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Thin Film Diblock Copolymers in Electric Field: Transition from Perpendicular to Parallel Lamellae

Summary: Thin Film Diblock Copolymers in Electric Field: Transition from
Perpendicular to Parallel Lamellae
Yoav Tsori and David Andelman*
School of Physics and Astronomy, Raymond and Beverly Sackler Faculty of Exact Sciences,
Tel Aviv University, 69978 Ramat Aviv, Israel
Received October 11, 2001
ABSTRACT: We examine the alignment of thin film diblock copolymers subject to a perpendicular electric
field. Two regimes are considered separately: weak segregation and strong segregation. For weakly
segregated blocks and below a critical value of the field, Ec, surface interactions stabilize stacking of
lamellae in a direction parallel to the surfaces. Above the critical field, a first-order phase transition
occurs when lamellae in a direction perpendicular to the confining surfaces (and parallel to the field)
become stable. The film morphology is then a superposition of parallel and perpendicular lamellae. In
contrast to Helfrich-Hurault instability for smectic liquid crystals, the mode that gets critical first has
the natural lamellar periodicity. In addition, undulations of adjacent intermaterial dividing surfaces are
out-of-phase with each other. For diblock copolymers in the strong segregation regime, we find two critical
fields E1 and E2 > E1. As the field is increased from zero above E1, the region in the middle of the film
develops an orientation perpendicular to the walls, while the surface regions still have parallel lamellae.
When the field is increased above E2, the perpendicular alignment spans the whole film. In another range
of parameters, the transition from parallel to perpendicular orientation is direct.
1. Introduction


Source: Andelman, David - School of Physics and Astronomy, Tel Aviv University


Collections: Materials Science; Physics