Revealing the Nano‐Level Molecular Packing in Chitosan–NiO Nanocomposite by Using Positron Annihilation Spectroscopy and Small‐Angle X‐ray Scattering

Chitosan–NiO nanocomposite (CNC) is shown to be a potential dielectric material with promising properties. CNCs containing NiO nanoparticles (0.2, 0.6, 1, 2, 5 wt %) are prepared through chemical methods. The inclusion of NiO nanoparticles in the chitosan matrix is confirmed by scanning electron microscopy (SEM) and X‐ray diffraction. The morphology of the NiO nanoparticles and the nanocomposites is investigated by transmission electron microscopy and SEM, respectively. Positron annihilation lifetime spectroscopy (PALS) and the coincidence Doppler broadening (CDB) technique are used to quantify the free volume and molecular packing in the nanocomposites. The triplet‐state positronium lifetime and the corresponding intensity show the changes in nanohole size, density, and size distribution as a function of NiO loading. Small‐angle X‐ray scattering indicates that the NiO aggregates are identical in all the CNCs. The momentum density distribution obtained from CDB measurements excludes the possibility of a contribution of vacant spaces (pores) available in NiO aggregates to the free volume of nanocomposites upon determination by using PALS. The results show systematic variation in free‐volume properties and nano‐level molecular packing as a function of NiO loading, which is presumed to play a vital role in determining the various properties of the nanocomposites.     

XRD,SAXS, and PALS investigations of three different polymers reinforced with tetraoctylammonium exchanged montmorillonite

Three different polymer nanocomposites were prepared using clay modified with tetraoctylammonium (4C8) surfactant. The dispersion of clay silicate layers was studied using X-ray diffraction and small-angle scattering. Free-volume cavity sizes were studied with positron annihilation lifetime spectroscopy. Scattering methods confirmed disruption of all polymer lamellae organization upon organoclay addition and the creation of partially intercalated systems for polyamide and polycaprolactone. The presence of organoclay in the polymers enlarges the lower value of average positronium lifetime τ₃ of polyamide and reduces the higher value of τ₃ for polycaprolactone and polyethylene.     

Positron annihilation spectroscopy in studying the basicity of a series of <Emphasis Type="Italic">n</Emphasis>-alcohols

A series of alcohols is studied by means of positron annihilation lifetime spectroscopy. The fourth lifetime component of the τ₄ positronium is determined.     

NiO的单层分散态及其载体效应

The NiO dispersion phase on three different supports have been studied by EXAFS in combination with XRD quantitative phase analysis. The result of XRD shows the threshold of NiO/γ-Al_2O_3 system is larger than that of NiO/TiO_2 system and the threshold of NiO/SiO_2 system is the least. From the FT functions of EXAFS for samples we can find that in NiO/γ-Al_2O_3 system a part of Ni changes its coordination polyhedra into tetrahedra, the Ni-Ni coordination peaks reduce markedly with the decrease of NiO content in samples. The result of EXAFS fitting and phase isolation shows Ni-Ni coordination number increases with the increase of NiO content in samples of NiO/γ-Al_2O_3 system and reaches a maximum value of 4.0 and remains constant when NiO content reaches the threshold obtained by XRD. It shows the structure of NiO dispersion phase is in better agreement with the NiO monolayer dispersion model than any other models.
In FT functions of EXAFS for samples at thresholds of two systems the Ni-Ni coordination peaks of NiO/TiO_2 system is much lower than those of NiO/γ-Al_2O_3 system. But they are similar when their NiO contents both close to the threshold of NiO/TiO_2 system. It shows that though the dispersion capacity of NiO in NiO/TiO_2 system is less than that in NiO/γ-Al_2O_3 system its dispersion performance is better.
In NiO/SiO_2 system the coordination peaks are similar to those of pure NiO and the Ni-Ni coordination number approaches 12 of the pure NiO. It shows it is difficult for NiO to disperse on the surface of SiO_2. In fact, for the NiO/SiO_2 system the threshold obtained by XRD is beyond the experiment accuracy so there is no threshold for this system.
Therefore above facts indicate that dispersion threshold and reducing of metal-metal coordination number of the dispersion phase is a quite characteristic feature of monolayer dispersion state.     

