Structure Control Of Polyazobenzene Hollow Microspheres And Responsive Behaviors

The fabrication methods of polymer microspheres, such as magnetic microspheres, fluorescent microspheres, microspheres sensitive to pH and temperature, are increasingly mature. These microspheres could be applied in biologic medication, electric information and new materials areas, etc. Light has attracted much attention since it can be localized in time and space and it can also be triggered from outside of the system, so it is of great importance in developing photo-responsive microspheres. At present, photo-responsive microspheres have been normally fabricated through amphiphilic copolymer containing photoresponsive moieties by self-assemble methods and their structures were not robust. In this thesis, photo-responsive azobenzene moieties, as a side group or a backbone unit, have been introduced to fabricate various structures of polyazobenzene microspheres via a novel distillation precipitation polymerization method. The hollow microspheres have been obtained after etching the silica templates by hydrofluoric acid, and their controlled release for fluorescent molecules has been investigated also.Firstly, we synthesized silica particles with diameter of 250 nm through Stober method. A series of PAzo@SiO2 microspheres under different polymerization conditions have been synthesized with 250 nm silica particles as a template, azobenzene containing vinyl chain as a monomer, divinylbenzene as a cross-linker, acetonitrile as a solvent. The optimized condition was chosed:200 mL acetonitrile,20 wt% DVB content relative to monomer, and 100mL acetonitrile was distilled out of the system during 4.5 h of polymerization time. The robust PAzo hollow microspheres with 250 nm core and 20 nm shell have been fabricated. They have excellent reversible photoisomerization behaviors and good thermal stability.Methyl methacrylate (MMA,5 mol%) as a co-monomer was introduced to improve the stiffness of shells. The result of nitrogen adsorption-desorption isotherm indicated the existence of pore channels in the shell. Photo-isomerization behaviors of azobenzene moieties grafted on the network would change the pore diameter within the shell. These photoresponsive pore channels have been successfully applied in photo-controlled release. The half release of Rhodamine B required 11.6 h under visible light and 7.8 h under UV-light, respectively. The diffusion coefficient under UV light was 44.2% larger than that under visible light. PAzo microspheres with different shell thickness could be synthesized by distillation of different amounts of solvent from the reaction system. Silica particles with diameters of 80,130 and 180 nm prepared by Stober method were also used as templates to synthesize PAzo hollow microspheres. PAzo particles prepared with 80 nm core conglutinated each other and didn't show any hollow structure. The other PAzo particles with uniform hollow structures have been fabricated and showed excellent thermal stability and reversible photoisomerization behaviors.PAA@SiO2 hybrid microspheres have been synthesized with silica diameters of 250, 180 and 130 nm as templates, azobenzene with double vinyl units as a cross-linker, acrylic acid (AA) as a monomer. After etching by hydrofluoric acid, PAA hollow microspheres were obtained. Transmission electron microscopy (TEM) and Scanning electron microscope (SEM) results indicated that all of PAA hollow microspheres had uniform and stable shells, and their dispersion indexes were low. Thermogravimetric analysis (TGA) indicated all of them had excellent thermal stability. PAA hollow microspheres with 250nm core had reversible photoisomerization behaviors which have been proved by UV spectra under both UV and visible irradiation. Under UV irradiation, due to the photo-isomerization behaviors, azobenzene moities transformed from trans- to cis- isomer and other units would have cooperative movement, which would be induced the shrinkage of the shell of the hollow microspheres. Dynamic light scattering (DLS) results indicated the diameters of PAA hollow microspheres were 361,481, and 640 nm when pH values were 5,7, and 9, respectively. These dual-responsive properties have been applied in controlled release. When pH value of the release solution was 5, the total release percent was 28.3%. When the pH changed to 7, some other RhB molecules have been released and the total release percent was 38.1%. When the pH increased to 9, the total release percent was up to 64.3%. For the photo-controlled release, the release of Rhodamine B could reach 34.7% during 320 min under UV-light,63.0% during 880 min under visible-light, and 75.1% during 880 min under alternate UV light and visible light, repectively. At last, the hollow microspheres have been successfully loaded with silver, which could broaden their application on catalyst, antibacterial materials, and so on.