On the Mechanisms of X-Ray-Induced Excitation of Aromatic Molecules in Polar Solution

Studies of the X-ray- and UV-induced luminescence of tryptophan and tyrosine in solution between 77°K and 300°K are reviewed. The fluorescence and phosphorescence spectra observed during X-irradiation are identical to those induced by UV light, but the phosphorescence to fluorescence ratios obtained with X-rays are much larger than those found with UV light. The experiments seem to show that charge recombination is the dominating mode of the X-ray-induced excitation at low temperatures, giving rise to roughly 85% of the X-ray-induced luminescence at 77°K. The fraction of the X-ray-induced excitation which results from charge recombination decreases with increasing temperature and becomes negligible above 220°K. This causes the yield of X-ray-induced excitation of the radiative levels of tryptophan to decrease from G = 5 excited molecules/100 eV absorbed in solute, at 77°K, to G = 0.4 at room temperature. The mechanisms for quenching of charge recombination are discussed.     

Thermal Aspects of Kumada Rearrangement in Symmetrical and Asymmetrical Polysilanes-Interdependence of Molecular Structure,Thermal Degradation,and Stability

Silicon - Thermal degradation behavior, exothermic patterns, and kinetic parameters of symmetrically/asymmetrically substituted polysilanes during the kumada thermal rearrangement in the...     

Synthesis and fluorescence properties of terbium-polybis(benzylsulfinyl) ethane complexes

A functional polymer polybis (benzylsulfiny) ethane (PBBSE) and Tb³⁺-PBBSE-L (where L is low-molecular-weight organic ligands) ternary complexes were synthesized. The structures of the polymer complexes were investigated by elementary analysis and by comparison of infrared (IR) and fluorescence spectra between the polymer complexes and the corresponding low-molecular-weight complexes. Emission bands of f → f transition of Tb³⁺ in the polymer complexes were characterized. There are 4–7 emission peaks in the fluorescence spectra of the polymer complexes at excitation of ultraviolet light at room temperature. Of particular interest is the strong fluorescence intensity of the polymer ternary complex Tb-PBBSE-Phen. The fluorescence intensity of the complexes was increased in the following order: Tb-PBBSE (27.0) < Tb-PBBSE-TTA (82.3) < Tb-PBBSE-Phen (172.2). The maximum excitation energy and the apparent Stokes shift were increased just as this order in the Tb-PBBSE-low-molecular-weight ligand system complexes. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 1575–1583, 1997     

Excitation Dependent Fluorescence Quantum Yield in Hydrocarbon Fuels Containing Polycyclic Aromatic Compounds

Fluorescence emission spectra and quantum yields have been measured for neat sample of six different hydrocarbon fuels over different possible excitation wavelengths. Hydrocarbon fuels show different excitation fluorescence cutoff depending on their commercial use and applications in the UV ～ visible region. The fluorescence emission maximum of these fuels shows a red shift with excitation wavelength. Resonance energy transfer and self-quenching through solvent collision of fluorophores play an important role on the fluorescence characteristics of multifluorophoric mixture like petroleum fuels at higher concentration. The fluorescence quantum yield of these fuels is a function of the excitation wavelength and is different for different fuels. The change in quantum yield with excitation wavelength for petroleum fuels is different from that of crude oils. The change in fluorescence quantum yield at lower excitation wavelength provides a promising tool to estimate contamination of diesel and petrol by kerosene.     

Temperature dependence of the intensity of fluorescence of poly(n-vinylcarbazole) films

Summary The fluorescence spectra of thin films of poly(N-vinylcarbazole), PVCA, were measured at temperatures between 103 K and 535 K by constant excitation light intensity. Below 300 K, the corrected spectra were divided into bands belonging to different emitting species of PVCA. Temperature changes of the intensity of fluorescence of the two main excitation energy traps in PVCA are discussed using the simplified kinetic model of Johnson. Local anomalies on the curves of intensity of excimer fluorescence versus temperature measured at constant wavelength reflex the - and - relaxation regions in PVCA.     

