The complexity of linear viscoelastic properties of polymer nanocomposites: polymer dynamics and nanoparticle rearrangement

Polymer nanocomposites (PNCs) are widely used as structural and functional materials, whose lifetime are typically evaluated from the time-temperature superposition. Rheological measurements of PNCs usually show >30 decades variation in the horizontal shift factor, a_T, over a finite temperature range [1], indicating an unrealistic fast polymer dynamics at high temperatures. In this work, we decoupled the dynamic spectra of PNCs through a combination of rheology and dielectric measurements. We found that polymer dynamics in PNCs at different time and length scales follow identical temperature dependence with the neat polymer, although the rheological measurements exhibit 30 decades variation in a_T. Further analyses suggest the local rearrangements of nanoparticles, especially at high temperatures, lead to the breakdown of the time-temperature superposition principle in PNCs. These results clearly show the shift factors by the time-temperature superposition principles cannot be directly translated to evaluate the dynamics features of PNCs at different temperatures.
[1]. Mujtaba, A.; et al. ACS Macro Letters 2014, 3 (5), 481-485.