Showing papers on "Thermal stability published in 1991"

Structural and thermodynamic consequences of burying a charged residue within the hydrophobic core of T4 lysozyme.

Abstract: To determine the energetic and structural consequences of placing a charged group within the core of a protein, two "buried charge" mutants, Met 102----Lys (M102K) and Leu 133----Asp (L133D) were constructed in phage T4 lysozyme. Both proteins fold at neutral pH, although they are substantially less stable than wild type. The activity of M102K is about 35% that of wild type, while that of L133D is about 4%. M102K could be crystallized, and its structure was determined at high resolution. The crystal structure (at pH 6.8) of the mutant is very similar to that of wild type except for the alpha-helix that includes residues 108-113. In wild-type lysozyme, one side of this helix is exposed to solvent and the other contacts Met 102. In the M102K structure this alpha-helix becomes much more mobile, possibly allowing partial access of Lys 102 to solvent. The stability of M102K, determined by monitoring the unfolding of the protein with CD, is pH-dependent, consistent with the charged form of the substituted amino acid being more destabilizing than the uncharged form. The pKa of Lys 102 was estimated to be 6.5 both by differential titration and also by NMR analysis of isotopically labeled protein with 13C incorporated at the C epsilon position of all lysines. As the pH is lowered below pH 6.5, the overall three-dimensional structure of M102K at room temperature appears to be maintained to pH 3 or so, although there is evidence for some structural adjustment possibly allowing solvent accessibility to the protonated form of Lys 102.

The effects of Cr additions to binary TiAl-base alloys

Abstract: The effects of Cr additions to y-base alloys have been investigated, using bulk materials consolidated from rapid solidification-processed ribbons. The composition ranges studied were 0 to 4 at. pet Cr and 44 to 54 at. pet Al. It was found that Cr additions do not affect the deformation behavior of single-phase γ alloys. However, they significantly enhance the plasticity of Al-lean duplex alloys which contain grains of single-phase γ and grains of lamellar γ/α2. Other Cr effects on microstructure, phase stability, site occupancy, and deformation sub-structures were characterized and correlated to the observed mechanical behavior. It was concluded that the ductilization effect of Cr in duplex alloys is partially due to the tendency of Cr to occupy Al lattice sites. Ductilization is also partially due to the ability of Cr to modify the Al partitioning and, therefore, the thermal stability of transformed α2 laths.

Properties and crystallization behaviors of TeO2LiNbO3 glasses

Abstract: (100−x)TeO2−xLiNbO3 (x = 10−60) glasses were prepared by using a conventional melt-quenching method, and some properties such as thermal stability, density and refractive index and crystallization behaviors of glasses were measured. The thermal stability of the glasses with x = 20 and 30 was high. The density of glasses decreased monotonically with increasing LiNbO3 content from 5.50 g/cm3 to 4.58 g/cm3. The values of the refractive index of glasses were 2.04-2.11 and showed an anomalous compositional dependence. It was found from Raman scattering spectra that the structure of glasses was composed of the structural units of TeO4 trigonal bipyramid, TeO3 trigonal pyramid and NbO6 octahedra. In the glasses with x = 10 and 20, α-TeO2 (paratellurite) crystals were precipitated by heat treatment above 450°C. In the glasses with x ≧ 40, LiNbO3 crystals were formed through the transformation of a metastable pyrochlore-type compound at temperatures above 500°C. From the present study, it is proposed that the fabrication of transparent TeO2-based glasses containing ferroelectric LiNbO3 microcrystallites may be possible.

Laminates of polymers having perfluorocyclobutane rings and polymers containing perfluorocyclobutane rings

Abstract: A laminate has at least two layers, at least one of which comprises a polymer having more than one perfluorocyclobutane group. Such polymers impart qualities of enviornmental or protection, chemical and solvent resistance, hydrolytic stability, lubricity, low dielectric, hydrostatic stability, weatherability, flame resistance, chemical resistance, hydrolytic stability, lubricity, environmental protection, scratch resistance, solvent resistance, surface passivation, water repellancy, lower surface refractive index, lower surface coefficient of friction, fluid barrier properties, oil repellancy, thermal stability, and/or reduced moisture pick-up. Additionally, the coatings are optically clear, easy to apply either neat, in a solvent or otherwise, have relatively low cure temperatures for their temperature resistance, and exhibit insulating and planarizing capabilities.

