The name is absent



segment density at the interface, due to the A-B repulsion. Such depletion is also
observed in the Monte Carlo simulations of the diblocks [151, 140]. As can be observed
from the figure, fixing the width (L) of the computational domain fixes the lamellar
period, which may not be the bulk equilibrium period of the copolymer. Hence
calculations are done for different widths of the computational domain. Figure 5.1b
shows the density profile of one of the blocks at different widths. The (grand) free
energy per volume for each of these cases is calculated and plotted against the width
of the computational domain as shown in figure 5.2. The width for which free energy
is at minimum, is the bulk equilibrium lamellar period (D⅛) and the free energy at
D⅛ is the equilibrium free energy (Clb) of the copolymer. From the figure Db = 10.2σ
and Clb/VkT = —0.2244. This equilibrium free energy is the balance of the free
energies due to the formation of the interfaces between the lamella of the two blocks
and stretching of the copolymer chains to form the lamellar structure. At smaller
lamellar period, higher number of interfaces increases the free energy. And at higher
lamellar period, the chains are highly stretched leading to increase in the free energy.
The density profile for Db = 10.2σ is shown fig. 5.lb. Lamella that are highly stretched
past their equilibrium period show depletion in the density at their centers, as shown
in fig. 5.1b for L = 15σ.

Figure 5.3a shows the equilibrium lamellar density profiles for diblock copolymers
with different incompatibilities between the two blocks (N = 8). The bulk equilib-
rium period, Db = 8.8σ, 9.2σ, 9.6σ, and 9.9σ for e∕kT = 0.17, 0.2, 0.231, and 0.25,

125



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