Physical crosslinking effects in α,ω-dihydroxy terminated polybutadienes

Structure and molecular dynamics of α,ω-dihydroxy terminated polybutadienes of varying number averaged molecular weight (1320-10?500 g/mol) have been investigated by Fourier-transformed infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, dielectric relaxation spectroscopy (DRS), differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS). DSC and DRS revealed an increase in the glass transition temperature upon decrease of the molecular weight, accompanied by an increasing dynamic fragility m (or steepness index) of the dielectric α-process. This correlation between T_g and m for different molecular weights indicates the presence of a physical network, where H-bonded end-group clusters act as temporary crosslinks. From the dielectric relaxation strength Δ?_α(M_n), the fraction of associated hydroxy groups (f_bond) was estimated showing a peak value for the two but shortest polymers, a behaviour that strongly resembles the molecular weight dependence of the fragility. By considering the quantity f_bond(M_n) in a modified Fox-Flory approach, the measured T_g(M_n) behaviour could be reproduced in a satisfying way. FTIR results support this general picture and show a considerable dependence of the extent of hydrogen bonding and formation of hydroxy groups associates on the molecular weight. Further, WAXS and DSC results disprove the idea of formation of pseudo-crystalline hydrophobic microdomains in these compounds as suggested by other authors.     

A correlation for the estimation of critical micellization concentrations and temperatures of polyols in aqueous solutions

An empirical correlation is presented for the estimation of critical micellization concentrations (CMC) and critical micellization temperatures (CMT) for poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) copolymers in aqueous solutions. The CMC and CMT are expressed as a function of the polyol molecular weight, composition, and temperature (for CMC determination) or concentration (in the case of CMT). The correlation was developed from experimental CMT data for a set of 12 polyols that covered a wide range of molecular weights (2900–14600) and poly(ethylene oxide) contents (30–80 wt%) and is based on a simple expression for the standard free energy of micellization. Such a correlation should be useful to practitioners of the field as it allows easy prediction of CMC and CMT for a wide range of polyols with a minimal number of input parameters.     

Elastic properties of adhesive polymers. II. Polymer films and bond lines by means of nanoindentation

Seven different polymers used frequently as adhesives and/or matrix polymers in wood, wood composites, and natural fiber‐reinforced composites were studied by uniaxial tensile tests and nanoindentation. It was shown that the elastic modulus, the hardness, the creep factor, and the elastic‐, plastic‐, and viscoelastic work of indentation of the seven different polymers is essentially the same regardless whether the polymers were tested in the form of pure films or in situ, i.e., in an adhesive bond line with spruce wood. An excellent correlation was found between the elastic modulus measured by tensile tests and the elastic modulus measured by nanoindentation. In spite of the good correlation, the elastic modulus measured by nanoindentation is significantly higher than the elastic modulus measured by tensile tests. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102:1234–1239, 2006     

Characterization of Pheromone Blend for Grapevine Moth, Lobesia botrana by Using Flight Track Recording

The behavioral responses of Lobesia botrana males to calling females, pheromone gland extracts, and synthetic sex pheromones were recorded in a wind tunnel. Gland extracts and synthetic pheromones were released from a pheromone evaporator. The numbers of males reaching the source and their flight tracks in response to calling females and pheromone gland extracts were compared to those of synthetic blends. Upwind flights to natural sex pheromone were straighter and faster than to a three-component blend of (E)-7,(Z)-9-dodecadienyl acetate (E7,Z9–12:Ac), (E)-7,(Z)-9-dodecadien-1-ol (E7,Z9–12:OH), and (Z)-9-docecenyl acetate (Z9–12:Ac) (100:20:5). The optimum ratio of E7,Z9–12:OH and Z9–12:Ac to E7,Z9–12:Ac was found to be 5% and 1%, respectively. An additional seven compounds identified in the sex pheromone gland were investigated for their biological activity. Two unsaturated acetates, i.e., (E)-9-dodecenyl acetate (E9–12:Ac) and 11-dodecenyl acetate (11–12:Ac), increased the number of males reaching the source as well as straightness, linear velocity, and decreased the track angle of upwind flight. Optimum response was obtained by releasing 10 pg/min E7,Z9–12:Ac in a mixture with 0.5 pg/min E7,Z9–12:OH, 0.1 pg/min Z9–12:Ac, 0.1 pg/min E9– 12:Ac and 1 pg/min 11-12–Ac. The saturated acetates previously identified in the female glands were biologically inactive.     

