Broadband Dielectric Relaxation Spectroscopy Research Articles

Polymer Chain Dynamics in Intercalated Poly(ε-caprolactone)/Nanoplatelet Blends

The dynamics of poly(ε-caprolactone) (PCL) blends with small amounts of Cloisite 30B nanoplatelets were investigated via broadband dielectric relaxation spectroscopy. The terminal relaxation, or normal mode of PCL, was found to be strongly influenced by the presence of the nanoplatelets, exhibiting a relaxation from PCL chains within the Cloisite platelets as well as a bulk-like relaxation. These results, as well as X-ray scattering and rheological results from the literature, help to form a clearer picture of how the structure and dynamics relate to the bulk physical properties in these systems. Cloisite was observed to have minimal impact on both the glassy state dynamics as well as the segmental dynamics, due to the relatively weak interactions between the polymer and the platelets.

Dielectric relaxation, ac conductivity and electric modulus in poly(vinylidene fluoride)/NaY zeolite composites

Poly(vinylidene fluoride)/zeolite composites prepared by solvent casting at 190°C have been investigated by scanning electronic microscopy, Fourier transformed infrared spectroscopy and broadband dielectric relaxation spectroscopy. The inclusion of the NaY zeolite filler in the polymer matrix induces the crystallization of poly(vinylidene fluoride), PVDF, in the electroactive γ phase. The dielectric function, ac conductivity and electric modulus of the composites were investigated as a function of zeolite content in the frequency range from 0.1Hz to 1×107Hz at temperatures from −120 to 150°C. The main relaxation process (β-relaxation) of the amorphous phase of the polymer matrix is not affected by the presence of the zeolite, as verified by the similar relaxation parameters obtained for the pure polymer and for the PVDF/NaY4% composite. In a similar way, the zeolite low temperature relaxation is not significantly affected by the polymer phase. On the other hand, the electric modulus formalism reveals significant contributions of the fillers to the electrical permittivity and conductivity of the composites. The presence of the zeolite particles increases ac conductivity and the Maxwell–Wagner–Sillars contribution that is predominant at low frequencies with respect to ohmic contribution to permittivity.

Hydrophilic and Hydrophobic Hydration of Sodium Propanoate and Sodium Butanoate in Aqueous Solution

Aqueous solutions of sodium propanoate (NaOPr) and n-butanoate (NaOBu) have been studied at concentrations of c ~/< 3 M by broadband dielectric relaxation spectroscopy over the frequency range 0.2 ≤ ν/GHz ≤ 89 at 25 °C. Three relaxation modes were resolved, centered at (approximately) 1, 8, and 18 GHz, for both sets of solutions. The two faster modes were assigned to the cooperative relaxation of "slow" and bulk water molecules. Detailed analysis of the spectra indicated that both OPr(-) and OBu(-) were strongly hydrated, with ~23 and ~33 slow water molecules per anion, respectively, at infinite dilution. These effective hydration numbers include ~6 water molecules hydrophilically bound to the carboxylate moiety, with the remainder arising from the hydrophobic hydration of the nonpolar alkyl chains. The latter shows a characteristic rapid decrease with increasing solute concentration, which facilitated the separation of the hydrophobic and hydrophilic contributions. The lowest frequency mode was a composite with contributions from ion-cloud, ion-pair, and anion relaxations. Although this low intensity mode provided specific evidence of weak ion pairing between Na(+)(aq) and the carboxylate anions, reliable estimates of the association constant could not be made because of its composite nature.

The Role of Soft Segment Molecular Weight on Microphase Separation and Dynamics of Bulk Polymerized Polyureas

The influence of poly(tetramethylene oxide) (PTMO) soft segment length on the phase-separated microstructure, state of hydrogen-bonded associations, and molecular dynamics was investigated in polyureas polymerized from the bulk. For higher PTMO molecular weights (1000 and 650 g/mol) hard segments self-assemble into ribbon-like domains, while incorporation of a 250 g/mol soft segment leads to a predominately mixed segment material. The degree of microphase separation of the hard and soft segments, however, is rather incomplete for polymers synthesized from 1000 and 650 g/mol PTMO and decreases with decreasing soft segment molecular weight. Broadband dielectric relaxation spectroscopy reveals two segmental relaxations: a soft segment rich (α) and slow segmental (α2) process. When the molecular weight is reduced from 1000 to 650 g/mol the mobility of these processes is reduced, consistent with findings from differential scanning calorimetry and dynamic mechanical analysis.

