Thermal Degradation Processes in Poly(xylylene sulfides) Investigated by Comparative Direct Pyrolysis MS and Flash Pyrolysis GC/MS Experiments

The thermal degradation processes of two sulfur polymers, poly(xylylene sulfide) (PXM) and poly(xylylene disulfide) (PXD), were investigated in parallel by direct pyrolysis mass spectrometry (DPMS) and flash pyrolysis GC/MS (Py-GC/MS). Thermogravimetric data showed that these polymers decompose with two separate steps in the temperature ranges of 250-280 and 600-650 degrees C, leaving a high amount of residue (about 50% at 800 degrees C). The pyrolysis products detected by DPMS in the first degradation step of PXM and PXD were terminated by three types of end groups, -CH3, -CH2SH, and -CH=S, originating from thermal cleavage reactions involving a series of homolytic chain scissions followed by hydrogen transfer reactions, generating several oligomers containing some intact xylylene sulfide repeating units. The presence of pyrolysis compounds containing some stilbene-like units in the first degradation step has also been observed. Their formation has been accounted for with a parallel cleavage involving the elimination of H2S from the PXM main chains. These unsaturated units can undergo cross-linking at higher temperatures, producing the high amount of char residue observed. The thermal degradation compounds detected by DPMS in the second decomposition step at about 600-650 degrees C were constituted of condensed aromatic molecules containing dihydrofenanthrene and fenanthrene units. These compounds might be generated from the polymer chains containing stilbene units, by isomerization and dehydrogenation reactions. The pyrolysis products obtained in the Py-GC/MS of PXM and PXD at 610 degrees C are almost identical. The relative abundance in the pyrolysate and the spectral properties of the main pyrolysis products were found to be in generally good agreement with those obtained by DPMS. Polycyclic aromatic hydrocarbons (PAHs) were also detected by Py-GC/MS but in minor amounts with respect to DPMS. This apparent discrepancy was due to the simultaneous detection of PAHs together with all pyrolysis products in the Py-GC/MS, whereas in DPMS they were detected in the second thermal degradation step without the greatest part of pyrolysis compounds generated in the first degradation step. The results obtained by DPMS and PSI-GC/MS experiments showed complementary data for the degradation of PXM and PXD and, therefore, allowed the unequivocal formulation of the thermal degradation mechanism for these sulfur-containing polymers.

Flow of metal into the flash gap in the last stages of a plane-strain closed-die forging operation

In closed-die forging the flash geometry should be such as to ensure that the cavity is completely filled just as the two dies come into contact at the parting plane. If metal is caused to extrude through the flash gap as the dies approach the point of contact — a practice generally resorted to as a means of ensuring complete filling — dies are unnecessarily stressed in a high-stress regime (as the flash is quite thin and possibly cooled by then), which reduces the die life and unnecessarily increases the energy requirement of the operation. It is therefore necessary to carefully determine the dimensions of the flash land and flash thickness — the two parameters, apart from friction at the land, which control the lateral flow. The dimensions should be such that the flow into the longitudinal cavity is controlled throughout the operation, ensuring complete filling just as the dies touch at the parting plane. The design of the flash must be related to the shape and size of the forging cavity as the control of flow has to be exercised throughout the operation: it is possible to do this if the mechanics of how the lateral extrusion into the flash takes place is understood for specific cavity shapes and sizes. The work reported here is part of an ongoing programme investigating flow in closed-die forging. A simple closed shape (no longitudinal flow) which may correspond to the last stages of a real forging operation is analysed using the stress equilibrium approach. Metal from the cavity (flange) flows into the flash by shearing in the cavity in one of the three modes considered here: for a given cavity the mode with the least energy requirement is assumed to be the most realistic. On this basis a map has been developed which, given the depth and width of the cavity as well as the flash thickness, will tell the designer of the most likely mode (of the three modes considered) in which metal in the cavity will shear and then flow into the flash gap. The results of limited set of experiments, reported herein, validate this method of selecting the optimum model of flow into the flash gap.

A laser flash photolysis study of pivalothiophenone triplets: steric and electronic effects in thione photoreaction kinetics

Upon laser pulse excitation (Aex = 532 nm) into the lowest-lying '(n,a*) band system, pivalothiophenones in benzene solutions give rise to short-lived triplets (Ama: = 325-335 nm, em: = (1 1-15) X lo3 M-' cm-I) with quantitative intersystem crossing efficiencies. The triplet yields decrease slightly (by 10-30%) upon changing A, to 308 nm (Le., upon excitation into S2). Kinetic data are presented for intrinsic triplet lifetimes, self-quenching, and quenching by oxygen, di-tert-butylnitroxy radical, and various reagents capable of interacting with the triplets via energy, electron, or hydrogen-atom transfer and by biradical formation (possibly leading to cycloaddition). The mechanisms of the quenching processes are discussed. Relative to rigid aromatic thiones, namely, xanthione and thiocoumarin, the interaction of pivalothiophenone triplets with most of the quenchers are kinetically inefficient. This is interpreted primarily as a manifestation of the steric crowding at positions a to the thiocarbonyl group.

