Secondary Relaxation Research Articles

Molecular dynamics of an epoxy resin studied by Thermally Stimulated Depolarization Currents

The Thermally Stimulated Depolarization Current (TSDC) technique has been used to study the molecular mobility of diglycidyl ether of bisphenol-A (DGEBA) in the temperature range from −160 °C up to 10 °C. The molecular mobility in the glassy state and in the supercooled liquid was then characterized. Two secondary relaxations were detected, one of them partially disguised by the main α-process. Differential Scanning Calorimetry (DSC) was used to analyse the dynamics of the structural relaxation around T g (from the heating rate dependence of the onset temperature of the glass transition signal). The values of the fragility index obtained in this work by TSDC and DSC are compared with the values reported in the literature obtained from other experimental techniques. A relaxation above the glass transition temperature was also observed. The controversy about the attribution of this upper- T g mobility at the molecular level is reported.

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.

Pressure-induced change in the relaxation dynamics of glycerol

Glycerol is one of the best studied and most widely used glass-forming liquids; however, its dynamic properties are still under discussion. The dielectric spectra of glycerol are studied in detail over wide ranges of temperatures and pressures up to 4.5 GPa. Starting from the pressures of 2–3 GPa, qualitative change in the dynamics of structural relaxation processes in glycerol has been revealed. It is accompanied by the appearance of secondary relaxation and a change in the asymptotic behavior of the pressure dependence of the fragility. The relation between the parameters for different relaxation mechanisms is analyzed.

Advancing the dynamic mechanical analysis of biomass: comparison of tensile-torsion and compressive-torsion wood DMA

Abstract In an effort to advance the dynamic mechanical analysis (DMA) of very small biomass specimens, and/or specimens having poor mechanical integrity, the functional equivalent of pendulum-torsion (tensile-torsion) DMA was compared to parallel-plate compressive-torsion DMA. The solvent-saturated lignin glass transition in yellow-poplar (Liriodendron tulipifera) was generally similar determined by both modes; however, direct data comparisons should be avoided or carefully considered. First-heat glass transition temperatures (T gs) were relatively similar; however, specimen densification elevated subsequent cooling-mode T gs by 5–8°C in compressive-torsion. Both modes revealed a first-heat tan δ shoulder; it was more prominent and had grain dependency in compressive-torsion. Below fiber saturation, subambient tensile-torsion DMA was superior; compressive-torsion resulted in an anomalous response, obscuring subambient secondary relaxations. With these differences and limitations in mind, compressive-torsion offers specific advantages. Solvent-submersion studies are simplified because solvent cups are easily devised for torsional rheometers. Specimens lacking mechanical integrity are more easily analyzed. Heavily biodegraded spruce (Picea sp.) was analyzed in the solvent-submersion mode as fibrous mats and the different actions of Gloeophyllum trabeum and Postia placenta were revealed. Very small specimens are easily analyzed in compressive-torsion; tissue maturity effects were revealed in minute sections of switchgrass (Panicum virgatum) stems. Applied appropriately, parallel-plate compressive-torsion DMA will provide new research opportunities.

NMR and dielectric studies of hydrated collagen and elastin: Evidence for a delocalized secondary relaxation

Using a combination of dielectric spectroscopy and solid-state deuteron NMR, the hydration water dynamics of connective tissue proteins is studied at sub-ambient temperatures. In this range, the water dynamics follows an Arrhenius law. A scaling analysis of dielectric losses, ‘two-phase’ NMR spectra, and spin-lattice relaxation times consistently yield evidence for a Gaussian distribution of energy barriers. With the dielectric data as input, random-walk simulations of a large-angle, water reorientation provide an approximate description of stimulated-echo data on hydrated elastin. This secondary process is quasi-isotropic and delocalized. The delocalization is inferred from previous NMR diffusometry experiments. It is emphasized that the phenomenology of this process is shared by many non-aqueous binary glasses in which the constituent components exhibit a sufficient dynamical contrast.

Relaxation Phenomena in Vitrifying Polymers and Molecular Liquids

Recent experimental results on the dynamics of glass-forming materials, particularly polymers, are surveyed. The focus is on aspects of the behavior that are connected to or correlated with structural relaxation. These results include the invariance to thermodynamic conditions (temperature, pressure, volume) of a number of properties—breadth of the relaxation dispersion, number of dynamically correlating molecules, Johari−Goldstein secondary relaxation time, onset of the dynamic crossover, and the product of temperature and specific volume with the latter raised to a material constant—provided the structural relaxation time is maintained constant. Additional salient experimental findings include the correlation of various high-frequency processes, usually measured in the glassy state, with properties of the equilibrium material above Tg. These correlations indicate that the glass transition, although conventionally defined by the relaxation time becoming larger than experimental time scales (>100 s), has ...

