the lamella are parallel to the two surfaces. There can be integer ‘n’ (symmetric) or
half-integer ⅛+∣, (anti-symmetric) number of lamella depending upon the separa-
tion between the surfaces. Walton et. al. [138] extended this phenomenological free
energy model for strong segregation to account for the vertical∕perpendicular mor-
phology where the lamella are oriented normal to the two surfaces. They found that
below a certain critical film thickness (or number of lamella), the lamellar morphology
could be either parallel symmetric or perpendicular. Above this critical number, only
parallel symmetric morphology is stable. Parallel anti-symmetric morphology is never
favored when the two surfaces are identical and meta-stable when the perpendicular
morphology is anticipated. However, when the two surfaces are dissimilar, parallel
anti-symmetric morphology can be realized for a limited number of lamella. Owing to
their previous successes with other inhomogeneous polymer systems, self-consistent
field theory (SOFT) has also been applied to study the lamellar morphologies in thin
films of symmetric diblock copolymers. Pickett and Balazs [139] did two dimensional
numerical SCFT calculations (on a spatial lattice) for diblock copolymers confined
by neutral and preferential (selective affinity towards one of the blocks) surfaces. For
neutral surfaces, the morphology is always perpendicular. However, for preferential
surfaces the morphologies can be either perpendicular or parallel depending upon the
film thickness. Matsen [135] used continuous space SCFT to account for the mixed
lamellar morphologies (perpendicular morphology at one and parallel at the other sur-
face) reported in experimental [131] and Monte Carlo simulation [140] results, in case
114
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