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  • 1
    Electronic Resource
    Electronic Resource
    A critical evaluation of models for complex molecular dynamics: Application to NMR studies of double- and single-stranded DNA (1983)
    New York : Wiley-Blackwell
    Biopolymers 22 (1983), S. 2703-2726 
    Details
    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The nature of internal and overall motions in native (double-stranded) and denatured (single-stranded) DNA fragments 120-160 base pairs (bp) long is examined by molecular-dynamics modeling using 13C-nmr spin-relaxation data obtained over the frequency range of 37-125 MHz. The broad range of 13C frequencies is required to differentiate among various models. Relatively narrow linewidths, large nuclear Overhauser enhancements (NOEs), and short T1 values all vary significantly with frequency and indicate the presence of rapid, restricted internal motions on the nanosecond time scale. For double-stranded DNA monomer fragments (147 bp, 24 Å diam at 32°C), the overall motion is that of an axially symmetric cylinder (τx = ∼10-6 s;τZ = ∼1.8 × 10-8s), which is in good agreement with values calculated from hydrodynamic theory (τx = ∼1.8 × 10-6 s; τZ = ∼2.7 × 10-8 s). The DNA internal motion can be modeled as restricted amplitude internal diffusion of individual C—H vectors of deoxyribose methine carbons C1′, C3′, and C4′, either with conic boundary conditions (τw = ∼4 × 10-9 s, θcone = ∼21°) or as a bistable jump (τA = τB = ∼2 × 10-9 s, θ = ∼15°). We discuss the critical role in molecular-dynamics modeling played by the angle (β) that individual C—H vectors make with the long axis of the DNA helix. Heat denaturation brings about increases in both the rate and amplitude of the internal motion (described by the wobble model with τW = ∼0.2 × 10-9 s, θcone = ∼50°), and overall motion is affected by becoming essentially isotropic (τx = τZ = ∼5 × 10-8 s) for the single-stranded molecules. Since 13C-nmr data obtained at various DNA concentrations for C2′ of the deoxyribose ring is not described well by the above models, a new model incorporating an additional internal motion is proposed to take into account the rapid, extensive, and weakly coupled motion of C2′.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/bip.360221214
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  • 2
    Electronic Resource
    Electronic Resource
    Condensation polymerizations in ideal continuous-flow-stirred tank reactors of monomers violating the equal reactivity hypothesis (1980)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Applied Polymer Science 25 (1980), S. 1049-1058 
    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: Molecular weight distributions, moments and the polydispersity indices were computed for AB-type condensation polymerization in an ideal continuous-flow-stirred tank reactor for monomers not following the equal reactivity hypothesis. The model considers the rate of reaction between monomer and monomer to be R times that characterizing the reaction between any other species in the reaction mass. It is found that the polydispersity indices in these reactors are substantially larger than values obtained in batch reactors. Also, the molecular weight distribution splits into two curves - one for odd-order homologs and the other for even-order ones for R 〉1. For R 〈 1, substantial amounts of unreacted monomer are present in the exit stream, and curve splitting does not occur.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/app.1980.070250607
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  • 3
    Electronic Resource
    Electronic Resource
    Mass transfer effects in polycondensation reactors wherein functional groups are not equally reactive (1982)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Applied Polymer Science 27 (1982), S. 1217-1231 
    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: Three simplified models of polycondensation reactors are considered in which the condensation product is continuously removed by application of vacuum. Reversible polycondensation reactions of monomers violating the equal reactivity hypothesis have been simulated in these reactors. The effect of various rate and reactor design variables on the molecular weight distribution (MWD) and its moments is studed. It is observed that when the reverse reactions are rapid, the results are fairly sensitive to the level of vacuum applied and to the mass transfer resistance; whereas when the forward reactions predominate, results lie very close to earlier plots for the corresponding irreversible polymerizations. These reactor variables then have relatively small influence on the MWD. Splitting of the MWD curves for odd and even values on n is observed under certain conditions, the effects being more pronounced in the presence of mass transfer than in its absence.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/app.1982.070270411
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  • 4
    Electronic Resource
    Electronic Resource
    Simulation of three-stage nylon 6 reactors with intermediate mass transfer at finite rates (1982)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Applied Polymer Science 27 (1982), S. 3089-3101 
    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 three-stage isothermal nylon 6 reactor with a kinetic scheme incorporating ring opening, polycondensation, polyaddition, cyclic dimer formation, and reaction with monofunctional acids has been modeled. In the first and third stages, removal of the condensation by-product, water, is prevented. The second stage of this sequence, however, involves finite rates of diffusion of water to cocurrently flowing inert gas bubbles. the number-average chain length of the polymer obtained in this reactor differs substantially from that obtained assuming instantaneous water removal and is a function of the various design variables. It is observed that several choices of these design variables can be made to obtain the same product, thus emphasizing the need for more comprehensive optimization studies than hitherto carried out.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/app.1982.070270831
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  • 5
    Electronic Resource
    Electronic Resource
    Analysis of wiped film reactors using the orthogonal collocation technique (1984)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Applied Polymer Science 29 (1984), S. 3217-3230 
    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: Transport equation for ARB polymerization in wiped film reactors have been written. These have been reduced to the moment generation equations and using a suitable moment closure approximation, the zeroth and the second moments of the polymer have been numerically solved using the finite difference as well as the orthogonal collocation techniques. In the numerical solution by the finite difference technique, it is necessary to divide the dimensionless film thickness into at least 250 grid points to obtain stable results. The use of nine collocation points by the orthogonal collocation technique gives results close to those by the finite difference method and leads to considerable computational saving. The transport equations for the bulk and the film are found to involve four dimensionless parameters, and their effect upon the polymer formed at the end of the reactor has been studied.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/app.1984.070291101
