Dielectric Relaxation Behavior Of Poly Research Articles

Effect of annealing treatment on crystalline and dielectric properties of PVDF/PEG-containing ionic liquid composites

To investigate the effect of poly(ethylene glycol)-containing ionic liquid (IL) and thermal annealing treatment on crystallization and dielectric relaxation behavior of poly(vinylidene fluoride) (PVDF), a series of PVDF/IL composites have been prepared using a solution-cast method. The interaction between imidazolium cation of ILs and >CF2 groups of PVDF can transfer nonpolar α phase to polar β and γ phase as a template at the primary crystallization, and then the interaction between PEG of ILs and >CF2 groups of PVDF can facilitate the PVDF chains in amorphous region to bring more polar phase and decrease the degree of crystallization for PVDF composites according to the template under thermal annealing treatment. The experimental dielectric data were analyzed within the formalisms of complex permittivity and electric modulus. In the frequency spectra of PVDF composites, dc conductivity, electrode and Maxwell-Wagner-Sillars (MWS) interfacial polarization resulted in high values of dielectric permittivity. The incorporation of ILs into the matrix and thermal annealing treatment can accelerate dc conductivity. The temperature dependence of relaxation time of dc conductivity follows the Arrhenius equation. The effect of annealing treatment on charge carrier movement mechanism was resulted from the increase of ion mobility induced by polymer chain segmental motion and polar phase crystals.

Enhanced dc conductivity and conductivity relaxation in PVDF/ionic liquid composites

To investigate the effects of 1-benzyl-3-methylimidazolium hexafluorophosphate on the crystal phase, morphology and dielectric relaxation behavior of poly(vinylidene fluoride) (PVDF), a series of PVDF/IL composites have been prepared using solution-cast method. The interaction between ILs and >CF2 groups of PVDF can facilitate the PVDF chains in amorphous area to form more polar phase for PVDF composites. In the frequency spectra of PVDF composites, MWS interfacial polarization, electrode polarization and dc conductivity resulted in high values of dielectric permittivity. The incorporation of ILs into the matrix can accelerate dc conductivity. The effect of IL on charge carrier movement mechanism was resulted from the increase of ion mobility induced by polar phase crystal and ion concentration.

Investigation of relaxation process in poly(vinylidene fluoride–hexafluoropropylene) using dielectric relaxation spectroscopy

Dielectric relaxation behavior of poly(vinylidene fluoride–hexafluoropropylene) [P(VDF–HFP)] is investigated on the basis of dielectric relaxation spectroscopy at 20–200 °C and 20–5 MHz after conversion to complex electric modulus formalism. It is found that imaginary modulus spectra exhibit asymmetry peak with peak-width much broader than that of the Debye peak. The peaks are skewed toward the high frequency sides due to the effect of the conductivity. The complex electric modulus data have been fitted using non-exponent Kohlrausch–Williams–Watts and Cole–Cole functions. The results show that the non-exponent parameter (β) and the shape parameter (α) are all lower than idealized Debye-type, indicating a wide relaxation distribution of P(VDF–HFP). Both the activation energies Ea from $${\text{M}}^{\prime \prime }$$ spectra and conductivity are all due to the contribution of dc and ac conductive relaxation process.

Isothermal Dielectric Relaxations of Poly(vinylidene Fluoride) Filled with Silicon Carbide Nanoparticles

The dielectric relaxation behavior of poly (vinylidene fluoride) based composites filled with beta silicon carbide nanoparticles were investigated over a wide frequency range and temperature intervals. The composites exhibited dielectric relaxations in the tested frequency range and the relaxations of composites can be well described via the modulus formalism of dielectric spectroscopy. Further, activation energy determined from the isothermal dielectric relaxations tended to decrease with increasing SiC indicating the promotion of SiC to the dipole relaxations of PVDF.