Dielectric and thermal studies of the segmental dynamics of poly(methyl methacrylate)/silica nanocomposites prepared by the sol–gel method

Abstract Poly(methyl methacrylate)/silica (PMMA/SiOx) nanocomposites were synthesized via sol–gel method and studied by various techniques. The dispersion of the silica particles (10–100 nm) in the matrix was probed by transmission electron microscopy (TEM), while solid‐state NMR and Raman spectroscopy detected the formation of an inorganic network with high degree of crosslinking. To elucidate the impact of the filler on the molecular dynamics of the PMMA, different methods were used; namely differential scanning calorimetry, thermally stimulated depolarization current and broadband dielectric relaxation spectroscopy. All three methods observed a significant impact of the nanoparticles on the segmental dynamics of the matrix, which was expressed as an increase of the glass transition temperature (Tg) in terms of calorimetry and as a shift of the α (segmental) relaxation to lower frequencies in terms of dielectric spectroscopy. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Probing Segmental Dynamics of Polymer Glasses during Tensile Deformation with Dielectric Spectroscopy

An advanced broadband dielectric relaxation spectroscopy technique was developed to measure dipolar reorientation dynamics in an actively deforming amorphous polymer below the glass transition temperature. The application of a weak oscillating electric field during deformation allows for direct probing chain segment mobility. Results show that the application of a monotonically increasing strain on a glassy poly(vinyl chloride) induces a significant increase of the out-of-phase dielectric permittivity, thereby reflecting enhancement of the transition activity of chain segments. Moreover, the strain-dependent relaxation spectra suggest that the bifurcation of the relaxation processes decreases together with an overall increase in molecular mobility upon active deformation. The dielectric results also display a highly pronounced sensitivity to the apparent strain rate used for the mechanical excitation and isothermal aging prior to the deformation.

Modeling Dynamics of Isotropic Dielectrics in a Laminar Heterogeneous Configuration

The predictive capabilities of models that satisfy the Weiner bounds and Hashin-Shtrikman (HS) bounds were studied for isotropic dielectrics in a laminar heterogeneous configuration oriented perpendicular to the electric field. The dynamics were investigated isothermally using broadband dielectric relaxation spectroscopy (DRS) in the frequency range of 1 MHz to 100 mHz. The molecules chosen for study were low molecular weight glass formers, glycerol, phenyl salicylate, imidazole, and dimethyl sulfoxide, and macromolecules, polymethylhydrosiloxane, polyvinylpyrrolidone-co-vinyl acetate, poly-dl-lactic acid, and poly l-lactic acid. It was found that none of the models were able to adequately predict in entirety the resultant dynamics. Of the models studied, the most successful were the HS upper bound (HSUB), the complementary universal Weiner equation (CWE), and the Lichtenecker model for the dimensional parameter, ζ = -1/2. The least successful models were the upper Weiner bound (UWB), the Neelakantaswamy, Turkman, and Sarkar (NTS) model for ζ = 1/2, and the Lichtenecker model for ζ = 1/2.

Correlation of Gas Diffusivity and Local Chain Mobility in Polycarbonate/POSS Nanocomposites as Determined by Broadband Dielectric Relaxation Spectroscopy

Correlation of Gas Diffusivity and Local Chain Mobility in Polycarbonate/POSS Nanocomposites as Determined by Broadband Dielectric Relaxation Spectroscopy

Water and polymer dynamics in poly(hydroxyl ethyl acrylate- co-ethyl acrylate) copolymer hydrogels

Water and polymer dynamics in hydrogels based on random copolymers of hydrophilic poly(hydroxyl ethyl acrylate) (PHEA) and hydrophobic poly(ethyl acrylate) (PEA), in wide ranges of composition, were investigated by means of two dielectric techniques, thermally stimulated depolarization currents (TSDC) and, mainly, broadband dielectric relaxation spectroscopy (DRS) at several levels of relative humidity/water content. Water sorption of the hydrogels was studied by equilibrium sorption isotherms (ESI). Two secondary relaxations ( γ and β sw ) and the primary (segmental) α relaxation associated with the glass transition of the copolymer matrix were followed and analyzed against copolymer composition and water content. The results show that the copolymers are homogeneous at nm scale, except at very high PEA content. Correlations were observed between results on the organization of water in the hydrogels and on water effects on polymer dynamics. Distinct changes in the dielectric response, in particular in the time scale and the dielectric strength of the β sw relaxation, at the water content of the completion of the first hydration layer indicate that water molecules themselves contribute to the dielectric response at higher water contents. Proton conductivity of the hydrogels at various levels of water content was also studied and correlation to segmental dynamics (decoupling) was analyzed.

