Assembly And Aggregated Structure Of Block Molecule/polyoxometalate Supramolecular Composites

Polyoxometalates (POMs) are a class of single molecular clusters with nanoscale sizes. POMs exhibit abundant chemical composition, various topologies and extensive physical and chemical properties, providing the potential for the applications in electro-optics, magnetics, medicine, and catalysis. POMs are good building blocks for the fabrication of supramolecular ordered structures and functional hybrid materials. However, POMs often exist as inorganic crystals and are difficult to process, which is unfavorable for integrating POMs into functional films and devices. Meanwhile, the properties of single molecular POM cluster also require the effective organization of POMs.The controllable combining of various accessory building blocks and POMs through supramolecular self-assembly is an effective methodology. This methodology not only can improve the workability of POMs, but also can optimize the properties of POMs through the synergy of POMs and other accessory building blocks. The supramolecular self-assembly of POMs with or without other building blocks has been developed well, and the controllable combining of various accessory building blocks and POMs has been realized through various supramolecular fabrication methods. As a consequence, a lot of ordered structures including POMs have been obtained, and some properties of POMs also have been optimized. However, preparing new supramolecular assemblies including POMs is still a challenge in this field.Amphiphilic or diblock molecules, i.e., surfactants and block copolymers, possess special advantages for directing the supramolecular self-assembly of POMs, exhibiting considerable flexibility. This viewpoint is obviously shown by the proposal and development of surfactant-encapsulated polyoxometalates (SEPs). Meanwhile, as the macromolecular counterpart to conventional amphiphiles, block polymers also have begun to exhibit their advantanges on the supramolecular self-assembly of POMs. In this dissertation, using the concept of supramolecular chemistry as a guide, we employed the amphiphiles or diblock molecules as the building blocks, focused on the supramolecular self-assembly of the diblock molecule–polyoxometalate composites, and prepared new supramolecular assembled structures.Firstly, we designed and synthesized a diblock molecule C18NEO12·Ts. The amphiphile is a substitute of octadecyltrimethylammonium p-toluenesulfonate, in which one of the methyl groups has been replaced by a poly(ethylene oxide) (PEO) chain. The diblock molecule–polyoxometalate hybrid C18NEO12/HSiW was prepared through adding ethanol into the mixed aqueous solution of C18NEO12·Ts and silicotungstic acid (HSiW). The chemical composition of the hybrid was characterized through 1H NMR, Fourier transform infrared spectroscopy (FT-IR), mass spectroscopy, elemental analysis and thermogravimetric analysis. The results indicate that the cationic C18NEO12 interacts with HSiW electrostatically through the ion replacement, and the inorganic polyanions and organic cations maintain their chemical structures. An approximate formula for the chemical composition of the hybrid complex is proposed to be (C18NEO12)1.6H2.4(SiW12O40). The structure of the hybrid was investigated by transmission electronic microscopy (TEM), scanning electronic microscopy (SEM), X-ray powder diffraction and contact angle measurement. The hybrid is a rod-like grain with the hexagonal mesostructure, which consists of one-dimensional column micelles in which alkyl chains of C18NEO12 locate at the center and PEO part at the outside, with HSiWs anchoring at the interface, but more close to the PEO part. Moreover, we investigated the evolution of aggregated structures in C18NEO12·Ts/HSiW mixture solution versus addition of ethanol by TEM. The observation shows that ethanol plays an important role in the self-assembly. More importantly, the hexagonal mesostructure can transform into nanofibers comprising nanofibrils in water through the disassembly. The nanofibers were characterized through TEM, atomic force microscopy, and FT-IR. According to the results of the control experiments, quaternary ammonium group plays a key role to stabilize the nanofibers in the disassembly. In this part, through the disassembly of the mesostructures, one dimensional amphiphile/POM hybrid aggregates in water are obtained for the first time.Secondly, we designed and synthesized a diblock molecule C18NEO3·Ts. The amphiphile is a substitute of octadecyltrimethylammonium p-toluenesulfonate, in