Slowing-down of positronium: analysis of the age–momentum correlation

The problem of deconvoluting measurements of the slowing-down of positronium by means of the age–momentum correlation technique based on the Doppler broadening of the annihilation-photon line is reduced to the solution of systems of linear equations. The solution gives directly the kinetic energy of positronium as function of the positronium age (=time passed since its formation). A novel time-resolution function consisting of one Gaussian times an arbitrary number of Hermite polynomials is proposed.     

Influence of deformation and chemical structure on elementary free volumes in glassy polymers

In the scope of the mechanism of positronium formation in polymers before trapping by elementary free volumes (EFV), we demonstrate an approach to quantitative analyses of experimental results. Correspondingly, a relation between the EFV concentration and annihilation characteristics (lifetimes and intensities) is derived. We use PATFIT and CONTIN analyses of positron annihilation lifetime spectra and obtain information on some heterogeneity of the structure of glassy polymers and on the mechanism of the plastic flow of glassy material.     

Positron age-momentum correlation studies of free volumes in toughened cyanate resin networks

Positron age-momentum correlation (AMOC) spectroscopy is applied to study the associations between the momentum distributions and positron annihilation in the toughened CE networks. The Doppler profile of the positron annihilation process is much broader for the toughened CE than pure CE network. This is due to positron trapping on high polar groups of the added elastomers or nano-CaCO₃ and annihilation with the electrons of high momenta. The broadening momentum distribution of ortho-positronium annihilation process indicates that the pick-off annihilation process is strongly influenced by the added high polar elastomers or nano-CaCO₃. The potential of AMOC spectroscopy for free volume analysis in polymer blends was demonstrated.     

Change in free volume of polyvinylidene fluoride

Measurement of positron lifetime was carried out on polyvinylidene fluoride as a function of temperature ranging from 193 to 293 K. The local molecular motion, the cooperative motion of long chain, and the thermal expansion are discussed based on the change in free volume concluded with the use of positron lifetime. The different temperature dependence of motions is suggested for an amorphous area constrained by crystalline regions and a free amorphous area.     

Positronium trapping in free volume of polymers

Positronium trapping process caused by slow thermalization of Ps inside intermolecular spaces of polymer is considered. The pickoff annihilation rate of Ps decreases from an initial value, characteristic for the delocalized Ps, to a final value, relating to its localized state. The elastic thermalization lifetime analysis of polymers spectra with a new ETLT program is proposed to determine the total Ps intensity (I_Ps), Ps trapping rate (γ), its initial (τ₀) and final (τ_∞) pickoff lifetimes as well as the relative e⁺–e⁻ contact density (η). The model is tested through the analysis of polyurethane spectra measured in a wide range of temperature, containing the glass transition point (T_g). In contrast to the conventional analysis, I_Ps does not display any change at T_g. The results indicated slow trapping rate (about 1 ns⁻¹) of Ps in the cavities.     

正电子湮没技术（PAT）在高分子材料中的应用（下）

正电子湮没技术（ＰＡＴ）在高分子材料中的应用（下）李桂芝，施良和，叶美玲（中国科学院化学研究所高分子物理开放实验室，北京，１０００８０）３．４高分子导体高分子导体有两种：（１）靠共轭分子链导电的高分子；（２）因掺入小分子而具有导电作用的高分子。现在已...     

Positron annihilation and Xe NMR studies of free volume in polymers

The existence and the average size of free volume in bisphenol-A polycarbonate (PC), low-density polyethylene (LDPE), poly (2,6-dimethyl-phenyleneoxide)(PPO), and polytetrafluoroethylene (PTFE) were studied by positron annihilation and ¹²⁹Xe NMR measurements. The ¹²⁹Xe NMR chemical shifts for xenon adsorbed in the polymers indicated that the average pore size of the free volume increased in the following order: PC, LDPE, PPO, and PTFE. This order of the pore size of the free volume agrees well with that estimated from the longest lifetime (τ₃) of ortho-positronium formed in the polymers. The unique correlation that δ⁻¹∝ r is established between the ¹²⁹Xe NMR chemical shift (δ) and the pore size (r), which is deduced from the positron annihilation measurements.     

Liquid–liquid transition in polymers and glass-forming liquids

It is shown that many simple glass-forming liquids exhibit a phenomenon known in the area of polymer science as the liquid–liquid transition. The phenomenon manifests itself as a third-order transition in the equilibrium liquid-specific heat data around approximately 1.2 T_g and also as a bifurcation of the liquid relaxation into primary and secondry processes. It is stressed that the above phenomenon is due to a smooth changeover of the liquid from one dynamic regime to the other and hence is not due to any real phase transition. It is suggested that a liquid cluster kind of picture for the supercooled liquid regime, is capable of explaining the above phenomenon and is consistent with observation made on polymers and monomeric liquids. © 1993 John Wiley & Sons, Inc.     