Investigation of Nanoscopic Free Volume and Interfacial Interaction in an Epoxy Resin/Modified Clay Nanocomposite Using Positron Annihilation Spectroscopy

Epoxy/clay nanocomposites are synthesized using clay modified with the organic modifier N,N‐dimethyl benzyl hydrogenated tallow quaternary ammonium salt (Cloisite 10A). The purpose is to investigate the influence of the clay concentration on the nanostructure, mainly on the free‐volume properties and the interfacial interactions, of the epoxy/clay nanocomposite. Nanocomposites having 1, 3, 5 and 7.5 wt. % clay concentrations are prepared using the solvent‐casting method. The dispersion of clay silicate layers and the morphologies of the fractured surfaces in the nanocomposites are studied using X‐ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The observed XRD patterns reveal an exfoliated clay structure in the nanocomposite with the lowest clay concentration (≤1 wt. %). The ortho‐positronium lifetime (τ₃), a measure of the free‐volume size, as well as the fractional free volume (f_v) are seen to decrease in the nanocomposites as compared to pristine epoxy. The intensity of free positron annihilation (I₂), an index of the epoxy–clay interaction, decreases with the addition of clay (1 wt. %) but increases linearly at higher clay concentrations. Positron age‐momentum correlation measurements are also carried out to elucidate the positron/positronium states in pristine epoxy and in the nanocomposites. The results suggest that in the case of the nanocomposite with the studied lowest clay concentration (1 wt. %), free positrons are primarily localized in the epoxy–clay interfaces, whereas at higher clay concentrations, annihilation takes place from the intercalated clay layers.     

Positron annihilation studies of poly(N‐isopropyl acrylamide) gel in mixed solvents

Positron annihilation lifetime spectroscopy was used to characterize the reentrant volume‐phase‐transition behavior of poly(N‐isopropyl acrylamide) hydrogel in an ethanol/water mixed solvent. The polymer gel was synthesized with γ irradiation. The ortho‐positronium lifetime (τ₃) in the gel slowly increased with an increase in the ethanol content in the mixed solvent. τ₃ was not influenced by the volume phase transition. The ortho‐positronium intensity decreased with the collapse of the gel in an approximately 10% ethanol/water mixture. When swelled in pure ethanol, τ₃ initially increased with the solvent amount in the gel, showing the destruction of intramolecular hydrogen bonding and the relaxation of polymer chains. The lower critical solution temperature of the gel in the 10% ethanol/water mixture was lower than that in pure water, and τ₃ for various solvent contents showed behavior similar to that seen in pure solvent. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1028–1036, 2002     

Three-gamma annihilation of ortho-positronium in NiO/γ-Al2O3 catalysts detected by positron lifetime and coincidence Doppler broadening measurements

The pore structure of NiO/γ-Al₂O₃ catalysts is characterized by positron lifetime and Doppler broadening measurements. A very long lifetime τ₄ of 92 ns is resolved from the positron lifetime spectrum measured for pure Al₂O₃, which could be attributed to the ortho-positronium (o-Ps) lifetime in large pores. It was also found that the fitted lifetime τ₄ and its corresponding intensity I₄ obtained from the lifetime spectra both decrease with narrowing energy window of the stop channel in the fast–fast coincidence lifetime measurement system. This suggests that the ultra long lifetime is primarily due to the self annihilation of o-Ps which emits three gamma-rays. Such 3γ annihilation is further evidenced by measuring the Doppler broadening of annihilation gamma rays in coincidence with the prompt gamma rays (1.28 MeV) emitted from the ²²Na positron source. In NiO/γ-Al₂O₃ catalysts both the lifetime τ₄ and its intensity I₄ decreases with increasing NiO content (from 3 wt% to 40 wt%), which indicates decreasing of the number of 3γ events. The 3γ annihilation parameter analyzed from the coincidence Doppler broadening spectrum shows consistent decrease with increasing NiO content.     

用正电子素探测叶绿素三线激发态浓度的变化

测量了在不同光强下叶绿素环已烷溶液(77 K)和酒精溶液(296 K)的正电子湮没寿命谱。提出正电子素光磁湮没模型以解释实验事实。在77 K固化的叶绿素环已烷溶液中, 正电子湮没寿命谱长寿命组分的湮没参数I_2和Λ_2随光强变化而呈现规律性变化, 表明可利用正电子素光磁猝灭效应来追踪光敏剂分子三线激发态浓度的变化。     

Measuring location, size, distribution, and loading of NiO crystallites in individual SBA-15 pores by electron tomography

By the combination of electron tomography with image segmentation, the properties of 299 NiO crystallites contained in 6 SBA-15 pores were studied. A statistical analysis of the particle size showed that crystallites between 2 and 6 nm were present with a distribution maximum at 3 and 4 nm, for the number-weighted and volume-weighted curves, respectively. Interparticle distances between nearest neighbors were 1-3 nm with very few isolated crystallites. In the examined pores, a local loading twice the applied average of 24 wt % NiO was found. This suggests that a very high local loading combined with a high dispersion is achievable.     

