Dynamic Dielectric Spectroscopy Research Articles

Tuning dielectric response of polyethylene by low gamma dose: Molecular mobility study improvement by dipolar probes implementation

Dynamic Dielectric Spectroscopy is a powerful method to study the molecular mobility of dipolar polymers. As Linear Low Density Polyethylene is globally non polar, the dielectric experimental signal needs to be seriously improved. The aim of this study is to implement carbonyl groups by gamma irradiation, that act as dipolar probes in order to increase the signal to noise ratio. A specific attention is paid to induce as less modification as possible. Films were gamma irradiated in dioxygen media with different absorbed doses from 0 kGy to 100 kGy. After irradiation, better signal to noise ratios have been obtain thanks to polar probes introduction. The study of dielectric relaxation modes is then possible, extended and much more precise. One should irradiate the polymer with the lowest dose, between 12.5 kGy and 25 kGy, to obtain satisfactory Dynamic Dielectric Spectroscopy results with virtually no chemical and physical changes in the gamma irradiated samples.

Dynamic dielectric properties of isotactic polypropylene-g-maleic anhydride crosslinked by capped-end polyether diamine and filled with native or functionalized nano-graphite particles

Dynamic dielectric properties of an isotactic polypropylene matrix grafted with maleic anhydride (CA 100) and then crosslinked by polyether amine molecules and reinforced with different weight percentages of graphite nanoplatelets (GNPs), KNG180, were studied for the first time and compared to those obtained by DMA (Dynamic Mechanical Analysis). The main objective of this work was to investigate the reinforcement effect of GNPs focusing on the GNPs/matrix interfacial adhesion using dynamic dielectric relaxation spectroscopy in the frequency range from 0.1 Hz to 1 MHz and temperature range from 20 to 140 °C. The obtained interfacial polarization increments \({\Delta \varepsilon }_{\mathrm{MWS}}\) from MWS (Maxwell Wagners Sillars) relaxation showed a threshold value of 3% in weight of KNG180. This analysis suggests that interfacial compatibility between matrix and fillers in the case of nanocomposite KNG180 3 wt% is higher than those of other nanocomposites. A new plasma treatment was used to modify graphite nano-fillers to produce different types of nanocomposites. The 5 wt% plasma treated graphite nanocomposite shows a good dispersion of the nano-fillers but also a high value of \({\Delta \varepsilon }_{\mathrm{MWS}}\), which is an indication of high graphite/graphite interaction. This evolution could show that this material can be close to the formation of an electrical percolation network.

Dynamic electric, dielectric, impedance, and modulus spectroscopy study with Rietveld refinement of Al-substituted NiCuZn bulk ceramics

Dynamic electric, dielectric, impedance, and modulus spectroscopy study with Rietveld refinement of Al-substituted NiCuZn bulk ceramics

Dynamic electrical and mechanical properties of epoxy/silver nanowires composites

AbstractThe aim of this work is to study electrically conductive composites from an epoxy matrix loaded with silver nanowires (AgNWs). Dynamic mechanical analysis (DMA) and dynamic dielectric spectroscopy (DDS) were used to study the molecular mobility of the epoxy matrix and its epoxy/AgNWs composites. The influence of the epoxy/amine ratio on the macroscopic properties was analyzed. The loss modulus of the epoxy matrix highlights two major relaxations, and . The relaxation is due to the localized mobility initiated by hydroxyl groups. The trelaxation is the anelastic effect liberated at the glass transition. The introduction of a small amount of AgNWs (<8 vol%) has practically no influence on the mechanical properties of the glassy state. For the rubbery state, AgNWs cause an increase in the storage modulus as a function of the filler content. The loss modulus profile is modified: The relaxation due to heterogeneities of the 3D network is observed independently from the hydration level of composites. The magnitude of the mode is increased due to the stick–slip phenomenon. Moreover, it is shifted toward lower temperatures upon the introduction of AgNWs. In other words, the activation enthalpy decreases for both modes. Electrical study through DDS and conductivity measurement of epoxy/AgNWs composites shows the consequence of a lower relative degree of conversion caused by the insertion of AgNWs. Epoxy/AgNWs composites allow us to obtain one of the highest values of the electrical conductivity for polymer‐based composites without any drastic evolution of mechanical properties.

