The
Glass Transition and Orientationally Disordered Crystals
Orientationally-disordered
crystals consist of molecules that lie on a crystalline lattice, but
which retain orientational degrees of freedom. At high temperatures
the molecules rotate freely (the so-called rotator phase). As the
temperature is lowered, the material can be supercooled, bypassing
the first-order transition to a perfectly-ordered crystal. Upon further
cooling, the rotational motions slow down and eventually freeze in
what is called an orientational glass transition. Because the molecules
lie on a lattice, this glass transition should be much easier to model
theoretically that the more complicated structural glass transition
seen in supercooled liquids. We have studied the cyclo-octanol and
ethanol, the latter being unique in that it is quite simple to prepare
it either as a supercooled liquid or as a rotator phase crystal. We
have compared the dynamics of these two phases using dielectric measurements
over a 10-decade frequency range. The relaxation times are very similar
in the two phases, but the relaxation time of the supercooled liquid
decreases faster with increasing temperature than that of the rotator
phase. We believe that this is due to the extra translational degrees
of freedom in the supercooled liquid that are not present in the rotator
phase.
Acknowledgement is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and to the Michigan State University Center for Fundamental Materials Research, for support of this research.