Magnetic quenching effecs on long-lived postronium states in zeosil

Positron annihilatioin data collected for zeosil (a zeoilite-type solid allowing abundant Psformation) are discussed, with particular attention payed to the longest-lived positronium states as observed in lifetime spectroscopy experiments. To unravel the nature of these components, additional information was sought by applying an extermal magnetic field and by reecording the total energy distributioin of the annihilation radiation. From both the lifetime and the extended Doppler spectra, it is shown that positronium presents a very long-lived component with a relative intensity of ca. 30% and a lifetime close to the intrinsic tripler lifetime (140ns), therefore undergoing almost no pick-off annihilation. From the magnetic field experiments, the contact density parameter has a value of essentially 1, which is characteristic of an unpoerturbed triplet Ps state in vacuo. This positronium state therefore does not appear to interact with the medium.     

A study on free‐volume properties of styrene‐acrylic ester type copolymers

The positron annihilation lifetime measurements have been performed on a number of amorphous styrene–methyl acrylate copolymers and styrene–butyl methacrylate copolymers. The densities of copolymers were obtained with immersion method by using a capillary pycnometer and the average molecular weights were determined by gel chromatography. The lifetime τ₃ of ortho‐positronium (o‐Ps) pick‐off annihilation have been found to correlate with side group volume and polarity of macromolecular chains in the copolymers, and relative intensity I₃ is attributed mainly to the electron‐attracting groups trapping the spur electrons and positrons. The experimental results have been discussed on the basis of the structural variation of macromolecular chains. In addition, the PALS measurement as a function of time for polystyrene and several styrene–methyl acrylate copolymers has also been performed. The result shows that an electric field is built in polymers during extended positron annihilation spectroscopy measurement, and the field effect is a main factor which causes the decrease in I₃ with time. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2476–2485, 1999     

Positron annihilation studies on radiation‐crosslinked poly(N‐isopropylacrylamide) hydrogels

The temperature‐sensitive poly(N‐isopropylacrylamide) hydrogels, prepared by γ and electron‐beam (EB) irradiation, were studied using positron annihilation lifetime spectroscopy (PALS). The effect of water content in the hydrogel on the ortho‐positronium (o‐Ps) lifetime and intensity was investigated. The observed positronium lifetime suggests microstructural differences between γ‐ and EB‐synthesized hydrogels. The distribution in positronium lifetime indicates nonhomogeneity in the distribution of free‐volume holes in EB‐synthesized hydrogels. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3462–3466, 2000     

Effect of water on glass transition in starch/sucrose matrices investigated through positron annihilation lifetime spectroscopy: a new approach

Glass transition is studied through positron annihilation lifetime spectroscopy (PALS) in maize starch matrices containing 10 (batch STS10) and 20 (STS20) w/w% sucrose, as a function of temperature (T) and water content (c(w)). To circumvent important losses of water upon heating while recording the PALS spectra, a new method is developed: instead of a series of measurements of τ(3), the triplet positronium lifetime, at different T, the latter is kept constant and the series relates to c(w), which is left to decrease at a constant rate. Similarly to the changes in τ(3) with T, the τ(3)vs. c(w) plots obtained show a smooth linear increase until a break, denoting the occurrence of glass transition, followed by a sharper increase. The gradients appear to be independent of T. The variation of the glass transition temperature, T(g), with c(w) shows a broad sigmoid with a large linear central part; as expected from the plasticising effect of sucrose, the plot for STS20 lies some 10 K below that for STS10. Results from differential scanning calorimetry for STS20 yield T(g) values some 15 K higher than from PALS. On the basis of the general shape of the τ(3)vs. T variations, a general equation is set for τ(3)(T, c(w)), leading one to expect a similar shape for τ(3)vs. c(w), as experimentally observed.     

Enhanced NiO Dispersion on a High Surface Area Pillared Heterostructure Covered by Niobium Leads to Optimal Behaviour in the Oxidative Dehydrogenation of Ethane

A Nb-containing siliceous porous clay heterostructure (PCH) with Nb contents from 0 to 30 wt %) was prepared from a bentonite and used as support in the preparation of supported NiO catalysts with NiO loading from 15 to 80 wt %. Supports and NiO-containing catalysts were characterised by several physicochemical techniques and tested in the oxidative dehydrogenation (ODH) of ethane. The characterisation studies on Nb-containing supports showed the presence of well-anchored Nb⁵⁺ species without the formation of Nb₂O₅ crystals. High dispersion of nickel oxide with low crystallinity was observed for the Nb-containing PCH supports. In addition, when NiO is supported on these Nb-containing porous clays, it is more effective in the ODH of ethane (ethylene selectivity of ca. 90 %) than NiO supported on the corresponding Nb-free siliceous PCH or on Nb₂O₅ (ethylene selectivities of ca. 30 and 60 %, respectively). Factors such as the NiO–Nb⁵⁺ interaction, the NiO particle size and the properties of surface Niⁿ⁺ species were shown to determine the catalytic performance.     

