Electronic absorption and resonance Raman spectra of large linear carbon clusters isolated in solid argon

Neutral and anionic carbon clusters have been generated via a laser-induced graphite-based plasma and deposited in a solid argon matrix. Anionic clusters were formed from neutral clusters by using crossed electron/carbon cluster beams. Thermal annealing (to 36 K) resulted in the aggregation of the smaller carbon species, leading to the formation of long chain neutral and anionic clusters. Spectroscopic measurements in the ultraviolet, visible, near-infrared and infrared regions revealed a series of bands attributable to a homologous set of odd-numbered C5-C29 neutral clusters and even-numbered C6(-)-C36- anionic clusters. Good agreement is found for the band positions of carbon chains containing odd C15-C21 neutrals and even C6(-)-C22- anions, with species previously identified by Maier and coworkers using mass selection or laser vaporization, followed by neon matrix isolation. Resonance Raman frequencies for the neutral C17, C21 and C23 species are shown to be consistent with the above attributions. Density functional theory calculations agree well with the observed bands. It is found that certain low frequency Raman stretching frequencies decrease in a predictable way with increasing chain length. Comparison of the 0(0)0 absorption transitions of the even C18(-)-C36- anionic clusters with the 'unidentified' infrared (UIR) interstellar emission bands suggests that the electronic emission from specific long chain carbon anions may contribute to the some of the UIR bands.     

Geometric Model Describing the Banded Morphology of Particle Films Formed by Convective Assembly

Nanoparticle films coated on smooth substrates by convective assembly from dilute suspensions in dip‐coating configuration are known to have discrete film morphologies. Specifically, the film morphology is characterized by alternating bands of densely packed particles and bands of bare substrate. Convective assembly is a frontal film‐growth process that occurs at the three‐phase contact line formed by the substrate, the suspension in which it is submersed, and the surrounding air. The bands are parallel to this contact line and can be either monolayered or multilayered. Monolayered bands result whenever the substrate is withdrawn from the suspension at a rate too high for particles to assemble into a continuous film. We report a new insight to the mechanism behind this banding phenomenon, namely, that inter‐band spacing is strongly influenced by the constituent particle size. We therefore propose a geometric model relating the inter‐band spacing to the particle size. By making banded films with systematically varied particle sizes (silica/zeolite, 20 to 500 nm), we are able to quantitatively validate our model. Furthermore, the model correctly predicts that multilayered banded films have higher inter‐band spacings than monolayered banded films comprising the same particles.     

PREPARATION AND PROPERTIES OF EPOXY RESIN MODIFIED BY LIQUID CHLOROPRENE-HYDROXYETHYLMETHACRYLATE COPOLYMER

The epoxy resin modified by liquid chloroprene-hydroxyethylmethacrylate copolymer (CP-HEMA) is a new kind of structural adhesives with good mechanical properties. By changing its mole-cular structure, it may also have such desirable properties as retardation of combustion, enduringweather aging, inert oils and chemicals. A series of the thermosets of the epoxy resin modifiedby CP-HEMA used as a toughener were prepared. The effects of CP-HEMA content, catalyst con-tent and curing temperature on the mechanical properties of modified epoxy resin system were studied.The thermal weight-loss curves were examined. The two-phase morphology was observed and dis-cussed.     

Rubber toughening of plastics: Rubber particle cavitation and its consequences

A newly-developed energy-based model for cavitation in rubber particles provides a quantitative basis for understanding the factors controlling toughness in rubber-modified plastics. Cavitation of the rubber is the initial step in the toughening mechanism, and is shown to accelerate yielding of the matrix, either by the formation of dilatation bands or by multiple crazing. Cavitation is thus a strain-softening effect. However, in some cases the cavitated rubber forms fibrils which become highly stretched as local strains increase, and thus begin to carry a significant fraction of the applied stress. The resulting strain-hardening ensures that yielding is not limited to a small deformation zone at a crack tip, but spreads outwards.     

