Sub-Rouse Modes in Polymers Observed by Dielectric Spectroscopy

One of the characteristic features of polymer dynamics is the existence of additional relaxation processes that are related to connectivity of monomers in polymer chain and are absent in small molecular liquids. These processes are usually described as purely entropic Rouse modes and appear on time scales much longer than segmental (structural) relaxation in polymers. They are commonly observed by mechanical relaxation spectroscopy and are detected as normal modes in dielectric spectra of some polymers (where dipole moment accumulates along the chain). Here we present the evidence of additional modes that appear in dielectric relaxation spectra of some polymers between the segmental and Rouse modes. We identified these modes as sub-house modes, the same as those previously found by mechanical spectroscopy. We also demonstrate that these modes are not detected in the dielectric loss spectra of many polymers. Moreover, existence of these modes does not correlate with the existence of dipole moment along the polymer chain. A possible explanation of this behavior is discussed.