Template wetting


Bidirectional template wetting


Complex tube and rod architectures are attracting increasing interest (S. J. Hurst, E. K. Payne, L. Qin, C. A. Mirkin, Angew. Chem. Int. Ed. 2006, 45, 2672). Nanorods and nanotubes consisting of segments of different functional materials have emerged as a new class of advanced 1D nanostructures. Their electronic, optical, mechanical and chemical properties may show controlled and adjustable variations along their long axes as required for a broad range of potential applications. For example, nanofibers with a gradient of their mechanical properties along their long axes may be used as building blocks for bioinspired adhesive structures. Bidirectional template wetting is a generic method to fabricate segmented nanofibers with adjustable segment length from materials that cannot be processed by conventional deposition methods. It enables the integration of many functional materials, such as tailor-made organic compounds and polymers, into segmented nanotubes and nanorods. Bidirectional template wetting involves two successive wetting steps starting from the opposite surfaces of a porous template. Exploiting different wetting mechanisms, it may yield segmented hybrid nanofibers consisting of a solid and a tubular segment (Fig. 1). The length of the solid segment can easily be adjusted.


Figure 1. Schematic diagram of the preparation of tube/rod hybrid nanofibers by bidirectional template wetting and exploitation of different wetting mechanisms. Light blue areas denote the pore walls of the template, red areas the component forming the solid rod-like segments with adjustable length, and black areas the component forming the tubular segments.



An example that demonstrates the versatility of this approach are polystyrene-block-poly(methyl methacrylate)/                      poly(vinylchloride) tube/rod hybrid nanofibers. In Fig. 2, rigid polystyrene- block- poly(methyl methacrylate) segments with a length of about 40 m are seen at the bottom. They exhibit the characteristic lamellar microphase structure of this symmetric block copolymer, which causes the high rigidity. At the top, flexible tubular segments consisting of  the homopolymer poly(vinylchloride) show up.


Figure 2. Scanning electron microscopy image showing the segmented nature of polystyrene-block-poly(methyl methacrylate)/poly(vinylchloride) tube/rod hybrid nanofibers. Rigid polystyrene-block-poly(methyl methacrylate) segments with a length of 40 m are seen at the bottom, and tubular poly(vinylchloride) segments at the top.



The rod to tube transition is obvious from Fig. 3 that shows the middle section of an ensemble of tube/rod hybrid nanofibers. In the upper section of the image, the nature of the nanofibers is apparently tubular, and in the lower section apparently columnar.


Figure 3. Scanning electron microscopy image of the middle section of a polystyrene-block-poly(methyl methacrylate)/poly(vinylchloride) tube/rod hybrid nanofiber array. Columnar polystyrene-block-poly(methyl methacrylate) segments are seen at the bottom, tubular poly(vinylchloride) segments at the top.

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