  The role of jamming (steric constraints) and its relationship to the
available volume is addressed by examining the effect that certain
modifications of a glass-former have on the ratio of its isochoric and isobaric
activation enthalpies. This ratio reflects the relative contribution of volume
(density) and temperature (thermal energy) to the temperature-dependence of the
relaxation times of liquids and polymers. We find that an increase in the
available volume confers a stronger volume-dependence to the relaxation
dynamics, a result at odds with free volume interpretations of the glass
transition.
