Dielectric Relaxation Of Poly Research Articles

Dielectric relaxation in poly(styrene-b-butadiene) copolymers with perfluorinated side chains

The dynamics of several poly(styrene-b-butadiene) (PS-PB) copolymers with fluorocarbon side chains have been studied with dielectric spectroscopy, covering two different mesophases with long spacings between 10 and 30 nm, and two different tail lengths. It is found that fluorination results in comparably rigid side chains in the PB domains with highly anisotropic properties. In the glassy state an Arrhenius relaxation was found with an activation energy of 7.4 kcal/mol reflecting some local side chain relaxation. Above the PB glass transition a bimodal relaxation exists, comprising two cooperative processes reflecting solely the anisotropic dynamics of the side chain. The dual relaxation resembles the anisotropic dielectric properties of side chain liquid crystalline polymers in the absence of macroscopic orientation.

Low frequency ac conduction and dielectric relaxation in poly(N-methyl pyrrole)

The ac conductivity and dielectric constant of poly(N-methyl pyrrole) thin films have been investigated in the temperature range 77–350 K and in the frequency range 10 2-102 Hz. The well defined loss peaks have been observed in the temperature region where measured ac conductivity approaches dc conductivity. These loss peaks are associated with the hopping of the charge carriers. The frequency and temperature dependence of ac conductivity have been qualitatively explained by considering the contribution from two mechanisms; one giving a linear dependence of conductivity on frequency and other having distribution of relaxation times giving rise to broad dielectric loss peak.

Dynamic mechanical and dielectric absorptions in poly(2-tert-butylcyclohexyl methacrylate) and poly(4-tert-butylcyclohexyl methacrylate)

Dynamic mechanical and dielectric relaxations of poly(2-tert-butylcyclohexyl methacrylate) (P2tBCHM) and poly(4-tert-butylcyclohexyl methacrylate) (P4tBCHM) have been measured. The tert-butyl group inhibits the chair-to-chair motions in the cyclohexyl ring. However, small motions of the cyclohexyl group are still possible giving rise to subglass relaxations in both polymers. The dielectric response is more sensitive than the mechanical one to the corresponding applied force fields

Microwave dielectric relaxation study of poly(propylene glycol) in dilute solution

Dielectric relaxation studies of poly(propylene glycol), average molecular weight 2000 g mol−1, in dilute solution of cyclohexane, decaline, benzene and carbon tetrachloride have been carried out at 10.10 GHz and 35 °C. Average relaxation time τ0, and relaxation times corresponding to segmental motion τ1, group rotations τ2 and dipole moment µ have been determined. It is found that τ0, τ1 and µ are influenced by the solvent environment while the τ2 value is solvent-independent. A comparison has been made with the dielectric behaviour of poly(ethylene glycol), average molecular weight 1500 g mol−1, in dilute solutions of benzene and carbon tetrachloride because both systems have an equal number of monomer units. The effect of methyl side-groups on dielectric relaxation in poly(propylene glycol) molecules is discussed. The Kirkwood correlation factor is also evaluated in dilute solutions with concentration variation and it is found that these molecules exist in cluster form due to intermolecular hydrogen bonding. © 2000 Society of Chemical Industry

Dynamic mechanical and dielectric relaxations of poly(difluorobenzyl methacrylates)

