Observation of single chain motion in a polymer melt

Molecular motion of single polymer chains has been investigated in a melt sample of polytetrahydrofuran and compared with results for the same polymer in dilute solution. Using a high resolution neutron scattering technique motion over distances up to 30 Å has been observed with an energy resolution of 0.01 μeV (～10⁷s^?1). The motion of the chains in the melt is described by the Rouse model and a friction factor per segment has been extracted from the data. This compares well with values obtained from viscosity and bulk relaxation measurements on similar polymers.     

Deuteron n.m.r. in relation to the glass transition in polymers

²H n.m.r. is introduced as a tool for investigating slow molecular motion in the glass transition region of amorphous polymers. In particular, we compare ²H spin alignment echo spectra of chain deuterated polystyrene with models for restricted rotational Brownian motion. Molecular motion in the polystyrene-toluene system has been investigated by analysing ²H n.m.r. of partially deuterated polystyrene and toluene, respectively. The diluent mobility in the mixed glass has been decomposed into ‘solid’ and ‘liquid’ components where the respective average correlation times differ by more than 5 decades.     

Oxidative dehydrogenation of ethane on a magnesium-vanadium aluminophosphate (MgVAPO-5) catalyst

Vanadium and/or magnesium substituted aluminophosphate with ALPO₄-5 structure have been prepared by hydrothermal synthesis. These catalysts have been tested for the oxidative dehydrogenation (ODH) of ethane. ALPO₄-5 has a low activity and low selectivity for the ODH of ethane. The presence of Mg²⁺ ions in MgAPO-5 increases the selectivity to ethene, while the presence of V⁵⁺ species in VAPO-5 increases both the activity and the selectivity for this reaction. The presence of Mg²⁺ and V⁵⁺ species in the vanadium-magnesium alumino-phosphate (MgVAPO-5) results in a more selective catalysts for the ODH of ethane. The behavior of MgVAPO-5 could be attributed to the presence of acid sites (Mg²⁺) near to the redox sites (V⁵⁺) in the molecular sieve framework.     

Synthesis and characterization of a new hyperbranched organic–inorganic solid polymer electrolyte with cyanuric chloride as a core element

A new hyperbranched organic–inorganic hybrid electrolyte based on the use of 2,4,6-trichloro-1,3,5-triazine (cyanuric chloride, CC) as the coupling core to couple with oligo(oxyalkylene)-amines, followed by condensation with (3-glycidoxypropyl)-trimethoxysilane (GLYMO) and complexed with LiClO₄, has been prepared and characterized. The Vogel–Tamman–Fulcher (VTF) like conductivity behavior is observed in the present organic–inorganic hybrid electrolytes with a maximum ionic conductivity value of 4.4 × 10⁻⁵ S cm⁻¹ at 30 °C. Multinuclear NMR techniques are used to provide a microscopic view for the specific interaction between the polymer chains and Li⁺ cations and their dynamic behaviors. The results of 2D ¹H–¹³C wide-line separation (WISE) and ⁷Li static line NMR width measurements divulge that the mobility of the ⁷Li cations is strongly related to a dynamic environment created by the polymer motion in the amorphous phase. The combined results of conductivity and ⁷Li pulse-gradient spin-echo (PGSE) NMR self-diffusion coefficient measurements reveal that the conductivity enhancement at low salt concentrations is mainly caused by the high mobility of the lithium cations.     

Effect of thermal molecular motion on pervaporation behavior of comb-shaped polymers with fluorocarbon side groups

Summary Thermal molecular motion of a series of comb-shaped polymers with heptadecafluorodecyl side chains (C^F ₈C₂-) has been studied based on dynamic viscoelastic measurement. The main chain of comb-shaped polymers was poly(fumarate), poly(methacrylate), and poly(acrylate). Poly(fluoroalkylfumarate) with heptadecafluorodecyl group, P(C^F ₈C₂-iPF) was amorphous polymer, whereas poly(acrylate) and poly(methacrylate) with C^F ₈C₂-, P(C^F ₈C₂-Acr) and P(C^F ₈C₂-MAcr) showed mesomorphic behavior. The pervaporation behavior of water/organic mixtures through amorphous polymer, P(C^F ₈C₂-iPF), and mesomorphic one, P(C^F ₈C₂-Acr), were investigated. The distinct increase in permselectivity has been observed at the mesomorphic transition of P(C^F ₈C₂-Acr).     

