Effects of temperature on chaotropic anion-induced shape transitions of star molecular bottlebrushes with heterografted poly(ethylene oxide) and poly(,-dialkylaminoethyl methacrylate) side chains in acidic water

• Published in: Soft matter 2021-07, Vol.17 (27), p.6566-6579
• Main Authors: Lewoczko, Evan M, Kelly, Michael T, Kent, Ethan W, Zhao, Bin
• Format: Article
• Language: English
• Subjects: Acidic oxides; Acidic water; Alkynes; Anions; Brushes; Cycloaddition; Ethylene; Ethylene oxide; Ferricyanide; Grafting; Heating; Iron cyanides; Polyethylene oxide; Polymers; Room temperature; Salts; Shape effects; Temperature effects; Transition temperature; Transition temperatures
• ISSN: 1744-683X; 1744-6848
• DOI: 10.1039/d1sm00728a

This article reports a study of the effects of temperature on chaotropic anion (CA)-induced star-globule shape transitions in acidic water of three-arm star bottlebrushes composed of heterografted poly(ethylene oxide) (PEO) and either poly(2-( N , N -dimethylamino)ethyl methacrylate) (PDMAEMA) or poly(2-( N , N -diethylamino)ethyl methacrylate) (PDEAEMA) (the brushes denoted as SMB-11 and -22, respectively). The brush polymers were synthesized by grafting alkyne-end-functionalized PEO and PDMAEMA or PDEAEMA onto an azide-bearing three-arm star backbone polymer using the copper( i )-catalyzed alkyne-azide cycloaddition reaction. Six anions were studied for their effects on the conformations of SMB-11 and -22 in acidic water: super CAs [Fe(CN) 6 ] 3− and [Fe(CN)6] 4− , moderate CAs PF 6 − and ClO 4 − , weak CA I − , and for comparison, kosmotropic anion SO 4 2− . At 25 °C, the addition of super and moderate CAs induced shape transitions of SMB-11 and -22 in pH 4.50 water from a starlike to a collapsed globular state stabilized by PEO side chains, which was driven by the ion pairing of protonated tertiary amine groups with CAs and the chaotropic effect. The shape changes occurred at much lower salt concentrations for super CAs than moderate CAs. Upon heating from near room temperature to 70 °C, the super CA-collapsed brushes remained in the globular state, whereas the moderate CA-collapsed brushes underwent reversible globule-to-star shape transitions. The transition temperature increased with increasing salt concentration and was found to be higher for SMB-22 at the same salt concentration, presumably caused by the chaotropic effect. In contrast, I − and SO 4 2− had small effects on the conformations of SMB-11 and -22 at 25 °C in the studied salt concentration range, and only small and gradual size variations were observed upon heating to 70 °C. The results reported here may have potential uses in the design of stimuli-responsive systems for substance encapsulation and release. While super chaotropic anion (CA)-collapsed protonated tertiary amine-containing three-arm star bottlebrushes remain globular upon heating from room temperature to 70 °C, moderate CA-collapsed bottlebrushes exhibit star-globule shape transitions.