AFM cross-sectional imaging of perpendicularly oriented nanocylinder structures of microphase-separated block copolymer films by crystal-like cleavage
AFM cross-sectional imaging of perpendicularly oriented nanocylinder structures of microphase-separated block copolymer films by crystal-like cleavage
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DOI：
10.1021/ma0704008
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发表时间：
2007-06-12
期刊：
影响因子：
5.5
通讯作者：
Iyoda, Tomokazu
中科院分区：
文献类型：
--
作者：
Komura, Motonori;Iyoda, Tomokazu
嵌段共聚物的微相分离导致的周期性有序纳米结构作为自组装过程的一种，已经推动人们制备纳米多孔材料、膜、光刻纳米模板和支架，作为未来电子和光子材料的基础技术。1-13特别令人感兴趣的是嵌段共聚物，其形成垂直取向并跨越两个最顶表面的圆柱形纳米结构域，这对于作为膜功能的穿过膜的圆柱形通道的质量传输应该是至关重要的。透射电子显微镜（TEM）的切片标本，小角度X射线散射的散装样品，和原子力显微镜（AFM）的表面形貌已被用于迄今为止的补充分析的周期性纳米结构。原子力显微镜是一种在大气环境下对表面形貌进行成像的有力工具，但这种成像仅限于表面，本研究可以将其解开，从而为微相分离纳米结构的表面和内部成像提供可能。最近，我们报道了一系列新设计的两亲性嵌段共聚物的薄膜，该共聚物由亲水性聚（环氧乙烷）（PEO）和疏水性聚甲基丙烯酸酯组成，侧链中具有偶氮苯介晶（PMA（Az））（图1）显示出高度有序的微相分离与六边形排列的圆柱体。特别地，当通过流延、旋涂、棒涂和浸渍方法在硅晶片、玻璃、云母和聚（对苯二甲酸乙二醇酯）（PET）基底上制备膜时，PEO纳米柱垂直于表面和基底取向。圆柱体的垂直方向已被证实的横截面TEM观察使用染色技术与RuO 4和显微镜切片术在以前报道的类似方式。15-23然而，薄膜样品内部的结构分析仅限于其区域被预先选择性染色的薄膜，适当的薄膜厚度低于约100 μ m。500 nm，和软基片如PET。在环境大气下还没有这种垂直取向的圆柱体的直接成像。在这里，为了解决这些问题，我们选择原子力显微镜（AFM）作为一种方法，用于评估横截面没有染色，完全不同于传统的使用图像表面形态。我们报道了首次成功地在大气环境下对垂直取向和六边形排列的PEO圆柱体结构进行了AFM截面成像，并采用简单的样品制备方法作为评价内部结构的新技术。PEO-b-PMA（Az）微相分离膜是在硅片上制备的（见支持信息）。如图1所示，微相分离膜的横截面仅通过将膜与带缺口的硅衬底一起劈开来制备。通过AFM测量放置在具有固定剂的AFM样品台上的基底上的裂开的膜。通过多模扫描探针显微镜Nanoscope IV（Digital Instruments）进行测量。我们使用标称谐振频率为300 kHz的轻敲模式硅杠杆（NanoDevices）。图2a中示出了从2重量%甲苯溶液棒涂的PEO 114-b-PMA（Az）51膜的表面的典型相移图像。大面积扫描图像及其部分缩放图像显示在支持信息中。在对应于晶体的（001）面的整个表面上观察到的六边形排列的点图案。
Periodically ordered nanostructures arising from the microphase separation of block copolymers, as one of the selfassembling processes, have driven one to fabricate nanoporous materials, membranes, lithographic nanotemplates, and scaffolds as fundamental technology of the coming electronic and photonic materials. 1-13 Of particular interest are block copolymers that form cylindrical nanodomains oriented perpendicularly and spanning to both the topmost surfaces, which should be crucial for mass transport through the cylindrical channels across the film as membrane function. Transmission electron microscopy (TEM) of microtomed specimen, small-angle X-ray scattering of bulk samples, and atomic force microscopy (AFM) of the surface morphology have been used so far to complementarily analyze the periodic nanostructures. The AFM is a powerful tool to easily image the surface morphology under ambient atmosphere, but the imaging has been limited only on the surface, which can be untied in this study to open the possibility for both surface and inside imaging of the microphase-separated nanostructures. Recently, we reported that the thin films of a series of newly designed amphiphilic block copolymer consisting of hydrophilic poly (ethylene oxide)(PEO) and hydrophobic polymethacrylate with azobenzene-mesogen in side-chain (PMA (Az))(Figure 1) show highly ordered microphase separation with hexagonally arranged cylinders. 14 Especially, the PEO nanocylinders are perpendicularly oriented to the surface and a substrate when the films were prepared on a silicon wafer, glass, mica, and poly (ethylene terephthalate)(PET) substrates by casting, spincoating, bar-coating, and dipping methods. The perpendicular orientation of the cylinders has been confirmed by crosssectional TEM observation using a staining technique with RuO4 and ultrathin microtomy in the similar manners reported previously. 15-23 However, the structural analysis inside the film samples has been limited to the films of which domains are selectively stained beforehand, adequate film thickness below ca. 500 nm, and soft substrates such as PET. There has been no direct imaging of such perpendicularly oriented cylinders under ambient atmosphere. Herein, in order to solve these problems, we selected atomic force microscopy (AFM) as a method for evaluating cross sections without staining, quite different from the conventional usage to image surface morphology. We report the first successful demonstration of the AFM cross-sectional imaging of the perpendicularly oriented and hexagonally arranged PEO cylinder structures under ambient atmosphere with a simple sample preparation as a new technique for evaluating inside structures.The PEO-b-PMA (Az) microphase-separated films were prepared on a silicon wafer (see Supporting Information). The cross sections of the microphase-separated films were prepared just by cleaving films together with the notched silicon substrate, as Figure 1 illustrates. The cleaved film on the substrate put up on an AFM sample stage with fixative was measured by AFM. The measurements were made by a Multi Mode scanning probe microscope, Nanoscope IV (Digital Instruments). We used Tapping Mode silicon cantilevers with a nominal resonance frequency of 300 kHz (NanoDevices). A typical phase-shift image of a surface of the PEO114-b-PMA (Az) 51 film bar-coated from a 2 wt% toluene solution is shown in Figure 2a. The large-area scanned image and its partly zoomed ones are shown in the Supporting Information. A hexagonally arranged dot pattern observed in the whole surface corresponding to a (001) plane of the …