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Electrospinning serves as a versatile means of understanding the effects of strong cylindrical confinement on encapsulated liquid crystals (LCs) and is a promising technique for developing functional fabrics and surfaces. In this work, we use electrospinning to create core-sheath nanofibers composed of a polyvinylpyrrolidone (PVP) sheath doped with a photochromic, azobenzene-based surfactant, and a low-molecular weight nematic liquid crystal core. The incorporation of an azobenzene surfactant into the polymer sheath allows for photochemical control over the nematic to isotropic transition temperature of the LC core resulting in ultra violet (UV) irradiation-induced depression of the phase transition temperature. Owing to the photochromic nature of the surfactant, the photoinduced decrease in the transition temperature is reversed with visible light irradiation with no notable fatigue resistance. At high surfactant loading rates, the temperature of the photoinduced phase transition is reduced to below room temperature allowing for the birefringence of the nanofibers “on” and “off” with irradiation and requires no external heating. Based on these oberservations, we propose photoisomerization of the azobenzene-based surfactant at the PVP/LC interface leads to surface induced disorder of the LC core changing the orientation from planar axial to random with UV irradiation and reversing that change with visible irradiation. This is further substantiated by examinig the LC alignment within nanofibers by imaging the fibers with the addition of a quarter-wave retardation plate. This method adds to the growing set of tools available for the control of the phase behavior of encapsulated liquid crystals and expands the functionality of electrospun materials.

The encapsulation of liquid crystals (LCs) within semiflexible, nanoscale media has potential use for developing flexible photodisplays, electronic devices, and sensors. Here, coaxial electrospinni...

Flexible large bandgap dielectric materials exhibiting ultra-fast charging-discharging rates are key components for electrification under extremely high electric fields. A polyoxafluoronorbornene (m-POFNB) with fused five-membered rings separated by alkenes and flexible single bonds as the backbone, rather than conjugated aromatic structure typically for conventional high-temperature polymers, is designed to achieve simultaneously high thermal stability and large bandgap. In addition, an asymmetrically fluorinated aromatic pendant group extended from the fused bicyclic structure of the backbone imparts m-POFNB with enhanced dipolar relaxation and thus high dielectric constant without sacrificing the bandgap. m-POFNB thereby exhibits an unprecedentedly high discharged energy density of 7.44 J/cm(3) and high efficiency at 150 °C. This work points to a strategy to break the paradox of mutually exclusive constraints between bandgap, dielectric constant, and thermal stability in the design of all-organic polymer dielectrics for harsh condition electrifications.

Since its initial discovery more than fifteen years ago, the thermodynamical scaling of the dynamics of supercooled liquids has been used to provide many new important insights in the physics of liquids, particularly on the link between dynamics and intermolecular potential. A question that has long been discussed is whether the scaling exponent γS is a constant or does it depends on pressure. An alternative definition of the scaling parameter, γI = ∂ ln T/∂ ln ρ|X has been presented in the literature, and has been erroneously considered equivalent to γS. Here we offer a simple method to determine the pressure dependence of γI using only the pressure dependence of the glass transition and the equation of state. Using this new method we find that for the six nonassociated liquids investigated, γI always decreases with increasing pressure. Importantly in all cases the value of γI remains always larger than 4. Liquids having γI closer to 4 at low pressure show a smaller change in γI with pressure. We argue that this result has very important consequences for the experimental determination of the functional form of the repulsive part of the potential in liquids. Comparing the pressure and temperature dependence of γS and γI we find, contrary to what has been assumed in the literature to date, that these two parameters are not equivalent and have very different pressure and temperature dependences.

cited By 4; International audience; Dynamic relaxation processes are fundamental to understand the mechanical and physical properties of metallic glasses. In the current work, mechanical relaxations in a La 30 Ce 30 Al 15 Co 25 bulk metallic glass were probed by dynamic mechanical analysis. In contrast to many metallic glasses, La 30 Ce 30 Al 15 Co 25 metallic glass shows a pronounced slow β relaxation peak. Physical aging below the glass transition temperature T g leads to an increase of the apparent activation energy and a decrease of the slow β relaxation magnitude. The correlation between the slow β relaxation and the main α relaxation is discussed. © 2019