On the features of the secondary relaxations: The case of cyclohexane derivatives

Recent results on the dynamics of ethylcyclohexane obtained using high resolution dielectric spectroscopy and mechanical spectroscopy at ultrasonic frequencies have been analyzed in comparison with experimental data on relaxations occurring in different materials having in common the presence of the cyclohexyl ring in their molecular structure. On the basis of experimental evidence concerning several cyclohexane-derived materials, some features of the secondary relaxations have been outlined that could be applicable to a wider class of materials.

Key experiments in highly viscous liquids

The flow of highly viscous liquids is still a theoretical no-man's-land, with many contradictory theories competing for the explanation of the flow process. On the other hand, a rapid experimental development, in particular in the field of broadband dielectric spectroscopy, supplies more and more information on the nature of the highly viscous flow. The flow process itself (the so-called primary or alpha-process) seems to be intimately related to faster precursor processes, which appear either as part of the wing of the flow process itself or as a secondary relaxation peak, the “Johari–Goldstein” peak, often five orders of magnitude faster than the primary process. An aging experiment shows that the secondary process occurs between strongly asymmetric states, with an energy difference of about 4 k B T. The paper describes some of these key experiments and their possible theoretical interpretation.

Origin of the Commonly Observed Secondary Relaxation Process in Saccharides

Broadband dielectric relaxation studies were performed on D-glucose and 1,6-anhydro-D-glucose. In the liquid phase of both systems, one can observe the cluster relaxation and structural relaxation. In the glassy state of D-glucose two secondary relaxations were recorded. The slower one, hardly detectable in the loss spectra, was identified as the Johari-Goldstein type (JG) relaxation. The faster one, the gamma-relaxation, is visible as a well pronounced peak. For the past few years the origin of this process has been a subject of hot debate. Different authors have speculated about the source of this relaxation, but no consensus was reached. Moreover, application of more sophisticated method, such as NMR and MD simulations have not resolved this problem yet. Comparison of the dielectric loss spectra measured for D-glucose and 1,6-anhydro-D-glucose, combined with other experimental findings described in detail in this paper, enabled us to certify unquestionably, that the rotation of hydroxymethyl group is the origin of gamma-relaxation in D-glucose as well as in the whole family of saccharides. Additionally, calculations of conformational changes with use of density functional theory (DFT) were performed to support our identification.

Dynamical Characterization of a Cellulose Acetate Polysaccharide

This work brings together dynamical and structural information at a molecular level for cellulose acetate being an original contribution to the general description of polysaccharide properties. In particular, it allowed reinterpreting the secondary relaxation mechanisms that are still controversial in the literature; a compilation of data provided by different authors is provided. Detailed dynamical information is provided by dielectric relaxation spectroscopy (DRS) (10(-1)-10(6) Hz) for cellulose acetate (CA) in the sub-T(g) region below ambient temperature; results were compared with cellulose acetate structured as an asymmetric membrane (CAmb). In samples with low water content, two secondary relaxation processes between 173 and 298 K were identified by DRS, associated with localized mobility. The process located at the lowest temperatures (process I) has a different mobility in CA relative to CAmb. The identical crystalline/amorphous state of both materials allowed rationalizing the distinct behavior in terms of polymeric arrangement and ability for water uptake. The looser structure of the CA relative to CAmb as confirmed by FTIR, TGA, and DSC analysis makes more sites accessible to water molecules, resulting in a higher water retention in CA (2.73% w/w) relative to CAmb (1.60% w/w) and an increased molecular mobility in the former due to a plasticizing effect. In both materials, process I is significantly influenced by hydration, shifting to higher frequencies and lower temperatures upon water uptake. This process seems to be associated with mobility occurring within the monomeric unit, which embraces the two anhydroglucose rings connected by the glycosidic linkage and the polar groups directly attached to it. It should involve a very limited length scale, as suggested by its location, far below the glass transition, and the tau(infinity) value with a low entropic effect. The relaxation process that emerges later, process II, is similar for both samples being much less influenced by water but experiencing a slight antiplasticizing effect shifting to lower frequencies and higher temperatures upon hydration. It should involve side group motions, strongly coupled to the mobility of the anhydroglucose rings, which become hindered probably due to establishment of H-bonds with water molecules. The plasticizing/antiplasticizing effect is being discussed only on the basis of the frequency position of the relaxation peak. Processes I and II merge into a broad relaxation (gamma(dry)) upon water removal in both CA and CAmb, however evolving slower in the former with drying, due to a more disordered structure of CA that allows water to interact with more internal sites in the polymer. At higher temperatures (T > or = 353 K), a process emerges in the high frequency side of the dynamic alpha-relaxation which is compatible with a beta(JG)-relaxation. The structured specimen CAmb provided an additional way to probe the morphological changes undergone by the material when annealed to temperatures higher than 353 K, originating an increase in the dielectric response. This effect can be associated with a skin densification and partial collapse of the membrane porous network, as observed by SEM.

