Inspired by stimuli-triggered unraveling of the von Willebrand factor from a nonsticky globular to a stretched linear shape with exposure of functional groups in blood clotting, this article reports on the synthesis of thermosensitive binary heterografted linear molecular brushes that exhibit temperature-induced shape changing between extended wormlike and collapsed yet stable globular conformations in water. The molecular brushes are composed of two distinct side chain polymers: thermosensitive poly(ethoxydi(ethylene glycol) acrylate) (PDEGEA), which undergoes a lower critical solution temperature (LCST) transition at 9 °C in water, and poly(ethylene oxide) (PEO), which serves as a stabilizer for the collapsed state. A "grafting to" method was developed to construct molecular brushes by clicking alkyne end-functionalized side chain polymers onto an azide-bearing backbone polymer. While a 1.0 mg/g aqueous solution of PDEGEA homografted molecular brushes turned cloudy upon heating from 0 to 22 °C, at the same concentration the aqueous solution of PEO/PDEGEA binary molecular brushes remained clear, indicating the stabilization of the collapsed state against aggregation by PEO side chains. Atomic force microscopy study revealed a stretched, wormlike morphology of brushes at 0 °C and compact, globular nano-objects at 40 °C. The thermally induced shape changing was exploited to regulate the binding of biotin, which was incorporated into the thermosensitive side chains along with a fluorescent resonance energy transfer (FRET) donor, and Rhodamine B (FRET acceptor)-labeled avidin. FRET study showed that when the molecular brushes changed from the globular to the wormlike state, the binding of biotin and avidin occurred and increased significantly with time.

The collapse of inner pH-responsive blocks drives cylindrical-to-globular shape transition while outer thermoresponsive blocks provide additional control of solution state.

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.

This Article reports on the synthesis of a series of well-defined, tertiary-amine-containing ABA triblock copolymers, composed of a poly(ethylene oxide) (PEO) central block and thermo- and pH-sensitive outer blocks, and the study of the effect of different tertiary amines on thermally induced sol-gel transition temperatures (T(sol-gel)) of their 10 wt % aqueous solutions. The doubly responsive ABA triblock copolymers were prepared from a difunctional PEO macroinitiator by atom transfer radical polymerization of methoxydi(ethylene glycol) methacrylate and ethoxydi(ethylene glycol) methacrylate at a feed molar ratio of 30:70 with ∼5 mol % of either N,N-diethylaminoethyl methacrylate (DEAEMA), N,N-diisopropylaminoethyl methacrylate, or N,N-di(n-butyl)aminoethyl methacrylate. The chain lengths of thermosensitive outer blocks and the molar contents of tertiary amines were very similar for all copolymers. Using rheological measurements, we determined the pH dependences of T(sol-gel) of 10 wt % aqueous solutions of these copolymers in a phosphate buffer. The T(sol-gel) versus pH curves of all polymers exhibited a sigmoidal shape. The T(sol-gel) increased with decreasing pH; the changes were small on both high and low pH sides. At a specific pH, the T(sol-gel) decreased with increasing the hydrophobicity of the tertiary amine, and upon decreasing pH the onset pH value for the T(sol-gel) to begin to increase noticeably was lower for the more hydrophobic tertiary amine-containing copolymer. In addition, we studied the effect of different tertiary amines on the release behavior of FITC-dextran from 10 wt % micellar gels in an acidic medium at 37 and 27 °C. The release profiles for three studied hydrogels at 37 °C were essentially the same, suggesting that the release was dominated by the diffusion of FITC-dextran. At 27 °C, the release was significantly faster for the DEAEMA-containing copolymer, indicating that both diffusion and gel dissolution contributed to the release at this temperature.

The photocleavage of o-nitrobenzyl moieties drives shape transitions from globular to wormlike in stimuli-responsive homografted and binary heterografted molecular bottlebrushes.