Catalysis by polyoxometalates. Part 3.—Influence of vanadium(V) on the thermal stability of 12-metallophosphoric acids from in situ infrared studies

Abstract: The 12-molybdo(tungsto)phosphoric acids, and the related compounds 1-vanado-11-molybdo(tungsto)phosphoric acids, all having the Keggin structure, have been studied at different temperatures in a special infrared cell allowing experiments in a controlled atmosphere. The effects of dehydration and the thermal stability are discussed. The anhydrous acids are evidenced only for the two 12-molybdo- and 12-tungsto-phosphoric acids: for the vanado compounds, the results are in agreement with the release of vanadium from the host Keggin structure, when increasing the temperature and duration of the treatments in a dry atmosphere. The effect of exposure to water vapour after the thermal treatments is also discussed.

High-temperature characteristics of nylon-11 and nylon-7 piezoelectrics

Abstract: : The piezoelectric strain constant, d31, stress constant, e31, and the electromechanical coupling coefficients, k31, of Nylon-11 and Nylon-7 films were measured up to a temperature close to their melting points. They were found to exhibit their highest values and, in addition, excellent thermal stability at temperatures between 100 C and 200 C. Maximum stable values were d312= 14pC/n and e31 = 21 mC/m2 for Nylon-11, and d31 = 17 pC/N and e31 = 27 mC/m2 for Nylon- 7. Corresponding values of k31 were 0.49 and 0.54. Poled and then annealed Nylon samples exhibit little decay of these values even after annealing at 185 C and 200 C for Nylon-11 and Nylon-7, respectively while poly(vinylidene fluoride) samples showed a large decay in response, decreasing to 4.9 pC/N, 11 mC/m, and k = 0.019 after annealing at 175 deg. C. The extraordinary stability of the piezoelectric activity of these Nylon samples was considered to be related to ferroelectric reorientation of the amide groups dipoles followed by closer packing of the hydrogen bonded sheet structure in the crystalline regions after the annealing treatment.

Conjugated ionic polyacetylenes. 3. Polymerization of ethynylpyridinium salts

Abstract: Les polymeres (CR 1 =CR 2 ) n (R 1 =H, C 5 H 4 N + R, R 2 =C 5 H 4 N + R, R=C 12 H 25 , C 2 H 5 ) et (CR 1 =CR 2 -CR 2 =CR 1 ) n (R 1 =C 5 H 4 N + (C 12 H 25 ); R 2 =(C=CH 2 CH 3 sont prepares par polyaddition de l'ethylnylpyridine ou de bis-pyridines avec un bromure d'alkyle; ils sont amorphes, solubles dans l'eau et les solvants organiques polaires, plus stables a l'air que le polyacetylene et leur conductivite electrique dont l'etat dope atteint 2.10 -4 -1.10 -2 S/cm

High temperature characteristics of Nylon-11 and Nylon-7 piezoelectrics

Abstract: : The piezoelectric strain constant, d31, stress constant, e31, and the electromechanical coupling coefficients, k31, of Nylon-11 and Nylon-7 films were measured up to a temperature close to their melting points. They were found to exhibit their highest values and, in addition, excellent thermal stability at temperatures between 100 C and 200 C. Maximum stable values were d312= 14pC/n and e31 = 21 mC/m2 for Nylon-11, and d31 = 17 pC/N and e31 = 27 mC/m2 for Nylon- 7. Corresponding values of k31 were 0.49 and 0.54. Poled and then annealed Nylon samples exhibit little decay of these values even after annealing at 185 C and 200 C for Nylon-11 and Nylon-7, respectively while poly(vinylidene fluoride) samples showed a large decay in response, decreasing to 4.9 pC/N, 11 mC/m, and k = 0.019 after annealing at 175 deg. C. The extraordinary stability of the piezoelectric activity of these Nylon samples was considered to be related to ferroelectric reorientation of the amide groups dipoles followed by closer packing of the hydrogen bonded sheet structure in the crystalline regions after the annealing treatment.

Synthesis of functionalized polypropylene

Abstract: La copolymerisation de Ziegler-Natta du propene avec le 6-(9-borabicyclo [331]nonane) hex-1-ene suivie d'un traitement des copolymeres par H 2 O 2 par le systeme NaI/chloramine T donne respectivement les copolymeres propene/1-hexen-6-ol et propene/6-iodo-1-hexene dont les proprietes (stabilite thermique, point de fusion, cristallinite) sont tres similaires a celles du poypropene isotactique

Synthesis, Thermal Properties, and Gas Permeability of Poly(N-n-alkylmaleimide)s

Abstract: Radical polymerization of N-n-alkylmaleimides (nRMI) with various n-alkyl groups (n=4–18) was carried out in the presence of 2,2′-azobisisobutyronitrile in benzene at 60°C. The high molecular weight polymer (Mn>105) was obtained in a high yield from all nRMIs. The resulting poly(nRMI) were soluble in organic solvents such as benzene, chloroform, and THF, and a thin and tough film was obtained from the solution. It was confirmed that poly(nRMI) showed excellent thermal-stability, i.e., they had a high glass transition temperature and decomposition temperatures. The side-chain crystallization was investigated by differential scanning calorimetry. From the measurement of permeation of oxygen and nitrogen through poly(nRMI), it was revealed that these polymers had a high permeability coefficient of 10−8–10−9 cm3(STP)cm cm−2 s−1 cm Hg−1, which depended intensively on the N-substituents.

