Preparation Of Polymer Grafted Silica Nanoparticles By Redox-Initiated Polymerization From Silica Surface

Silica nanopartiele is one of the most important fillers in rubber,plastic,painter industry. However,the silica nanoparticles have a poor wettability and dispersibility in organic medium due to their hydrophilic surfaces,which confines its application.Thus,the modification for silica,which enhances their dispersibility and compatibility in organic medium and improves its performance,is of great importance.This study describes a facile and versatile method for preparing polymer grafted silica nanopartieles by surface-initiated graft polymerization,using a redox initiation system comprising ceric ion(Ce⁴⁺)and reducing agents immobilized on the nanoparticle surfaces. Different from the living free radical polymerizations,the present method has some merits, e.g.it is able to allow a facile and mild grafting polymerization at low temperatures in aqueous solutions,and it involves only inexpensive,commercially available reagents,and is versatile for many monomers,especially for water soluble monomers.Silica nanoparticles were treated with 3-aminopropyltriethoxysilane(APTES),and subsequently modified in two different routes.(1)The aminated silica was reacted with poly(ethylene glycol)acrylate through the Michael addition reaction,so that terminal hydroxyl was introduced onto the silica surface.Two model monomers,methyl methacrylate(MMA)and N-isopropylacrylamide(NIPAAm), were used for graft polymerization from the silica surface initiated by the Ce⁴⁺/-OH redox system.(2)An emulsion polymerization of MMA and St initiated by the Ce⁴⁺/-NH₂ redox system was performed for modification of the aminated silica,respectively.Surfactants used were sodium dodecyl sulfate(SDS)and hexadecyl trimethyl ammonium bromide(CTAB). The polymer grafted silica nanoparticles were characterized by dynamic light scattering, infrared spectroscopy,thermogravimetric analysis,transmission electron microscopy and micro-Vickers hardness tester.IR spectra confirm that PMMA and PNIPAAm are successfully grafted onto the silica surface initiated by the Ce⁴⁺/-OH redox system.On the contrary,graft polymerization does not occur on bare silica nanoparticles.The hydrophobicity of polymer plays a crucial role in determining the suspendability of silica@polymer particles in the aqueous solutions.The silica@PMMA nanoparticles cannot be suspended stably and finally precipitate from water,while the suspension of silica@PNIPAAm particles has bluish color and remains stable for almost 24 h.The silica@PNIPAAm particles exhibit a reversible change in response to temperature changes. The mean diameter sharply decreased from 194 nm to 155 nm when temperature was increased from 31℃to 32℃,which was consistent with the previously reported LCST of pure PNIPAAm.The hydrophobicity of polymer has large effect on the amount of polymer grafted onto silica surface.According to the TGA results,the grafted contents of PMMA and PNIPAAm on 1 g of silica are calculated to be about 0.18g and 0.50g,respectively.The TEM images show that the average thickness of PMMA and PNIPAAm shell on the silica surface is about 15 nm and 25 nm,respectively.This is consistent with the result of TGA,which shows that the amount of PMMA grafted onto the silica surface is less than that of PNIPAAm.As to the emulsion polymerization,IR spectra confirm that PMMA and PS are successfully grafted onto silica initiated by the Ce⁴⁺/-NH₂ redox system.Compared with the former system without surfactant,the amount of polymer grafted onto the silica surface increased.Thus,the addition of surfactant can significantly improve the graft ratio.The solubility of monomer has effect on the amount of grafted polymer.The grafted amount of PMMA is larger than that of PS under the use of either SDS or CTAB.When SDS was used,the mean diameter of SiO₂/PMMA particles was much smaller than that of SiO₂/PS particles.When CTAB was used,the mean diameter of SiO₂/PMMA particles was larger by 25 nm than that of SiO₂/PS particles,and both polymers reduced the aggregation of the silica nanopartieles.The electric charge of surfactant has effect on the amount of grained polymer. When SDS was used,the grained amounts of both polymers are larger.Because SDS is negative charged and is electrostatically attracted with the Ce⁴⁺ions,graft polymerization can be effectively initiated.While CTAB is positive charged and electrostatically repulsed with the Ce⁴⁺ions,graft polymerization cannot be effectively initiated on the surface of silica nanoparticles.The Vickers hardness of SiO₂/PMMA and SiO₂/PS is higher than that of pure PMMA and PS.