Flow-modified permittivity of solutions of poly(n-butyl isocyanate) and poly(n-hexyl isocyanate)

The dielectric behaviour of solutions of poly(n-butyl isocyanate) and poly(n-hexyl isocyanate) in flow have been examined, including, for the latter polymer, fractions of differing molecular weight. The data are analysed in terms of established theories for changes in the permittivity of solutions of rigid macromolecules undergoing shear and are critically compared to dielectrically based information obtained from studies of still solutions. Rotational diffusion coefficients and dipole moments have been measured and the influence of heterogeneity in molecular weight on flow-modified permittivity is examined. The results obtained support the view that the theories for the flow-modified permittivity of rigid macromolecules are generally satisfactory in providing rotational diffusion coefficients and resolved dipoles, but that the influence of molecular shape on the effect is not simply obtained when polydisperse material is involved.     

Enthalpic relaxation in semi-crystalline PEEK

The effect of crystallisation on the glass transition temperature and enthalpic relaxation in poly(ether ether ketone) (PEEK) has been investigated. The increase in glass transition temperature and the activation enthalpy of ageing is explained in terms of the amorphous phase being constrained by the crystallites. The extent of enthalpic relaxation with time has been analysed in terms of the Cowie-Ferguson model and the β value was found to be dependent on both temperature and the crystalline morphology, changes in the co-operativity of the relaxations are used to explain this observation.     

Dielectric properties of poly(vinyl chloride) in tetrahydrofuran and cyclohexanone

The dielectric absorption and dispersion of poly(vinyl chloride), M M_v = 49 000, in THF and in cyclohexanone have been studied over a frequency range of 120 kHz to 11·0 MHz, at temperatures from ?22·5 to 35°C, and at concentrations ranging from 4 to 12·0(w/v)% PVC/THF and from 2 to 8·0(w/v)% PVC/cyclohexanone. The viscosities of the two systems have also been measured at temperatures from 20 to 50°C. A single relaxation time was found (β = 0·8–1·0), which indicates that relaxation occurs by segmental rotation. The dielectric and viscous activation energies have been calculated. The dipole moment associated with the relaxation process has also been calculated. The relaxation time, dielectric and viscous activation energies and the dipole moment were found to be dependent on the type of solvent. The dipole moment also showed a molecular weight dependence. The influence of the concentration and temperature on the apparent dipole moment and on the relaxation time is discussed.     

Effect of water on relaxations in the glassy and liquid states of poly(propylene oxide) of molecular weight 4000

The complex relative permittivity of poly(propylene oxide) (PPO) of molecular weight 4000 containing 1.23 wt% water has been measured in the temperature range 77 to 325 K and frequency range 12 Hz to 500 kHz, and the results are compared with the corresponding study of pure PPO-4000. On the addition of water, all the three processes, namely the β-process (at T < T_g) and the - and ′-processes (at T > T_g), are shifted to higher temperatures. The strength of the β-process remained unchanged but that of the and ′-processes increased. The halfwidths of the three processes remained unchanged on dilution with water. The decrease in the relaxation rate of the β-process is suggested to be due to hydrogen bonding of the ---CH(CH₃)---O---CH₂--- group with water molecules. Water antiplasticizes PPO-4000 and this is interpreted as due to the increased chain length when the chain ends become linked via hydrogen bonds. The static permittivity is increased by 30% on addition of 1.23 wt% water.     

Tensile mechanical properties of PEEK films over a wide range of strain rates. II

Tensile mechanical properties of poly(aryl ether ether ketone) (PEEK) films showing different thermal histories have been investigated at room temperature to point out the main key microstructural features governing properties over a wide strain rate range, i.e., from 10⁻⁵ to 300 s⁻¹. The strain rate sensitivity of the mechanical properties of amorphous PEEK films significantly depends on the analyzed strain rate range: i.e., 1) from 10⁻⁵ to 10 s⁻¹, the strain rate dependence of both apparent Young's modulus and yield stress is weak; and 2) from 10⁻¹ to 200 s⁻¹, both parameters significantly increase. Thus, based on the definition of the relationships between temperature, strain rate, and frequency respectively used for tensile tests and dynamic mechanical spectrometry, it was shown that the mechanical behavior of PEEK films at room temperature could be governed by similar molecular mechanisms as those giving rise to the β₁ and β₂ transitions. The Eyring analysis shows that motions of five or six monomers are implied at the beginning of the plastic deformation of amorphous and semi-crystalline PEEK films, while at higher strain rates, shorter chain segments are concerned. Thus, the crystalline phase only induces an increase in the stress level because of the reinforcement effect but does not modify the molecular mechanisms governing the plastic deformation of PEEK films at room temperature. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 1053–1059, 1997     

All-organic Poly(butyl methacrylate)/Poly(vinylidenefluoride-trifluoroethylene) Dielectric Composites with Higher Permittivity and Low Dielectric Loss for Energy Storage Application

With the rapid development of electronic industry and technology, there are increasingly stringent requirements on dielectric materials with higher permittivity, lower loss, and enhanced breakdown field. Since single dielectric material cannot meet the demands of industrialization, polymer-based dielectric materials with better quality have been widely applied. This study focused on a poly(butyl methacrylate/poly(vinylidene fluoride-trifluoroethylene) dielectric composite thin film, which were prepared by solution casting method. X-ray diffraction and differential scanning calorimetry data indicated that introducing poly(butyl methacrylate) led to increase in the crystallinity. The result of scanning electron microscopy showed the good compatibility between poly(vinylidene fluoride-trifluoroethylene) and poly(butyl methacrylate). Besides, the dielectric film remained good mechanical property. The dielectric properties were studied as a function of filler content and frequency. The results showed that the permittivity was as high as 13.3, while the breakdown field was 322?MV m^?1 when the fraction of poly(butyl methacrylate) was 10%. Blending poly(butyl methacrylate) improved the dielectric performance of the poly(vinylidene fluoride-trifluoroethylene).     