NiO在NaY分子筛上的分散研究

用ＸＲＤ和ＩＲ对ＮｉＯ在ＮａＹ分子筛上的分散进行了研究，结果表明，采用多次浸渍能提高ＮｉＯ的分散量，而焙烧温度的改变对此没有明显的影响，得到ＮｉＯ的最大分散量为２５％，同时发现，ＮｉＯ分散相与ＮａＹ之间存在着较强的相互作用，使分子筛的热稳定性降低；随着ＮｉＯ分散量的增大，ＮａＹ结构遭到破坏程度加大，对ＮｉＯ在ＮａＹ表面存在的状态提出了选择分布的模型。     

低温等离子体处理对NiO/A12O3吸附NOx的促进作用

采用共沉淀法制备了不同NiO负载量的NiO/A12O3催化剂,并用于NOx储存还原技术去除NOx反应中.结果表明,当Ni/Al摩尔比为0.3时,NiO/A12O3样品的NOx吸附量最大;随着Ni含量的增加,NiAl2O4晶相减少.沉淀物在高温煅烧前利用低温等离子体处理后,所得催化剂吸附NOx的能力明显增强,当Ni/Al...     

Positron lifetime diagnostics of β-diketonates of bivalent metals

Positron lifetime diagnostics was used to study manganese(II), cobalt(II), nickel(II), copper(II), and zinc(II) acetylacetonates, γ-chloro and γ-bromo chelate complexes of cobalt and nickel, and similar halo and nitro chelate complexes of copper. The dependences of the annihilation lifetime τ₂ on the metal nature and the stability constant of the metal chelate ring were established. The annihilation lifetime and intensity were found to depend not only on the electronic effect of the γ-substituent in the chelate ring but also on the structure of the complex. A comparison of the dependences of the frequencies of the C-O and M-O stretching vibrations on τ₂ revealed that a positron interacts simultaneously with the γ-carbon atoms and the O atoms of the chelate ring, whose accessibilities are determined by the structures of bis(chelates). The plots of the ionization potentials of the n-orbitals of the bis(chelates) vs. τ₂ were similar to the plots of the quenching rate constant of the benzophenone triplet vs. τ₂ in the presence of the bis(chelates).     

On the measurement of structural relaxation in polymers using positron annihilation lifetime spectroscopy

Positron annihilation lifetime spectroscopy has been identified as an effective means of characterizing the free volume content of amorphous polymers. The lifetime and intensity of the ortho-positronium (o-Ps) pick-off annihilation has been found to correlate with the average size and density of free volume sites, respectively. Recently, PALS has been used to evaluate and monitor the physical aging and structural relaxation of polymers in terms of both initial state and evolution in state with time. However, during extended PALS measurements in insulating materials, an electric field can build up due to positron-electron annihilation and can effectively reduce the probability of positronium formation. In this paper, an observed decrease in intensity associated with the o-Ps annihilation component in the glassy polymers polycarbonate and polystyrene is found to be unrelated to structural relaxation of the materials over the time periods examined as reported earlier by others, and, instead, to be more likely a result of electric charge build-up. © 1993 John Wiley & Sons, Inc.     

三组分(NiO、La2O3、SiO2)负载型加氢催化剂中La2O3的作用

用X射线衍射分析、FT-IR、小角X光散射等方法研究了浸渍法制备的NiO/SiO_2和NiO/(La_2O_3+SiO_2)样品中组分之间的相互作用。La_2O_3在SiO_2上可形成近乎密置单层的分散。在负载了La_2O_3的SiO_2上,NiO的分散程度比在SiO_2上有明显提高,也更容易被还原,而且还原后所得金属镍粒子的平均粒度降低,小粒子所占百分比提高。这些结果表明,NiO/(La_2O_3+SiO_2)比NiO/SiO_2样品具有更高的加氢还原硝基普鲁卡因为胺基普鲁卡因的活性,与Ni/(La_2O_3+SiO_2)比Ni/SiO_2活性高的情形相一致。     

Free volume changes on optical switching in azobenzene‐polymethylmethacrylate blends studied by a pulsed low‐energy positron beam

Polymers including chromophores, which can be switched by light, have been studied extensively during the last years due to a host of potential applications which arise from the marked changes in physical properties on switching. Even though there is clear evidence that the free volume has a significant influence on the isomerization kinetics, the question of free volume changes on switching was only addressed recently. Using a pulsed low‐energy positron beam the ortho‐positronium lifetime τ₃ was taken as a very sensitive free volume probe, and no change in free volume was detected on isomerization in an azobenzene‐polymethylmethacrylate (PMMA) copolymer containing about 8 wt % of the azobenzene moiety. Here, we report for the first time on free volume changes in an azobenzene‐PMMA blend with an azobenzene moiety concentration as high as 40 wt %. Using the same pulsed low‐energy positron beam, small but significant changes of τ₃ were observed between the structurally relaxed dark and the UV‐illuminated states suggesting a decrease in free volume of the order of 10%. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011