Isoconversional kinetics of degradation of polyvinylpyrrolidone used as a matrix for ammonium nitrate stabilization

Thermogravimetric analyzer (TGA) has been applied to measure the kinetics of the thermal degradation of virgin polyvinylpyrrolidone (PVP) and a phase stabilized PVP–ammonium nitrate (AN) material. The PVP–AN samples have been prepared by using 20 wt.% of AN and PVP of three different molecular weights. Virgin PVP undergoes a major mass loss in the region 380–550 °C leaving a small amount of nonvolatile residue. The application of an advanced isoconversional method to the respective degradation process demonstrates that its effective activation energy increases from 70 kJ mol⁻¹ to a plateau value at 250–300 kJ mol⁻¹, which is independent of the molecular weight. The PVP–AN materials lose spontaneously 20% of their mass on heating above the glass transition temperature of the PVP matrix (160–180 °C). After the escape of AN, the remaining PVP matrix degrades in the same temperature region as virgin PVP, however, the effective activation energy of this degradation is 150–200 kJ mol⁻¹.     

A computational study and fragment analysis of the back-bonding in Titanocenyl Complexes containing a five-member L,L′-cyclic ligand, L,L′ = O,O′; S,S′ or Se,Se′

A DFT computational study is performed on different Cp₂Ti^IV(L,L′-BID) complexes with L,L′-BID = dioxolene, dithiolene or diselenolene. A fragment analysis of the titanocene-ligand bonding in the DFT optimized geometries showed that out of plane folding for maximum Ti ← L π donation increases Cp₂Ti^IV(O,O′-BID) (35°) < Cp₂Ti^IV(S,S′-BID) (43–49°) < Cp₂Ti^IV(Se,Se′-BID) (48–53°).     

Effect of small amount of poly(ethylene naphthalate) on isothermal crystallization and spherulitic morphology of poly(trimethylene terephthalate)

The effect of a small amount of poly(ethylene naphthalate) (PEN) in its blends with poly(trimethylene terephthalate) (PTT) on isothermal melt-crystallization kinetics and spherulitic morphology of the blends was thoroughly investigated. The maximum PEN content in the blends was 9 wt%. Due to the single composition-dependent glass transition temperature (T_g) that was observed for each blend, these blends appeared to be miscible in the amorphous state. After isothermal crystallization from the melt state, the neat PTT and its blends with PEN exhibited either double or triple melting endotherms. The triple endothermic peaks were observed in both the neat PTT and the blends when being crystallized at crystallization temperatures (T_c) of less than or equal to 195 °C. The equilibrium melting temperature () for the neat PTT was determined based on the linear Hoffman–Weeks extrapolative method to be 248 °C. Such values for the blends were found to decrease with the addition and increasing amount of PEN. Both the neat PTT and the blends were isothermally crystallized over the T_c range of 190–205 °C. At a given T_c, the 97PTT/3PEN blend exhibited a half-time of crystallization (t_0.5) value that was lower, while it exhibited reciprocal half-time (), Avrami rate constant (K_A), and spherulitic growth rate (G) values that were greater, than those of the neat PTT. With further increase in the PEN content, the t_0.5 value increased, while the , K_A, and G values decreased. Analysis of the G values based on the Lauritzen–Hoffman's (LH) secondary nucleation theory showed that the neat PTT and the 91PTT/9PEN blend exhibited a regime II→III transition at 194 °C (467.2 K), while no regime transition was observed for the other two blends. The lateral and the fold surface free energies (σ and σ_e) and the work of chain folding (q) for the neat PTT and the blends were 19.4, 30.2–46.3 erg cm⁻², and 2.4–3.6 kcal mol⁻¹, respectively. Lastly, the effect of both the T_c and the PEN content on morphology and texture of the PTT spherulites was also investigated and the results showed that the texture of the spherulites became coarser with increasing T_c and PEN content.     

TGA–FTIR study of a lead zirconate titanate gel made from a triol-based sol–gel system

A Pb(Zr,Ti)O₃ precursor gel made from a sol prepared using 1,1,1,-tris(hydroxymethyl)ethane, lead acetate and zirconium and titanium propoxides, stabilised with acetylacetone, was analysed using TGA–FTIR analysis. Decomposition under nitrogen (N₂) gave rise to evolved gas absorbance peaks at 215 °C, 279 °C, 300 °C and 386 °C, but organic vapours continued to be evolved, along with CO₂ and CO until 950 °C. The final TGA step in N₂ is thought to relate to decomposition of an intermediate carbonate phase and the final elimination of residues of triol or acetylacetonate species which form part of the polymeric gel structure. By contrast, heating in air promoted oxidative pyrolysis of the final organic groups at ≤450 °C. In air, an intermediate carbonate phase was decomposed by heating at 550 °C, allowing Pb(Zr,Ti)O₃ to be produced some 400 °C below the equivalent N₂ decomposition temperature.     

