Mechanical Relaxation Studies of Sub-Rouse Modes in Amorphous Polymers

Mechanical spectroscopy is a powerful tool for the investigation of molecular dynamics of amorphous polymers over a large temperature range and frequency scale. In this work, by using high precision shear mechanical spectroscopy tool, we have investigated the segmental dynamics from local segmental relaxation to sub-Rouse modes in a series of amorphous polymers. We have demonstrated the existence of sub-Rouse modes slower than the local segmental motion in amorphous polymers. The sub-Rouse modes exhibit a similar change of dynamics at the same temperature TB ~1.2 Tg, as the local segmental relaxation through the temperature dependence of relaxation time and relaxation strength. Furthermore, the crossover relaxation time of the sub-Rouse modes at TB is almost the same for all the polymers investigated, i.e. τα'(TB) = 10-1±0.5 s, which is independent of molecular weight and molecular structure. This remarkable finding indicates that solely the time scale of the relaxation determines the change in dynamics of the sub-Rouse modes. According to the coupling model, the crossover is suggested to be caused by the onset of strong intermolecular cooperativity below TB. Hence the results suggest that the sub-Rouse modes and their properties are generally found in amorphous polymers by mechanical spectroscopy, and reveal the cooperative nature of the sub-Rouse modes.