Hygrothermal aging of rubber‐modified and mineral‐filled dicyandiamide‐cured DGEBA epoxy resin. III. Dielectric spectroscopy investigation

The effects of moisture absorption on the dielectric properties of a rubber‐modified, mineral‐filled, epoxy resin based on the diglycidyl ether of bisphenol A cured with dicyandiamide are reported. Samples of the resin were aged by immersing in deionized water, or 5% w/w NaCl solution, at elevated temperatures. Dielectric measurements were carried out over the frequency range 10^?1 to 6 × 10⁵ Hz. A featureless dielectric spectrum was observed with both real and imaginary dielectric permittivity increasing with the amount of absorbed water. The change in the dielectric properties with absorption of water was independent of presence of salt, temperature of exposure, and aging history, although a hysteresis of the hydration–dehydration process was observed at low frequencies. Two types of absorbed water were observed—water molecularly dispersed within the epoxy matrix and clustered water in spherical microcavities. The time dependence of the real dielectric permittivity measured at 10 kHz was found to closely resemble that of the water absorption, which allowed the activation energy of diffusion to be calculated from both dielectric and gravimetric data. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 1011–1024, 2002; DOI 10.1002/app.10368     

Dielectric studies of water in epoxy resins

Dielectric measurements are reported on amine-cured epoxy resin samples over a frequency range from 200 Hz to 200 kHz and a temperature range from ?60°C to 70°C as a function of molecular weight of the diglycidyl ether and water content. The effects of change of the molecular weight of the diglycidyl ether on the dielectric relaxation are small in comparison with the changes observed on the introduction of water into the matrix. Analysis of the data indicates the presence of cluster—free and bound—molecularly dispersed water. The former are presumed to be found in voids and cavities which arise in curing powder samples. The conductivity of the water-doped samples reflects the mobility of the water and is compared with the predictions of theories for amorphous materials.     

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     

Fumed SiO2 nanoparticle reinforced biopolymer blend nanocomposites with high dielectric constant and low dielectric loss for flexible organic electronics

In the present study, fumed silica (SiO₂) nanoparticle reinforced poly(vinyl alcohol) (PVA) and poly(vinylpyrrolidone) (PVP) blend nanocomposite films were prepared via a simple solution‐blending technique. Fourier transform infrared spectroscopy (FTIR), ultraviolet–visible spectroscopy (UV–vis), X‐ray diffraction (XRD), and scanning electron microscopy (SEM) were employed to elucidate the successful incorporation of SiO₂ nanoparticles in the PVA/PVP blend matrix. A thermogravimetric analyzer was used to evaluate the thermal stability of the nanocomposites. The dielectric properties such as dielectric constant (?) and dielectric loss (tan δ) of the PVA/PVP/SiO₂ nanocomposite films were evaluated in the broadband frequency range of 10^?2 Hz to 20 MHz and for temperatures in the range 40–150 °C. The FTIR and UV–vis spectroscopy results implied the presence of hydrogen bonding interaction between SiO₂ and the PVA/PVP blend matrix. The XRD and SEM results revealed that SiO₂ nanoparticles were uniformly dispersed in the PVA/PVP blend matrix. The dielectric property analysis revealed that the dielectric constant values of the nanocomposites are higher than those of PVA/PVP blends. The maximum dielectric constant and the dielectric loss were 125 (10^?2 Hz, 150 °C) and 1.1 (10^?2 Hz, 70 °C), respectively, for PVA/PVP/SiO₂ nanocomposites with 25 wt % SiO₂ content. These results enable the preparation of dielectric nanocomposites using a facile solution‐casting method that exhibit the desirable dielectric performance for flexible organic electronics. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134 , 44427.     

