In situ investigation of SBR vulcanization: A combined study of1H-NMR and vulcametry

For the first time, in situ NMR measurements were performed during sulfur vulcanization of unfilled SBR. A vulcanization device was constructed for use in combination with a standard microimaging probe in a vertical bore NMR magnet.¹H-linewidth measurements are correlated with cure simulations in a vulcameter to explain the increase of the linewidth during the vulcanization. Inhomogeneous heating conditions in the sample result in an inhomogeneous course of the vulcanization as a function of time. The spatial dependence of this process was monitored by NMR imaging.     

高介电常数、低介电损耗的聚合物基复合材料

考虑到高介电常数、低介电损耗聚合物基复合材料在电气、电子行业的广泛重要应用,本文对其研究进展进行了回顾。重点讨论了功能填料、聚合物基体及它们两者的相互作用等因素对聚合物基复合材料介电性能的影响规律,试图为建立高介电常数、低介电损耗的聚合物基复合材料的设计和制备原理理清思路。指出该领域在未来的主要发展方向是涉及制备结构、形貌、尺寸可控的新的功能填料,探索新型简单的复合工艺和界面控制技术,从理论上分析建立功能填料的结构和介电性能的关系模型等。     

纤维素基介电材料

作为电荷的高效存储设备,介电电容器受到学术界和产业界越来越多的关注,其中介电材料是介电电容器的核心。长久以来,双向拉伸聚丙烯和聚对苯二甲酸乙二醇酯等石油基聚合物作为介电材料广泛应用于商用电容器设备。然而,随着服役环境的变化(如工作温度升高等),聚合物材料的介电常数与充放电效率下降,同时介电损耗与电流漏导增加,导致储能密度大幅降低,亟待开发具有高储能密度的新型可持续聚合物介电材料。纤维素是地球上储量最丰富的天然聚合物,具有可持续、价格低廉、可生物降解等特点,是理想的候选生物基材料。近年来,通过将纤维素纳米纤维、氰乙基纤维素、再生纤维素及醋酸纤维素分别与不同填料进行复合,显著提高了介电常数、击穿场强与充放电效率,获得了一系列具有高储能密度的生物基介电材料。本文总结了上述纤维素基介电材料的研究进展,并对该领域面临的挑战与发展前景进行了探讨与展望。     

Dielectric differential thermal analysis

Dithionates (CaS₂O₆·4H₂O, SrS₂O₆·4H₂O, BaS₂O₆·2H₂O, MnS₂O₆·4H₂O, MgS₂O₆·6H₂O, CoS₂O₆·6H₂O, NiS₂O₆·6H₂O, ZnS₂O₆·6H₂O and CuS₂O₆·4H₂O) were subjected to thermodielectric analysis. The thermoanalytical curves show low temperature effects from 60 to 350°. These are related with the dehydration and decomposition of the dithionates, which could be fully correlated with the knowledge of the thermal behavior of these compouds obtained with other thermal methods.     

Dielectric differential thermal analysis

Thermodielectric analysis has been used to test some very well-characterized kaolins from Cuban deposits. The samples were analysed by thermal analysis, X-ray diffraction, infrared spectrometry, electron microscopy and chemical analysis.The dielectrical thermograms show that the most evident effect for the hydrated versions is the water peak. The second dielectric effect, at high temperatures, is related to the cationic conductions. A relation involving the K content of the sample was observed.

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Zusammenfassung Mittels thermodielektrischer Analyse wurden einige gut charakterisierte Kaoline aus kubanischen Lagerstätten untersucht. Die Proben wurden mittels Thermoanalyse (TA), Röntgendiffraktion (XRD), Infrarotspektroskopie (IR), Elektronenmikroskopie (EM) und Elementaranalyse untersucht. Die Dielektrothermogramme zeigen, daß bei den hydratierten Versionen der augenscheinlichste Effekt der Wasserpeak ist. Der zweite dielektrische Effekt bei höheren Temperaturen wird mit der Kationenleitung in Zusammenhang gebracht, wodurch es möglich wird, Informationen über den K-Gehalt der Proben zu erlangen.

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Dielectric differential thermal analysis

The present paper deals with the role of guest molecules in the moleculer sieves systems.     

Dielectric properties of emulsions

Summary Dielectric constants of O/W emulsions were measured over a wide range of concentration and at frequencies ranging from 20 cps. to 3 mc. No dielectric dispersion due to the interfacial polarization was observed in the experimental range of frequency, while the electrode polarization was observed below 100 kc. Experimental results were compared with theoretical values for spherical dispersions. It was concluded that the dielectric constants of O/W emulsions were expressed best byBruggeman's equation over the whole range of concentration.