Confinement of liquid crystals (LCs) in polymeric fibers offers a promising strategy to control liquid crystal response to external stimuli. Here, the confinement of 4-cyano-4'-pentylbiphenyl (5CB), a nematic liquid crystal, within the core of coaxially electrospun fibers composed of poly(vinylpyrrolidone) (PVP) containing different surfactants is discussed. The effects of surfactant type, surfactant concentration, and core flow rate (confinement) on the LC behavior were demonstrated using polarized optical microscopy, scanning electron microscopy, differential scanning calorimetry, Raman, and dielectric spectroscopy. Introduction of surfactant dopants of varying hydrophilic and hydrophobic components into the sheath altered the interfacial interaction between the PVP sheath and the 5CB core of the fibers. Significant effects on the LC nematic to isotropic phase transition were attributed to changes in surface anchoring between the sheath and core. Confinement of nematic LCs in surfactant doped polymeric fibers demonstrates a facile method for tuning LC phase behavior.

Dielectric heating in commercial insulation materials is approximated using a simple method. Orders of magnitude differences in dielectric heating are predicted for cured VPI resins, polymer compositions used as wire coatings, and several composite insulation materials. The recommended materials characterization utilized Broadband Dielectric Thermal Analysis (DETA) on idealized laboratory samples. At increased power switching frequencies, dielectric heating may add to the total heat load in electric machinery. The concern is that the resulting increased local temperature could indicate the onset of thermal aging.

Encapsulation of the nematic liquid crystal N-(4-methoxybenzylidene)-4-butylaniline (MBBA) into the core of poly(vinylpyrrolidone) (PVP) microfibers has been accomplished via coaxial electrospinning for the first time. Data from optical microscopy, two-dimensional Raman mapping, differential scanning calorimetry, and dielectric spectroscopy have been employed to gain detailed insights into the confinement effects on MBBA in a semiflexible polymer sheath. The electrospun fiber diameters could be tuned easily by modifying the flow rate of MBBA, and both the MBBA core and PVP sheath diameters were determined via Raman line cuts. The PVP sheath induced alignment of MBBA, where its long axis is parallel to the fiber axis. Further, the alignment of MBBA led to iridescence in the fibers, which was found to be temperature-sensitive and reversible. Surprisingly, at temperatures below 0 °C, the phase-transition temperatures of confined MBBA are different than the neat and one of its smectic phases is inhibited by t...

This paper describes a fast and robust procedure for calibrating a two factor Gaussian model (G2++) with piecewise-constant volatility to a large set of options on interest rates swaps. As far as we know the algorithm implemented is novel as it overcomes some issues affecting highly parameterized models like model identification and the need to employ heuristic optimizations in order to get a good overall fitting. In this sense, our procedure extends the G2++ model ability to reproduce the term structure of volatility implied by interest rate options, similarly to how the Hull and White model extended the Vasicek model in order to perfectly fit the term structure of interest rates.

International audience; The attainable lateral resolution of electrostatic force microscopy (EFM) in an ambient air environment on dielectric materials was characterized on a reference sample comprised of two distinct, immiscible glassy polymers cut in a cross-section by ultramicrotomy. Such a sample can be modeled as two semi-infinite dielectrics with a sharp interface, presenting a quasi-ideal, sharp dielectric contrast. Electric polarizability line profiles across the interface were obtained, in both lift-mode and feedback-regulated dynamic mode EFM, as a function of probe/surface separation, for different cases of oscillation amplitudes. We find that the results do not match predictions for dielectric samples, but comply well or are even better than predicted for conductive interfaces. A resolution down to 3 nm can be obtained by operating in feedback-regulated EFM realized by adopting constant-excitation frequency-modulation mode. This suggests resolution is ruled by the closest approach distance rather than by average separation, even with probe oscillation amplitudes as high as 10 nm. For better comparison with theoretical predictions, effective probe radii and cone aperture angles were derived from approach curves, by also taking into account the finite oscillation amplitude of the probe, by exploiting a data reduction procedure previously devised for the derivation of interatomic potentials.

Recently Sanz et al. [A. Sanz, T. Hecksher, H.W. Hansen, J.C. Dyre, K. Niss, and U.R. Pedersen, Phys. Rev. Lett. 122, 055501 (2019).] reported that the scaling exponent gamma for tetramethyl-tetraphenyl-trisiloxane (DC704) varied with temperature; i.e., was not a material constant. Such a finding is at odds with previously published results on this compound and on more than 100 other liquids and polymers. The result of Sanz et al. comes from their measurement of a pressure dependence of the relaxation time at low temperature that becomes weaker with increasing pressure. Such a result is unphysical and contrary to the behavior of all known liquids. We re-measured this pressure dependence for DC704 and find it to be linear over the studied range. Thus, the conclusion of Sans et al. is incorrect; gamma for DC704 is state-point independent, in accord with other simple liquids and polymers., 2 pages, 2 figures