Influence of epoxy addition on the thermal,mechanical, and dielectrical properties of polycyanurate films

In the study, polycyanurate (PCN)/epoxy resin (ER) blends are prepared to enhance the physical properties of cyanate ester resins. The effects of curing schedule and blend composition on their thermal, mechanical, and dielectrical properties of cured PCN/epoxy blend films are examined. FTIR analysis of the cured blend films exhibits the expected cyanurate and oxazolidinone peaks in all blend compositions except the film thermally treated for 1 h in the presence of 1% phenol. TGA results show that the thermal stability decreases with epoxy content in the blend film. From SEM analyses, it is observed that all films have very dense, smooth, and bubble free surface without phase separation. For the pure PCN, the dielectric constants are found to be 3.54–5.91 in the range of 10^?1–10⁷ Hz between 20°C and 200°C. PCN/epoxy blends up to 50% epoxy resin show a good stability of dielectric constant in this frequency band for 200°C, which is close to the dielectric constant of the homopolymerized PCN. Beyond this percentage of epoxy resin, dielectric constants of PCN/epoxy blends greatly increase at low‐frequency region (0.1–10³ Hz) due to the interfacial polarization governed by Maxwell–Wagner–Sillars effect. POLYM. ENG. SCI., 58:820–829, 2018. © 2017 Society of Plastics Engineers     

Synthesis and some properties of anthracene and anthraquinone incorporated poly(vinyl chloride) and the ion exchange resin therefrom

Poly(vinyl chloride) has been modified by chlorine displacement reaction with 2-anthrol and anthraquinone-2-ol. The condensates (PVC-ACOL and PVC-AQOL) are insoluble in all solvents common to PVC and have been characterized by elemental and IR spectra analysis. The initial decomposition temperature of these condensates follow the trend: PVC-AQOL (250°C) > PVC-ACOL (200°C) > PVC(190°C) and the overall thermal stabilities beyond 60% decomposition follows the same trend. Permittivity and dielectric loss of these condensates sharply fall with increasing applied frequency (10–1.3 ×10⁷ Hz) to a limiting constant value. In contrast, for unmodified PVC these values are low and remain independent of frequency in the same range. Sulfonation of these PVC condensates affords a weak acid resin with ? COOH and ? OH ionogenic groups, but no strong sulfonic acid groups due presumably to oxidative degradation of the PVC matrix.     

Mechanical, rheological, and thermal behavior assessments in HDPE/PA-6/EVOH ternary blends with variable morphology

Visco-elastic and dielectric spectra of multiwalled carbon-nanotube reinforced silicon elastomer nanocomposites were used to study relaxation behavior. SEM photomicrographs shows well dispersion of MWCNT in elastomer matrix. In visco-elastic analysis primary relaxation was studied as a function of temperature (?100 to 100 °C) at frequency 1Hz and strain 1 %. The effect of MWCNT loadings on storage modulus, loss modulus, and loss tangent has been studied. The non-linearity in loss tangent, storage modulus and loss modulus was explained on the basis of MWCNT-elastomer interaction and the inter-aggregate attraction of MWCNT. The secondary β relaxation was studied using dielectric relaxation spectra in the frequency range of 0.1 Hz to 10⁶ Hz. The effect of MWCNT loadings on the complex and real parts of impedance was distinctly visible which has been explained on the basis of interfacial polarization of fillers in a heterogeneous medium and relaxation dynamics of polymer chains in the vicinity of fillers. The dielectric formalism has been utilized to further investigate the conductivity and relaxation phenomenon. The ‘percolation limit’ of the MWCNT in the silicon elastomer was found to be in the range of 4 phr loading.     

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.     

Effects of Carbon Nanofiber on Dielectric Properties of PMN/CNFs/EP Composites

A kind of new composite material composed of piezoelectric ceramic lead magnesium niobate-lead zirconate-lead titanate (PMN)/carbon nanofibers (CNFs)/epoxy resin (EP) were prepared by a resin casting method. The effects of carbon nanofibers on dielectric properties of PMN/CNFs/EP composites were investigated in this paper. The concetration of CNFs had significant effects on dielectric constant, dielectric loss, dielectric frequency dependence and dielectric temperature dependence of PMN/CNFs/EP composites. When the content of CNFs increased from 0 to 0.8 wt% of the epoxy resin, the dielectric constant at 1 kHz increased sharply from 13.6 to 158, and the dielectric loss increased from 0.0524 to 2.59. In plots of the dielectric constant against frequency and dielectric loss against frequency, the dielectric constant and dielectric loss reduced dramatically at lower frequency ranging from zero to about 10⁵ Hz, then approach to be stable at frequency higher than 10⁵ Hz. Moreover, as to the effects of temperature on dielectric constant and dielectric loss of PMN/CNFs/EP composites, the dielectric constant of composite increased ranging from 25°C to 160°C, and the dielectric loss of composite also showed increase besides that with CNFs content 0.8 wt%.     