Effect of interfacial interaction on free volumes in phenol-formaldehyde resin-carbon nanotube composites: positron annihilation lifetime and age momentum correlation studies

The phenol-formaldehyde-carbon nanotube composites were characterized for their free volume properties and interfacial interactions between nanotubes and the polymer matrix. The base polymeric material was a novolac type phenol-formaldehyde (PF) condensation resin cross-linked with para-toluene sulfonic acid. Multi-wall carbon nanotubes (MWCNTs) were synthesized using a catalytical chemical vapor deposition method and characterized using high-resolution transmission electron microscopy. The PF resin-carbon nanotubes composites having 2, 5, 10 and 20% (w/w%) MWCNTs were prepared. The crystallinity and morphology of the samples were characterized using X-ray diffraction and scanning electron microscopy. The free volume size in the polymer nanocomposites was observed to increase with the increase in nanotube content. Positron age momentum correlation (AMOC) studies revealed the electronic environment around different positron annihilation sites. The studies showed that ortho-positronium principally annihilates from interfacial regions of polymer and nanotubes in the nanocomposite. The positron lifetime studies together with AMOC measurements indicate an increase in the free volumes at the interface of polymer and MWCNTs in the composite. The free positron intensities showed that the polymer and nanotubes are weakly interacting in this system.     

Small band gap conjugated polymers based on thiophene–thienopyrazine copolymers

In this study, five small band gap thiophene ( TH )–thienopyrazine ( TP ) conjugated copolymers were synthesized by Stille‐coupling reaction. The polymer structures consisted of one to four thiophene rings with the TP of different side groups provided a systematical investigation on the structure–electronic property relationship. The absorption maxima of the polymer films decreased from 850 to 590 nm as the thiophene moieties increased from thiophene to quaterthiophene. The optical and electrochemical band gaps of the studied poly[2,3‐didodecyl‐5‐(thiophen‐2‐yl)thieno[3,4‐b]pyrazine] ( PTHTP‐C12 ) were 0.97 and 0.78 eV, respectively, indicating a significant intramolecular charge transfer. The theoretical geometry and electronic properties of the TH ‐ TP copolymers by the density functional theory at the B3LYP level and 6‐31G(d) basis set suggested that the bond length alternation enlarged with enhancing the thiophene content and resulted in the variation on the polymer band gap. The relatively small theoretical effective mass of poly( TH ‐alt‐ TP ) also indicated its potential applications for field transistor applications. Our study demonstrates the tunable electronic properties of small band gap copolymers by the thiophene content and the resulted geometry variation. Such polymers could be potentially used for near‐infrared electronic and optoelectronic devices. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5872–5883, 2007     

Thiol–norbornene materials: Approaches to develop high Tg thiol–ene polymers

The ability to prepare high T_g low shrinkage thiol–ene materials is attractive for applications such as coatings and dental restoratives. However, thiol and nonacrylated vinyl materials typically consist of a flexible backbone, limiting the utility of these polymers. Hence, it is of importance to synthesize and investigate thiol and vinyl materials of varying backbone chemistry and stiffness. Here, we investigate the effect of backbone chemistry and functionality of norbornene resins on polymerization kinetics and glass transition temperature (T_g) for several thiol–norbornene materials. Results indicate that T_gs as high as 94 °C are achievable in thiol–norbornene resins of appropriately controlled chemistry. Furthermore, both the backbone chemistry and the norbornene moiety are important factors in the development of high T_g materials. In particular, as much as a 70 °C increase in T_g was observed in a norbornene–thiol specimen when compared with a sample prepared using allyl ether monomer of analogous backbone chemistry. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5686–5696, 2007     

Spectroscopical UV–Vis implications of an intramolecular η2–Mg coordination in bacteriochlorophyll–a from the Fenna–Matthews–Olson complex

Chlorophylls and their related compounds prominently feature a Mg²⁺ ion in the center of a porphyrine, with an intermolecular fifth coordination usually observed to place the ion out of the macrocyclic plane. Herein, we assess the role of a potential intramolecular η²–(C = C)Mg interaction and compare it to the intermolecular coordination from the Hystidine groupt to Mg²⁺ for Bacterichlorophyll–a (Bchl–a), the main photosynthetic pigment in the Fenna–Matthews–Olson complex present in green and purple bacteria. The influence of this fifth coordination on the UV‐Vis spectroscopy (CAM‐B3LYP/cc‐pVDZ), and the concomitant change in geometry around Mg in Bchl–a from planar to pyramidal is assessed by the quantum theory of atoms in molecules based non–covalent interactions scheme and through energetic analysis via natural bond orbital population methods at the M06‐2X/cc‐pVDZ and compared to the reference multi–hapto compound, magnesocene, Cp₂Mg.     