Glass transition and polymer dynamics in silver/poly(methyl methacrylate) nanocomposites

Dynamic mechanical–thermal analysis (DMTA), differential scanning calorimetry (DSC), thermally stimulated depolarization currents (TSDC) and, mainly, broadband dielectric relaxation spectroscopy (DRS) were employed to investigate in detail glass transition and polymer dynamics in silver/poly(methyl methacrylate) (Ag/PMMA) nanocomposites. The nanocomposites were prepared by radical polymerization of MMA in the presence of surface modified Ag nanoparticles with a mean diameter of 5.6 nm dispersed in chloroform. The fraction of Ag nanoparticles in the final materials was varied between 0 and 0.5 wt%, the latter corresponding to 0.055 vol%. The results show that the nanoparticles have practically no effect on the time scale of the secondary β and γ relaxations, whereas the magnitude of both increases slightly but systematically with increasing filler content. The segmental α relaxation, associated with the glass transition, becomes systematically faster and stronger in the nanocomposites. The glass transition temperature T g decreases with increasing filler content of the nanocomposites up to about 10 °C, in good correlation by the four techniques employed. Finally, the elastic modulus decreases slightly but systematically in the nanocomposites, both in the glassy and in the rubbery state. The results are explained in terms of plasticization of the PMMA matrix, due to constraints imposed to packing of the chains by the Ag nanoparticles, and at the same time, of the absence of strong polymer–filler interactions, due to the surface modification of the Ag nanoparticles by oleylamine at the stage of preparation.

Poly(ethylene oxide)–silica hybrids entrapping sensitive dyes for biomedical optical pH sensors: Molecular dynamics and optical response

Polymer–silica hybrid nanocomposites prepared by sol–gel process based on triethoxisilane-terminated poly(ethylene oxide) chains and tetraethoxysilane as silica precursor, doped with organic pH sensitive dyes, have been prepared and their suitability for use as sensors coupled with plastic optic fibers has been evaluated. Sensors were prepared by immobilizing a drop of the hybrid materials onto the tip of a multi-mode poly(methyl methacrylate) optical fiber. The performance of the optical sensor in terms of sensitivity and response time was tested in different experimental conditions, and was found to be markedly higher than analogous sensors present on the market. The very fast kinetic of the hybrid’s optical response was supported by studies performed at the molecular level by broadband dielectric relaxation spectroscopy (DRS), investigated over a wide range of frequency and temperature, showing that poly(ethylene oxide) chains maintain their dynamics even when covalently bonded to silica domains, which decrease the self-association interactions and promote motions of polymer chain segments. Due to the fast response kinetic observed, these pH optical sensors result suitable for the fast-detection of biomedical parameters, i.e. fast esophageous pH-metry.

Dynamics near the filler surface in natural rubber-silica nanocomposites

We study the effect of in situ synthesized 10 nm silica nanoparticles on the glass transition and dynamics of natural rubber networks using differential scanning calorimetry, broadband dielectric relaxation spectroscopy and thermally stimulated depolarization currents. Even in the absence of specific polymer-filler interactions, polymer segments within a few nanometers of the filler particles exhibit relaxation times up to 2–3 orders of magnitude slower and reduced heat capacity increment at the glass transition compared to bulk natural rubber. These effects are only observed when the nanoparticles are uniformly distributed in the polymer matrix.

Polymer dynamics in epoxy/alumina nanocomposites studied by various techniques

AbstractEpoxy/alumina nanocomposites of various compositions were prepared by dispersing modified and nonmodified boehmite nanoparticles in diglycidyl ether of bisphenol‐A using diethylenetriamine as curing agent. Measurements of the viscosity of the nanodispersions provided information on particle–particle and particle–resin interactions. The structure of the nanocomposites was studied by scanning electron microscopy on fractured samples. Effects of nanoparticles on polymer dynamics was studied in detail by dynamic mechanical thermal analysis and two dielectric techniques, broadband dielectric relaxation spectroscopy and thermally stimulated depolarization currents. Three secondary relaxations, γ, β, and ω, the segmental α relaxation associated with the glass transition, and an interfacial relaxation, in the order of increasing frequency/decreasing temperature, were observed and studied. A correlation between viscosity (of the nanodispersions), storage modulus, glass transition temperature, real part of dielectric permittivity, and ductility of the nanocomposites was observed. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Dielectric Relaxation Study of the Ion Solvation and Association of NaCF3SO3, Mg(CF3SO3)2, and Ba(ClO4)2inN,N-Dimethylformamide