The local segmental and global dynamics of a series of polypropylene glycol / silica nanocomposites were studied using rheometry and mechanical and dielectric spectroscopies. The particles cause substantial changes in the rheology, including higher viscosities that become non-Newtonian and the appearance of stress overshoots in the transient shear viscosity. However, no change was observed in the mean relaxation times for either the segmental or normal mode dynamics measured dielectrically. This absence of an effect of the particles is due to masking of the interfacial response by polymer chains remote from the particles. When the unattached polymer was extracted to isolate the interfacial material, very large reductions in the relaxation times were measured. This speeding up of the dynamics is due in part to the reduced density at the interface, presumably a consequence of poorer packing of tethered chains. In addition, binding of the ether oxygens of the polypropylene glycol chains truncates the normal mode, which shifts the corresponding relaxation peak to higher frequencies., 6 figures

We studied the segmental dynamics of poly(methyl methacrylate) (PMMA) in multi-layered films with polycarbonate (PC) having layer thicknesses as small as 4 nm. Such a design provides macroscopic dimensions enabling macroscopic measurements, while maintaining nm-scale confinement. Of particular significance, we were able to determine correlation length scales for the polymer under nanoconfinement. Increases of the local segmental relaxation time, ta, and glass transition temperature, Tg, were observed with decreasing layer thickness. However, neither the fragility (Tg-normalized temperature dependence of ta) nor the breadth of the relaxation dispersion were affected by the geometric confinement. More significantly, the dynamic correlation volume, a measure of the degree of cooperativity of the dynamics, was also unaffected; that is, values of the correlation volume for confined PMMA were equal to those of the bulk polymer when compared at the same ta. This absence of an effect of geometric confinement on dynamic correlation, even when the confinement length scale approaches the correlation length scale, suggests a nonspherical correlation volume. The slowing of the segmental dynamics of PMMA confined to thin layers is due to admixing of the high Tg polycarbonate. A negligible mixing enthalpy gives rise to an extended interfacial region, which comprises a significant fraction of each layer. Published by Elsevier Ltd.

cited By 22; Although the atomic structure of amorphous alloys, which lacks long-range translational symmetry, may appear homogeneous at the macroscopic scale, their local dynamic and/or static properties however vary significantly according to the recent experimental and simulation results. In the literature of amorphous alloys, the nature of such local heterogeneities is currently an issue under debate. More importantly, since amorphous alloys are in a thermodynamically nonequilibrium state, their local structures constantly evolve during structural relaxation, physical aging and mechanical deformation. As such, local structural heterogeneities, which vary with the thermal and mechanical history of amorphous alloys, could provide a key to understand the structural origin of their mechanical behavior, such as anelasticity, viscoelasticity, plasticity and fracture. In this review article, we first review mechanical spectroscopy or dynamic mechanical analyses as an important tool to study the relaxation dynamics in amorphous alloys, with a focus on the possible correlation between the secondary (also called β)relaxation and the local structural heterogeneities of amorphous alloys. After that, we discuss the recent advances on the understanding of structural heterogeneities in metallic supercooled liquids and the influence of the structural heterogeneities on the overall mechanical properties of the corresponding amorphous alloys. Finally, we briefly discuss the further development of research on this subject. © 2019 Elsevier Ltd

In this paper we present an overview of results in the literature regarding the thermodynamical scaling of the dynamics of liquids and polymers as measured from high-pressure measurements. Specifically, we look at the scaling exponent gamma, and argue that it exhibits the limiting behavior gamma=4 in regimes for which molecular interactions are dominated by the repulsive part of the intermolecular potential. For repulsive potentials of the form U(r)= r^n, gamma has been found to be related to the exponent n via the relation gamma = n/3. Therefore, this limiting behavior for gamma would suggest that a large number of molecular systems may be described by a common repulsive potential U(r)~ r^n with n~12., Comment: accepted for publication in Journal of Chemical Physics in 2019

Interest rate models are a workhorse for risk and asset managers, in particular for those overseeing long term investments. This paper discusses the suitability of admissible Affine Term Structure Model for risk management applications, and presents some results of two models: A parsimonious specification of the A0(3) model of Dai and Singleton, and the Arbitrage-Free Nelson-Siegel model of Christensen, Diebold and Rudebush. We believe they both have sufficient desirable characteristics to be adopted by risk managers of buy-side institutions, such as insurance companies, as a benchmark for their interest rate models.