Temperature dependence of free volume in atactic polypropylene

Positron annihilation lifetime spectroscopy data are often used to get information on the typical sizes of sub nanometric holes forming the free volume in polymers. To this purpose, the cavities are modelled as spheres or, more generally, using geometries which assume an isotropic expansion with the temperature. However, this guess could be unrealistic owing to the irregular shape of holes and the constrained movements of the macromolecules.In this work positron and dilatometric data are used to estimate the free volume fraction in atactic polypropylene. Comparison with the prediction of the Simha–Somcynsky theory supplies information on the thermal dependence of the volume of holes, whose behaviour can be interpreted in terms of anisotropic expansion.     

Petrographic/geochemical studies of primary and alteration–weathering minerals in garnetiferous ultramafic xenoliths–basanite, Tartous Province, NW Syria

Kimberlitic–pyropic peridotite–xenolites, probably of Jurassic–Cretaceous age, were found mixed with a younger Upper Tertiary basanitic diabase, as flow texture of plagioclase laths and ilmenite rods around those xenoliths indicated. Mafic–ultramafic rocks were crushed, sheared, and cropped along a creek about 15–18 km NE of the town of Dreikeesh, NW Syria. ⁴⁰K–⁴⁰Ar isotopic dating of a pure fresh black cpx sample, collected from the peridotite xenoliths, yielded an age of about 70 Ma. This age is concurrent to the time when Africa, Eurasia, and America were part of the super continent Pangaea. It also suggests that kimberlite–pyropic peridotitic rocks were located within a cratonic pipe prior to their 2000-km eastward journey (starting from the Mid-Atlantic Rift). Sampled outcrops were located within a ring of about 40 km diameter, considered to be a dome consisting of one or more clusters of kimberlitic pipes. The dome structure, mostly covered with Upper Jurassic–Cretaceous carbonate beds, was tilted westward, and rifted and sheared along its eastern edge. Tilting and crushing were accomplished after the opening of the Red Sea in the Miocene, and the counterclockwise movements of the Arabian plate, which folded the carbonate beds to form the N–S-trending Alawite mountain range along the Syrian coast. Olivine, cpx, and pyrope were the major phases in peridotite. Pyrope, including its Cr–Ni-contents, was found to be the best indicator to decipher the temperature–pressure (T–P) conditions for the system. The inferred temperature was found to be about 1460 °C, at a pressure of 62 kb (around 207 km-depth). Petrographic studies revealed many zoned, resorbed, octahedral and non-metasomatized tiny grains, associated with partly to wholly metasomatized and iddingsitized olivines.     

Formulation of unrestricted and restricted Hartree–Fock–Bogoliubov equations

The Hartree–Fock–Bogoliubov (HFB) method, dealing with Bogoliubov orbitals, which consist of particle and hole part, can provide states with pair correlations associated with Cooper pairs. The dimension of HFB Fock matrices can be reduced by restrictions of spin states of Bogoliubov orbitals similarly to ordinary Hartree–Fock (HF) equations such as restricted HF (RHF), unrestricted HF (UHF), and generalized HF (GHF). However, there are few studies of moderate restricted HFB equations such as UHF‐based HFB equations. In this article, formulation and calculations of restricted HFB equations are described. The solutions of general and restricted HFB equations are compared. Pair correlations taking account of restricted and general HFB equations are discussed. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Analysis of thermal degradation process of Nafion‐117 with age‐momentum correlation method

The thermal degradation of Nafion membrane was analyzed with age‐momentum correlation (AMOC) measurement and four‐electrode AC impedance measurement. In the heated sample, the decrease in proton conductivity was observed. The lifetimes and corresponding relative intensities showed fairly good agreement between heated and nonheated samples within the experimental error. In the analysis of the photopeak of annihilation γ‐ray, on the other hand, the difference between those two kinds of samples was observed, and this difference was found to be caused by the annihilation of free positrons with low energy electrons by using AMOC method. The decrease in proton conductivity was caused by the low energy electrons, namely sulfonic radicals. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1–7, 2008     

Effect of nitrogen on mechanical,oxidation and structural behaviour of Ti–Si–B–C–N nanocomposite hard coatings deposited by DC sputtering

