Thermal stability of model compounds related to thermostable polymers by pyrolysis-gas chromatography

Thermal stabilities of eighteen model compounds related structurally to thermostable polymer systems were determined accurately by pyrolysis-gas chromatography. This approach offers distinct advantages over previously used indirect methods which measure parameters other than actual starting material remaining undecomposed. Both the oxadiazole ring and the aromatic amide link were found to have poor thermal stabilities in comparison with other compounds studied. Since polymers based on these two features have been shown to be valuable for use in air at 200–400°C., the data suggest that oxidation resistance rather than the ultimate in pure thermal resistance should be sought in all-purpose thermostable materials.     

Correlation of urea structure with thermal stability in model compounds

Initial decomposition temperatures of various ureas have been determined using thermogravimetry and infrared analyses. Ranking of the compounds in order of thermal stability allows correlation between urea structure and resistance to thermal degradation. N-alkyl substitution of an arylurea results in the most significant lowering of thermal stability (50–70°C) as compared to the standard (N,N′-diphenylurea) whereas orthosubstituent effects and electronic effects are present but lesser in magnitude. The large steric effect is postulated to indicate the necessity of rotaion of the nitrogen–carbonyl bond during dissociation in order to establish the proper geometry for isocyanate bond formation.     

The electric resistivity of compacts of graphite intercalation compounds with antimony pentafluoride and with antimony pentachloride

The electric resistivities were measured for textured compacts of the intercalation compounds of graphite with SbF₅ and with SbCl₅. Starting materials were coarse flakes of natural graphite and graphite foils (0.35 and 2 mm thick). First stage compounds and mixtures of first and second stage compounds were studied. The resistivities of the compacts from flakes were strongly pressure dependent, maximum conductivity was not yet reached at 600 bar. The resistivities of preparations from graphite foils were much less pressure dependent. The resistivity of the foils decreased to 36 × 10^?8 Ωm, that is 3–4% of the resistivity of the parent foils. In general, the intercalation compounds with SbF₅ showed somewhat higher resistivities than the SbCl₅ compounds. This seems to be caused by formation of an insulating layer on the surface of the particles due to partial fluorination of carbon atoms with SbF₅. CF bonds were detected by IR spectroscopy and by XPS. The first stage SbF₅ compound is formed already at room temperature, contrary to literature reports, provided sufficient time is given.     

Plasmodium telomeric sequences: structure, stability and quadruplex targeting by small compounds

The increasing resistance of Plasmodium falciparum to the most commonly used antimalarial drugs makes it necessary to identify new therapeutic targets. The telomeres of the parasite could constitute an attractive target. They are composed of repetitions of a degenerate motif ((5')GGGTTYA(3'), where Y is T or C), different from the human one ((5')GGGTTA(3')). In this report we investigate the possibility of targeting Plasmodium telomeres with G-quadruplex ligands. Through solution hybridisation assays we provide evidence of the existence of a telomeric 3' G-overhang in P. falciparum genomic DNA. Through UV spectroscopy studies we demonstrate that stable G-quadruplex structures are formed at physiological temperature by sequences composed of the degenerate Plasmodium telomeric motif. Through a FRET melting assay we show stabilisation of Plasmodium telomeric G-quadruplexes by a variety of ligands. Many of the tested ligands display strong quadruplex versus duplex selectivity, but show little discrimination between human and Plasmodium telomeric quadruplexes.     

Thermal stability of polyaniline

Summary Among conducting polymers with metallic characteristics, polyaniline (PANI) is claimed to have one of the highest environmental stability. From an industrial point of view it would be useful to be able to thermally process this material into useful products using conventional techniques such as extrusion, stretching, rolling, etc., which generally are carried out at elevated temperatures. Somewhat surprisingly, relatively few studies have addressed the thermal stability of PANI, in particular that of the non-protonated emeraldine base form. Therefore, a study of the thermal properties was performed to examine the thermal stability of this material and to identify the optimum processing and maximum application temperatures. Received: 8 July 1999/Revised version: 7 November 1999/Accepted: 7 December 1999     

Thermal stability of polypyrroles

The effect of dopants and level of doping on the thermal stability of polypyrrole at 90, 120 and 150°C in dry air and nitrogen was investigated by monitoring the decay of conductivity. Polymers doped with aromatic anions (p-toluene sulphonate and p-chlorobenzene sulphonate) exhibit better stability than polymers doped with an aliphatic anion (dodecyl sulphate). The conductivity decay appears to follow diffusion controlled kinetics. After an initial decrease in conductivity, polypyrrole doped with p-toluene sulphonate anion maintains a constant conductivity at 150°C in air for at least 4 weeks. Dedoping results in materials of lower conductivity but greater thermal stability. Differential scanning calorimetry (DSC) and thermogravimetry (TG) were found to be useful techniques to characterize and investigate thermal stability. Oxidation of polypyrrole films, monitored by DSC, shows diffusion controlled kinetics. Although both oxidation and conductivity decay show typical diffusion kinetics, oxidation is a necessary but not sufficient condition for the decay.     

