Correlation between Sub-Tg relaxation processes and mechanical behavior for different hydrothermal ageing conditions in epoxy assemblies

The aim of this study is to understand aging phenomena by monitoring physical parameters after real and simulated aging experiments. This study focuses on aluminum-epoxy assemblies, which are commonly used on spacecraft structures. Different samples are submitted to simulated aging tests. Influence of temperature and moisture is analyzed. Evolution with aging is characterized at two different scales. The macroscopic behavior of the assemblies is studied by single lap shear test. A decrease in the shear rupture stress is observed with increasing temperature and relative humidity. It is demonstrated that temperature has more important influence. The molecular behavior in the adhesive joint is studied by dynamic dielectric spectroscopy measurements. This experiment gives access to molecular mobility in the adhesive. Dipolar entities are identified as evolving with aging conditions. The temperature is more effective than moisture at this scale. An interpretation of the molecular mobility before and after aging shows that water is an important parameter of this study. A link between mechanical and molecular behavior with hydrothermal aging is found. The decrease of mechanical properties occurs while failures become interfacial. In the same time, the interactions between hydroxyether and water increase. The evolution of the macroscopic behavior of the bonded assemblies is due to this combination observed at different scales

Microwave Heating of Water, Ice and Saline Solution: Molecular Dynamics Study

In order to study the heating process of water by the microwaves of 2.5-20GHz frequencies, we have performed molecular dynamics simulations by adopting a non-polarized water model that have fixed point charges on rigid-body molecules. All runs are started from the equilibrated states derived from the I$_{c}$ ice with given density and temperature. In the presence of microwaves, the molecules of liquid water exhibit rotational motion whose average phase is delayed from the microwave electric field. Microwave energy is transferred to the kinetic and inter-molecular energies of water, where one third of the absorbed microwave energy is stored as the latter energy. The water in ice phase is scarcely heated by microwaves because of the tight hydrogen-bonded network of water molecules. Addition of small amount of salt to pure water substantially increases the heating rate because of the weakening by defects in the water network due to sloshing large-size negative ions.Comment: 21 pages, 13 figure

Spectral responses in granular compaction

The slow compaction of a gently tapped granular packing is reminiscent of the low-temperature dynamics of structural and spin glasses. Here, I probe the dynamical spectrum of granular compaction by measuring a complex (frequency-dependent) volumetric susceptibility $\tilde{\chi}_v$. While the packing density $\rho$ displays glass-like slow relaxations (aging) and history-dependence (memory) at low tapping amplitudes, the susceptibility $\tilde{\chi}_v$ displays very weak aging effects, and its spectrum shows no sign of a rapidly growing timescale. These features place $\tilde{\chi}_v$ in sharp contrast to its dielectric and magnetic counterparts in structural and spin glasses; instead, $\tilde\chi_v$ bears close similarities to the complex specific heat of spin glasses. This, I suggest, indicates the glass-like dynamics in granular compaction are governed by statistically rare relaxation processes that become increasingly separated in timescale from the typical relaxations of the system. Finally, I examine the effect of finite system size on the spectrum of compaction dynamics. Starting from the ansatz that low frequency processes correspond to large scale particle rearrangements, I suggest the observed finite size effects are consistent with the suppression of large-scale collective rearrangements in small systems.Comment: 18 pages, 17 figures. Submitted to PR

Power-law decay in first-order relaxation processes

Starting from a simple definition of stationary regime in first-order relaxation processes, we obtain that experimental results are to be fitted to a power-law when approaching the stationary limit. On the basis of this result we propose a graphical representation that allows the discrimination between power-law and stretched exponential time decays. Examples of fittings of magnetic, dielectric and simulated relaxation data support the results.Comment: to appear in Phys. Rev. B; 4 figure

Study of the dielectric relaxation of poly(phenylpropyl acrylate) and poly(phenylpropyl methacrylate): effect of slight differences in chemical structure

A comparative study of the dielectric relaxational behaviour of two structurally close polymers, containing aromatic side groups, was carried out in order to analyse how slight differences in the chemical structure affect the molecular responses to perturbation field. Specifically, poly(phenylpropyl acrylate) (P3Ph1PA) and poly(phenylpropyl methacrylate) (P3Ph1PM) were studied using differential scanning calorimetry and dielectric relaxation spectroscopy in the frequency range 10−2 106 Hz and temperature window of −80 to 120 ∘C. Both techniques show one glass rubber transition for P3Ph1PA and two for P3Ph1PM, which evidence the great effect of the methyl groups on the segmental motions of the polymer. Phenomenological analysis of the datawas carried out in order to establish the strength, width and fragility parameters of the glass rubber transitions. In the case of P3Ph1PA, the strength is found to be larger than for P3Ph1PM, pointing out that themethyl group disturbs the mobility. Conductive processes dominate the dielectric spectra at high temperatures and low frequencies.BR-F, PO-S, MC and MJS gratefully acknowledge CICYT for grant MAT2012-33483. DR and AL acknowledge Fondecyt grant 1120091.Ortiz Serna, MP.; Sanchis Sánchez, MJ.; Redondo Foj, MB.; Carsí Rosique, M.; Díaz Calleja, R.; Leiva, A.; Gargallo, L.... (2015). Study of the dielectric relaxation of poly(phenylpropyl acrylate) and poly(phenylpropyl methacrylate): effect of slight differences in chemical structure. Polymer International. 64(12):1733-1740. https://doi.org/10.1002/pi.4973S17331740641