Molecular fine structure and transition dipole moment of NO2 using an external cavity quantum cascade laser

Rotational vibrational fine structure and transition dipole moment of NO₂ is measured using Doppler free saturation spectroscopy with an external grating cavity quantum cascade laser (QCL). The QCL wavelength is calibrated using a 310 cm long internally coupled Fabry–Perot interferometer. We obtain a frequency splitting of 139.68 ± 0.06 MHz (0.0047 cm⁻¹) between the spin doublets (17) of 000 → 001 transition of NO₂. The resolution of the QCL based saturation spectrometer is limited by the QCL linewidth of 3.99 MHz ( 0.00013 cm⁻¹) deduced from the half width of the Lamb dips. The Lamb dip spectroscopy is utilized to obtain a vibrational dipole moment of 0.37 Debye for the (17) transitions.     

An octane-fueled low temperature solid oxide fuel cell with Ru-free anodes

A Ru-free anode was developed for the direct utilization of iso-octane in low temperature solid oxide fuel cells (SOFCs). The anode was consisted of a Ni framework and a nano-sized oxygen–ion conductor, samaria-doped ceria (SDC), which was coated onto the inner surface of the framework via an ion impregnation process. Compared with the cells based on conventional Ni–SDC anodes, single cells with the SDC-coated Ni anodes exhibited improved stability and enhanced electrochemical activity. Peak power density of 400 mW cm⁻² was achieved at 600 °C, and power generation was relatively stable over 260 h when iso-octane–air mixture was directly used as the fuel. The performance is comparable with those obtained using ceria-Ru as an internal reforming catalyst.     

Enhanced tungstate electrochromism via formation of transparent conductive networks

Indium tin oxide (ITO) nanopowder was added to a polymer film containing WO₃ · H₂O particles to enhance electron conductivity and complimentary Li ion kinetics in an electrochromic device. Film conductivity increased dramatically with ITO content, suggesting the formation of conductive ITO networks in the film. The improved electron conductivity leads to a substantial increase of the effective Li⁺ ion diffusion coefficient in the composite film, from 10⁻¹¹ to 10⁻⁹ cm²/s. Electrochromic contrast studies revealed that the presence of the ITO networks leads to enhanced blue/green color contrast.     

Adsorption of 1,4-Benzenedithiol on Gold and Silver Surfaces: Surface-Enhanced Raman Scattering Study

The adsorption behavior of 1,4-benzenedithiol (1,4-BDT) on colloidal gold and silver surfaces has been investigated by means of surface-enhanced Raman scattering (SERS). 1,4-BDT chemisorbed dissociatively on both gold and silver surfaces but as mono- and dithiolate, respectively. Regardless of the bulk concentration of 1,4-BDT, only a monolayer was assembled on the silver surface with a flat orientation by forming two Ag–S bonds. On the gold surface, the monothiolate species,1,4-BDT⁻¹, appeared to assume a rather flat orientation at a very low surface coverage, but as the surface coverage was increased, the adsorbate took a perpendicular orientation. Furthermore, when the bulk concentration of 1,4-BDT was close to that required for a full-monolayer coverage limit, a band assignable to the S–S stretching vibration appeared at 536 cm⁻¹ in the gold sol SERS spectra. A separate ellipsometry measurement performed with vacuum-evaporated gold substrates revealed that up to tetralayers could be assembled on gold in 1 mM n-hexane solution of 1,4-BDT while at best a bilayer formed in either methanol or ethanol solution. The different adsorbate structure of 1,4-BDT on gold and silver was overall quite comparable to that of p-xylene-α,α′-dithiol.     

Preparation and characterization of stable monodisperse silver nanoparticles via photoreduction

Silver nanoparticles were synthesized by UV irradiation of [Ag(NH₃)₂]⁺ aqueous solution using poly(N-vinyl-2-pyrrolidone) (PVP) as both reducing and stabilizing agents. The formation of silver nanoparticles was confirmed from the appearance of surface plasmon absorption maxima around 420 nm. It was found that the formation rate of silver nanoparticles from Ag₂O was much quicker than that from AgNO₃, and the absorption intensity increased with PVP concentration as well as irradiation time. The maximum absorption wavelength (λ_max) was blue shift with increasing PVP content until 8 times concentration of [Ag(NH₃)₂]⁺ (wt%). The transmission electron microscopy (TEM) showed the resultant particles were 4–6 nm in size, monodisperse and uniform particle size distribution. X-ray diffraction (XRD) demonstrated that the colloidal nanoparticles were the pure silver. In addition, the silver nanoparticles prepared by the method were stable in aqueous solution over a period of 6 months at room temperature (25 °C).     

