利用無規則共聚物表面改質調控嵌段共聚物薄膜之方向性

Abstract

In this study, we used random copolymers to modify the surface of substrates and the modified surfaces were used to control the microdomain orientation of block copolymer thin films. We synthesized a series of random copolymers PS-r-P4VP by nitroxide-mediated controlled radical polymerization and the random copolymers were coated on substrates to form stable cross-linked mats where PS-b-P4VP and PS-b-PMMA thin films were spun-cast. The interfacial interactions between the blocks and the mats can be tuned by adjusting the compositions of PS-r-P4VPs and the optimal conditions for inducing perpendicular microdomains were investigated. Previous studies have shown that PS-r-PMMA mats can efficiently induce the perpendicular microdomains of PS-b-PMMA thin films due to the balance of the interfacial interactions. Here, we found that on PS-r-P4VP mats, the perpendicular microdomains of PS-b-P4VP can be induced only for films with small thickness, ~ one natural period of the block copolymer. Instead, PS-r-P4VP mats can be used to effectively control the orientation of PS-b-PMMA thin films. We attribute these results to the much higher surface tension of P4VP compared to that of PMMA, which greatly prevents the exposure of P4VP to air so that parallel microdomains are preferred on the free surface of the films. We also studied the effects of molecular weight of PS-b-P4VP and PS-b-PMMA on the orientation control and found that high molecular weights are required to produce perpendicular microdomains using the methods of surface modification. This can be well explained by the theory of chain end effect and nematic effect.