Dielectric studies of water absorption and desorption in epoxy resins: influence of cure process on behaviour

Little is known about the way in which the chemical structure of an epoxy resin influences its ability to absorb and desorb moisture. This issue is addressed in a study of dicyandiamide‐ and amine‐cured epoxy resins. The dicyandiamide‐cured material will have a significantly lower preponderance of pendant hydroxyl groups than the amine‐cured material and may exhibit different behaviour when exposed to moisture. The uptake and loss of moisture was monitored gravimetrically, using broad band dielectric, dynamic mechanical thermal analysis and thickness measurements performed as a function of time at various temperatures. A comparison of the uptake and loss profiles for the first and subsequent cycles indicated significant differences in behaviour attributed to the way in which water can plasticise the matrix. Stresses frozen into the matrix during the cure process are allowed to relax as a consequence of the water hydrating the matrix and create voids and also allow matrix densification. These processes occur during the first hydration cycle and are not reversible. Subsequent hydration and dehydration appear to be reversible after the first hydration cycle. Water in the polymer is distributed between free water which is to be found in microvoids and bound water which is attached to the polymer chain. The amine‐cured epoxy resin which contains pendant hydroxyl groups has a greater capacity for water absorption than the ether‐containing backbone of the dicyandiamide‐cured material. Copyright © 2008 Society of Chemical Industry     

Elaboration and dielectric property of modified PZT/epoxy nanocomposites

Composites of lead zirconate titanate (PZT) 50 nm nanoparticles and epoxy resins have been produced with various PZT contents from 0 to 20 wt%. The morphology and thermal properties of prepared composites were characterized using scanning electron microscopy and differential scanning calorimetry. The PZT nanoparticles were found to be well dispersed in the epoxy resin matrix. The glass transition temperature (T_g) value of the nanocomposites increases from 164 to 178°C with increasing the PZT weight fraction. The dielectric composites properties dependences were studied via a wide range of frequency from 10 Hz to 100 kHz. The dielectric constant of PZT/epoxy composite was increased from 5.56 to 6.29 (at f = 1 kHz, T = 30°C), respectively to the incorporated PZT amount, and these values are higher than that the dielectric constant of pure cured epoxy resin, ε = 4.86. POLYM. COMPOS., 37:455–461, 2016. © 2014 Society of Plastics Engineers     

Electrical,Mechanical, and Thermal Properties of Mg(OH)<Subscript>2</Subscript>/PI Nanocomposite Films

Nano-Mg(OH)₂ particles were prepared by reverse precipitation. Nano-Mg(OH)₂/PI composite films with varying contents of Mg(OH)₂ were successfully prepared by in situ polymerization and thermal imidization. The surface morphology, thermal stability, and the mechanical and dielectric properties of the films were characterized. Results indicated the following: the nanoparticles were homogeneously dispersed in the matrix, thermal stability was reduced, and the Young’s modulus of PI-3% was 2851.6 N/mm² and increased by 155% relative to that of neat PI. The real and imaginary parts of relative permittivity increased, and electric modulus formalism was introduced to determine the dielectric relaxation behavior of the composites in the study. The result from the Cole–Davidson semicircle showed that the increase in nano-Mg(OH)₂ concentration led to an increase in the inhomogeneous distribution of relaxation time. Direct current illustrates that the activation energy of the hopping process exhibits an increasing trend. The aging threshold increased in accordance with the theory of space-charge limited current. Breakdown increased to 323 kV/mm, whereas the pure film was is 292 kV/mm, and the electric property of polyimide improved.     

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.     