Rf‐GDOES analysis of composite metal/ceramic electroplated coatings with nano‐ to microceramic particles' size: issues in plasma sputtering of Ni/micro‐SiC coatings

Ni matrix composite coatings reinforced with nano‐ and microceramic particles were analyzed by radio frequency glow discharge optical emission spectrometry (Rf‐GDOES). An interesting phenomenon related to the sputtering and excitation modes of this technique was observed. During plasma sputtering with Rf‐GDOES, the micro‐SiC particles were detached from metal matrix and did not contribute to the analytical signals. The same was not found in composite coatings containing nanoceramic particles. This anomalous behavior was confirmed by atomic force microscopy (AFM) investigation and scanning electron microscope (SEM) observations into Rf‐GDOES craters that showed the presence of residual non‐sputtered microparticles. Various attempts were done in order to minimize this problem, mainly by varying the analysis parameters of the used instrumentation, but without any relevant success. Some suggestions were then proposed for explaining the observed phenomenon, moreover possible solutions (e.g. by using a strong magnetic field or changing plasma gas to be more energetic) are discussed. Copyright © 2011 John Wiley & Sons, Ltd.     

ASSOCIATION OF CHLOROPHYLL WITH AMIDES ON PLASTICIZED POLYETHYLENE PARTICLES–III. UNUSUAL SPECTRA OF CHLOROPHYLL a WITH N-METHYLMYRISTAMIDE*

—The absorption spectrum of chlorophyll a, adsorbed with the amphiphilic amide, N-methylmyristamide. to particles of polyethylene swollen with tetradecane, is unusual in that the red band apparently consists of three main components, which by Gaussian deconvolution are located at 664.5, 678 and 687 nm. The last is very narrow, with a band width of only 5-6 nm at half maximum. At low amide concentration, the 744 nm band of chlorophyll hydrate is also observed. Room temperature fluorescence is weak, and indistinctly resolved into bands. However, on gradual cooling to 80 K, the fluorescence intensifies greatly and becomes resolvable into at least eight bands. The circular dichroism spectrum of the red band region shows optical rotatory strength in two narrow bands at 677 and 686 nm which is enormous, compared to that of monomeric chlorophyll or even the 744 nm hydrate. It is suggested that cyclic oligomer structures, in which adjacent chlorophylls are linked through the amide group of N-methylmyristamide, might be responsible for the spectral phenomena. Unsubstituted myristamide and N.N-dimethylmyristamide do not produce these narrow-banded phenomena with chlorophyll at all.     

Experimental study of the dynamics of front propagation in the Co(OH)2/NH4OH Liesegang system using spectrophotometry

In this paper we study the temporal dynamics of the Co(OH) 2/NH 4OH Liesegang system with redissolution by complex formation with ammonia using UV-vis spectrophotometry with a special setup. The formation of precipitate bands is accompanied with band redissolution at the top, and because of such precipitation-redissoultion dynamics, the bands appear as a propagating wave. The spectrophotometric technique developed in this study allows us to study at the kinetics of formation of the bands and their redissolution in great details. The formation, growth, and dissolution of multiple bands are monitored by the time evolution of the absorbance. It was found that the individual band formation is sudden and takes between 15 min to half an hour to form before the next band appears. The speed of formation of bands was different for different bands and the maxima of these speeds fit a Gaussian curve. The content of cobalt hydroxide in these bands was calculated and is shown to increase to a maximum away from the interface and then decreases. The bands later grow by further precipitation. This growth was demonstrated to be nonlinear in time. On the other hand, the dissolution of bands was shown to take place simultaneously and collectively among the multiple bands under study. The effect of the concentration of Co (+2) ions on the dynamics of band formation and dissolution was studied. A time law for this Liesegang system was also determined. The system was also found to be very sensitive to temperature fluctuations.     

Fracture mechanisms in rigid core-shell particle modified high performance epoxies

The fracture mechanisms of a high performance epoxy system modified with two types of preformed rigid core-shell particles (RCSP) were investigated. The use of the preformed RCSP anables the control of the dispersion of the toughener phase in the epoxy, which, in turn, allows the mechanical properties of the modified epoxy to be optimized. The toughening effect via the RCSP modification is found to be as good as that via the core-shell rubber modification. The moduli andT _g of these RCSP-modified epoxies are virtually unaltered via the RCSP modification, when compared with the neat epoxy resin equivalent. The toughening mechanisms in these toughened systems appear to be predominantly crack deflection, crack bifurcation, and microcracking. Approaches for effective toughening of high performance polymers via rigid polymers are discussed.     

