Dielectric relaxation of poly(phenylene sulfide) containing a fraction of rigid amorphous phase

The dielectric relaxation behavior of poly(phenylene sulfide), PPS, has been investigated from room temperature to 180°C. This study was undertaken to examine the mobility of the amorphous phase through the glass transition region, to determine the contribution that rigid amorphous phase material makes to the relaxation process. Semicrystalline samples contain a fraction of the rigid amorphous phase, which was determined from the heat capacity increment at the glass transition, using degree of crystallinity determined from x-ray scattering. In the dielectric experiment, we measured the temperature and frequency dependence of the real and imaginary parts of the dielectric function. ε″ vs. ε′ was used to determine the dielectric relaxation intensity, δε = ε_s–ε∞, at temperatures above the glass transition. For amorphous PPS, δε decreases as temperature increases, while for all semicrystalline PPS, δε increases with temperature. The ratio of semicrystalline intensity to amorphous intensity determines the total fraction of dipoles which are already relaxed at a given temperature. Results indicate that more and more rigid amorphous phase material relaxes as the temperature is increased. This provides the first evidence that rigid amorphous phase material in PPS contains chains that possess different levels of molecular mobility. Finally, to the temperature of the loss peak maximum, at a given frequency, we assign the value of the dielectric T_g. For both melt and cold crystallization, the dielectric T_g systematically decreases as the crystallization temperature increases, and as the fraction of rigid amorphous phase decreases.     

Thermally stimulated current study of the microstructure of peek

Thermally Stimulated Current (TSC) spectroscopy and Differential Scanning calorimetry (DSC) have been applied to the characterization of the microstructure of Poly (Ether Ether Ketone)/PEEK. the dielectric relaxation spectra show two modes, dependent upon crystallinity:

* for the mode stuated in the vicinity of the glass transition temperature, two components have been distinguished and attributed to the molecular mobility in the ‘true amorphous phase’ and in the ‘rigid amorphous region’.

* below 0°C, two sub-modes appear, situated around-110°C and-75°C, due to the two different crystal entities, beads and laths.

     

Glass transition in polyacrylonitrile: Analysis of dielectric relaxation data

Dielectric relaxation of polyacrylonitrile (PAN) samples, in which the presence of an amorphous phase is evidenced by x-ray diffraction, has been studied over the temperature range 30–150°C and frequency range 10²–10⁵ Hz. These data as well as those reported by other authors, reveal several points useful to the understanding of the dielectric relaxations of PAN in relation to its structure. A glass transition in PAN is evidenced by at least two of the four data sets investigated; the third shows combined effect of two relaxations, whereas the fourth clearly shows a relaxation process different from the glass transition. The glass-transition behavior of the dielectric relaxation data is confirmed by Williams-Landel-Ferry theory and a recent theory of Phillips, both of which lead to consistent conclusions.     

Molecular analysis of the crystallization behavior of poly(aryl-ether-ether-ketone)

The crystallization behavior, via annealing, of PEEK was investigated using FTIR spectroscopy. It was found that predominantly amorphous PEEK (e.g., quenched polymer) follows two distinct mechanisms when it is annealed. One is a lower energy relaxation process that involves the partial rotation of the ether linkages, allowing the molecular chains in the disordered phase to become better packed, and occurs from 50 to 140°C (slightly below T_g). This relaxation process is also observed by dynamic mechanical analyses. The other process is a crystallization that takes place above the glass transition, with its onset characterized by the rotation of the benzophenone linkages near T_g. Isothermal crystallization kinetics data on PEEK can be described by an Avrami equation with an Avrami constant n of about 1.     

Characterization of polymers using the thermally stimulated current technique

Thermally Stimulated Current (TSC) technique is a relaxation technique, such as DMA, but works at very low equivalent frequency (10^?3–10^?4Hz) and thus clearly resolves the peaks related to the molecular mobility. The effect of orientation on Polypropylene fibers, the relaxation of internal stresses on Polycarbonate disks, and the effect of aging on acrylic airplane windows have been investigated using the TSC method.     

