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. But block copolymer melts are remarkably responsive to flow stresses; indeed, solid-on-solid contact typically imparts stresses far more than the minimum required. Instead, 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.

Schematic of the rheometer experiment and imaging the film by TM-AFM — 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 (Wu *et al*., *Phys. Rev. E*, 74, 040801R (2006)).

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. These projects were carried out in collaboration with Professor Paul Chaikin (Physics, Princeton; later, Physics, New York University) and Dr. Douglas Adamson (Princeton Institute for the Science and Technology of Materials).

Supported by the National Science Foundation, Materials Research Science and Engineering Center (MRSEC) Program, through the Princeton Center for Complex Materials

Group Members Involved, and Their Project Titles:

Andrew Marencic PhD *11 – “Well-Ordered Block Copolymer Thin Films Using Shear-Alignment Techniques”
Vincent Pelletier PhD Phys *05 – “Physics and Technology of Sheared Cylinder-Forming Diblock Copolymer Thin Films”
Mingshaw Wu PhD Phys *05 – “Progress in the Physics of Block Copolymer Lithography”
Dan Angelescu PhD Phys *03 – “Physics and Applications of Diblock Copolymer Thin Films”
Judith Waller Oxford MEng Materials *03 – “Long Range Orientation of Block Copolymer Microdomains”
Ranulfo Allen ‘08 – “Oscillatory Shear Alignment of Block Copolymer Thin Films”

Relevant Group Publications:

A.P. Marencic, M.W. Wu, R.A. Register, and P.M. Chaikin, “Orientational Order in Sphere-Forming Block Copolymer Thin Films Aligned Under Shear”, Macromolecules, 40, 7299-7305 (2007).

V. Pelletier, D.H. Adamson, R.A. Register, and P.M. Chaikin, “Writing Mesoscale Patterns in Block Copolymer Thin Films through Channel Flow of a Nonsolvent Fluid”, Appl. Phys. Lett., 90, 163105 (2007).

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, J.H. Waller, R.A. Register, and P.M. Chaikin, “Shear-Induced Alignment in Thin Films of Spherical Nanodomains”, Adv. Mater., 17, 1878-1881 (2005).

D.E. Angelescu, 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].