Dielectric enhancement in polymer-nanoparticle composites through interphase polarizability

Dielectric measurements on polyimide-oxide nanoparticle composite thin films show a composite dielectric constant (composite) that increased monotonically with increasing oxide content well above the value predicted by Maxwell's rule for dielectric mixtures below the percolation threshold. Above certain volume fractions, the measured epsilon composite values were found to exceed the corresponding nanoparticle epsilon such that epsilon polymer< epsilon particle< epsilon composite contrasted to conventional composites where epsilon polymer< epsilon composite< epsilon particle. The epsilon composite was independent of frequency to 10 MHz with dielectric loss of <0.005 throughout this range, indicating that the observed enhancement in epsilon does not originate from space-charge related contributions and hence should be due to dipolar contributions. The observed epsilon enhancement (epsilon composite- epsilon Maxwell) showed a correlation with the total surface area of the nanoparticles. The dielectric model of Vo and Shi [Microelectron. J. 33, 409 (2002), and references therein] showed that the enhanced dielectric behavior originates from significant interfacial nanoparticle-polymer interactions and the critical role of additional contributions to polarizability through specific physicochemical interactions within the interphase region. An interphase epsilon int considerably higher than that of the nanoparticle and a high interface interaction constant of 3.24 for the nanocomposite suggest a strong interaction between the functional groups of the polymer and the nanoparticle surface. Although modeling suggests a maximum of epsilon~65 vol %, loss in micromechanical stability occurred above 20% due to incomplete polymer wetting films arising from the high nanoparticle surface areas.