  We investigate the effects of chain flexibility on the self-assembly behavior
of symmetric diblock copolymers (BCPs) when they are confined as a thin film
between two surfaces. Employing worm-like chain (WLC) self-consistent field
theory, we study the relative stability of parallel (L$_{\parallel}$) and
perpendicular (L$_{\perp}$) orientations of BCP lamellar phases, ranging in
chain flexibility from flexible Gaussian chains to semi-flexible and rigid
chains. For flat and neutral bounding surfaces (no surface preference for one
of the two BCP components), the stability of the L$_{\perp}$ lamellae increases
with chain rigidity. When the top surface is flat and the bottom substrate is
corrugated, increasing the surface roughness enhances the stability of the
L$_{\perp}$ lamellae for flexible Gaussian chains. However, an opposite
behavior is observed for rigid chains, where the L$_{\perp}$ stability
decreases as the substrate roughness increases. We further show that as the
substrate roughness increases, the critical value of the substrate preference,
$u^{*}$, corresponding to an L$_{\perp}$-to-L$_{\parallel}$ transition,
decreases for rigid chains, while it increases for flexible Gaussian chains.
Our results highlight the physical mechanism of tailoring the orientation of
lamellar phases in thin-film setups. This is of importance, in particular, for
short (semi-flexible or rigid) chains that are in high demand in emerging
nanolithography and other industrial applications.