TiO2纳米带表面光伏特性的研究

近年来, 一维结构的纳米带由于其不同于管、线材料的新颖结构以及独特的光电性能而受到广泛关注. 人们通过各种方法合成了氧化锌、硫化镉和氧化锡等纳米带材料[1,2]. 纳米TiO2以其优异的光电性能和高的化学稳定性而被广泛应用于太阳能电池、光催化降解等诸多领域[3], 从而成为研究热点, 最近其纳米带的制备也有报道[4].     

Asymmetric band profile of the Soret band of deoxymyoglobin is caused by electronic and vibronic perturbations of the heme group rather than by a doming deformation

We measured the Soret band of deoxymyoglobin (deoxyMb), myoglobin cyanide (MbCN), and aquo-metmyoglobin (all from horse heart) with absorption and circular dichroism (CD) spectroscopies. A clear non-coincidence was observed between the absorption and CD profiles of deoxyMb and MbCN, with the CD profiles red- and blueshifted with respect to the absorption band position, respectively. On the contrary, the CD and absorption profiles of aquametMb were nearly identical. The observed noncoincidence indicates a splitting of the excited B state due to heme-protein interactions. CD and absorption profiles of deoxyMb and MbCN were self-consistently analyzed by employing a perturbation approach for weak vibronic coupling as well as the relative intensities and depolarization ratios of seven bands in the respective resonance Raman spectra measured with B-band excitation. The respective B(y) component was found to dominate the observed Cotton effect of both myoglobin derivatives. The different signs of the noncoincidences between CD and absorption bands observed for deoxyMb and MbCN are due to different signs of the respective matrix elements of A(1g) electronic interstate coupling, which reflects an imbalance of Gouterman's 50:50 states. The splitting of the B band reflects contributions from electronic and vibronic perturbations of B(1g) symmetry. The results of our analysis suggest that the broad and asymmetric absorption band of deoxyMb results from this band splitting rather than from its dependence on heme doming. Thus, we are able to explain recent findings that the temperature dependences of CO rebinding to myoglobin and the Soret band profile are uncorrelated[Ormos et al., Proc. Natl. Acad. Sci U.S.A. 95, 6762 (1998)].     

X-ray and UV photoelectron spectroscopy of Ag nanoclusters

The main purpose of the present work is to analyze a series of Ag nanoparticles (NPs) with different size or ligand functionalization by using X-ray photoelectron spectroscopy (XPS) and to identify the differences in the band-shape and energy peak position of photoemission spectra due to the particle dimension. A transmission electron microscopy characterization was performed, to verify the consistency of the results. Three types of samples were prepared starting from AgNO₃ water solution and adding different capping agents. In the first two cases, the formation of NPs was promoted by the reduction of silver ions Ag⁺¹ to metallic Ag⁰ through the addition of sodium borohydride, whereas in the last case, it was triggered by the exposure to UV light. Depending on the size of the NPs, a different physical behavior can be recognized. NPs with diameter of about 5 nm are characterized by the phenomenon of localized surface plasmon resonance (LSPR). The other type of samples having a diameter of about 1.5 nm presents discrete energy levels instead of electronic bands, and in this case, a typical fluorescence phenomenon can be observed. In the latter case, we can refer to such systems as nanoclusters. The XPS analyses were focused on the Ag 3D spectra looking for the possible shifts of the Ag doublet as a function of the particles size. The ultraviolet photoelectron spectroscopy with He II source was used for the investigation of possible changes in the valence band.     

层层组装法制备核壳SiO_2/Pt粒子及其CO电氧化和原位电化学FTIR光谱

采用层层组装法合成了核壳SiO2/Pt粒子,用电化学循环伏安法(CV)和原位电化学傅里叶变换红外(FTIR)光谱研究了SiO2/Pt粒子对CO分子的氧化和吸附行为.透射电子显微研究表明:包覆SiO2的Pt壳是由团聚的Pt纳米微粒构成,其平均厚度大约为26nm.CO在SiO2/Pt粒子修饰的玻碳(GC)电极上的主氧化峰为0.49V(vsSCE),表现出比本体Pt金属好的催化性能.电化学FTIR光谱研究发现:线性CO在SiO2/Pt粒子上的IR吸收带的方向发生倒反,而且在不同的研究电位下,每个吸收带劈裂为两个间隔约为14cm-1的吸收带,这种劈裂现象在饱和吸附CO的Pt金属表面上是很难观察到的.这些异常的红外吸收现象可能是由SiO2/Pt粒子的结构效应导致的.     

