On the size and dielectric properties of the interphase in epoxy-alumina nanocomposite

In this work we have proposed a combined experimental-numerical technique to elicit important quantitative information about the dielectric properties of the interphase region surrounding a nanoparticle embedded in a polymer matrix. The method involves performing permittivity measurements through dielectric spectroscopy and combining with concurrent unit cell based Finite Element solutions of the electrostatic fields. The analysis has to be supplemented with some information about the degree of agglomeration in the nanocomposites which are estimated from electron microscopy done at several resolutions. For an epoxy matrix with 40 nm alumina nanoparticles, our analysis indicates the presence of an interphase region about 200 nm thick and having permittivity lower than that of epoxy. Heating the nanoparticles prior to the synthesis of the nanocomposite makes the interphase region thicker and lowers its permittivity slightly. At higher volume fractions of the nanoparticles, agglomeration sets in and creates regions of even higher volume fraction within the matrix. Our analysis also revealed that at 2% volume fraction of alumina nanoparticles, 600 nm sized agglomerates exist in the matrix leading to local particle volume fraction as high as 5%.