  We present a model addressing the orientation transition of symmetric block
copolymers such as PS/PMMA on smooth and rough surfaces. The distortion free
energy of parallel and perpendicular lamellar phases in contact with a rough
solid surface is calculated as function of the surface roughness amplitude and
wavelength, as well as the polymer lamellar periodicity (molecular weight). We
find an analytical expression for the orientation transition. This expression
is compared and agrees well with recent experiments done with six different
polymer molecular weights and surface preparations.
