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  • Articles  (223)
  • Chemistry  (203)
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  • 1990-1994  (180)
  • 1955-1959  (43)
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  • Articles  (223)
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  • 1
    Electronic Resource
    Electronic Resource
    Stamford, Conn. [u.a.] : Wiley-Blackwell
    Polymer Engineering and Science 33 (1993), S. 1110-1121 
    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: The dielectric behavior of glassy thermoplastic polymer in the microwave frequency range was investigated. Specifically, the relationship between energy absorption and temperature for several thermoplastic systems was examined to test the theoretical basis for heating under microwave irradiation. Irradiation under traveling and standing wave conditions were explored. The heating rate versus temperature data at a frequency of 2.45 GHz yielded a microwave calorimetry procedure for examination of the dielectric and relaxation behaviors. Correlations were drawn between (a) the apparent activation energy and the critical temperature, and (b) the shape of the dielectric spectra at 2.45 GHz and its shape in the kHz region. WLF relationships were examined for glassy thermoplastics to show the difference in changing activation energy with temperature.
    Additional Material: 17 Ill.
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  • 2
    Electronic Resource
    Electronic Resource
    Stamford, Conn. [u.a.] : Wiley-Blackwell
    Polymer Engineering and Science 33 (1993), S. 1092-1109 
    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: The objective of this effort has been to investigate the relationship between polymer structure and microwave absorptivity. Dielectric loss factor, ε″, loss tangent, tan δ, and oscillator strength, (εS -- ε∞), were used to evaluate potential material processability under applied microwave radiation. Numerous polymeric materials varying in chemical and physical structures were irradiated in a low power (≤ 100W) electric field at 2.45 GHz. Electromagnetic radiation was applied as either traveling or resonant wave modes in cylindrical and rectangular waveguides. In general, heatability was found to be a direct function of the dielectric loss dispersion dependence on temperature and frequency. The dielectric loss factor obtained at low frequency measurements was found to be directly proportional to the heatability of polymers. A WLF plot was used to predict the shift of dielectric loss maxima into or out of the microwave frequency range.
    Additional Material: 19 Ill.
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  • 3
    Electronic Resource
    Electronic Resource
    Stamford, Conn. [u.a.] : Wiley-Blackwell
    Polymer Engineering and Science 33 (1993), S. 1122-1131 
    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: The overall objective of these studies was to investigate the relationship between polymer structure and microwave absorptivity. In this paper, the microwave processing of semicrystalline polymers such as poly(ether ether ketone) (PEEK), nylons, and poly(ethylene terephthalate) (PET), via a cylindrical resonance wave cavity and a rectangular standing wave applicator is described. These polymeric materials were irradiated in low power (〈 50W) electric fields at 2.45 GHz. Silicone flexible molds were necessary for improved processing of nylons and PEEK at temperatures below their Tc Rapid heating rates were observed between the glass transition temperature, Tg, and the melting temperature, Tm, for all these polymers provided that Tc was exceeded. Both dynamic mechanical thermal analysis (DMTA) and dielectric thermal analysis (DETA) spectra were utilized to predict the heating phenomena between amorphous and semicrystalline materials and to explain the rapid crystallizing rate of PEEK. above its glass transition temperature. Correlations were drawn between (a) the apparent activation energy and the critical temperature (Tc) and (b) the shape of the dielectric spectra at 2.45 GHz and its shape in kHz region.
    Additional Material: 15 Ill.
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  • 4
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Chemical Kinetics 23 (1991), S. 197-202 
    ISSN: 0538-8066
    Keywords: Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We have performed a product and kinetic study of the hydrolysis of 3,4-diphenyl-1,2,5-thiadiazole-1,1-dioxide in aqueous solution. Benzil and sulfamide are the only products of hydrolysis and are formed in equimolar yields.The kinetic results indicate that a first order law is followed up to 90% conversion. The observed rate constant is independent of substrate concentration.Ionic strength and buffer concentration do not affect the rate constant, but the reaction is acid-base catalyzed. The rate-pH profile has been determined and a mechanism that fits the experimental data satisfactorily is proposed. Corresponding rate constants and equilibrium constants for the protonation of the substrate are reported.
    Additional Material: 1 Ill.
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  • 5
    ISSN: 0003-3146
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Molecular theories of rubberlike elasticity are surveyed, starting with the earliest attempts carried out in the 1930's and ending with work still very much in progress. The models are variously based on phenomenological arguments, affinely-deforming networks, phantom networks, constrained junctions, constrained chains, slip-links, molecular tubes, van der Waals concepts, or rotational isomeric chains. Most of the theories are analytical, but some involve computer simulations based on Monte Carlo methods. The theories are generally for networks of Gaussian chains, but there have been significant advances for non-Gaussian networks as well, and these are particularly important with regard to a better understanding of ultimate properties. The major outstanding problem on the theory side, however, seems to be a quantitative treatment of chain entanglements.There have also been numerous advances on the experimental side. These include preparation of model networks of known structure, accumulation of elasticity data for deformations other than the much-studied simple elongation, development of new characterization techniques (such as fluorescence polarization, and X-ray and neutron scattering), investigation of the phenomenon of gel collapse, borrowing of ideas from the area of bioelastomers, and preparation of novel reinforcing fillers.
    Additional Material: 8 Ill.