Bismuth-induced effects on optical,lattice vibrational,and structural properties of bulk GaAsBi alloys

Bulk GaAs_{1 - x}Bi_x/GaAs alloys with various bismuth compositions are studied using power- and temperature-dependent photoluminescence (PL), Raman scattering, and atomic force microscopy (AFM). PL measurements exhibit that the bandgap of the alloy decreases with increasing bismuth composition. Moreover, PL peak energy and PL characteristic are found to be excitation intensity dependent. The PL signal is detectable below 150 K at low excitation intensities, but quenches at higher temperatures. As excitation intensity is increased, PL can be observable at room temperature and PL peak energy blueshifts. The quenching temperature of the PL signal tends to shift to higher temperatures with increasing bismuth composition, giving rise to an increase in Bi-related localization energy of disorders. The composition dependence of the PL is also found to be power dependent, changing from about 63 to 87 meV/Bi% as excitation intensity is increased. In addition, S-shaped temperature dependence at low excitation intensities is observed, a well-known signature of localized levels above valence band. Applying Varshni’s law to the temperature dependence of the PL peak energy, the concentration dependence of Debye temperature (β) and thermal expansion coefficient (α) are determined. AFM observations show that bismuth islands are randomly distributed on the surface and the diameter of the islands tends to increase with increasing bismuth composition. Raman scattering spectra show that incorporation of Bi into GaAs causes a new feature at around 185 cm^-1 with slightly increasing Raman intensity as the Bi concentration increases. A broad feature located between 210 and 250 cm^-1 is also observed and its intensity increases with increasing Bi content. Furthermore, the forbidden transverse optical (TO) mode becomes more pronounced for the samples with higher bismuth composition, which can be attributed to the effect of Bi-induced disorders on crystal symmetry.

PACS

78.55Cr 78.55-m 78.20-e 78.30-j     

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.     

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.     

Effect of polymer–nanoparticle interactions on the glass transition dynamics and the conductivity mechanism in polyurethane titanium dioxide nanocomposites

We report on the glass transition dynamics and the conductivity properties of a nanodielectric system composed of pre-synthesized TiO₂ nanoparticles embedded in thermoplastic polyurethane. Increase of TiO₂ loading results in enhanced segmental mobility of the composites and less steep temperature dependence, i.e., lower fragility index. The decrease in the fragility index and glass transition temperature is discussed based on the FTIR results. We observe different behavior of conductivity for temperatures above and below the glass transition temperature. At high temperatures the composites exhibit conductivity values more than 2 orders of magnitude higher than those in the pristine matrix. At the same time, at sub-T_g temperatures composites are characterized by superior electrical insulation properties compared to pristine matrix material. Such drastic temperature dependence of the conductivity/insulating ability of the flexible and light-weight, low-T_g composite material can be utilized in various applications including sensing and temperature switching materials.     

Effect of viscoelasticity on the spherical and flat adhesion characteristics of photopolymerizable acrylate polymer networks

This research is the first of its kind to study the comparison between spherical and flat probe adhesion behavior as a function of viscoelasticity. Viscoelastic properties were tailored through the use of acrylate networks synthesized from tert-butyl acrylate and poly(ethylene glycol) dimethacrylate (PEGDMA) solutions. The molecular weight and the weight fraction of PEGDMA crosslinker was altered to maintain a constant glass transition temperature of approximately 57 °C, but systematically vary the viscoelastic properties and the rubbery moduli (1–62 MPa). Dynamic mechanical analysis was performed to characterize the low-strain thermo-mechanical behavior of the materials. Viscoelastic behavior of the materials was characterized by creep testing and was observed to inversely correlate with crosslinking density. The samples tested with the spherical probe exhibited low pull-off forces at temperatures well above and well below the glass transition temperature of the material. A maximum in pull-off force was observed in the vicinity of the glass transition temperature owing to the viscoelastic energy dissipative processes. The peak in pull-off force was observed to decrease with an increase in crosslinking density and modulus. Adhesion measurements using the flat probe demonstrated a strong dependence of pull-off force on the modulus of the material above the glass transition temperature. It is concluded that viscoelasticity is a dominating factor in increasing the pull-off force values in the vicinity of the glass transition, while it plays a little or no role for temperatures +/−20 °C away from transition region , opening the possibility of thermally switchable adhesives.     

Fluorescence upconversion properties of a chiral polybinaphthyl induced by two‐photon absorption

The fluorescence properties of a chiral polymer based on optically active polybinaphthyls were studied in tetrahydrofuran solution. One‐photon excited fluorescence of the polymer was located in the range of ～ 596 nm and the corresponding lifetime was ～ 4.38 ns. From the excitation spectra and emission spectra excited at 800 nm, the upconversion fluorescence emission was observed. When excited by using 800 nm fs laser pulses with different input irradiances, the peak fluorescence intensity of the solution provides square dependence with the input irradiance power, giving an evidence for two‐photon excited fluorescence. Furthermore, open aperture Z‐scan measurements were performed at different irradiation intensities to confirm two‐photon absorption property of the solution at 800 nm excitation. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Prepreg aging in relation to tack

