Thermal and electrical properties of some epoxy based composites

Thermally stimulated creep compliance, differential scanning calorimetric behavior, thermal degradation, AC dielectric permittivity and loss (between 120 Hz and 100 kHz) and thermally stimulated polarization and aepolarization currents were studied in a cycloaliphatic epoxy resin (B), in a conventional bisphenol-A-diglycidyl ester type epoxy resin (C) and in composites consisting of: resin B/wollastonite (B/W), resin B/quartz (B/Q) and resin C/wollastonite (C/W). The filler content was 60 wt%. Resin B exhibited higher T_g and lower rubbery deformability than resin C due to its more compact structure. Fillers reduced the rubbery deformation and thermal expansion and shifted the transition temperatures by a few degrees. The shift depended on the method used. Composites B/W and C/W exhibited higher thermal stability than the corresponding pure resins, while sample B/Q was less stable than resin B. Resins B and C exhibited a low temperature β transition (in the case of resin B a doublet) and a high temperature α or glass transition. AC dielectric losses were fairly similar in samples B and B/W, while the high temperature loss of sample B/Q was determined by a space charge process probably due to the matrix/filler interface. In samples C and C/W the α transition is visible but it is superposed on a strong space charge process due to the resin/electrode interface. Thermally stimulated currents show a behavior qualitatively in agreement with the AC results but the very low effective frequency and the nonlinear field strength dependence of the space charge processes cause some minor differences.     

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

The 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 T_g. 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 T_g is detected. The dielectric behaviour of this poly(thiocarbonate) is compared with the corresponding poly(carbonate).     

Multiple transitions in atactic 1,2-polybutadienes observed from dynamic mechanical and dielectric relaxation data

Detailed investigation by dynamic mechanical, dielectric and ¹³C-NMR relaxation spectroscopies over a temperature range from ?180 to 100°C reveals the multiple transitions in atactic 1,2-polybutadienes, which are denoted as T₁₁, α, β, γ, and δ in conformity with reducing temperature. The spectroscopic data obtained show that α transition is the glass transition of the samples, β-and γ-relaxations are likely to be related to the flexibility of the main chain and the side chain motion, respectively, and δ loss process seems to be associated with the defects and inhomogeneity in structure. T₁₁ transition observed in the dielectric relaxation spectra may be manifested by the abrupt broadening of the resonance peaks in solid-state ¹³C-NMR spectra.     

Effect of Hydrothermic Ageing on Dielectric and Mechanical Properties of Rigid Poly(vinyl chloride)

An accelerated hydrothermic ageing (according to N.F.T. 54043 method) was performed on samples of rigid poly(vinyl chloride). The test consisted of sample immersion in boiling water at 100°C for different exposure periods up to 480 h. The samples were removed from the boiling water every two hours for mechanical and dielectric characterization and color test. The dielectric measurements carried out on aged samples, up to 40 h, showed that the permittivity remained almost constant and its value was found to be 2.3 in the range –100 to +62.8°C. However, as the temperature approaches the glass transition (T_g = 88.3°C), the permittivity was observed to increase rapidly. It was also found that the permittivity of aged samples was lower than that of the non‐aged samples. The dielectric loss factor (tan δ) measurements with respect to temperature have confirmed the presence of two relaxations: β at low temperature (around –35°C) and α near the T_g. The combined action of water and temperature reduced the intensity of the corresponding relaxation peaks. The sample color index was evaluated up to 480 h using the SYNMERO scale in order to estimate the degradation extent. Unexpectedly, elongation at break under uniaxial traction remained unaffected by the hydrothermic ageing, meaning that two competing processes were involved simultaneously (sample degradation via chain scission and sample plastification via water absorption).     

Morphology and physical properties of poly(butylene terephthalate)

Liquid nitrogen-quenched PBT samples produce much larger spherulites of an optic axis orientation different from the of the air-cooled samples. Optical and scanning electron microscopy show that glass fibers in the glass-reinforced PBT sample nucleate the growth of well-defined spherulites along the glass fiber axis. Fracture studies at temperatures below and above the T_g indicate, respectively, brittle and ductile interspherulite boundary fracture. From dynamic mechanical studies, three transitions designated by α (flow transition), β (T_g), and γ (secondary relaxation) are observed. The magnitudes of the β and γ transitions are larger for the more amorphous quenched sample than the air-cooled sample, suggesting their amorphous phase origin. Addition of glass fibers raises the dynamic modulus and flow temperature, but suppresses the γ transition without significantly affecting the melting and glass transition temperatures.     