Analytical representations for relaxation functions of glasses

Analytical representations in the time and frequency domains are derived for the most frequently used phenomenological fit functions for non-Debye relaxation processes. In the time domain the relaxation functions corresponding to the complex frequency dependent Cole-Cole, Cole-Davidson and Havriliak-Negami susceptibilities are also represented in terms of $H$-functions. In the frequency domain the complex frequency dependent susceptibility function corresponding to the time dependent stretched exponential relaxation function is given in terms of $H$-functions. The new representations are useful for fitting to experiment.Comment: 6 pages, Late

Origin of temperature dependent conductivity of α\alpha-polyvinylidene fluoride

The conductivity of $\alpha$-polyvinylidene fluoride ($\alpha$-PVDF) is obtained from dielectric measurements performed in the frequency domain at several temperatures. At temperatures above the glass-transition, the conductivity can be interpreted as an ionic conductivity, which confirms earlier results reported in the literature. Our investigation shows that the observed ionic conductivity is closely related to the amorphous phase of the polymer

Experimental search for dynamic heterogeneities in molecular glass formers

We have measured the linear dielectric susceptibility of two molecular glass formers close to Tg in order to estimate the size of the dynamically correlated clusters of molecules which are expected to govern the physics of glass formation. This size has been shown to be related to the dynamic dielectric susceptibility dEps(w)/dT (Eps : dielectric susceptibility, T : temperature, w&#61472;: frequency). To allow for an accurate determination of the T derivative, we scanned the interval 192 < T < 232 K every 1 K for glycerol and 159 < T < 179 K every 0.5 K for propylene carbonate. The resolution on T variations was about 1 mK. The result for glycerol is that the number of correlated molecules increases by a factor 3 when T goes from 226 to 195 K. It has been shown that the non-linear susceptibility provides a direct measurement of dynamic correlations. To measure it, we used a standard Lockin technique yielding the third harmonic of the current flowing out of a capacitor. We obtained only an upper limit on the ratio of the third to the first harmonic, due to the non-linear response of standard electronics.Comment: 7 page

Dielectric relaxation of DNA aqueous solutions

We report on a detailed characterization of complex dielectric response of Na-DNA aqueous solutions by means of low-frequency dielectric spectroscopy (40 Hz - 110 MHz). Results reveal two broad relaxation modes of strength 20<\Delta\epsilon_LF<100 and 5<\Delta\epsilon_HF<20, centered at 0.5 kHz<\nu_LF<70 kHz and 0.1 MHz<\nu_HF<15 MHz. The characteristic length scale of the LF process, 50<L_LF<750nm, scales with DNA concentration as c_DNA^{-0.29\pm0.04} and is independent of the ionic strength in the low added salt regime. Conversely, the measured length scale of the LF process does not vary with DNA concentration but depends on the ionic strength of the added salt as I_s^{-1} in the high added salt regime. On the other hand, the characteristic length scale of the HF process, 3<L_HF<50 nm, varyes with DNA concentration as c_DNA^{-0.5} for intermediate and large DNA concentrations. At low DNA concentrations and in the low added salt limit the characteristic length scale of the HF process scales as c_DNA^{-0.33}. We put these results in perspective regarding the integrity of the double stranded form of DNA at low salt conditions as well as regarding the role of different types of counterions in different regimes of dielectric dispersion. We argue that the free DNA counterions are primarily active in the HF relaxation, while the condensed counterions play a role only in the LF relaxation. We also suggest theoretical interpretations for all these length scales in the whole regime of DNA and salt concentrations and discuss their ramifications and limitations.Comment: 15 pages, 9 figure

AC and DC conductivity correlation: The coefficient of Barton--Nakajima--Namikawa relation

It has been some time since an empirical relation, which correlates DC with AC conductivity and contains a loosely defined coefficient thought to be of order one, was introduced by Barton, Nakajima and Namikawa. In this work, we derived this relation assuming that the conductive response consists of a superposition of DC conductivity and an AC conductivity term which materialized through a Havriliak--Negami dielectric function. The coefficient was found to depend on the Havriliak--Negami shape parameters as well as on the ratio of two characteristic time scales of ions motion which are related to ionic polarization mechanism and the onset of AC conductivity. The results are discussed in relation to other relevant publications and they also applied to a polymeric material. Both, theoretical predictions and experimental evaluations of the BNN coefficient are in an excellent agreement, while this coefficient shows a gradual reduction as the temperature increases.Comment: 15 pages plain latex2e, 5 eps figures (new figures added). In this revised version the manuscript has been rewritten extensively due to motivational comments and suggestions made by the referee. Accepted for publication by the Journal of Non--Crystalline Solid