Morphology & Exchange Kinetics in Polymer Brushes: The Physical Significance of Hydrodynamic Transitions and Stretched Exponential Evolutional Forms

ABSTRACTThis work reexamines equilibrium and kinetic features of polymer brushes end-adsorbed onto solid substrates from solution, providing a new interpretation of previously-published experimental results and insight into the comparison between experiments and existing kinetic models. The study centers on an end-modified polyethylene oxide (PEO) which may adsorb to form a polymer brush on a hydrophobic polystyrene latex, and whose main backbone also exhibits an attraction for the surface. In a previous work, Gao and Ou-Yang claim a sharp pancake-to-brush transition with increasing surface coverage of the samples with the strongest end group attraction to the substrate, and only a delicate increase in the hydrodynamic thickness of layers whose chain ends are weakly attracted to the surface. Our reevaluation of this data, however, collapses all molecular architectures to a single curve describing the hydrodynamic thickness at different surface loadings, and which shows a relatively sharp transition for all samples. Gao and Ou-Yang have also performed kinetic exchange experiments where low molecular weight species replace preadsorbed high molecular species (with the same endgroup-surface adsorption energy), and observed stretched exponential behavior in the evolution of the hydrodynamic thickness of the adsorbed layer. We employed Milner's treatment for brush exchange kinetics, to extract equivalent phenomenological parameters for more direct comparison between fundamental parameters and experiment.