Self-assembly of Diblock Copolymer Films upon Cyclic Solvent Annealing: A Simulation Study

Solvent (-vapor) annealing (SA) has been widely used to promote the formation of perpendicularly oriented structures in self-assembly of block copolymer thin films. In SA, as-prepared block copolymer films are exposed to the vapor of solvent to form a swollen and mobile polymer film atop the substrate, and finally the solvent is evaporated. Tranditional SA contains only one swollen-evaporation process, while cyclic solvent annealing (CSA) contains multiple swollen-evaporation processes in a cyclic means. The existence of solvent affects the effective interaction between the blocks, the effective film thickness, the effective surface preference, and the effective volume fractions of the blocks, and hence affects the resulting morphology. The morphologies induced by the presence of solvent may be trapped during the solvent evaporation process. Hence, the resulting morphology after SA/CSA may be quite different from that without SA/CSA treatment. Furthermore, the swollen-evaporation processes in CSA can be designed to facilitate the formation of perpendicularly-orientated structures. In this study, we present our lattice Monte Carlo simulation results of the self-assembly for cylinder-forming diblock copolymer films under CSA. Our simulations are based on the "single-site bond fluctuation" model, and it is assumed that the maximum swelling ratio of the film in each swollen-evaporation cycle decreases linearly with the increase of the cycle number. This assumption ensures that relatively large rearrangement of microdomains occurs when the number of cycles is relatively small, and the resulting characteristic structure can be kept effectively in the following cycles. The results show that the surface preference window (beta-window) of perpendicular cylinders is affected by the solvent evaporation rate, solvent selectivity and commensurability between the film thickness and the bulk period. When the solvent evaporation rate is intermediate, the beta-window enlarges with increasing it. A stronger solvent selectivity for the majority block as well as dismatch between the film thickness at a swelling ratio of 1.5 and the period of the bulk cylindrical phase enlarges the beta-window. By analyzing the mean-square displacements, it is found that the diffusion of the polymer segments in the perpendicular direction is always faster than that in the parallel direction, which leads to that segments are preferentially gathered along the perpendicular direction, and hence promotes the formation of the perpendicular cylinders. Compared with that in the SA treatment for the same system, the beta-window of perpendicular cylinders is wider and the annealing time can be greatly reduced under CSA treatment.