Dielectric relaxation spectroscopy of amorphous and liquid-crystalline side-chain polycarbonates

The molecular dynamics of amorphous and liquid-crystalline (LC) side-chain polycarbonates was studied by dielectric spectroscopy at frequencies from 10⁻² to 10⁶ Hz and at temperatures from −160 to 180°C. ‘Model’ compounds containing no mesogenic side-groups showed two relaxations, which originate from the carbonate group (α, β_m-relaxation). By contrast, in LC-polycarbonates bearing a mesogenic nitrostilbene side group around and above the glass transition temperature T_g up to three relaxation modes were distinguished (α-, λ₁-, λ₂-process); below T_g four secondary relaxations (γ-, β_m-, β_s-, β_sc-relaxation) were observed. The γ-relaxation was found only in compounds possessing an aliphatic spacer linked to the backbone by an ether bond. Apart from β_m-, two additional β-processes were identified as relaxations associated with the mesogenic unit in the glassy (β_s) or in the crystalline state (β_sc).     

Relaxation behavior of aliphatic-aromatic poly(ether amide)s as revealed by dynamic mechanical and dielectric methods

The mechanical and dielectric relaxation of a set of aromatic-aliphatic polyamides containing ether linkages have been examined as a function of temperature (−140 to 190°C) and frequency (3 to 10⁶ Hz). The polymers differ in the orientation (meta and para) of the aromatic rings, in the length of the aliphatic chain, and in the number of ether linkages per repeating unit. Dynamic mechanical experiments showed three main relaxation peaks related to the glass transition temperature of the polymers (α relaxation), the subglass relaxations associated to the absorbed water molecules (β) and to the motion of the aliphatic moieties (γ). Dielectric experiments showed two subglass relaxation processes (β and γ) that correlates with the mechanical β and γ relaxations, and a conduction process (σ) above 50°C that masks the relaxation associated to the glass transition. A molecular interpretation is attempted to explain the position and intensity of the relaxation, studying the influence of the proportion of para- or meta- oriented phenylene rings, the presence of ether linkages and the length of the aliphatic chain. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys, 35: 457–468, 1997     

Dielectric relaxations of chitosan: The effect of water on the α‐relaxation and the glass transition temperature

In this work thermal relaxations of chitosan are reported by using a novel methodology that includes subtraction of the dc conductivity contribution, the exclusion of contact and interfacial polarization effects, and obtaining a condition of minimum moisture content. When all these aspects are taken into account, two relaxations are clearly revealed in the low frequency side of the impedance data. We focus on the molecular motions in neutralized and non‐neutralized chitosan analyzed by dielectric spectroscopy in the temperature range from 25 to 250 °C. Low and high frequency relaxations were fitted with the Havriliak and Negami model in the 10^?1 to 108 Hz frequency range. For the first time, the low frequency α‐relaxation associated with the glass‐rubber transition has been detected by this technique in both chitosan forms for moisture contents in the range 0.05 to 3 wt % (ca. 18–62 °C). A strong plasticizing effect of water on this primary α‐relaxation is observed by dielectric spectroscopy and is supported by dynamic mechanical analysis measurements. In the absence of water (<0.05 wt %) the α‐relaxation is obscured in the 20–70 °C temperature range by a superposition of two low frequency relaxation processes. The activation energy for the σ‐relaxation is about 80.0–89.0 kJ/mol and for β‐relaxation is about 46.0–48.5 kJ/mol and those values are in agreement with that previously reported by other authors. The non‐neutralized chitosan possess higher ion mobility than the neutralized one as determined by the frequency location of the σ‐relaxation. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2259–2271, 2009     

Dielectric relaxation behavior of poly(methyl methacrylate) under high‐pressure carbon dioxide

An in situ dielectric measurement for atactic poly(methyl methacrylate) (at‐PMMA) was performed under high‐pressure CO₂ under various pressures and temperatures. The at‐PMMA has the acetate side group with a large dipole moment. In the glassy state, a local relaxation process (β‐process) can be observed using dielectric measurement. In the rubbery state, the micro‐Brownian motion of main chain (α‐process) occurs, and the β‐process changes into αβ‐process coordinated with the α‐process. The dielectric loss (ε″) spectrum of at‐PMMA in the glassy state is asymmetric because of the density fluctuation for the amorphous structure. The loss peak frequency shifted to higher frequencies, and the relaxation strength increased with increasing CO₂ pressure. In the glassy state, the shape of ε″ spectrum became more symmetric with increasing CO₂ pressure. These show that the molecular mobility enhanced by the plasticization effect of CO₂ allows the dipolar side groups in the high‐density region to contribute to the relaxation process. We also found that the apparent activation energy decreased under high‐pressure CO₂. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2951–2962, 2005     