Fluorescence of poly(di-n-alkylsilane)s in room-temperature solution

A comprehensive study of the electronic absorption and emission spectra and the fluorescence quantum yield and lifetime of seven poly(di-n-alkylsilane)s and of three isotopically labelled poly(di-n-hexylsilane)s in hydrocarbon solution at room temperature is reported. Also reported are fluorescence polarization and carbon tetrachloride quenching of fluorescence of poly(di-n-hexylsilane). The observed fluorescence spectra, quantum yield, and polarization depend on the selected excitation energy in a very characteristic fashion, whereas the fluorescence lifetime does not; however, it depends on the selected emission energy. These characteristic dependencies are qualitatively accounted for by the previously proposed segment distribution model if one assumes that the photophysical behavior at higher excitation energies is strongly affected by the presence of a low-lying weakly allowed state in short-segment chromophores and the behavior at lower excitation energies is dictated by the selective excitation of emitting long-segment chromophores.     

Verification of randomized consensus algorithms under round-rigid adversaries

International Journal on Software Tools for Technology Transfer - Randomized fault-tolerant distributed algorithms pose a number of challenges for automated verification: (i) parameterization in...     

UR: SMART–A tool for analyzing social media content

Information Systems and e-Business Management - The digital transformation, with its ongoing trend towards electronic business, confronts companies with increasingly growing amounts of data which...     

High-voltage surge protection by a varistor-filled air gap

The electric discharge across a varistor granule filled air gap under a fast-rising voltage pulse was investigated for surge protection applications. The effects of temperature and pressure on the arc and the electrical conduction were analyzed by the characteristic changes in voltage waveforms triggered by a fast-rising high voltage pulse. In addition to the gap size, experimental results show that competing mechanisms among arc conduction, conduction through the varistor granule network, thermionic emission from Joule heating at granule-to-granule contact points, and the magnitude of the switching voltage dictate the maximum surge protection voltage for the filled air gap. Experimental evidence indicated that accumulated degradation was created at small contact points between varistor granules by repetitive assaults from longer duration, high voltage pulses. The uniqueness of using varistor over other dielectric granules in an air gap for surge protection is identified and discussed.     

Comparison of 2D simultaneous multi-slice and 3D GRASE readout schemes for pseudo-continuous arterial spin labeling of cerebral perfusion at 3 T

Objective

In this perfusion magnetic resonance imaging study, the performances of different pseudo-continuous arterial spin labeling (PCASL) sequences were compared: two-dimensional (2D) single-shot readout with simultaneous multislice (SMS), 2D single-shot echo-planar imaging (EPI) and multishot three-dimensional (3D) gradient and spin echo (GRASE) sequences combined with a background-suppression (BS) module.

Materials and methods

Whole-brain PCASL images were acquired from seven healthy volunteers. The performance of each protocol was evaluated by extracting regional cerebral blood flow (rCBF) measures using an inline morphometric segmentation prototype. Image data postprocessing and subsequent statistical analyses enabled comparisons at the regional and sub-regional levels.

Results

The main findings were as follows: (i) Mean global CBF obtained across methods was were highly correlated, and these correlations were significantly higher among the same readout sequences. (ii) Temporal signal-to-noise ratio and gray-matter-to-white-matter CBF ratio were found to be equivalent for all 2D variants but lower than those of 3D-GRASE.

Discussion

Our study demonstrates that the accelerated SMS readout can provide increased acquisition efficiency and/or a higher temporal resolution than conventional 2D and 3D readout sequences. Among all of the methods, 3D-GRASE showed the lowest variability in CBF measurements and thus highest robustness against noise.