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

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 epsilon(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 epsilon(r)' owing to Maxwell-Wagner polarization of the resultant free electrons. On the other hand, the dissipation factor (tan delta) 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 epsilon(r)' for PMMA-ZnO. Thus, PMMA-ZnO is a promising material for high sensitivity and low loss UV light sensors using the change in epsilon(r)'.