Phase and Rheological Behavior of Colloidal Particles with Polymer-Induced Bridging Interactions

Suspensions containing micron-sized colloidal particles and polymers are widely used in many commercial and industrial applications such as paints, consumer products, pharmaceuticals, and separation processes. The addition of polymer to the suspensions can cause varying interactions between the colloidal particles and the polymers. The types of interactions that arise from the addition of polymer can affect the phase and rheological of these suspensions. The suspension attractive strength is affected by polymer size, polymer dispersity, polymer charge, polymer concentration, type of colloid-polymer interaction, solution pH, and salt concentration - also affecting the phase and rheological behavior. By understanding how these parameters affect the interactions between the colloidal particles and polymers can provide insight on flocculation processes such as wastewater treatment and creation of dense markers for indirect detection of specific cell surface molecules. In this thesis, we developed a bridging colloid – polymer model system in which the bridging strength can be controlled via the solution pH. The bridging strength as a function of polymer concentration, particle volume fraction, and solution pH showed to affect the formation of clusters and colloidal networks (Chapter 2). Next, we explored the effects of polymer molecular weight on our pH – tunable bridging model system at low particle volume fraction at varying normalized polymer concentration in the free volume without changing the solution pH. We found that the polymer size affected the size of cluster formation and steric stabilization limit in our colloid – polymer model system. At low polymer molecular weight, small clusters formed at low normalized polymer concentration which decreased in size as normalized polymer concentration increased until particles appeared nearly-hard-sphere due to steric stabilization. At high polymer molecular weight, large dense clusters formed at low normalized polymer concentration in which the size of the clusters decreased as normalized polymer concentration increased. Although a decrease of cluster size was observed, polymer concentrations were not large enough for all the particles to be sterically stabilized (Chapter 3). Next, we examined how polymer size affects the rheological behavior on solid-like bridging suspensions using the same colloid – polymer model system. We investigated the oscillatory shear rheology of high particle volume fraction suspensions at constant normalized polymer concentration across polymer molecular weights 130 and 450 kDa. We ran shear stress and flow sweeps on these suspensions to obtain the linear viscoelastic range and range of yield stress (Chapter 4). Finally, we provide a summary of the work presented in this thesis as well as a discussion of open concepts and inquiries to be still investigated in Chapter 5.