Poly(butylene succinate) and graphene nanoplatelet–based sustainable functional nanocomposite materials: structure-properties relationship

Sustainable functional polymer nanocomposites from renewable resources are extremely promising materials that can provide the next-generation of lightweight, multifunctional materials for several applications including energy storage, automotive, construction, defense, aerospace, consumer products, biomedical and functional coatings to name few. There is limited information on the use of sustainable polymers and graphene nanoplatelets (GNs), as well as the combinations of these two can provide reduced water permeability or enhanced electrical conductivity and improved thermal properties, and so on. Building upon this hypothesis, biobased poly(butylene succinate)/few-layer GN nanocomposites were prepared via a solventless melt-blending technique. Different characterization techniques such as differential scanning calorimetery, thermogravimetric analysis, dynamic mechanical analysis, dielectric spectroscopy, X-ray diffraction (XRD) and hot stage optical microscopy were used to study the thermal and structural characteristics. The melt blending was characterized by torque and temperature curves which showed that torque was reduced by up to 15 Nm, and melt temperature was improved by up to 5 °C. The improved crystallization of the composites in low concentrations of GN was observed. Graphene has been found to increase the crystallization temperature up to 10 °C and yielded pronounced spherulite structure, whereas peak shift was observed in XRD. High filler loading from 0.5 to 6.0 wt% was used to obtain more insights for few-layer graphene applications for thermoplastic polymer processing applications.

Label-Free Whole Cell Biosensing for High-Throughput Discovery of Activators and Inhibitors Targeting G Protein-Activated Inwardly Rectifying Potassium Channels.

Dynamic mass redistribution (DMR) and cellular dielectric spectroscopy (CDS) are label-free biosensor technologies that capture real-time integrated cellular responses upon exposure to extra- and intracellular stimuli. They register signaling routes that are accompanied by cell shape changes and/or molecular movement of cells proximal to the biosensor to which they are attached. Here, we report the unexpected observation that robust DMR and CDS signatures are also elicited upon direct stimulation of G protein-activated inwardly rectifying potassium (GIRK) channels, which are involved in the regulation of excitability in the heart and brain. Using ML297, a small-molecule GIRK activator, along with channel blockers and cytoskeletal network inhibitors, we found that GIRK activation exerts its effects on cell shape by a mechanism which depends on actin but not the microtubule network. Because label-free real-time biosensing (i) quantitatively determines concentration dependency of GIRK activators, (ii) accurately assesses the impact of GIRK channel blockers, (iii) is high throughput-compatible, and (iv) visualizes previously unknown cellular consequences downstream of direct GIRK activation, we do not only provide a novel experimental strategy for identification of GIRK ligands but also an entirely new angle to probe GIRK (ligand) biology. We envision that DMR and CDS may add to the repertoire of technologies for systematic exploitation of ion channel function and, in turn, to the identification of novel GIRK ligands in order to treat cardiovascular and neurological disorders.

Dynamic relaxations in a bio‐based polyamide with enhanced mechanical modulus

ABSTRACTA new grade of bio‐based polyamide (PA)—PA meta‐xylylene diamine 10 (PA mXD 10)—was investigated. Its first interest is that it permits mild processing conditions at about 200 °C. The calorimetric study shows the existence of two cold crystallizations indicative of slow crystallization rate. The glass transition stabilizes at 55 °C. By combining calorimetry with dynamic mechanical analysis and dynamic dielectric spectroscopy, we found a perfect consistency between the set of data giving the molecular mobility. The localized relaxations follow Arrhenius equations while the viscoelastic transition follows a Vogel–Fulcher–Tammann law. The compilation of all the relaxation times determined by means of the different analyses highlights a good correlation. This result is perfectly explained by the polarity of the macromolecular chain. The dynamic mechanical behavior showed a storage modulus higher than for the corresponding aliphatic PA and nearly constant until room temperature. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46846.

Microstructure aspects of radiation-cured networks: Cationically polymerized aromatic epoxy resins

The thermo-mechanical properties and nanostructural features of epoxy aromatic resins cationically cured by UV–visible or electron beam radiation have been studied by FT-NIR spectroscopy, dynamic mechanical analysis (DMA), dielectric spectroscopy (DS), and atomic force microscopy (AFM). The influence of formulation (nature and content of onium salt) and of curing parameters (doses, thermal treatment) on the thermophysical have been investigated. The presence of several relaxation domains observed by DMA and DS analysis confirms the presence of heterogeneities in the cured materials. Network formation is described by the percolation of glassy nanoclusters which are evidenced by AFM analyses. AFM probing by quantitative nanomechanical measurements confirms the gradual build-up of the local Young's modulus in good agreement with the macroscopic value.

