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  • Chemistry  (14)
  • 60J65  (2)
  • SPACE SCIENCES
  • 1990-1994  (16)
  • 1
    Electronic Resource
    Electronic Resource
    On the existence of positive solutions of semilinear elliptic equations with Dirichlet boundary conditions (1994)
    Springer
    Mathematische Annalen 298 (1994), S. 543-556 
    Details
    ISSN: 1432-1807
    Keywords: 35J65 ; 60J65 ; 53C21
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01459750
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  • 2
    Electronic Resource
    Electronic Resource
    Existence and uniqueness of semimartingale reflecting Brownian motions in an orthant (1993)
    Springer
    Probability theory and related fields 96 (1993), S. 283-317 
    Details
    ISSN: 1432-2064
    Keywords: 60J60 ; 60J65 ; 60G44 ; 60K25 ; 58G32
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Summary This work is concerned with the existence and uniqueness of a class of semimartingale reflecting Brownian motions which live in the non-negative orthant of ℝ d . Loosely speaking, such a process has a semimartingale decomposition such that in the interior of the orthant the process behaves like a Brownian motion with a constant drift and covariance matrix, and at each of the (d-1)-dimensional faces that form the boundary of the orthant, the bounded variation part of the process increases in a given direction (constant for any particular face) so as to confine the process to the orthant. For historical reasons, this “pushing” at the boundary is called instantaneous reflection. In 1988, Reiman and Williams proved that a necessary condition for the existence of such a semimartingale reflecting Brownian motion (SRBM) is that the reflection matrix formed by the directions of reflection be completely-L. In this work we prove that condition is sufficient for the existence of an SRBM and that the SRBM is unique in law. It follows from the uniqueness that an SRBM defines a strong Markov process. Our results have potential application to the study of diffusions arising as approximations tomulti-class queueing networks.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01292674
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  • 3
    Electronic Resource
    Electronic Resource
    Cure of epoxy resins containing epoxidized polyols (1992)
    Stamford, Conn. [u.a.] : Wiley-Blackwell
    Polymer Engineering and Science 32 (1992), S. 836-840 
    Details
    ISSN: 0032-3888
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: A commercial epoxy resin, consisting of a mixture of diepoxides based on diglycidyl ether of bisphenol-A (DGEBA) and containing an epoxidized polypropylene glycol as reactive diluent, was characterized by 1HNMR, FTIR, SEC, and chemical analysis. The kinetics of the cure with ethylenediamine (EDA) was catalyzed by the (OH) groups present in a large amount in the commercial formulation. A second order kinetic behavior gave an accurate fitting of results obtained by different experimental techniques (DSC in dynamic and isothermal modes and SEC in the pregel stage). The activation energy was E = 59.1 kJ/mol (14.1 kcal/mol), in very close agreement with values reported for the catalytic mechanism of the DGEBA-EDA polymerization. From the gel conversion and the critical stoichiometric ratio for samples containing an epoxy excess it was found that the average functionality of epoxidized species was f = 1.58.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/pen.760321211
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  • 4
    Electronic Resource
    Electronic Resource
    Criteria for selecting cure cycles in autoclave processing of graphite/epoxy composites (1990)
    Stamford, Conn. [u.a.] : Wiley-Blackwell
    Polymer Engineering and Science 30 (1990), S. 1140-1145 
    Details
    ISSN: 0032-3888
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Room temperature mechanical properties, such as flexural strength and impact resistance, of epoxies and graphite/epoxy composites go through a maximum as a function of epoxy conversion. For tetraglycidyl 4,4′-diaminodiphenylmethane (TGDDM)-4,4′-diamlnodiphenylsulfone (DDS) formulations, the recommended cure cycle prescribes a maximum temperature close to 177°C. The maximum extent of reaction that may be obtained at this temperature is determined from the vitrification curve. At this maximum conversion, balanced mechanical and physical properties are attained in the partially cured specimen. However, if the standard cycle is used to cure thick parts, the maximum temperature inside the sample increases beyond 177°C. This leads to a complete conversion in most of the part and a consequent impairment of resulting physical and mechanical properties. It is shown how numerical solutions of differential energy and mass balances may be used to propose alternative cure cycles such that the maximum conversion at every point remains bounded by the vitrification curve. An illustration for a particular thickness is provided.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/pen.760301805
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  • 5
    Electronic Resource
    Electronic Resource
    Glass transition temperature versus conversion relationships for thermosetting polymers (1990)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part B: Polymer Physics 28 (1990), S. 85-95 
    Details
    ISSN: 0887-6266
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Owing to enthalpy relaxation, values of the glass transition temperature (Tg) for partially reacted polymers may depend on the thermal history of samples and the heating rate used for measurements. Use of theoretical relations between Tg and the extent of reaction (x) of a thermoset must take this fact into account. The original DiBenedetto equation has been reevaluated as a convenient constitutive equation for expressing Tg versus x. An extension of Couchman's approach for the expression of the compositional variation of Tg enabled us to derive the same functionality as given by the DiBenedetto equation. Thus, the DiBenedetto equation may be regarded as based on entropic considerations applied to a model of the thermosetting polymer consisting of a random mixture of a fully reacted network with the initial monomers in an amount which depends on the particular conversion level. These two equations have been applied with success to different diepoxy-diamine copolymers.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/polb.1990.090280107
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  • 6
    Electronic Resource
    Electronic Resource
    The glass transition temperature of nonstoichiometric epoxy-amine networks (1991)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part B: Polymer Physics 29 (1991), S. 1503-1511 
    Details
    ISSN: 0887-6266
    Keywords: glass transition of non-stoichiometric epoxy/amine networks ; epoxy/amine non-stoichiometric networks, Tg of ; networks of non-stoichiometric epoxy/lamine, Tg of ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Glass transition temperatures (Tg) of nonstoichiometric epoxy-amine networks based on the diglycidylether of bisphenol A (DGEBA), are analyzed in terms of the network structure. In most cases reasonable predictions of Tg can be made using an empirical equation reported by L. E. Nielsen together with the experimental Tg value of the stoichiometric network and statistical calculations of the concentration of elastic chains. It is stated that in these rigid networks the concentration of elastic chains is the main structural factor associated to the variations of Tg with stoichiometry. For flexible networks based on the diglycidylether of butanediol (DGEBD), the effect of elastic chains on the Tg value is much less significant.