Preparation, characterization, structure, and dynamics of carboxymethyl chitosan grafted with acrylic acid sodium salt

AbstractGraft copolymerization of different vinyl monomers onto chitosan (Cs) and its derivatives can alter their properties and consequently expands their potential applications. In this study, carboxymethyl chitosan (CMCs) was prepared and characterized. Graft copolymerization of acrylic acid sodium salt (AAs) onto CMCs was accomplished by using ammonium persulphate (APS)‐induced free radical polymerization in aqueous medium under nitrogen atmosphere. Occurrence of grafting was confirmed and the effects of [AAs], [APS], reaction time, and temperature on the extent of grafting were studied. The influence of grafting yield on the structure and dynamics was also investigated by using differential scanning calorimetry (DSC), two‐dimensional wide‐angle X‐ray scattering (2D‐WAXS), scanning electron microscopy (SEM), and dielectric spectroscopy. Preliminary study, using dielectric spectroscopy, was carried out to the CMCs‐g‐AAs copolymer with grafting percent ∼ 2900%, as an example, in a wide range of temperature (from 299 to 473 K) and frequency (from 1 × 10−2 to 1 × 105 Hz). One relaxation process with Arrhenius type was detected. This process has the characteristics of a secondary relaxation process related to local chain dynamics with activation energy of 53 ± 3 kJ/mol. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Dynamic mechanical properties of a dual-phase Zr28Y28Al22Co22 metallic glass

In order to investigate the mechanical relaxation behaviors of a Zr28Y28Al22Co22 phase separating metallic glass, dynamic mechanical measurements of the tensile storage and loss moduli were performed for broad temperature and frequency ranges. The glass transition of each separated phase in the as-spun ribbon was manifested by the double decreasing of the storage modulus and two loss modulus peaks. The master curves for tanδ of the as-spun and heat-treated specimens were constructed according to the time-temperature superposition principle. From the temperature dependence of the shift factor, the activation energies for the primary and secondary relaxation of both specimens were calculated and the effects of the interconnected structure of the dual-phase as-spun ribbon and partially crystallized ribbon on the mechanical relaxation behaviors were studied.

Relaxation Processes of Poly(tert‐butyl acrylate) Chemically Confined via Hydrogen Bonds

AbstractIn this work, tailored copolymers of tert‐butyl acrylate and acrylic acid (AA) with an ABA type structure were systematically analyzed via dynamic‐mechanical analysis with regard to their relaxations, in which the main focus was put on a chemical confinement (cc) mode caused by hydrogen bonds. Four different relaxation modes were detected, whereupon two modes seem to depend on the length of the inner B‐block and one on the length of the outer A‐blocks. One mode below room temperature could be identified as a secondary relaxation which belongs to the B‐block and is most likely induced by a cc. A softening of the polymer corresponding to the glass transition is found near to room temperature and surprisingly shifts to lower temperatures with increasing chain length of the B‐block. A second softening of the ABA type copolymers is caused by the AA containing A‐blocks and was found at higher temperatures.magnified image

Dipolar and Ionic Relaxations of Polymers Containing Polar Conformationally Versatile Side Chains

This work reports a comparative study of the response of poly(2,3-dimethoxybenzyl methacrylate), poly(2,5-dimethoxybenzyl methacrylate), and poly(3,4-dimethoxybenzyl methacrylate) to electrical perturbation fields over wide frequency and temperature windows with the aim of investigating the influence of the location of the dimethoxy substituents in the phenyl moieties on the relaxation behavior of the polymers. The dielectric loss isotherms above Tg exhibit a blurred relaxation resulting from the overlapping of secondary relaxations with the glass−rubber or α relaxation. At high temperatures and low frequencies, the α relaxation is hidden by the ionic conductive contribution to the dielectric loss. As usual, the real component of the complex dielectric permittivity in the frequency domain increases with decreasing frequency until a plateau is reached corresponding to the glass−rubber (α) relaxation. However, at high temperatures, the real permittivity starts to increase again with decreasing frequency unt...

Secondary relaxations and electrical conductivity in hyperbranched polyester amides

AbstractBroadband dielectric spectroscopy is used to investigate molecular dynamics and charge transport in three hyperbranched polyester amides with hydroxyl, phenyl, and stearate terminal groups. At higher temperatures, the dielectric spectra are interpreted in terms of hopping conduction in a spatially randomly varying energy landscape, whereas two secondary dipolar relaxations attributed to librations of the terminal and amide groups dominate the low temperature regime. Despite a shift of more than 3 decades in the dc conductivity upon variation of the end groups, the Barton–Nakajima–Namikawa relation is shown to hold. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1651–1657, 2010