Nonlinear dielectric measurements were carried out on two strongly polar liquids, 4-vinyl-1,3-dioxolan-2-one (VPC) and 4-ethyl-1,3-dioxolan-2-one (EPC), having chemical structures differing from propylene carbonate (PC) only by the presence of a pendant group. Despite their polarity, the compounds are all non-associated, "simple" liquids. From the linear component of the dielectric response, the α relaxation peak breadth was found to be invariant at a fixed value of the relaxation time, τα. From spectra from the nonlinear component, the number of dynamically correlated molecules was determined; it was also constant at fixed τα. Thus, two manifestations of dynamic heterogeneity depend only on the time constant for structural reorientation. More broadly, the cooperativity of molecular motions for non-associated glass-forming materials is connected to (i.e., reciprocally governs) the time scale. The equation of state for the two liquids was also obtained from density measurements made over a broad range of pressures and temperatures. Using these data, it was determined that the relaxation times of both liquids conform to density scaling. The effect of density, relative to thermal effects, on the α relaxation increases going from PC < VPC < EPC.

Scattering from a cavity in a soft elastic medium, such as silicone rubber, resembles scattering from an underwater bubble in that low-frequency monopole resonance is obtainable in both cases. Arrays of cavities can therefore be used to reduce underwater sound transmission using thin layers and low void fractions. This article examines the role of cavity shape by microfabricating arrays of disk-shaped air cavities into single and multiple layers of polydimethylsiloxane. Comparison is made with the case of equivalent volume cylinders which approximate spheres. Measurements of ultrasonic underwater sound transmission are compared with finite element modeling predictions. The disks provide a deeper transmission minimum at a lower frequency owing to the drum-type breathing resonance. The resonance of a single disk cavity in an unbounded medium is also calculated and compared with a derived estimate of the natural frequency of the drum mode. Variation of transmission is determined as a function of disk tilt angle, lattice constant, and layer thickness. A modeled transmission loss of 18 dB can be obtained at a wavelength about 20 times the three-layer thickness, and thinner results (wavelength/thickness ∼ 240) are possible for the same loss with a single layer depending on allowable hydrostatic pressure.

The entry into force of the Solvency II regulatory regime is pushing insurance companies in engaging into market consistence evaluation of their balance sheet, including the financial options and guarantees embedded in life with-profit funds. The robustness of these valuations is crucial for insurance companies in order to produce sound estimates and good risk management strategies, in particular for liability driven products such as with-profit saving and pension funds. This paper introduces a simulation approach for Monte Carlo evaluation of insurance assets and liabilities, which is more suitable for risk management of liability driven products than common approaches generally adopted by insurance companies, in particular with respect to the assessment of valuation risk.

The thermal, rheological, and mechanical and dielectric relaxation properties of exfoliated dispersions of montmorillonite clay in a molecular liquid, polychlorobiphenyl (PCB), were studied. The viscosity enhancement at low concentrations of clay (≤5%) exceeded by a factor of 50 the increase obtainable with conventional fillers. However, the effect of the nanoclay on the local dynamics, including the glass transition temperature, was quite small. All materials herein conformed to density-scaling of the reorientation relaxation time of the PCB for a common value of the scaling exponent. A new relaxation process was observed in the mixtures, associated with PCB molecules in proximity to the clay surface. This process has an anomalously high dielectric strength, suggesting a means to exploit nanoparticles to achieve large electrical energy absorption. This lower frequency dispersion has a weaker dependence on pressure and density, consistent with dynamics constrained by interactions with the particle surface.

Relaxation properties of three isomers of atactic polychlorostyrene, having the chlorine atom in the ortho, meta, and para positions (P2CS, P3CS, and P4CS, respectively), were measured by mechanical and dielectric spectroscopies and modulated scanning calorimetry. For the α relaxation, the more spatially extended pendant group in P4CS gives rise to steric constraints on the segmental dynamics that cause both the temperature dependence of its relaxation time (i.e., the fragility) and the number of dynamically correlated segments to be largest. For the secondary β-relaxation in P4CS, the chlorophenyl moiety can rotate (spin) without involving the backbone; that is, the motion involves internal degrees of freedom. Consequently, this secondary relaxation is high in frequency and has a weak relaxation strength, especially as measured dielectrically. On the other hand, the pendant group in P2CS cannot reorient without adjustments in position of the backbone segments, causing its β relaxation strength measured m...