Ti–Si–B–C–N film was deposited by DC magnetron sputtering at different argon and nitrogen ratios such as N₂/Ar = 1 : 5, 2 : 4, 3 : 3, 4 : 1 and 5 : 0. The formation of TiN and TiB phases was observed because of incorporation of nitrogen. The hardness, modulus, microstructure, structure and bond formation with different nitrogen contents during the deposition were studied by nanoindentation, scanning electron microscope, X‐ray diffraction and X‐ray photoelectron spectroscopy, respectively. The oxidation kinetics of Ti–Si–B–C–N was investigated. The nitrogen incorporation during deposition influences different properties of the coating. Hardness and modulus decreased, and microstructure showed very fine grain presence, and film changes to fully amorphous because of incorporation of nitrogen in the film. Copyright © 2016 John Wiley & Sons, Ltd.     

Supramolecular hydrogen-bonded liquid–crystalline polymer complexes. Design of side-chain polymers and a host–guest system by noncovalent interaction

Supramolecular liquid–crystalline polymeric complexes based on a backbone that contains vinyl pyridine units and azobenzene or biphenyl derivatives that posses alkyl chains terminated by carboxylic acid have been obtained by the formation of intermolecular hydrogen bonds between the carboxylic acid and the pyridyl moieties. The polymeric complexes behave as side-chain liquid–crystalline polymers and exhibit smectic phases. A new type of H-bonded host-guest liquid–crystalline system is also reported. The liquid–crystalline host copolymers contain both mesogenic acrylate and 4-vinylpyridine units. The guest molecule is an azobenzene that has a carboxylic acid moiety at one of its extremities. The H-bonded polymeric host–guest complexes exhibit nematic phases. Sequential UV and visible light irradiation of the polymeric complex causes reversible photochemically induced phase transitions. The isothermal nematic–isotropic and isotropic–nematic transitions result from the trans-cis and cis-trans photoisomerization of the guest azobenzene in the host–guest system. © 1996 John Wiley & Sons, Inc.     

Study of pancreatic lipase inhibition kinetics and LC–QTOF–MS‐based identification of bioactive constituents of Momordica charantia fruits

The different parts of Momordica charantia have been reported to have several therapeutic applications against hyperglycemia and hypercholesterolemia associated with pancreatic lipase (PL). Inhibition of this enzyme prevents the absorption of dietary triglyceride in the intestine, and thus exerts an anti‐obesity effect. This study aimed to investigate the bioactive constituents of the fruits of M. charantia (MCF) extract and fractions against pancreatic PL followed by study of their inhibition kinetics. The PL inhibitory assay was performed spectrophotometrically by measuring the change in absorbance of the products at 405 nm, using p‐nitrophenylcaprylate as substrate. The results indicated that the ethyl acetate fraction of MCF (EFMC) offered significant, dose‐dependent inhibition against PL, compared with the positive control, Orlistat. The enzyme kinetics study revealed the inhibition to be a mixed type in nature. Additionally, the total phenol and flavonoid content of the fractions was estimated. A positive correlation between phenolic content of EFMC and its PL inhibitory activity was established statistically, which implied that higher inhibition potential was contributed by the phenolic compounds. The identification of the bioactive constituents was further confirmed by LC–QTOF–MS study. This finding suggested that phenolic compounds of MCF can serve as functional food components to address obesity‐related disorders linked with PL.     

Jacket‐like structure of donor–acceptor chromophores‐based conducting polymers for photovoltaic cell applications

Through the Stille coupling polymerization, a series of soluble acceptor/donor quinoxaline/thiophene alternating conducting polymers with a hole‐transporting moiety of carbazole as a side chain ( PCPQT ) has been designed, synthesized, and investigated. The UV–vis measurement of the charge‐transferred type PCPQT s of different molecular weights with low polydispersity exhibits a red shifting of their absorption maximum from 530 to 630 nm with increasing chain length (M_n: from 1100 to 19,200). The HOMO and LUMO energy levels of PCPQT can be determined from the cyclic voltammetry measurement to be ?5.36 and ?3.59 eV, respectively. Solar cells made from PCPQT/PCBM bulk heterojunction show a high open‐circuit voltage, V_oc of ～0.75 V, which is significantly higher than that of a solar cell made from conventional poly(3‐hexyl thiophene)/ PCBM as the active polymer PCPQT has lower HOMO level. Further improvements are anticipated through a rational design of the new low band‐gap and the structurally two‐dimensional donor–acceptor conducting polymers. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1607–1616, 2010