Dielectric relaxation study of gamma irradiated oriented low-density polyethylene

The influence of drawing, gamma irradiation and accelerated aging on the dielectric relaxation of low-density polyethylene has been studied using dielectric loss tangent measurements in the temperature range from 25 to 325 K and in the frequency range from 10³ to 10⁶ Hz. The intensity, position and activation energy of the γ- and β-dielectric relaxations were found to be strongly dependent upon the changes in the microstructure of the amorphous phase induced by uniaxial orientation, oxidation and crosslinking.     

Thermally Stimulated Current spectroscopy for the study of thermoplastics

Relaxation phenomena in polycarbonate have been explored by Thermally Stimulated Current (TSC) spectroscopy. A comparative study of transitions by differential scanning calorimetry has also been undertaken. In the sub-T _g region, the observed relaxations have been associated with the diffusion of local defects along the chains. In the glass transition region, the annealing induces the segregation of two relaxation modes: the lower temperature component associated with the unstressed amorphous phase, the higher temperature component attributed to stressed amorphous phase responsible for the modules observed in electron microscopy and X-ray diffraction.

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Zusammenfassung RelaxationsphÄnomene von abgeschrecktem und getempertem Polycarbonat (Lextan) wurden mit Thermisch Stimulierten Feldpolarisations Spektroskopie [Thermally Stimulated Current (TSC) Spectroscopy] untersucht. Unterhalb der Glasumwandlung wird eine Relaxation beobachtet, die mit einem molekularen Kettenmechanismus in Zusammenhang gebracht wird. Temperung verursacht eine Trennung in eine spannungsfreie und eine unter Belastung durch die Kristalle stehende amorphe Phase. Experimentelle Befunde werden diskutiert und Modellbetrachtungen zur dielektrischen Relaxation vorgestellt.

     

Complex dielectric constant and relaxation parameters of aqueous solutions of barium nitrate

Microwave dielectric properties of aqueous solutions of barium nitrate are studied as a function of concentration at frequencies from 3.4 to 23.5 GHz and temperatures from 283 to 313 K. The following dielectric parameters of the solutions are determined: static dielectric constant ε_s, dielectric relaxation time τ, and enthalpy of activation of dielectric relaxation ΔH _ε ⁺⁺ . In transfer from water to the solutions, the static dielectric constant decreases for all test solutions. Decreases in τ and ΔH _ε ⁺⁺ are induced by the increasing mobility of water molecules in ion hydration shells. The ionic effect disappears as temperature increases.     

Temperature and frequency dependencies of the complex dielectric constant of poly(ethylene oxide) under hydrostatic pressure

Electrical impedance measurements have been made in the frequency range 5 Hz to 10 MHz in pure poly(ethylene oxide) having a molecular weight of 600,000 from 12 K nearly up to the melting point of the crystalline phase (about 330 K). A pronounced relaxation peak in the dielectric loss and a corresponding step in the dielectric constant have been observed at about 240 K, which can be readily related to the glass-rubber transition in the amorphous region of the polymer. As the temperature approaches the melting point there are large increases in the real ϵ′ and imaginary e′ parts of the dielectric constant. The frequency dependence of ϵ′ is characterized by a primary relaxation process, whose frequency increases with increasing temperature as a consequence of decrease of the average structural relaxation time. There is strong evidence that this low-frequency dispersion arises mainly from the diffusive transport of ionic charge carriers rather than a purely orientation relaxation process. In addition, the effects of hydrostatic pressures (0–0.25 GPa) on the frequency dependencies of the real ϵ′ and imaginary ϵ′ parts of the dielectric constant have been measured in the temperature range from 254 to 329 K. An advantage of applying pressure is that it shifts the α_𝒶 relaxation peak into an experimentally accessible frequency window of the equipment; the lowering of frequency results from a decrease in the relaxation volume and a consequent reduction in the mobility of the molecular units. Results are discussed in terms of theoretical models of the effect of pressure on the glass transition, providing information on the cooperative dynamics. © 1996 John Wiley & Sons, Inc.     

