Cylindrical microdomain orientation of PS-b-PMMA on the balanced interfacial interactions: Composition effect of block copolymers

The microdomain orientations in thin films of cylinder-forming PS-D-PMMAs, having PS volume fraction (Φ _PS) = 0.72,0.76, and 0.80, were investigated as a function of film thickness and the composition of the P(S-r-MMA) anchored to the substrate. From the thickness window where the cylindrical microdomains orient normal to the film surface, a larger range of thickness windows was observed for X _s = 0.62 to 0.66, with X _s = 0.64 being the optimal conditions for the balanced interfacial interactions on the substrate. Especially for the thickness window of cylinder-forming PS-fr-PMMA-80, the more extensive compositional range of X _s from 0.48 to 0.85 as well as the larger thickness window in the vicinity of the neutral surfaces (X _s ∼ 0.64) indicates that the greater the asymmetry of the PS-b-PMMA (the shorter PMMA), the greater is the tendency for the microdomain orientation normal to the neutral surfaces. This compositional asymmetry effect in BCP thin films on the microdomain orientation, significant in the thin films of highly asymmetric cylinder-forming PS-b-PMMA, might be dominantly correlated to the enhanced neutrality by the decrease of polymer density near surface favoring the perpendicular microdomain orientation on the neutral surfaces.