Dielectric relaxation measurements, in combination with density determinations, on tetramethyl tetraphenyl trisiloxane (DC704) over an unusually broad range of temperatures and pressures revealed a state-point dependency in its density scaling exponent. This is the first unambiguous experimental demonstration of a breakdown of density scaling in a nonassociated glass-forming material, and unanticipated for DC704, among the "simplest" of liquids, having a constant breadth of the relaxation dispersion and a Prigogine-Defay ratio near unity characteristic of approximate single-parameter systems. We speculate that the anomalous behavior has origins in the large value of its scaling exponent and relative flexibility of the chemical structure.

Enhancing dipole polarization has been demonstrated as an effective approach to increase the dielectric constant of polymer dielectrics and thus to improve their discharged energy density for energy storage applications in electrical power and electronic systems. However, further optimization to get a higher dielectric constant and lower polarization loss hinges upon a more insightful understanding of the dynamics of dipole relaxation. Here, we demonstrate an approach, based on the Dissado-Hill dielectric response model, to probe the dynamics of dipole polarization in an all organic polythiourea (PDTC-HK511) composed of p-phenylene diisothiocyanate (PDTC) as rigid segments and Jeffamine® HK511 as flexible segments. Our results reveal that PDTC-HK511 possesses strong subglass transition β and γ relaxation processes in conjunction with the quasi-DC diffusion process at relatively high temperature and a low frequency range. The γ relaxation enhances the dielectric constant by 20%–25% but causes no apparent loss at the operating temperature and frequency conditions due to the high flexibility and short relaxation time of the dipole orientation. In comparison to β relaxation, the weak interactions and much shorter relaxation time of dipoles in γ relaxation evidenced the large flexibility of dipole movement. This work provides deeper insight into the dipole movement and aids future rational designs of polymers for dielectric energy storage.

The morphology, relaxation properties, and conductivity of the statistical copolymer polystyrenic (alkoxy 1H-tetrazole-co-alkoxy nitrile), an anhydrous proton conductor, were measured. The material...

Dielectric spectra were obtained on a low molecular weight poly(methyl methacrylate) (PMMA) over a range of temperatures (331 < T (K) < 386) at pressures approaching 0.8 GPa. The α relaxation times...

An investigation of the recombination kinetics and the density of states distribution in bulk heterojunction organic photovoltaic cells was undertaken using impedance spectroscopy measurements under varying levels of illumina- tions. Three organic polymer solar cells were investigated which differed only by the p-type polymer used in the active layers while the device architecture and acceptor molecule (PC70BM) were kept the same. We found that the density of states profiles for the three cells are similar and can be superimposed by a horizontal shift due to a difference between the polymers' HOMO and the PC70BM's LUMO levels. The recombination mechanisms for the three cells showed the same behavior, suggesting an important role of the acceptor in this process; however, large differences in the absolute value of carrier lifetime τeff related to the choice of the polymer are evident, indicating that the recombination kinetics depends on a combination of the properties of both materials and the morphology of their blend.

The magnitude of the high frequency, static dielectric permittivity is used to determine the density of tetramethyl tetraphenyl trisiloxane, a non-associated glass-forming liquid, as a function of temperature and pressure. We demonstrate that the properties in the glassy state are affected by the pressure applied to the liquid during vitrification. This behavior is normal for hydrogen-bonded liquids and polymers, but unanticipated by models of simple liquids., Comment: 7 pages, 3 figures

An experimental study was carried out of various factors affecting the ballistic penetration resistance of elastomer/steel bilayers. For blunt penetrators, the contribution of the coating to performance is optimized using the hardest substrates, front surface placement of the elastomer, and (when normalizing by added weight) thin, ca. 2–3 mm, coatings. These results, none of which are predicted by existing models, evince the marked coupling of coating and substrate in the impact response of the bilayer. We also show that nanoparticle fillers have a modest effect on ballistic performance of polyurea coatings, changing the penetration velocity by a few percent or less. This contrasts with the linear dynamic mechanical behavior, which shows much more significant increases in energy absorption due to nano-reinforcement.

