School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, USA.
J Am Chem Soc. 2011 Jun 22;133(24):9592-606. doi: 10.1021/ja203106c. Epub 2011 Jun 1.
We report the unique layer-by-layer (LbL) assembly behavior of pH-sensitive star-shaped polyelectrolytes with both linear and exponential growth modes controlled by star architecture and assembly conditions. Cationic poly[2-(dimethylamino)ethyl methacrylate] and anionic poly(acrylic acid) stars were synthesized via "core-first" atom-transfer radical polymerization (ATRP) based on multifunctional initiators, in addition to their linear analogues. We demonstrated the LbL growth behavior as a function of deposition pH (ranging from 5 to 7), number of layers (up to 30 bilayers), and the method of assembly (dip- vs spin-assisted LbL). The spin-assisted LbL assembly makes it possible to render smoother and thinner LbL films with parameters controlled by the shear rate and pH conditions. In contrast, for dip-assisted LbL assembly, the pH-dependent exponential growth was observed for both linear and star polyelectrolytes. In the case of linear/linear pair, the exponential buildup was accompanied with a notable surface segregation which resulted in dramatic surface nonuniformity, "wormlike" heterogeneous morphology, and dramatic surface roughening. In contrast, star/linear and star/star LbL films showed very uniform and smooth surface morphology (roughness below 2.0 nm on the scale of 10 μm × 10 μm) with much larger thickness reaching up to 1.0 μm for 30 bilayers and rich optical interference effects. Star polyelectrolytes with partially screened charges and high mobility caused by compact branched architecture appear to facilitate fast diffusion and exponential buildup of LbL films. We suggest that the fast buildup prevents long-range lateral diffusion of polyelectrolyte star components, hinders large-scale microphase separation, and thus leads to unique thick, smooth, uniform, transparent, and colorful LbL films from star polyelectrolytes in contrast to mostly heterogeneous films from traditional linear counterparts.
我们报告了具有线性和指数增长模式的 pH 敏感星型聚电解质的独特逐层（LbL）组装行为，这种行为由星型结构和组装条件控制。阳离子聚[2-（二甲氨基）乙基甲基丙烯酸酯]和阴离子聚（丙烯酸）星型聚合物是通过基于多功能引发剂的“核先”原子转移自由基聚合（ATRP）合成的，此外还有它们的线性类似物。我们展示了作为沉积 pH 值（范围从 5 到 7）、层数（高达 30 个双层）和组装方法（浸泡-与旋转辅助 LbL）函数的 LbL 生长行为。旋转辅助 LbL 组装使得可以通过剪切速率和 pH 条件控制的参数来制备更光滑、更薄的 LbL 薄膜。相比之下，对于浸泡辅助 LbL 组装，无论是线性还是星型聚电解质都观察到 pH 依赖性的指数增长。在线性/线性对的情况下，指数增长伴随着明显的表面分相，导致显著的表面不均匀性、“蠕虫状”不均匀形态和剧烈的表面粗化。相比之下，星型/线性和星型/星型 LbL 薄膜显示出非常均匀和光滑的表面形态（在 10 μm×10 μm 的尺度上粗糙度低于 2.0 nm），厚度更大，达到 30 个双层时可达 1.0 μm，并具有丰富的光学干涉效应。部分屏蔽电荷和紧凑支化结构导致的高迁移率的星型聚电解质似乎有利于 LbL 薄膜的快速扩散和指数增长。我们认为，快速增长阻止了聚电解质星型成分的长程横向扩散，阻碍了大规模的微相分离，从而导致了独特的厚、光滑、均匀、透明和多彩的 LbL 薄膜，与传统的线性对应物主要形成不均匀的薄膜形成对比。
