Polymer Gels: Modeling the Swelling Behavior
Abstract: This thesis is concerned with the swelling behavior of polyelectrolyte gels. The thesis can be divided into two parts. Part one deals with the modeling and computer simulation of the gel swelling. The second part contains experimental results from the binding of oppositely charged surfactant to a polyacrylate network and a theoretical model for the swelling in the presence of a "skin" around the gel core, which is formed of polyelectrolyte and surfactant. The description of the gel in the first part of the thesis is based on the primitive model and treats the polyelectrolyte as a chain of charged hard spheres connected by harmonic springs. The topology of the network is diamond-like, and the counterions are taken into account explicitly. The interactions in the model are (i) excluded volume, (ii) bond and angle, (iii) Coulomb, and (iv) short-range attractive interactions. The role of the counterion contribution to the osmotic pressure was investigated, and it was found to be the dominating contribution to the gel swelling. The electrostatic interactions between network charges and counterions lead to a reduction of the osmotic pressure. The influence of the charge density, cross-linking density, chain stiffness, and counterion valence was also investigated and good agreement with experiment was found. Discontinuous volume transitions were observed in the simulation of networks with short-range attractive interactions, and at high electrostatic coupling for gels without any additional attractive potential. The influence of salt on the swelling and the distribution of the salt ions between the gel and the surrounding solution have been investigated. In the experimental part, the swelling of the polyelectrolyte gel depending on the amount of bound surfactant was measured and a model for the swelling in the presence of a polyelectrolyte-surfactant "skin" is presented. The structure and the size of the surfactant aggregates was determined using small-angle X-Ray scattering and time-resolved fluorescence quenching. Hexagonal and cubic (Pm3n) structures were found.
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