Preparation and photoelectrochemical characterization of nano-particulate TiO2 and polyaniline composite films on Au

TiO₂ and polyaniline(PANI) composite film was obtained by electrochemical methods^[1] and investigated for solar energy conversion application. A strong rectifying effect was found in the cyclic voltammogram of the film in dark when the scan speed was slow enough (5 mV/s) and a quasi-reversible voltammogram of the film was observed when the scan speed was fast (100 mV/s). The results of both the cyclic voltammograms and the SEM images of the composite film show that the PANI film is almost completely covered with nano-particulate TiO₂. Two anodic photocurrent bands and a cathodic band was observed in the spectra of the photocurrent of the TiO₂/PANI/PATP(p-aminothiophenol)/Au film in 0.05 mol/L Fe(CN)₆^3-/Fe(CN)₆^4- solution. The band at 300~400 nm (3.1~4.1 eV) can be ascribed to the photocurrent band of TiO₂ particles and the bands at 450~730 nm (1.7~2.8 eV) can be ascribed to those of the PANI. The partially-oxidized PANI has internal photoemission function and the insulating matrix of PANI is verified to be reduced PANI with a 3.33 eV bandgap energy. The flat-band potentials of partially-oxidized PANI and TiO₂/PANI composite film in 1.0 mol/LHClO₄ solution are determined as 0.87 V and 0.09(SHE) from Mott-Schottky plots, respectively.     

Microphase separation in polybenzoxazine thermosets containing benzoxazine-terminated poly(ethylene oxide) telechelics

Benzoxazine-terminated poly(ethylene oxide) telechelics (Ba-terminated PEO) was synthesized and incorporated into polybenzoxazine to obtain the nanostructured thermosets. The morphology of the thermosets was investigated by means of atomic force microscopy (AFM), small angle X-ray scattering (SAXS) and dynamic mechanical analysis (DMA). The formation of the nanophase structures in the thermosetting composites was addressed on the basis of the mechanism of reaction-induced microphase separation (RIMPS), which was in marked contrast to the case of the binary thermosetting blends of polybenzoxazine with hydroxyl-terminated poly(ethylene oxide). The occurrence of RIMPS resulted from the copolymerization reaction of the end groups of Ba-terminated PEO telechelics with the precursor of thermosetting matrix (i.e., benzoxazine), which suppressed the occurrence of the macroscopic phase separation. It was found that the formation of the nanostructures has a significant effect on the melting behavior of PEO in the thermosets, thermal transition properties of the PBZ thermosets.     

ELASTOMERIC REACTIVE MICROGELS AS TOUGHENER FOR EPOXY ADHESIVES

Elastomerie reactive microgels were evaluated as toughener for epoxy adhesives. It turned out that elastomeric microgels can improve lap shear strength and peel strength greatly. The glass transition temperature of the resin matrix was not affected by the presence of dispersed mierogel particles, and thus the microgel-toughened adhesives have a good bond strength at elevated temperatures.     

Processing,microstructure and properties of bulk metallic glass composites

A Zr-Nb-Cu-Ni-Al bulk metallic glass was reinforced with up to 80 volume-percent (% V_f) continuous fibers, short fibers or particles. Characterization based on X-ray diffraction, differential scanning calorimetry, electron microprobe and scanning electron microscopy is presented. The metallic glass matrix remains amorphous after adding reinforcements. Reactions at the matrix/reinforcement interfaces were examined using transmission electron microscopy. A narrow band of crystalline particles typically forms adjacent to the reinforcement. The composites were tested in compression. Compressive strain-to-failure increased by up to factor of 12 compared to the unreinforced bulk metallic glass. The increase in compressive strain-to-failure is due to the particles restricting shear band propagation, promoting the generation of multiple shear bands and additional fracture surface area.     

