Synthesis and characterization of sulfonated homo‐ and co‐polyimides based on 2,4 and 2,5‐diaminobenzenesulfonic acid for proton exchange membranes

A series of sulfonated homo‐ and random co‐polyimides (co‐SPI) based on 2,4‐diaminobenzenesulfonic acid (2,4‐DABS) and 2,5‐diaminobenzenesulfonic acid (2,5‐DABS) has been synthesized via conventional two‐step polyimidization method. 2,4‐DABS and 2,5‐DABS were used as sulfonated diamine compounds, 4,4′‐oxydianiline (ODA) and 4,4′‐diaminodiphenyl sulfone (DDS) were used as non‐sulfonated diamine compounds. Mixtures of sulfonated and non‐sulfonated diamine compounds were reacted with benzophenonetetracarboxylic dianhydride (BTDA) to obtain co‐SPI membranes. Molar ratios of sulfonated to non‐sulfonated diamine were systematically varied to produce copolymers of controlled compositions. The co‐SPIs were evaluated for thermal oxidative stability, ion exchange capacity (IEC), water uptake, proton conductivity, solubility, and hydrolytic stability. Proton conductivity and hydrolytic stability of the co‐SPIs were compared with the fully aromatic polyimide, homo‐SPIs (BTDA/2,4‐DABS and BTDA/2,5‐DABS). Regarding thermogravimetric analysis (TGA) analysis, it is concluded that desulfonation temperature in the range of 200–350°C suggests high stability of sulfonic acid groups. co‐SPIs with 40 mol% of 2,4‐DABS showed similar or higher proton conductivity than Nafion® 117 in water. Proton conductivity values of the co‐SPIs were mainly a function of IEC and water uptake. Consequently, the optimum concentration of 2,4‐DABS was found to be in the range of 30–40 mol% from the viewpoint of proton conductivity, IEC, and hydrolytic stability. Copyright © 2008 John Wiley & Sons, Ltd.     

Numerical study of exchange bias in antiferromagnetic/ferromagnetic core/shell nanoparticles with non-magnetic defects

We perform Monte Carlo simulations for an antiferromagnetic/ferromagnetic core/shell nanoparticle with a doubly inverted structure. We investigate the dependence of the exchange bias field and coercivity on the magnetic dilution of the shell-interface and shell part. It is demonstrated that exchange bias and coercivity can exhibit monotonic or non-monotonic behavior depending on the location of the non-magnetic components. Also, temperature dependence of the exchange bias and coercivity of the system are studied for a particular defect concentration value. Our results provide an alternative way for tunning the magnetic properties of doubly inverted nanoparticles.     

The crystallization behavior of Cis-polyisoprenes extracted from the guayule plant

The crystallization kinetics and resultant morphologies of thin films of cis-polyisoprenes from guayule rubber have been studied using osmium tetroxide staining and transmission electron microscopy. Polymers used were extracted from the plant either on a laboratory scale or in a pilot plant operation. The polymers show significantly different behavior with regard to both morphological features and to lamellar growth rates. Freshly extracted cis-polyisoprene as well as freshly prepared specimens of pilot plant rubber show only α growth, as in cis-polyisoprene from hevea, but a slightly faster lamellar growth rate. Aged pilot plant rubber shows both α and β growth, both at higher rates than in natsyn, with the rate of α growth being lower than in hevea. Freshly extracted and aged specimens show a maximum lamellar growth rate at ?20°C, whereas the maximum for pilot plant rubber occurs at ?24°C. The data are interpreted in terms of oxidation of double bonds during aging to produce pendant ester groups. Lamellar growth rates and infrared analyses are both consistent with 1.2–1.5% of the double bonds being affected. Data from guayule and natsyn are now consistent in showing that the effects of microstructural impurities on the crystallization of cis-polyisoprene are to reduce the rates of α-lamellar growth and to induce the β growth, which would not otherwise exist. Secondary nucleation analyses show the regime II–III transition, as in hevea; however, the temperature of the transition, as well as the ratio of the slopes, is different for this linear polymer.     

硼氢化钠还原樟脑制备异冰片——国外本科生实验的介绍与讨论

冰片作为一种自然界广泛存在的次级代谢产物,可以通过樟脑的还原反应制备。但由于冰片极易氧化为樟脑,实验室中常使用硼氢化钠还原樟脑制备冰片的方法。该法可能生成2种冰片异构体,可以通过动力学控制获得其中一个异冰片从而使其成为不可逆反应。该有机化学实验设计在美国与欧洲国家均有涉及,但在国内教材中只是单纯设计为环己酮等单分子的还原反应,缺乏体现化学在解决实际问题中的实用性以及课堂的趣味性。该实验的引进不失为改进国内有机化学实验设计的良好素材。     

Effective field investigation of dynamic phase transitions for site diluted Ising ferromagnets driven by a periodically oscillating magnetic field

Dynamic behavior of a site diluted Ising ferromagnet in the presence of a periodically oscillating magnetic field has been analyzed by means of the effective field theory (EFT). The dynamic equation of motion has been solved for a honeycomb lattice (z=3$z=3$) with the help of a Glauber type stochastic process. The global phase diagrams and the variation of the corresponding dynamic order parameter as a function of the Hamiltonian parameters and temperature has been investigated in detail and it has been shown that the system exhibits reentrant phenomena, as well as a dynamic tricritical point which disappears for sufficiently weak dilution.     

Effects of the Quenched Random Crystal Field on the Dynamic Spin-1 Blume-Capel Model

In this study, we have analyzed the dynamical phase transitions of spin-1 Blume-Capel model with quenched random crystal field under the effect of a time dependent oscillating magnetic field. We have obtained the magnetic field, temperature (h,T) cross sections of the global phase diagram for constant values of the concentration and the amplitude of the single-ion anisotropy within mean field approximation. There are regions of the phase space where both ordered and disordered phases coexist. In addition, the dynamic phase transition from one regime to the other can be a first- or a second-order depending on the region in the phase diagram. Hence, the system exhibits a number of interesting phenomena and a rich variety of phase diagrams with type being according to the concentration p of active local crystal fields.     

Tunning exchange bias in inverted antiferromagnetic/ferromagnetic core/shell nanoparticles by binary alloy shells

A detailed investigation of exchange bias properties of an inverted nanoparticle with an antiferromagnetic core and a ferromagnetic binary alloy shell of the type $B_x{C}_{1?x}$ is presented by benefiting from Monte Carlo simulations. Exchange bias displays a non-monotonic behavior with the varying value of the concentration the type-B magnetic components, x. Coercivity exhibits a monotonic or a non-monotonic variation with x depending on the relative strength between unlike magnetic components in the shell. Particular attention has also been given to determine the effects of the cooling field process on the magnetic properties of the nanoparticle. Numerical results obtained in this work present a different physical mechanism and an alternative way for tuning the exchange bias and coercivity of bimagnetic core/shell nanoparticles.     

On semigroup presentations and efficiency

The purpose of this paper is twofold. First we determine some forms of the relations in a finite semigroup presentation with zero deficiency which does or does not define a group. Moreover, we conclude that a finite Rees matrix semigroup M [G; I, Λ; P] is efficient when G is efficient and the index sets I, Λ are finite.