To analyze the Johari–Goldstein (JG) secondary process, dielectric relaxation measurements were carried out on poly(methyl methacrylate) (PMMA) and poly(thiomethyl methacrylate) (PTMA). The latter has a sulfur atom replacing the oxygen in the ester of PMMA, making the pendant group much less polar. This weakens the intensity of the secondary relaxation, enabling facile resolution of the JG and primary structural relaxation peaks. We find that the JG relaxation time of PTMA has a substantially larger activation energy (factor of 2) in the liquid compared to the glassy state. Although deconvolution of the peaks in PMMA is more ambiguous due to the large dielectric strength of its JG relaxation, applying the same analysis leads to a very similar result. In light of previous results, we conclude very generally from their temperature-dependence that JG secondary motions sense the glass transition, consistent with their role as precursor to structural relaxation.

The effects of multiwall carbon nanotubes (MWCNT), nanolayered silicate (nanoclay), and trisilanolphenyl-functionalized polyhedral oligomeric silsesquioxane (POSS) on the rheology and mechanical properties of an oligomeric polydiamine and the polyurea formed by its reaction with isocyanate were measured. The MWCNT and nanoclay increase the viscosity of the polydiamine and form a flocculated filler network at very low concentrations (

Dielectric relaxation and complex heat capacity were carried out on high molecular weight poly(methyl acrylate) (PMA). The temperature was found to exert a stronger effect than density on the segme...

Under standing the origin of the dramatic temperature and density dependence of the relaxation time of glass-forming liquids is a fundamental challenge in glass science. The recently established ‘density-scaling’ relation quantifies the relative importance of temperature and density for the relaxation time in terms of a material-dependent exponent. We show that this exponent for approximate single-parameter liquids can be calculated from thermoviscoelastic linear-response data at a single state point, for instance an ambient-pressure state point. This prediction is confirmed for the van der Waals liquid tetramethyl-tetraphenyl-trisiloxane. Consistent with this, a compilation of literature data for the Prigogine‐Defay ratio shows that van der Waals liquids and polymers are approximate single-parameter systems, whereas associated and network-forming liquids are not.

Dielectric relaxation times were measured for 1,4-polyisoprenes (PI) of different molecular weight. From the data, the number of dynamically correlated segments, Nc, was calculated using an approximation to the dynamic susceptibility. Nc increases with approach to the glass transition in the usual fashionandalsoincreaseswithincreasingmolecularweightofthePI.Thelattereffectisascribedtothelossof the configurational mobility conferred by the chain ends. The correlation volume was also estimated from calorimetry and,because PI hasadielectrically active normal mode,from theintersection oftheextrapolated segmentalandnormalmoderelaxationtimes.Thethreemethodsyieldconsistentresults,althoughthelasthas large uncertainty due to the ambiguous connection between dynamic correlation lengths and volumes. Using the equation of state for the polymers, the dependence of the relaxation times on the scaling variable TV γ , where V is specific volume and γ is a material constant, was calculated. For the lowest molecular weight PI, there is a small difference in γ for the segmental and chain modes. The scaling exponent is also marginally smaller for the lower molecular weight sample, suggesting, in contrast with the behavior of other polymers, that in PI the volume dependence becomes weaker with decreasing molecular weight.

Dielectric relaxation of a low molecular weight PMMA was measured in both the equilibrium and glassy states. The intense β process, due to motion of the pendant ester moiety, exhibits a change in T -dependence of both its relaxation time and dielectric strength at T g , reflecting the coupling of this high frequency process to the backbone conformational changes underlying the α dynamics. Consequently, we can exploit the change in the properties of the β process during physical aging to quantify the α relaxation time and its T -dependence for glassy PMMA. The obtained values, τ α > 10 4 , are too long for direct measurement.

The effect of network formation on the secondary (Johari–Goldstein) β-relaxation was investigated for polyvinylethylene (PVE). Crosslinking affects the segmental (α-) process in the usual fashion, the networks exhibiting slower and more temperature-sensitive dynamics. However, the effect on the β-process is the opposite. The secondary relaxation becomes faster and the activation energy slightly decreases with crosslinking. The strength of the intermolecular cooperativity governing the behavior of the α-process was assessed using the coupling model, with consistent results obtained from analysis of both the timescale separating the α- and β-relaxations and the activation energy for the latter. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 582–587, 2010

Decahydroisoquinoline (DHIQ) appears to be a unique material-the only non-associated, simple liquid with dynamics deviating from density scaling. To examine whether this anomaly is real, the density, ρ, of DHIQ was measured at temperatures, T, as low as 214 K and pressures up to ∼1.2 GPa. This enabled the equation of state (EoS) to be determined, without extrapolation, over the range of thermodynamic conditions for which the relaxation times had been reported. Using this less ambiguous EoS, we find that within the precision of the available relaxation times, the latter are a function of T/ρ(3.9), contrary to previous reports. Thus, the behavior of DHIQ is unexceptional; similar to every non-associated liquid tested to date, its dynamics comply with density scaling.

