1,721,090 research outputs found
Including Plastic Strain Into the Discrete Preisach-Mayergoyz Space: Application to Granular Media
Full text linkThe Preisach-Mayergoyz (PM) model describes hysteretic behavior in several fields. For fractured rocks, a discretized version of the PM model successfully models nonlinear hysteretic elasticity under multiple loading cycles. In addition to hysteresis, granular media are subjected to large irreversible (plastic) deformation. To account for plastic deformation, we propose a modification of the PM density matrix, in which we include negative opening pressure. We associate negative opening pressure with rearrangements of the contact network. We apply the model to three sand samples undergoing multiple isotropic loading cycles. Calibrating the model parameters from quasi-static measurements of volumetric deformation, we estimate the quality of prediction of the dynamic bulk modulus. When this elastoplastic PM model is compared to the classic PM model, strong improvements are found both in matching the strain path and in the estimation of the dynamic bulk modulus
Tuning-fork-based piezoresponse force microscopy
No full textSurface displacements of a few picometers, occurring after application of an electric potential to piezoelectric materials, can be detected and mapped with nanometer-scale lateral resolution by scanning probe methods, the most notable being piezoresponse force microscopy (PFM). Yet, absolute determination of such displacements, giving access for instance to materials' piezoelectric coefficients, are hindered by both mechanical and electrostatic side-effects, requiring complex experimental and/or post-processing procedures for carrying out reliable results. The employment of quartz tuning-fork force sensors in an intermittent contact mode PFM is able to provide measurements of electrically-induced surface displacements that are not influenced by electrostatic side-effects typical of more conventional cantilever-based PFM. The method is shown to yield piezoeffect mapping on standard ferroelectric test crystals (periodically-poled lithium niobate and triglycine sulfate), as well as on a ferroelectric polymer (PVDF), with no visible influence from the applied dc electric potential
LOCAL DIELECTRIC SPECTROSCOPY AND ITS APPLICATION TO POLYMERS
No full textThe advent of nanodielectrics, nanocomposite materials based on a polymeric matrix, and materials with physical properties ruled by interfacial effects in general demands techniques to characterize functional properties on a local scale with high spatial resolution. Scanning probe microscopies (SPMs), in their electrical modes, have emerged as indispensable tools to access physical quantities such as dielectric constant, surface potential, and static charge, with nanometer-scale lateral resolution and with surface selectivity, being influenced mainly by the outermost layer of the specimen. In this tribute, the development of various SPM electrical modes is illustrated, focusing on the measurement of dielectric permittivity and its spectroscopic extension to access the local, frequency-dependent dielectric function (local dielectric spectroscopy [LDS]). The application to nanostructured polymers in the form of ultrathin films, nanometer-scale-separated blends, and self-assembled block copolymer structures is described. LDS appears to be a promising technique for characterizing the electric properties of polymers and their composites as well as other glass formers and nanostructured systems
Reconsidering the relation of the JG β-relaxation to the α-relaxation and surface diffusion in ethylcyclohexane
No full textAccording to the Coupling Model (CM), secondary relaxation belonging to a special class has strong connection to the α-relaxation. These Johari-Goldstein (JG) β-relaxations are predicted to be ubiquitous, and its relaxation time is approximately equal to the primitive relaxation time and the surface diffusion time. These predictions cannot be tested directly in ethylcyclohexane (ECH) because the JG β-relaxation is unresolved and overlapping the α-relaxation to modify its dispersion. To capture the actual dispersion of the α-relaxation we considered surface diffusion data, adiabatic calorimetry data of the JG β-relaxation, and dielectric data of cyanocyclohexane (CNCH). We demonstrated the dispersion of the α-relaxation is actually narrower than known before. The primitive relaxation times calculated are now in agreement with τJG(T) and with τsurface(T), and in accord with the CM predictions
Coupling of Caged Molecule Dynamics to JG β-Relaxation II: Polymers
Full text linkAt temperatures below the nominal glass transition temperature Tgα, the structural α-relaxation and the Johari-Goldstein (JG) β-relaxation are too slow to contribute to susceptibility measured at frequencies higher than 1 GHz. This is particularly clear in the neighborhood of the secondary glass transition temperature Tgβ, which can be obtained directly by positronium annihilation lifetime spectroscopy (PALS) and adiabatic calorimetry, or deduced from the temperature at which the JG β-relaxation time τβ reaches 1000 s. The fast process at such high frequencies comes from the vibrations and caged molecules dynamics manifested as the nearly constant loss (NCL) in susceptibility measurements, elastic scattering intensity, I(Q, T), or the mean-square-displacement, «u2(T)», in quasielastic neutron scattering experiment. Remarkably, we find for many different glass-formers that the NCL, I, or «u2» measured in the glassy state changes its temperature dependence at temperature THF near Tgβ. In paper I (Capaccioli, S.; et al. J. Phys. Chem. B 2015, 119 (28), 8800-8808) we have made known this property in the