Dielectric behaviour of pure and nickel-doped cellulose acetate films

Pure and Ni (0·05%, 0·1% and 0·15%)-doped cellulose acetate (CA) films were formed on well-cleaned glass substrates using a solution growth technique. Metal–insulator–metal (MIM) sandwich structures of thin film capacitors were fabricated. Dielectric studies were carried out in the frequency range 1kHz to 10MHz in the temperature range 303–450K. The variation of capacitance and dielectric loss with temperature were studied at various frequencies. The variation of dielectric constants (ε′ and ε″) with frequency and temperature is discussed. An increase in capacitance with temperature was observed and this may be due to the chaotic oscillations of molecules in the polymer matrix. β (at lower temperature) and α (at higher temperature) relaxation loss peaks were observed. Doping results in the formation of charge transfer complexes/molecular aggregates in the polymer, which cause the shift of C_max value and the β and α relaxation loss peaks towards higher temperature. An appreciable dispersion of tan δ at lower frequency was noticed in all the samples studied. © 1998 SCI.     

Studies on dielectric relaxation and ac conductivity of cellulose acetate hydrogen phthalate–poly (vinyl pyrrolidone) blend

The dielectric constant, dielectric loss, and ac conductivity of polyblends of cellulose acetate hydrogen phthalate (CAP) and poly (vinyl pyrrolidone) (PVP) of different compositions were measured in the temperature range of 300–430 K and in the frequency range of 50 Hz–100 kHz. In the blends, the dielectric constant as well as the dielectric loss as a function of the temperature display a single peak corresponding to the glass transition temperature (T_g) in the region between the T_g values of the pure polymers. The T_g values observed agree well with those values obtained from DSC. Dielectric studies show that CAP forms a miscible blend with PVP. Ac conductivity values were calculated from the dielectric data and the conduction mechanism is discussed. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 1702–1708, 2002     

Electrical properties of polyimide composite films containing TiO2 nanotubes

A study on the dielectric behavior of polyimide composite films containing different amounts of TiO₂ nanotubes (TNs) was performed. The films were prepared by casting solutions resulting from direct mixing of a poly(amic acid) and TNs onto glass plates, followed by thermal imidization. The influence of TNs content on the properties of polyimide composites was investigated. AFM and SEM analyses showed good compatibility between the filler and polymer matrix. Dynamic mechanical analysis and broadband dielectric spectroscopy were used to evidence relaxation processes into the films. The electrical properties were evaluated on the basis of dielectric constant and dielectric loss, and their variation with frequency and temperature. At moderate temperature a secondary β relaxation was observed while incorporation of TNs decreased the activation energy and facilitated the appearance of an additional β ₁ process. An α relaxation and a conductivity process were evidenced at higher temperatures. The values of dielectric constant and dissipation factor increased with TNs amount, and the maximum of σ relaxation peak shifted to higher temperatures. POLYM. COMPOS., 38:2584–2593, 2017. © 2015 Society of Plastics Engineers     

Determination of the glass transition of polymer by the autovibron

The automated Rheovibron (Autovibron) has been useful for determining the glass transition behavior of polymers. The standard (manual Rheovibron viscoelastometer has been used sufficiently small intervals, and so, some accuracy on transition location must be sacrificed when the temperature intervals are taken 5 to 10°C apart. However, the Autovibron can be observed over very small temperature intervals (<1.5°C)), which essentially provide a continous monitoring of the changes in storage modulus, loss modulus, and tan δ. An improved accuracy and sensitivity of the Autovibron are provided by a combination of multiprogrammer, programmable calculator, and lock-in analyzer. The Autovibron are provided by a combination of multiprogrammer, programmable calculator, and lock-in analyzer. The Autovibron shows a good capability for dynamic thermomechanometry. The glass transition bility for dynamic thermomechnometry. The glass transition of polymers can be determined by analyzing the loss tangent peak. The maximum value in loss tangent, peak temperature, and the newly introduced terms, ΔT₁ and ΔT₂ which indicate the widths of tan δ peak, are useful indicators of the glass transition. The ΔT₁ and ΔT₂ show the distribution of the glass transition and are related to order and crystallinity in the structure.     

