Generating Long-Range Order by Shear-Alignment
Alignment of polymer thin films is often achieved by “rubbing”or “buffing”: contacting the film with another solid, as practiced commercially for the polyimide alignment layers in liquid-crystal displays. We have employed a similar process to align supported block copolymer thin films, using a soft silicone rubber (PDMS) pad, as schematized below:
Apparatus for the shear-alignment of supported block copolymer thin films. Shear stress is applied through the PDMS pad, held in contact with the film by a simple dead load; temperature control is provided by a digital hot plate.
This simple approach is remarkably effective for the alignment of monolayers (or multilayers) of cylinder-forming block copolymers, as well as bilayers (or thicker stacks) of sphere-forming diblocks. Examples of successful alignment are shown in the tapping-mode atomic force microscope (TM-AFM) images below:
TM-AFM images of shear-aligned block copolymer films. Shear direction indicated by red arrows; the alignment is coincident with the shear direction, over the entire centimeter-squared sheared area (set by the area of the PDMS pad). Both are polystyrene-poly(ethylene-alt-propylene) diblocks, S/EP, with a minority S block; the rubbery EP matrix and glassy S inclusions provide excellent contrast for TM-AFM imaging at room temperature. Left: monolayer of cylinder-forming S/EP 5/13. Right: bilayer of sphere-forming S/EP 3/24 (only top layer of spheres is revealed by TM-AFM).
Block copolymer melts are remarkably responsive to flow stresses; indeed, the sort of solid-on-solid contact employed in the apparatus above is far more than the minimum required. The necessary stresses can be transmitted through a viscous nonsolvent fluid: think of a shearing flow where the block copolymer film coats one “wall”; the stress applied to the film is the “wall” shear stress in whatever flow geometry is employed. By using a controlled-stress rheometer with parallel-plate fixtures, as shown below, we can apply—to a single film—a gradient of stress values, providing a high-throughput method to quantify how the alignment quality depends on stress, time, and temperature.
Top: Schematic of the rheometer experiment (layer thicknesses not to scale). A thin block copolymer film containing two or more layers of spherical nanodomains is covered with a thick layer of silicone oil, which is rotationally sheared from the top to transmit the stress to the block copolymer film. Bottom: Imaging the film by TM-AFM after shearing reveals the stress needed to align the film; colors indicate the orientation of the spheres in the top layer. At low stresses (187 Pa, left), the spheres exist in a polygrain arrangement typical of unaligned films, while at high stresses (1363 Pa, right), the entire film is highly oriented.
Examination of the sheared film at different distances from the rotation axis reveals the state of order achieved at that stress/time/temperature combination. We find that the stress required for alignment decreases progressively as the block copolymer’s order-disorder transition temperature is approached, making this the optimal window for the shearing process. We have also developed a simple melting-recrystallization model which describes how the alignment evolves with continued shearing.
Supported by the National Science Foundation through the Princeton Center for Complex Materials
Recent/Current People and Projects:
Andy Marencic GS - Shear-Alignment of Block Copolymer Thin Films
Mingshaw Wu *05 - Progress in Block Copolymer Lithography
Dan Angelescu *03 - Physics and Applications of Diblock Copolymer Thin Films
Judith Waller (Oxford) - Long Range Orientation of Block Copolymer Microdomains
Selected Recent Publications:
M.W. Wu, R.A. Register, and P.M. Chaikin, “Shear Alignment of Sphere-Morphology Block Copolymer Thin Films with Viscous Fluid Flow”, Phys. Rev. E, 74, 040801R (2006).
D.E. Angelescu, J.H. Waller, M.W. Wu, P.M. Chaikin, and R.A. Register, “Method and Apparatus for Providing Shear-Induced Alignment of Nanostructures in Thin Films”, U.S. Patent Application 2006/0013956 A1, published January 19, 2006.
D.E. Angelescu, D.E., J.H. Waller, D.H. Adamson, P. Deshpande, S.Y. Chou, R.A. Register, and P.M. Chaikin, “Macroscopic Orientation of Block Copolymer Cylinders in Single-Layer Films by Shearing”, Adv. Mater., 16, 1736-1740 (2004) [cover article].