Thermal diffusivity of rubber compounds

Abstract

During the vulcanisation process, rubber compounds are heated to temperatures of ～200°C and then cooled. An increase in temperature occurs in rubber tyres during use as a result of heat produced by repeated deformation. This temperature increase can be so high that it can cause tyre destruction. From this point of view, knowledge of thermal diffusivity data of rubber compounds and reinforced rubber is very important. Despite thermal diffusivity being a crucial value, which determines the rate of heating or cooling of solid bodies, only limited data of this kind are available in the literature for elastomers. A new method, developed especially for determining the thermal diffusivity of thick fibre composite materials and reinforced rubber, has been used to measure the thermal diffusivity of rubber compounds. Both the content and the type of rubber and carbon black determine the thermal diffusivity. The content of other components, however, has a very limited influence on this parameter.     

Thermal stability of phthalic esters

Phthalic esters, as typical plasticizers for vinyl plastics, were thermally decomposed by a flow reactor system and their decomposition products were analyzed in detail. The thermal decomposition products were olefin, alcohol, hydrogen phthalate, benzoates with alkyl groups corresponding to these of the original phthalate esters and phthalic anhydride. We found from the main thermal decomposotion products-olefin, alcohol and hydrogen phthalate-that phthalic esters were thermally decomposed throughcis-elimination in the same way as in the general case for esters. In the presence of polyvinyl chloride (PVC), hydrochlorinated products were identified in the decomposition products. A good relationship was found between the amount of chloroalkanes produced and the reaction temperature. Thecis-elimination reaction of phthalic esters was found to be promoted by PVC.     

Thermal stability of polyethylene terephthalate

The kinetics of thermal and thermal-oxidative degradation of polyethylene terephthalate (PET) have been investigated as a function of melt temperature, melt residence time, melt environment, and drying environment. Rates of thermal and thermal-oxidative degradation were measured in terms of: weight loss of volatile degradation products, decreasing inherent viscosity, and increasing carboxyl end group concentration. Thermal-oxidative degradation was also investigated by Differential Scanning Calorimetry. Calorimetric results show that thermal-oxidative degradation of PET is an exothermic reaction, with an apparent activation energy of 117 kJ/mol. Melt temperature, melt residence time, melt environment, and drying environment have all been found to affect the degradation of PET. Analysis of inherent-viscosity kinetic data has been carried out, utilizing existing theories based on a random chain scission mechanism. Activation energies of 117 and 159 kJ/mol have been calculated for air-dried and vacuum dried samples respectively.     

Thermal stability of polyquinolineterphenyl fibres

Conclusions The build-up of internal stresses in polyquinolineterphenyl fibres during spinning and heat-stretching causes a decrease in their thermal stability. The formic acid used as a solvent in fibre prepration brings about a similar action.The thermal stability of the fibre can be increased if it is subjected to the action of temperature under a current of inert gas.Translated from Khimicheskie Volokna, No. 4, pp. 20–22, July–August, 1988.     

Thermal stability of polymerization reactors

Temperature rises which occur in the early stages of polymerization in batch reactors are discussed. Comparisons between experimental results and a reactor model shows that realistic predictions of temperature rises can be made when the model allows for changes in density and specific heat of the reaction fluid. In the case of styrene polymerization, the neglect of density and specific heat changes leads to the prediction of large temperature increases, which are not found in practice. When allowance for these changes in physical properties is made, agreement between theoretical prediction and experimental results is good.     

Thermal stability of grafted fibers

This paper presents the experimental results on the study of thermal stability of grafted fibers, i.e., polypropylene-, polyester-, and rayon-grafted fibers. These fibers were obtained by radiation grafting processes using hydrophylic monomers such as 1-vinyl 2-pyrrolidone, acrylic acid, N-methylol acrylamide, and acrylonitrile. The thermal stability of the fibers was studied using a Shimadzu Thermal Analyzer DT-30. It was found that the thermal stability of the fibers, which can be indicated by the value of the activation energy for thermal degradation, was not much improved by radiation grafting. The degree of improvement depends on the thermal stability of the monomers used for grafting. The thermal stability of a polypropylene fiber, either a grafted or an ungrafted one, was found to be inferior compared to the polyester of a rayon fiber, which may be due to the lack of C?O and C?C bonds in the polypropylene molecules. The thermal stability of a fiber grafted with acrylonitrile monomer was found to be better than that of an ungrafted one. However, no improvement was detected in the fibers grafted with 1-vinyl 2-pyrrolidone monomer, which may be due to the lower thermal stability of poly(1-vinyl–2-pyrrolidone), compared to the polypropylene or polyester fibers.     

Thermal stability of phosphonitrilic fluoroelastomers

The thermal stability of the polymer prepared by substituting poly(dichlorophosphazene) with an excess of a 64/36 mole-% mixture of the sodium salts of 2,2,2-trifluoroethanol/2,2,3,3,4,4,5,5-octafluoropentanol were studied by thermogravimetric and gel permeation chromatography techniques. The primary mode of degradation of this polymer was shown to be via random scission. The overall degradation reaction corresponds to quasi-zero-order kinetics.     

Thermal stability and flammability of organic polymers

Attempts have been made to assess the importance of the thermal instability of organic polymers as a factor governing their flammability. In general, however, there is little correlation between the susceptibility of polymers to undergo thermal decomposition and the readiness with which these materials burn. Although certain additives which exhibit flame-retardant activity do raise the temperature at which polymers start to break down, the ability of these compounds to increase polymer stability makes no appreciable contribution to their inhibiting action on the combustion of polymeric materials. Nevertheless studies of the thermal behaviour of additives incorporated into polymers can provide useful information aboút the interactions which occur between polymers and additives during the combustion process.