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.
A novel ferroceneylazobenzene self-assembled monolayer (SAM) has been constructed on an indium-tin oxide (ITO) electrode via the covalent attachment of 4-(4'-11-ferrocenyl-undecanoxyphenylazo)benzoic acid ( FcAzCOOH) onto a silanized ITO substrate surface and verified by reflectance infrared spectroscopy and water contact angle. Atomic force microscopy (AFM) and cyclic voltammogram (CV) indicated that the FcAzCOOH formed a uniform and reproducible SAM on the ITO electrode with a surface coverage of ca. 1.9 x 10 (-10) mol/cm (2) (87 A (2)/molecule). The reversible photoisomerization behavior of the SAM was characterized by UV-vis spectra. The azo pi-pi* transition band intensity of the SAM gradually decreased with UV (365 nm) irradiation and was almost recovered again when subsequent exposure to ambient room light (400-800 nm). The increased tilt angle of the molecules on the ITO substrate after UV irradiation further confirmed the trans-to- cis isomerization of azobenzene moieties. The CV of the trans- FcAzCOOH modified ITO electrode showed a pair of waves due to redox of the ferrocene groups in the potential range of 0 to +800 mV (vs SCE), and the peak separation of the redox wave became larger after UV irradiation and almost returned to its original value after subsequent exposure to the visible light. Rate-dependent CV curves indicated that the charge transfer rate between the ferrocene species in the SAM and the ITO electrode was slowed down after UV irradiation due to the smaller porosity of the monolayer film and the more compact barrier layer between the redox species and the ITO electrode. It is the first time to directly observe the influence of photoisomerization of the azobenzene moiety on the redox behavior of redox species in the ferroceneylazobenzene-functionalized SAM. The present results provide profound insight into the role of redox microenvironment on electron transfer kinetics and also provide a simple and facile approach to the preparation of photocontrollable electrodes.
Ionic complexes of linear poly(ethylenimine) ( lPEI) and poly(allylamine hydrochloride) (PAH) with 3,4,5-tris(n-alkan-1-yloxy)benzoic acid [(3,4,5)nG1-COOH, n = 8, 10, 12, where n is the number of carbon atoms in the alkyl tail) or 3,4,5-tris[(p-(n-dodecan-1-yloxy)benzyloxy]benzoic acid [(4-3,4,5)12G1-COOH] dendrons [or the corresponding potassium salts (3,4,5)nG1-COOK and (4-3,4,5)12G1-COOK] were prepared. The complexes were characterized with XRD, FTIR, TG, DSC, and polarized optical microscopy (POM). The complexes of lPEI-(3,4,5)nG1 were found to be in the lamellar smectic A or C (SmA and SmC) phase, while the PAH-(3,4,5)nG1 complexes were in the hexagonal columnar (Φh) phase. All of these complexes were in the ionic thermotropic liquid crystal state at room temperature because their melting temperature, if had, was much lower. Interestingly, the complexes lPEI-(4-3,4,5)12G1 and PAH-(4–3,4,5)12G1 were also in the same SmA or SmC phase and Φh phase, respectively, regardless of whether there was an additional mesogen unit benzenyloxy moiety (−C6H4CH2O−) in the dendron, increasing the long period and adjustability of the alkyl tails. This study demonstrates that the binding site plays an important role in determining the mesomorphous structure of the polymer−dendritic amphiphile complexes. Furthermore, the PAH complexes exhibited a higher clear point than the corresponding lPEI complexes due to the different binding sites in these two polymers. The alkyl chain length (n ≤ 12) of the dendron and the difference in dendron chemical structure had little effect on the mesomorphous structure and clear point of the polymer−dendritic amphiphile complexes. The present results provide a profound insight into the role of polymer topological structure in controlling the supermolecular structure for the polymer−dendritic amphiphile complexes.
Polymer-amphiphile complexes based on specific noncovalent interactions have received considerable attention in recent years due to its simple preparation and interesting properties. However, little attention has been paid to the complex composition effect on the crystal and mesomorphous structure of amphiphile side chains in nonequimolar polymer-amphiphile complexes. The structure of a series of nonequimolar linear poly(ethylenimine)-octadecanoic acid complexes, lPEI-OA-x, with the molar ratio x of OA to the lPEI amino group ranging from 0.66 to 1.45, was investigated with WAXD, SAXS, FTIR, DSC, fluorescence spectrum, and polarized optical microscope. Two crystalline modifications β O (βorthorhombic) and βT (β-triclinic) of OA side chains have been found to coexist in the complexes varying with x, and only 8-10 CH2 groups in an OA molecule participate in the crystallization. The crystalline OA tails with amorphous lPEI form the lamella stacking structure. The complexes of x > 1.0 are predominant with βO form and stacked into an end-to-end bilayer lamella with the long period of ∼5.6 nm. While for the complexes of x < 1.0, the βT form is dominantly stacked in the interdigitating monolayer structure with the long period of ∼2.8 nm. The fluorescence emission from pyrene-doped complexes indicates a decrease in the microenvironment polarity with increasing x. The thermotropic liquid crystal state has been observed from the complexes of x < 1.0 at temperatures above the melting point of the OA tail crystal. This study demonstrates that the crystalline and mesomorphous structures of polyelectrolyteamphiphile complexes can be effectively tuned by changing the relative amount of bound amphiphiles. In other words, the stacking structure of amphiphile molecules depends on the amount of added binding polyelectrolytes.
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