Department of Chemical and Biological Engineering, University of Colorado Boulder, Boulder, Colorado 80309, United States.
ACS Nano. 2015 Mar 24;9(3):2465-76. doi: 10.1021/acsnano.5b01321. Epub 2015 Mar 13.
The topology and transport behavior of the lamellar morphology self-assembled by block copolymers in thin films are shown to depend on the length scale over which they are characterized and can be described by percolation in a network under confinement. Gold nanowires replicating the lamellar morphology were fabricated via self-assembled poly(styrene-block-methyl methacrylate) thin films and a lift-off pattern transfer process. The lamellar morphology exhibits long-range connectivity (macroscopic scale); however, characterization of electrical conduction over confined areas (5-500 μm) demonstrates a discrete probability of disconnection that arises due to the underlying network structure and a lack of self-similarity at these microscale dimensions. In particular, it is proved that the lamellar network morphology under confinement has a conductance that is nonlinear with channel length or width. The experimental results are discussed in terms of percolation theory, and a simple, two-dimensional Monte Carlo model is shown to predict the key trends in the network topology and conductance in lamellar block copolymers, including the dependencies on composition, extent of spatial confinement, and confinement geometry. These results highlight the need to exquisitely control or engineer the self-assembled nanostructured pathways formed by block copolymers to ensure consistent device performance for any application that depends upon percolating material, ionic, or electrical transport, especially when confined in any dimension. It is also concluded that the two most promising approaches for enhancing conductivity in block copolymer materials may be achieved either at the limits of (1) perfectly oriented, single-crystalline or (2) high defect density, polycrystalline microphase separated morphologies and that nanostructured systems with intermediate defect densities would be detrimental to transport in confined systems.
层状形态自组装嵌段共聚物在薄膜中的拓扑和输运行为取决于其特征的长度尺度，并可通过受限网络中的渗流来描述。通过自组装的聚苯乙烯嵌段甲基丙烯酸甲酯薄膜和剥离图案转移工艺，制造了复制层状形态的金纳米线。层状形态表现出长程连接性（宏观尺度）；然而，对受限区域（5-500μm）的电传导特性的表征表明，由于底层网络结构和微尺度下缺乏自相似性，出现了不连续的断开概率。特别是，证明了受限下层状网络形态的电导率与通道长度或宽度呈非线性关系。实验结果根据渗流理论进行了讨论，并展示了一个简单的二维蒙特卡罗模型，该模型预测了层状嵌段共聚物中网络拓扑和电导率的关键趋势，包括对组成、空间受限程度和受限几何形状的依赖性。这些结果强调了需要精细控制或设计嵌段共聚物形成的自组装纳米结构路径，以确保任何依赖于渗流材料、离子或电传输的应用的一致设备性能，特别是在任何维度受限的情况下。还得出结论，提高嵌段共聚物材料电导率的两种最有前途的方法可能是在（1）完美取向的单晶或（2）高密度缺陷的多晶微相分离形态的极限下实现的，而具有中间缺陷密度的纳米结构系统则不利于受限系统中的传输。