Bromate Destruction by UV Irradiation and Electric Arc Discharge

Bromate ion destruction by UV irradiation using either a low pressure mercury lamp or a medium pressure mercury lamp has been evaluated. A low pressure lamp which emits radiation predominantly at < 200 nm was more effective than the UV lamp which emits radiation at 254 nm, since bromate ion has a peak absorbance of about 195 nm. Bromate ion was shown to be reduced to bromide ion with bromine as an intermediate. Bromate ion destruction using a low pressure mercury lamp (< 200 nm) ranged from 3 to 38% for doses ranging from 23 to 228 mW-s/cm²; 7-46% destruction was achieved using a medium pressure lamp with initial bromate ion concentrations of 11-38 μg/L and doses ranging from 60 to 550 mW-s/cm². A new innovative electric arc discharge method also has been evaluated and compared with UV irradiation. The electric arc discharge method destroyed 12-45% bromate ion for doses ranging from 130 to 1300 mW-s/cm².     

Dielectric spectroscopy of poly(butylene succinate) films

Dielectric properties of poly(butylene succinate) crystallized under different conditions have been reported in the temperature range of 163-383 K and in the frequency range of 0.01-10⁵ Hz. Both the dipolar α and β processes have been identified at low temperatures: the α process is associated with the amorphous fraction while the β with the relaxations in both the amorphous and crystalline fractions. The space charge effect dominates the high temperature dielectric spectra. These spectra have been analyzed in the light of an equivalent circuit model. The Maxwell-Wagner-Sillars polarization, electrode polarization and free charge motion are well resolved. At 383 K, near the melting temperature (387 K), massive melting and subsequent recrystallization have been observed. The peculiar evolution of the spectra is also analyzed using the same equivalent circuit model. The relationship between the fitting parameters and the evolved microstructures is discussed.     

Particle flow in a hydrocyclone investigated by positron emission particle tracking

The flow of a particle through a hydrocyclone acting on water has been studied by positron emission particle tracking (PEPT). The positron-emitting radioactive tracer was ¹⁸F. It was found that the activity on an ion-exchange resin particle labeled with ¹⁸F did not leach out into the water during the duration of an experiment. In the state-of-the-art PET camera it is shown to be possible to locate the centroid of the tracer particle with a standard deviation of only about 0.2 mm once per ms, making both the temporal and spatial resolution high enough to trace the particle in its very fast motion through the hydrocyclone. The design of the hydrocyclone was a modified Stairmand high-efficiency geometry with a long cone. The results are, among other things, shown as spatial tracks of the tracer particle as it moves through the hydrocyclone. Several interesting features were seen. The particle path, although the particle was much larger than the cut size of the cyclone, exhibited excursions into the inner, upwardly directed, part of the vortex giving rise to recirculatory loops. Moreover, at a particular position low in the cyclone, the particle exhibited a complicated flowpattern moving up and down repeatedly across this position. Careful analysis of the motion is presented, particularly of the motion low in the hydrocyclone, on basis of which it is made likely that this position represents the end of the vortex in the hydrocyclone.     

Molecular motion in polyisobutylene and poly(propylene oxide) studied by 13C nuclear magnetic relaxation

The ¹³C spin-lattice relaxation times of low molecular weight (up to 2500) samples of polyisobutylene and poly(propylene oxide) have been measured as a function of molecular weight, temperature and concentration in chloroform solution. For both polymers there is little dependence on molecular weight indicating a flexible conformation in the liquid state, but the relaxation time increases with increasing dilution in CHCl₃. The motion of the polymer backbone therefore depends on the microviscosity of the solution, rather than the bulk viscosity. In polyisobutylene the methyl re-orientation rate increases in parallel with the backbone re-orientation rate, showing that the two motions are interlinked. In poly(propylene oxide) the methyl re-orientation rate is independent of the backbone motion.     