    Additional Material: 11 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/polb.1991.090291207
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  • 7
    Electronic Resource
    Electronic Resource
    Rubber-modified epoxies. III. Analysis of experimental trends through a phase separation model (1991)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Applied Polymer Science 42 (1991), S. 717-735 
    Details
    ISSN: 0021-8995
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: A phase separation model was used to simulate the morphologies obtained in a system consisting of a diepoxide based on bisphenol-A diglycidylether cured with a cycloaliphatic diamine, in the presence of an epoxy-terminated butadiene-acrylonitrile random copolymer (ETBN). A detailed analysis of experimental factors affecting resulting morphologies was previously reported. The model, based on a thermodynamic description through a Flory-Huggins equation, and constitutive equations for polymerization and phase separation rates, could explain most of the observed trends. A nucleation-growth mechanism was believed to take place because of the very low values of interfacial tensions for this type of systems. Conditions which would lead to spinodal demixing are also discussed.
    Additional Material: 27 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/app.1991.070420316
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  • 8
    Electronic Resource
    Electronic Resource
    Citric acid-diethylenetriamine salts as latent curing agents for epoxy resins (1992)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Applied Polymer Science 45 (1992), S. 607-610 
    Details
    ISSN: 0021-8995
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Citric acid (CA)-diethylenetriamine (DETA) salts (CADETA) were prepared by using a 4.5 : 1 molar ratio of DETA-CA and removing the DETA excess. The structure of CADETA was analyzed by 13C-NMR, IR, and DSC associated with weight loss. One-step formulations consisted on dispersions of CADETA (variable amounts) in an epoxy resin based on diglycidylether of bisphenol A (DGEBA, EEW = 185.5 g/eq). The cure was followed in the pressure cell of a DSC (N2 at 2.5 MPa), to avoid volatilization of DETA in the temperature range where decomposition of CADETA and beginning of reaction took place (T 〉 175°C). A very small heat of reaction was observed, (-ΔH) ∼ 10 kJ/eq, resulting from the simultaneous endothermic salt decomposition and exothermic network formation. A stoichiometric formulation showed a Tg = 180°C, i.e., some 60°C higher than the one observed for the usual DGEBA/DETA system.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/app.1992.070450407
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  • 9
    Electronic Resource
    Electronic Resource
    Cure of epoxy novolacs with aromatic diamines. I. Vitrification, gelation, and reaction kinetics (1993)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Applied Polymer Science 47 (1993), S. 1361-1371 
    Details
    ISSN: 0021-8995
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: The cure reaction of a commercial epoxidized novolac with 4,4' diaminodiphenylsulfone (DDS) was studied at constant cure temperatures in the range 120-270°C, as well as at constant heating rates (differential scanning calorimetry, DSC). Stoichiometric formulations did not attain complete conversion due to the presence of topological restrictions. The limiting conversion was xmax = 0.8. Samples containing an amine excess (≥ 20%) could be completely reacted, whereas this was not possible for formulations containing an epoxy excess. Samples containing a 20% amine excess showed the maximum value of the glass transition temperature (Tg230°C). Cure took place by epoxy-amine hydrogen reactions catalyzed by (OH) groups. A reactivity ratio of secondary to primary amine hydrogens equal to 0.2 was found. The activation energy was E = 61 kJ/mol, as arising from Tg versus time shift factors and time to gel measurements. A unique relationship between Tg and x could be obtained. Gelation took place at xgel = 0.45 and the maximum Tg for the stoichiometric system was Tgmax = 215°C for x = 0.8. A conversion versus temperature transformation diagram was used to represent conditions where gelation, vitrification, degradation, and topological limitations took place. © 1993 John Wiley & Sons, Inc.
    Additional Material: 13 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/app.1993.070470806
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  • 10
    Electronic Resource
    Electronic Resource
    The cure of epoxy novolacs with aromatic diamines. II. Statistical parameters of the resulting networks (1993)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Applied Polymer Science 47 (1993), S. 1373-1379 
    Details
    ISSN: 0021-8995
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: The buildup of the network arising from the reaction of an epoxy novolac (derived from a phenol - formaldehyde novolac resin), with 4,4'diaminodiphenylsulfone (DDS), was modeled using a kinetic-statistical procedure that takes into account (i) the complex structure of the starting novolac, (ii) the actual polymerization kinetics including the different reactivity of primary and secondary amine hydrogens, and (iii) the formation of intramolecular cycles. Predictions of gel conversion and sol fraction were in excellent agreement with experimental results. Glass transition temperatures Tg's of networks prepared at different stoichiometries were predicted using Nielsen's equation (J. Macromol. Sci.-Rev. Macromol. Chem., C3, 69, 1969) and the concentration of elastic chains arising from the model. Theoretical predictions explained the presence of a maximum in the experimental Tg for formulations containing a 20% amine excess. Experimental values of Tg as a function of conversion, for stoichiometric samples, were correlated using a model proposed in the literature. © 1993 John Wiley & Sons, Inc.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/app.1993.070470807
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