Department of Chemical Engineering, Columbia University , New York, New York 10027, United States.
ACS Nano. 2017 Jul 25;11(7):7028-7035. doi: 10.1021/acsnano.7b02657. Epub 2017 Jun 21.
It is now well-accepted that controlling the spatial dispersion of nanoparticles (NPs), which can be achieved by grafting them with polymers of different chain lengths and grafting densities, is central to optimizing the thermomechanical properties of the resulting polymer nanocomposites. In general, there are two methods for creating such polymer-grafted NPs: "grafting to" and "grafting from". The conventional wisdom is that the "grafting from" mechanism, where monomer-sized initiator/functional groups are attached to the surface followed by growing the chains, allows for higher polymer grafting densities and hence a more uniform polymer coverage of the NP surface. Here, we perform calculations and instead show that the "grafting to" strategy surprisingly leads to a more uniform polymer coverage of the NP surface at a given grafting density since the brush is formed while respecting the excluded volume constraints of the previously grafted chains. This conclusion is especially clear in the limit of low-to-moderate grafting density. Thus, at a given grafting density, the "grafting to" mechanism leads to an enhanced miscibility of the NPs in the matrix (which has the same chemistry as the grafts) and lower propensity to create self-assembled structures. Another important factor is that the dispersity in the number of grafted chains on the NPs is also smaller in the case of "grafting to" systems, thus leading to better defined materials. These two conclusions imply that the "grafting to" mechanism may provide better control over the NP dispersion state and hence the thermomechanical properties of polymer nanocomposites.
现在人们普遍认为，控制纳米粒子（NPs）的空间分散是优化所得聚合物纳米复合材料的热机械性能的关键，这可以通过用不同链长和接枝密度的聚合物接枝 NPs 来实现。一般来说，有两种方法可以制备这种聚合物接枝的 NPs：“接枝到”和“从接枝”。传统观点认为，“从接枝”机制，即单体大小的引发剂/官能团附着在表面上，然后生长链，可以实现更高的聚合物接枝密度，从而使 NP 表面的聚合物覆盖更均匀。在这里，我们进行了计算，而不是显示出，在给定的接枝密度下，“接枝到”策略会导致 NP 表面的聚合物覆盖更均匀，因为在遵守先前接枝链的排除体积限制的情况下形成了刷。在低至高中等接枝密度的限制下，这一结论尤其明显。因此，在给定的接枝密度下，“接枝到”机制导致 NPs 在基质（与接枝相同化学性质）中的混溶性增强，并且形成自组装结构的倾向降低。另一个重要因素是，在“接枝到”体系中，接枝到 NPs 上的链的数量分散性也更小，从而导致更好定义的材料。这两个结论意味着“接枝到”机制可能更好地控制 NP 的分散状态，从而控制聚合物纳米复合材料的热机械性能。
