Preparation And Application Of Acrylate Polymer/nanometer SiO₂ Composite Particles Via In-situ (Mini)emulsion Polymerization

Nanometer silica (nano-SiO₂) particles have been widely used in the modification of polymer materials to improve the comprehensive properties of materials. However, nano-SiO₂ particles, especially being held as powder, are easy to aggregate and are difficult to be dispersed homogeneously in the polymer matrix due to the high surface energy and the great polarity difference between silica particles and polymer.In this thesis, the aqueous dispersion or monomer dispersion of nano-SiO₂, in which nano-SiO₂ particles were dispersed in the primary particles form, were used to prepare polyacrylate(ACR)/nano-SiO₂ composite particles. The nano-SiO₂ particles were firstly treated by absorbing the initiator or coupling with acrylic-like agent to introduce the reactive groups onto the surface of SiO₂ particles, and then were in-situ coated and grafted by acrylate polymer via in-situ (mini)emulsion polymerization process. The treatment of nano-SiO₂ particles, the kinetics and stability of in-situ (mini)emulsion polymerization in the presence of modified nano-SiO₂ particles, the control of the size and size distribution of composite latex and the morphology of the composite particles, and the formation mechanism of the composite particles were investigated in this thesis. The ACR/nano-SiO₂ composite particles with the greater combination and grafting degrees, and with well dispersion ability of nano-SiO₂ particles, were prepared and applied in the modification of acrylate polymer and PVC.The nano-SiO₂ particles dispersed in water were used to prepare the ACR/nano-SiO₂ composite particles via in-situ emulsion polymerization. 2,2'-Azobis(2-amidinopropne)dihydrochloride (AIBA) initiator were effectivelyadsorbed onto the surface of nano-SiC^ particles when the pH value of the dispersion medium was adjusted to be greater than the isoelectric potential point of SiC>2. It was found that the dispersion stability of silica particles was decreased as the pH value of medium decreased and as the amount of added AIBA increased. An upper critical AIBA absorption amount existed for the stable dispersion of nano-SiC>2 at a certain pH value of the dispersion medium and it increased as pH increased. The absorption efficiency of AIBA was more than 80% and the activity of adsorbed AIBA was greater than that of unabsorbed AIBA determined by DSC.In the in-situ emulsion polymerization process, it was found that the rate of polymerization increased as the amount of adsorbed AIBA increased and decreased as the content of nano-SiC?2 increased. The stability of composite latex particles also decreased as the content of nano-SiC>2 particles increased. Adding sodium lauryl sulfate (SDS) at the final stage of polymerization could remarkably increase the polymerization and latex stability. Comparing with the ACR/nano-SiO2 composite latex particles prepared by emulsion polymerization without the pre-absorption of AIBA onto nano-SiO2 particles and by the direct mixing of ACR latex with aqueous dispersion of nano-SiC^, the ACR/nano-SiO2 composite latex particles prepared by in-situ emulsion polymerization exhibited a higher combination degree between ACR and nano-SiO2, and a greater grafting degree of ACR onto nano-SiO2 particles.The morphology of the ACR/nano-SiO2 composite latex particles was observed by using TEM, and the combination condition of ACR latex particles with SiC>2 particles was characterized by the centrifugation/redispersion and HF etching experiments. It was found that the ACR/SiO2 composite particles exhibited a "raspberry-like" morphology with a part of SiC>2 particles inserted at the surface of ACR latex particles. When the mass fraction of SiO^ in the composite particles ranged from 14.6 to 22.6%.more than 60% SiO2 particles with about 13.4% of ACR grafting percent were enriched at the surface of composite particles, and about 30% SiC?2 particles with about 16.8% of ACR grafting percent were encapsulated in the inner of composite particles.During the in-situ emulsion polymerization, the polymerization initiated by the absorbed radicals led to the formation of polymer coating (partial grafting) at the surface of SiCh primary particles. The hydrophilicity of nano-SiC?2 particles decreased as more ACR polymer coated (partial grafted) at the surface of SiC?2 primary particles, they would be aggregated to form the latex particles containing many SiC>2 primary particles as the main loci of polymerization. The SiC>2 particles trended to diffuse from polymer phase to water phase, while the anchored polymer chains at the surface of S1O2 particles would hinder this diffusion, which led to the "raspberry-like" morphology and the distribution of SiO2 particles in the composite latex.Nano-SiC>2 particles dispersed in acrylate monomer were used to prepare the ACR/nano-SiO2 composite particles via miniemulsion polymerization. Concerning the serious escape of nano-SiO2 particles from acrylate monomer to water phase during the miniemulsification of the commercial available acrylate/ nano-SiC>2 dispersion, the treatment of nano-SiC^ with 3-(trimethoxysilyl)propyl methacrylate (MPS) was firstly carried out. The coupling reaction between MPS and nano-SiC>2 was confirmed by IR and elemental analysis. It was found that the amount of coupled MPS increased as the amount of added MPS and reaction time increased. The encapsulation degree of nano-SiO2 particles in the miniemulsion droplets increased as the coupled MPS increased, and the encapsulation degree was close to 100% when0.08g MPS/gSiO2 was coupled.The size and size distribution index of the miniemulsion droplets can be controlledby varying SDS concentration, nano-SiC>2 content, ultrasonic power and time. Thesize and size distribution index increased as the nano-SiCVmonomers weight ratio and nano-SiC>2 dispersion content increased, and as the SDS concentration decreased. The stable ACR/SiC^ nanocomposite latex with more than 20 wt% nano-SiC?2 content and a high encapsulation efficiency of nano-SiO2 (about 100%) was obtained by the miniemulsion polymerization process. HF etching experiments and TEM micrographs showed that the resultant ACR/S1O2 nanocomposite particles exhibited a "guava-like" morphology with the most of silica particles encapsulated in the inner of the composite particles. It was also found that the amount of coupled MPS onto the nano-SiC>2 particles would greatly influence the amount of ACR grafted and the structure of composite particles. Too much coupled MPS (i.e.0.20g/g SiO2) would lead to the formation of the composite particles with crosslinked ACR. Therefore, the amount of coupled MPS and grafted ACR should be controlled at an appropriate range.ACR/nano-SiO2 composite particles prepared by in-situ (mini)emulsion polymerization were applied in the modification of acrylate polymer and PVC. The dispersion of nano-SiO2 particles in polymer matrix and the effect of nano-SiO2 particles on the thermal and mechanical properties of nanocomposites were investigated. It was found that nano-SiC>2 particles with grafted polymer chains (but not crosslinked ) could be dispersed in the primary particle level in the polymer matrix. The nano-SiC?2 particles dispersed in the acrylate polymer in the primary particle level enhanced the storage modulus and heat resistance of materials. Especially, the nano-SiO2 particles in the minemulsion polymerized composite particles could remarkably enhance the impact strength of PVC because it had more grafted ACR and better interfacial adhesion with PVC than that of in-situ emulsion polymerization product. It was also found that the brittle-ductile transition of material was achieved as a small amount of modified nano-SiO2 added. The impact strength of PVC increased 454% when just 1.0wt% nano-SiO2 particles with 59.7% grafted ACR were added. Finally, on the base of percolation model, a new equation of matrix ligament thickness was deduced from theory and fitted from experimental result toexplain the toughening mechanism of nano-SiC>2 particles to PVC.