Dynamic molecular mobility of polyurethane by a broad range dielectric and mechanical analysis

The dynamic molecular mobility of poly(2-hydroxypropyl methacrylate) (PHPMA) base polyurethane (PU) was studied over a broad range of frequency and temperature by combining dynamic dielectric spectroscopy and dynamic mechanical analysis. Two hydrated levels were considered in this study: dry and room humidity states. In dry state, two secondary relaxations γ and β are identified. And in room humidity state, a βsw mode is pointed out. These modes are well known in poly(hydroxylalkyl methacrylate)s. The main α relaxation is influenced by humidity and crosslinks. The Kramers-Kronig transform is used to reveal an ionic conductivity σionic. Relaxation times extracted from dielectric and mechanical analyses are coherent for PU and PHPMA. This correlation allows us to propose an interpretation of relaxations at a molecular level.

Relaxation processes in an aromatic polyamide-imide and composites on its basis with hydrosilicate nanoparticles

Features of the relaxation behavior of an aromatic polyamide-imide and a composite with nanotubes of magnesium silicate with the structure of chrysotile have been studied by dynamic mechanical analysis, dielectric spectroscopy, and differential scanning calorimetry. Two secondary relaxation (β1 and β2) transitions have been found, the activation energies of these processes have been determined, and the solvent effect on the cooperativeness degree has been studied. Changes in the value of the apparent activation energy of the β1-process caused by the subsequent heating of polymeric and composite samples have been analyzed according to the Starkweather procedure. It has been shown that, as the solvent is released from the polyamide-imide film, the polymer exhibits increasing local mobility, which are predetermined by the structure of the molecular unit. Using the GAMESS software, we have assumed the most probable dimer conformations that correspond to two repeating units of the polyamide-imide molecule.

Atomistic details of protein dynamics and the role of hydration water

BackgroundThe importance of protein dynamics for their biological activity is now well recognized. Different experimental and computational techniques have been employed to study protein dynamics, hierarchy of different processes and the coupling between protein and hydration water dynamics. Yet, understanding the atomistic details of protein dynamics and the role of hydration water remains rather limited. Scoop of reviewBased on overview of neutron scattering, molecular dynamic simulations, NMR and dielectric spectroscopy results we present a general picture of protein dynamics covering time scales from faster than ps to microseconds and the influence of hydration water on different relaxation processes. Major conclusionsInternal protein dynamics spread over a wide time range from faster than picosecond to longer than microseconds. We suggest that the structural relaxation in hydrated proteins appears on the microsecond time scale, while faster processes present mostly motion of side groups and some domains. Hydration water plays a crucial role in protein dynamics on all time scales. It controls the coupled protein-hydration water relaxation on 10–100ps time scale. This process defines the friction for slower protein dynamics. Analysis suggests that changes in amount of hydration water affect not only general friction, but also influence significantly the protein's energy landscape. General significanceThe proposed atomistic picture of protein dynamics provides deeper understanding of various relaxation processes and their hierarchy, similarity and differences between various biological macromolecules, including proteins, DNA and RNA.This article is part of a Special Issue entitled “Science for Life” Guest Editor: Dr. Austen Angell, Dr. Salvatore Magazù and Dr. Federica Migliardo”.

Interfacial effects on dielectric properties of ethylene propylene rubber–titania nano- and micro-composites

The influence of titania particle size on dielectric properties of ethylene–propylene rubber (EPR) composites was investigated, with an emphasis on the chain dynamics in the interphase region. We hypothesized that a reduction in the conformational entropy of rubber chains in the interphase region would make the restricted chains more prone to orient along the applied electric field and increase dielectric permittivity than the chains in the bulk. The morphology and microstructure of the composites was characterized using scanning electron microscopy and atomic force microscopy, and a notable difference was detected in the average aggregate size and microstructure of the interphase between the nano- and micro-composites. We predicted that the physical attraction between nano- and micro-fillers and polymer, as characterized by the surface energy of fillers, would be similar. However, differences in the chain dynamics investigated by dynamic mechanical thermal analysis and dielectric spectroscopy confirmed the presence of more restricted chains, quantitatively and qualitatively, in the interphase region of the nanocomposites compared to the micro-composites. We concluded that the higher dielectric properties of the nanocomposites could be explained by the lower conformational entropy of chains in these composites compared to those in the micro-composites.

Electrical behaviour of a silicone elastomer under simulated space environment

The electrical behavior of a space-used silicone elastomer was characterized using surface potential decay and dynamic dielectric spectroscopy techniques. In both cases, the dielectric manifestation of the glass transition (dipole orientation) and a charge transport phenomenon were observed. An unexpected linear increase of the surface potential with temperature was observed around Tg in thermally-stimulated potential decay experiments, due to molecular mobility limiting dipolar orientation in one hand, and 3D thermal expansion reducing the materials capacitance in the other hand. At higher temperatures, the charge transport process, believed to be thermally activated electron hopping with an activation energy of about 0.4 eV, was studied with and without the silica and iron oxide fillers present in the commercial material. These fillers were found to play a preponderant role in the low-frequency electrical conductivity of this silicone elastomer, probably through a Maxwell–Wagner–Sillars relaxation phenomenon.