This work analyses the common industry practice used to evaluate financial options written on with-profit policies issued by European insurance companies. In the last years regulators introduced, with the Solvency II directive, a market consistent valuation framework for determining the fair value of asset and liabilities of insurance funds. A relevant aspect is how to deal with the estimation of sovereign credit and liquidity risk, that are important components in the valuation of the majority of insurance funds, which are usually heavily invested in treasury bonds. The common practice is the adoption of the certainty equivalent approach (CEQ) for the risk neutral evaluation of insurance liabilities, which results in a deterministic risk adjustment of the securities cash flows. In this paper, we propose an arbitrage free stochastic model for interest rate, credit and liquidity risks, that takes into account the dependences between different government bond issuers. We test the impact of the common practice against our proposed model, via Monte Carlo simulations. We conclude that in the estimation of options whose pay-off is determined by statutory accounting rules, which is often the case for European traditional with-profit insurance products, the deterministic adjustment for risk of the securities cash flows is not appropriate, and that a more complete model such as the one described in this article is a viable and sensible alternative in the context of market consistent evaluations.

cited By 8; International audience; Zn38Mg12Ca32Yb18 bulk metallic glass (BMG) presents higher corrosion resistance, lower glass transition temperature and lower density than most BMGs. Dynamic properties of Zn38Mg12Ca32Yb18 BMG were investigated by mechanical spectroscopy. The structural (α) process in the isothermal spectra is well described by a Kohlrausch-Williams-Watts (KWW) function with βKWW = 0.505. At high frequency, an extra contribution to the α process (i.e. an excess wing) is observed with a much smaller frequency dependence of the loss shear modulus G″(f) ∼ f- 0.37, indicating the presence of a submerged secondary peak. The temperature behaviour of the α relaxation time, τα, is characterized by a very small steepness index (i.e. strong behaviour). We find that results for this BMG show a deviation of the correlation between fragility and βKWW generally found in glass formers. Results in the literature for other BMGs suggest that this deviation may not be an exception for BMGs, since very similar values of βKWW (∼ 0.5) have been found for BMGs independently of their steepness index. © 2016 Elsevier B.V. All rights reserved.

Dielectric spectroscopy is used to measure the local segmental relaxation times for the soft segments of a polyurea as a function of temperature and pressure. In combination with the equation of state determined for the material, we show that the relaxation times are uniquely defined by the product of temperature times specific volume, with the latter raised to the power of 2.35 ± 0.10. This superpositioning of the relaxation times enables both the local segmental and the global chain dynamics to be calculated for any combination of temperature and pressure, using only measurements at ambient pressure. Since this polyurea finds applications as a coating to mitigate the damage from impact loading, its response to high frequencies and elevated hydrostatic pressure is of some import.

Pressure−volume−temperature measurements, calorimetry, and dielectric spectroscopy at ambient and elevated pressures were carried out on poly(cyclohexyl methacrylate) (PCHMA) and its blend with poly(α-methylstyrene) (PaMS). Both the glass transition temperature and local segmental relaxation times, τα, go through a maximum as a function of blend composition, plausibly due to a negative excess volume. Using a nonlinear function for the composition dependence of Tg, a value of the self-concentration parameter of the Lodge−McLeish model is determined, which is close to that calculated from the PCHMA chain statistics. For both neat PCHMA and the blend, the τα superpose as a function of TVγ. Since the breadth of the local segmental relaxation dispersion is a unique function of τα, this means that the parameter γ defines both the magnitude and distribution of relaxation times. The thermodynamic scaling for the blend was corroborated by calculating the exponent γ from the isochronal thermal expansion coefficient...

Pressure–volume–temperature (PVT) measurements were obtained on decahydroisoquinoline (DHIQ). From the resulting equation of state in combination with previously reported dielectric relaxation data, an analysis of the volume contribution to the dynamics was carried out for this very fragile glass-former (isobaric fragility, mP = 163). We find that the ratio of the isochoric and isobaric fragilities equals 0.71 ± 0.02, reflecting a strong temperature effect, consistent with the large fragility. The relaxation times superpose when plotted versus temperature times the specific volume raised to the 3.55 power. This is a larger value of the exponent than expected based on the fragility of DHIQ. The implication is that for molecular glass-formers, the classical Lennard-Jones 6–12 repulsive potential may represent the limit for fragile liquids.

