419 research outputs found
Comparison of methods for estimating continuous distributions of relaxation times
The nonparametric estimation of the distribution of relaxation times approach
is not as frequently used in the analysis of dispersed response of dielectric
or conductive materials as are other immittance data analysis methods based on
parametric curve fitting techniques. Nevertheless, such distributions can yield
important information about the physical processes present in measured
material. In this letter, we apply two quite different numerical inversion
methods to estimate the distribution of relaxation times for glassy \lila\
dielectric frequency-response data at 225 \kelvin. Both methods yield unique
distributions that agree very closely with the actual exact one accurately
calculated from the corrected bulk-dispersion Kohlrausch model established
independently by means of parametric data fit using the corrected modulus
formalism method. The obtained distributions are also greatly superior to those
estimated using approximate functions equations given in the literature.Comment: 4 pages and 4 figure
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
Ion and polymer dynamics in polymer electrolytes PPO-LiClO4: II. 2H and 7Li NMR stimulated-echo experiment
We use 2H NMR stimulated-echo spectroscopy to measure two-time correlation
functions characterizing the polymer segmental motion in polymer electrolytes
PPO-LiClO4 near the glass transition temperature Tg. To investigate effects of
the salt on the polymer dynamics, we compare results for different ether oxygen
to lithium ratios, namely, 6:1, 15:1, 30:1 and infinity. For all compositions,
we find nonexponential correlation functions, which can be described by a
Kohlrausch function. The mean correlation times show quantitatively that an
increase of the salt concentration results in a strong slowing down of the
segmental motion. Consistently, for the high 6:1 salt concentration, a high
apparent activation energy E_a=4.1eV characterizes the temperature dependence
of the mean correlation times at Tg < T< 1.1T_g, while smaller values E_a=2.5eV
are observed for moderate salt contents. The correlation functions are most
nonexponential for 15:1 PPO-LiClO4, whereas the stretching is reduced for
higher and lower salt concentrations. A similar dependence of the correlation
functions on the evolution time in the presence and in the absence of ions
indicates that addition of salt hardly affects the reorientational mechanism.
For all compositions, mean jump angles of about 15 degree characterize the
segmental reorientation. In addition, comparison of results from 2H and 7Li NMR
stimulated-echo experiments suggests a coupling of ion and polymer dynamics in
15:1 PPO-LiClO4.Comment: 14 pages, 12 figure
From caging to Rouse dynamics in polymer melts with intramolecular barriers: a critical test of the Mode Coupling Theory
By means of computer simulations and solution of the equations of the Mode
Coupling Theory (MCT), we investigate the role of the intramolecular barriers
on several dynamic aspects of non-entangled polymers. The investigated dynamic
range extends from the caging regime characteristic of glass-formers to the
relaxation of the chain Rouse modes. We review our recent work on this
question, provide new results and critically discuss the limitations of the
theory. Solutions of the MCT for the structural relaxation reproduce
qualitative trends of simulations for weak and moderate barriers. However a
progressive discrepancy is revealed as the limit of stiff chains is approached.
This disagreement does not seem related with dynamic heterogeneities, which
indeed are not enhanced by increasing barrier strength. It is not connected
either with the breakdown of the convolution approximation for three-point
static correlations, which retains its validity for stiff chains. These
findings suggest the need of an improvement of the MCT equations for polymer
melts. Concerning the relaxation of the chain degrees of freedom, MCT provides
a microscopic basis for time scales from chain reorientation down to the caging
regime. It rationalizes, from first principles, the observed devations from the
Rouse model on increasing the barrier strength. These include anomalous scaling
of relaxation times, long-time plateaux, and non-monotonous wavelength
dependence of the mode correlators.Comment: 15 pages, 14 figure
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
Loading rate and contraction type effects on the human Achilles tendon force-elongation relationship
IntroductionWhile it is accepted that tendons are viscoelastic, the loading rate of contractions is often not strictly controlled when assessing human tendon mechanical properties in vivo. Given the potential benefits of sustained constant load isometric contractions for in vivo tendon property assessment, we aimed to determine if sustained submaximal isometric plantarflexion contractions result in a similar force-elongation relationshipand stiffness of the Achilles tendon (AT) to other loading methods.MethodsThe AT mechanical properties (elongation and stiffness) of the dominant leg inten male adults (26.5±5.5y) were assessed during isometric plantarflexion contractions by integrating dynamometry and ultrasonography (Aloka α7, Tokyo, Japan).Measurements were taken on two consecutive days and the results from all participants on day one and seven participants on day two (three excluded due to measurement problems) were pooled for the analysis. Maximum voluntary contractions (MVC; high loading rate), ramp maximum force contractions with three seconds loading (RAMP; lower loading rate), and sustained contractions (held for three seconds) at forces of 25%, 50% and 80% of the maximal tendon force with the lower loading rate (SUS) were conducted.ResultsA two way repeated measures ANOVA with method and tendon force level as factors revealed a significant method (P<0.001) effecton tendon elongation. Post hoc tests with Bonferroni corrections revealed significantlygreater tendon elongation in SUS compared withMVC (P=0.001) and RAMP(P=0.002),but no differences in tendon elongationbetween MVC and RAMP(P=0.077).A one way ANOVA with method as a factor did not reveal a significant method effect on tendon stiffness(P=0.079; MVC: 653.6±220.9 N/mm; RAMP: 694.8±190.3 N/mm; SUS: 564.2±148.1 N/mm).ConclusionSustained plantarflexion contractions appear to lead to an increased AT elongation for a given force, presumably due to the reduced influence of the loading rate on the viscoelastic behaviour of the tendon during the sustained contractions. However,AT stiffness was not significantly different between methods, suggesting that the differences in the rate of elongation occurred prior to the linear region of the force elongation relationship. Sustained isometric contractions appear to be appropriate for assessing AT stiffness in vivo, although potential differences in tendon elongation should be considered when comparing results with other methods
Extracting spectral density function of a binary composite without a-priori assumption
The spectral representation separates the contributions of geometrical
arrangement (topology) and intrinsic constituent properties in a composite. The
aim of paper is to present a numerical algorithm based on the Monte Carlo
integration and contrainted-least-squares methods to resolve the spectral
density function for a given system. The numerical method is verified by
comparing the results with those of Maxwell-Garnett effective permittivity
expression. Later, it is applied to a well-studied rock-and-brine system to
instruct its utility. The presented method yields significant microstructural
information in improving our understanding how microstructure influences the
macroscopic behaviour of composites without any intricate mathematics.Comment: 4 pages, 5 figures and 1 tabl
Direct observation of molecular cooperativity near the glass transition
We describe direct observations of molecular cooperativity near the glass
transition in poly-vinyl-acetate (PVAc), through nanometer-scale probing of
dielectric fluctuations. Molecular clusters switched spontaneously between two
to four distinct configurations, producing complex random-telegraph-signals
(RTS). Analysis of the RTS and their power spectra shows that individual
clusters exhibit both transient dynamical heterogeneity and non-exponential
kinetics.Comment: 14 pages pdf, need Acrobat Reade
Free volume dilatation in polymers by ortho-positronium
The possibility of positronium induced free volume cavity expansion in some polymers above the glass transition temperature was investigated using experimental positron annihilation lifetime data from the literature for polydimethylsiloxane, polyisobutylene, and polybutadiene as function of temperature. The results suggest that free volume sites can expand towards an equilibrium size, determined as the equilibrium Ps-bubble size defined earlier for low-molecular-weight liquids. The expansion can be explained by the increase of molecular mobility and hence decrease of relaxation times, which at the higher temperatures approach the o-Ps lifetimes. Nanoscale viscosities were estimated using Navier-Stokes equation and were found to be several orders of magnitude lower than the macroscopic viscosity at the same temperature
Additive Manufacturing and Performance of Architectured Cement-Based Materials
There is an increasing interest in hierarchical design and Additive Manufacturing (AM)of cement-based materials. However, the brittle behavior of these materials and the presence of interfaces from the additive manufacturing process represent the current major challenges. Our work focuses on harnessing the heterogeneous interfaces by employing clever designs from bio-inspired Bouligand architectured materials. In this paper, we aim to demonstrate some key mechanisms that can allow brittle hardened cement-based materials to gain flaw-tolerant properties. Mechanisms such as cracktwisting at the interfaces have been previously observed in naturally-occurring orsynthetic composite Bouligand architectures. In this paper, a heterogeneous interface with porous characteristics in 3D-printed solid hardened cement paste (hcp)architectures were characterized. We hypothesize that the presence of heterogeneous interface in 3D-printed hardened cement paste (hcp) elements, in conjunction with clever architectures, promote key damage mechanisms such as interfacial cracking and crack twisting that lead to damage delocalization. This delocalization can be energetically favorable and allow energy dissipation and promote toughening and flaw-tolerant properties. We found that these architectures can enhance the properties from the typical strength-porosity relationship, classically known for brittle hcp materials
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