Dynamics in polymer-silicate nanocomposites as studied by dielectric relaxation spectroscopy and dynamic mechanical spectroscopy

Nanocomposites of organically modified clay nanoparticles and a polyisoprene (PI) matrix were prepared by solution-mediated intercalation, and their dynamics were investigated over a broad range of frequency and temperature by dielectric relaxation spectroscopy (DRS) and dynamic mechanical spectroscopy (DMS). The principal goal was to address the effect of geometric confinement and elucidate how the dynamics vary as a function of the type and concentration of clay and the molecular weight of PI. Dielectric spectra of nanocomposites with low-molecular-weight PI reveal no effect of clay loading on the average relaxation time for segmental and normal mode relaxation, but dc conductivity and interfacial polarization are affected. In nanocomposites with high molecular weight PI (in the entangled regime), however, a clear effect of clay loading on the average relaxation time for the normal mode process is observed. Most interestingly, it is found that the normal mode becomes faster with increasing clay content, and an explanation is offered in terms of the preferential suppression of the longer scale (lower frequency) portion of the normal mode spectrum. The average relaxation times for segmental and normal mode calculated from dielectric and viscoelastic measurements are in excellent agreement. DMS measurements also reveal an increase in the magnitude of storage and loss modulus with increasing clay loading up to the threshold value of 8 wt %. The observed increase originates from the "filler effect".