  The coupling behavior of time scales of structural relaxation is investigated
on the basis of five different response functions for 1,2,6-hexanetriol, a
hydrogen-bonded liquid with a minor secondary contribution, and
2,6,10,15,19,23-hexamethyl-tetracosane (squalane), a van der Waals bonded
liquid with a prominent secondary relaxation process. Time scales of structural
relaxation are derived as inverse peak frequencies for each investigated
response function. For 1,2,6-hexanetriol, the time-scale indices are
temperature-independent, while a decoupling of time scales is observed for
squalane in accordance with literature. An alternative evaluation approach is
made on the squalane data, extracting time scales from the terminal relaxation
mode instead of the peak position, and in this case temperature-independent
coupling is also found for squalane, despite its strong secondary relaxation
contribution. Interestingly, the very same ordering of
response-function-specific time scales is observed for these two liquids, which
is also consistent with the observation made for simple van der Waals bonded
liquids reported previously [Jakobsen \textit{et al.}, J. Chem. Phys.
\textbf{136}, 081102 (2012)]. This time-scale ordering is based on the
following response functions, from fast to slow dynamics: shear modulus, bulk
modulus, dielectric permittivity, longitudinal thermal expansivity coefficient,
and longitudinal specific heat. These findings indicate a general relation
between the time scales of different response functions and, as inter-molecuar
interactions apparently play a subordinate role, suggest a rather generic
nature of the process of structural relaxation.