Dynamics of polymer molecules using neutron spin—echo

The dynamics of single polymer chains have been investigated in dilute solution and in the melt using the neutron spin—echo technique. In dilute solution the intramolecular motion is described to a first approximation by that for a Rouse chain incorporating hydrodynamic interactions (Zimm). It is characterised by an inverse correlation time which may be normalised by temperature, solvent viscosity and segment size, and which for long chains varies as Q³ at small scattering angles (Q is the change in wavevector on scattering). At very low Q vectors the predicted ‘universal’ regime is observed, but over most of the accessible range chemical structure also becomes important. For short chains, deviations from this Q³ behaviour are associated with overall molecular diffusion. In the melt, chain entanglements come into play and modify the simple Rouse-type motion. The correlation functions are described by a combination of Rouse motion over short distances and times and a long-time slow-motion predicted by the reptation model of melt dynamics.     

Dynamics of hydrated iron(II) cations in nafion polymer membranes

Mössbauer studies of Fe²⁺ in water-soaked nafion polymer membranes in the temperature range between 90 K and 250 K have been performed. Above a critical temperature (～ 180 K) the spectra exhibit both elastic narrow absorption lines and quasielastic broad lines. These spectra are typical of bounded diffusion phenomena observed by Mössbauer spectroscopy in macromolecular systems like haemoglobin, myoglobin and ferritin. Similar spectral shapes have been observed by quasielastic neutron scattering from water in nafion membranes. Within 50 K above the critical temperature the total Mössbauer absorption area decreases by an order of magnitude whereas the narrow absorption line decreases by two orders of magnitude. The results are interpreted in terms of bounded diffusive motion of the iron. Using a model based on overdamped harmonically bound Brownian motion, the essential parameters of the iron motion can be derived as a function of temperature. The iron motion most probably reflects the motion of a large Fe²⁺ complex, e.g. Fe(H₂O)²⁺₆, which is attached to the polymer side chains via the sulphonic group.     

Poiy(hexamethyienebigyanidiniym chloride) as a reagent for the determination of water-soluble naturai carboxylate and sulphate polymers

The poly(hexamethylenebiguanidinium chloride) [PHMBH ⁺ Cl^?] assay method developed originally for sodium alginate determination in solution (0 01-0 5 %) has been applied to a wide range of other acidic polysaccharides. The simple procedure consists of the addition of a known excess of PHMBH ⁺ Cl^? to precipitate the sample quantitatively and thereafter measuring the u.v. absorption of residual PHMBH ⁺ Cl^? in the supernatant. Response to the method is dependent upon the acidic group-monosaccharide molar ratio and can be conveniently altered for polysaccharides with different anionic charge densities by simple adjustment of assay conditions. For acidic polysaccharides with very low charge densities, such as propylene glycol alginates with a high degree of esterification, the response to the assay can be increased by addition of borate ions which complex with the polymer molecules to increase the negative charge. The electrostatic nature of the response also allows the assay method to be used for the determination of extent of esterification in propylene glycol alginates with up to ～70% derivatisation. The method has been shown to be suitable for low molecular weight samples. Therefore, it is more reliable than other assay methods for acidic polysaccharides which involve a precipitation step with doubtful quantitative recovery for low molecular weight material. The simplicity, rapidity and reliability of the method makes it a suitable procedure for both carboxylated and sulphated polysaccharides in a range of applications.     

A Layered Perovskite EuBaCo2O5+δ for Intermediate‐Temperature Solid Oxide Fuel Cell Cathode

A layered perovskite EuBaCo₂O_5+δ (EBCO) has been prepared by a solid‐state reaction, and evaluated as potential cathode for intermediate‐temperature solid oxide fuel cells. Structural characterizations are determined at room temperature using powder X‐ray diffraction and transmission electron microscopy technique. The good fits to the XRD data by Rietveld refinement method are obtained in the orthorhombic space group (Pmmm). The lower average thermal expansion coefficient, 14.9 × 10^–6 °C^–1 between 100 and 800 °C, indicates its better thermal expansion compatibility with conventional electrolytes, compared with the other cobalt‐containing cathode materials. The high electrical conductivity and large oxygen nonstoichiometry at intermediate temperatures suggest the effective charge transfer reactions including electron conduction and oxide‐ion motion in cathode. As a result, a highly electrochemical activity towards the oxygen reduction reaction is achieved between 600 and 700 °C, as evidenced by low area‐specific resistances, e.g. 0.14–0.5 Ω cm². In addition, cathodic overpotential and oxygen reduction kinetics of the EBCO cathode have also been studied.     

