phenomena in entangled polymer solutions studied by quasielastic light scattering

Abstract Quasielastic light scattering (QLS) analyses at small scattering vector (KRg < 1) of two polymer-solvent systems in the semi-dilute concentration regime are reported. A series of polystyrenes (M = 0.18, 0.39, 1.8, 3.0 and 10.0×106g/mole) were studied in a good solvent, tetrahydrofuran. The results indicate that QLS is sensitive to two hydrodynamic transition effects. The first occurs at coil overlap and represents the onset of entanglement effects; a second transition occurs at higher concentration and represents a transition to molecular-weight-independent-(pseudo-gel) dynamics. The correlation functions of the three samples of highest molecular weight are close to single exponential decays, and the concentration-dependence of the first moments in the pseudo-gel regime are in agreement with the theory of deGennes based on a flexible coil network. A breakdown of the scaling theory is observed for the samples of lower molecular weight. The correlation functions above the overlap concentration are non-exponential and in fact appear to exhibit bimodal relaxation behavior. These properties are in agreement with a theoretical model based on a transient network. A study of the biological polysaccharide, xanthan gum is also reported. The dilute solution properties of this polymer are consistent with a structural model of a rigid thin rod of dimensions 6,000Å×60Å. At higher concentrations, a transition is observed to highly non-exponential correlation functions spanning several decades of time. These correlation functions can also be interpreted as bimodal decays. Strong similarities between these results and the theory of Doi-Edwards for semi-dilute solutions of rigid rod macromolecules are noted.