Physical and electrical properties of trimetallic nitride template endohedral metallofullerenes and their polymer nanocomposites

Hanaa Mohammed Ahmed

Abstract

The main objective of this study was characterization of pure metallic nitride fullerene, MNF, and MNF containing polymers to evaluate these materials as suitable devices for tunable applications. Polymer-fullerene nanocomposites consisting of linear polyurethane (PU) segments crosslinked via polyhydroxylated fullerenes (C 60 and Sc3 N@C 80 , a metallic nitride fullerene) were prepared and characterized for their mechanical and dielectric properties using dynamic mechanical analysis (DMA) and broadband dielectric spectroscopic techniques. Polyhydroxylated fullerenes C60 (OH)29 and Sc3 N@C80 (OH) 18 were synthesized in a high yield through a solid-state high sheer ball-milling procedure and were characterized using a verity of techniques, such as FT-R, mass spectroscopy (MS) and thermal gravimetric analysis (TGA), to elucidate their structures. A difunctional isocyanate-terminated prepolymer was prepared from the reaction of poly(tetramethylene oxide) glycol (PTMO, ∼2000 g/mol) and methylene bis(4-isocyanatobenzene) (MDI) followed by the addition of the crosslinking fullerene agent. Fullerene-polymer networks [C 60 -PU and Sc3 N@C80 -PU] having high gel fractions and good mechanical properties and thermal stabilities were produced. Dynamic mechanical analyses of (C60 or Sc3 N@C80 )-PU networks indicated a glass transition temperature, Tg , of -50°C with a sub-Tg relaxation due to local chain motions. Broadband dielectric spectroscopic analyses of the nanoparticles prior to incorporation into the networks revealed one relaxation and large [varepsilon]' values in hydroxylated C60 relative to unfunctionalized C60 . The analogous hydroxylated Sc3 N@C80 exhibited two relaxations, and the extra relaxation may be due to reorientations of cage-encapsulated Sc 3 N clusters. Permittivity values ([varepsilon]') for Sc3 N@C 80 -PU were found to be higher than the corresponding values for C 60 -PU, likely because of the rotationally mobile dipoles. For temperature < 0°C there was a dielectric loss peak due to the glass transition of the PU matrix and another at a lower temperature due to short range chain motions. The loss-frequency spectra of all prepared samples were analyzed sing the Kramers-Kronig transformation and Havriliak-Negami (HN) equation to extract information about relaxation processes taken place in these samples. Capacitance-voltage characteristics of the fullerene-PUs did not show any significant change with the applied dc bias voltage in the range of our instrument window (-30 to +30 volt). A general conclusion is that this class of materials can be rendered quite polarizable.