Effect of pressure on relaxation dynamics at different time scales in supercooled systems

For over 10 decades, the dynamics of two glass-forming systems have been investigated by dielectric spectroscopy under cooling (from melting point to well below the glass transition temperature) and under compression (from atmospheric pressure up to 700 MPa). The alpha-relaxation time tau(alpha) was significantly affected by both thermodynamic variables, showing equal roles in slowing down the dynamics. Some similarities have been found; for instance, the dispersion of the alpha-process was shown to increase with decreasing temperature T and increasing pressure P. Furthermore, the same shape for relaxation dynamics over a broad time-scale was found by comparing two dielectric loss spectra obtained at different T and P but characterized by the same tau(alpha)(T, P). Additionally, it is noteworthy that the effect of T and P on slowing down the time scale of fast relaxation processes (beta-relaxation and excess wing), although less strong than in the case of alpha-process, was again comparable. The evidence demonstrates that in the investigated systems: (a) slow and fast relaxations are strongly related; (b) the shape of alpha-relaxation and (c) the separation between alpha- and alpha-relaxation time scale are controlled by tau(alpha)(T, P) and not by separate thermodynamic variables.