Local Dynamics of Bismaleimide Adhesives in an Aggressive Environment

Molecular aspects of chemical and physical changes in bismaleimide (BMI) adhesive joints caused by absorbed moisture were investigated. The focus was on the early (pre-damage) stage that precedes the formation of voids and microcracks. Local dynamics were investigated by broad-band dielectric relaxation spectroscopy (DRS) and the changes in the chemical state of the matter were monitored by Fourier transform infrared spectroscopy (FTIR). Absorbed water interacts with the BMI network and gives rise to a fast relaxation process (termed γ*), characterized by an increase in the dielectric relaxation strength, an Arrhenius temperature dependence of the average relaxation time, and an activation energy of 50 kJ/mol. The γ* dynamics are slower than the relaxation of bulk liquid water because of the interactions between the absorbed water and various sites on the network (the ether oxygen, the hydroxyl group, the carbonyl group, and the tertiary amine nitrogen). One particularly significant finding is that the average relaxation time for the γ* process above 20°C is of the order of nanoseconds or less and, hence, the detection and monitoring of this process hinges upon the ability to perform high precision DRS at frequencies above 1 MHz. This is an important consideration in the ongoing efforts aimed at the implementation of DRS as a non-destructive inspection (NDI) tool for adhesive joints. FTIR spectra reveal the presence of non hydrogen-bonded water and hydrogen-bonded water, the latter bonded to one and/or two sites on the BMI network. A good agreement was found between the calculated ratio of non hydrogen-bonded to total absorbed water from DRS and FTIR data.     

Cleaner preparation of poly(vinylidene fluoride)/barium titanate composites: Thermal,mechanical, and dielectric properties and relaxation dynamics

Traditional polymer composite preparation techniques often employ organic solvents, which can damage the environment, to disperse inorganic fillers. In this article, classic nanocomposites with poly(vinylidene fluoride) (PVDF) polymer matrices and BaTiO₃ nanoparticle (BTP) fillers were created by a clean method combining planetary ball milling with an ultracentrifugal mill and then hot pressed into thin films. The microstructures, properties and relaxation dynamics of the thin films were characterized and analyzed. Scanning electron microscopy results demonstrated that BTP was homogeneously dispersed in the PVDF matrix. The thermal, mechanical, and dielectric properties were comparable to those of composite films prepared by solution mixing. Dielectric analysis revealed that the dielectric constant of the thin films reached 14 (10⁴ Hz) when the volume fraction of BTP was 30%; however, the dielectric loss was 0.1 (10⁴ Hz). Additionally, the dielectric loss spectra fitted with the Havriliak−Negami (H−N) and Vogel Fulcher equations were employed to analyze the relaxation dynamics of the nanocomposites. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47254.     

Structural characterization and electrical properties of carbon nanotubes/epoxy polymer composites

The purpose of this study is to identify the relationship between the electrical and structural characteristics of multiwalled carbon nanotubes dispersed into the polymer matrix of a resin. In a first step, the composites were characterized by small‐angle neutron scattering, which provide information about the bulk dispersion of nanotubes in the matrix and form three‐dimensional networks with a surface fractal behavior. In the second step, a dielectric and electrical study was carried out in the frequency range between 1 Hz and 10 MHz at room temperature. We have found that the electric and dielectric behavior of these composites can be described by Jonscher's universal dielectric response. We show that the critical exponents describing the concentration dependence of the conductivity and the dielectric constant, obtained in the vicinity of the percolation threshold, are in good agreement with the theoretical values. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44514.     

Structure and properties of ethylene—methacrylic acid copolymers and their sodium salts: Dielectric and proton magnetic relaxation studies

Dielectric and proton magnetic relaxation data have been obtained for an ethylene-methacrylic acid copolymer (containing about 4 mole% methacrylic acid units) and its 53% ionised sodium salt. The degrees of crystallinity and percentage ionisation of the samples investigated were estimated by infra-red methods. The dielectric results were obtained principally in the frequency range 100 Hz to 10 kHz and at temperatures ranging from 80° to – 120°. A few results are also reported at frequencies down to 0·1 Hz and up to 100 MHz. For the acid copolymer, two dielectric loss regions are observed and these are correlated with the reported mechanical β′- and γa-processes respectively. The partly ionised copolymer exhibits three dielectric relaxation regions which correlate with the mechanical α-, β- and γa-relaxations respectively. In addition, a dielectric peak appears at about —40° in the presence of absorbed water, a result similar to that found in the polyamides. The proton magnetic relaxation results were obtained by pulse methods which yielded the spin-lattice relaxation times T₁ (at 30 MHz) and T_1p (at kilohertz frequencies) as a function of temperature from ?180° to 100°. Two components were generally observed for T_1p. For the acid copolymer the β′- and γ-processes have been observed from these results, as well as a lower-temperature (δ) process which has not been detected by the mechanical or dielectric methods. For the sodium salt the γ- and δ-processes are also found, in addition to a high-temperature process in the region of the merged α- and β-processes. The present data are consistent with previous assignments for the β′-, α-, β- and γ-processes. The ‘water’ relaxation appears to involve some rotation of water molecules, or of ionic segments to which water molecules are attached, in the proposed ionic domains. The δ-process is ascribed to the rotations of methyl groups present in the methacrylic acid units.     

