Department of Chemical Engineering, Columbia University, 500 West 120th Street, New York, New York 10027, USA.
Soft Matter. 2016 Sep 14;12(34):7241-7. doi: 10.1039/c6sm01396a. Epub 2016 Aug 9.
We compare the self-assembly of silica nanoparticles (NPs) with physically adsorbed polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) copolymers (BCP) against NPs with grafted bimodal (BM) brushes comprised of long, sparsely grafted PS chains and a short dense carpet of P2VP chains. As with grafted NPs, the dispersion state of the BCP NPs can be facilely tuned in PS matrices by varying the PS coverage on the NP surface or by changes in the ratio of the PS graft to matrix chain lengths. Surprisingly, the BCP NPs are remarkably better dispersed than the NPs tethered with bimodal brushes at comparable PS grafting densities. We postulate that this difference arises because of two factors inherent in the synthesis of the NPs: In the case of the BCP NPs the adsorption process is analogous to the chains being "grafted to" the NP surface, while the BM case corresponds to "grafting from" the surface. We have shown that the "grafted from" protocol yields patchy NPs even if the graft points are uniformly placed on each particle. This phenomenon, which is caused by chain conformation fluctuations, is exacerbated by the distribution function associated with the (small) number of grafts per particle. In contrast, in the case of BCP adsorption, each NP is more uniformly coated by a P2VP monolayer driven by the strongly favorable P2VP-silica interactions. Since each P2VP block is connected to a PS chain we conjecture that these adsorbed systems are closer to the limit of spatially uniform sparse brush coverage than the chemically grafted case. We finally show that the better NP dispersion resulting from BCP adsorption leads to larger mechanical reinforcement than those achieved with BM particles. These results emphasize that physical adsorption of BCPs is a simple, effective and practically promising strategy to direct NP dispersion in a chemically unfavorable polymer matrix.
我们比较了二氧化硅纳米粒子（NPs）的自组装与物理吸附的聚苯乙烯嵌段-聚（2-乙烯基吡啶）（PS-b-P2VP）共聚物（BCP），后者由长而稀疏接枝的 PS 链和短而密集的 P2VP 链组成的双模态（BM）刷组成。与接枝 NPs 一样，通过改变 NP 表面上 PS 的覆盖率或 PS 接枝与基质链长度的比例，可以轻松调节 BCP NPs 在 PS 基质中的分散状态。令人惊讶的是，在可比 PS 接枝密度下，BCP NPs 的分散性明显优于用双模态刷固定的 NPs。我们推测，这种差异源于 NPs 合成中固有的两个因素：在 BCP NPs 的情况下，吸附过程类似于链“接枝到”NP 表面，而 BM 情况对应于“从表面接枝”。我们已经表明，即使接枝点均匀地放置在每个粒子上，“从表面接枝”的方案也会产生有斑点的 NPs。这种现象是由链构象波动引起的，并且由于与每个粒子上的接枝数（小）相关的分布函数而加剧。相比之下，在 BCP 吸附的情况下，每个 NP 更均匀地被 P2VP 单层覆盖，这是由强烈有利的 P2VP-二氧化硅相互作用驱动的。由于每个 P2VP 块都与 PS 链相连，我们推测这些吸附系统比化学接枝的情况更接近空间均匀稀疏刷覆盖的极限。最后，我们表明，BCP 吸附导致的更好的 NP 分散导致更大的机械增强，这比使用 BM 颗粒实现的增强更大。这些结果强调了 BCP 的物理吸附是一种简单、有效且具有实际前景的策略，可以在化学上不利的聚合物基质中指导 NP 的分散。