Dynamic heterogeneity in a miscible poly(vinyl acetate)/poly(ethylene oxide) blend

We studied viscoelastic and dielectric relaxation behavior of poly(vinyl acetate)/poly(ethylene oxide) (PVAc/PEO) blends in the miscible state. Especially for the viscoelastic measurements, we used highly asymmetric molecular weight MW combinations to examine the component dynamics separately. Time–temperature superposition principle did not hold since the two components have different shift factors (aT). We determined the aT of both components in the blends for the terminal relaxation modes. Concerning the dielectric relaxation, we observed single α-relaxation peak and found that the α-relaxation mainly reflected the segmental motion of PVAc in the blends. The dielectric and viscoelastic aT of the PVAc component showed almost the same temperature dependence compared at the same composition. From these aT data, we determined effective glass transition temperatures (Tg,eff) as a function of blend composition for both components. The results were compared with the literature data for a similar miscible blend, poly(methyl methacrylate) (PMMA)/PEO [T.R. Lutz, Y. He, M.D. Ediger, H. Cao, G.Lin, A.A. Jones, Macromolecules 36 (2003) 1724]. The Tg,eff’s of PEO in the two blend systems were almost the same at any compositions even though the blend partners have very different glass transition temperatures Tg’s: 311K for PVAc and 405K for PMMA. If we use the Lodge–McLeish (LM) model to explain the similar Tg,eff’s of PEO in both blends, the self-concentration of PEO should be dependent on the blend partner. This suggests the necessity to take into account the effect of the inter-molecular interaction in the LM model.

Dielectric relaxation behavior of poly(acrylonitrile- co-methacrylonitrile) microcapsules dispersed in a silicone matrix

The dielectric relaxation behavior of poly(acrylonitrile- co-methacrylonitrile) dispersed in a cured polydimethyl siloxane (PDMS) matrix as microcapsules was investigated over multiple thermal cycles and at varying concentrations. The copolymer microcapsules contained an isopentane core. In the PDMS matrix this copolymer displayed a pronounced relaxation signal at temperatures above the glass transition of the copolymers due to Maxwell–Wagner–Sillars (MWS) relaxation. The mechanism of MWS relaxation interpreted by the Havriliak–Negami and Kohlrausch–Williams–Watts relaxation functions was found to be very similar to previous studies of neat polyacrylonitrile and its copolymer. The activation energy of the relaxation decreased over successive thermal cycling coincident with a decreasing strength of the relaxation. These observations were attributed to the decreasing concentration of nitrile groups due to intramolecular cyclizations.

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

AbstractAn 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

Influence of oxycycloaliphatic groups in the relaxation behavior of acrylic polymers

A comparative study on the mechanical and dielectric relaxation behavior of poly(5-acryloxymethyl-5-methyl-1,3-dioxacyclohexane) (PAMMD), poly(5-acryloxymethyl-5-ethyl-1,3-dioxacyclohexane) (PAMED), and poly(5-methacryloxymethyl-5-ethyl-1,3-dioxacyclohexane) (PMAMED) is reported. The isochrones representing the mechanical and dielectric losses present prominent mechanical and dielectric β relaxations located at nearly the same temperature, approximately −80°C at 1 Hz, followed by ostensible glass–rubber or α relaxations centered in the neighborhood of 27, 30, and 125°C for PAMMD, PAMED, and PMAMED, respectively, at the same frequency. The values of the activation energy of the β dielectric relaxations of these polymers lie in the vicinity of 10 kcal mol−1, ∼ 2 kcal mol−1 lower than those corresponding to the mechanical relaxations. As usual, the temperature dependence of the mean-relaxation times associated with both the dielectric and mechanical α relaxations is described by the Vogel–Fulcher–Tammann–Hesse (VFTH) equation. The dielectric relaxation spectra of PAMED and PAMMD present in the frequency domain, at temperatures slightly higher than Tg, the α and β relaxations at low and high frequencies, respectively. The high conductive contributions to the α relaxation of PMAMED preclude the possibility of isolating the dipolar component of this relaxation in this polymer. Attempts are made to estimate the temperature at which the α and β absorptions merge together to form the αβ relaxation in PAMMD and PAMED. Molecular Dynamics (MD) results, together with a comparative analysis of the spectra of several polymers, lead to the conclusion that flipping motions of the 1,3-dioxacyclohexane ring may not be exclusively responsible for the β-prominent relaxations that polymers containing dioxane and cyclohexane pendant groups in their structure present, as it is often assumed. The diffusion coefficient of ionic species, responsible for the high conductivity exhibited by these polymers in the α relaxation, is semiquantitatively calculated using a theory that assumes that this process arises from MWS effects, taking place in the bulk, combined with Nernst–Planckian electrodynamic effects, due to interfacial polarization in the films. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2486–2498, 1999

