Phase behavior of block copolymer solutions in thin films studied by Monte Carlo simulations

Abstract

The phase behavior of an A–B diblock copolymer in a selective solvent of type A in thin films is examined by Monte Carlo simulations, using the coarse-grained lattice model with the Cooperative Motion Algorithm (CMA). This behavior is compared with that of the bulk (3-dimensional) and the 2-dimensional solutions. While we focus the simulations on symmetric 16–16 copolymers with a chain length of N = 32, we also simulate asymmetric 8–24 copolymers and chains with N = 48 and 64. At a relatively low copolymer volume fraction, approximately ϕ ≈ 0.3, the self-assembled micelles lose their long range order, and a solution of disordered micelles is obtained. We simulate the phase behavior of copolymer solutions both for thin films and 2-dimensional systems. Relevant properties, such as squared end-to-end distance, energy, specific heat and structure factor are measured as a function of the reduced temperature, T^*. We also characterize the observed micelles by counting the average number of copolymer chains that constitute a single micelle. We observe three phases: layers, hexagonally packed disks and disordered micelles. The phase diagrams from ϕ = 1.0 to 0.1 in the (ϕ, T^*) parameter space are shown for both the symmetric and asymmetric copolymer. We find that the phase behavior of the thin film solution is intermediate between that of the 3D and 2D solutions, but it is more similar to 2D because the film thickness is relatively small.