Phase transition of block copolymer/homopolymer binary blends under 2D confinement

Abstract

Constraints imposed by nanometer scale confinement lead to the changes in bulk equilibrium behavior of block copolymers (BCPs). Cylindrical pores with diameter corresponding to the equivalent length of several copolymer chains have been employed to investigate the influence of two-dimensional (2D) confinement on the behavior of BCPs. Herein, we reported the microdomain transition behavior of block copolymer/homopolymer (AB/A) binary blends in cylindrical confinement. Lamellae forming poly(styrene-b-butadiene) (PS-b-PBD) and PS homopolymers (hPS) were drawn into the pores of anodized aluminum oxide (AAO) and isolated by selective etching of the AAO templates. Concentric ring morphologies of neat PS-b-PBD in the cylindrical confinement varied with on the addition of hPS. Given the volume fraction of homopolymers, the phase behavior of polymer blends was dependent on the molecular weight of homopolymers. When the ratio between the molecular weight of hPS and PS block was much lower than unity (α«1) (i.e., wet brush regime), the concentric ring structure was transformed into helical or spherical structure depending on the volume fraction of hPS. For α≈1 (i.e., dry brush regime), additional hPS chains were localized at the center of concentric ring where the entropic penalty of copolymer chains induced by confinement is maximized. Over the capability of hPS in the BCP domains in radial axis, the phase transition into microemulsions occurred in the dry brush regime. In both wet and dry brush regimes, the effect of hPS on the phase transition of BCP was significantly enhanced in the nanoscale 2D confinement compared to the bulk state due to the loss of conformational entropy of polymer chains.