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

Abstract

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.

Breaking translational symmetry via polymer chain overcrowding in molecular bottlebrush crystallization

“…The PEO mBBs used in this study were synthesized by a grafting-to method, where alkyne end-functionalized PEO was grafted onto an azidebearing backbone polymer using the highly efficient copper(I)catalyzed azide-alkyne cycloaddition click reaction (Supplementary Fig. 1) 21,[28][29][30][31] . Detailed synthesis and characterization of mBBs are summarized in the supporting information (SI, Supplementary Figs.…”

Stimuli-Induced Star-Globule Shape Transitions of Dually Responsive Binary Heterografted Three-Arm Star Molecular Brushes in Aqueous Solution

“…When stimuli-responsive polymers are employed as side chains, the resultant MBBs can undergo dramatic changes in size and shape in response to external stimuli. − ,− Schmidt et al provided the first example of thermoresponsive MBBs, prepared by growing poly­( N -isopropylacrylamide) (PNIPAm) side chains from a backbone polymer using atom transfer radical polymerization (ATRP) . They observed that the PNIPAm brushes underwent a worm-to-globule shape transition upon heating to above the lower critical solution temperature (LCST) of PNIPAm.…”

“…We recently developed two methods to stabilize collapsed globular states of MBBs. ,,, This was achieved by introducing poly­(ethylene oxide) (PEO), a permanently water-soluble polymer, as the second type of side chains in thermoresponsive linear binary heterografted MBBs or another (stimuli-responsive) polymer as the outer block of diblock copolymer side chains in linear homografted brushes. The introduced second polymer served as a stabilizer when the brush molecules collapsed into the globular state; for the brush polymers with dually responsive diblock copolymer side chains, the stimuli-responsive outer block provided another level of control over the solution state of the brushes .…”

“…These multicomponent MBBs were prepared by grafting-to using the CuAAC click reaction. By increasing the PEO chain length relative to that of thermoresponsive side chains, we further showed that worm-to-globule shape transitions could be achieved in moderately concentrated aqueous solutions due to the shielding effect provided by the long PEO side chains …”

“…By increasing the PEO chain length relative to that of thermoresponsive side chains, we further showed that worm-to-globule shape transitions could be achieved in moderately concentrated aqueous solutions due to the shielding effect provided by the long PEO side chains. 50 Compared with linear MBBs, star-shaped brushes are much less investigated, partly because of the synthetic challenges. 51−54 Star MBBs can be envisioned to possess their own unique structural features and behavior, such as more compact molecular structures, lower solution viscosity, and even higher entanglement molecular weight, compared with linear MBBs.…”