Long-Range Ordered Thin Films of Block Copolymers Prepared by Zone-Casting and Their Thermal Conversion into Ordered Nanostructured Carbon Chuanbing Tang, ² Adam Tracz, ‡ Michal Kruk, ² Rui Zhang, ² Detlef-M. Smilgies, § Krzysztof Matyjaszewski, ² and Tomasz Kowalewski* ,² Department of Chemistry, Carnegie Mellon UniVersity, 4400 Fifth AVenue, Pittsburgh, PennsylVania 15213, Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, 90-363 Lodz, Sienkiewicza 112, Poland, and CHESS, Cornell UniVersity, Ithaca, New York 14853 Received February 10, 2005; E-mail: tomek@andrew.cmu.edu One of the main challenges in the utilization of self-assembly of block copolymers to fabricate well-defined periodic nanostruc- tures of technological relevance (e.g., for magnetic data storage 1a,b or for lithography 1c ) is the achievement of long-range ordered structures. Some recently developed strategies toward this goal typically involve the use of external stimuli, such as electric field 2a or shear (e.g., roll casting), 2b or employ such phenomena as eutectic solidification, 2c orientation during solvent evaporation, 2d graphoepi- taxy, 2e and alignment on lithographically patterned substrates. 2f In the present work, the technique of directional casting, also referred to as “zone-casting”, which had been originally developed to facilitate the oriented growth of molecular crystals, 3 has been used to achieve large-scale alignment of nanoscale domains of a phase- separated diblock copolymer. Oriented block copolymer films were then converted into carbon films with well-defined, long-range ordered texture by applying the recently developed approach to the synthesis of nanostructured carbon. 4 This approach is based on the use of block copolymers containing polyacrylonitrile (PAN), which serves as a carbon precursor and a sacrificial block, immiscible with PAN (e.g., poly(n-butyl acrylate), PBA). Nanoscale PAN domains are first stabilized by heating under air to 200-300 °C and are then carbonized through heating to about 800 °C under nitrogen, with concomitant volatilization of the sacrificial block. The current study was carried out with PBA-b-PAN diblock copolymers described by the structural formula (BA) 240 (AN) 124 . 5 Zone-casting was performed by depositing the copolymer solution in N,N-dimethylformamide (DMF) onto a moving substrate with the aid of a syringe equipped with a flat nozzle (Figure 1a). The syringe plunger and the substrate were displaced at 6 μm/s using two separate computer-controlled stepper motors. To achieve the desirable solvent evaporation rate, the temperatures of the copoly- mer solution and of the substrate were maintained at 90 °C. Adjustment of the solution concentration under constant solution supply rate and constant rate of substrate withdrawal made it possible to vary the film thickness in the range from 100 nm to 1 μm (c.f. Supporting Information). The surface morphology of thin films of PBA-b-PAN block copolymers was visualized using tapping mode atomic force microscopy (TMAFM). Phase contrast images of films prepared by zone-casting exhibited characteristic long-range ordered striped morphology with elongated rigid PAN domains appearing as brighter stripes alternating with more compliant and mechanically lossy PBA phase (Figure 1b, left half). The periodicity (domain spacing) of this oriented texture was determined by 2-D Fourier transform (FT) of AFM images and was equal to 37 nm. The high degree of orientation was reflected by the shape of azimuthal profiles of the FT magnitude maximum corresponding to this periodicity; the azimuthal profiles could be well fitted by a Lorentzian with a half-width at half-height, ∆φ ) 1.4° (Figure 1e). Imaging of adjacent 3 × 3 μm areas revealed that the orientation was almost perfectly preserved over the distances of tens of micrometers, as shown in the accompanying animation, constructed from overlaid images covering the range of over 30 μm in two directions: parallel and perpendicular to the orientation direction (movie, Supporting Information). Random inspection of other regions revealed that the orientation persisted over the entire sample area (3 cm × 5 cm). Grazing incidence small-angle X-ray scattering (GISAXS) pat- terns of the copolymer sample prepared by zone-casting (Figure 1c, top) were consistent with the lamellar structure with lamellae perpendicular to the surface and oriented perpendicular to the casting direction (see scheme in Figure 1d, left). The periodicity determined from the position of the first maximum in the GISAXS ² Carnegie Mellon University. ‡ Polish Academy of Sciences. § Cornell University. Figure 1. Long-range order in thin films of PBA-b-PAN block copolymers prepared by zone-casting (a) and in nanostructured carbons prepared by subsequent pyrolysis. (b) AFM phase images (left, copolymer; right, carbon); (c) GISAXS patterns acquired at 90° to casting direction (top, copolymer; bottom, carbon); (d) schematic illustration of lamellar order (left, copolymer; right, carbon); (e) azimuthal profiles of maxima in 2-D Fourier transforms of AFM images and maxima in GISAXS patterns corresponding to the lamellar period. Published on Web 04/21/2005 6918 9 J. AM. CHEM. SOC. 2005, 127, 6918-6919 10.1021/ja0508929 CCC: $30.25 © 2005 American Chemical Society