and the lamella are compressed. For lɪ lamella at H = 16.4σ, the lamellar period is
higher and the lamella are stretched. At higher film thickness of 19.4σ, 2∣ lamella
of both A and B are observed. One of the advantages of modified iSAFT is that
these different lamellar phases can be calculated at any film thickness. The lamellar
phase with the lowest excess surface free energy, Ωs is the equilibrium structure at
that film thickness. The excess free energy curve as a function of the effective film
thickness for the anti-symmetric phases is calculated and overlaid onto the free energy
curve for the symmetric phases. The equilibrium free energy curve for the given
symmetric diblock copolymer (N = 8 and e∕kT = 0.289) confined between the two
planar surfaces (ew∕kT = 0.1) is shown in fig. 5.8a. Till Heyγ < 4∣, the symmetric L^n
lamellar phases are stable around integer (n) values of He∕/ and anti-symmetric b2n+1
lamellar phases are stable around half-integer (n + ɪ) values of Heyγ. The sole reason
for observing the stable anti-symmetric lamellar phases even though the energetically
unfavorable B block is at one of the surfaces is that the lamellar period is closer to the
(preferred) bulk equilibrium period. The range of film thickness for which these anti-
symmetric phases are stable becomes smaller for higher film thicknesses. And when
He// gets larger than 4∣, only symmetric phases are stable. For symmetric lamellar
phases at higher film thickness, the lamella are not highly stretched or compressed as
the lamellar period are closer to the (preferred) bulk equilibrium lamellar period as
shown in figure 5.8b. Note that in this figure, a reduced lamellar period Dejγ of the
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