The dielectric behaviour of shellac-urea-formaldehyde resins

The Cole-Cole method has been employed for an analysis of the dielectric relaxation data, obtained by Bhattacharya, of a shellac-urea-formaldehyde resin (SUF) in the temperature range 30–100°C. Three relaxation processes were observed for temperatures 30 to 50°C. At 60°C, two relaxation processes were noticed. The relaxations at 70°C and above represented typical Cole-Cole type. The static dielectric constant, relaxation time and distribution parameter of SUF were found to be greater than those of the unmodified shellac. Marked changes in the above mentioned parameters were observed during a rise of temperature from 70 to 100°C. The results of the present study indicate that some rearrangement in the tertiary disposition of the resin molecules takes place during the rise of temperature from 30 to 100°C.     

The dielectric behavior of the natural resin manila copal

The dielectric relaxation data of the natural resin Manila copal, obtained by Bhattacharya,¹ has been analyzed by the Cole–Cole method at temperatures from 30° to 150°C. Two distinct relaxation processes were found for temperatures of 70°, 80°, and 90°C as opposed to only one as observed by Bhattacharya.¹ The relaxation at 100°C and at higher temperatures could be represented by the typical Cole–Cole patterns. The variation by temperature of the static dielectric constant (ε₀) exhibited three distinct slopes, which indicated the different phases of the resin during thermal transformation. Two “transformation points”, around 70° and 100°C, were found instead of only one at 105°C as observed by Bhattacharya.¹ The static dielectric constant, the dielectric increment, the Cole–Cole distribution parameter, and the relaxation time decreased markedly owing to the increase of temperature from 100° to 150°C. These indicated some changes in conformation of the resin molecules during transition from the solid to the liquid state.     

Dielectric relaxations of shellac/amino resin blends

The dielectric constant (ε′) and loss (ε″) of shellac/melamine resin blends have been determined at temperatures between 20° and 120°C and frequencies between 0.1 and 100 kHz. ε′ decreases with increase in the percentage of melamine resin in the blend. Two relaxations have been observed, of ΔH = 4.99 kcal/mole and 11.1 kcal/mole. The glass transition is observed between 60° and 70°C. The cole-cole parameter increases with temperature and becomes constant above T_g.     

Effect of high temperatures on the dielectric behaviour of viscose fibres at low frequency

Commercial and purified viscose fibres exhibit dielectric dispersion and a loss peak within the frequency range 0.05–1 k c/s and at temperatures of 80–140°C. Such high temperature relaxation is local in nature and is attributed to side group motion. The characteristics of the absorption bands of these samples are compared and the results obtained are interpreted. The variation of the dielectric constant with temperature showed a second order transition at about 110°C with these samples, which disappears on hydrolysis.     

Beta dielectric dispersion in natural rubber latex

The dielectric dispersion of natural rubber latex has been investigated over a frequency range of 0.1–100 MHz and at six temperatures from 20° to 70°C. A computer analysis of the dispersion data showed that the Cole–Cole structural equation gave a better fit than the Debye. The large values of the Cole-Cole α spread parameter suggest heterogeneity of structure in rubber latex. The latex viscosity and the relaxation time were both found to be influenced by changes in temperature. Application of the Arrhenius temperature equation to the data yielded a mean activation enthalpy of 32.6 ± 2.8 kJ/mol for the relaxation process.     

Curing behavior and dielectric properties of hyperbranched poly(phenylene oxide)/cyanate ester resins

