The Structure and Properties of Ultra-high density Polymer Brushes

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Living radical polymerizations (LRPs) such as atom transfer radical polymerization (ATRP) and reversible addition-fragmentation transfer (RAFT) polymerization were applied to various solids (silicon wafer, polymer films, silica or metal nano-particles, etc.) with initiating sites chemically bound on their surfaces, establishing experimental techniques to prepare graft polymer films (polymer brushes) of differing monomers with controlled chain length and chemical structure and with a surface density as high as 0.7 chains/nm^2.These graft films were recognizable as a new molecular organization having characteristic properties, in both swollen and dry states, different from those of the equivalent cast films. For example, the polymer chains in the high-density film swollen by a good solvent are highly extended normally to the film surface, thus giving a film thickness of 80 to 90% of the fully stretched chain length. The plate compressibility of the swollen graft film disobeys the scaling theory, and that of the dry (melt) film is about 50% larger than the cast film. Moreover, the glass transition temperature T_g of the dry graft film is essentially different from that of the equivalent cast film in both low-molecular weight (film thickness < 50nm) and high-molecular weight regions. In particular, the experimental data indicate that the Tg of poly(methyl methacrylate) (PMMA) high-density graft films is about 8K higher than that of bulk PMMA even in the limit of high molecular weight, meaning that the high-density grafting has an effect not only to thin films but also to thick films and hence to the bulk of the films.