Viscoelastic and Dielectric Relaxation Behavior of Substituted Poly(p-phenylenes)

The thermal, dynamic mechanical, and dielectric relaxation behavior of poly(p-phenylenes) substituted with benzoyl and 4-phenoxybenzoyl groups and of a copolymer of benzoyl-1,4-phenylene and 1,3-phenylene were investigated. These amorphous materials were found to have flexural moduli higher than any other reported unoriented thermoplastic. Benzoyl-substituted poly(p-phenylene) which possesses the greatest rigid-rod character also displayed the highest modulus. The temperature dependence of viscoelastic and dielectric relaxation times were well described by the Williams-Landel-Ferry and Vogel-Fulcher equations, respectively. The temperature sensitivity or «fragility» of viscoelastic shift factors α T and apparent dielectric relaxation times τ * was greatest for the copolymer. The glass transition temperature of these polymers varied as a function of the total free volume. The polymer containing the 4-phenoxybenzoyl side group exhibited the lowest T g due to internal plasticization, but the highest Vogel energy attributable to the large intramolecular bond rotational barrier associated with the bulky pendant group. The shape of frequency plane dielectric relaxation spectra of the substituted polyphenylenes could be accurately fitted to the Kolrausch-Williams-Watts (KWW) correlation function. The KWW stretched exponential term β displayed a very weak dependence on molecular structure and remained constant within experimental error for each material over the temperature range studied. Since β should be proportional to the degree of intermolecular coupling, the structural variation in «fragility», albeit small, of the substituted polyphenylenes may not be fully described by the coupling model for relaxation. Assuming the macroscopic expansion coefficient is proportional to the free volume expansion coefficient, the temperature dependence of τ * and α T may be explained more simply from free volume considerations. The 4-phenoxybenzoyl-substituted polymer displayed the highest expansion coefficient and largest total free volume, but the benzoyl-1,4-phenylene/1,3-phenylene copolymer exhibited the largest relative change in free volume with temperature, which explained the greater «fragility» of the copolymer compared to either homopolymer

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

AbstractThe 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 = εs–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 Tg. For both melt and cold crystallization, the dielectric Tg systematically decreases as the crystallization temperature increases, and as the fraction of rigid amorphous phase decreases.

Concentrated solutions of polydisperse rodlike polymers in the isotropic and lyotropic liquid‐crystalline phases. II. Model simulation of the dielectric relaxation behavior of poly(n‐alkylisocyanates) in toluene and comparison with experimental results

AbstractA simulation has been made of the dielectric relaxation behavior of poly(n‐hexylisocyanate) in solution covering the isotropic, biphasic, and anisotropic ranges. The simulation incorporates the Flory‐Abe statistical mechanical theory for the phase behavior of rodlike macromolecules in solution and the Warchol, Vaughan, Wang, and Pecora theory for the dynamics of a rodlike molecule in a virtual cone prescribed by the neighboring molecules. It is shown that asymmetric Gaussian, Gaussian, or Poisson distributions of molecular weight do not lead to dielectric behavior of the type observed experimentally by Moscicki, Williams, and Aharoni but addition of a high‐molecular‐weight “tail” to such distributions and taking account of the dependence of relaxation time on molecular length gives a simulation of the dielectric increment Δε, the loss maximum ε, and frequency of maximum loss fm, which vary with polymer concentration in a manner entirely consistent with the experimental data.

Relaxation properties of linear polyethers: Dielectric behavior of poly(oxyhexamethylene)

AbstractThe dielectric relaxation behavior of poly(oxyhexamethylene) was studied over the temperature range 50 to −56°C at different frequencies lying in the interval 102 to 105 Hz. A single absorption and dispersion is observed in the region of −40°C which shows the same frequency temperature location as the α mechanical relaxation. Therefore, it is postulated that the mechanisms that give rise to both the dielectric and mechanical processes are of the same nature. The α relaxation is associated with micro‐Brownian motions in the amorphous phase of the polymer and seems to shift to higher temperature as the number of methylene groups in the repeating unit increases. From the analysis on the flexibility of polyethers, including the limiting case of polyethylene, it is concluded that the lower limit for the glass transition of polyethylene should be in the vicinity of −80°C.