Ffects Of Chain Stiffness On Properties Of Polyelectrolyte Multilayer

In this dissertation, by use of the combined techniques of quartz crystal microbalance with dissipation (QCM-D), surface zeta potential analyzer (ZPA), atomic force microscopy (AFM), and optical fixed-angle reflectometry (OR), we have systematically investigated the influence of chain stiffness on self-assembly behavior of polyelectrolyte multilayer (PEM), protein adsorption behavior on surface of PEM, and the osmotic pressure induced swelling and collapse of PEM. The results demonstrated that the chain stiffness plays an important role in the interfacial behavior of PEM. The main results are as follows:(1) We have systematically studied the influence of chain stiffness on self-assembly behavior of PEM formed by flexible and semi-flexible polyelectrolytes by using QCM-D and AFM. The results demonstrate that the growth of PEM with salt concentration is determined by the delicate balance between the weakening of electrostatic repulsion between the identically charged groups and the decrease of electrostatic attraction between the neighboring layers. For the flexible/flexible polyelectrolyte pairs, the multilayer growth is dominated by the weakening of electrostatic repulsion between the identically charged groups with the increasing salt concentration. For other polyelectrolyte pairs, the multilayer growth exhibits two different regimes. For example, in the cases of semi-flexible/semi-flexible and flexible/semi-flexible polyelectrolyte pairs, as the salt concentration increases, the multilayer growth is dominated by the weakening of electrostatic repulsion between the identically charged groups and by the decrease of electrostatic attraction between the neighboring layers at the low and high salt concentrations, respectively. The results also indicate that the surface roughness of the multilayers is significantly influenced by the chain stiffness on the surface.(2) We have systematically investigated the effect of polyelectrolyte chain stiffness on the adsorption of bovine serum albumin (BSA) on the PEM surface by use of OR and QCM-D. The results demonstrate that BSA can absorb on the surface of PEM no matter the surface carries the same charge or opposite charge with the protein molecules. The BSA adsorption behavior is influenced not only by the electrostatic interactions but also by the conformation of polyelectrolyte chains. For the flexible/flexible multilayer, a large amount of BSA can be adsorbed on the surface of PEM, which is due to the encapsulation of the BSA molecules by the flexible loops or tails induced by the matching of electrostatic interactions. On the other hand, for the semi-flexible/semi-flexible multilayer, only slight adsorption of BSA can be observed on the surface of PEM because a polyzwitterion-like structure is formed on the surface. In addition, there is no obvious layer number dependence of the adsorbed amount of BSA on the surface of flexible/flexible multilayer. However, for the cases of semi-flexible/semi-flexible and flexible/semi-flexible multilayers, the adsorbed amount of BSA decreases as layer number increases.(3) On the basis of a highly swollen flexible/semi-flexible multilayer, we have used CP-AFM to in situ study the swelling and collapse of PEM induced by the osmotic pressure with dextran solution of different concentrations. The relationship between surface force and osmotic pressure of bulk solution is quantitatively established according to the change in surface force. The results demonstrate that the free water in PEM is sucked out by the high osmotic pressure accompanied by the collapse of PEM with the increasing concentration of dextran. As the concentration of dextran decreases, the water diffused into PEM, resulting in a swelling to the original state. This process can be repeated many cycles without disturbing the inner structure of PEM. The force measurements also demonstrate that the osmotic pressure induced diffusion of water is a fast process. Our study also indicates that some dextran molecules may diffuse into the multilayer and contribute to surface force.