Shear Alignment of Block Copolymer Thin Films with a Nonsolvent Fluid

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. By contrast, self-assembled block copolymer films useful as nanofabrication templates require only small stresses to align; these stresses can be transmitted through a viscous fluid, avoiding the need for solid-on-solid contact. By using a controlled-stress rheometer with parallel-plate fixtures, 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. 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.