Dielectric spectroscopy was used to follow the component dynamics in the miscible blend of poly(vinyl methyl ether) (PVME) and poly(2-chlorosytrene) (P2CS). For the PVME component (which has the more intense loss peak due to its higher polarity), the shape of the segmental relaxation peak depends only on the relaxation time and is otherwise independent of thermodynamic conditions. This is in accord with the general behavior of neat materials. By measuring the spectra as a function of both temperature and pressure, the relative effect of temperature and volume on the segmental relaxation times was quantified from ratio of the isochoric and isobaric activation enthalpies. This ratio is essentially the same for the neat polymers, but blending has a disparate effect of the components: volume effects become stronger for the PVME but are diminished for P2CS. Similarly, the fragilities of the neat components are quite close but change markedly in the blend. The P2CS component has a fragility of 32, which is low...

We employ temperature modulated DSC (TMDSC) to determine the dependence of the fictive temperature on cooling rate for a series of polychlorinated biphenyls (PCB). From the slopes of semi-logarithmic plots of cooling rate vs. fictive temperature, the latter normalized by the fictive temperature for an arbitrary cooling rate, we determine the enthalpic fragilities. Despite significant differences in glass transition temperature and chemical structure (specifically chlorine content), the PCB have the same fragility. The value of the fragility determined using TMDSC is consistent with the fragility previously determined using dielectric relaxation spectroscopy.

The relative effect of volume and thermal energy on the local segmental dynamics, as reflected in the ratio of the isochoric, EV, and isobaric, EP, activation enthalpies, is determined for blends of polystyrene (PS) with poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) and with poly(vinyl methyl ether) (PVME). We find that neat PPO near Tg has the lowest value of EV/EP = 0.25 ± 0.02 reported for any polymer, indicating volume-dominated dynamics. Addition of the lower Tg PS alleviates constraints on local motion, resulting in a weaker volume effect. The opposite situation prevails with PS/PVME blends. PS has a higher Tg, and in blends segmental relaxation of the PVME becomes more controlled by volume than for neat PVME. We also show herein that the relaxation times for the PVME/PS blends measured at various T and P superpose when plotted vs Tυγ, where υ is the specific volume and γ is a material constant. This scaling, which has previously been demonstrated for various neat glass-formers and is reported herein...

The different nature of multiple secondary (β- and γ-) relaxations for the glass-formers dipropyleneglycol-dibenzoate and benzoin-isobutylether has been investigated by varying temperature and pressure in the equilibrium liquid and glassy states, as well as by monitoring the structural recovery after a rapid quenching of the liquid to form a glass. For both systems the behavior of the slower β-relaxation mimicked that of the α-relaxation: its time scale was strongly affected by pressure variation and its behavior below T g was sensitive to thermal history and to aging. On the contrary the γ-relaxation time was negligibly dependent on pressure and on aging. The correlation between the dynamic properties of the α-process and those of the slower β-process indicates that the latter was likely the genuine Johari–Goldstein (JG) intermolecular relaxation. Moreover, a good agreement was found between the β-relaxation time and the independent relaxation time τ 0 of the coupling model (CM), leading to predictive quantitative relations between the JG and the structural relaxation, supported also by the experimental data of many other systems, in both liquid and glassy state.

A central question concerning glass-formation has been what governs the kinetic arrest of the quenched liquid – cooling reduces the thermal energy which molecules need to surmount local potential barriers, while the accompanying volume contraction promotes molecular crowding and congestion (and thus alters the potential). Recent experimental findings have shown that both thermal energy and density contribute significantly to the temperature-dependence of vitrifying liquids. Herein, we show that the scaling (superpositioning) of the relaxation times near the glassy state, by expressing them as a function of temperature and the specific volume, leads to a modification of the usual fragility curves, whereby differences in the extent of departure from Arrhenius behavior can be rationalized. More intriguingly, the characteristic changes in the relaxation properties (i.e., the ‘dynamic crossover’), occurring well above the liquid-to-glass transition, are shown to be related to the same function that superposes the relaxation data.

An intriguing problem in condensed matter physics is understanding the glass transition, in particular the dynamics in the equilibrium liquid close to vitrification. Recent advances have been made by using hydrostatic pressure as an experimental variable. These results are reviewed, with an emphasis in the insight provided into the mechanisms underlying the relaxation properties of glass-forming liquids and polymers.