63 research outputs found
Dielectric and Dilatometric Studies of Glass Transitions in Thin Polymer Films
Dielectric relaxation and thermal expansion spectroscopy were made for thin
polystyrene films in order to measure the temperature
corresponding to the peak in the loss component of susceptibility due to the
-process and the -relaxation time as functions of film
thickness . While the glass transition temperature decreases
with decreasing film thickness, and were found to remain
almost constant for and decrease drastically for
for high temperatures. Here, is a critical thickness. Near the
glass transition temperature, the thickness dependence of and
is more prominent. The relation between the fragility index and
non-exponentiallity is discussed for thin films of polystyrene.Comment: 4 pages, 5 figure
Aging phenomena in polystyrene thin films
The aging behavior is investigated for thin films of atactic polystyrene
through measurements of complex electric capacitance. During isothermal aging
process the real part of the electric capacitance increases with aging time,
while the imaginary part decreases with aging time. This result suggests that
the aging time dependence of the real and imaginary parts are mainly associated
with change in thickness and dielectric permittivity, respectively. In thin
films, the thickness depends on thermal history of aging even above the glass
transition. Memory and `rejuvenation' effects are also observed in the thin
films.Comment: 4 pages, 2 figure
Fracture energy of gels
To clarify effects of crack speed and cross-link density on fracture energy
of acrylamide gels, we evaluated the roughness of the fracture surface and
measured the fracture energy taking into account the roughness. The fracture
energy increases linearly with crack speed in a fast crack speed region,
and the increasing rate of fracture energy with decreases with increasing
cross link density in the gels. In a slow crack speed region the fracture
energy depends on crack speed more strongly than in the fast crack speed
region. This indicates that a qualitative change exists in fracture process of
the gels.Comment: 7 pages, 8 figures. Some errors in the first version have been
correcte
Glassy dynamics in thin films of polystyrene
Glassy dynamics was investigated for thin films of atactic polystyrene by
complex electric capacitance measurements using dielectric relaxation
spectroscopy. During the isothermal aging process the real part of the electric
capacitance increased with time, whereas the imaginary part decreased with
time. It follows that the aging time dependences of real and imaginary parts of
the electric capacitance were primarily associated with change in volume (film
thickness) and dielectric permittivity, respectively. Further, dielectric
permittivity showed memory and rejuvenation effects in a similar manner to
those observed for poly(methyl methacrylate) thin films. On the other hand,
volume did not show a strong rejuvenation effect.Comment: 7 pages, 7 figures. Phys. Rev. E (in press
Glass transition and alpha-relaxation dynamics of thin films of labeled polystyrene
The glass transition temperature and relaxation dynamics of the segmental
motions of thin films of polystyrene labeled with a dye,
4-[N-ethyl-N-(hydroxyethyl)]amino-4-nitraozobenzene (Disperse Red 1, DR1) are
investigated using dielectric measurements. The dielectric relaxation strength
of the DR1-labeled polystyrene is approximately 65 times larger than that of
the unlabeled polystyrene above the glass transition, while there is almost no
difference between them below the glass transition. The glass transition
temperature of the DR1-labeled polystyrene can be determined as a crossover
temperature at which the temperature coefficient of the electric capacitance
changes from the value of the glassy state to that of the liquid state. The
glass transition temperature of the DR1-labeled polystyrene decreases with
decreasing film thickness in a reasonably similar manner to that of the
unlabeled polystyrene thin films. The dielectric relaxation spectrum of the
DR1-labeled polystyrene is also investigated. As thickness decreases, the
-relaxation time becomes smaller and the distribution of the
-relaxation times becomes broader. These results show that thin films
of DR1-labeled polystyrene are a suitable system for investigating confinement
effects of the glass transition dynamics using dielectric relaxation
spectroscopy.Comment: 10 pages, 11 figures, 2 Table
Discretized Wiener-Khinchin theorem for Fourier-Laplace transformation: application to molecular simulations
The Wiener-Khinchin theorem for the Fourier-Laplace transformation (WKT-FLT)
provides a robust method to calculate numerically single-side Fourier
transforms of arbitrary autocorrelation functions from molecular simulations.
However, the existing WKT-FLT equation produces two artifacts in the output of
the frequency-domain relaxation function. In addition, these artifacts are more
apparent in the frequency-domain response function converted from the
relaxation function. We find the sources of these artifacts that are associated
with the discretization of the WKT-FLT equation. Taking these sources into
account, we derive the new discretized WKT-FLT equations designated for both
the frequency-domain relaxation and response functions with the artifacts
removed. The use of the discretized WKT-FLT equations is illustrated by a flow
chart of an on-the-fly algorithm. We also give application examples of the
discretized WKT-FLT equations for computing dynamic structure factor and
wave-vector-dependent dynamic susceptibility from molecular simulations
History Memorized and Recalled upon Glass Transition
The memory effect upon glassification is studied in the glass to rubber
transition of vulcanized rubber with the strain as a controlling parameter. A
phenomenological model is proposed taking the history of the temperature and
the strain into account, by which the experimental results are interpreted. The
data and the model demonstrate that the glassy state memorizes the time-course
of strain upon glassification, not as a single parameter but as the history
itself. The data also show that the effect of irreversible deformation in the
glassy state is beyond the scope of the present model.
Authors' remark: The title of the paper in the accepted version is above. The
title appeared in PRL is the one changed by a Senior Assistant Editor after
acceptance of the paper. The recovery of the title was rejected in the
correction process.Comment: 4 pages, 4 figure
Slow dynamics near glass transitions in thin polymer films
The -process (segmental motion) of thin polystyrene films supported
on glass substrate has been investigated in a wider frequency range from
10 Hz to 10 Hz using dielectric relaxation spectroscopy and thermal
expansion spectroscopy. The relaxation rate of the -process increases
with decreasing film thickness at a given temperature above the glass
transition. This increase in the relaxation rate with decreasing film thickness
is much more enhanced near the glass transition temperature. The glass
transition temperature determined as the temperature at which the relaxation
time of the -process becomes a macroscopic time scale shows a distinct
molecular weight dependence. It is also found that the Vogel temperature has
the thickness dependence, i.e., the Vogel temperature decreases with decreasing
film thickness. The expansion coefficient of the free volume is
extracted from the temperature dependence of the relaxation time within the
free volume theory. The fragility index is also evaluated as a function of
thickness. Both and are found to decrease with decreasing film
thickness.Comment: 9 pages, 7 figures, and 2 table
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