Preparation And Characterization Of Polyurea Nanofibers And Microspheres

With continuous development, new materials with better performance are urgentlyneeded, which bring about new technologies in polymer manufacturing, particularly forpolymer materials with specific structure, morphology and functionality. Polyureas, asspecific functional polymer materials, have been known for its outstanding high temperatureand wearing resistance and are widely used in different applications. Rapid development andgreat interests have been seen worldwide. In an earlier work, precipitation polymerization ofisophorone diisocyanate (IPDI) as the only monomer was carried out in our Lab usingwater-acetone as the solvent without any other additives. Highly uniform microspheres withneat surface bearing abundant amine groups were prepared. As a continuation of the work,polyurea materials with special structure and morphology different from the microspheres,such as polyurea nanofibers, hollow particles and porous microspheres are prepared in thiswork. First of all, polyurea nanofibers were prepared by one-step precipitation polymerizationof toluene diisocyanate (TDI) and4,4’-diaminodiphenyl ether (ODA), using acetone assolvent. Results demonstrated that the monomer concentration and polymerizationtemperature played important roles in the process. The morphology of the outcome polymerwas readily adjustable by varying monomer concentration, polymerization temperature andmechanical agitation. Fibrous polymer was obtained when monomer concentration was keptunder2.0wt%; both polymer nanofibers and spherical nanoparticles appeared whenmonomers concentration was increased to about3.0wt%; whereas only nanoparticles wereobserved when monomers concentration was further increased to5.0wt%or higher. Sphericalnanoparticles appeared when polymerization temperature was0℃, with temperature between30and70r/min, nanofibers were well formed. Polymerization was also conducted with thebottle reactor installed into a thermostatic water bath with oscillation. Oscillation frequencyvaried from0to150osc/min did not show significant effect on the morphology of polyureananofibers; while mechanical agitation imposed significant effect on the morphology of thepolymer. With agitation rates between100and300r/min, nanofibers were well formed.Microspheres appeared with increased agitation. And when it went up to above600r/min,only nanoparticles were observed. Moreover, the glass transition temperature (Tg) of polyureananofibers was detected to be around230~250℃. These fibers, not soluble in acetic acid,toluene, tetrahydrofuran, strong alkaline, acetonitrile, m-cresol and other solvents tested, alsoshowed good solvent resistance.In a second part of the work, hollow polyurea microspheres were prepared using CaCO3 particles as the templates. Polyurea was formed on the surface of CaCO3, leading to formationof CaCO3/polyurea core/shell composite microspheres. Hollow polyurea microspheres werethen obtained by dissolution of the CaCO3core by immersing in an acide solution. To this end,CaCO3particles with size varied from nano-to micron-meter and different shape and crystalstructures were prepared using different crystal growth inhibitors, concentrations of CaCl2andNaCO3, and changing the way of dispersing during the precipitation reaction. CaCO3particleswere characterized using electronic microscopy, dynamic light scattering and X-raydiffraction. The results indicate that, without any inhibitor, the product was of cubic blocks ofabout4~5μm and the cubic crystals were more uniform with ultrasonic dispersion than withmechanical stirring. Crystal morphology and structures were easily altered by changing thecrystal growth inhibitors and their amount used. With sodium triphosphate (STP) as theinhibitor, amorphous spherulites were usually formed except with very low amount wherevaterite crystals were detected. With carboxymethylcellulose sodium (CMC) as the inhibitor,relatively uniform ellipsoidal particles, mostly calcite with small portion of vateritespherulites of size about3μm, were observed, and the size of the spherulites changed withCaCl2concentration. While with sodium polystyrene sulfonateas as the inhibitor, uniformcrystal spherulites were easily obtained by adjusting the concentrations of the inhibitor andthe reactants. Opposite to the CMC, the crystalline structure was mostly vaterite with calcitein small portion.CaCO3/polyurea core/shell composite particles were prepared by precipitationpolymerization of IPDI as the only monomer in water-acetone as the solvent using CaCO3particles with different shape and size as templates. Polyurea hollow particles were obtainedby immersing the composite microspheres in hydrochloric acid. The results showed that thesize of polyurea hollow particles was dependent on the size of the template. The volume ofthe hollow cavity was in accordance with the size of the templates. The wall thickness ofhollow microspheres can be controlled by adjusting monomer amount used and water-acetoneratio of the mixed solvent. The hollow particles shell was much thinner when more water usedin the mixed solvent.Finally,"raspberry-like" polyurea/SiO2composite microspheres were prepared simply byadding monodisperse SiO2nanospheres into the monomer-solvent in the precipitationpolymerization system of IPDI in water-acetone mixed solvent. SiO2nanospheres enwrappedin polyurea microspheres were etched by NaOH solution. Monodisperse "golf ball-like"porous polyurea microspheres were finally obtained. In addition, control of porous polyureamicrospheres size was successfully achieved by changing polymerization monomer concentration, so were the pore density and the pore distribution of the porous polyureamicrospheres by varying the mass ratio of monomer/SiO2nanospheres.