Pyrolysis of polyvinylsilazane precursors to silicon carbonitride

Abstract: The pyrolysis of liquid or solid polyvinylsilazanes, cross-linked according to four different routes, was studied in the 20–1400°C temperature range. The similar behaviour and the high ceramic yields (⋍85%) are related to the reactivity of the functional groups (CHCH2, SiH, NH) which lead to highly cross-linked solids before the mineralization step. The major gases evolved above 450°C are hydrogen, methane and ethylene. During the thermal treatment SiN bonds are unaffected. The silicon carbonitride samples are dense and amorphous up to 1400°C. The thermal stability of the microtexture is due to the high carbon content.

Polyvinylidene Fluoride (PVDF) Appearance, General Properties and Processing

Abstract: SOLEF1 polyvinylidene fluoride (PVDF or PVF2). characterized by a high crystallinity, possesses certain basic properties shared by all the varieties of PVDF: chemical inertness, thermal stability, ...

Structural modifications of hydroxyapatite induced by lead substitution for calcium

Abstract: Continuous series of solid solutions of lead and calcium hydroxyapatites have been prepared by solid-state reaction as well as from aqueous solutions and characterized by X-ray diffraction and IR spectroscopic analyses. While the lattice constant a of the solid solutions varies linearly with composition, the variation of the lattice constant c does not follow Vegard's law but shows a discontinuity at about 50% lead atom content. A similar discontinuity has been observed in the variation of the IR frequencies with composition. Although lead incorporation seems to destabilize the apatite structure, the solid solutions prepared from aqueous media at a lead content around 50% exhibit a high thermal stability. The variation of c, thermal stability, as well as the frequencies and intensities of the IR bands of lead–calcium hydroxyapatite are interpreted on the basis of the inhomogeneous lead distribution in the two non-equivalent cation sites of the hydroxyapatite structure.

On-line solid phase extraction - thermal desorption for introduction of large volumes of aqueous samples into a gas chromatograph

Abstract: A new approach to the introduction of large aqueous samples into a gas chromatograph, solid phase extraction - thermal desorption, is presented. Carrier gas pushes the sample through a packed liner mounted in a programmed temperature vaporizer; analytes retained by the packing material are thermally desorbed after drying of the adsorbent. The sorption properties and thermal stability of some packing materials have been studied off-line. Tenax GC®, and octyl-modified silica silylated with diphenyltetramethyldisilazane to improve its thermal stability, appeared to be suitable materials. The drying period and the desorption temperature are critical for satisfactory performance of the method. When Tenax GC® is used as packing material, thermal desorption at 250°C for 15 min gives quantitative recovery for methyl esters up to hexacosanoic acid methyl ester. With the silylated octyl-modified silica, the application range is limited to the methyl esters of decanoic to octadecanoic acids.

Hydrogen stability in amorphous germanium films

Abstract: The thermal stability of hydrogen in hydrogenated amorphous germanium films has been studied by hydrogen evolution and by deuterium and hydrogen interdiffusion experiments. Similar to observations on hydrogenated amorphous silicon, the hydrogen stability in amorphous germanium is found to depend strongly on the film microstructure and on the position of the Fermi level.

Thermal and hydrothermal stability of molecular sieve VPI-5 by in situ x-ray powder diffraction

Abstract: The framework stability of the molecular sieve VPI-5 is studied at various conditions of temperature, pressure, and humidity. Dehydration of VPI-5, by either rapid heating or pressure reduction, leads to a contraction in «a» and an expansion in «c» (hexagonal cell). These changes are reversible upon rehydration. The transformation of VPI-5 into AlPO 4 -8 depends on the rate at which water is removed from the pores. Slow heating results in the irreversible formation of AlPO 4 -8 from VPI-5 at about 100°C. However, VPI-5 rapidly heated to at least 550°C can withstand exposure to steam at these elevated temperatures with little loss of structural integrity

Preparation of substantially crystalline silicon carbide fibers from polycarbosilane

Abstract: This invention relates to the preparation of thermally stable, substantially polycrystalline silicon carbide ceramic fibers derived from a polycarbosilane resin. The unexpected thermal stability of these fibers is achieved by the incorporation of boron prior to or during ceramification.