Showing papers on "Thermal stability published in 1977"

Induced Smectic Mesomorphism by Incompatible Nematogens

Abstract: Non-linear behaviors of nematic-isotropic transition temperatures and the appearance of the “solid-like” phase found in the mixtures between MBBA and 4-cyano-4-n-pentylbiphenyl (CPB)1 are now found to be quite general among many other nematogens. The cyano-group containing nematogens do not have to be biphenyl derivatives, nor do the “donner” type molecules need to be limited to Schiff Bases and azo-type compounds. This type of phase diagram has also been obtained in mixtures of CPB with phenyl benzoates, azoxy and tolan nematogens. Other cyano-group containing nematogens, such as substituted 4′-cyanophenyl benzoates and 4′alkoxybenzylidene-4-aminobenzonitriles gave rise to behavior identical with the above nematogens. A detailed study on the phase diagram of mixtures containing 4-cyano- 4′-n-octyloxybiphenyl and 4′n-hexyloxy-phenyl-4-n-butyl blenzoate has yielded further insight into the nature of the “solid-like” phase. The smectic thermal stability of the alkoxycyanobiphenyl decreased sharply ...

Superconducting and mechanical properties of in situ formed multifilamentary Cu‐Nb3Sn composites

Abstract: A systematic experimental study of the variations of superconducting transition temperature and critical‐current density is reported for in situ formed multifilamentary Cu‐Nb3Sn composites containing 10 at.% Nb and 2–3 at.% Sn annealed at either 650 or 700 °C. A particular emphasis was placed on the evaluation of uniformity, thermal stability, and mechanical strength. Critical‐current density was measured as a function of transverse magnetic field and was found to increase in samples measured in a bent position. The overall critical‐current performance is comparable to that of reinforced stabilized conventional composites. Large residual resistivity ratios are indicative of a clean high‐conductivity matrix surrounding each individual filament, an important requirement for thermal stability. High resistance to plastic flow in these composites is attributed to strong filament‐to‐matrix bonding and small interfilament spacing. The ultimate tensile strength at 77 °K reached a value of ∼100 ksi (690 MPa). The ...

Poly(mono n-alkyl itaconic acid esters): Their preparation and some physical properties†

Abstract: A series of poly(mono n-alkyl itaconic acid esters), with ester chain lengths varying from methyl to decyl, were prepared by emulsion polymerisation. The materials are all hard brittle glassy polymers at ambient temperatures and undergo a dehydration reaction in the range 430 K to 450 K. Dynamic mechanical measurements showed the presence of at least three other damping maxima: an Δ-transition at 90−95 K, a δ-transition at 190–200 K and a γ-transition at 240–260 K. The e is associated with motion in the hydrocarbon part of the side chain, while the δ and γ transitions are associated with relaxations involving the carboxyl and alkoxycarbonyl groups respectively. Glass transitions were only detectable in higher esters. The thermal stability of the polymers is also briefly discussed.

End capped polyalkylene carbonates having improved thermal stablity

Abstract: Polycarbonates, of the type formed by reacting an aliphatic or cycloaliphatic 1,2-monoepoxide with carbon dioxide and having substantially alternating units of epoxide and carbon dioxide, are improved in thermal stability by reacting the free hydroxyl groups thereon with a hydroxyl reactive organic compound.

Method for preparing a thermal-stability additive and a thermally stabilized methylpolysiloxane and compositions therefrom

Abstract: Endblocked methylpolysiloxane fluids, such as trimethylsiloxane-endblocked polydimethylsiloxanes, are made more resistant to thermally induced siloxane rearrangement by incorporating therein certain amounts of titanium-, zirconium-, or hafnium-containing compounds. Incorporation of these compounds may be accomplished either by mixing organosiloxy derivatives of Ti, Zr, or Hf with the fluid or by mixing certain organic derivatives of Ti, Zr, or Hf with the fluid and heating the mixture to decompose the added organic derivative. Furthermore, incorporation may be accomplished either by mixing the correct amount of the Ti, Zr, or Hf compound with the fluid or by mixing an excessive amount of the Ti, Zr, or Hf compound with the fluid to form a thermal-stability additive which may be diluted with additional fluid to achieve the correct amount of Ti, Zr, or Hf compound. Further improvements in the thermal stability of the fluid may be obtained by mixing with the titanium-, zirconium-, or hafnium-containing fluid a small amount of an organosilicon hydride compound. The thermally stabilized fluids are particularly useful as heat-transfer fluids in non-oxidative, anhydrous environments.