烷基聚葡糖苷C9.6G1.3/n-C4H9OH/n-C6H14/H2O微乳液体系的介电弛豫谱研究

The pseudotertiary phase diagram of the microemulsion system alkyl polyglucoside/n-butanol/n-hexane/water was plotted at （30.0±0.1） ℃. The dielectric measurements, including permittivity, conductivity, relaxation strength, characteristic relaxation time, etc,, were applied to investigate the microstructure of the system. Unique dielectric relaxations were observed over the frequency range of 5-10^7 Hz, taking place possibly through an interracial polarization mechanism. According to the results obtained from dielectric spectroscopy, the structures of the microemulsion O/W, BC and W/O were determined, and some dielectric and phase parameters were calculated.     

Columnar phases of achiral vanadyl liquid crystalline complexes

Two liquid crystalline vanadyl complexes have been studied by frequency domain dielectric spectroscopy over the range 10 mHz to 13 MHz. The materials exhibit two or three columnar phases denoted Col_ro, Col_rd, and Col_hd that were identified by X-ray diffraction. In the higher temperature Col_rd phase, a relaxation process in the kHz range is observed that is attributed to the reorientation about the molecular short axis. A pronounced dielectric relaxation process shows up in the low temperature Col_ro phase at hertz and sub-hertz frequencies. This slow relaxation is assigned to reorientation of the molecular dipoles within the polar linear chains, which are aligned along the column's axis. Triangular wave switching studies at low frequency reveal processes inside the Col_ro phase which are most probably due to ionic/charges relaxations but a ferroelectric switching for an achiral discotic system cannot be ruled out completely. Below the Col_ro phase there is an orientationally disordered crystalline Cr_x phase with disordered side chain dipoles. A dielectric relaxation process connected with the intramolecular relaxation of the alkoxy side chains, similar to the beta-process of polymers, has been found in the lower temperature Cr_x phase.     

Mechanical relaxations and moduli of oriented nylon 66 and nylon 6

The mechanical relaxations of dry and wet nylon 66 and nylon 6 with draw ratios λ = 1–3 have been studied from ?180 to 160°C and in the frequency range of 1 Hz to 10 MHz. The five independent elastic moduli C_11, C_12, C_13, C_33, and C₄₄ have also been determined by an ultrasonic method at 10 MHz. Wide-angle x-ray diffraction and birefringence measurements reveal that the crystalline orientation rises sharply at low λ and becomes saturated near λ = 3; the amorphous orientation function increases continuously, reaching values of 0.3–0.5 at λ = 3. The alignment of molecular chains and the presence of taut tie molecules in the amorphous regions lead to a lowering of segmental mobility, thereby reducing the magnitude and increasing the peak temperature and activation energy of the α relaxation. Water absorption weakens the interchain bonding and so gives rise to effects opposite to those of drawing. At low temperature, the development of mechanical anisotropy is largely determined by the overall chain orientation, with the c-shear mechanism contributing a small additional effect. However, above the α relaxation, where the amorphous region is rubbery, the stiffening effect of taut tie molecules becomes dominant and leads to increases in all moduli.     

Broadening of the glass transition in blends of poly(aryl ether ketones) and a poly(ether imide) as studied by thermally stimulated currents

Blends of poly(aryl ether ketones) (PAEKs) and an amorphous poly(ether imide) (PEI) were used as model systems to study the broadening of the glass transition due to crystallization and the resulting depletion of PAEK from the amorphous phase. Two different PAEKs were studied, which are completely miscible with PEI in the amorphous state; poly(aryl ether ether ketone) (PEEK) and a slower crystallizing poly(aryl ether ketone ketone)(PEKK). Relatively rapid crystallization conditions were chosen in order to trap a significant fraction of PEI between the PAEK crystal lamellae or between bundles of lamellae. The broad glass transitions are apparently a result of the nonuniform nature of this process. The breadth of the glass transition was quantified by thermally stimulated currents (TSC) applied in the thermal sampling (TS) mode. The results compared favorably with DSC data. The magnitude of the apparent activation energy obtained by the TS method allows one to assign the relaxations as cooperative (glass transition-like) or non-cooperative and to define the limits of the glass transition with a higher degree of precision than other techniques. Cooperative relaxations can be resolved with this technique, even if they are only a small fraction of the overall relaxing species at a given temperature. In some cases the glass transition region was found to broaden to ca. 60°C after crystallization. © 1993 John Wiley & Sons, Inc.     