Laser flash photolysis study of triplets of cyclobutanethiones

Five cyclobutanethiones with different chromophores at the 3-position were examined for triplet state behaviour in benzene using laser excitation into their low lying nπ*1 band systems. A weak transient absorption attributable to the triplet state is observed in all these cases. Results concerning triplet lifetimes, intersystem crossing yields (S1 → T1), self-quenching kinetics and kinetics of energy transfer to all-trans-1,6-diphenyl-1,3,5-hexatriene and oxygen and quenching by di-t-butyl nitroxide (DTBN) are presented. Intersystem crossing yields estimated with reference to p,p′-dimethoxythiobenzophenone are roughly unity in all five cases. Self-quenching rates are found to be less than diffusion limited and this is attributed to steric crowding at the α positions (dimethyl group). The rates of oxygen and DTBN quenching compare well with those reported for several other thiones in the literature. No transients other than the triplet were detected in the above cyclobutane-thiones.

Triplet-state Photophysics and Transient Photochemistry of Cyclic Enethiones A Laser Flash Photolysis Study

The triplets of four cyclic enethiones, including thiocoumarin, have been investigated by nanosecond laser flash photolysis. Data are presented for transient spectra and kinetics associated with triplets, quantum yields of intersystem crossing and singlet oxygen photosensitization. The quenching of the thiocoumarin triplet (A:, = 485 nm, E:,, = 8.8 x lo3 dm3 mol-' cm-'in benzene) by several olefins, amines and hydrogen donors occurs with rate constants of 107-5 x lo9 dm3 mol-' s-'; the lower limits of quantum yields ( c#+~) for the related photoreactions, estimated from ground-state depletion, are generally small (0.0-0.1 1 in benzene, except for good hydrogen donors, namely, p-methoxythiophenol and tri-n-butylstannane) . The radical anion of thiocoumarin (A,,, = 405-435 nm) is formed in two stages upon triplet quenching by triethylamine in acetonitrile; the fast component is the result of direct electron transfer to the triplet and the slower component is assigned to secondary photoreduction of the thione ground state by the a-aminoalkyl radical derived from the triethylamine radical-cation.

Role of glutamine synthetase in citric acid fermentation byAspergillus niger

The activity of glutamine synthetase fromAspergillus niger was significantly lowered under conditions of citric acid fermentation. The intracellular pH of the organism as determined by bromophenol blue dye distribution and fluorescein diacetate uptake methods was relatively constant between 6•0–6•5, when the pH of the external medium was varied between 2•3–7•0.Aspergillus niger glutamine synthetase was rapidly inactivated under acidic pH conditions and Mn2+ ions partially protected the enzyme against this inactivation. Mn2+-dependent glutamine synthetase activity was higher at acidic pH (6•0) compared to Mg2+-supported activity. While the concentration of Mg2+ required to optimally activate glutamine synthetase at pH 6•0 was very high (≥ 50 mM), Mn2+ was effective at 4 mM. Higher concentrations of Mn2+ were inhibitory. The inhibition of both Mn2+ and Mg2+-dependent reactions by citrate, 2-oxoglutarate and ATP were probably due to their ability to chelate divalent ions rather than as regulatory molecules. This suggestion was supported by the observation that a metal ion chelator, EDTA also produced similar effects. Of the end-products of the pathway, only histidine, carbamyl phosphate, AMP and ADP inhibitedAspergillus niger glutamine synthetase. The inhibitions were more pronounced when Mn2+ was the metal ion activator and greater inhibition was observed at lower pH values. These results permit us to postulate that glutamine synthesis may be markedly inhibited when the fungus is grown under conditions suitable for citric acid production and this block may result in delinking carbon and nitrogen metabolism leading to acidogenesis.

Solid phase fermentation of leaf biomass to biogas

This paper describes a simple technique for the fermentation of untreated or partly-treated leafy biomass in a digester of novel design without incurring the normal problems of feeding, floating and scum formation of feed, etc. The solid phase fermentation studied consists of a bed of biomass frequently sprinkled with an aqueous bacterial inoculum and recycling the leachate to conserve moisture and improve the bacterial dispersion in the bed. The decomposition of the leaf biomass and water hyacinth substrates used in this study was rapid, taking 45 and 30 days for the production of 250 and 235 l biogas per kg total solids (TS) respectively, for the above mentioned substrates at a daily sprinkled volume of 26 ml cm−2 of bed per day sprinkled at 12 h intervals. Very little volatile fatty acid (VFA) intermediates accumulated in the liquid sprinkled, suggesting acidogenesis to be rate-limiting in this process. From the pattern of VFA and gas produced it is concluded that most of the biogas produced is from the biomass bed, thus making the operation of a separate methanogenic reactor unnecessary.