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  • 6
    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Aggregation of filipin in aqueous medium and filipin-induced changes in cholesterol micelles have been studied using intensity and dynamic light scattering. The dependencies of filipin aggregate dimensions on concentration, solvent, and temperature were studied, and revealed that the aggregates do not have a well-defined geometry, i.e., a critical micelle concentration cannot be detected and stable structures are not formed. The aggregates are of size Rg ≈ 110 nm and Rh ≈ 63 nm, referring to the radius of gyration and hydrodynamic radius, respectively. In the concentration range studied (1 μM 〈 C 〈 30 μM), a low molecular weight species (monomer/dimer) is always present together with the aggregates. In ethanol/ water mixtures, large (Rg ≈ 500 nm), narrow distribution aggregates are formed in the water volume fraction range 0.45 〈 ΦH2O 〈 0.65. Aggregation also occurs on changing the temperature; In the range 7-37°C, smaller aggregates (10-30 nm) form and the process is only partially reversible.No pronounced effect of filipin on the structure of the cholesterol micelles was observed (a small increase in Rg and Rh is noted). These results rule out any “specificity” for the filipin interactions with cholesterol, which has been considered a key event in the filipin biochemical mode of action. A reevaluatiori Of this question is suggested and some alternatives are advanced. © 1994 John Wiley & Sons, Inc.
    Additional Material: 12 Ill.
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  • 7
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Chemie in unserer Zeit 24 (1990), S. V 
    ISSN: 0009-2851
    Keywords: Chemistry ; Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 8
    Electronic Resource
    Electronic Resource
    Stamford, Conn. [u.a.] : Wiley-Blackwell
    Polymer Engineering and Science 32 (1992), S. 1372-1378 
    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: Polymer coatings are widely used to protect glass from indentation damage. A model for the strength degradation that occurs when a sharp indenter penetrates through the coating is developed by accounting for the indentation load shared by the coating and substrate. This model accounts for the additional load supported by the coating due to the pile-up of coating material underneath the indenter. The model predicts the strength degradation as a function of indentation load, coating and substrate hardnesses, and coating thickness. Comparison of the model to experimental data for a wide range of polymer coatings (two epoxies, epoxy acrylate, and urethane acrylate) on soda-lime glass substrates shows good agreement.
    Additional Material: 10 Ill.
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  • 9
    Electronic Resource
    Electronic Resource
    New York : Wiley-Blackwell
    Die Makromolekulare Chemie 192 (1991), S. 499-506 
    ISSN: 0025-116X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Bimodal elastomeric networks are unusually tough materials in that they generally show large increases in modulus at high elongations. In the present study such networks were prepared by end-linking mixtures of very short and relatively long chains of poly(dimethylsiloxane) (PDMS) [—Si(CH3)2O—]n, which is known to have easily deformable Si—O—Si bonds. Their stress-strain isotherms were then determined in order to locate the range of elongations over which the upturns were most pronounced. Raman and infrared spectra were then obtained in this range using both a double monochromator Raman spectrophotometer and a Fourier transform infrared spectrophotometer with attenuated total reflection. The elongations imposed were found to cause small but not negligible frequency shifts for the stretching and bending modes of the Si—O—Si backbone. These shifts are probably due to rotational isomerization of the PDMS skeletal bonds, but there could also be a small contribution from bond-angle deformations. The results thus support the usual assumption that the increases in modulus observed for bimodal networks are primarily due to the very limited number of spatial configurations available to the short network chains, particularly in the case of chains having skeletal bonds that are less easily deformed than those in PDMS.
    Additional Material: 7 Ill.
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  • 10
    ISSN: 0887-624X
    Keywords: thermoplastic elastomer ; carbocationic polymerization ; polyisobutylene ; living polymerization ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: New linear triblock thermoplastic elastomers (TPEs) comprising a rubbery polyisobutylene (PIB) midblock flanked by two glassy endblocks of various styrenic polymers have been synthesized by living carbocationic polymerization by sequential monomer addition. First isobutylene (IB) was polymerized by a bifunctional tert-ether (dicumyl methyl ether) initiator in conjunction with TiCl4 coinitiator in CH3Cl/methylcyclohexane (MeCHx) (40/60 v/v) solvent mixtures at -80°C. After the living narrow molecular weight distribution PIB midblock ($\[\bar M_n\]$ = 1.1-1.2) has reached the desired molecular weight, the styrenic monomers together with an electron pair donor (ED) and a proton trap (di-tert-butylpyridine, DtBP) were added to start the blocking of the glassy segments from the living ⊕PIB⊕ chain ends. While p-methylstyrene (pMeSt), p-t-butylstyrene (ptBuSt) and indene (In) gave essentially 100% blocking to the corresponding glassy endblocks, the blocking of 2,4,6-trimethylstyrene (TMeSt) and α-methylstyrene (αMeSt) were ineffective. Uncontrolled initiation by protic impurities was prevented by the use of DtBP. In the simultaneous presence of DtBP and the strong ED N,N-dimethylacetamide (DMA), TPEs with good mechanical properties (10-20 MPa tensile strength, 300-600% elongation) were prepared. The products exhibit a low and a high temperature Tg characteristic of phase separated rubbery and glassy domains. The service temperature of these new TPEs exceeds that of PSt-PIB-PSt triblock copolymers due to the higher Tgs (PpMeSt = 108, PptBuSt = 142 and PIn = 220-240°C) of the outer blocks. The Tg of the glassy blocks can be regulated by copolymerizing two styrene derivatives; a triblock copolymer with outer blocks of poly(pt-butylstyrene-co-indene) showed a single glassy transition Tg = +165°C, i.e., in between that of PptBuSt and PIn. Virgin TPEs have been repeatedly compression molded without deterioration of physical properties. The high melt flow index obtained with a TPE containing PptBuSt endblocks suggests superior processability relative to those with PSt end-blocks. The tensile strength retention at 60°C of the former TPE is far superior to that of a PSt-PIB-PSt triblock of similar composition.
    Additional Material: 4 Ill.
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