Simultaneous determination of chloride, nitrate and sulphate in vegetable samples by single-column ion chromatography

A new ion chromatography method is described for the simultaneous determination of Cl⁻, NO₃⁻ and SO₄²⁻, using a selected eluent 1.3-mM sodium gluconate/1.3-mM borax (pH 8.5). The extraction methods of Cl⁻, NO₃⁻, SO₄²⁻ in vegetables are studied. The determination limits of Cl⁻, NO₃⁻, SO₄²⁻ are 0.17 μg/ml, 0.63 μg/ml and 0.81 μg/ml. The linear ranges are 060 μg/ml, 090 μg/ml and 090 μg/ml. The relative S.D. are <2.5%. The mean recoveries of Cl⁻, NO₃⁻, SO₄²⁻ in vegetables range from 97.0 to 104%.     

Existence of clathrate-like structures in supercooled water: X-ray diffraction evidence

X-ray diffraction study of supercooled water has been performed using an imaging-plate X-ray detector down to −15 °C. The peak at 10.8 Å, which grows with decreasing temperature, in the radial distribution function {D(r) − 4πr²ρ₀} indicates the existence of clathrate-like structures in supercooled water. It is suggested that anomalous properties of water, which become more pronounced at low temperatures, are closely linked to the development of clathrate-like structures in water at low temperatures.     

Transitions between P21, , and P6322 modifications of SrAl2O4 by in situ high-temperature X-ray and neutron diffraction

The results of in situ high-temperature X-ray and neutron powder diffraction experiments reconcile inconsistencies in previous reports on the symmetry of high-temperature phases of SrAl₂O₄. The material undergoes two reversible phase transitions and at 680 and 860 °C, respectively, and the latter one is experimentally observed and characterized for the first time. The higher symmetry above the transition is gained by disordering off-center split site of oxygen atoms around trigonal axis rather than by unbending Al–O–Al angle to the ideal value 180°. The analysis of the literature suggests that it is a common feature of the P6₃22 phases of stuffed tridymites.     

Electrochemical properties of polyaniline in p-toluene sulfonic acid solution

Polyaniline was deposited potentiodynamically on a stainless steel substrate in the presence of an inorganic acids (sulfuric acid). The electrochemical characterization of the electrode was carried out by means of cyclic voltammetry and electrochemical impedance spectroscopy in the organic acids (p-toluene sulfonic acid) solution. The results show that polyaniline has a high specific capacitance of 431.8 F g⁻¹ at 1 mV s⁻¹, high coulombic efficiency of 95.6% at 20 mV s⁻¹, and exhibits a high reversibility. This indicates the promising feasibility of the polyaniline used as an electrochemical capacitor material in the electrolyte of p-toluene sulfonic acid solution especially at high charge–discharge process.     

Fabrication and superhydrophobicity of fluorinated titanium dioxide nanocoatings

This study demonstrates the fabrication of a stable superhydrophobic surface with low contact angle hysteresis (CAH) using an arrangement of nanoscale TiO₂ spheres. The control of precursor quantity is selected as the key factor in determining surface roughness that significantly intensifies water contact angle (CA) of TiO₂ films. After surface fluorination treatment, the anatase-type crystalline surfaces exhibit good water repellency (CA 166.1°), low CAH (6°), and superhydrophobic stability (>60 min). Enhanced water repellency is attributed to the fact that the higher density of TiO₂ spheres results in more tortuous three-phase contact line, leading to the self-cleaning effect. Such a unique textured surface imparts many promising potentials for engineering and the development of optics devices with robust superhydrophobic materials.     

An investigation of the chemiluminescence reaction in the sodium hypochlorite–folic acid–emicarbazide hydrochloride system

A chemiluminescence method for the determination of folic acid by the sodium hypochlorite–folic acid–semicarbazide hydrochloride system with a new flow injection technique has been established. The new method can perform simple, sensitive and rapid determinations of folic acid. The response to the concentration of folic acid, in the range of 1.0×10⁻⁷5.0×10⁻⁵ g/ml, is linear. The relative standard deviation of the method is 2.3% (C_s=4.0×10⁻⁶ g/ml, n=11). The detection limit is 2.7×10⁻⁸ g/ml. This method is suitable for automatic and continuous analysis, and has been successfully tested for the determination of folic acid in a folic acid tablet.     