Correlation between dipolar TSDC and AC dielectric spectroscopy at the PVDF glass transition

The α_a-mode (associated to the dynamic glass transition) in PVDF-α has been studied by Thermally Stimulated Depolarization Currents (TSDC) and Dielectric Spectroscopy (DS) techniques. The distribution of relaxation parameters, reorientation energies, characteristic temperature, and preexponential factors of the Vogel–Tammann–Fulcher relaxation times have been precisely determined by using the Simulated Annealing Direct Signal Analysis applied to a partially discharged TSDC α_a peak. This distribution has been used to predict the variation of the dielectric loss, ε″(ω, T), in the temperature and frequency range where the DS measurements were made on the same material. The simulated ε′(T, ω) for various ω, are compared to the experimental values. The width of the peak is always too low, due to the restricted distribution used for the generation of the curves. A relaxation map including the TSDC results is used to determine the relaxation time variation. In the limited frequency range where the AC DS experiments are performed (10² ≤ f ≤ 10⁵ Hz) a master curve is drawn and the exponents of the frequency dependence are found at low and high frequency; also, a fitting to the Havriliak–Negami distribution is successfully performed. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 2483–2493, 1997     

Relaxations in thermosets. XII. Dielectric effects during curing of nonstoichiometric DGEBA-based thermosets

The dielectric permittivity ε′ and loss ε″ of diglycidyl ether of bisphenol-A thermosets cured with nonstoichiometric amounts of diamino-diphenyl methane have been measured during the course of their chemical reactions from the sol to gel to glass-formation regions. ε′ monotonically decreases with time and ε″ initially decreases, increases to a peak value, and finally decreases to extremely low values characteristic of the glassy state. The initial decrease in ε″ is due to the decrease in the dc conductivity, and the peak is due to the dipolar reorientation. The appearance of these features shifts to longer time when the thermoset is stoichiometrically starved by decreasing the amount of the curing agent and, at a molar ratio of 4 : 1 of the epoxide to diamine, the ε″ peak does not appear during the curing process. Complex plane plots of ε′ and ε″ have the shape of an arc in all cases except when the molar ratio of the epoxide to diamine is 4 : 1. The dielectric consequences of the chemical changes with time during the crosslinking of a thermoset are analogous to the frequency dependence of ε″ of a condensed phase. The time dependence of ε″ follows a stretched exponential decay, ?(t = exp ? [(t/τ)^γ], where 0 < γ < 1. The parameter decreases with decreasing amine content. ε″ has been analyzed to obtain the increase in the relaxation time as curing progresses. A representation of ε^* in terms of electrical modulus M^* shows the occurrence of, first a Maxwell relaxation due to dc conductivity, and second a dipolar relaxation, during the period of a typical isothermal cure. Changes in the features of the isothermal cure that occur on changing the amount of the curing agent are discussed in terms of network formation in the thermoset, and the change in the electrical conductivity with curing time has been analyzed in terms of both a power law for gel formation and by a new equation that suggests an approach toward a singularity.     

Contribution of Temperature Modulated DSC® to the Study of the Molecular Mobility in Glass Forming Pharmaceutical Systems

The temperature modulated differential scanning calorimetry (MDSC^®) technique has been used to characterise the low frequency molecular mobility of indomethacin and maltitol just above their respective calorimetric glass transition temperatureT _g. Analysis has been made using the concept of complex specific heat. Spectroscopic information are thus obtained through the temperature dependence of the isochronal real and imaginary parts C′ and C″. This gives access to the fragility index m and the stretched exponent β. The comparison with dielectric spectroscopy has been performed to check the coherence of spectroscopic information. Measurements on maltitol enable to demonstrate the useful complementarity of the technique when the low frequencies dielectric relaxations are occulted by the presence of conductors default.