Nearly constant dielectric loss behavior in ionomers

The electrical conductivity of a series of ionomers has been characterized by measuring the electrical conductivity in a relatively broad range of frequencies and temperatures. At low frequencies, the conductivity of the ionomers exhibits a universal Jonscher power law (JPL), and at higher frequencies a nearly constant loss (NCL) behavior. The NCL for the ionomers is qualitatively similar to that observed for other inorganic ionic conductors. However, the magnitude of NCL for ionomers is lower than that observed for inorganic ionic conductors. The analysis of the conductivity master curves suggests that the conduction mechanism, which includes both the NCL and the JPL behaviors, is governed by ion hopping of the mobile ions.     

How Does the Trans–Cis Photoisomerization of Azobenzene Take Place in Organic Solvents?

The trans–cis photoisomerization of azobenzene‐containing materials is key to a number of photomechanical applications, but the actual conversion mechanism in condensed phases is still largely unknown. Herein, we study the ${{\rm{n}}{\rm{,{\rm \pi} ^\ast }}}$ isomerization in a vacuum and in various solvents via a modified molecular dynamics simulation adopting an ab initio torsion–inversion force field in the ground and excited states, while allowing for electronic transitions and a stochastic decay to the fundamental state. We determine the trans–cis photoisomerization quantum yield and decay times in various solvents (n‐hexane, anisole, toluene, ethanol, and ethylene glycol), and obtain results comparable with experimental ones where available. A profound difference between the isomerization mechanism in vacuum and in solution is found, with the often neglected mixed torsional–inversion pathway being the most important in solvents.     

Some results about a theorem of Shidov

Abstract The authors characterize the finite groups in which , the intersection of the maximal non-nilpotent subgroups of G, is nilpotent, but different from Φ(G). Further, if is a saturated formation and if is the intersection of all maximal subgroups of G not belonging to , a necessary and sufficient condition is given for to be nilpotent different from Φ(G). Keywords: Frattini subgroup, Maximal subgroups, Saturated formation Mathematics Subject Classification (2000): 20B05, 20D10, 20D25,20E28     

On the “Shidov Property” and a Formation Satisfying It

If G is a nonsoluble finite group, the intersection of the maximal subgroups of G which are not nilpotent is the Frattini subgroup of G. This was proved by Shidov (1971 Shidov , L. I. ( 1971 ). On maximal subgroups of finite groups . Sibirsk. Mat. Zh. 12 ( 3 ): 682 – 683 . [Google Scholar]). The authors here present a new formation ? larger than the formation of the nilpotent groups for which the analogous of the theorem of Shidov holds. The theorem makes use of the classification of finite simple groups.     

Organic sonochemistry. Synthesis and use of reissert compounds under PTC-ultrasound

Use of phase transfer catalysis and ultrasound stirring improved the synthesis, alkylation and hydrolysis of several Reissert derivatives.     

Expected and unexpected behavior of the orientational order and dynamics induced by azobenzene solutes in a nematic

We have explored the changes in the phase stability, orientational order, and dynamics of the nematic 4-cyano-4'-n-pentylbiphenyl (5CB) doped with either the trans or the cis form of different p-azobenzene derivatives using the ESR spin-probe technique. In particular, we have studied the effects induced by each of the seven nonmesogenic 4-R-phenylazobenzenes (R = H, F, Br, CH3, CF3, On-Bu, Ot-Bu) at 1% and 7% mole fraction on the order parameter and on the shift of the nematic-isotropic transition temperature (TNI), as reported by a nitroxide spin probe, and we have tried to relate them to the solute shape and charge distribution. In all the cases the presence of the azo-derivative causes a depression of T(NI), more pronounced for the cis isomers. The dependence of on the reduced temperature T* = T/T(NI) remains the same as that of pure 5CB in all trans-doped samples at 1% and 7% and decreases only slightly in the cis at 1%. However, we observe different and in some cases large variations (up to 25%) in for the cis at 7%, showing solute effects that go beyond the shift in T(NI). Surprisingly enough, even at the highest concentration, the probe dynamics appears to be essentially independent of the nature, the configuration, and the concentration of the different solutes and very similar to that observed in the pure 5CB.     

