Radiation modification of silica nanoparticles through grafting of vinyl monomers and polymers

Introduction: Radiation-induced grafting is a simple but effective method for modifying the surface of particles, films, and fibers. Since it is applicable not only to organic materials but also to inorganic materials, organic/inorganic hybridized materials can be prepared by this method. Recently inorganic nanoparticles attract significant attention because of their potential application to nanotechnology. When various functional polymers are tethered to their surface, new functionality can be given to the nanoparticle, leading to the useful nanomaterials. We paid attention to γ-ray-induced surface modification of silica nanoparticles with thermoresponsive PNIPAM, or poly(N-isopropylacrylamide). In order to investigate effective grafting by γ-ray-induced reaction, we compared two grafting methods: monomer grafting and polymer grafting. Here we report the dependence of grafting yields on 60Co γ-ray irradiation condition and the difference between the grafting with N-isopropylacrylamide (NIPAM) monomer and that with prepolymerized PNIPAM. Experimental: Silica nanoparticles (AEROSIL200, 12 nm in diameter), washed with pure water under stirring prior to use, were dispersed in methanol in a glass tube with a concentration of 1% and then a NIPAM monomer or PNIPAM was dissolved into it. After the glass tube was evacuated and sealed, this dispersion was irradiated with 60Co γ-rays under stirring. Products were purified by repeating procedure of centrifugation, decantation and re-dispersion in methanol. Grafting yield, defined by the ratio of PNIPAM weight to silica weight, was evaluated from IR spectra with reference to a calibration curve predetermined with the mixture of silica particles and PNIPAM. Results and Discussion: Figure 1 shows radiation dose dependence of the grafting yield for the monomer and polymer grafting. In both cases, it is seen that the grafting yield increases with increasing dose rate and levels off at higher doses. However, the grafting rate at the initial rise is quite higher for the monomer grafting than for the polymer grafting. Moreover, the grafting rate is strongly dependent on the dose rate for the monomer grafting, whereas it is almost constant for the polymer grafting. Maximum grafting yields are higher for the polymer grafting. The difference in the grafting rate at the initial rise between the monomer grafting and the polymer grafting can be explained by the diffusion of grafted molecules to the surface of silica particles. Small NIPAM molecules easily approach the surface, while large PNIPAM chains require longer time to reach the surface and radicals formed on the chain vanish before that. In contrast, the polymer grafting results in high grafting yield. Homopolymerization of NIPAM as a side reaction which lowers the concentration of NIPAM to be grafted is responsible for this result. For the polymer grafting, higher plateau value is obtained at a low dose rate. This is due to multiple anchoring of each chain. When the dose rate is high, it is likely that many radicals are formed on one chain and polymer chains are anchored to the surface at several points. The particle surface can be covered with fewer chains, resulting in low grafting yields. The molecular weight of the grafted polymer and the reaction temperature are also proved to affect the grafting yield. Although radiation-induced grafting is a simple technique, effective and efficient modification can be achieved by the proper irradiation condition and adequate selection of grafted molecules. (authors)