pH responsive MWCNT–star terpolymer nanohybrids

Abstract

We present a unique multisegmented, heteroarm star block terpolymer that can be used as an effective dispersing agent for multiwalled carbon nanotubes in aqueous media. The (polystyrene)₂₂(poly(2-vinylpyridine)-b-poly(acrylic acid))₂₂, PS₂₂(P2VP-b-PAA)₂₂, star terpolymer exhibits a pH responsive self-assembly behavior in aqueous solution, due to the ampholytic nature of its diblock copolymer arms. Depending on the pH of the medium, two different self-assemblies were observed. At a low pH (pH 2), core–shell unimolecular micelles, with a hydrophobic PS core and P2VP, PAA segments in the shell, as well as multicore large compound micelles were formed. At pH 8.5, the star was transformed from a bis-hydrophilic into a bis-hydrophobic state, leading to a network-like intermicellar assembly. This star terpolymer was capable of dispersing MWCNTs, through non-covalent interactions, thus leading to stable MWCNT–star nanohybrids in water. More importantly, the resulted MWCNT–star nanohybrids adopted the “smart” pH responsive properties of the physisorbed stars, i.e. the nanohybrids can be positively or negatively charged at low and high pH respectively, while at an intermediate pH window, phase separation occurs due to neutralization of the oppositely charged segments.