Thermoanalytical measurements and tack tests were both performed using a commercially available carbon fiber/epoxy prepreg system (Hercules 3501–6) to examine changes caused by aging as they affect handling and processability of thermosetting matrix-based composites. Combining these techniques, a relationship between prepreg bulk and surface characteristics in relation to aging was investigated. Isothermal kinetic studies at low temperatures showed maximum conversions (α_m) that increased with increasing cure temperatures. In addition, a linear relationship between glass transition temperatures (T_g) and conversions (α) was observed regardless of aging (or cure) temperatures. Energy of separation of prepreg stacks, which may be viewed as a measure of prepreg tack, showed a maximum value at a specific temperature. The maximum energy of separation was observed in the temperature range of 20–25°C above the glass transition temperature for a given sample. However, the maximum energy of separation values decreased with increasing aging times (or conversions), implying that prepreg tack was a viscoelastic property rather than a viscous property of the resin matrix in the prepreg.     

Relaxation time and cooling rate of a liquid in the glass transition range

The transfer of kinetic units (atoms or groups of atoms) in amorphous materials from one quasiequilibrium position to another quasi-equilibrium position is governed by the fluctuations of both the energy and the entropy of the system. In the glass transition range of liquids, the entropy mechanism plays a dominant role: fluctuations of the particle packing appear to be more significant than the accumulation of the energy. At high temperatures above the glass transition range, the energy mechanism plays a decisive role. The physical meaning of the parameter involved in the Bartenev equation relating the relaxation time to the cooling rate at the glass transition temperature is discussed. A technique is proposed for calculating this parameter with allowance made for the temperature dependence of the activation energy in the liquid-glass transition range. A variant of the modification of this equation is considered.     

Optical Absorption in Sol-Gel-Derived Crystalline Barium Titanium Fine Particles

Pure, translucent, and highly crystallized barium titanate (BaTiO₃) monolithic gels were synthesized via a sol-gel technique at temperatures down to 90°C by using a high-concentration solution of barium alkoxide and titanium alkoxide. The gels consisted of fine particles with an average diameter of ~10 nm and showed the X-ray diffraction patterns of a pseudo-cubic BaTiO₃ system at room temperature; however, in bulk crystal, the tetragonal structure was more stable. The optical transmission spectra of the polycrystalline gels were similar to those for single-crystalline BaTiO₃. From the excitation energy dependence of the absorption-edge structures, the optical-gap energies for the gels were estimated to have values that were ~0.1 eV higher than those for the single crystal; these higher optical-gap energies may be attributed to size effects.     

Synthesis, functionalization and bioimaging applications of highly fluorescent carbon nanoparticles

Highly fluorescent crystalline carbon nanoparticles (CNPs) have been synthesized by one step microwave irradiation of sucrose with phosphoric acid at 100 W for 3 min 40 s. This method is very simple, rapid and economical and hence can be used for large scale applications. The average particle sizes are 3 to 10 nm and they emit bright green fluorescence under the irradiation of UV-light. Therefore, the particles can be used as a unique material for bioimaging as well as drug delivery. To further increase the fluorescence property of the synthetic carbon nanoparticles we simply functionalized them by using different organic dyes, such as fluorescein, rhodamine B and α-naphthylamine; the maximum fluorescence intensity was observed for the particles functionalized with fluorescein. It is very interesting to note that all of those compounds show maximum fluorescence intensity at 225 nm excitation wavelength and for any excitation wavelength the peak positions are exactly same the position as that of CNPs itself, which is completely different from the individual precursors (dyes). All of the above compounds, including CNPs, have also been successfully introduced into the erythrocyte enriched fraction of healthy human blood cells with minimum cytotoxicity.     

Molecular motions and segmental size of vulcanized natural and acrylonitrile-butadiene rubbers by the spin-probe method

Nitroxide radicals were dispersed as paramagnetic probes in natural and acrylonitrile-butadiene rubbers, differing in the content of combined sulphur, and their molecular motions were investigated through the motion of the probe inspecting e.s.r. spectra. Broad e.s.r. spectra at low temperatures changed into sharp ones around or above T_g of the rubbers, showing the rapid narrowing of the extreme line separation of triplet spectra. From the activation energy of the rotational motion of the probe, calculated from the temperature dependence of the rotational correlation frequency, it was deduced that the self-diffusion of the probe through polymer matrices and polymer segments had occurred around or above T_g. The temperature narrowing of the line separation was correlated to the glass transition of the rubbers and a relation between the temperature narrowing, T_g and the molar volume ratio of the segment to the probe on the basis of the hole theory, was proposed. From this theory, the size of segments of the rubbers was estimated.     

