Thickness-Controlled Phase Transitions of AB Diblock Copolymers in Asymmetric Ultrathin Films

Abstract

An unusual hexagonal dot pattern has been observed in thin films of symmetric diblock copolymers experimentally. In order to verify the stability of the hexagonal dot pattern and understand its formation mechanism, we investigate the self-assembly of AB diblock copolymers in ultrathin films with a neutral top surface and a B-selective bottom surface using self-consistent field theory (SCFT) and dissipative particle dynamics (DPD). Our SCFT results reveal that ideally symmetric diblock copolymers with A-block volume fraction f = 0.5 can indeed form a hexagonal dot pattern in a certain range of film thickness (h), which is actually a half-period perforated lamellar (hPL A ) morphology. The hPL A morphology transfers to a patternless half-period parallel lamella (hL ∥ ) with increasing h and to a stripe pattern (L ⊥ ) with decreasing h. The phase diagram with respect to f and h further demonstrates that the stable region of hPL A shifts to small f as h increases and disappears at a critical value of h. The formation of the hPL A region is mainly caused by the competition between the A/B interfacial energy and the overall surface energy (including top and bottom). In addition, the formation of hPL A is also verified by DPD simulations. Therefore, our work confirms that the experimentally observed dot pattern in thin films of symmetric diblock copolymers is an equilibrium morphology, the formation of which requires ultrathin thickness and asymmetric surface affinities.

Boundary Frustration in Double-Gyroid Thin Films

“…In summary, in the absence of preferential wetting, the DG termination plane that accommodates the preferred wetting angle (roughly 85°) with the least distortion relative to the bulk morphology will be the most stable termination plane, which explains why the (211)-oriented plane with a double-wave pattern is so common in DG thin films. The phenomenological model developed here in the context of a frustrated boundary also provides an explanation for observations made in very different systems; for instance, the morphology of a step edge in parallel lamellae , clearly exhibits the effect of a preferred wetting angle, as does the morphology and phase behavior of ultrathin films, − and our results help explain why a twin boundary in DG forms on the (211)-oriented plane with the double-wave pattern …”

Directed Self-Assembly by Sparsely Prepatterned Substrates with Self-Responsive Polymer Brushes

“…The chemoepitaxial strategy, in which the chemical pattern is typically prepared by grafting polymer brushes onto a silicon substrate and then periodically modifying the chemistry of the brushes, ,,, is relatively simple and therefore more compatible with conventional semiconductor processes. Earlier experimental studies of chemoepitaxy were mainly based on the understanding of the self-assembly behavior of simple diblock copolymers in thin films, − which allowed the estimation of some key parameters but not all of them, and it was particularly difficult to identify the processing window of each parameter.…”

“…While the thermodynamic study serves as the basis for setting the parameters of DSA, ,,,,,− dynamic simulations can in principle better evaluate the directing effect. ,,− In the past several decades, a considerable amount of effort has been devoted to computer simulations of DSA by chemical patterns. − ,− de Pablo and co-workers used coarse-grained Monte Carlo simulations to rationalize many experimental observations of DSA. ,− ,, Müller et al applied single-chain-in-mean-field (SCMF) Monte Carlo simulations to study DSA. − , These pioneering simulations have provided an insightful understanding of DSA. Recently, Evangelio et al explicitly included the grafted polymer brushes in their SCMF model to investigate DSA with density multiplication for the formation of perfectly ordered lines .…”