18,213 research outputs found
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
Thermodynamic scaling of diffusion in supercooled Lennard-Jones liquids
The manner in which the intermolecular potential u(r) governs structural
relaxation in liquids is a long standing problem in condensed matter physics.
Herein we show that diffusion coefficients for simulated Lennard-Jones m-6
liquids (8<m<36) in normal and moderately supercooled states are a unique
function of the variable rho^g/T, where rho is density and T is temperature.
The scaling exponent g is a material specific constant whose magnitude is
related to the steepness of the repulsive part of u(r), evaluated around the
distance of closest approach between particles probed in the supercooled
regime. Approximations of u(r) in terms of inverse power laws are also
discussed.Comment: 4 pages, 3 figure
Optical data storage and metallization of polymers
The utilization of polymers as media for optical data storage offers many potential benefits and consequently has been widely explored. New developments in thermal imaging are described, wherein high resolution lithography is accomplished without thermal smearing. The emphasis was on the use of poly(ethylene terephthalate) film, which simultaneously serves as both the substrate and the data storage medium. Both physical and chemical changes can be induced by the application of heat and, thereby, serve as a mechanism for high resolution optical data storage in polymers. The extension of the technique to obtain high resolution selective metallization of poly(ethylene terephthalate) is also described
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
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
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