A novel hyperbranched poly(phenylene oxide) (HBPPO) modified 2,2′‐bis(4‐cyanatophenyl) isopropylidene (BCE) resin system with significantly reduced curing temperature and outstanding dielectric properties was developed, and the effect of the content of HBPPO on the curing behavior and dielectric properties as well as their origins was thoroughly investigated. Results show that BCE/HBPPO has significantly lower curing temperature than BCE owing to the different curing mechanisms between the two systems, the difference also brings different crosslinked networks and thus dielectric properties. The dielectric properties are frequency and temperature dependence, which are closely related with the content of HBPPO in the BCE/HBPPO system. BCE/2.5 HBPPO and BCE/5 HBPPO resins have lower dielectric constant than BCE resin over the whole frequency range tested, while BCE/10 HBPPO resin exhibits higher dielectric constant than BCE resin in the low frequency range (<10⁴ Hz) at 200°C. At 150°C or higher temperature, the dielectric loss at the frequency lower than 10² Hz becomes sensitive to the content of HBPPO. These phenomena can be attributed to the molecular relaxation. Two relaxation processes (α‐ and β‐relaxation processes) are observed. The β‐relaxation process shifts toward higher frequency with the increase of temperature because of the polymer structure and chain flexibility; the α‐relaxation process appears at high temperature resulting from the chain‐mobility effects. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Dielectric behavior of the constituents of the natural resin shellac

The dielectric relaxation data of the constituents of natural resin shellac have been analyzed by the Cole–Cole method at temperatures between 10° and 90°C in the frequency range of 50 Hz to 500 kHz. The functional groups present in hard and soft lac appeared to be responsible for the dielectric properties exhibited by shellac below and above its glass transition and melting temperature, respectively. A combination of 70% hard lac and 30% soft lac was found to exhibit the dielectric properties of shellac satisfactorily.     

Dielectric relaxation spectroscopy of poly[(R)‐3‐hydroxybutyrate] (PHB) during crystallization

The dielectric response of poly [(R)‐3‐hydroxybutyrate] (PHB) was investigated as a function of time after quenching a film from the melt to a crystallization temperature of 20 °C. In the frequency range investigated (20 to 10⁶ Hz) a relaxation maximum was observed–attributable to the glass transition–which could be analysed by the Havriliak–Negami relation. Changes in the complex dielectric constant were monitored both during spherulite growth (primary crystallization) and subsequently during a period of progressive crystallization (secondary crystallization) at room temperature. The relaxation strength changed only slightly during primary crystallization and its peak position remained at a constant frequency. Subsequently a continuous decrease in relaxation strength occurred, indicating considerable changes in molecular mobility after spherulite growth had been completed. The results provide further evidence that molecular mobility in the amorphous regions decreases significantly with time, and that this would be the reason why PHB shows embrittlement on ageing at room temperature. Copyright © 2004 Society of Chemical Industry     

Mechanical and dielectric properties of bulky side chain poly(methacrylates). Analysis of the low frequency phenomena. 1: Poly(5-indanyl methacrylate)

This paper composes the dynamic mechanical and dielectric relaxation properties for Poly (5-indanyl methacrylate) (P51M), a polymer with a bulky side chain. Measurements were carried out from about ?100°C to near 250°C. A secondary loss peak was observed in dielectric measurements at room temperature (20°C at 0.1 Hz.), whereas nothing was resolved in the mechanical spectrum. A prominent α relaxation associated with the glass transition was also observed near 107°C at 0.1 Hz. in the dielectric and the mechanical spectra. Dipolar dielectric loss overlapped with conductivity at high temperatures and low frequencies. A new method to split the conductive, interfacial, and dipolar contributions to the spectrum is proposed.     

Molecular dynamics of poly(glycolide‐co‐L‐lactide) copolymer during isothermal cold crystallization

