  We present the high pressure dielectric spectroscopy (up to 4.2 GPa) and
ultrasonic study (up to 1.7 GPa) of liquid and glassy propylene carbonate (PC).
Both of the methods provide complementary pictures of the glass transition in
PC under pressure. No other relaxation processes except $\alpha$-relaxation
have been found in the studied pressure interval. The propylene carbonate
liquid is a glassformer where simple relaxation and the absence of
$\beta$-relaxation are registered in the record-breaking ranges of pressures
and densities. The equation of state of liquid PC was extended up to 1 GPa from
ultrasonic measurements of bulk modulus and is in good accordance with the
previous equations developed from volumetric data. We measured the bulk and
shear moduli and Poisson's ratio of glassy PC up to 1.7 GPa. Many relaxation
and elastic properties of PC can be qualitatively described by the soft-sphere
or Lennard-Jones model. However, for the quantitative description of entire set
of the experimental data, these models are insufficient. Moreover, the Poisson
coefficient value for glassy PC indicates a significant contribution of
non-central forces to the intermolecular potential. The well-known correlation
between Poisson's ratio and fragility index (obtained from dielectric
relaxation) is confirmed for PC at ambient pressure, but it is violated with
pressure increase. This indicates that different features of the potential
energy landscape are responsible for the evolution of dielectric response and
elasticity with pressure increase.