Solutions of sodium trifluoromethanesulfonate, magnesium trifluoromethanesulfonate, and barium perchlorate in N,N-dimethylformamide (DMF) have been investigated using broadband dielectric relaxation spectroscopy at 25 °C. All spectra were dominated by a solvent relaxation process centered at ∼15 GHz but also exhibited one (for NaCF(3)SO(3)) or two (for the 2:1 salts) low-amplitude processes, centered at frequencies below 2 GHz, that could be attributed to the presence of ion pairs. Effective solvation numbers calculated from the solvent relaxation amplitudes indicated strong solvation of all three cations, with evidence for the formation of a second solvation sheath for Mg(2+) and possibly Ba(2+). Detailed analysis of the solute-related processes showed that solvent-shared ion pairs (SIPs) were formed in NaCF(3)SO(3) solutions in DMF. The data for Mg(CF(3)SO(3))(2) and Ba(ClO(4))(2) solutions were not definitive but, consistent with the solvation evidence, favored the presence of double solvent-separated ion pairs and SIPs. Overall association constants, K(A), were small for all three salts in DMF and increased in the order: NaCF(3)SO(3)<Ba(ClO(4))(2)<Mg(CF(3)SO(3))(2).

Glass Transition and Dynamics in Lysozyme–Water Mixtures Over Wide Ranges of Composition

Differential scanning calorimetry (DSC) and two dielectric techniques, broadband dielectric relaxation spectroscopy and thermally stimulated depolarization currents (TSDC), were employed to study glass transition and water and protein dynamics in mixtures of water and a globular protein, lysozyme, in wide ranges of water content, both solutions, and hydrated solid samples. In addition, water equilibrium sorption isotherms (ESI) measurements were performed at room temperature. The main objective was to correlate results by different techniques to each other and to determine critical water contents for various processes. From ESI measurements the content of water directly bound to primary hydration sites was determined to 0.088 (grams of water per grams of dry protein), corresponding to 71 water molecules per protein molecule, and that where clustering becomes significant to about 0.25. Crystallization and melting events of water were first observed at water contents 0.270 and 0.218, respectively, and the amount of uncrystallized water was found to increase with increasing water content. Two populations of ice crystals were observed by DSC, primary and bulk ice crystals, which give rise to two separate relaxations in dielectric measurements. In addition, the relaxation of uncrystallized water was observed, superimposed on a local relaxation of polar groups on the protein surface. The glass transition temperature, determined by DSC and TSDC in rather good agreement to each other, was found to decrease significantly with increasing water content and to stabilize at about −90 °C for water contents higher than about 0.25. This is a novel result of this study with potential impact on cryoprotection and pharmaceutics.

Influence of the Degree of Sulfonation on the Structure and Dynamics of Sulfonated Polystyrene Copolymers

The structure and dynamics of sulfonated polystyrene (SPS) acid copolymers were investigated using X-ray scattering and broadband dielectric relaxation spectroscopy (DRS). Evidence of acid group aggregation was found at 3.5, 6.7, and 9.5 mol % sulfonation. An increase in the segmental relaxation time and dielectric relaxation strength was observed with increasing acid content. The average state of dipolar interactions was quantified as a function of temperature using the Onsager equation and the Kirkwood−Frohlich correlation factor. Two high-temperature relaxation processes at frequencies 4−7 decades below the segmental relaxation were identified and are proposed to originate from Maxwell−Wagner−Sillars interfacial polarization and association dynamics of various assemblies of acid groups.

Local Relaxation Behavior and Dynamic Fragility in Hydrogen Bonded Polymer Blends

The dynamics of intermolecularly hydrogen-bonded polymer blends of poly(p-(hexafluoro-2-hydroxyl-2-propyl)styrene) with poly(vinyl acetate), poly(ethylene[30]-co-vinyl acetate[70]) and poly(ethylene[55]-co-vinyl acetate[45]) are investigated by broadband dielectric relaxation spectroscopy and Fourier transform infrared spectroscopy. Each blend component exhibits a glassy state (β) relaxation, and these relaxations are affected by the formation of intermolecular associations. The glassy state behavior of the blends can be modeled using the Painter−Coleman association model. All blends exhibit a single Tg and a single dielectric segmental (α) relaxation, indicative of strong segmental-level coupling. The fragility of the glass-formers depends on the volume fraction of intermolecularly associated segments, and the association model predicts which compositions have the highest fragilities. A relaxation related to the breaking and reforming of hydrogen bonds is observed at temperatures above the α process, and its temperature dependence varies systematically with ethylene content.