Band splitting in overloaded isocratic elution chromatography I. The experimental evidence

A series of samples of increasing volume (from 0.001 to 4.0 cm3), containing the same constant concentration (40 g/l) of two simple compounds, ethylbenzoate and 4-tert.-butylphenol were injected on a Kromasil-C18 column with methanol-water (62:38. v/v) as the mobile phase. Complex band profiles were observed when the volume of the sample became large enough and strong band interference took place. The analysis of the fractions collected during the elution of the mixed band demonstrates that, for samples larger than 2 cm3, the band of 4-tert.-butylphenol is split into two separate bands, one eluted before and the other eluted after the band of ethylbenzoate. Such a phenomenon has never been observed yet in RPLC, under isocratic elution conditions.     

Flow electrification in nonaqueous colloidal suspensions, studied with video microscopy

Flow electrification in nonaqueous suspensions has been scarcely reported in the literature but can significantly affect colloidal stability and (phase) behavior, perhaps even without being recognized. We have observed it in shear flow experiments on concentrated binary suspensions of hydrophobized silica particles in chloroform. In this low-polarity solvent, electrical charges on the large-particles' surfaces manifest themselves via long-ranged forces, because hardly any screening can take place through counterions. By shearing the suspension for a prolonged time, we could demonstrate that the effective interactions between the large particles change from weakly attractive (due to the small particles) to strongly repulsive (due to acquired Coulomb interactions). One of the conditions required for flow electrification was the presence of a glass surface in the shear cell. A spectacular manifestation of the phenomenon was observed with confocal video microscopy. First, the formation of large-particle aggregates was seen, while subsequently (over a long shearing time) the aggregates disintegrated into small entities, mostly primary particles. The spatial distribution of these entities in the quiescent state after stopping the flow showed evidence for acquired long-range repulsion. The occurrence of flow electrification was further corroborated by control experiments, where no flow was imposed, antistatic agent was added, or the glass bottom was coated with a conducting (indium tin oxide, ITO) layer: here, the aggregates kept growing until they became very large. To further diagnose the phenomenon, we have also done experiments in which an external electric field was applied (via the ITO layer) to an aggregated suspension. When the lower electrode was given the lowest potential, the aggregates were found to move away from the bottom and disintegrate. The qualitative similarity hereof with the flow electrification experiment suggests that in the latter, the glass acquired negative charges. After prolonged application of an external electric field, we observed segregation into regions enriched in large particles and regions completely depleted of them. In the quiescent fluid these regions exist as isolated units, but in shear flow they merge into bands, a behavior which resembles shear banding.     

Anisotropic thermoset networks based on ring-forming monomers (cyanate esters and isocyanates)

Cyanate esters and isostructural isocyanates have been synthesized and investigated in terms of their thermal and phase behaviour. Model cyclotrimers of both were synthesized to study the influence of the triazine rings on the mesophase formation. Liquid crystalline properties (calamitic mesophases) were found only for cyanurates. Polycyclotrimerization of 4-cyanatophenyl 4-cyanatobenzoate starts in an isotropic phase with a transition to a mesophase after reaching the gel point. The gel point observed by rheological measurements is temperature-dependent and probably caused by association of the reactive oligomers as no gel particles are found below the chemical gel point. The gel point of the commercial 2,2-bis(4-cyanatophenyl)propane was found at 50 % conversion by the same method. Due to the higher rigidity of the mesogenic cyanate the final conversion is below 100 %, as opposed to the conventional material. Under appropriate conditions isotropic networks can be obtained from the mesogenic monomer, which sets the basis for a quantitative investigation of the influence of liquid crystalline order on physical properties of the thermosets.     

Early barriers in the matrix photochemical formation of syn-anti randomized FC(O)SeF from the OCSe:F(2) complex

The photochemically induced reaction of OCSe and F(2), isolated together in an Ar matrix at about 15 K, leads to formation of the hitherto-unknown fluorocarbonylselenyl fluoride FC(O)SeF. The reaction occurs via a van der Waals complex O=C=Se...F-F that favors very early formation of the anti conformer. The presence and subsequent decay of a band assigned to the F-F vibration correlated with perturbed OCSe bands seems to confirm this hypothesis. Subsequent irradiation of the matrix leads to randomized FC(O)SeF by a photochemically induced conformational equilibrium between syn and anti forms. Another photochemical reaction channel is the formation of CO and SeF(2) molecules, which are produced in the same matrix cage and then form a loose complex. The changes were monitored and the products characterized experimentally by their IR spectra, and the spectra analyzed in the light of the results of theoretical calculations.