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

An unusual hexagonal dot pattern has been observed inthin filmsof symmetric diblock copolymers experimentally. In order to verifythe stability of the hexagonal dot pattern and understand its formationmechanism, we investigate the self-assembly of AB diblock copolymersin ultrathin films with a neutral top surface and a B-selective bottomsurface using self-consistent field theory (SCFT) and dissipativeparticle dynamics (DPD). Our SCFT results reveal that ideally symmetricdiblock copolymers with A-block volume fraction f = 0.5 can indeed form a hexagonal dot pattern in a certain rangeof film thickness (h), which is actually a half-periodperforated lamellar (hPL(A)) morphology. The hPL(A) morphology transfers to a patternless half-period parallel lamella(hL(& PAR;)) with increasing h and toa stripe pattern (L-& BOTTOM;) 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 increasesand disappears at a critical value of h. The formationof the hPL(A) region is mainly caused by the competitionbetween 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 workconfirms that the experimentally observed dot pattern in thin filmsof symmetric diblock copolymers is an equilibrium morphology, theformation of which requires ultrathin thickness and asymmetric surfaceaffinities.