The Structure and Properties of Ultra-high density Polymer Brushes

原子转移自由基聚合（ATRP）和可逆加成-破碎转移聚合（RAFT）等活性自由基聚合（lrp）被应用于各种固体（硅片、聚合物薄膜、二氧化硅或金属纳米颗粒等），其表面化学结合了引发位点。建立不同单体的接枝聚合物薄膜（聚合物刷）的实验技术，控制其链长和化学结构，表面密度高达0.7链/nm^2。这些接枝膜被认为是一种新的分子组织，在肿胀和干燥状态下具有不同于等效铸膜的特征性质。例如，高密度薄膜中的聚合物链被良好的溶剂膨胀后，会高度正常地延伸到薄膜表面，从而使薄膜厚度达到完全拉伸链长度的80 - 90%。膨胀接枝膜的板压缩性不符合结垢理论，干（熔）接枝膜的板压缩性比铸型接枝膜大50%左右。此外，干接枝膜的玻璃化转变温度T_g与等效铸膜在低分子量区（膜厚< 50nm）和高分子量区都有本质差异。特别是，实验数据表明，即使在高分子量的极限下，聚甲基丙烯酸甲酯（PMMA）高密度接枝膜的Tg也比体积PMMA高约8K，这意味着高密度接枝不仅对薄膜有影响，而且对厚膜也有影响，从而对薄膜的体积也有影响。

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.