Kinetic study of aqueous polymerization of methyl methacrylate initiated by Ce(IV)-maltose redox system

Polymerization of methyl methacrylate initiated by ceric ammonium nitrate-maltose has been investigated in aqueous nitric acid under nitrogen in the temperature range 20.5-35°C. The dependence of the initial rate of polymerization and the initial rate of ceric ion consumption on maltose, Ce(IV), and monomer concentrations has been determined. The reaction orders were found to depend on ceric ion concentration. At a moderately high Ce(IV) concentration (1 × 10^?3mol litre^?1) the orders were 1/2 and 3/2 with respect to maltose and monomer concentration, respectively, and independent of Ce(IV) concentration. But at a low Ce(IV) concentration (4 × 10^?4mol litre^?1) the orders with respect to monomer and Ce(IV) changed to 1 and 1/2, respectively. The effect of temperature was also examined. The average molecular weight, as determined by size-exclusion chromacography, was found to depend on maltose, Ce(IV), and monomer concentrations, as well as on temperature.     

Electrical properties of biomorphic SiC ceramics and SiC/Si composites fabricated from medium density fiberboard

A study has been made of the dependences of the electrical resistivity and the Hall coefficient on the temperature in the range 1.8-1300 K and on magnetic fields of up to 28 kOe for the biomorphic SiC/Si (MDF-SiC/Si) composite and biomorphic porous SiC (MDF-SiC) based upon artificial cellulosic precursor (MDF - medium density fiberboards). It has been shown that electric transport in MDF-SiC is effected by carriers of n-type with a high concentration of ∼10²⁰ cm⁻³ and a low mobility of ∼0.4 cm² V⁻¹ s⁻¹. The specific features in the conductivity of MDF-SiC are explained by quantum effects arising in disordered systems and requiring quantum corrections to conductivity. The TEM studies confirmed the presence of disordering structural features (nanocrystalline regions) in MDF-SiC. The conductivity of MDF-SiC/Si composite originates primarily from Si component in the temperature range 1.8-500 K and since ∼500 to 600 K the contribution of MDF-SiC matrix becomes dominant.     

Fluorescent polyolefins by free radical post-reactor modification with functional nitroxides

The 4-(1-naphthoate)-2,2,6,6-tetramethylpiperidine-1-oxyl (NfO-TEMPO) has been synthesized and successfully grafted in the melt onto a random poly(ethylene-co-1-ottene) copolymer. Functionalized polyolefins have been prepared by coupling reaction between NfO-TEMPO free radicals and macroradicals which have been formed by H-abstraction induced by the presence of a peroxide. In order to deepen insight into the functionalization mechanism, the reaction has been investigated by Electron Paramagnetic Resonance (EPR). EPR spectra collected during the reaction run, have evidenced the decrease of TEMPO signal as a consequence of temperature increasing. This decrease has been attributed to the formation of a covalent bond between macroradicals and nitroxide free radical. The resulting functionalized polyolefins, PO-g-(NfO-TEMPO), have been characterized by FT-IR and ¹H NMR which has allowed to evaluate their functionalization degree, whereas UV–Vis and fluorescence spectroscopy have been used to investigate their optical properties. The comparison with low molecular weight model compounds, has allowed to state that our methodology can be conveniently adopted to prepare fluorescent polyolefins where the optical properties of the chromophore has been completely transferred to polymer backbone both in solution and in the condensed phase.     

