Morphology characterization of brush-linear diblock copolymers with variable brush density and chain length

Abstract

Brush-linear block copolymers are capable of self-assembly to from microphase separated domain morphologies, and their phase behavior shows characteristics of both coil-coil and rod-coil block copolymers. Previous studies have investigated the domain morphologies of brush-linear diblock copolymers at fixed brush density but with varying compositions and chain length. However, the effect of brush density on block copolymer phase behavior is much less explored. As brush density is directly related to molecular asymmetry, it is expected to influence the block copolymer domain morphology. In this study, we investigate the self-assembly of brush-linear block copolymers where both the brush density and chain length are systematically varied. A poly(pentafluorophenyl acrylate-b-styrene) (PPFPA-b-PS) linear diblock copolymer is first prepared as the precursor polymer. By utilizing the PFP ester groups on the PFPA repeating unit, amine-functionalized poly(ethylene glycol) (PEG) are then covalently grafted onto the PPFPA block to yield brush-linear diblock copolymers with the structure (PPFPA-g-PEG)-b-PS. The domain morphologies of the diblocks as a function of PEG density and chain length are examined using small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). At a high PEG side-chain density, the PPFPA-g-PEG block acts as a brush block, and the block copolymer morphology is determined by the composition of the brush block. More interestingly, at a low PEG side-chain density, the polymer microstructure is set by the segregation of both PS and PEG away from PPFPA, leading to lamellae and cylinder domains with unconventional chain packing. Our study illustrates the rich domain morphology that can be achieved by controlling the brush density and chain length in brush-linear block copolymers.