Poly(glycolide‐co‐l ‐lactide) (PGA/PLLA) is a random copolymer with 92 wt % PGA, being the basic resin for Vicryl® suture. Molecular dynamics of PGA/PLLA in its wholly amorphous state and during isothermal cold crystallization at 70 and 80°C have been analyzed. Experimental results were generated over a wide range of frequency and temperature by broad‐band dielectric spectroscopy (DRS). The variation of the average relaxation time (defined as τ = [1/2]πf_max where f_max is the frequency at maximum loss for the main α relaxation) has been studied during cold crystallization and the temperature dependence of this average relaxation time for wholly amorphous and crystallized samples has been analyzed. This behavior has been modeled by Havriliak–Negami, Vogel–Fulcher–Tammann, and Kohlrausch–Williams–Watts equations. The evolution profile of the dynamics (frequency at which the maximum loss appears, f_max) depends on the crystallization temperature, being different at 80°C relatively to 70°C, which could reflect different progress of the spherulitic morphology, as it is shown by the evolution of the morphologies obtained during the crystallization processes, followed by optical microscopy. While the loss maximum (and consequently relaxation times) remains almost unmodified during the crystallization process at 70°C, for the process at 80°C the maximum first moves slightly to higher frequencies (shortening of relaxation times) and at the final stages of crystallization it moves to lower frequencies (increasing the relaxation times). Supporting evidence about the thermal behavior of the polymers has been obtained with DSC. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Revisiting the thermal relaxations of poly(vinyl alcohol)

The molecular dynamics of poly(vinyl alcohol) (PVA) were studied by dielectric spectroscopy and dynamic mechanical analysis in the 20–300°C range. The well-established plasticizing effect of water on the glass-transition temperature (T_g) of PVA was revisited. Improper water elimination analysis has led to a misinterpretation of thermal relaxations in PVA such that a depressed T_g for wet PVA films (ca. 40°C) has been assigned as a secondary β relaxation in a number of previous studies in the literature. In wet PVA samples, two different Vogel–Fulcher–Tammann behaviors separated by the moisture evaporation region (from 80 to 120°C) are observed in the low- (from 20 to 80°C) and high- (>120°C) temperature ranges. Previously, these two regions were erroneously assigned to two Arrhenius-type relaxations. However, once the moisture was properly eliminated, a single non-Arrhenius α relaxation was clearly observed. X-ray diffraction analysis revealed that the crystalline volume fraction was almost constant up to 80°C. However, the crystallinity increased approximately 11% when temperature increased to 180°C. A secondary β_c relaxation was observed at 140°C and was related to a change in the crystalline volume fraction, as previously reported. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Dielectric characteristics of CuO-Na2O-SiO2 glasses with cole-cole plots technique and ac conductivity study

A series of glasses with formula (SiO₂)_{0.7 ? x} (Na₂O)_0.3(CuO) _x with (0.0 ≤ x ≤ 0.20) were prepared and studied by means of ac measurements in the frequency range 20 kHz-10 MHz at room temperature. The study of frequency dependence of both dielectric constant ?′ and dielectric loss ?″ showed a decrease of both quantities with increasing frequency indicating a normal behavior of dielectrics. Cole-Cole plots are drawn with ?′ and ?″. From the Cole-Cole plot parameters like optical dielectric constant, static dielectric constant, average relaxation time, molecular relaxation time are evaluated. Besides, the ac conductivity were analyzed by considering a power relation σ(ω) = A ω ^s (s ≤ 1). The observed behavior of the ac conduction is treated in terms of the quantum-mechanical tunneling (QMT) model.     

Investigation on gradient dielectric characteristics of bamboo (Dendrocalamus strictus)

Dielectric constant (ε′) and tan δ dependence on distance from the outer most skin to the center of bamboo has been determined. Dielectric measurements have been done in the temperature range from 24 to 120°C and in the frequency range from 4 to 100 kHz. Gradient behavior in dielectric constant (ε′) and tan δ has been found in bamboo. It has also been observed that the dielectric constant (ε′) and tan δ increase with the increasing temperature and decrease with the increasing frequency. Relaxation times have been calculated for the four samples at 80, 90, and 100°C temperatures, which show that relaxation time decreases with the increase of temperature due to the increased molecular mobility. A continuous increase in the hardness from the center 48 to the outer surface 70 and density from 0.45 to 0.80 g/cc has been observed. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3489–3494, 2006     

Investigation on gradient dielectric characteristics of bamboo (Dentroclamus strictus)

