The Structural Evolvementof Spherical Triblock Copolymer In Confinement

Triblock copolymer can form various ordered microstructures different from those of their bulk when confined in certain geometric conditions and acted by substrates. This is of great importance in new technology area such as the preparation of nano-materials. In this paper, the phase behaviors of spherical triblock copolymer A₈B₄A₈ were simulated by using of MesoDyn code based on dynamic mean-field density functional theory.With selection of optimal parameters for forming spherical microstructure in their bulk: interactional parameter of block A and block B isεAB =8kJ/mol, coefficient of compressibility is D=8, and equidensity value is 0.2, a bulk period of about 6nm were obtained for the spherical triblock copolymer A₈B₄A₈ through a series of simulations and analysis.For one dimension confinement (film), many types of ordered microstructures have been obtained through a mass of simulations with modulating the thickness of film and action parameter of substrate. The typical results are: (1) When the film thickness matches approximatively with the bulk period, for 6nm≤H≤7nm, single-layer microstructures are formed; and for H=7nm, some perpendicular cylinder phases are obtained; for 11nm≤H≤12nm, almost the same double-layer microstructures are formed, in which the phase structures are wetting surface layer with interior spheres, hexagonal close-packed spheres, cylinders paralleled to substrate, perforated lamellae and lamellae. (2) When the film thickness mismatches with the bulk period, for H=8nm and effective action parameterεM is about 4kJ/mol,some perpendicular cylinder phases are obtained,and hexagonal close-packed spheres phase are not obtained; for H=10nm, some perpendicular cylinders and hexagonal close-packed spheres are obtained; for H=15nm andεM=7kJ/mol, upper-layer spheres with under-layer cylinders perpendicular to the substrate can be obtained, and for 9kJ/mo≤εM≤13kJ/mol, interior spheres with surface cylinders paralleled to substrate are obtained. The dynamic processes of forming steady microstructure for effective action parameterεM=5kJ/mol were studied with various film thickness H. Two phase diagrams of triblock copolymer A₈B₄A₈ film are plotted according to the simulation results.For two-dimension confinement (column), the simulations were completed with cylinder of diameter d=15nm and d=30nm, respectively. It was found that the substrate actions and the diameter of cylinder have important influence on the surface microstructure of triblock copolymer A₈B₄A₈.