Influence of molecular mobility on the physical stability of amorphous pharmaceuticals in the supercooled and glassy States.

We investigated the correlation between molecular mobility and physical stability in three model systems, including griseofulvin, nifedipine, and nifedipine-polyvinylpyrrolidone dispersion, and identified the specific mobility mode responsible for instability. The molecular mobility in the glassy as well as the supercooled liquid states of the model systems were comprehensively characterized using dynamic dielectric spectroscopy. Crystallization kinetics was monitored by powder X-ray diffractometry using either a laboratory (in the supercooled state) or a synchrotron (glassy) X-ray source. Structural (α-) relaxation appeared to be the mobility responsible for the observed physical instability at temperatures above Tg. Although the direct measurement of the structural relaxation time below Tg was not experimentally feasible, dielectric measurements in the supercooled state were used to provide an estimate of the α-relaxation times as a function of temperature in glassy pharmaceuticals. Again, there was a strong correlation between the α-relaxation and physical instability (crystallization) in the glassy state but not with any secondary relaxations. These results suggest that structural relaxation is a major contributor to physical instability both above and below Tg in these model systems.

Bottom-up self-organizing dissipative polymer nanostructures

Based on scaling concepts a methodology of research of non-equilibrium polymer systems has been elaborated. Polymers with flexible chains (melt-crystallized linear high density polyethylene is chosen as an example) are solutions in melt as well as in solid state, the ends of a chain serving as a solvent for it. At critical polymerization degree all phases (melt, solid isotropic or oriented state) are identical. The square of the neck draw ratio is equal to the product of the square of the draw ratio at break and the chain ends collision probability. This probability in its turn is proportional to the average thickness of amorphous layers in the isotropic material. Depending on the type of polymer statistics (Gauss or Lévy–Khinchin) and the number of components of an ordering field for the second case, the melt viscosity and the self-diffusion coefficient vs. the molecular weight of linear flexible-chain polymer follow the power laws with the 3.50, 3.41 or 3.33 and −2.50, −2.05 or −1.65 exponents, respectively, within the reptation model near the critical point. Vibrational–rotational Brownian motions of chain ends about the polymer melt flow direction were taken into account to find better agreement with the experiment. The recent experimental results of dynamic mechanic and dielectric spectroscopy show the value 3.5±0.1 for the viscosity exponent of long chains, while NMR data result in −2.3±0.1 for the self-diffusion coefficient exponent of short chains. Possible reasons are discussed.

Multiwalled-Carbon-Nanotube-Induced Miscibility in Near-Critical PS/PVME Blends: Assessment through Concentration Fluctuations and Segmental Relaxation

The effects of multiwalled carbon nanotubes (MWNTs) on the concentration fluctuations, interfacial driven elasticity, phase morphology, and local segmental dynamics of chains for near-critical compositions of polystyrene/poly(vinyl methyl ether) (PS/PVME) blends were systematically investigated using dynamic shear rheology and dielectric spectroscopy. The contribution of the correlation length (ξ) of the concentration fluctuations to the evolving stresses was monitored in situ to probe the different stages of demixing in the blends. The classical upturn in the dynamic moduli was taken as the rheological demixing temperature (T(rheo)), which was also observed to be in close agreement with those obtained using concentration fluctuation variance, <(δϕ)(2)>, versus temperature curves. Further, Fredrickson and Larson's approach involving the mean-field approximation and the double-reptation self-concentration (DRSC) model was employed to evaluate the spinodal decomposition temperature (T(s)). Interestingly, the values of both T(rheo) and T(s) shifted upward in the blends in the presence of MWNTs, manifesting in molecular-level miscibility. These phenomenal changes were further observed to be a function of the concentration of MWNTs. The evolution of morphology as a function of temperature was studied using polarized optical microscopy (POM). It was observed that PVME, which evolved as an interconnected network during the early stages of demixing, coarsened into a matrix-droplet morphology in the late stages. The preferential wetting of PVME onto MWNTs as a result of physicochemical interactions retained the interconnected network of PVME for longer time scales, as supported by POM and atomic force microscopy (AFM) images. Microscopic heterogeneity in macroscopically miscible systems was studied by dielectric relaxation spectroscopy. The slowing of segmental relaxations in PVME was observed in the presence of both "frozen" PS and MWNTs interestingly at temperatures much below the calorimetric glass transition temperature (T(g)). This phenomenon was observed to be local rather than global and was addressed by monitoring the evolution of the relaxation spectra near and above the demixing temperature.