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  • 6
    Electronic Resource
    Electronic Resource
    Simulation of ARB type reversible step growth polymerization in semibatch reactors (1985)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Applied Polymer Science 30 (1985), S. 445-460 
    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 polymerization of ARB type reversible step growth polymerization in semibatch reactors has been simulated. It is assumed that the condensation byproduct W and the monomer P1 both can vaporize, but P1 is refluxed back to the reactor. Raoults law as well as Flory-Huggins theory have been used to account for vapor-liquid equilibrium. The degree of polymerization, μn, and the polydispersity index DN have been computed for various situations. The results are found to be most sensitive to the total pressure and the equilibrium constant. A limiting total pressure history PT(T) has been obtained, going below which does not help to increase μn. The final set of equations incorporate both equalities as well as inequalities, and a simple and efficient computational scheme has been suggested to solve them.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/app.1985.070300201
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  • 7
    Electronic Resource
    Electronic Resource
    Computational scheme for the step growth polymerization of multifunctional monomers in presence of intramolecular reactions (1987)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Applied Polymer Science 33 (1987), S. 1835-1842 
    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
    Additional Material: 4 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/app.1987.070330535
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  • 8
    Electronic Resource
    Electronic Resource
    Algorithm for computation of molecular weight distribution and its moments in reversible step-growth polymerization in batch reactors (1987)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Applied Polymer Science 34 (1987), S. 571-585 
    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 differential equations governing the molecular weight distribution (MWD) in step-growth polymerization are coupled and nonlinear and a large number of them must be solved simultaneously to keep the truncation error low. In this work, these equations have been decoupled so that they can be solved sequentially. The solution of these is independent of the truncation error and there is considerable saving of computation time. To demonstrate the efficiency of the algorithm, the formation of polyethylene terephthalate (PET) in batch reactors with ethylene glycol evaporating has been analyzed. The feed to the reactor is taken as polymer with its oligomers present according to the Flory's distribution. The effect of pressure and temperature of the reactor on the progress of polymerization has been modelled and evaluated. The amount of ethylene glycol distilled, the concentrations of the first five oligomers Q1 to Q5, the number average chain length, and the polydispersity index of the polymer have been determined. It is shown that the reduced pressure and increased temperature reduce the concentration of the condensation product in the reaction mass, thus pushing the polymerization in the forward direction. Lastly the CPU time on Dec 1090 using this algorithm is only 0.40 s compared to about 10 min for similar computations using other existing methods.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/app.1987.070340212
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  • 9
    Electronic Resource
    Electronic Resource
    Polymerization of melamine and formaldehyde in homogeneous continuous-flow stirred-tank reactors using functional group approach: Part A: Conversion of functional groups (1987)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Applied Polymer Science 34 (1987), S. 1367-1382 
    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 comprehensive kinetic model using the functional group approach has been proposed for the polymerization of melamine and formaldehyde. The kinetic model is consistent with the basic chemistry of polymerization and involves five rate constants which have been estimated using the experimental data of Tomita. Homogeneous continuous-flow stirred-tank reactors (HCSTRs) have been modelled and the mole balance relations for various functional groups have been written. The performance of HCSTRs is governed by algebraic equations and, for any specified residence time, is found by the method of successive substitution using the Brown's algorithm. The computations show that as long as free formaldehyde is present, the reaction mass would consist predominantly of substituted melamine molecules. However, after formaldehyde is completely reacted, larger oligomers are formed in larger concentrations. On comparison of results with batch reactors, it is found that for the same reaction time HCSTRs yield polymer with higher branching.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/app.1987.070340404
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  • 10
    Electronic Resource
    Electronic Resource
    Polymerization of melamine and formaldehyde in homogeneous continuous-flow stirred-tank reactors using functional group approach: Part B: Molecular weight distribution (1987)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Applied Polymer Science 34 (1987), S. 1383-1397 
    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: Polymerization of melamine and formaldehyde in homogeneous continuous-flow stirred-tank reactors (HCSTRs) is reversible and leads to formation of branched polymer due to the hexafunctional nature of melamine. The reversed reaction of branched molecules depends upon the chain structure, and herein a simple model is presented to account for this. The functional group analysis of part A of this series has been extended and the molecular weight distribution (MWD) relations for HCSTRs have been solved. The MWD of the polymer is extremely sharp and the molecules are more branched compared to those for batch reactors and can be explained as follows. A given reacted bond can react with water as well as free formaldehyde through the reversed reaction, and the rate constant for the latter step is much larger. Consequently, the chain growth is limited as long as there is free formaldehyde in the reaction mass. Lower conversion of melamine and formaldehyde in HCSTRs is used to explain the behavior observed.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/app.1987.070340405
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