This work reports the mechanical and dielectric relaxation spectra of three difluorinated phenyl isomers of poly(benzyl methacrylate), specifically, poly(2,4-difluorobenzyl methacrylate), poly(2,5-difluorobenzyl methacrylate) and poly(2,6-difluorobenzyl methacrylate). The strength of the dielectric glass-rubber relaxation of the 2,6 difluorinated phenyl isomer is, respectively, nearly three and two times larger than the strengths of the 2,5 and 2,4 isomers. The 2,4 isomer presents a mechanical a peak the intensity of which is nearly two times that of the other two isomers. Both the mechanical and dielectric relaxation spectra display a subglass process, called γ relaxation, centered in the vicinity of -50 °C at 1 Hz and, in some cases, a subglass β absorption is detected at higher temperature partially masked by the glass-rubber relaxation. The mean-square dipole moments per repeating unit, /x, measured at 25 °C in benzene solutions, are 2.5 D 2 , 1.9 D 2 , and 5.0 D 2 for poly(2,4-difluorobenzyl methacrylate), poly(2,5-difluorobenzyl methacrylate) and poly(2,6-difluorobenzyl methacrylate), respectively. These results, in conjunction with Onsager type equations, permit to conclude that auto and cross-correlation contributions to the dipolar correlation coefficient may have the same time-dependence. On the other hand, dipole intermolecular interactions, rather than differences in the flexibility of the chains, seem to be responsible for the relatively high calorimetric glass-transition temperature of the 2,6 diphenyl isomer, which is, respectively, nearly 36 °C and 32 °C above the T g 's of the 2,4 and 2,5 isomers. Molecular Mechanics calculations give a good account of the differences observed in the polarity of the polymers.

Dynamic mechanical and dielectric relaxations in poly(dibenzyl itaconate) and poly(diethylphenyl itaconate)

Dynamic mechanical and dielectric behaviour of poly(dibenzyl itaconate), and poly(diethylphenyl itaconate) were studied. The study was performed by determining the components of the complex relaxation modulus E∗ and the complex dielectric permittivity ε∗. The results are compared with those previously reported for poly(benzyl methacrylate), poly(monobenzyl itaconate) and poly(monoethylphenyl itaconate). The results are discussed in terms of the effect of flexible spacer groups and of the different steric hindrance between mono and disubstituted polymers.

Dielectric relaxation of poly-(β-hydroxybutyrate) relating to microstructure

The dielectric relaxation behavior of a high molecular weight poly(β-hydroxybutyrate) (PHB) synthesized through Azotobacter chroococcum 23 was investigated by measuring the complex dielectric permittivity over wide frequency and temperature ranges. PHB exhibits two dielectrically active processes in the temperature and frequency ranges investigated. The higher temperature relaxation α has been attributed to motions that appear at temperatures above the glass transition temperature T g. The low-temperature process β is thought to be due to local motion of carboxyl groups and local rotations around the C─O bond. The phenomenological Havriliak-Negami (HN) description has been applied to analyze the dielectric loss data and to extract the shape parameters of the above relaxations in an attempt to correlate experimental data to dynamics of relaxation processes.

Dynamic mechanical and dielectric relaxations in poly(pentachlorophenyl methacrylate)

In the present work, a comparative study of the dynamic mechanical and dielectric properties of two related polymers, poly(pentachlorophenyl methacrylate) (PPCPM) and poly(phenyl methacrylate) (PPhM), was carried out. In both polymers a similar relaxational behaviour was observed. Dielectric relaxation measurements give an improved resolution as compared with that obtained from dynamic mechanical techniques for a given process. The polymers show a secondary loss peak at approximately room temperature and a prominent peak overlapped with the conductivity electrode polarization contributions at temperatures above the glass transition temperature, studied by dielectric relaxation measurements. The activation energy associated with the relaxation process at low temperature is about 77 kJ.mol -1 (which is similar to that observed in poly(alkyl methacrylates)) whose origin is attributed to the hindered partial rotation of the carboxymethyl group. An improved method of separating dipolar contributions from the non-dipolar ones was used to characterize the high temperature relaxation process and to obtain information from the diffusion coefficient of the conducting species.

Dynamic mechanical and dielectric relaxations in poly(pentachlorophenyl methacrylate)

Dynamic mechanical and dielectric relaxations in poly(pentachlorophenyl methacrylate)

Dynamic mechanical and dielectric relaxations in poly(monoethylphenyl itaconate)

Dynamic mechanical and dielectric relaxational behavior of poly(monoethylphenyl itaconate) at different frequencies and temperatures was studied. Three relaxation zones are found. The dynamic mechanical response is dominated by a relaxation peak at room temperature, labeled β relaxation. Two prominent shoulders labelled as γ and α relaxations are observed. Because of the overlapping of the α and γ with the β relaxation, a deconvolution method to improve the understanding of these phenomena is proposed. In spite of the complexity of the experimental spectra, the proposed deconvolution method seems to be a convenient approach to interpret the relaxational behavior of this polymer. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2749–2756, 1997

Dielectric Relaxation Behaviour of Ethyl Methacrylate and Vinyl Acetate Grafted Poly[tetrafluoroethylene-co-(perfluorovinyl ether)] Films Prepared by Direct Radiation Grafting Method

Dielectric relaxation of poly(tetrafluoroethylene-co-perfluorovinyl ether) (PFA) grafted by ethyl methacrylate (EMA) and vinyl acetate (VAc) of different compositions prepared by direct radiation was studied at 0.1, 1, 10, and 100 kHZ between 273 and 550 K. Two well resolved relaxation peaks are observed in all films except that with EMA/VAc of compositon 10/90 wt% in which the second peak is masked. In PFA film grafted by EMA/VAc of 50/50 wt% at low temperature the third relaxation was observed. The activation energy for the relaxation was determined from Arrhenius plots. The ac conductivity of the films was investigated and the activation energy of conduction was evaluated. X-Ray diffraction patterns and IR spectra were also measured.

The effect of side groups on the dielectric relaxation of poly(methylhydrogensiloxane)

AbstractThe α‐relaxation of a linear poly(methylhydrogensiloxane) has been investigated as a function of temperature and frequency using dielectric relaxation spectroscopy. Replacement of the hydrogen with a hexyl group increased the temperature and broadness of the α‐process, and this is interpreted in terms of a slowing of the motions of the Si—O dipoles and plasticisation of the siloxane chain by the side groups. Replacement of the hydrogen with a mesogenic group produced a side‐chain liquid crystal which exhibited dielectric relaxation behaviour characteristic of the glass and liquid crystalline states.

Dielectric relaxation in poly(n-alkyl methacrylate)s and their mixtures with p-nitroaniline

In the work described in this paper the dielectric properties of solid solutions of p-nitroaniline in poly(methyl methacrylate), poly(ethyl methacrylate) and poly(n-butyl methacrylate) were measured in the frequency range from 20 to 10 6 Hz and the results obtained were compared with those of the pure polymers. It is shown that the presence of the solute has a strong influence on the relaxation process of poly(methyl methacrylate), and this is ascribed to the formation of hydrogen bonds between the amino group of the solute and the side groups of the polymer. In poly(ethyl methacrylate) this effect is less pronounced and it is absent in the case of poly(n-butyl methacrylate), suggesting that the increasing size of the n-alkyl group prevents hydrogen bond formation between the solute and the polymer.

Dielectric relaxation properties of poly(dimethylphenyl methacrylate)s

The dielectric relaxations of poly(dimethylphenyl methacrylate)s were studied over a wide frequency and temperature range by two methods. The α relaxation, associated with the glass transition temperature, was studied by classical dielectric measurements. Thermal peak cleaning was used to deconvolute the global thermally stimulated current spectra over the range of low temperatures, corresponding to the β relaxation. The α relaxation involves long-range, generalized and highly cooperative motions of the main chain. They are influenced by the steric hindrance and dipole-dipole interactions relative to the position of the CH 3 group of the aryl ring in the polymer repeat unit. As a first approximation it was assumed that the β process was related only to local motions of the side group, but the activation parameters are too high to be consistent with this assumption. Therefore it was concluded that intermolecular factors and local motions of the main chain could also be involved.

Dipole moment and dielectric relaxation of poly(dimethylsilazane) oligomers

AbstractThe polarity of polysilazane chains is studied using as models hexamethyldisilazane (HMDSZ) and an oligomer of amine‐terminated poly(1,1‐dimethylsilazane) (PDMSZ). Solvent effects and the contribution of the atomic polarization to the dipole moment of both PDMSZ and HMDSZ are discussed. A qualitative interpretation of the dipole moment of the oligomer suggests that gauche states about the skeletal bonds may be disfavored with respect to the alternative trans states. The dielectric spectrum of PDMSZ presents a glass‐rubber relaxation followed by two weak secondary absorptions. The experimental results show that the relaxation strength Δε of PDMSZ is lower than that corresponding to poly(1,1‐dimethylsiloxane) (PDMS). The analysis of the glass‐rubber relaxation by using the coupling scheme suggests that strong intermolecular interactions occur in the chains that presumably arise from hydrogen bonding interactions between amine groups. © 1993 John Wiley & Sons, Inc.

Dielectric relaxations in poly(ethylene oxide) based polymers

Int the present work the poly(ethylene oxide) complexed with the NaSCN salt dielectric response is analysed at various salt contents (3%, 11% and 22%) with temperature ranging from 90 to 300 K and frequency from 20 Hz to 2 kHz. Previous studies of the ϵ″ experimental data have shown the γ-peak followed by a rapid increase of the dielectric function due to the conductivity relaxation peak. Between the γ and the conductivity relaxation peaks a weak shoulder has often been observed. The analysis of this feature in terms of the electric modulus reveals that it corresponds to a broad “peak”, usually masked by the conductivity contribution. The position of the shoulder shifts to higher temperatures by increasing both salt content and frequency. A quantitative analysis in terms of a relaxation times distribution with a Gaussian distribution function for the activation energies gives a good fit of the experimental data.

Dielectric relaxations of poly(fluoroalkyl methacrylates) and poly(fluoroalkyl .alpha.-fluoroacrylates)

In two homologous series of poly(fluoroalkyl methacrylate) (PFM) and poly(fluoroalkyl α-fluoroacrylate) (PFF)there were observed α above T g and γ relaxations below T g , which are assigned to reorientational motion of long segments and local molecular motion of fluoroalkyl side groups, respectively. As the length of the fluoroalkyl side groups are longer, the a relaxation gradually shifted to lower temperatures and the γ relaxation rapidly moved to higher temperatures, being very consistent with the well-known results in the homologous series of poly(alkyl methacrylate)

Dielectric relaxations in poly [2,2‐propane‐bis‐(4‐phenyl thiocarbonate)

AbstractThe dielectric relaxation properties of poly[2,2‐propane‐bis‐(4‐phenyl thiocarbonate)] (PTC) have been studied. The existence of crystallinity, which can be eliminated by quenching, is detected. The degree of crystallinity of polymer samples was determined by differential scanning calorimetry in order to investigate the effect of this factor on the dielectric behaviour of this polymer. The thermal degradation of the samples was studied by thermogravimetry. The degradation of the polymer begins before the glass transition temperature Tg. The dielectric spectrum is complex showing several relaxation phenomena. With increasing temperature a γ relaxation can be observed at ‐ 100°C (5 kHz). The activation energy obtained from an Arrhenius plot (lnfvs T−1) is 6 kcal mol−1. At 160°C the α relaxation which is associated with the glass transition temperature Tg is detected. The dielectric behaviour of this poly(thiocarbonate) is compared with the corresponding poly(carbonate).

Subglass relaxations: mechanical vs. dielectric relaxation in poly(vinyl acetate) and its ethylene copolymers

The complex shear moduli were measured in the subglass relaxation temperature region using a torsion pendulum. Dielectric relaxation measurements were also carried out. The dielectric and mechanical relaxation features are in accord with similarities and differences expected on the basis of a previous interpretation of dielectric relaxation.

Dielectric relaxation of poly(benzyl methacrylate)

AbstractThe relaxation processes in atactic poly(benzyl methacrylate) were studied by means of dielectric measurements. Beside the α‐ and β‐relaxation processes due to the segmental motions of the main chain and of the entire side chain, respectively, a process designated as γ was found to occur in the temperature range between −60°C and 0°C. For the γ‐relaxation, the time‐temperature superposition principle is applicable. A linear relationship between temperature and the relaxation frequency fm or shift factor aT was found. Apparent activation energies estimated from the temperature dependence of fm and aT are 50 and 53 kJ · mol−1, respectively. The γ‐process is considered to result from the movement of the benzyl group in the side chain.