Dielectric relaxation of poly(tetrafluoroethylene–perfluorovinyl ether) films

Dielectric constant and dielectric loss have been studied for poly(tetrafluoroethylene–perfluorovinyl ether) (PFA) films over a wide temperature range in the frequency range 0.1–100 kHz. Two relaxation peaks were observed, one at room temperature (α_a-relaxation) and the other in the range 170–140 K (β-relaxation), with activation energies of 143·2 and 16·4 kcal/mol, respectively. The β-absorption is attributed to the short segmental local mode motion of the main chains. The α_a-relaxation can be interpreted as due to large-scale conformational rearrangement. The Cole–Cole diagrams are given at different temperatures and the distribution parameters (ϵ₀–ϵ_∞) and (1–α) of the relaxation times were calculated. The X-ray diffraction pattern of PFA shows both a diffuse halo and sharp reflections, characteristic of amorphous and crystalline phases of conventional semicrystalline polymers. Also, no evidence of crystallinity in the films due to thermal treatment during dielectric measurements was observed. IR spectra revealed the absence of any new peaks after the heat treatment.     

Universal dielectric relaxation induced giant dielectric permittivity in Mn-doped PbZrO3 ceramics

Manganese-doped lead zirconate ceramics were prepared by a conventional solid reaction method. Although the doping content varies from 5% (mole) to 20% (mole), significantly enhanced universal dielectric relaxation occurs only in 5% Mn-doped lead zirconate. The behavior cannot be properly explained by Maxwell-Wagner relaxation, which is commonly used for explaining giant dielectric permittivity in electroceramics. It is found that the enhanced giant dielectric permittivity is related to hetero-valence cation doping induced universal dielectric relaxation.     

Electric and Dielectric Properties of Some Fluorescent Dye/PMMA Solar Concentrators

The effect of dye concentrations of perylene, RH-6G and K₁ doped in polymethylmethacrylate (PMMA) as matrix material, on the electrical properties of some fluorescent solar concentrator (FSC) have been investigated. The samples were prepared by dissolving grains of both PMMA and dyes in chloroform, which were then left at room temperature to evaporate the solvent. The samples were characterized by differential scanning calorimeter (DSC). The results of both dc conductivity (σ_dc) and ac conductivity (σ_ac) showed that the total conductivity σ_tot(w) is higher than σ_dc and the activation energy of σ_tot is lower than that of (σ_dc) due to the increase of the applied field frequency, which enhances the carrier jumping and subsequently the conductivity value. The dielectric properties (dielectric constant (?′), dielectric loss (?″), and dielectric tangent (tanδ) have been studied. They show that ?′ increases by increasing the concentration of the dyes doped in PMMA. All the dielectric constants, the dielectric loss and loss tangent temperature dependence, show a peak value affected by the dye concentration as well the frequency changes. The temperature dependence of the exponent S shows that at low temperature the conduction obeys a quantum mechanical tunnel model, while at high temperature the conduction obeys the correlated barrier-hopping model.     

Blends of poly(ether imide) and an aromatic poly(ether amide): Phase behavior and CO2 transport properties

Blends of poly(ether imide) (PEI, Ultem 1000) and an aromatic poly(ether amide) were studied. Although homogeneous or heterogeneous blends can be obtained depending on the blend preparation method, the inherent miscibility of the mixture was finally established. The so-called enthalpy relaxation method was used to detect one or two glass transition temperatures in the blends in spite of the similarity of the pure component transitions. Fourier transform infrared analysis provided additional evidence of the specific interactions, which could be in the origin of the miscibility. A preliminary study of the influence of the homogeneity level in the transport properties of the blend films was also undertaken. Carbon dioxide at 1 bar was used as a penetrant. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 2141–2149, 1998     

聚醚砜纤维及织物性能研究

将聚醚砜(PES)进行熔融纺丝,制得PES纤维,并织成布样,对纤维及其织物的性能进行了研究。结果表明:PES纤维的力学性能不稳定,放置一定时间,其强度下降,需经140℃高温热定型加以改善。纤维回潮率为2．33%;织物极限氧指数为26．9%,紫外吸光度低于1．6,体积比电阻为8．13×10~(13)Ω·cm,摩擦静电压为7 272 V,1 min后衰减电压为5 519 V。     

Giant permittivity and low dielectric loss of Fe doped BaTiO3 ceramics: Experimental and first-principles calculations

Fe doped BaTiO₃ ceramics with giant permittivity and low dielectric loss were synthesized in N₂/H₂ atmosphere started with BaTiO₃ powders and iron powders. XRD analysis exhibited the tetragonal-pseudocubic phase transition when the Fe content is 3 mol%. XPS spectra confirmed the iron oxides with mixed-valence structure of Fe²⁺/Fe³⁺, while Ti-ions maintain Ti⁴⁺3d⁰ states without any oxidization-reduction. For the case of ceramics with 5 mol% Fe, the dielectric constant was 66,650 at 1000 Hz at room temperature, 19 times higher than that of pure BaTiO₃ ceramics, while the dielectric loss tangent was 0.13. Comparison with other giant-permittivity materials demonstrated the superior potential of present ceramics. First-principles calculations investigated the interfacial interaction of Fe-[TiO₂] interface and Fe-[BaO] interface. Giant dielectric constant was induced by the interfacial polarization between insulating ferroelectrics and semiconducting iron oxides with mixed-valence states, as well as the contribution from the generated electron hopping conduction.     

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