Adhesive bonding of glassy polymer surfaces by an ultrathin layer of a semicrystalline polymer

To develop a greater understanding of interfacial interactions between a semicrystalline polymer and a glassy polymer, adhesion tests were performed on very thin layers of poly(ethylene oxide) (PEO) sandwiched between two layers of poly(tetramethyl bisphenol A polycarbonate) (TMPC). The tests were designed to provide intimate contact between the surfaces while they were heated above the melting point of the PEO and cooled back to room temperature. A contact mechanics approach, based on the Johnson, Kendall, and Roberts theory, was used to determine values of the energy release rate describing the energetic driving force for crack propagation within the interfacial region. The ability to measure crack propagation at large values of the energy release rate was limited by rupture of the silicone elastomer that was used to provide a sufficiently compliant matrix for the adhesion experiment. By cycling the tensile stress at relatively low loading levels, we were able to measure fatigue crack propagation at values of the energy release rate that did not result in failure of the elastomer. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3809–3821, 2004     

The influence of the liquid crystalline core geometry on the mesogenicity of novel chiral 2-(4-substituted-phenoxy)propanonitriles

The synthesis and characterization of seven novel (R)-2-(4-substituted-phenoxy)propanonitriles are described. The propanonitriles were prepared to evaluate their potential use as thermochromics and ferroelectric dopants, as well as to determine their twist sense properties. The materials exhibit smectic and chiral nematic phases of high thermal stability; the mesogenic behaviour of the nitriles is directly related to the type of two-ring core unit employed. The effects of the different molecular geometries and polarizabilities of the liquid crystalline cores on mesophase stability are discussed, particularly in relation to other members of this series. The chiral nematic phase of the propanonitriles is assigned as having a left-handed twist sense from contact preparation studies, and this is in agreement with rules relating absolute configuration and molecular structure to helical twist sense.     

Mechanical endurance of polymer electrolyte membrane and PEM fuel cell durability

The life of proton exchange membrane fuel cells (PEMFC) is currently limited by the mechanical endurance of polymer electrolyte membranes and membrane electrode assemblies (MEAs). In this paper, the authors report recent experimental and modeling work toward understanding the mechanisms of delayed mechanical failures of polymer electrolyte membranes and MEAs under relevant PEMFC operating conditions. Mechanical breach of membranes/MEAs in the form of pinholes and tears has been frequently observed after long‐term or accelerated testing of PEMFC cells/stacks. Catastrophic failure of cell/stack due to rapid gas crossover shortly follows the mechanical breach. Ex situ mechanical characterizations were performed on MEAs after being subjected to the accelerated chemical aging and relative humidity (RH) cycling tests. The results showed significant reduction of MEA ductility manifested as drastically reduced strain‐to‐failure of the chemically aged and RH‐cycled MEAs. Postmortem analysis revealed the formation and growth of mechanical defects such as cracks and crazing in the membranes and MEAs. A finite element model was used to estimate stress/strain states of an edge‐constrained MEA under rapid RH variations. Damage metrics for accelerated testing and life prediction of PEMFCs are discussed. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2346–2357, 2006     

Gmelin handbook of inorganic chemistry, 8th edit., S — Sulfur-Nitrogen Compounds,Part 2: Compounds with Sulfur of Oxidation Number IV: Springer-Verlag,Berlin, Heidelberg,New York,Tokyo, 1985, xiv + 333 pages,DM 1144, ISBN 3-540-93512-6

A close examination of the ESR spectrum of a paramagnetic complex formed from bis(ethoxythiocarbonyl)sulfide and dimanganese decacarbonyl has revealed, for the first in this type of radical adduct, ₁₃C hyperfine coupling in natural abundance from the carbonyl ligands.     