Batch foaming of SAN/clay nanocomposites with scCO2: A very tunable way of controlling the cellular morphology

This paper aims at elucidating some important parameters affecting the cellular morphology of poly(styrene-co-acrylonitrile) (SAN)/clay nanocomposite foams prepared with the supercritical CO₂ technology. Prior to foaming experiments, the SAN/CO₂ system has first been studied. The effect of nanoclay on CO₂ sorption/desorption rate into/from SAN is assessed with a gravimetric method. Ideal saturation conditions are then deduced in view of the foaming process. Nanocomposites foaming has first been performed with the one-step foaming process, also called depressurization foaming. Foams with different cellular morphology have been obtained depending on nanoclay dispersion level and foaming conditions. While foaming at low temperature (40 °C) leads to foams with the highest cell density (∼10¹²-10¹⁴ cells/cm³), the foam expansion is restricted (d∼0.7-0.8 g/cm³). This drawback has been overcome with the use of the two-step foaming process, also called solid-state foaming, where foam expansion occurs during sample dipping in a hot oil bath (d∼0.1-0.5 g/cm³). Different foaming parameters have been varied, and some schemes have been drawn to summarize the characteristics of the foams prepared - cell size, cell density, foam density - depending on both the foaming conditions and nanoclay addition. This result thus illustrates the huge flexibility of the supercritical CO₂ batch foaming process for tuning the foam cellular morphology.     

The environment of lithium ions and conductivity of comb-like polymer electrolyte with a chelating functional group

The behavior of lithium ions in a comb-like polymer electrolyte with a chelating functional group have been characterized by differential scanning calorimeter (DSC), dynamic mechanical analysis (DMA), Fourier transform infrared (FTIR) spectroscopy, ac impedance and ⁷Li solid-state NMR measurements. The comb-like copolymer is synthesized by poly(ethylene glycol-methyl ether methacrylate) (PEGMEM) and (2-methylacrylic acid 3-(bis-carboxymethylamino) -2-hydroxy-propyl ester) (GMA-IDA). FTIR and ⁷Li solid-state NMR spectra demonstrate the interactions of Li⁺ ions with both the ether oxygen of the PEGMEM and the nitrogen atom of the GMA-IDA segments. Moreover, ⁷Li solid-state NMR shows that the lithium ions are preferentially coordinated to the GMA-IDA segment. The T_g increases for the copolymers doped with LiClO₄. These results indicate the interactions of Li⁺ with both PEGMEM and GMA-IDA segments form transient cross-links. The Vogel-Tamman-Fulcher (VTF)-like behavior of conductivity implies the coupling of the charge carriers with the segmental motion of the polymer chains. The dependence of the maximum conductivity on the composition of the copolymers and the doping lithium ion concentration was determined. The GMA-IDA unit in the copolymer improves the dissociation of the lithium salt, the mechanical strength and the conductivity.     

1-Methyl-1-Vinyl-3,3,5,5-Tetraphenylcyclotrisiloxane: An Organofunctional Cyclotrisiloxane

The characterization of 1-methyl-1-vinyl-3,3,5,5-tetraphenylcyclotrisiloxane by NMR(¹H,¹³C,²⁹Si) and X-ray crystallography is reported. The central siloxane ring is planar with Si–O bond lengths and angles which are similar to those found in the structures of other cyclotrisiloxanes. There is significant thermal motion of the methyl and vinyl groups. Very low incorporation of the siloxane is observed in the radical catalyzed copolymerization of the siloxane with styrene.     

Studies on chromium(III) complexes with active nitrogen,oxygen and sulfur donor ketimines synthesized under microwave conditions

Heterocyclic ketimines, 1-(2-furanyl)ethanonehydrazincarbothioamide (L¹H), 1-(2-furanyl)ethanone- hydrazincarboxamide (L²H), 1-(2-thienyl)ethanone hydrazincarbothioamide (L³H) and 1-(2-thienyl) ethanonehydrazincarboxamide (L⁴H), were prepared by the condensation of thiosemicarbazide and semicarbazide hydrochloride (in the presence of sodium acetate) in ethanol with the respective ketones by using microwave as well as conventional methods. Chromium(III) complexes have been prepared by mixing CrCl₃·6H₂O in 1:1 and 1:2 molar ratios with monobasic bidentate ketimines. The authenticity of the ligands and their complexes has been established by elemental analyses, melting point determinations, molecular weight determinations, EPR, infrared and UV spectral and X-ray powder diffraction studies. These studies showed that the ligands coordinated to the metal atom in a monobasic bidentate mode, coordinating through the nitrogen and sulfur/oxygen donor system. Thus, an octahedral environment around the metal atoms has been proposed. The growth-inhibiting potential of the ligands and complexes has been assessed against a variety of fungal and bacterial strains.