Temperature Dependence of Polymer Diffusion in MWCNT/PS Nanocomposites

Polymer tracer diffusion in multiwalled carbon nanotubes (MWCNT)/polystyrene (PS) nanocomposites as a function of MWCNT concentration was measured from 152 to 214 °C. At 170 °C, we previously reported a minimum in the tracer diffusion coefficient that occurs at ∼2 wt % MWCNT; this concentration corresponds to the onset of solidlike behavior as measured by linear viscoelasticity. The minimum in the tracer diffusion coefficient in the nanocomposites (D) normalized by the tracer diffusion coefficient without MWCNTs (D₀) is shallower at higher temperatures, namely, (D/D₀)_min = 0.9 at 214 °C and (D/D₀)_min = 0.6 at 152 °C. Using our trap model, this implies that the difference between the tracer diffusion parallel and perpendicular to the MWCNT is smaller at higher temperatures. Finally, at fixed MWCNT concentrations (0.5, 2, and 6 wt %) the temperature dependence of the tracer diffusion coefficient follows the Williams–Landel–Ferry (WLF) equation, indicating that polymer dynamics in nanocomposites is captured by changes in free volume with temperature.