Thermal Effects on Electron-Phonon Interactions in Silicon Nanostructures

Raman spectroscopy of silicon nanostructures, recorded using an excitation laser power density of 1.0 kW/cm ² , was employed here to reveal the dominance of thermal effects at temperatures higher than room temperature. The room temperature Raman spectrum showed only phonon confinement and Fano effects. Raman spectra recorded at higher temperatures showed an increase in FWHM and a decrease in asymmetry ratio with respect to its room temperature counterpart. Experimental Raman scattering data were analyzed successfully using theoretical Raman line-shapes generated by incorporating the temperature dependence of a phonon dispersion relation. The experimental and theoretical temperature dependent Raman spectra are in good agreement. Although quantum confinement and Fano effects persist, heating effects start dominating at temperatures higher than room temperature.     

Synthesis and thermo-responsive behavior of fluorescent labeled microgel particles based on poly(N-isopropylacrylamide) and its related polymers

Poly(N-isopropylacrylamide) (PNIPAM) microgel particles labeled with 3-(2-propenyl)-9-(4-N,N-dimethylaminophenyl)phenanthrene (VDP) as an intramolecular fluorescent probe were prepared by emulsion polymerization. The thermo-responsive behavior of the VDP-labeled PNIPAM microgel particles dispersed in water was studied by turbidimetric and fluorescence analyses. The transition temperature of the VDP-labeled PNIPAM microgel particles in water determined by turbidimetric analysis was ca. 32.5 °C. The wavelength at the maximum fluorescence intensity of the VDP units linked directly to the microgel particles dramatically blue-shifted around the transition temperature. In addition it gradually blue-shifted even below the transition temperature where there was no change observed in the turbidity. These findings suggest that the gradual shrinking of microgel particles occurs with increasing temperature and the subsequent dramatic shrinking results in the increasing in the turbidity. The transition temperatures of VDP-labeled poly(N-n-propylacrylamide) and poly(N-isopropylmethacrylamide) microgel particles determined by turbidimetric analysis were ca. 23 and ca. 42.5 °C, respectively, and their thermo-responsive behavior was similar to that for the VDP-labeled PNIPAM system. In these three systems the microenvironments around the fluorescent probes above the transition temperatures became more hydrophobic than those below the transition temperature, and the estimated values of microenvionmental polarity around the VDP units on their collapsed states were almost the same.     

The apparent activation energy and dynamic fragility of ancient ambers

According to the literature, different types of materials have different glass transition temperature (T_g) dependences of apparent activation energy (E_g) and dynamic fragility (m). In previous work we found that for different ambers, there were different glass transition temperatures. These same samples provide an opportunity to study the T_g dependence of E_g and m for amber, which has not been reported previously in the literature. In this work, nine pieces of amber from different locations and having different ages were investigated by differential scanning calorimetry. Six cooling rates were used to provide the different thermal histories, and the corresponding limiting fictive temperatures were determined using Moynihan's area matching method. From the cooling rate dependence of the limiting fictive temperature both the apparent activation energy and dynamic fragility were calculated. We find that as glass transition temperature increases, both the apparent activation energy and dynamic fragility increase. The T_g dependence of m for amber shows a similar trend with temperature as do the metallic glass formers and compares favorably with the m–T_g dependence of other aromatic polymers.     

温度对4种典型生物质成型特性的影响

以玉米秸秆、桉木屑、竹屑、硬杂木屑4种典型生物质为原料进行成型实验，研究了温度对其成型特性的影响。在差示扫描量热仪上测定了4种原料的热转变特征温度，显示玻璃态转变特征温度T_g在80~100℃之间。40℃时松弛密度（D_RS）和径向最大抗压力（M_RS）较低，成型比能耗（E_SC）较高；70~100℃时松弛密度和径向最大抗压力增大，成型比能耗降低；100~160℃时松弛密度增加，径向最大抗压力无一致性变化规律，比能耗增大；100℃左右接近4种原料玻璃态转化的终止温度，存在比能耗最低点，说明粒子达到最大软化程度，易于发生形变；原料木质素含量越高成型后颗粒的径向最大抗压力越大，成型时的比能耗越高。成型颗粒内部横断面的微观形貌测试结果显示低于原料玻璃态转变温度时，粒子间存在较大缝隙，结合不紧密；高于原料玻璃态转变温度时，粒子间缝隙较少，结合紧密。