Study Of Self-assembly Of Polyoxometalate And Polyhedral Oligomeric Silsesquioxane Hybrid Systems

The phenomenon of self-assembly is very common in nature.The ordered structures obtained in the process of self-assembly are the result of the spontaneous action of the elementary units by means of non-covalent forces.In order to understand the process of self-assembly and the various non-covalent forces,new types of’molecular nanoparticles’can also be used as objects of experimental investigation,in addition to common atoms and molecules as the basic units.Molecular nanoparticles often have a precise chemical structure and surface functionality,which allows them to be precisely modified.Common molecular nanoparticles include fullerene(C₆₀),polyoxometalate（POMs）,polyhedral oligomeric silsesquioxanes（POSS）and so on.These molecular nanoparticles have the advantage of rigidity,precise surface modifications and high symmetry.In the previous studies,rigid molecular nanoparticles were covalently bonded to obtain a wide range of amphiphilic hybrid molecules that can self-assemble in solution to give different assembly structures.In non-covalent bond modification,self-assembly of rigid molecular nanoparticles coated with a surfactant with charge flexibility can also lead to a wide variety of self-assembled structures.On the basis of the above studies,the solution self-assembly of bi-rigid molecular nanoparticles in the presence of non-covalent bonds can be further investigated.The molecular nanoparticles studied in this paper are Silicotungstic acid(H₄Si W₁₂O₄₀,denoted as HSiW)from Keggin-type heteropoly acids,aminopropyl isobutyl polyhedral oligomeric siloxanes(Si₈O₁₂NC₃₁H₇₁,abbreviated as BPOSS-NH₂),and the modified Propylammonium trimethyl iodide hepta-isobutyl polyhedral oligomeric siloxanes(Si₈O₁₂NC₃₄H₇₈I,abbreviated as BPOSS-N（CH₃）₃I).The specific studies are as follows:1.Synthesis and characterization of BPOSS-NH₂/HSiW and BPOSS-N（CH₃）₃I/HSiW hybridsBPOSS-NH₂is dissolved in chloroform and HSiW in methanol,then the solutions are mixed according to the molar mass ratio of BPOSS-NH₂to HSiW of 4:1,3:1,2:1,1:1.The amino of BPOSS-NH₂in solution reacts with H⁺on HSiW in a reversible reaction of NH₂+H⁺（?）NH₃⁺.BPOSS-NH₂and HSiW are protonated to give a BPOSS-NH₂/HSiW-type hybrid molecule.BPOSS-N（CH₃）₃I was mixed with HSiW in an acetone solution in the ratio of 4:1,3:1,2:1,1:1 by molar mass.The BPOSS-N（CH₃）₃I in solution is complexed with HSiW to give BPOSS-N（CH₃）₃⁺/HSiW-type hybrid molecules with HI.Using infrared,nuclear magnetic and thermogravimetric analysis for characterisation,it was found that nanoparticles in hybrids can maintain their structural integrity in the presence of non-covalent forces.BPOSS-NH₂and HSiW are combined with each other at an optimum molar ratio of 3.8 at a feed ratio of 4:1 and BPOSS-N（CH₃）₃I and HSiW are combined with each other at an optimum molar ratio of 4 at a feed ratio of 4:1.2.Solution self-assembly of BPOSS-NH₂/HSiW-type hybridsObservation of the self-assembly of BPOSS-NH₂/HSiW hybrid system solutions revealed that different self-assembly structures were obtained for these hybrids in different solvent systems.At a volume ratio of acetone to water of 1:9,two-dimensional nanosheets were obtained which were stacked in layers with a spacing of 3.02 nm.However,when the solvent system was changed to a volume ratio of tetrahydrofuran to water of 3:7,the hybrids were able to self-assemble for a long enough time（1 week）to obtain a more structurally complete vesicles structure.However,by observing the intermediate processes,it was found that the hybrid molecules first assemble into a rod-like structure or a spherical shell structure in a relatively short time（30 minutes）.As the assembly time increases,the molecules within the assembled body are rearranged,the gap in the nanospheres is gradually reduced（24 hours）and as the time continues to increase,hybrid molecules form vesicle structures with a lamellar structure.The experiments found that the molar ratio of the two components was changed,but the resulting self-assembly structure was consistent.TEM and XRD characterisation revealed that one HSiW molecule tends to react with four BPOSS-NH₂during the assembly process,and the self-assembly of hybrids in solution was carried out at a BPOSS-NH₂to HSiW molar ratio of 4.There are also differences in the solvent forces to which the hybrids are subjected in different solvent systems,which leads to different assembly structures in different solvent conditions.3.Solution self-assembly of BPOSS-N（CH₃）₃I/HSiW-type hybridsSolution self-assembly of hybrids formed by the complex decomposition of BPOSS-N（CH₃）₃I and HSiW was also studied and observed using different solvent systems.It was found that at a volume ratio of acetone to water of 1:9,stacked 2D nanosheets with a layer spacing of 3.04 nm could be obtained..When the solvent system is tetrahydrofuran to water ratio of 3:7,the hybrids self-assemble to obtain a one-dimensional ribbon structure with a layer spacing of 3.04nm,too.When the solvent system is further changed to tetrahydrofuran to water ratio of 5:95,the hybrids can self-assemble in a very short time to form nanospheres with an irregular internal structure.Continuing to change the solvent system,i.e.a volume ratio of acetone to n-hexane of4:6,results in shuttle-shaped assemblies with laminate structures inside.The experiments show that with different solvent systems,the resulting amphiphilic molecules interact with the solvent,which in turn optimises the molecular arrangement parameters and results in different assembly structures.