Time-dependent pump-probe spectra of NeBr2

Time- and frequency-resolved pump-probe measurements on NeBr2 have been performed to better characterize its fragmentation dynamics on the B electronic state for vibrational levels in the energy region of the transition from direct vibrational predissociation to intramolecular vibrational relaxation dynamics. Above nu'=20 of the Br2 stretching mode, it was observed that the dependence of lifetime on the vibrational quantum number deviates from the energy-gap law by leveling off in the range of 10 psE transitions of the complex. These transitions are shifted 20 cm(-1) to lower energy from the free Br2 resonances, indicating an E state Ne-Br2 bond energy of 82 cm(-1). Measurements of NeBr2 vibrational predissociation via the delta nu=-2 channel were also performed for nu'=27, 28, and 29. The closing of the delta nu=-1 channel leads to an increase in the lifetimes of these vibrational levels. A new Nd:yttrium aluminum garnet pumped dual optical parametric oscillator/optical parametric amplifier system is described that allows us to conveniently record time-delayed pump-probe spectra with 2-cm(-1) spectral resolution and 15-ps time resolution.     

Bistable ferroelectric liquid crystal photoswitch triggered by a dithienylethene dopant

The spontaneous polarization (PS) of a ferroelectric liquid crystal is modulated reversibly by photocyclization of the dopant 1,2-bis[5'-(4' '-heptyloxyphenyl)-2'-methylthien-3'-yl]perfluorocyclopentene. The magnitude of PS photomodulation increases with dopant concentration up to 3 mol %, and the resulting photoswitch is fatigue resistant and bistable. To the best of our knowledge, this is the first example of a bistable ferroelectric liquid crystal photoswitch to be reported in the literature.     

Variable step molecular dynamics: An exploratory technique for peptides with fixed geometry

A new algorithm is presented for performing molecular dynamics simulations of peptides with fixed geometry, with the aim of simulating conformational changes and of exploring conformational space. The principle of the method is to expand the potential energy as a Taylor's series in the coordinates around the current point, retaining the force and its first two derivatives, and obtain a series solution of the resulting differential equations using a method due to Lyapunov. By choosing the time step so that the second term in the series is small compared to the first, the true solution can in principle be approximated to any desired degree of accuracy. The algorithm has been used to solve numerically Lagrange's equations of motion for N-acetyl alanine amide and N-acetyl methionide amide, regarded as fixed at their C-termini, under the influence of the ECEPP/2 potential energy function, and time steps of 15–30 fsec have been achieved with little variation in the total energy. Possible directions for future development are discussed.     

A new class of heterocyclic betaines: 1,1-dimethyl-3-phenylpyrazolium-5-oxide

1,1-Dimethyl-3-phenylpyrazolium-5-oxide (1) was isolated as a minor product (3%) in the reaction of ethyl benzoylacetate with 1,1-dimethylhydrazine and also was obtained as the major product (58%) from the reaction of methyl p-toluenesulfonate with ethyl benzoylacetate dimethylhydrazone. The structural assignment was made on the basis of chemical reactions and spectral data and the formula is represented by a resonance structure for which there are two principal canonical forms. Reaction of 1 with methanol at room temperature afforded methyl benzoylacetate dimethylhydrazone. Ethyl benzoylacetate dimethylhydrazone was shown to exist as a tautomeric mixture containing 65% of the imine form at room temperature.     

Potential energy surfaces for HenNe+ ions: ab initio and diatomics-in-molecule results

The potential energy surface of He2Ne+ has been reinvestigated using a combination of ab initio and diatomics-in-molecule (DIM) calculations. In contrast to the reports of two recent studies the ion is found to have an asymmetric linear He-Ne-He structure, with no barrier to formation from the separated atoms on the ground-state surface. The He-Ne+ bond lengths at the potential minimum are 1.51 and 1.81 A, and the total bonding energy is 0.717 eV. Comparing the He2Ne+ energy to that of HeNe+, the bonding energy for the second helium atom is 0.06 eV, about 10% of that of the first He atom. The saddle point between the two equivalent minima is a symmetric structure, 0.0074 eV above the potential minimum. A symmetric geometry becomes the overall potential minimum if the 2s hole on the Ne is excluded from the reference states of a multireference configuration interaction calculation. A DIM potential was created for the HenNe+ family of ions. The DIM potential is consistent with the asymmetric He2Ne+ ion serving as a core; it predicts a slightly more asymmetric geometry than the ab initio results. Additional helium atoms form five-membered rings around the bonds of the core ion to fill the first shell and then add to the ends of the cluster. The asymmetric core ion and the highly compact structure help to account for the lack of apparent shell structure in the mass spectrometry of HenNe+ clusters. Finally, we recommend that the value De=0.63+/-0.04 eV be adopted for the ground state of HeNe+.