In this thesis three specific cases of competitive adsorption of polymers are investigated. The major experimental technique used is dynamic light scattering. The investigations used a water soluble poly(ethylene oxide) (PEO) chain with several modifications to introduce a competitive element to the adsorption. Data from other investigators' studies, on the same systems, are analyzed along with the dynamic light scattering data to clarify the structure of the adsorbed polymer layer.
In the first study, experiments and data analysis are conducted to investigate the outstanding issues of the predicted pancake - quasi-brush transition These issues are: (1) are the observed dramatic increase in the hydrodynamic layer thickness and vertical rise in the adsorption isotherm associated with multi-layer formation; or (2) if they are not associated with multi-layer formation are they a phase transition; and (3) if they are a phase transition is it a first order phase transition? The hydrophobically modified PEO in this study has end-groups which adsorb strongly onto polystyrene (PS) colloids while the PEO backbone adsorbs weakly. The difference in adsorption strengths provide the competition needed for the chain to undergo a phase transition in the conformation of the adsorbed chain, a pancake - quasi-brush transition, when the adsorbed mass is increased. This study eliminates the possibility of using multilayer formation as an explanation of the dramatic increase in the hydrodynamic layer thickness, and shows experimental evidence consistent with a first order phase transition. Atomic force microscopy measurements were performed on dried adsorbed polymer layers in an attempt to visualize the coexistence of the pancake and quasi-brush phases.
In the second study, the investigation centered on the flocculation of colloids by bridging. In particular, we studied the decay of a non-equilibrium, transient bridged state. The investigation used hydrophobically end-modified PEO to create a transient flocculated system which was studied by simultaneous measurements of the polarized and depolarized scattered light. Three time regimes of the decay out of the non-equilibrium state were identified, corresponding to the initial multiple scattering regime, an intermediate large cluster regime, and a long time doublet cluster regime. Temperature dependent measurements identified the intermediate regime to follow an activation process with an activation energy of about 30 k$\sb{B}$T, several times the end-group adsorption energy.
In the third study, the investigation focuses on a competition between micellization and adsorption of a diblock copolymer. In particular, a unique approach was demonstrated to tip the balance so that diblock copolymers can form a stable brush on a colloidal surface even when thermodynamically, the micelle formation is dominant. The competition was tipped in favor of adsorption by introducing a co-solvent, tetrahydrofuran (THF), which is a good solvent for the micelle core (PS). By tuning the solubilities of the block through the use of THF, we were able to induce the adsorption of a brushlike layer onto the PS particles. Stability tests indicated that the adsorbed layer is quite robust. (Abstract shortened by UMI.)
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