copolymer film is defined as
_ ∏eff
eff- M
(5.18)
where Λ∕^ is the number of lamella. Hence, only the symmetric lamellar phases with
energetically favorable A block at both the surfaces are stable at higher film thickness.
Figure 5.8: (a) Excess surface free energy of the copolymer as a function of the effective
film thickness, (b) Reduced lamellar period (for symmetric phases) of the copolymer as a
function of the effective film thickness. For the diblock copolymer, N = 8 and e∕kT — 0.289.
For surfaces, ew∕kT = 0.1.
In other words, the balance between having the lamellar period close to the (pre-
ferred) bulk equilibrium period and unfavorable interactions of the B block of the
copolymer at one of the surfaces, determines the stability of the anti-symmetric lamel-
lar phases. To test this, the preference of the both the surfaces towards the A block
is increased by increasing ew∕kT to 0.3. Figure 5.9 shows the symmetric lamellar
135
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