Pervaporation dehydration of ethylene glycol with chitosan–poly(vinyl alcohol) blend membranes: Effect of CS–PVA blending ratios

Chitosan–poly(vinyl alcohol), CS–PVA, blended membranes were prepared by solution casting of varying proportions of CS and PVA. The blend membranes were then crosslinked interfacially with trimesoyl chloride (TMC)/hexane. The physiochemical properties of the blend membranes were determined using Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR), X-ray diffraction (XRD), differential scanning calorimetry (DSC), tensile test and contact angle measurements. Results from ATR-FTIR show that TMC has crosslinked the blend membranes successfully, and results of XRD and DSC show a corresponding decrease in crystallinity and increase in melting point, respectively. The crosslinked CS–PVA blend membranes also show improved mechanical strength but lower flexibility in tensile testing as compared to uncrosslinked membranes. Contact angle results show that crosslinking has decreased the surface hydrophilicity of the blend membranes. The blend membrane properties, including contact angle, melting point and tensile strength, change with a variation in the blending ratio. They appear to reach a maximum when the CS content is at 75 wt%. In general, the crosslinked blend membranes show excellent stability during the pervaporation (PV) dehydration of ethylene glycol–water mixtures (10–90 wt% EG) at different temperatures (25–70 °C). At 70 °C, for 90 wt% EG in the feed mixture, the crosslinked blend membrane with 75 wt% CS shows the highest total flux of 0.46 kg/(m² h) and best selectivity of 986. The blending ratio of 75 wt% CS is recommended as the optimized ratio in the preparation of CS–PVA blend membranes for pervaporation dehydration of ethylene glycol.     

Enhanced sensitivity for Cu(II) by a salicylidine-functionalized polysiloxane carbon paste electrode

A new approach for decreasing the detection limit for a copper(II) ion-selective electrode (ISE) is presented. The ISE is designed using salicylidine-functionalized polysiloxane in carbon paste. This work describes the attempts to develop the electrode and measurements of its characteristics. The new type of renewable three-dimensional chemically modified electrode could be used in a pH range of 2.3–5.4, and its detection limit is 2.7 × 10⁻⁸ mol L⁻¹ (1.2 μg L⁻¹). This sensor exhibits a good Nernstian slope of 29.4 ± 0.5 mV/decade in a wide linear concentration range of 2.3 × 10⁻⁷ to 1.0 × 10⁻³ mol L⁻¹ of Cu(II). It has a short response time (8 s) and noticeably high selectivity over other Cu(II) selective electrodes. Finally, it was satisfactorily used as an indicator electrode in complexometric titration with EDTA and determination of copper(II) in miscellaneous samples such as urine and various water samples.     

Ultrafast dynamics of heat flow across molecules

An ultrafast flash thermal conductance apparatus is used to study heat flow through aliphatic and aromatic molecules arranged in self-assembled monolayers (SAMs). The apparatus consists of a thin metal film which can be flash-heated by many hundreds of degrees in 1 ps using a femtosecond pulse. Heat flow from the metal surface into the SAM molecules is detected using vibrational sum-frequency generation (SFG) spectroscopy. The SAMs studied were alkanethiolates (AT) ranging from C6 to C24, benzenethiolate (BT) and benzylmercaptide (BMT). SFG in the CH-stretch region selectively probes transitions of the terminal methyl groups of AT and the CH moiety at the 4-position of the phenyl ring of BT and BMT (opposite the thiolate-surface bond). The SFG signal is sensitive to temperature-jump induced thermal disorder of the SAM and also to vibrational frequency shifts induced by the changing intramolecular vibrational populations. The SFG probe functions as a thermometer, and this thermometer is 1.5 Å thick with a response time of 1 ps. In the AT chains, a study of the length dependence is used to determine the rate heat flows across the metal–SAM interface and the rate of heat flow through the AT chains. The interface thermal conductance is 220 GW m⁻² s⁻¹. The AT molecular conductance is 50 pW K⁻¹ or 0.3 eV s⁻¹ K⁻¹. Heat flow through the AT chains is ballistic with a velocity of 1 km/s. Heat flow into BMT is slower than in BT because BMT has one additional methylene linker group. The BT and BMT structures evidence a thermally-initiated surface rearrangement occurring in a few tens of picoseconds. These SAMs are strained and the phenyl rings cannot adopt the most stable staggered herringbone structure. After the T-jump, the SAM molecules have enough freedom to relax into more favorable configurations.     

Poly(arylene ether sulfone)s containing pendant sulfonic acid groups as membrane materials for direct methanol fuel cells

A series of poly(arylene ether sulfone)s containing pendant sulfonic acid groups have been prepared by an aromatic substitution polymerization reaction using 4,4-difluorodiphenylsulfone, 6,7-dihydroxy-2-naphthalenesulfonate, and various hydroxyl terminated monomers in the presence of potassium carbonate. The synthesized sulfonated polymers have been characterized by nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, gel permeation chromatography, ion exchange capacity, thermogravimetric analysis, and proton conductivity measurements. With a molecular weight of 50,000–59,000 g/mol and an ion exchange capacity of 1.17 meq./g, these polymers are thermally stable up to 250 °C. They are found to exhibit better performance at 65 and 80 °C in direct methanol fuel cells than Nafion 115 membrane despite lower proton conductivity due to a significantly lower methanol crossover.