Modeling of heat and mass transfer for solid state fermentation process in tray bioreactor

A mathematical model is developed to simulate oxygen consumption, heat generation and cell growth in solid state fermentation (SSF). The fungal growth on the solid substrate particles results in the increase of the cell film thickness around the particles. The model incorporates this increase in the biofilm size which leads to decrease in the porosity of the substrate bed and diffusivity of oxygen in the bed. The model also takes into account the effect of steric hindrance limitations in SSF. The growth of cells around single particle and resulting expansion of biofilm around the particle is analyzed for simplified zero and first order oxygen consumption kinetics. Under conditions of zero order kinetics, the model predicts upper limit on cell density. The model simulations for packed bed of solid particles in tray bioreactor show distinct limitations on growth due to simultaneous heat and mass transport phenomena accompanying solid state fermentation process. The extent of limitation due to heat and/or mass transport phenomena is analyzed during different stages of fermentation. It is expected that the model will lead to better understanding of the transport processes in SSF, and therefore, will assist in optimal design of bioreactors for SSF.

'Wet N2O oxidation' process and interface state density characterization of nanoscale nitrided SiO2 for flash memory application

In this paper we first present the 'wet N2O' furnace oxidation process to grow nitrided tunnel oxides in the thickness range 6 to 8 nm on silicon at a temperature of 800 degrees C. Electrical characteristics of MOS capacitors and MOSFETs fabricated using this oxide as gate oxide have been evaluated and the superior features of this oxide are ascertained The frequency response of the interface states, before and after subjecting the MOSFET gate oxide to constant current stress, is studied using a simple analytical model developed in this work.

Duration of a lightning flash

The mean duration of a lightning flash is observed to exhibit systematic variation with the growth and decay of the activity of a thundercloud and reaches a minimum value when the radio noise level and rate of flashing are at their maximum values.

Microbial Production Of Amino-Acids .3. Nutritional Studies And Effects Of Organic Growth Promoters And Thiamine On Dl-Alanine Fermentation By Arthrobacter Strain C19d

The study of the nutritional requirements of Arthrobacter strain C19d which accumulates alanine in large amounts in the culture medium. 1evealed that the organism needs thiamine for its growth. A Iso the alanine accumulation by this strain was found to be related to thiamine concentration in the medium. The optimum concentration of thiamine for alanine accumulation (20 tJ.g/mJ) Was also optimum for the growth of the organism indicating thereby that alanine accumulation by this strain is a growth associated process rather than far removed from it. Among the various growth promoters tried yeast extract was found to be superior from the point of view of alanine yield and it wa5 also superior to giving thiamine alone in the medium. A concentration of 0.02% yeast extract was found to be optimum for alanine occumulation.

Dynamics of charge transfer in the excited amine complexes of the fullerenes C60 and C70.: A picosecond laser flash photolysis study

In benzene solution, C60 and C70 interact weakly in the ground state with amines having favourable oxidation potentials. Picosecond time-resolved absorption measurements show that on photoexcilation, the weak complexes undergo charge separation to produce ion pairs which in turn undergo fast geminate recombination either to produce the triplet state of the fullerenes or give back the ground slate of the complex, depending on the oxidation potential of the amine. Free-ion yield is generally negligible.

Synthesis of A1PO4, LaPO4 and KTiOPO4 by flash combustion

Fine particle AlPO4, LaPO4 and KTiOPO4 have been prepared by the flash combustion of aqueous solutions containing metal nitrate, ammonium hydrogen phosphate, ammonium nitrate or ammonium perchlorate and carbohydrazide or tetraformal trisazine. When rapidly heated at 400 °C, the solution containing the redox mixtures ignites to undergo self-propagating, gas-producing, exothermic reactions. Formation of crystalline phosphates was confirmed by powder X-ray diffraction patterns and IR spectra. The metal phosphates formed are fine and have 20�78 m2 g?1 surface area.

Thermal degradation products of poly(styrenesulfides) investigated by direct pyrolysis�mass spectrometry and flash pyrolysis�gas chromatography/mass spectrometry

The thermal degradation products of two sulfur polymers, poly(styrenedisulfide) (PSD) and poly(styrenetetrasulfide) (PST), were investigated in parallel by direct pyrolysis-mass spectrometry (DPMS) and by flash pyrolysis-GC/MS (Py-GC/MS). The time-scale of the two pyrolysis techniques is quite different, and therefore they were able to detect significantly different products in the pyrolysis of PSD and PST because of the thermal lability of sulfur-containing compounds. However, the results obtained are not contradictory, and satisfactory mechanisms for the thermal degradation of PSD and PST have been derived from the overall evidence available. Pyrolysis compounds containing sulfur, styrene, and a number of cyclic styrene sulfides and diphenyldithianes have been observed by DPMS. However, in flash pyrolysis-GC/MS, styrene, sulfur, only one cyclic styrene sulfide, and two isomers of diphenylthiophene have been detected. These thiophene derivatives were indeed absent among the compounds obtained by DPMS because they were the terminal (most thermally stable) species arising from further decomposition of the cyclic styrene sulfides formed in the primary thermal degradation processes of PSD and PST.