Dependence of dielectric constant (ε′) and dielectric dissipation factor (tan δ) on distance from outermost skin to the center of bamboo has been determined. Dielectric measurements have been done in the temperature range of 24–120°C and in the frequency range, 4–100 kHz. Gradient behavior in ε′ and tan δ has been found in bamboo. It has also been observed that ε′ and tan δ increase with increasing temperature and decrease with increasing frequency. Relaxation times have been calculated for the four samples at 80, 90, and 100°C temperatures, which show that relaxation time decreases with the increase of temperature because of the increased molecular mobility. A continuous increase in the hardness from center (48) to outer surface (70) and density from 0.45 to 0.80 g/cc has been observed. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 380–386, 2006     

Screen printable epoxy/BaTiO3 embedded capacitor pastes with high dielectric constant for organic substrate applications

In this article, embedded capacitor pastes (ECPs) with various BaTiO₃ (BTO) powder contents were formulated and screen‐printed on PCBs to fabricate capacitors. The material properties of the ECPs that included their rheology, curing behavior, and dielectric constant were investigated. Embedded capacitors were fabricated for reliability tests related to the thermal cycling and high temperature and humidity potential of optimized ECPs. Additionally, changes in the dielectric properties were discussed. ECPs were formulated with various powder contents from 0 to 70 vol %. ECP resin was cured at temperatures ranging from 130 to 220°C. All ECPs had the viscosities below 30 Pa · s at a shear rate of 100 s^?1 to be easily screen‐printable. The dielectric constant of the cured ECPs increased to 60 at 70 vol %, and the dielectric loss was approximately 0.023 for all ECPs regardless of BTO volume content. For the reliability test, ECPs with 50, 60, and 70 vol % BTO powder contents were selected and embedded capacitors were fabricated. After a thermal cycling test with a temperature range from ?55 to 125°C for 1000 cycles, capacitance decreased by approximately 5 ～ 10%, but the dielectric loss did not change. After a 85°C/85RH% test for 1000 h, the capacitance and dielectric loss increased by nearly 20%. Cyanoresin (CRS) was used to form the high dielectric polymer/ceramic composite material. The newly formulated resin system had a dielectric constant that is double that of a conventional epoxy resin system. Additionally, the dielectric constant of the polymer/ceramic composite material increased 50% at 50 vol %. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

TSD-Untersuchungen zum Einfluß von Wasser auf die molekularen Beweglichkeiten in Polyamid-6

Molecular motions in nylon 6 are studied by thermally stimulated discharge (TSD) techniques. The effect of absorbed water on the relaxation behaviour of nylon 6 is discussed. The γ relaxation process at ?145°C is attributed to local segmental motions in the amorphous phase. The intensity of the γ relaxation is reduced and broadened by absorbed water. The molecular origin of the β relaxation process in the temperature range between ?80 and ?60°C is associated with localized reorientational motions of chain segments involving nonchain-bonded or weakly bonded amide groups. It is quite likely that also rotational rate processes of water molecules bonded to CO-groups contribute to the β process. The intensity increases are due to added water, since associated water molecules are involved with those molecular motions. The α relaxation process at 70°C is associated with large-scale segmental motions at the glass transition. The chain mobility is increased by added water and gives rise to an α peak near 50°C. The α′ relaxation process in the temperature range between 100 and 110°C may either be associated with reorientational motion of stretched chain segments or with the beginning of a crystalline transformation of the γ phase into the monoclinic α form. The temperatures of TSD-peaks correspond to alternating current loss data at a frequency of 5·10^?2 Hz.     

