pH- and chaotropic anion-induced conformational changes of tertiary amine-containing binary heterografted star molecular bottlebrushes in aqueous solution

Abstract

This article reports on the conformational behavior of binary heterografted three-arm star molecular bottlebrushes composed of poly(ethylene oxide) (PEO) and either poly(2-(N,N-dimethylamino)ethyl methacrylate) (PDMAEMA, the brushes denoted as SMB-1) or poly(2-(N,N-diethylamino)ethyl methacrylate) (PDEAEMA, the brushes denoted as SMB-2) side chains in aqueous solutions in response to pH changes and addition of salts containing chaotropic anions (CAs). PEO was introduced into the brushes as a stabilizer when the tertiary amine-containing side chains collapsed. While a small size decrease of SMB-1 was observed with increasing pH from acidic to basic, SMB-2 exhibited a large and abrupt size transition caused by the pH-induced solubility change of PDEAEMA. Atomic force microscopy imaging revealed a star-to-globule shape transition of SMB-2 upon increasing pH across the pK_a; in contrast, SMB-1 stayed in the starlike state at both low and high pH values. Intriguingly, both SMB-1 and -2 displayed star-to-globule shape transitions in acidic solutions upon addition of salts containing sufficiently strong CAs such as ClO₄⁻, with SMB-2 showing a greater sensitivity to moderate CAs than SMB-1. Moreover, superchaotropic anions (e.g., Fe(CN)₆³⁻ and S₂O₈²⁻) were significantly more efficient in inducing shape changing than common CAs. The CA-induced shape transitions resulted from the ion pairing of CAs and protonated tertiary amine groups and the high propensity of CAs to associate with hydrophobic moieties in the brushes, which decreased the solubility of the tertiary amine-containing side chains and caused the brushes to collapse. The findings reported here may enable potential applications of molecular bottlebrushes in, e.g., encapsulation and release of ionic substances.