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Zusammenfassung Die DK von ?l/Wasser-Emulsionen wurden über einen weiten Konzentrationsbereich und für Frequenzen zwischen 20 Hz und 3 MHz gemessen. Es war keine dielektrische Dispersion verursacht durch Oberfl?chenpolarisation innerhalb des experimentellen Frequenzbereiches zu beobachten. Dagegen wurde unterhalb 100 kHz Elektrodenpolarisation bemerkbar. Die experimentellen Ergebnisse wurden mit den theoretischen Darstellungen der Misch-DK für Dispersionen mit kugligen Teilchen verglichen. Der Verlauf der DK der ?l/Wasser-Emulsion wird am besten und über den ganzen Konzentrationsbereich durch die Gleichung vonBruggeman dargestellt.

     

Dielectric relaxation of polydimethylsiloxane

Dielectric studies were performed on crystallized and amorphous polydimethylsiloxane which had been characterized by differential thermal analysis and polarized light microscopy. The crystallized specimen displayed one relaxation near 160 K at 1 kHz while the amorphous specimen showed absorption peaks at 155 and 165 K. For the latter material the high-temperature peak was not due to a true relaxation but resulted from crystal nucleation at 160 K and the subsequent growth of spherulites. The low-temperature peak at 155 K resulted from the relaxation associated with the glass transition. A sharp decrease of dielectric constant was observed for both specimens at the melting point (235 K). For the dielectric relaxation associated with the glass transition in crystallized specimens, the values of the dispersion amplitude, the apparent activation energy at 160 K, and the half-width of the absorption curve are 0.43 and 29 kcal/mole, and 5.6 decades, respectively, which are in marked contrast to the corresponding values of 0.82, 18, and 2.2 for amorphous specimens.     

Dielectric properties of moist cellulose

The dielectric constant ?′ and the dielectric loss ?″ for cellulose fiber were measured over a frequency band 0.2 to 10 Mc/sec and a temperature range from ?20 to 80°C. Also, the variation of the dielectric behavior with relative humidity was measured at 25°C. From these data, both the specific resistivity R_s and the dissociation energy U₀ were calculated. The results showed that the dielectric constant increased with frequency and temperature. This may be due to the increase in the rotation and the polarization of the flexible part in the fiber. The variation of the dielectric loss with temperature showed a maximum absorption corresponding to the β-relaxation. For the moist fiber, it is found that as the relative humidity increases, the dielectric constant and the dielectric loss increase. This increase may be due to the presence of polar water molecules, to the freeing of the polar groups, and to the freeing of the ions in the fiber molecule as well as to the increase in the number of OH^? and H⁺ ions resulting from the ionization of water. A relation between the dielectric constant and resistivity at different humidities is represented graphically. From this relation, it is found that the dissociation energy is equal to 0.318 × 10^?12 and 5.46 × 10^?12 erg below and above 52% RH, respectively.     

Dielectric characterization of substituted diols

Dielectric properties of four diols having different chemical structures of the hydrophobic part were determined by the time domain spectroscopy method in the frequency range from c. 15MHz to c. 7GHz. The static permittivity clearly reflects the changes in the position of the dipole moments in the molecules. The dielectric relaxation times characterizing the diol part of the molecules are similar for all the substances studied. In the case where the phenylthiadiazole unit is built into the alkyl chain, two well separated processes were observed: one connected with the 'hydrophobic' and the second with the 'hydrophilic' part of the molecule. This emphasizes the amphotropic character of such molecules.     

Variations in Proteins Dielectric Constants

Using a new semi-empirical method for calculating molecular polarizabilities and the Clausius−Mossotti relation, we calculated the static dielectric constants of dry proteins for all structures in the protein data bank (PDB). The mean dielectric constant of more than 150,000 proteins is with a standard deviation of 0.04, which agrees well with previous measurement for dry proteins. The small standard deviation results from the strong correlation between the molecular polarizability and the volume of the proteins. We note that non-amino acid cofactors such as Chlorophyll may alter the dielectric environment significantly. Furthermore, our model shows anisotropies of the dielectric constant within the same molecule according to the constituents amino acids and cofactors. Finally, by changing the amino acid protonation states, we show that a change of pH does not have a significant effect on the dielectric constants of proteins.     

Temperature Calibration in Dielectric Measurements

Dielectric experiments are often performed in non-isothermal conditions. Thus, there is a difference between the temperature of the sample and the sensor temperature. In this work we propose and compare three temperature calibration methods based on the detection of transitions or relaxations: i) the melting of high-purity metallic standards (indium and tin), ii) the 2^nd order phase transition of a ferroelectric crystal (TGS); iii) the -relaxation of an amorphous polymer (poly(carbonate)). The results obtained from the three different methods were used to construct a calibration curve for a given heating rate.This revised version was published online in November 2005 with corrections to the Cover Date.     

Dielectric relaxation time of polymers

The temperature dependence of the dielectric relaxation time of amorphous polymers can be described quite satisfactorily by an expression derived from the theory of the relaxation time for local conformational transitions in a polymer chain. This theory was recently developed using the Kramers theory of the rate constant together with the free-volume theory.