A dielectric study of water uptake in epoxy resin systems

The influence of epoxy resin formulation on the nature and extent of moisture is studied using dielectric and gravimetric analysis for samples cured at 40, 50, 60, and 70 °C and aged at 50 °C and 70 °C. The equilibrium moisture uptake depends on the difference between the glass transition and aging temperature. Dielectric relaxation data measured from 1 to 3 × 10¹⁰ Hz indicates the presence of water in at least two different environments. The high frequency relaxation ca. 1 × 10¹⁰ Hz is associated with water clustered in nano‐voids, whereas the relaxation at 10⁵ Hz arises from a combination of OH pendant group reorientation motion coupled with that of molecularly dispersed water molecules. A correlation exist between the dielectric permittivity and the amount of moisture absorbed. Water initially resident in voids is re‐dispersed with aging into the resin matrix aiding plasticization and allowing densification of the matrix. The extent to which changes occur depends on the chemical functions forming the matrix and densification leads to a drop in the amount of water absorbed. In the complex resin systems, water interacts with both OH groups and polyether of the amine curing agent which is not possible with the aliphatic diamine in the simple system.] © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44717.     

Dielectric dispersion studies of poly(vinyl alcohol) in aqueous solutions

The dielectric constant ε′ and loss factor ε″ of deionized water and poly(vinyl alcohol) in aqueous solutions are measured in the frequency region 200 MHz to 20 GHz at four different temperatures (25, 35, 45 and 55 °C). Complex plane plots (ie ε″ vs ε′) are drawn to obtain the static dielectric constant ε₀, high frequency dielectric constant ε_∞, distribution parameter α and average relaxation time τ₀. The variations of dielectric constants with increasing solvent concentration and temperature are discussed in terms of solute–solvent and solute–solute interactions. The average relaxation time τ₀ of poly(vinyl alcohol) aqueous solutions is found to the very short. It is also observed that the relaxation time is almost independent of the viscosity of the solution. The effect of water concentration on macromolecular size, shape and flexibility of the molecular chain are discussed using the observed values of dielectric relaxation times at different temperatures. The possibility of multiple dielectric dispersion is also discussed with concentration variation. © 2000 Society of Chemical Industry     

Effect of fillers on the relaxation behavior of chlorobutyl vulcanizates

The effect of addition of fillers (carbon black (CB), carbon silica dual phase filler (CSDPF), and nanoclays) on the relaxation behavior of chlorobutyl vulcanizates has been studied. The primary relaxation (α‐transition, the glass transition) was studied by dynamic mechanical analysis as a function of temperature (?60 to +100°C) and positron annihilation life time spectroscopy (?70 to +110°C). Irrespective of the filler and its loading, all the composites showed the glass transition temperature in the range of –29 to –33°C, which was explained on the basis of relaxation chain dynamics of polyisobutylene in the vicinity of fillers. The secondary relaxation (α* or β relaxation) was studied using dielectric relaxation spectra in the frequency range of 100–10⁶ Hz. Nanoclays had a profound influence on the secondary relaxation, whereas CSDPF and CB had a marginal effect. The nonlinear strain dependent dynamical parameters were also evaluated at double strain amplitudes of 0.07–5%. The nonlinearity in tan δ and storage modulus has been explained on the concept of filler–polymer interactions and the interaggregate attraction (filler networking). The “percolation limit” of the fillers in the composites has been studied by DC conductivity measurements. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3161–3173, 2006     

Structural and impedance analysis of Bi0.5Na0.5Ti0.80Mn0.20O3 ceramics

The Bi_0.5Na_0.5Ti_0.80Mn_0.20O₃ ceramic was synthesized by a conventional solid-state reaction technique. Rietveld refinement of X-ray diffraction data confirms the rhombohedral crystal structure of the compound with R3c space group. The optical band gap energy of the compound is found to be 1.93 eV. The substitution of 20% Mn ions at the Ti sites results in the improved dielectric characteristics and a shift in the ferroelectric to paraelectric electric phase transition peak from 330 °C to 370 °C in the material. The frequency dispersion of dielectric constant and its footprint in the Nyquist and Cole-Cole plots have been analyzed. The analysis of complex impedance and modulus spectroscopy confirms the non-Debye type of relaxation mechanisms in the material with contributions from both the grain and grain boundary to the electrical properties. The frequency dependence of AC conductivity data exhibits overlapping large polaron tunneling conduction mechanism in the compound.