| In recent years,the preparation of high-performance organic-inorganic nanocomposites has become a research hotspot in the fields of materials science and nanotechnology,and the related researches are attracting more and more attentions.Compared with the traditional bulk materials,nanoparticles exhibit a range of unique effects including surface-interface effect,small size effect,quantum size effect and macroscopic quantum tunneling effect.However,due to the high surface energy of nanoparticles,it is very easy to cause agglomeration between particles,which makes it difficult to give full play to the above effects in nanocomposites.Therefore,as the key unit of nanocomposites,the dispersity of nanoparticles in polymer matrix has an important impact on both the mechanical and applied properties of nanocomposites.In order to overcome this difficulty that stands in the way of large-scale preparation and application of high-performance organic-inorganic nanocomposites,it is of great value to develop a method for preparing nanoparticles with low cost,high efficiency and excellent dispersity.In this thesis,a variety of metal hydroxide nanoparticles,which can be stably dispersed or even monodispersed in different liquids,and their transparent dispersions have been prepared by a direct reaction precipitation or coprecipitation method,combined with high gravity technology,surface modification process or hydrothermal treatment.The as-prepared nanodispersions can effectively avoid the particle agglomeration,which is a common phenomenon in practical applications of nanopowders.Based on the above obtained transparent nanodispersions,several kinds of transparent organic-inorganic nanocomposites have been fabricated and explored at the same time on their mechanical,optical and flame-retarding properties.The main research contents and results of this thesis are as follows:(1)MgCl2·6H2O and NaOH were used as raw materials to prepare magnesium hydroxide nanoparticles and their transparent dispersions by a direct reaction precipitation method,combined with surface modification process.The effects of each factor on the preparation of nanodispersions have been investigated and the optimal preparation conditions are also determined.According to the results,anhydrous ethanol was adopted as reactant solvent.The reaction temperature and final solution pH at the end of precipitation were 30℃ and 10,respectively.In addition,silane coupling agent KH-570 was used as modifier and added into the mixed slurry after precipitation with a dosage of 10 wt%.The temperature and duration of modification process were 70℃ and 3 h,respectively.Under the optimal conditions,the as-prepared magnesium hydroxide nanoparticles have irregular lamellar morphology,with an average particle size of about 65 nm as well as a thickness of 1-2 nm.These nanoparticles can be stably dispersed into methanol,ethanol and ethylene glycol for a relatively long period(≥ 6 months)to form transparent and stable nanodispersions.Furthermore,the rotating packed bed(RPB)was adopted as a reactor to prepare magnesium hydroxide nanoparticles with smaller particle size and more uniform size distribution.The results indicate that the particle size decreased with increasing of RPB rotating speed,and an average particle size of 32 nm was achieved when the rotating speed was increased to 2500 rpm.Compared with the product obtained from the stirred tank reactor(STR),the RPB-prepared particles have much smaller particle size and narrower size distribution,and the dispersions show a better transparency.More importantly,the preparation process changed from batch to continuous,and the production efficiency was obviously improved.FT-IR result indicates the KH-570 molecules were attached to the surface of magnesium hydroxide nanoparticles and played the role of preventing particle agglomeration through the steric-hindrance effect.Therefore,the obtained nanodispersions can be stably maintained for a long time.(2)AlCl3·6H2O and NaOH were used as raw materials to prepare pseudo-boehmite nanoparticles and their transparent dispersions by a direct reaction precipitation method without surface modification.The pseudo-boehmite nanoparticles can be stably dispersed in both aqueous and alcohol phases.The effects of each factor on the preparation of nanodispersions have been investigated and the optimal preparation conditions are also determined.According to the results,water was adopted as reactant solvent,and the reaction was conducted at 30 0C with reactant molar ratio n(OH-)/n(Al3+)=3.After that,the slurry was aged at 70℃ for 3 h to get the final product.The as-prepared pseudo-boehmite nanoparticles belong to a kind of amorphous aluminium hydroxide,and they present irregular rectangular flake or curly rod morphology with a long diameter size of 60-80 nm and a short diameter size of 15-20 nm,respectively.The corresponding nanodispersion shows obvious thixotropy.A stable network can be formed among particles by hydrogen bond force,which can prevent the agglomeration of particles and keep the nanodispersion stable for a long time.Furthermore,after a hydrothermal treatment at 220℃ for 12 h,the pseudo-boehmite nanoparticles can be transformed into boehmite nanoflakes,which have higher crystallinity and more regular morphology.The average particle size and thickness of these nanoflakes are about 58 nm and 12.5 nm,respectively,and they can be monodispersed into water.After hydrothermal treatment,the aqueous nanodispersion still maintained good optical transparency and high stability.(3)Based on the as-prepared transparent dispersions of single metal hydroxide nanoparticles,transparent dispersions of layered double hydroxides(LDHs)nanoparticles was further obtained.Similarly,the nanoparticles and transparent dispersions of MgAl-LDHs were achieved by a coprecipitation method without surface modification and with the usage of Mg(NO3)2·6H2O,Al(NO3)3·9H2O and ammonia hydroxide as reactants.The effects of each factor on the preparation of nanodispersions have been investigated and the optimal preparation conditions are also determined.According to the results,water was adopted as reactant solvent,and the reaction was conducted at 30℃ with the molar ratio n(Mg)/n(Al)=3.In addition,the pH value of solution at the end of reaction was controlled to 9.Under the optimal conditions,the MgAl-LDHs nanoparticles were prepared with irregular lamellar morphology and an average particle size of about 57 nm.The XRD and FT-IR results indicate the intercalated anion was NO3-.After that,RPB was used to strengthen the micromixing and mass transfer of this reaction.As a result,the products with smaller particle size,narrower size distribution and higher dispersion transparency were achieved.The average particle size of MgAl-LDHs nanoparticles was measured as 31 nm,when the rotating speed of RPB was set to 2400 rpm,and the reactant feeding rate and flow ratio were set as 350 mL/min and 1:1,respectively.Compared with the product obtained from STR,the average particle size of MgAl-LDHs nanoparticles prepared by RPB was significantly decreased,and the dispersion transparency was further improved.Afterwards,the RPB-prepared nanodispersion was treated with a hydrothermal process at 130℃ for 12 h.This resulted in the change of morphology of MgAl-LDHs nanoparticles from irregular nanoplates to hexagonal nanoflakes,and the average particle size was increased to 71 nm.However,there was no significant decrease in the optical transparency of dispersion,and the hydrothermally-treated MgAl-LDHs nanoparticles can be monodispersed in aqueous solution.According to the particle surface potential test result,the high zeta potential(55.4 mV)was supposed to be the key of long-term existence for the as-prepared nanodispersion.(4)On the basis of the as-prepared transparent dispersions of metal hydroxide nanoparticles,several kinds of transparent metal hydroxide/polymer nanocomposites were fabricated through solution mixing or mechanical spin-coating method.In addition,the mechanical,optical and flame-retarding properties of these nanocomposites were also studied.According to the results,nanocomposite films were prepared with excellent optical transparency and material flexibility,when the filling content of metal hydroxide nanoparticles was as low as 10 wt%.Specifically,the magnesium hydroxide/polymer nanocomposite films still displayed excellent optical transparency even with a high solid content of 30 wt%,because of the better compatibility of surface-modified magnesium hydroxide nanoparticles with polymer,compared with aluminium hydroxide and MgAl-LDHs nanoparticles.Furthermore,the as-prepared nanocomposites exhibited excellent flame retardancy(LOI>26)when the solid content was further increased to 40-60 wt%. |
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