13,862 research outputs found
Study of the glass transition in the amorphous interlamellar phase of highly crystallized poly(ethylene terephthalate)
Poly(ethylene terephthalate) (PET) is a semi--crystalline polymer that can be
crystallized to different degrees heating from the amorphous state. Even when
primary crystallization has been completed, secondary crystallization can take
place with further annealing and modify the characteristics of the amorphous
interlamellar phase. In this work we study the glass transition of highly
crystallized PET and in which way it is modified by secondary crystallization.
Amorphous PET samples were annealed for 4 hours at temperatures between 140C
and 180C. The secondary crystallization process was monitored by differential
scanning calorimetry and the glass transition of the remaining interllamelar
amorphous phase was studied by Thermally Stimulated Depolarization Currents
measurements. Non--isothermal window polarization is employed to resolve the
relaxation in modes with a well--defined relaxation time that are subsequently
adjusted to several standard models. Analysis of experimental results, show
that cooperativity is reduced to a great extend in the interlamellar amorphous
regions. The evolution of the modes on crystallization temperature reveals that
large scale movements are progressively replaced by more localized ones, with
higher frequency, as crystallization takes place at higher temperatures. As a
consequence, the glass transition temperature of the amorphous interlamellar
phase tends to lower values for higher annealing temperatures. Evolution of
calorimetric scans of the glass transition are simulated from the obtained
results and show the same behaviour. The interpretation of these results in
terms of current views about secondary crystallization is discussed.Comment: 30 pages, 5 tables, 12 figures; figure 5 modifie
Relaxation of isolated quantum systems beyond chaos
In classical statistical mechanics there is a clear correlation between
relaxation to equilibrium and chaos. In contrast, for isolated quantum systems
this relation is -- to say the least -- fuzzy. In this work we try to unveil
the intricate relation between the relaxation process and the transition from
integrability to chaos. We study the approach to equilibrium in two different
many body quantum systems that can be parametrically tuned from regular to
chaotic. We show that a universal relation between relaxation and
delocalization of the initial state in the perturbed basis can be established
regardless of the chaotic nature of system.Comment: 4+ pages, 4 figs. Closest to published versio
Lyapunov decay in quantum irreversibility
The Loschmidt echo -- also known as fidelity -- is a very useful tool to
study irreversibility in quantum mechanics due to perturbations or
imperfections. Many different regimes, as a function of time and strength of
the perturbation, have been identified. For chaotic systems, there is a range
of perturbation strengths where the decay of the Loschmidt echo is perturbation
independent, and given by the classical Lyapunov exponent. But observation of
the Lyapunov decay depends strongly on the type of initial state upon which an
average is done. This dependence can be removed by averaging the fidelity over
the Haar measure, and the Lyapunov regime is recovered, as it was shown for
quantum maps. In this work we introduce an analogous quantity for systems with
infinite dimensional Hilbert space, in particular the quantum stadium billiard,
and we show clearly the universality of the Lyapunov regime.Comment: 8 pages, 6 figures. Accepted in Phil. Trans. R. Soc.
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