case of the polyalcohols and a pharmaceutical glass former, flufenamic acid studied by THz dielectric spectroscopy, and explained it by the coupling of the NCL to the JG β-relaxation, and the density dependence of these processes. In this paper II, we extend the consideration of the high frequency response to broader range from 100 MHz to THz in the glassy state of many polymers observed by quasielastic light scattering, Brillouin scattering, quasielastic neutron scattering, and GHz-THz dielectric relaxation. In all cases, the NCL changes its T-dependence at some temperature, THF, below Tgα, which is approximately the same as Tgβ. The latter is independently determined by PALS, or adiabatic calorimetry, or low frequency dielectric and mechanical spectroscopy. The property, THF Tgβ, had not been pointed out before by others or in any of the quasielastic neutron and light scattering studies of various amorphous polymers and van der Waals small molecular glass-formers over the past three decades. The generality and fundamental importance of this novel property revitalize the data from these previous publications, making it necessary to be reckoned with in any attempt to solve the glass transition problem. In our rationalization, the property arises first from the fact that the JG β-relaxation and the caged dynamics both depends on density and entropy. Second, the JG β-relaxation is the terminator of the caged dynamics, and hence the two processes are inseparable or effectively coupled. Consequently, the occurrence of the secondary glass transition at Tgβ necessarily is accompanied by corresponding change in the temperature dependence of the NCL, I, or «u2» of the fast caged dynamics at THF =Tg
Piezoelectric displacement mapping of compliant surfaces by constant-excitation frequency-modulation piezoresponse force microscopy
No full textA simple experimental method for piezoresponse force microscopy (PFM) measurements for reliable evaluation of piezoelectric surface displacements even on compliant surfaces is proposed based on atomic force microscopy (AFM) operated in frequency-modulation (FM) dynamic mode with constant excitation (CE), by using non-contact mode cantilevers. Surface displacement by piezoelectric effect after application of an electric potential to the conductive AFM probe translates into a likewise variation of the probe oscillation amplitude, while the related electrostatic forces mainly affect the oscillator resonant frequency, and cantilever bending is limited due to their high stiffness. Our non-contact CE-FM-PFM method is shown to reduce electrostatic force contributions as compared to contact-PFM modes. Converse piezoelectric effect mapping is demonstrated on poly(vinylidenefluoride) nanofibers obtained by electrospinning
The isomorphic dynamic properties of biomolecular matters and glass-forming materials
No full textGlass formation is commonly found in many different kinds of materials and systems. The conventional dynamic and thermodynamic properties considered are usually associated with the structural relaxation and the transport coefficient such as viscosity. Our studies of widely different classes of glass-forming materials over several decades have led to the discovery of processes faster than the structural relaxation are strongly connected to and inseparable from the structural relaxation in dynamic and thermodynamic properties. These faster processes include the caged molecular dynamics, and a special kind of secondary relaxation with the primitive relaxation of the Coupling Model as its precursor. Overwhelming evidences from experiments and simulations supporting this universal finding can be found in the review entitled "Universal Properties of Relaxation and Diffusion in Complex Materials: Originating from Fundamental Physics with Rich Applications", published in Prog. Mater. Sci. 2023, 139, 101130. Consequently any theory of glass transition is neither complete nor fundamental if these important faster processes have not been considered. In this paper we examine the dynamics and thermodynamic properties of dry, hydrated, and solvated proteins and biomolecules to find the presence of the faster processes and verify their strong connections to the structural relaxation. Thus the dynamics and thermodynamics of the processes in the biomolecular systems considered are isomorphic to those in ordinary glass-forming material
Electrical measurements in the 100 Hz to 10 GHz frequency range for efficient rock wettability determination
No full textFast differential scanning calorimetry: new solutions in data treatment and applications to molecular glass-formers
Full text linkFast scanning calorimetry is an experimental technique very appreciated for its capability of suppressing reorganization processes, thanks to its wide interval of scanning rates, several orders higher than that of conventional calorimeters; nevertheless, drawbacks still exist. In this paper we propose a novel way to estimate the dynamical thermal lag by using the temperatures of maximum slope of the heat flow through the glass transition when we are not in the optimal conditions to apply the existing methods based on a reference material added on both cells of the chip or on the fictive temperature. Moreover, a novel interpretation of the heat flow losses due to the sample depending on the scanning rate sign is provided, in order to rescale the measured specific heat capacity to that from conventional calorimetry. Finally, the use of the glass to liquid transition measured on heating is shown as a new manner to reveal static thermal gradients
Uso di oligonucleotidi antisenso per il trattamento di degenerazioni e neoplasie retiniche.
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