Effect of thermal history on the tensile yield stress of polycarbonate in the β transition range

The tensile yield behaviour of three forms of polycarbonate differing in their thermal history is investigated, at constant strain rate, over a wide range of temperatures (?140° to + 80°C). The plot of the yield stress versus temperature exhibits a thermal history dependence over the whole range explored and reveals the β transition which is located at the same temperature T_β for the three forms. The yield behaviour is described assuming that two activated processes α and β requiring additive stresses are involved. The β component of the yield stress is found to be insensitive to thermal history as well as the related loss peak. Only the α process is affected; details concerning this dependence are discussed.     

Influence of the thermal history on dielectric properties of poly(vinyl chloride)

In this paper, we report dielectric permittivity and loss of poly (vinyl chloride) samples that have received three different thermal treatments: (a) as received, (b) quenched from 110°C to 20°C and (c) slow cooled at 5°C/h. There are several observations: first, the secondary (β) loss peak-is not representative of a simple mechanism of transition, in agreement with results of other authors (10), second, in the glass transition zone, there are clearly two peaks (α₁ and α₂)—α₁, is a typical peak of an amorphous glass transition; the second, α₂, has possibly a crystalline origin—and, third (and the most interesting fact), there is an increase of the loss tangent in the intermediate zone between α and β peaks showing a new relaxational peak with high activation energy (70 Kcal/mole), in agreement with dynamic mechanical results (6).     

Improvement of mechanical performance of epoxy resins filled with cobalt and chromium powders

Epoxy/ powder metal composites have interesting electrical properties, becoming conductors above the percolation threshold. To complete this study, mechanical investigations have been carried out to show the influence of the fillers on the mechanical performance of these composites. In this framework, different epoxy/metallic powders (Cobalt, Chromium) composites were prepared. Scanning Electron Microscopy showed that the dispersion of the metallic fillers in the matrix is almost homogeneous. The dynamic mechanical thermal analysis (DMTA) measurements showed the dependence of the viscoelastic parameters with the frequency, temperature, nature, and content of fillers. The main relaxations observed are the primary α relaxation (associated to the glass transition, T_g) and a secondary β relaxation. A second DMTA run on the same samples showed a slight increase of the T_g. It clearly showed that the used metallic fillers improve the mechanical properties of the obtained composites. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Effect of the cooling rate in the formation of glass on the α and β relaxations of some amorphous polymers

It has been observed in dynamic mechanical measures that the range of temperatures in which relaxation properties are affected by the diminishing of the free volume has, as a lower limit the temperature at which the maximum of G″ or t_gδ, corresponding to the β relaxation, occurs. In dielectric measures on polymers in which the β relaxation is not predominant over the α relaxation, this temperature range is scarcely affected by changes in frequency, whereas the peak shifts according to its activation energy. This fact leads to the conclusion that the decrease in free volume due to a slow cooling from a temperature above T_g mainly affects mobility of the main chains under T_g, without disturbing the groups that give rise to the secondary relaxations.     

Effects of strain rate and comonomer on the brittle–ductile transition of polymers

The effect of rate on the brittle–ductile transition of polymers can be given by an Arrhenius-type equation with activation energy between those of α and β transitions and given by where E_b is the activation energy for brittle-ductile transition, E_α is that for α transition, E_β is that for β transition, T_g is the glass transition temperature, T_b is the brittle–ductile transition temperature at 0.1 min.^?1, T_α is the α transition temperature at 1 cps, and T_β is the β transition temperature at 1 cps. The plots of T_b versus the weight fraction (w) of comonomer are sigmoidal, with an inflection point at w = 0.5.     

Low-frequency thermomechanical spectrometry of polymeric materials: Tactic poly(methyl methacrylates)

A computerized and automated torsional pendulum has been used to characterize amorphous poly(methyl methacrylates) at about 1 Hz in the temperature sequence 473° → 93° → 473°K. The effects of thermal prehistory, temperature cycling, water content, and tacticity are demonstrated. In particular, a comparison of the out-of-phase shear modulus (G″) versus temperature for “syndiotactic,” “atactic,” and “isotactic” polymer specimens shows that the intensity of the glassy-state β loss peak decreases with increasing isotactic content while the temperature of its location remains the same. Extrapolation suggests that completely isotactic polymer would not display a β loss peak. The shape and location of the G″ data at low temperatures indicate that the basic mechanism of the β process is the same for the three polymer samples and support the validity of the extrapolation. The effect of tacticity is reflected also in the glass transition region; the isotactic sample has its T_g about 65°C lower with greater intensity than the syndiotactic polymer.     

