Charged Polymers at Spherical Surfaces

Sammanfattning: Two separate topics in the area of colloidal chemistry have been investigated by the use of molecular modeling: the structural properties of polyion?macroion complexes and of diblock polyampholyte brushes. The main results were obtained by the use of Monte Carlo simulation in the framework of the primitive model, and in the case of polyampholyte brushes the simulation results were also complemented by lattice mean-field calculations. The complexation of a polyion chain and an oppositely charged spherical macroion in a salt-free aqueous solution has been studied. The complexation dependence on the polyion stiffness, length and charge density as well as on the macroion size has been examined. At increasing polyion stiffness, the following complex structures were obtained: disordered adsorption of the whole chain, wrapping arrangements, multi- and single-loop structures as well as U-turn and single-point contact. For short chains the strongest complexation was observed for slightly semiflexible chains, while with increase of the chain length the fully flexible polyions formed the strongest complexes. The complexation of long and highly charged chains resulted in a macroion overcharging and an asymmetric complex structure, where the polyion bound to the macroion by one end with the other end stretching away from the like-charged complex. The properties of diblock polyampholytes grafted by one end to an uncharged spherical surface have been investigated. The dependence on the relative charge of the blocks, chain composition, block stiffness, absolute charge of the blocks as well as on the chain grafting density has been studied. Keeping the charge of the outer block fixed, the charge of the inner block was gradually increased from zero up to a full block charge compensation. Uncharged inner blocks resulted in a stretched polyelectrolyte star, while at compensated block charge, all the chains were collapsed into a dense polyelectrolyte complex. At intermediate charge conditions, a coexistence of stretched and coiled chains was observed. Polyampholytes with stiff blocks and flexible block junction lead to a polyampholyte-star structure with the outer blocks being backfolded onto the inner ones. The brush structure was found to be only weakly dependent on the polyampholyte grafting density.