Structure–dynamics relationships of the α‐relaxation in flexible copolyesters during crystallization as revealed by real‐time methods

The evolution of the α‐relaxation during an isothermal crystallization process of a series of flexible copolyesters of hydroxybutyrate (HB) and hydroxyvalerate (HV) has been followed in real‐time by wide‐angle X‐ray scattering and dielectric complex permittivity measurements. The change of the dielectric parameters with crystallization time can be phenomenologically described in terms of the Havriliak‐Negami equation. The dielectric strength follows a sigmoidal‐shaped pattern similar to that shown by the crystallinity. A reduction of the overall mobility with crystallization time of the polymeric chains in the amorphous phase has been observed. This slowing down effect depends on the HV molar content. The influence of the chain flexibility on the crystalline‐induced restriction has been discussed in the light of similar studies carried out with more rigid polymers. Dielectric experiments suggest that the progressive immobilization of polymer segments as crystallization proceeds cannot be exclusively associated with the amount of crystalline material. Differences in microstructure, depending on the HV molar content, seem to be responsible for the observed behavior. The progressive broadening and symmetrization of the α‐relaxation with increasing crystallization time has been explained as due to a restriction of the large‐scale motions of the polymeric chains, as the material is being filled in with crystals. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 37–49, 1999     

Phase behavior of a substituted poly(paraphenylene): Poly(para-2,5-didecyl-p-phenylene)

The thermal behavior of poly(para-2,5-didecyl-p-phenylene) has been investigated by differential scanning calorimetry and real time X-ray diffraction. Poly(para-2,5-didecyl-p-phenylene) is a semicrystalline material that crystallizes in a layered structure. The system exhibits two thermal transitions in the investigated temperature range. The first one, occurring at lower temperatures, provokes a reduction of the layered spacing accompanied by an appreciable disordering of the lateral side chains. Above the first transition the material is shearable, highly viscous, and birefringent. Thus, we have associated this transition to the formation of a layered mesophase. The higher temperature transition exhibits a twofold endothermic DSC peak and is characterized by the disappearance of X-ray diffracted intensity. At temperatures above the second transition the system presents the characteristics of an isotropic melt. Consequently, we have associated this transition with the complete disordering of the polymeric backbones. By following an appropriate thermal treatment it has been shown that the twofold shape of the endotherm characterizing the higher temperature transition can be changed into a single endotherm. This effect has been interpreted as being due to the kinetics of main-chain ordering. This ordering seems to proceed by the initial growth of domains with a high level of order followed by the subsequent increase of these domains through the inclusion of less ordered material. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36 : 49–54, 1998     

Stacking of main chain-crown ether polymers in thin films

Thin films (9-70 nm) of a series of polymers containing in the main chain dibenzo-18-crown-6 ether unit (DB18C6) linked to an aliphatic spacer of different length (10C and 14C) and nature have been prepared, from chloroform solutions, by spin coating on a silicon substrate. The quality and homogeneity of the polymer coatings was revealed by their reflectivity spectra and atomic force microscopy (AFM). The grazing incidence small-angle X-ray scattering (GISAXS) patterns show an out-of-plane structure correlation (interference maximum near the horizon) of scale size related to the polymer repeating unit length. Above this Bragg reflection, the shape of the scattering observed, in the GISAXS pattern, reveals an orientation of the stacked molecular columns in the coated polymer. A thermal treatment of the samples improves the nanostructure by increasing the lamellar coherence size (in y-direction) as well as the vertical orientation of the molecular columns.     

On the electrical conductivity of PVDF composites with different carbon-based nanoadditives

Composites based on poly(vinylidene fluoride) (PVDF) and different carbon additives, such as carbon nanofibers (CNF), graphite (G), expanded graphite (EG), and single-walled carbon nanotubes (SWCNT) have been prepared by nonsolvent precipitation, from solution, and subsequent melt processing. From a structural point of view, the α-crystal phase is the predominant crystal form in all the nanocomposites. However, those containing CNF, G, and EG at high nanoadditive content present also β-crystal phase. Even though the intrinsic thermal properties of PVDF are hardly affected, the nanoadditives act as nucleating agents for the crystallization. In regard to the electrical properties, all nanocomposites exhibit a percolating behavior. Moreover, the fact that the nanocomposites present both high dc conductivity and high dielectric constant, in a certain nanoadditive concentration range below the percolation threshold, suggests that a tunneling conduction mechanism for charge transport is present. With regard to the ac electrical properties, depending on the morphology of the different additives, the charge transport above percolation threshold can be explained taking into account the anomalous diffusion effect for high nanoadditive content or an intercluster polarization mechanism when the nanoadditive concentration decreases.