Synthesis and characterization of a polyfluorene containing carbazole and oxadiazole dipolar pendent groups and its application to electroluminescent devices

We have synthesized a blue‐light‐emitting polyfluorene (PF) derivative ( PF‐CBZ‐OXD ) that presents bulky hole‐transporting carbazole and electron‐transporting oxadiazole pendent groups functionalized at the C‐9 positions of alternating fluorene units. The results from photoluminescence and electrochemical measurements indicate that both the side chains and the PF main chain retain their own electronic characteristics in the copolymer. An electroluminescent device incorporating this polymer as the emitting layer was turned on at 4.5 V; it exhibited a stable blue emission with a maximum external quantum efficiency of 1.1%. Moreover, we doped PF‐CBZ‐OXD and its analogue PF‐TPA‐OXD with a red‐light‐emitting iridium phosphor for use as components of phosphorescent red‐light emitters to investigate the effect of the host's HOMO energy level on the degree of charge trapping and on the electrophosphorescent efficiency. We found that spectral overlap and individual energy level matching between the host and guest were both crucial features affecting the performance of the electroluminescence devices. Atomic force microscopy measurements indicated that the dipolar nature of PF‐CBZ‐OXD , in contrast to the general nonpolarity of polydialkylfluorenes, provided a stabilizing environment that allowed homogeneous dispersion of the polar iridium triplet dopant. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2925–2937, 2007     

Elastic scattering of slow positrons on atoms

The results of calculations of the elastic scattering cross section of positrons on noble gas and alkali atoms are presented. The calculations are performed within the one-electron Hartree-Fock approximation with multielectron correlations in the so-called random phase approximation with exchange taken into account. Virtual positronium formation is taken into account and proved to be very important. Arguments are presented that the positron polarization potential is repulsive for alkali atoms. The results obtained are in a reasonable agreement with experiment and with some previously reported calculations.     

Numerical approximation of generalized Newtonian fluids using Powell–Sabin–Heindl elements: I. theoretical estimates

In this paper we consider the numerical approximation of steady and unsteady generalized Newtonian fluid flows using divergence free finite elements generated by the Powell–Sabin–Heindl elements. We derive a priori and a posteriori finite element error estimates and prove convergence of the method of successive approximations for the steady flow case. A priori error estimates of unsteady flows are also considered. These results provide a theoretical foundation and supporting numerical studies are to be provided in Part II. Copyright © 2003 John Wiley & Sons, Ltd.     

Highly enantioselective fluorescent recognition of α-amino acid derivatives

Bisbinaphthyl-based macrocycles are found to carry out enantioselective fluorescent recognition of α-amino acid derivatives. It is observed that one enantiomer of a N-protected phenyl glycine can increase the fluorescence intensity of the binaphthyl fluorophores by over 4-fold but the other enantiomer does not cause much fluorescence enhancement. This highly enantioselective fluorescent response makes the binaphthyl macrocycles practically useful for the enantioselective fluorescent recognition of the amino acid substrate.     

Evidences of phase separation in moisture‐cured poly(urethane urea)s by means of temperature modulated differential scanning calorimetry

The degree of phase separation in several moisture‐cured poly(urethane urea)s (PUUs) was studied by FTIR spectroscopy, wide angle X‐ray diffraction (WAXD), and temperature‐modulated differential scanning calorimetry (TMDSC). This latter technique was shown to be particularly useful in analysing the degree of phase separation in PUU polymers. Both phase mixing and phase segregation coexisted in the PUUs and the degree of phase separation increased as the urea hard segment (HS) content in the PUU increased. The maximum solubility of urea HSs into the polyol soft segments (SSs) was achieved for 50 wt % urea HS content in diol‐based PUUs, whereas for triol‐based PUUs the highest solubility between HS and SS was reached for lower urea HS amount. Finally, the higher the urea HS content the higher the extent of phase separation in the PUU. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3034–3045, 2007     

Polarization dynamics of intracavity frequency-doubled Nd:YAG laser

Experimental study of low-frequency dynamics of an intracavity frequency-doubled Nd:YAG laser demonstrates the influence of the interaction of orthogonally polarized modes, participating in frequency doubling (type II phase matching), on the stability of the laser output. At a sufficiently low pump rate and low conversion efficiency, the laser shows stable operation with a low noise level at the frequencies of relaxation oscillations. At a high pump power and/or a high conversion efficiency, the laser emission becomes unstable as a result of Hopf bifurcation at the frequencies of relaxation oscillations that are responsible for the anti-phase polarization dynamics of the laser.     

Characterizations of Tb:Zn2SiO4 films on silicon wafer prepared by sol-gel dip-coating and solid-phase reaction

Terbium-doped Zn_2SiO_4 films were successfully prepared on Si wafers by a simple sol-gel dip-coating and solid-phase reaction method of ZnO and SiO_2. X-ray diffraction (XRD) and UV－Vis absorption results revealed that films processed below 850℃ were ZnO in wurzite structure, and films processed above 850℃ were Zn_2SiO_4 in wellimite structure. Photoluminescence measurements of the Tb-doped Zn_2SiO_4 films showed two strong emission bands at 490 and 545nm. The photoluminescence lifetime was 4.6ms.     

Effect of short glass fiber on structure and viscoelastic behavior of olefinic polymers synthesized with metallocene catalyst

Several composites were prepared on the basis of an ethylene homopolymer and different copolymers of ethylene and 1‐hexene, synthesized with a metallocene catalyst, as matrices and a content of a 5 wt % of short glass fiber. The effect of the fiber incorporation on the structure and mechanical and viscoelastic behaviors was analyzed for the different samples. The glass fibers induced a slightly higher crystallinity, and the crystallite morphology significantly changed (long spacings and crystal orientation). The incorporation of fibers did not reinforce the different matrices under study at this low content; consequently, the mechanical parameters, such as Young's modulus, yielding stress, and microhardness, were lower in the composites as compared with those values found in the neat polyolefins. The location and apparent activation energies of distinct relaxation processes are also discussed. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1244–1255, 2003     

A case of concomitant polymorphism and spontaneous resolution: the tetragonal phase of 5‐hydroxymethyl‐7,7,N‐trimethyl‐6‐oxabicyclo[3.2.1]octane‐1‐carboxamide

The title compound, C₁₂H₂₁NO₃, crystallizes in two polymorphic forms, viz. the tetragonal form described here and the monoclinic form described previously [Foces‐Foces, López‐Rodríguez, Pérez, Martín & Pérez‐Hernández (2007). Cryst. Growth Des. 7 , 905–911]. The differences in the conformations of the hydroxymethyl and methylaminocarbonyl substituents have important consequences in the hydrogen‐bond interaction motifs and, therefore, in the packing arrangements. These forms are concomitant polymorphs with melting points differing by 3 K.