The controlled self-assembly of self-propelled Janus micromotors may give the micromotors some potential applications in many fields. In this work, we design a kind of SiO2-Pt Janus catalytic micromotor functionalized by spiropyran (SP) moieties on the surface of the SiO2 hemisphere. The spiropyran-modified SiO2-Pt Janus micromotor exhibits autonomous self-propulsion in the presence of hydrogen peroxide fuel in N,N-dimethylformamide (DMF)/H2O (1:1 in volume) mixture. We demonstrate that the self-propelled Janus micromotors can dynamically assemble into multiple motors because of the electrostatic attractions and π-π stacking between MC molecules induced by UV light irradiation (λ = 365 nm) and also quickly disassemble into mono motors when the light is switched to green light (λ = 520 nm) for the first time. Furthermore, the assembled Janus motors can move together automatically with different motion patterns propelled by the hydrogen peroxide fuels upon UV irradiation. The work provides a new approach not only to the development of the potential application of Janus motors but also to the fundamental science of reversible self-assembly and disassembly of Janus micromotors.

Novel linear-dendritic amphiphilic block copolymers with hydrophilic poly(ethylene glycol) (PEG) block and hydrophobic Percec-type dendrons containing ferrocenyl terminals were synthesized by the esterification reaction of poly(ethylene glycol) methyl ether with ferrocenyl-terminated alkyl-substituted benzoic acid dendrons. On the basis of the results that the critical aggregation concentration (CACox) of the oxidation state polymer is much higher than CACred of the corresponding reduction state, these polymers can reversibly self-assemble into various aggregates, such as spherical, wormlike micelles, and vesicles, and also disassemble into irregular fragments in aqueous solution by redox reaction when changing the polymer concentrations. Copolymer PEG45-b-Fc3 (3) with 3,4,5-tris(11-ferrocenylundecyloxy) benzoic acid (2) can self-assemble into nanoscale wormlike micelles when the polymer concentration in aqueous solution is above its CACox. These wormlike micelles can be transformed into nanosized vesicles by Fe2(SO4)3 and regained by vitamin C. Interestingly, copolymer PEG45-b-Fc2 (5) with 3,5-bis(11-ferrocenylundecyloxy) benzoic acid (4) can reversibly self-assemble into spherical micelles with two different sizes by redox reaction above the CACox, indicating that the terminal hydrophobic tail number of dendrons plays a key role in determining the self-assembled structures. Furthermore, rhodamine 6G (R6G)-loaded polymer aggregates have been successfully used for the oxidation-controlled release of loaded molecules, and the release rate can be mediated by the concentrations of oxidant and copolymers. The results provide an effective approach to the reversible self-assembly of linear-dendritic amphiphilic block copolymers and also promise the potential of these novel redox-responsive amphiphilic block copolymers in drug delivery systems, catalyst supports, and other research fields.

Novel dendron hydrophobically modified ethoxylated urethanes (DHEUR) with almost the same molecular weights, molecular weight distributions, and identical hydrophilic portion but different terminal hydrophobic group numbers were prepared by using Percec-type alkyl substituted benzyl alcohol dendrons as new end-cappers. These DHEUR polymers in solution possess interesting associative and rheological behavior. For DHEUR-1 with 4mono(decyloxy)benzyl alcohol (2), the solutions are dominantly composed of the isolated and separated micelles and exhibit Newtonian behavior in a wide shear rate range accompanied by shear thinning at high shear rate region. DHEUR-2 with 3,5-di(decyloxy)benzyl alcohol (4) in solutions form a relatively more complete network through dominant micellar junctions and process a relatively higher solution viscosity and similar solution viscosity behavior to DHEUR-1. However, shear thinning behavior shifts to a lower shear rate region due to a relatively longer relaxation time. Interestingly, the solutions of DHEUR-3 with 3,4,5-tri(decyloxy)benzyl alcohol (6) have developed a complete physical network and show pronounced shear thinning behavior over the whole shear rate range. The oscillatory measurements further confirm that a gradually developing associative network leads to their different solution rheological behavior, i.e., viscous fluid (DHEUR-1), viscoelastic fluid (DHEUR-2), and elastic body (DHEUR-3) with increasing the hydrophobic tail number of dendrons. Furthermore, the rheological activation energy of these DHEUR polymers increases with the increase of terminal hydrophobic group numbers, indicating that DHEUR polymers with more hydrophobic tail chains need more energy potential barrier for the disengagement of hydrophobes from micelles due to stronger association strength. In general, the results demonstrate that the terminal hydrophobic tail number of dendrons plays a key role in determining the associative and rheological behavior of DHEUR in solutions. This work opens a new perspective for more efficient thickeners and also promises the potential of these DHEUR polymers in waterborne coating, cosmetics, dyestuff, medicines, and so on for the first time.

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Hydrophobically modified ethoxylated urethanes (HEURs) belong to an important class of telechelic associative polymers for improving solution rheological properties. We designed and prepared a novel azobenzene end-functionalized HEUR polymer (AzoHEUR), which was used to investigate the effects of hydrophobicity change of end hydrophobes induced by photoisomerization of azobenzene on the solution aggregation and rheological properties. The concentrated AzoHEUR solutions show a reversible rheological property change upon alternative exposure to UV and visible light. We have demonstrated that a reversible change in hydrophilic−lipophilic balance of polymer followed by photoisomerization of azobenzene induces a reversible rearrangement of micellar junctions through loop−bridge or bridge−loop transitions, which reversibly changes not only the network connectivity but also the solution relaxation behavior. Moreover, a structural model is proposed to describe the rearrangement of micellar junctions induced by photoisomerization of azobenzene. The work will not only provide new insights into the effect of hydrophobicity change of stimuli-responsive end groups on the aggregation and rheological behavior of HEUR aqueous solutions but also open a new perspective for development of some special applications of HEURs in fabrication and transmission of soft materials, medicines, cosmetics, inks for inkjet printers, and flow rate controlling systems. ■ INTRODUCTIONThe hydrophobically modified ethoxylated urethanes (HEURs) as a representative class of associative polymers have been widely used as thickeners in many fields in which careful control of the rheology of the solution is required, such as water-borne coatings, inks, medicines, and cosmetics over the past decades. 1 Like telechelic amphiphilic polymers, HEURs in aqueous solution can form flowerlike micelles composed of the flower loops (hydrophilic backbone) and the micellar cores (hydrophobic end groups) above a critical aggregation concentration (cac). When the polymer concentration (C) exceeds a critical percolation concentration (C p ), the extra hydrophobic end groups will come into the micellar cores through the bridge connection of the hydrophilic chains to form large aggregates of micelle, and a dynamical physical network of micellar junctions is formed eventually, leading to the solution viscosity rises sharply with increasing C. 2−6 Usually, HEUR aqueous solution possesses a Newtonian plateau at low shear rates, followed by shear thinning at high shear rates, with shear thickening at intermediate shear rates for some samples. The research interests include the mechanism of association, the association structure of the polymer in solution, and the response of this structure under shear stress.Up to now, a lot of theories or models have been developed to describe the aggregation and rheological behavior of HEUR aqueous solution, such as the loop−bridge model developed by Winnik 7,8 and the transient network theory developed by Tanaka and Edward et al. 3,9 Accord...