Are Nanoscale Ion Aggregates Present in Aqueous Solutions of Guanidinium Salts?

A detailed investigation using broadband dielectric relaxation spectroscopy (DRS) has been made of the aqueous solutions of guanidinium chloride and carbonate, GdmCl(aq) and Gdm₂CO₃(aq), at 25 °C. The spectra indicate that Gdm(+) ions, C(NH₂)₃(+), do not bind strongly to water nor are they hydrophobically hydrated; rather they appear to have a most unusual ability to dissolve in water without altering its dynamics. Although DRS is particularly sensitive to the presence of ion pairs, only weak ion pairing was detected in Gdm₂CO₃(aq) solutions and none at all in GdmCl(aq). Surprisingly, no evidence was found for the existence of the higher order homo- and heteroionic nanoscale aggregates that have been identified in recent years by Mason and co-workers using molecular dynamics simulations and neutron diffraction. Possible reasons for this discrepancy are discussed. The present DR spectra and other solution properties of GdmCl(aq) and Gdm₂CO₃(aq), such as apparent molar volumes and electrical conductivities, are shown to have strong similarities to those of the corresponding Na+ salts. However, such solutions also differ remarkably from their Na(+) analogues (and all other simple electrolytes in aqueous solution) in that their average water relaxation times correlate strongly with their bulk viscosities. The biological implications of the present results are briefly discussed.

Positron annihilation and broadband dielectric spectroscopy: A series of propylene glycols

We report a thorough joint analysis of the behavior of the ortho-positronium lifetime as obtained from positron annihilation lifetime spectroscopy and of the dipolar relaxation spectra investigated by broadband dielectric relaxation spectroscopy in a series of glass-forming propylene glycols including propylene glycol, dipropylene glycol and tripropylene glycol. A number of empirical correlations between the temperature dependence of the ortho-positronium annihilation lifetime, τ 3( T), and the various spectral and relaxational quantities have been found. The phenomenological evaluation of the quasi-sigmoidal τ 3( T) dependence reveals three characteristic temperatures: T g PALS, T b1 = (1.23 − 1.27) T g PALS and T b2 = (1.46 − 1.53) T g PALS, which are found to decrease with increasing fragility. The slighter change of slope in the PALS response at T b1 in this series of propylene glycols appears to be related to the crossover from the α-process to the excess wing or secondary relaxation, found in the dielectric spectra. The onset of the high-temperature plateau in the τ 3( T) plot at T b2 occurs when τ 3 matches the average relaxation time of the primary α process. Moreover, the plateau region lies in the vicinity of the crossover in the dielectric parameters of the structural relaxation in all the samples, i.e. spectral width and relaxation strength. In addition it is approximately related to a crossover of the α relaxation time τ α ( T) from non-Arrhenius to Arrhenius regime. In summary, all the empirical correlations support further very close connections between the PALS response and the dielectric relaxation behavior in the series of propylene glycols.

Comparative studies on effects of silica and titania nanoparticles on crystallization and complex segmental dynamics in poly(dimethylsiloxane)

Effects of in situ synthesized silica and titania nanoparticles, 5 and 20–40 nm in diameter, respectively, on glass transition and segmental dynamics of poly(dimethylsiloxane) networks were studied by employing differential scanning calorimetry, thermally stimulated depolarization currents and broadband dielectric relaxation spectroscopy techniques. Strong interactions between the well dispersed fillers and the polymer suppress crystallinity and affect significantly the evolution of the glass transition in the nanocomposites. Next to the α relaxation associated with the glass transition of the bulk amorphous polymer fraction, two more segmental relaxations were recorded, originating from polymer chains restricted between condensed crystal regions (αc-relaxation) and the semi-bound polymer in an interfacial layer with strongly reduced mobility due to interactions with hydroxyls on the nanoparticle surface (α′ relaxation), respectively. Interactions with the polymer were found to be stronger in the case of titania than silica, leading to an estimated interaction length of around 2 nm for silica and at least double for titania nanocomposites.