Adsorption Of Block Copolymers On A Surface

The thesis deals with the adsorption of block copolymers on a surface. First, we have synthesized polystyrene-b-poly(N-isopropylacrylamide) (PS-b-PNIPAM) diblock copolymers via reversible addition fragmentation chain transfer (RAFT) polymerization. Such diblock copolymers can form micelles or vesicles in water, depending on the relative length of the two blocks. On the other hand, the dithiobenzoate group at the end of PNIPAM block can be reduced into thiol by NaBH₄ in aqueous solution.By use of quartz crystal microbalance with dissipation monitoring (QCM-D) and other techniques, we have investigated the adsorption kinetics and the structural transition of the micelles or vesicles with and without thiol groups on a gold surface. The changes of frequency (â–³f) and dissipation (â–³D) in QCM-D reveal that the micelles and vesicles without thiol groups are intact with some deformation when they are deposited on the surface. Due to the strong coupling between the thiol group and gold, the micelles or vesicles with thiol groups can be strongly tethered on gold surface and transform into a sandwich structure with one thin PNIPAM layer contacting the gold surface and one thick PNIPAM layer on the top, in between is the PS layer. Atomic force microscopy (AFM) images further indicate such a structural deformation and transformation.We have also investigated the adsorption of PEO-b-PPO-b-PEO triblock copolymers on a hydrophobic surface. The copolymers studied (PE10100, PE 10300 and PE10500) have the same PPO block (M_wppo=3250) but different PEO blocks. The PEO contents in the copolymers are 10 wt%, 30 wt% and 50 wt%, respectively. A combination of QCM-D and SPR experiments demonstrates that the copolymers with longer PEO blocks (PE10300 and PE10500) quickly adsorbed on the surface and form a monolayer. When the concentration is below the CMC, the adsorption of PE10100 is similar to those of PE10300 and PE10500. However, the adsorption markedly increases at a concentration above CMC, leading to a multilayer. The adsorption kinetics exhibits three-stage; namely, the initial adsorption, limited adsorption and readsorption. This is probably because the shorter PEO blocks on the monolayer can not shield the hydrophobic PPO in the initial stage, which drives more chains to reside on the surface. On the other hand, the release of some chains from PE10100 micelle leads the micelle to be unstable and spilt, so that more chains are the readsorbed on the surface forming multilayer structure.