Zhao Dan, Di Nicola Matteo, Khani Mohammad M, Jestin Jacques, Benicewicz Brian C, Kumar Sanat K
Department of Chemical Engineering, Columbia University, 500 West 120th Street, New York, New York 10027, United States.
School of Science and Technology, Università di Camerino, Via Sant'Agostino 1, 62032, Camerino, Italy.
ACS Macro Lett. 2016 Jul 19;5(7):790-795. doi: 10.1021/acsmacrolett.6b00349. Epub 2016 Jun 16.
We systematically compare the dispersion and self-assembly of silica nanoparticles (NPs) grafted with either a sparse monomodal long chain length polystyrene (PS) brush or a bimodal brush comprised of a sparse grafting of long PS chains and a dense carpet of short poly(2-vinylpyridine) (P2VP) chains. These two different types of NPs are placed in pure PS matrices of varying molecular weights in a series of experiments. We first show that NP dispersion is generally improved in the case of bimodal brushes. More interestingly, at low PS grafting densities the bimodal brushes give different self-assembled structures relative to the monomodal brushes; we conjecture that the presence of the short P2VP chains in the bimodal brush reduces the effective core-core attractions and thus allows these bidisperse NPs to display self-assembly behavior that is less likely to be kinetically trapped by the strong intercore attractions that control the behavior of monomodal NPs. In this low PS grafting density limit, where we expect the spatial coverage of the brush to be the most nonuniform, we find the formation of "vesicular" structures that are representative of highly asymmetric ("tadpole") surfactants. Our results therefore show that reducing the inter-NP attractions gives rise to a much richer ensemble of NP self-assemblies, apparently with a smaller influence from kinetic traps (or barriers).
我们系统地比较了接枝有稀疏单峰长链聚苯乙烯（PS）刷或由长PS链的稀疏接枝和短聚（2-乙烯基吡啶）（P2VP）链的致密层组成的双峰刷的二氧化硅纳米颗粒（NPs）的分散和自组装情况。在一系列实验中，将这两种不同类型的纳米颗粒置于不同分子量的纯PS基质中。我们首先表明，在双峰刷的情况下，纳米颗粒的分散通常会得到改善。更有趣的是，在低PS接枝密度下，双峰刷相对于单峰刷会产生不同的自组装结构；我们推测，双峰刷中短P2VP链的存在降低了有效的核-核吸引力，从而使这些双分散纳米颗粒表现出的自组装行为不太可能被控制单峰纳米颗粒行为的强核间吸引力动力学捕获。在这个低PS接枝密度极限下，我们预计刷的空间覆盖率是最不均匀的，我们发现形成了代表高度不对称（“蝌蚪”）表面活性剂的“囊泡”结构。因此，我们的结果表明，降低纳米颗粒间的吸引力会产生更丰富的纳米颗粒自组装集合，显然受动力学陷阱（或势垒）的影响较小。
