We report on the pressure dependence of the local segmental and normal mode
s of a type-A polymer (polyisoprene) using dielectric spectroscopy. We find
that external pressure exerts a stronger influence on the dynamics of the
segmental mode as compared to the longest normal mode which results in the
crossing of the two modes at higher pressures. The predictions of the Rouse
model are not adequate to explain the different shift factors even at elev
ated temperatures. (C) 1999 American Institute of Physics. [S0021- 9606(99)
50135-9].