Dielectric relaxation processes in stoichiometric Ge:Sb:Te amorphous films

The aim of this work is to investigate the relaxation processes of amorphous thin films by dielectric spectroscopy in films with stoichiometric compositions Ge 1Sb 4Te 7, Ge 1Sb 2Te 4 and Ge 2Sb 2Te 5. Experimental results fit well to an empiric Havriliak–Negami model. Two relaxation processes have been observed in all materials: an alpha relaxation in the low frequency range (0.1–10 kHz) and a secondary beta relaxation in the high frequency range (100 kHz–10 MHz). The temperature dependence of the relaxation time for the alpha relaxations is described by the Vogel–Fulcher–Tammann relation with Vogel temperature increasing from 280 K for Ge 1Sb 4Te 7 to 291 K for Ge 2Sb 2Te 5. The secondary beta or Johari–Goldstein relaxation observed in the high frequency range has an Arrhenius-type dependence and approximately the same activation energy 0.422 ± 0.009 eV for all studied Ge:Sb:Te films. A plausible explanation for relaxation processes observed in Ge:Sb:Te films can be related to cooperative and local rearrangements of molecules or clusters of GeTe and/or Sb 2Te 3.

Blends of chitosan and poly(sodium-4-styrene sulphonate). Compatibilization by lysine and glutamic acid

Polymer blends of chitosan (CS) and poly(sodium-4-styrene sulphonate) (PSS) have been compatibilized using lysine (LYS) and glutamic acid (GLU). Blends of CS/PSS containing LYS and GLU have been studied by thermal analysis. The values of T g for CS/PSS-LYS blends could not be detected by thermal analysis but one single T g was found for CS/PSS-GLU blends. Thermogravimetric analysis (TGA) showed that blends decompose at lower temperatures than that of pure components. Displacements in the main absorption bands in the FT-IR spectra were not clear to discern compatibility. Atomic force microscopy (AFM) indicated that addition of lysine or glutamic acid clearly modified the compatibility of the blends. The globular domains existing in the unmodified blend changed to more or less linear and smooth crystalline domains with parallel periodicity. Dielectric analysis revealed that compatibilizer addition changed the secondary relaxation process of PSS to higher temperatures as a result of the modification on interactions. No variation on the segmental dynamics of both polymers was observed as their main relaxations remained unchanged after the addition of lysine or glutamic acid.

Dielectric characteristics of polyimide CP2

High-performance aerospace-grade polyimides such as CP2 fulfill many important roles in a wide range of applications. A thorough understanding of the polymer matrix’s physiochemical properties is an important consideration when developing polymer nanocomposite materials. In this work, we report the dielectric properties of polyimide CP2 including the primary and two secondary dipole relaxations, and their thermal characteristics by way of temperature variable impedance spectroscopy (10−2 to 106 Hz, −40 to 225 °C). Special emphasis has been placed on detailing the characteristic phenomena near CP2′s glass transition (199 °C). The consequences of residual DMAc solvent on CP2′s overall loss and relaxation characteristics are also discussed.

Molecular mobility studies on the amorphous state of disaccharides. I—thermally stimulated currents and differential scanning calorimetry

The relaxational processes in amorphous solid gentiobiose and cellobiose are studied by thermally stimulated depolarization currents (TSDC) in the temperature region from 108 K up to 423 K. The slow molecular mobility was characterized in the crystal and in the glassy state. The features of different motional components of the secondary relaxation have been monitored as a function of time as the glass structurally relaxes on aging. It is concluded that some modes of motion of this mobility are aging independent, while others are affected by aging. The value of the steepness index or fragility ( T g–normalized temperature dependence of the relaxation time) was obtained by differential scanning calorimetry (DSC) from the analysis of the scanning rate dependency of T g.

Dielectric and mechanical relaxation in isooctylcyanobiphenyl (8*OCB)

The dynamics of isooctylcyanobiphenyl (8*OCB) was characterized usingdielectric and mechanical spectroscopies. This isomer of the liquid crystallineoctylcyanobiphenyl (8OCB) vitrifies during cooling or on application of pressure,exhibiting the typical features of glass-forming liquids: non-Debye relaxationfunction, non-Arrhenius temperature dependence of the relaxation times,τα, a dynamiccrossover at T ∼ 1.6Tg.This crossover is evidenced by changes in the behavior of both the peak shape and the temperature dependenceof τα. The primary relaxation time at the crossover, 2 ns at ambient pressure, is the smallest value reportedto date for any molecular liquid or polymer. Interestingly, at all temperatures below this crossover,τα and the dc conductivity remain coupled (i.e., conform to the Debye–Stokes–Einsteinrelation). Two secondary relaxations are observed in the glassy state, one of which isidentified as the Johari–Goldstein process. Unlike the case for 8OCB, no liquid crystallinephase could be attained for 8*OCB, demonstrating that relatively small differences inchemical structure can effect substantial changes in the intermolecular potential.