Study on wholly aromatic polyamides containing methyl‐substituted phenylene linkage

Abstract: In order to study the influence of chemical modification on properties of polyamides, wholly aromatic polyamides have been synthesized from p-phenylenediamine, its 2,5-dimethyl and 2-methyl derivatives, m-phenylenediamine, its 2-methyl and 4-methyl derivatives, and terephthaloyl and isophthaloyl dichlorides by solution polycondensation at low temperature. The thermal stability and solubility of the methyl-substituted polyamides were compared with those of unsubstituted ones. The introduction of methyl groups in a polymeric chain led to a decrease in their thermal stability to different degrees depending on the positions of methyl groups, accompanied by an increase in their solubility. The unsymmetrical introduction of methyl groups in benzene rings had a greater effect on the increase in solubility of polyamide than did symmetrical methyl groups.

End capped polyalkylene carbonates

Abstract: Polycarbonates, of the type formed by reacting an aliphatic or cycloaliphatic 1,2-monoepoxide with carbon dioxide and having substantially alternating units of epoxide and carbon dioxide, are improved in thermal stability by reacting the free hydroxyl groups thereon with a hydroxyl reactive phosphorus compound.

Thermal behavior of cotton grafted with vinyl monomers individually and in mixture compositions

Abstract: Thermal analysis of cotton samples grafted with acrylamide, acrylonitrile, methyl acrylate, and methyl methacrylate individually and in mixture compositions has been carried out. Additional endothermic peaks in the DTA curves characteristic of the polymers grafted were observed. Graft copolymerization of acrylamide and acrylonitrile makes cotton thermally more stable, while in the case of methyl acrylate-and methyl methacrylate-grafted cottons, the initial decomposition starts at higher temperatures, but subsequent decomposition is faster and the overall thermal stability is lowered. In the case of binary mixtures of acrylamide and acrylonitrile, inception of decomposition starts earlier, but subsequent decomposition takes place at much higher temperatures than for individual monomer-grafted cottons. Interaction between monomers during grafting is indicated. When fabric samples containing polyacrylamide grafts are methylolated and subsequently cross-linked, there is a reduction in the thermal stability of the treated cotton.

Preparation and properties of polycarbodiimides

Abstract: A series of polycarbodiimides was prepared from isocyanates using 1-phenyl-3-methyl-2-phospholene-1 oxide as catalyst. The molecular weight of the series was varied by employing controlled amounts of phenyl isocyanate as a terminating agent. The resultant polymers showed improved processing over very high molecular weight unterminated polymer, while still maintaining good thermal stability and equivalent mechanical properties. A study of the viscoelastic properties of these polymers using modulus–temperature plots showed that the materials underwent a crosslinking reaction at elevated temperatures (200–250°C). These reactions could be explained on the basis of a model, diphenylarbodiimide, and most likely include the formation of carbodiimide dimers and trimers.

Some aspects of the thermal stability of poly(alkyl itaconic acid esters)

Abstract: The thermal stability of members of several series of polyitaconates has been assessed using thermogravimetric and thermal volatilisation techniques. From these data general degradation patterns have been established. The poly (di-n-alkyl itaconates) are stable up to 570K before chain breakdown occurs. The poly(mono-n-alkyl itaconates) are less stable and undergo a dehydration/de-esterification reaction at 430K which results in the formation of anhydride structures. This reaction can be suppressed by the formation of the salts. In the copolymers, salt formation also leads to increased stability but the range of copolymers studied was too restricted to draw general conclusions about their thermal behaviour.

Thermal stability and molecular weight of two new boron–nitrogen polymers

Abstract: Molecular weight and thermal stability measurements on the presumed polymers of aminoborane, H2BNH2, and aminodifluoroborane, F2BNH2, were explored using a laser light scattering technique for the molecular weight measurements and both differential thermal analyses and thermogravimetric analyses for the thermal stability studies. Weight-average molecular weight of polymeric aminodifluoroborane in water was determined as 23,000 g/mole, whereas the molecular weight of polymeric aminoborane could not be determined since a suitable solvent was not found. Both DTA and TGA observations showed partial decomposition of polymeric aminoborane between 135° and 200°C, and the vaporization of polymeric aminodifluoroborane between 250° and 360°C.

