Conformalional analysis of azetidines-evidenee for a non-planar ring

The proton coupling constants and particularly chemical shifts of a series of 1-alkylazetidines, 1-alkylazeudin-3-ols, 1-alkyl-3-methylazetidin-3-ols, and C-methylazetidines are rationalized in terms of a conformationally mobile, non-planar azetidine ring. Preferred conformations for these compounds are discussed.     

Polybenzopinacolones. Acid-catalyzed rearrangement of aromatic polybenzopinacols

Polybenzopinacols resulting from the photocondensations of p,p′-dibenzoyldiphenyl ether, p,p′-dibenzoyldiphenylmethane, and p,p′-dibenzoyl-1,2-diphenylethane were rearranged in sulfuric acid–dioxane solutions. The inherent viscosities of the polybenzopinacolones did not differ significantly from the inherent viscosities of the polybenzopinacols after rearrangement. A model compound study by NMR and infrared methods indicated that the polymer chain was probably the main migrating group for each of the polymer rearrangements.     

Analysis of superposed fluids by the finite element method: Linear stability and flow development

A Galerkin finite element method is described for studying the stability of two superposed immiscible Newtonian fluids in plane Poiseuille flow. The formulation results in an algebraic eigenvalue problem of the form Aλ² + Bλ + C = 0 which, after transforming to a standard generalized eigenvalue problem, is solved by the QR algorithm. The numerical results are in good agreement with previous asymptotic results. Additional results show that the finite element method is ideally suited for studying linear stability of superposed fluids when parameters characterizing the flow fall outside the range amenable to perturbation methods. The applicability of the finite element method to similar eigenvalue problems is demonstrated by analysing the steady-state spatial development of two superposed fluids in a channel.     

Cardiovascular applications of magnetic resonance imaging

Magnetic resonance imaging (MRI) is a completely noninvasive modality that has shown significant promise for the evaluation of the cardiovascular system. Our imaging technique employed electrocardiographic (ECG) gating, which resulted in well-resolved images of the cardiac structures. Patients and animals with a variety of cardiovascular abnormalities were also assessed with this technique; the abnormalities included acute and remote myocardial infarctions and their sequelae, atherosclerotic plaques, hypertrophic cardiomyopathy, pericardial diseases, and aneurysms. The diagnostic utility of MRI includes direct tissue characterization, and such utility may be further extended by the use of paramagnetic contrast media. In addition, metabolic imaging of elements other than hydrogen may further increase the clinical potential of MRI for assessment of the cardiovascular system.     

Polycarbonate carbon nanofiber composites

Carbon nanofiber (CNF) composites have the potential for creating inexpensive, semiconducting polymers. These composites require a homogeneous dispersion within the polymer. Many groups have focused on high shear methods such as twin screw extrusion. Although high shear methods produce a homogeneous dispersion, the aspect ratio of the nanofibers is reduced by the mechanical force. In this report, we present results for low shear composite formation via in situ polymerization of cyclic oligomeric carbonates. The composites were characterized by thermal gravimetric analysis, electrical conductivity, scanning electron microscopy and transmission electron microscopy. The composites exhibit minimal aggregation of the carbon nanofibers even at high weight percents. The polycarbonate/CNF composites exhibit an electrical conductivity percolation threshold of 6.3 wt% which is higher compared with similar CNF composites. The composites also show an increase in thermal stability of 40 °C as the CNF loading increases from 0 to 9 wt%.     

Cationic crosslinking of solid dgeba resins with ytterbium(III) trifluoromethanesulfonate as initiator

Solid bisphenol-A epoxy resin of medium molecular mass was cured using a Lewis acid initiator (ytterbium(III) trifluoromethanesulfonate) in three different proportions (0.5, 1 and 2 phr). A kinetic study was performed in a differential scanning calorimeter. The complete kinetic triplet was determined (activation energy, pre-exponential factor, and integral function of the degree of conversion) for each system. A kinetic analysis was performed with an integral isoconversional procedure (free model), and the kinetic model was determined both with the Coats-Redfern method (the obtained isoconversional value being accepted as the effective activation energy) and through the compensation effect. All the systems followed the same isothermal curing model simulated from non-isothermal ones. The growth-of-nuclei Avrami kinetic model A_3/2 has been proposed as the polymerization kinetic model. The addition of initiator accelerated the reaction especially when 2 phr was added. 0.5 and 1 phr showed very few kinetic differences between them.     

Protonation and subsequent intramolecular hydrogen bonding as a method to control chain structure and tune luminescence in heteroatomic conjugated polymers

We report the effects of protonation on the structural and spectroscopic properties of 1,4-dimethoxy-2,5-bis(2-pyridyl)benzene (9) and the related AB coploymer poly(2,5-pyridylene-co-1,4-[2,5-bis(2-ethylhexyloxy)]phenylene) (7). X-ray crystallographic analysis of 9, 1,4-dimethoxy-2,5-bis(2-pyridyl)benzene bis(formic acid) complex 10, and 1,4-dimethoxy-2,5-bis(2-pyridinium)benzene bis(tetrafluoroborate salt) (11) establishes that reaction of formic acid with 9 does not form an ionic pyridinium salt in the solid state, rather, the product 10 is a molecular complex with strong hydrogen bonds between each nitrogen atom and the hydroxyl hydrogen in formic acid. In contrast, reaction of 9 with tetrafluoroboric acid leads to the dication salt 11 with significant intramolecular hydrogen bonding (N-H.O-Me) causing planarization of the molecule. The pyridinium and benzene rings in 11 form a dihedral angle of only 3.9 degrees (cf. pyridine-benzene dihedral angles of 35.4 degrees and 31.4 degrees in 9, and 43.8 degrees in 10). Accordingly, there are large red shifts in the optical absorption and emission spectra of 11, compared to 9 and 10. Polymer 7 displays a similar red shift in its absorption and photoluminescence spectra upon treatment with strong acids in neutral solution (e.g. methanesulfonic acid, camphorsulfonic acid, and hydrochloric acid). This is also observed in films of polymer 7 doped with strong acids. Excitation profiles show that emission arises from both protonated and nonprotonated sites in the polymer backbone. The protonation of the pyridine rings in polymer 7, accompanied by intramolecular hydrogen bonding to the oxygen of the adjacent solubilizing alkoxy substituent, provides a novel mechanism for driving the polymer into a near-planar conformation, thereby extending the pi-conjugation, and tuning the absorption and emission profiles. The electroluminescence of a device of configuration ITO/PEDOT/polymer 7/Ca/Al is similarly red-shifted by protonation of the polymer.