Effect of Cross-Link Density on Interphase Creation in Polymer Nanocomposites

The thermal and dynamic behavior of epoxy nanocomposites with varying cross-link density was tested via dynamic scanning calorimetry (DSC) in order to determine the influence of cross-link density on the creation of “interphase”, zones of altered polymer properties near polymer−particle interfaces. The results show that the inclusion of nanoparticles creates an increase in the glass transition temperature (Tg) at low cross-link density and a decrease in Tg at higher cross-link densities. This phenomenon suggests that two mechanisms work in tandem to alter the Tg of epoxy systems, with relative magnitudes determined by cross-link density: (1) network disruption at the nanotube−polymer interfaces leading to lower Tg and (2) interphase creation leading to retarded dynamics, resulting in higher Tg. Results show that as cross-link density increases, the length scale of cooperatively rearranging regions (CRRs) decreases. This decrease hinders communication of the dynamics between adjacent CRRs, thereby reducing interphase penetration into the bulk matrix. Moreover, increasing cross-link density leads to increased network disruption due to the presence of nanoparticle obstacles in an otherwise densely connected network.