Dielectric properties of nylon 6/clay nanocomposites from on-line process monitoring and off-line measurements

Nylon 6/clay nanocomposites were studied by dielectric relaxation spectroscopy (DRS) to correlate morphology and microstructure with relaxation behavior of the polymer matrix at the molecular level. Partially exfoliated clay microstructure was achieved by extruding nylon 6 with surfactant-treated montmorillonite clays. A new on-line dielectric slit die sensor was used to examine the melt state properties during extrusion compounding. Solid state properties were probed by off-line DRS over a temperature range from −50 to 180 °C in a frequency range from 10⁻³ to 10⁶ Hz. Using non-linear regression methods in conjunction with the temperature-frequency positions of relaxations observed in the dielectric loss data, the experimental data were fit with the Havriliak-Negami and Cole-Cole dielectric relaxation functions corrected for electrode polarization and DC conductivity. Characteristic frequency, relaxation strength, and DC conductivities were extracted from curves with overlapping relaxation modes. Two dielectric relaxations were observed in the composite melt: the α relaxation associated with molecular segmental motion, and a Maxwell-Wagner relaxation (MW) resulting from interfacial polarization at the resin/clay interface. Analysis of the solid-state data yielded a comprehensive master plot of dielectric relaxations attributed to segmental and local molecular dynamics and other relaxations resulting from water and Maxwell-Wagner interfacial polarization. The impact of clay fillers is seen in nearly all relaxation processes changing both characteristic frequency and strength of the relaxation.     

Room-temperature densification of MgO bulk ceramics with dispersed nitride phosphor particles

Wave conversion materials with high thermal conductivity are necessary for high-power semiconductor lighting. Ceramics have higher thermal conductivity than existing matrices such as resin or glass in which phosphor particles are dispersed. However, the high densification of ceramics generally requires high-temperature sintering, which degrades and alters the phosphor particles. In this study, we aimed to achieve the high densification of MgO ceramics at room temperature. Applying high hydrostatic pressure with water addition improved the sample packing ratio and promoted the formation of Mg(OH)₂. As a result, the relative density was ≥95%. Additionally, various nitride phosphor particles (CaAlSiN₃:Eu²⁺, β-SiAlON:Eu²⁺, and α-SiAlON:Eu²⁺) were dispersed in the MgO matrix at room temperature without degrading the luminescence property. The thermal conductivity of the obtained sample was about 8 W m^?1K^?1, 40 times higher than that of the epoxy matrix.     

Local Dynamics of Adhesives in Aggressive Environment in the Pre-Damage Stage

Molecular aspects of chemical and physical changes in adhesive joints caused by absorbed moisture were investigated. The focus was on the pre-damage stage that precedes the formation of voids and microcracks. A model and a commercial epoxy-amine formulation were studied. Local dynamics were monitored by broad-band dielectric relaxation spectroscopy (DRS). One portion of the absorbed water does not form hydrogen bonds with the network and gives rise to a fast relaxation process (termed γ) with activation energy of 28 kJ/mol. The local β dynamics are slowed down by the interactions between water and various sites on the network that include the ether oxygen, the hydroxyl group and the tertiary amine nitrogen. One particularly significant finding is that the average relaxation time for the β process above 20°C is of the order of nanoseconds or less and, hence, the detection and monitoring of this process hinges upon the ability to perform high-precision DRS at frequencies above 1 MHz. This is an important consideration in the ongoing efforts aimed at the implementation of DRS as nondestructive inspection (NDI) tool for adhesive joints.