  Dielectric spectroscopy measurements over a broad range of temperature and
pressure were carried out on poly(oxybutylene) (POB), a type-A polymer
(dielectrically-active normal mode). There are three dynamic processes
appearing at lower frequency, the normal and segmental relaxation modes, and a
conductivity arising from ionic impurities. In combination with
pressure-volume-temperature measurements, the dielectric data were used to
assess the respective roles of thermal energy and density in controlling the
relaxation times and their variation with T and P. We find that the local
segmental and the global relaxation times are both a single function of the
product of the temperature times the specific volume, with the latter raised to
the power of 2.65. The fact that this scaling exponent is the same for both
modes indicates they are governed by the same local friction coefficient, an
idea common to most models of polymer dynamics. Nevertheless, near Tg, their
temperature dependences diverge. The magnitude of the scaling exponent reflects
the relatively weak effect of density on the relaxation times. This is usual
for polymers, as the intramolecular bonding, and thus interactions between
directly bonded segments, are only weakly sensitive to pressure. This
insensitivity also means that the chain end-to-end distance is invariant to P,
conferring a near pressure-independence of the (density-normalized) normal mode
dielectric strength.
