Worm-to-Sphere Shape Transition of Thermoresponsive Linear Molecular Bottlebrushes in Moderately Concentrated Aqueous...

Molecular bottlebrushes have been shown to exhibit intriguing worm-to-sphere shape transitions in response to external stimuli. However, such shape changing has been restricted to dilute solutions, typically <1.0 mg/g, or at interfaces. Here we report a method for achieving worm-to-sphere shape transitions of linear molecular bottlebrushes in moderately concentrated aqueous solutions by using binary heterografted molecular brushes composed of a poly(ethylene oxide) (PEO) and a thermoresponsive polymer as side chains. The PEO was designed to be significantly longer so that the thermoresponsive side chains were well shielded by PEO to avoid intermolecular association during the lower critical solution temperature transition. To facilitate the analysis by dynamic light scattering (DLS) and atomic force microscopy (AFM), a suitable amount of cross-linkable cinnamate groups was introduced into the thermoresponsive polymer, allowing fixing of the brush shape at higher temperatures by UV irradiation. To demonstrate the effect of relative chain lengths of the two side chain polymers, three brush polymers, BMB-5k, -2k, and -750, were synthesized by grafting an alkyne end-functionalized thermoresponsive, UV-cross-linkable polymer with a DP of 43 and a PEO with a DP of 114, 45, or 17, respectively, in a molar ratio of 1:1 onto an azide-bearing backbone polymer via a copper(I)-catalyzed click reaction. While BMB-2k and -750 underwent intermolecular aggregation in water at a concentration of 10 mg/g upon heating, DLS and AFM studies showed that BMB-5k collapsed intramolecularly and transformed from a wormlike to a spherical shape at concentrations of 10 and 25 mg/g. Even at a concentration of 100 mg/g, at least 95% of brush molecules underwent a worm-to-sphere transition from AFM analysis of the UV-cross-linked BMB-5k at a higher temperature. The method reported here may enable new opportunities for potential applications of shape changing molecular brushes.