Thermal stability of NH4-chabazite

Abstract: The thermal stability of NH4-chabazite has been examined by i.r. spectroscopy and thermal analysis combined with mass spectrometry. Dehydration occurs in two and deammoniation in four steps. Disregarding a very small water loss during deammoniation, dehydroxylation is completely separable from deammoniation and occurs in two distinct steps. Two OH stretching bands at 3540 and 3630 cm–1 appear during deammoniation and could be related to distinct ammonium ions having bending vibrations at 1420 and 1460 cm–1 respectively. The hydroxyls showing a band at 3540 cm–1 are formed at lower temperatures and are less thermostable than other hydroxyls.

D2‐m‐carborane‐siloxanes. III. Preparation and properties of trifluoropropyl‐modified polymers

Abstract: Linear D2-m-carborane-siloxanes with one, two, and three trifluoropropyl moieties per repeat unit were prepared by the condensation reaction between bisureidosilanes and carborane disilanol. Molecular weights between 100,000 and 220,000 were obtained. Compared to fluorosilicones, the trifluoropropyl-modified carborane-siloxanes exhibited greater thermal and oxidative stability and in certain cases comparable solvent resistance. The swelling index for carborane-siloxane elastomers decreased with increasing trifluoropropyl content, although this fluorocarbon moiety did compromise the thermal stability of the carborane system.

Suspension Polymerization of Vinyl Chloride with Programmed Temperature: Morphological Characteristics and Thermal Stability of Polyvinyl Chloride)

Abstract: Polyvinyl chloride) obtained by intermittent polymerization in suspension with temperature programming, has been characterized from the point of view of morphological properties and of thermal stability. The data have been compared with the values of the morphological characteristics and thermal stability for the PVC samples obtained under identical polymerization conditions, but at constant temperature. The polyvinyl chloride) obtained by polymerization with temperature programming has the same properties as the polyvinyl chloride) obtained by simple polymerization at constant temperature, with an identical average molecular weight. However, the former polymer shows improved thermal stability, as well as a decrease in the volumetric properties.

D2‐m‐carborane siloxanes. V. Structure–property relationships

Abstract: D2-m-decacarborane dimethylsiloxane has unusual thermal stability and resistance towards reversion, however, this material exhibits a crystalline phase, Tm at 68°C. To obtain an elastomeric material, this crystallinity is disrupted by replacing 30 to 50% of the m-carborane with p-carborane moieties or by the incorporation of phenyl moieties on the polymer backbone. The latter approach is preferred since improved thermo-oxidative stability accompanies this modification. Correlation of the glass transition temperature and thermo-oxidative stability with the polymer structure are presented.

Solution grown isotactic polystyrene crystals. I. Degree and distribution of crystallinity; thermal stability

Abstract: The subject of this paper is the degree of crystallinity and annealing behavior of solution grown single crystals of isotactic polystyrene (IPS) in relation to the fold length, an enquiry which acquires special significance in view of the fact that previously the fold length had been found to be identical over a wide range of crystallization temperatures (Tc). It was found that both crystallinity and thermal stability increase with Tc even over the range of constant fold length thus invalidating the usual assumption that the fold length and crystal properties are uniquely correlated. Further, the crystallinity figures as measured by conventional calorimetry are very low (<50% throughout) which by conventional ideas would require an unrealistically thick amorphous surface layer. However, the “linear crystallinity” (crystal core thickness as determined from x-ray linewidths) is much larger, commensurate with crystallinities in single crystals from other materials. It is suggested that this is the more relevant figure for the subdivision of the lamellas into crystal core and surface layer. The additional amorphous content being otherwise accommodated. Further, this “linear crystallinity” is broadly unaffected by fold length changes induced by heat annealing. The thermal stability (including annealing ability) of the crystals differs markedly whether Tc is above or below ∼60°C, a Tc value which is in the range where the fold length is constant, but corresponds to a maximum in the crystallization rate. Possible connections between crystallization conditions and the stability of the resulting crystals (fold length considerations apart) are pointed out.

Interactions of Nonaqueous Solvents with Textile Fibers: Part IX: Thermal Stability of Solvent-Induced Modifications in Polyester Fiber Structure

Abstract: The thermal stability of heat- and dimethylformamide (DMF)-induced structural modifications in polyester yarns has been investigated by differential scanning calorimetry (DSC) and other techniques....