EQUILIBRIUM SURFACE ORIENTATION OF LAMELLAE

We theoretically investigate the equilibrium orientation of lamellae arising from symmetric diblock copolymers in the presence of a hard wall. Firstly, the wall may preferentially wet with one of the species of the copolymer, thus favoring lamellar planes parallel to the wall. Secondly, chains are more easily stretched along the wall than they are in bulk; this favors the lamellar planes being perpendicular to the wall. Finally, the enhanced chain end density near the wall in the parallel orientation favors the planes flat along the wall. As molecular weight M approaches infinity, the most important effect is the wetting approximately M0, followed by the nematic effect approximately M-2/3, and lastly followed by the end effect approximately M-8/9. In experimentally available copolymer domains, we find that the nematic and end effects have comparable magnitude. This magnitude appears more than adequate to influence the orientation of the lamellae. Lastly, we discuss effects of substrate roughness.