Multilayer films ageing under ultraviolet radiations: Complementary study by dielectric spectroscopy

The effect of ultraviolet (UV) irradiation on a multilayer film made of poly(ethylene terephthalate)/Polyamide 6/poly(ethylene terephthalate) was investigated by uniaxial tractions, UV-visible-NIR and Fourier transformed infra-red-attenuated total reflection spectroscopy, differential scanning calorimetry (DSC), and dynamic dielectric spectroscopy (DDS). The multilayer was exposed to ultraviolet radiations (filtered at 270 nm) for 7 days, in air. The complexity of the multilayer thermograms recorded by DSC and DDS has required the study of each film constituting the multilayer to assess each the contribution of each one of them. A deterioration in mechanical properties and a decrease in UV transmission for low wavelengths are observed. These evolutions seem to result to the photo-oxidation of the poly(ethylene terephthalate) film mainly localized at the exposed layer. This layer acts as a UV protection filter for the other layers. However, the DDS analyses show a plasticization effect of the primary mode in the Polyamide 6, which is evidence of photo-oxidation.

Environmental ageing of aerospace epoxy adhesive in bonded assembly configuration

Aerospace epoxy adhesive has been subjected to various environmental ageing processes. A comparative study is performed between ageing under vacuum and atmospheric pressure for various ageing temperatures. The macroscopic behaviour of the assemblies is investigated by single lap shear test. Glass transition temperature of the bulk adhesive is followed by differential scanning calorimetry. Molecular mobility of the adhesive in service configuration is studied by dynamic dielectric spectroscopy. No significant difference is noticed due to an ageing under vacuum. A comparison between test results after each isotherm ageing highlights the temperature influence. For ageing of adhesive in a vitreous state, the crosslink density increases. For ageing of adhesive in a rubbery state, a severe decrease of glass transition is observed. The influence of additives and physical ageing is null. This plasticisation effect might be associated with the homogenisation of the structure of epoxy due to the molecular mobility of loose chains.

Molecular mobility interpretation of the dielectric relaxor behavior in fluorinated copolymers and terpolymers

Thermo Stimulated Current and Dynamic Dielectric Spectroscopy have been applied to investigate dielectric relaxation modes of poly(vinylidene-fluoride-trifluoroethylene) copolymer and poly(vinylidene-fluoride-trifluoroethylene-chlorofluoroethylene) terpolymer. The aim of this work is to check the molecular origin of the ferroelectric relaxor behavior of the terpolymer. The combination of data obtained by both dielectric methods allows us to describe the molecular mobility of the amorphous phase and the cooperativity of the order/disorder dipolar transition in the crystalline phase. The introduction of 1,1-chlorofluoroethylene units in the main chain induces an increase of the Cooperative Rearranging Region size associated with less ordered and smaller crystallites. This morphological evolution is responsible of a lack of cooperativity and it explains the dielectric relaxor behavior of the poly(vinylidene-fluoride-trifluoroethylene-chloroethylene) terpolymer.

Correlation between Molecular Mobility and Physical Stability of Amorphous Itraconazole

The goal was to investigate the correlation between molecular mobility and physical stability in amorphous itraconazole and identify the specific mobility mode responsible for its instability. The molecular mobility of amorphous itraconazole, in the glassy as well as the supercooled liquid state, was comprehensively characterized using dynamic dielectric spectroscopy. Isothermal frequency sweeps in the 5-40 °C temperature range revealed a β-relaxation which exhibited Arrhenius temperature dependence. As the temperature approached T(g), β-relaxation became progressively less resolved due to interference from the high frequency tail of the α-relaxation and then transformed into an excess wing. Above T(g), nonlinear temperature dependence of the α-relaxation was described by the Vogel-Tammann-Fulcher (VTF) model. Itraconazole was found to be a fragile glass former with a VTF strength parameter of ∼4. Isothermal crystallization kinetics, at several temperatures over the range of 75 to 95 °C, was best described by the 3-dimensional nucleation and growth model. Primary relaxation appeared to be the mobility responsible for the observed physical instability at temperatures above T(g) as indicated by the linear correlation of α-relaxation with both crystallization onset and kinetics (represented by the inverse of the crystallization rate constant). A strong coupling between global mobility and crystallization onset was evident. However, for growth kinetics, the coupling was less pronounced, indicating the involvement of factors other than global mobility.