Physical Ageing in Polymethylmethacrylate — A Dielectric Investigation

Abstract

Polymethylmethacrylate aged for various times at temperatures below the glass transition temperature, T_g, were studied using dielectric spectroscopy. The study uses dipole relaxation as a probe for the physical ageing process. Isothermal measurements indicate that the dominant changes occurring in the dielectric loss spectrum at temperatures between the T_g and the side chain (β) relaxation process are associated with changes in the dipole relaxation distribution and the ionic charge mobility. Differences between the spectra for aged and unaged samples are used to assess the nature of the changes occurring at a molecular level. The rate of change of dielectric loss was estimated as a function of the ageing temperature and the observed behaviour was found to be similar in form to that observed from creep and structural relaxation.     

The microwave processability of semicrystalline polymers

The overall objective of these studies was to investigate the relationship between polymer structure and microwave absorptivity. In this paper, the microwave processing of semicrystalline polymers such as poly(ether ether ketone) (PEEK), nylons, and poly(ethylene terephthalate) (PET), via a cylindrical resonance wave cavity and a rectangular standing wave applicator is described. These polymeric materials were irradiated in low power (< 50W) electric fields at 2.45 GHz. Silicone flexible molds were necessary for improved processing of nylons and PEEK at temperatures below their T_c Rapid heating rates were observed between the glass transition temperature, T_g, and the melting temperature, T_m, for all these polymers provided that T_c was exceeded. Both dynamic mechanical thermal analysis (DMTA) and dielectric thermal analysis (DETA) spectra were utilized to predict the heating phenomena between amorphous and semicrystalline materials and to explain the rapid crystallizing rate of PEEK. above its glass transition temperature. Correlations were drawn between (a) the apparent activation energy and the critical temperature (T_c) and (b) the shape of the dielectric spectra at 2.45 GHz and its shape in kHz region.     

Dynamic mechanical studies of thermal aging in poly(vinyl chloride)

The effects of thermal aging on the dynamic mechanical properties of rigid poly(vinyl chloride) (PVC) and chlorinated poly(vinyl chloride) (CPVC) are examined in the glass transition (T_g) and β transition (T_β) regions. PVC, when quenched through T_g and then annealed at 40°C, exhibits a sub-T_g peak in the loss modulus function. The position of the peak moves to higher temperatures with increasing annealing time. The effect is analogous to sub-T_g endothermic peaks previously observed by differential scanning calorimetry (DSC). A sub-T_β peak in the tan δ or the loss modulus functions can be produced by quenching through T_β. The effect of heating rate on the sub-T_β peak is explored.     

Oxidative skeleton breaking in epoxy–amine networks

The photo and thermal oxidation of various epoxide–amine tridimensional polycondensates is studied by IR spectrophotometry and DSC. The network degradation results in a decrease of the glass transition temperature (T_g), which is correlated with the quantity of amide groups formed. For each system, the relative yield in skeleton breaks increases with the intensity of internal stresses. A mechanism involving the β scission of α amino alkoxy macroradicals resulting from oxidation of constrained segments of the network is proposed.     

Study on high‐performance poly(etheretherketone)/Si3N4 nanocomposites: New electronic substrate materials

The morphology, thermal, mechanical, and dielectric properties of high‐performance poly(etheretherketone)/Si₃N₄ nanocomposites fabricated by hot pressing were investigated. It was found that the coefficient of thermal expansion (CTE) and dissipation factor decreased significantly with increasing Si₃N₄ content, whereas thermal stability was affected slightly. A nanocomposite with 30 wt% Si₃N₄ exhibited about 45% and 23% decrease in CTE, below and above T_g, respectively. The glass‐transition temperature (T_g) was increased up to 20°C. Microhardness was improved by 20% at 10 wt% Si₃N₄ content and thereafter it improved slightly. Modified rule of mixture with β = 0.1 or Halpin–Tsai model with ξ = 4 fits well the microhardness. The dielectric constant and loss factor of the nanocomposites are quite low, and thermal stability is much higher compared with commercial products. Various models were also used to correlate CTE and dielectric constant. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers