Depolarization currents in illite

Journal of Thermal Analysis and Calorimetry - In ceramic materials, depolarization effects take place when an external electrical field is removed. Therefore, an understanding of mechanisms of...     

Persistent polarization in polymers. I. Relationship between the structure of polymers and their ability to become electrically polarized

The ability of various polymers to acquire persistent polarization (i.e., to become electrets) was investigated. Polarization was induced in the polymers by two methods: (a) by cooling under a voltage drop and (b) by cooling with flow under pressure. There was found to be an optimum temperature for electret formation by either method. This optimum temperature was roughly 37°C. above T_g when polarization was produced by application of a voltage drop and roughly 57°C. when polarization was produced by flow under pressure. Crystallinity and the nonhomogeneities present in blended polymers were harmful to electret formation, but a small critical amount of ionic impurity was helpful.     

Depolarization thermocurrents in amorphous polymers

The depolarization thermocurrent (DTC) method gives the dependence of the dielectric relaxation time on temperature. It has been used for investigations of relaxations obeying an Arrhenius-like law in crystalline polymers. The analysis of this method shows it is possible to study mechanisms described by the Williams-Landel-Ferry (WLF) equation. The critical temperature appearing in the free-volume theory of Cohen and Turnbull and also in the statistical thermodynamic theory of Adam and Gibbs can then be measured with good accuracy. The thermal coefficient of expansion of the free-volume and the WLF coefficient for any reference temperature can also be obtained. Since analysis of the experimental DTC spectrum is particularly simple, this method seems to be a very useful tool for examination of relaxation transitions in amorphous polymers. As an example, results obtained for poly(methyl methacrylate) are presented; they are consistent with published data.     

Persistent polarization in poly(vinylidene fluoride). III. Depolarization and pyroelectricity of poly(vinylidene fluoride) thermoelectrets

Persistent polarization in poly(vinylidene fluoride) thermoelectrets prepared under high electric field has been studied by measurements of depolarization and pyroelectricity. Various polarizations are examined in detail; the polarizations related to a characteristic molecular motion near 60°C and the polarizing temperature are not responsible for the major piezoelectric effect in β-form electrets. The piezoelectricity is attributed to a polarization appearing near the melting temperature. The persistent polarization corresponding to d₃₁ of 2 × 10^?11 coul/N is about 5 × 10^?6 coul/cm². The pyroelectricity of β-form electrets is linearly correlated with the piezoelectricity.     

Persistent polarization in poly(vinylidene fluoride). II. Piezoelectricity of poly(vinylidene fluoride) thermoelectrets

The piezoelectricity of PVDF thermoelect rets formed with vacuum-coated aluminum electrodes has been investigated in detail. The piezoelectricity depends on the β-form crystal structure of PVDF homopolymer and copolymers. However, the piezoelectricity is not attributed to the stress dependence of the spontaneous polarization of β-form crystals, but rather to the persistent polarization arising from trapped charges. The trapping mechanism is discussed.     

Depolarization field effect on the ferroelectric behavior of polymers

The effect of the depolarization field in crystallites on the ferroelectric behavior of polymers is discussed on the assumption that the crystallite is a prolate ellipsoid with its major axis directed along the electric field. The theory relates the polarization P and the electric field E in the crystallite to the overall values P and E for the polymer. The determination of the P -E hysteresis of a crystallite from the P-E hysteresis is given with examples for poly(vinylidene fluoride) (PVDF) and vinylidene fluoride (VDF)–trifluoroethylene (TrFE) copolymer. The ratio R = J/(P_s ? P), with J the switching current density and P_s the saturation polarization of polymer, is proved to be free from the depolarization field effect and the plot of logR against logarithmic time is shown to have merit for characterization of switching behavior. Examples of the curves are given for PVDF and VDF-TrFE copolymers. The temporal change of local electric field in the unreversed domains in the crystallite in the course of polarization reversal is predicted by the theory and this change is proved to be a significant mechanism of switching acceleration.     

Diffusion in glassy polymers. II

The Fickian diffusion coefficient of methylene chloride in a glassy epoxy polymer is calculated with the use of Crank's model of discontinuous change of D with concentration C. The diffusion constant is obtained as 1.93 × 10^?6 cm²/sec. The swollen layer behind the advancing solvent front is essentially in the rubbery state of the same polymer. The case II swelling by benzene is discussed in terms of a convective transport arising from the partial stress (internal) tensor of the penetrant. The superposition of Fickian and case II diffusion found with mixtures of methylene chloride and benzene is also discussed briefly.     

Investigations on electric properties of polymers—II. Dielectric relaxation in atactic polystyrene determined by thermally stimulated depolarization currents

The thermally stimulated depolarization currents (TSDC) from atactic polystyrene and d.c. conductivity of the same material have been studied over the temperature ranges 200–395 K and 363–393 K respectively. TSDC spectra are complex and consist of four peaks, β₁, β₂, β₃ (β₃ of reduced intensity) and α in order of increasing temperature. The first three peaks appeared below the glass transition temperature T_g of the polymer. The β₁ peak seems to arise from a single dipolar relaxation process. The β₂ peak and probably β₃ arise from a distribution in activation energy of dipolar relaxation processes. On the basis of the molecular origin of the β relaxation process, it was suggested that β₁, β₂ and β₃ peaks involve motions of backbone chain fragments of various lengths. The last peak α appeared at T_g and could be considered as a result of dipole relaxation and electric conductivity. TSDC peaks and d.c. conductivity have been related to the second order transition in the polymer.     

Electronic currents in anodic polarization of oxide-covered valve metals

A model for the electron current in anodic oxidation of valve metals is presented by considering the film-forming electrolyte solution as a doped semiconductor which injects electrons into the oxide layer. The model enables us to explain the available experimental facts concerning the dependence of the current on the field in the oxide and the nature and concentration of the solution.     

Chemorheology of linear polymers. II

The chemical stress relaxation of crosslinked polymers had long been studied. The chemorheology of linear polymers is difficult to study because molecular flow by diffusion is generally much more rapid than relaxation or flow caused by chemical reaction. Our previous paper dealt with the chemorheology of linear polymers on taking such physical flow into consideration. By using another modified expression suggested by Tobolsky, good consistency between the theoretical chemical stress relaxation curve and an experimental one was obtained for polydisperse polystyrene and the commercial polystyrene used.     

Orientation and molecular dynamics in uniaxial polymers. I. Theory of fluorescence polarization

A theory of polarized fluorescence is developed for uniaxial physical systems in which micro-Brownian motion is not negligible. All experimental information is shown to reduce, with reasonable assumptions, to four quantities characterizing molecular orientation and reorientational molecular mobility. The geometrical properties of the mobility in uniaxial systems are studied and methods are given for correcting for the effects of the birefringence and for a possible delocalization of the fluorescence transition moments.     

Metal coordination polymers. II. Molecular weight studies of beryllium phosphinate polymers in toluene

The number-average molecular weights of beryllium 4-biphenyl(phenyl)phosphinate, di-n-pentylphosphinate, di-n-heptylphosphinate, and trifluoromethyl(phenyl)phosphinate are degraded by the presence of water in toluene. It is proposed that easily hydrolyzable P? O? P bonds contribute, in part, to the bonding of these polymers.     

Dehydrochlorination reactions in polymers. Part II. Chlorinated polystyrene

The dehydrochlorination of chlorinated polystyrene was studied in the temperature range 120–220°C., when HCl was the sole volatile product. The dehydrochlorination was accompanied by the slow development of color and chain scission. The elimination rate fell too rapidly with reaction extent to be accounted for by reduction of reagent. Solution studies in o-dichlorobenzene indicated that the reaction was initially first-order in polymer concentration but was retarded by the polyene reaction products. The overall elimination was interpreted as a radical process in which the product was an active retarder. These observations are also valid for the elimination of HBr from brominated polystyrene.     

Theoretical models for diffusion in glassy polymers. II

The author refines and generalizes a model for diffusion in glassy polymers which he previously introduced. The model unifies many diverse observations by explicity formulating the common property of a glassy polymer in all its various modes, namely the finite relaxation time due to its slow response to changing conditions. An integral approximation method is used to study the motion of the penetrant front and the glass-gel interface and a useful polynomial approximation method is introduced for use in special simple situations.     

Macroreticular redox polymers. II. Further synthesis and properties of some redox polymers

A series of redox polymers was prepared by the addition of different redox groups to preformed, chloromethylated macroreticular styrene–divinylbenzene copolymers. These polymers contained the hydroquinone, hydroquinonesulfonic acid, methylhydroquinone, 2,5-dimethylhydroquinone, 2,5-dimethylhydroquinonesulfonic acid, 2,3,5-trimethylhydroquinone, tert-butylhydroquinone, chlorohydroquinone, benzyl mercaptan, anthraquinone, and the pyrogallol redox groups. Thus, a set of redox polymers is available having redox potentials that may range from approximately 150 to 700 mv.     

Inorganic coordination polymers. VIII. Cobalt(II) and zinc(II) phosphinate polymers and copolymers

The preparation, properties, x-ray powder patterns, and TGA curves are given for cobalt(II) dimethyl-, methylphenyl- and diphenylphosphinate. The polymeric character of cobalt(II) methylphenylphosphinate is demonstrated by its colligative properties and melt indexes. These cobalt(II) phosphinates along with hybrid copolymers of zinc(II) or cobalt(II) were prepared by the reaction of the metal acetate with the appropriate phosphinic acid or mixture of phosphinic acids. A consideration of the hybrid copolymers leads to the conclusion that both the zinc(II) and cobalt(II) dimethyl- and diphenylphosphinates are likewise polymeric. Thermal stability is discussed in terms of the structures suggested, and it is shown that crystallinity is related to polymer symmetry.     

Stretching globular polymers. II. Macroscopic cross-linked networks

We expand upon the results for the force-extension behavior of single-collapsed polymer chains to consider the mechanical response of networks of cross-linked globular polymers in poor solvent. Force-strain curves are obtained under the affine deformation approximation for networked globules with both disordered and ordered globule conformations. Due to their large stored lengths, these networks would be capable of reaching extremely large strains. They also show anomalous nonmonotonic force-strain response, as a consequence of the nonmonotonic force-extension curves of their constituent globules. Finally, we consider the stability of ordered and disordered globules in these networks and propose means taken from biological and colloid science to stabilize networked globules.