  Published dielectric relaxation measurements on polypropylene glycol and
1,4-polyisoprene are analyzed to determine the relative effect that thermal
energy and volume have on the temperature dependence of the normal mode
relaxation times, and compare this to their effect on the temperature
dependence of the local segmental relaxation times. We find that for both
polymers at temperatures well above Tg, both relaxation modes are governed more
by thermal energy than by volume, although the latter's contribution is not
negligible. Such a result is consistent with an assumption underlying models
for polymer viscoelasticity, such as the Rouse and tube models, that the
friction coefficient governing motions over large length scales can be
identified with the local segmental friction coefficient. We also show that
relaxation data for both the segmental and the normal mode superimpose, when
expressed as a function of the product of the temperature and the volume, the
latter raised to a power. This scaling form arises from an inverse power form
for the intermolecular potential. The value of the exponent on the volume for
these two polymers indicates a relatively "soft" potential.
