Controllable Fabrication And Characterization Of Cyclomatrix Polyphosphazene Micro/Nano Materials

Cyclomatrix polyphosphazenes was synthesized under relatively mild conditions via one–pot precipitation polymerization between multi–functional cyclotriphosphazene and bi/multi monomer. Due to their highly cross–linked spatial structure and their ability to be integrated with a variety of monomers, cyclomatrix polyphosphazene showed excellent thermal stability, biocompatibility, and structural stability. Besides, different morphologies of cyclomatrix polyphosphazene could be prepared simply by adjusting the reaction conditions. Therefore, it possesses a great potential and prospects for the applications in many fields.In this study, cyclomatrix polyphosphazenes with different chemical compositions and morphologies were prepared by polymerizing with the selected monomers and adjusting the reaction conditions. To develop a deeper understanding of as–obtained materials, the mechanism for their formation was studied. The important details of this study and results are showed as follows:(1) Cyclomatrix polyphosphazene microtubes, poly [(cyclotriphosphazene–co–melamine)](PZM), have been synthesized through one–pot precipitation polymerization between hexachlorocyclotriphosphazene(HCCP) and melamine with pyridine as a solvent and an acid–acceptor in absence of any surfactant or template agent. The as–synthesized microtubes, with a closed end on one side, had a rough surface, a length of about 3–10 ?m, an outer diameter of 200–300 nm, and two inner diameters from both ends of the microtubes as 5–10 nm and 100–200 nm, respectively. An optimum condition for preparing PZM microtubes was determined by varying the temperature, monomer concentration, ratio of monomer, and feeding methods and observing their effect on the morphology change in the as–obtained product accordingly. In this particular study, the optimum the concentration of HCCP was varied between 5.0–7.5 g/L, the molar ratio between HCCP and melamine was kept at 1:3, initial temperature was varied between 0–10°C, and the reaction temperature was kept at 120°C.The progress in the formation of PZM microtubes was monitored at variable stages by using scanning electron microscopy(SEM) to elucidate the proposed formation mechanism as template-induced self–assembly. During the self–assembly process, melamine acted as a template to guide the formation of microtubes. The morphology of PZM microtubes had undergone a transformation starting from melamine fibers to the formation of oligomers on the surface of microfibers. As the production of oligomers with low molecular weight increased the melamine was slowly dissolved to the solvent. In the reaction process, the pyridine hydrochloride was generated briefly and redissolved in pyridine, playing a role of promoting the reaction rather than templates.PZM microtubes possessed of outstanding thermal stability as the initial significant weight loss occurred when temperature reached 245°C under a nitrogen atmosphere. Furthermore, the total weight loss was less than 60% when the temperature reached to 800°C. In addition, PZM microtubes could be used as an effective adsorbent, adsorbing methylene blue(MB) and promoting the photocatalytic degradation of MB in water.(2) Monodispersed PZM microspheres with smooth surface were synthesized by employing HCCP and melamine as co-monomers completely dissolved in pyridine under a reflux condition(120°C). The diameter of PZM microspheres varied from 200 to 400 nm. PZM microspheres showed a more superior thermal stability when compared with PZM microtubes and the micro/nano materials obtained by self–assembly. The thermal degradation temperature for PZM microspheres was determined to be 475°C yet PZM microspheres still maintained spherical structure at 800°C under a nitrogen atmosphere. In this study, the monodispersed PZM microspheres could be prepared in a relatively wide reaction conditions. The optimum conditions for preparing PZM microspheres were obtained as follows: initial temperature at 120°C, concentrations of HCCP between 2–3 g/L, molar ratio of –Cl:–NH2 ranging from 2:1 to 2:3. Overall, the preparation of PZM microspheres was simple, suitable for industrial-scale production and post-treatment.The formation mechanism was studied by using SEM to observe the effects of the factors, such as monomer concentration, feeding modes and reactant ratio, on the morphology of as–obtained product and the progress in the formation of PZM microspheres. The progress started with a great amount of oligomers generated and self–assembled into laminated structure in which oligomer segments and nano-size oligomers were nucleated and grew, with a steady increase in molecular weight and crosslinking to form PZM microspheres.(3) An amine–terminated cyclotriphosphazene hexakis(4–aminophenoxy) cyclotriphosphazene(HACP) was synthesized. Monodispersed cyclomatrix polyphosphazene microspheres with active amino groups on their rough surface, denoted as poly [cyclotriphosphazene–co–hexakis(4–aminophenoxy)] microspheres(PZAA–MS), have been prepared with HCCP and HACP in absence of surfactant and without any stirring and ultrasonic agitation. The experiment found that the diameter of the PZAA–MS could be adjusted from 50 nm to 2 ?m. The products at variable stages of polymerization were characterized by TEM to elucidate the formation mechanism as the oligomeric species absorption self–assembly.The study showed that the concentration of HCCP had a significant effect on the diameter and the agglomeration state of PZAA microspheres. The three dimensional PZAA nanofiber–networks were formed with nanospheres as nodes when the concentration of HCCP was 1 g/L. For the property perspective, PZAA microspheres also showed excellent thermal performance and the thermal degradation temperature was determined to be 460°C, which was attributed to their highly cross–linked structure and cyclotriphosphazene rings. The weight loss was less than 20% when the temperature reached to 800°C.