711 research outputs found
Effect of entropy on the dynamics of supercooled liquids: New results from high pressure data
We show that for arbitrary thermodynamic conditions, master curves of the
entropy are obtained by expressing S(T,V) as a function of TV^g_G, where T is
temperature, V specific volume, and g_G the thermodynamic Gruneisen parameter.
A similar scaling is known for structural relaxation times,tau = f(TV^g);
however, we find g_G < g. We show herein that this inequality reflects
contributions to S(T,V) from processes, such as vibrations and secondary
relaxations, that do not directly influence the supercooled dynamics. An
approximate method is proposed to remove these contributions, S_0, yielding the
relationship tau = f(S-S_0).Comment: 10 pages 7 figure
Determination of the Thermodynamic Scaling Exponent from Static, Ambient-Pressure Quantities
An equation is derived that expresses the thermodynamic scaling exponent, g,
which superposes relaxation times and other measures of molecular mobility
determined over a range of temperatures and densities, in terms of static,
physical quantities. The latter are available in the literature or can be
measured at ambient pressure. We show for 13 materials, both molecular liquids
and polymers, that the calculated g are equivalent to the scaling exponents
obtained directly by superpositioning. The assumptions of the analysis are that
the glass transition is isochronal and that the first Ehrenfest relation is
valid; the first assumption is true by definition, while the second has been
corroborated for many glass-forming materials at ambient pressure. However, we
find that the Ehrenfest relation breaks down at elevated pressure, although
this limitation is of no consequence herein, since the appeal of the new
equation is its applicability to ambient pressure data.Comment: 9 pages, 3 figures, 1 tabl
What can we learn by squeezing a liquid
Relaxation times for different temperatures, T, and specific volumes, V,
collapse to a master curve versus TV^g, with g a material constant. The
isochoric fragility, m_V, is also a material constant, inversely correlated
with g. From these we obtain a 3-parameter function, which fits accurately
relaxation times of several glass-formers over the supercooled regime, without
any divergence below Tg. Although the 3 parameters depend on the material, only
g significant varies; thus, by normalizing material-specific quantities related
to g, a universal power law for the dynamics is obtained.Comment: 12 pages, 4 figure
Density Scaling and Dynamic Correlations in Viscous Liquids
We use a recently proposed method [Berthier L.; Biroli G.; Bouchaud J.P.;
Cipelletti L.; El Masri D.; L'Hote D.; Ladieu F.; Pierno M. Science 2005, 310,
1797.] to obtain an approximation to the 4-point dynamic correlation function
from derivatives of the linear dielectric response function. For four liquids
over a range of pressures, we find that the number of dynamically correlated
molecules, Nc, depends only on the magnitude of the relaxation time,
independently of temperature and pressure. This result is consistent with the
invariance of the shape of the relaxation dispersion at constant relaxation
time and the density scaling property of the relaxation times, and implies that
Nc also conforms to the same scaling behavior. For propylene carbonate and
salol Nc becomes constant with approach to the Arrhenius regime, consistent
with the value of unity expected for intermolecularly non-cooperative
relaxation.Comment: revisio
Thermodynamic Scaling of the Viscosity of Van Der Waals, H-Bonded, and Ionic Liquids
Viscosities and their temperature, T, and volume, V, dependences are reported
for 7 molecular liquids and polymers. In combination with literature viscosity
data for 5 other liquids, we show that the superpositioning of relaxation times
for various glass-forming materials when expressed as a function of TV^g, where
the exponent g is a material constant, can be extended to the viscosity. The
latter is usually measured to higher temperatures than the corresponding
relaxation times, demonstrating the validity of the thermodynamic scaling
throughout the supercooled and higher T regimes. The value of g for a given
liquid principally reflects the magnitude of the intermolecular forces (e.g.,
steepness of the repulsive potential); thus, we find decreasing g in going from
van der Waals fluids to ionic liquids. For strongly H-bonded materials, such as
low molecular weight polypropylene glycol and water, the superpositioning
fails, due to the non-trivial change of chemical structure (degree of
H-bonding) with thermodynamic conditions.Comment: 16 pages 7 figure
- …