which one of the methyl groups has been replaced by a tri(ethylene oxide) chain. We also prepared the diblock molecule C18NEO3·PF6 through exchanging the p-toluenesulfonate in C18NEO3·Ts for hexafluorophosphate. By metathesis reaction and recrystallation, four Keggin-type POMs including phosphotungstic acid, silicotungstic acid, pentapotassium dodecatungstoborate(III), and phosphomolybdic acid were complexed with C18NEO3·Ts to give SEP-P, SEP-Si, SEP-B, and SEP-PMo, respectively. All the SEPs were characterized through FT-IR, 1H NMR, thermogravimetric analysis, and elemental analysis. The results indicate that the inorganic polyanions and organic cations maintain their chemical structures, and the inorganic cations in POMs have been totally exchanged by the organic cation C18NEO3 in SEPs. The thermal properties of C18NEO3·Ts, C18NEO3·PF6, and SEPs were investigated by differential scanning calorimetry, polarized optical microscopy, and variable-temperature X-ray diffraction. C18NEO3·Ts shows a SmA phase with low transition temperatures, and C18NEO3·PF6 is a near-room temperature ionic liquid. SEP-P and SEP-B show a typical SmB phase, whereas SEP-PMo decomposes before melting, and SEP-Si decomposes obviously when reaching the isotopic liquid state. The microphase segregation of incompatible parts and the aggregation of compatible parts, the long alkyl chains, and the electrostatic interactions with enough strength are key conditions to endow C18NEO3·Ts, SEP-P, and SEP-B with typical liquid crystalline behavior. To adjust the packing efficiency of the ions through introducing the tri(ethylene oxide) chains into SEPs is an efficient way to influence the thermal properties of SEPs, especially the transition temperatures to isotropic liquid state, although SEP-Si and SEP-PMo do not show typical liquid crystalline behavior. The present results indicate that combination of proper cationic amphiphiles and POMs can afford typical thermotropic liquid crystalline behavior to the obtained SEPs, and mesogenic groups or hydrogen banding precursors are not necessary to access the thermotropic liquid crystalline materials based on SEPs.At last, through dissolving polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) into its non-selective solvent (DMF) and then adding the solution of phosphotungstic acid (HPW) or HSiW, we prepared two block copolymer–polyoxometalate composites (HPW/BC and HSiW/BC), respectively. The composite micelles were characterized through 1H NMR, FT-IR, TEM, energy-dispersive X-ray mapping, and SEM. The results indicate that POMs keep their structures well in the composites. The heteropoly acids have protonated the pyridine groups in the P4VP blocks and cross-linked P4VP blocks through electrostatic interactions. The morphologies of the composite micelles can be adjusted by changing the charge of POMs. In detail, HPW/BC can form the spherical micelles, whereas HSiW/BC can form the wormlike micelles. Upon increasing the content of POMs in the composites, the core diameters of the micelles increase, that is, the size of the micelles can also be adjusted. The charge or the content of POMs affects the stretching degree of the core blocks and the micellar morphologies. HPW/BC micelles include a more swollen core than that of HSiW/BC micelles due to the less coupling points provided by HPW relatively to HSiW. In addition, the stable micelles can form only when the concentration of the block copolymers and the content of POMs are high enough. The obtained micelles are highly dynamic and can perform self-dissociation when the composite solution was diluted or placed for a long time, or added with pyridine. In addition, HSiW/BC micelles form wormlike network structures with with T- or Y-junctions, loops, and end-caps. In this part, we have firstly realized the micellization of polystyrene-block-poly(4-vinylpyridine) in DMF through noncovalent cross-linking of the core blocks bridged by POMs.In conclusion, we have researched the supramolecular self-assembly of the diblock molecule–polyoxometalate composites in this dissertation. We have realized the application of mesostructure disassembly in amphiphile/inorganic cluster systems, fabricated the hexagonal mesostructure as well as nanofibers in water, obtained SEPs which exhibit typical thermotropic liquid crystals using no mesogenic groups, and induced the micellization of polystyrene-block-poly(4-vinylpyridine) in DMF using POMs and adjusted the morphologies of the composite micelles. We believe that the obtained aggregated structures may provide a new way to favor the applications of POMs.