Shape-changing linear molecular bottlebrushes with dually pH- and thermo-responsive diblock copolymer side chains

Abstract

This article reports on the synthesis and responsive behavior of shape-changing linear molecular bottlebrushes containing dually pH- and thermo-responsive diblock copolymer side chains. Using a “click” grafting to method, we synthesized linear homografted molecular brushes composed of diblock copolymer side chains with pH-responsive poly(2-(N,N-diethylamino)ethyl methacrylate) (PDEAEMA) as inner block and thermoresponsive poly(methoxytri(ethylene glycol) acrylate) (PTEGMA) as outer block. Dynamic light scattering studies showed that the brushes exhibited a sharp decrease in hydrodynamic size with increasing pH across the pK_a of PDEAEMA and the size leveled off at higher pH. This is in contrast to the molecular brushes with only PDEAEMA homopolymer side chains, which underwent aggregation and precipitation in basic solution even at a concentration of 0.2 mg g⁻¹. Atomic force microscopy confirmed the cylindrical morphology of the brushes at acidic pH and pH-induced cylinder-to-globule transitions. In addition, the use of PTEGMA as outer block provided another means to control the solution state of the collapsed brushes; above the lower critical transition temperature of PTEGMA, the brushes clustered at higher concentrations but remained unaggregated at lower concentrations. The switching of molecular shape, stabilization of collapsed nano-objects, and control of solution state reported here may enable new potential applications of stimuli-responsive shape-changing molecular brushes.