Poly(methyl methacrylate)-grafted ZnO nanocomposites with variable dielectric constants by UV light irradiation

Abstract

A series of poly(methyl methacrylate)-grafted ZnO nanoparticles (PMMA–ZnO) were synthesized using a surface-initiated polymerization technique and the optical and dielectric properties of PMMA–ZnO were studied. The dielectric constant ε_r′ of PMMA–ZnO thin films was highly increased by irradiation of UV light. It is indicated that electrons in the ZnO nanoparticles are excited from a valence band to a conductive band by absorption of UV light, resulting in a large increase in ε_r′ owing to Maxwell–Wagner polarization of the resultant free electrons. On the other hand, the dissipation factor (tan δ) of PMMA–ZnO is very low and almost constant during UV irradiation because PMMA–ZnO is electrically insulated by the grafted PMMA chains on the ZnO nanoparticles. Also, it was confirmed that due to the grafted PMMA chains, PMMA–ZnO nanocomposites exhibited low light scattering in addition to strong absorption of UV light. The low light scattering of PMMA–ZnO would enhance the absorption efficiency of UV light and therefore contribute to the large increase in ε_r′ for PMMA–ZnO. Thus, PMMA–ZnO is a promising material for high sensitivity and low loss UV light sensors using the change in ε_r′.