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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 10583-10594 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An efficient continuum model for simulating polymer blends and copolymers is presented. In this model, the interactions are short-range and purely repulsive, thus allowing for excellent computational performances. The driving force for phase separation is a difference in the repulsive interaction strength between like and unlike mers. The model consists essentially of a molecular-dynamics algorithm, supplemented by an appropriate Monte Carlo exchange process. To demonstrate the effectiveness of the model we study two systems, a symmetric binary blend of polymers and a symmetric diblock copolymer system. For the binary blend, we determine the phase diagram and find, as predicted by theory, that the critical interaction parameter scales with the inverse of the chain length of the polymers. For the diblock copolymer system, we study both the one-phase region and the microphase separated lamellar region. For the latter, we show that constant-pressure algorithms are more appropriate since, contrary to recent lattice simulations, the lamellar spacing can self-adjust in such an ensemble. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Macromolecules 26 (1993), S. 2455-2463 
    ISSN: 1520-5835
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 2097-2108 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have developed a new algorithm that simulates the structure buildup process during homopolymerization of Af monomers. The algorithm is an off lattice percolation solution on a cube with periodic boundary conditions. All monomers are treated as point particles. After choosing any two nodes at random, the probability of their reaction is computed and compared with a random number. We present results where the probability rule is a step function over the interunitary distance between the chosen pair. We reason that this simulates, in a simplistic but effective fashion, the importance of diffusion vs reaction times in the problem. Thus, the case where units react in immediate neighborhoods corresponds to a diffusion limited growth and where they react with equal probability with all other units to the mean field or kinetic limited growth. We present results for five step reactive potentials with careful analysis of finite size effects. We find that the gel conversion is delayed as a consequence of diffusion limitations. We find that cyclization is only partially responsible for this effect. We also find that small mobilities, such as might be available from rotational and translational degrees of freedom, give results realistically close to the mean-field results, thus explaining the experimental success of the mean-field theory. We find that the (effective) scaling exponent γ depends on the reactive radius and thus on diffusion. It is not entirely clear to us if this suggests nonuniversal behavior or is simply a crossover problem. We conclude that the dependence of the critical exponent on diffusion over a wide range of conversions suggests that some of the "nonuniversal'' experimental observations might be due to diffusional effects.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 1588-1596 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a self-consistent-field structure for the thermodynamic description of concentrated solutions of liquid-crystal polymers. The polymers are assumed to be locally stiff but capable of curvature over large distances. The formulation of the chain geometry is that of a wormlike polymer, but the final analysis is simpler than the Kratky–Porod form. The phase behavior is analyzed for a cylindrically symmetric mean-field nematic ordering interaction. As expected, in two dimensions the system undergoes a second-order phase transition and in three dimensions the nematic transition is first-order uniaxial. The three relevant physical parameters which describe the phase behavior of the polymers are the molecular mass of the polymers, a measure of the chain stiffness and strength of the nematic interaction. The formulation in this paper is for infinitely long polymers. If the chain stiffness is measured by the bending energy ε, with u as the strength of the hardcore repulsion and U that of the soft interactions, we find the following relationship at the isotropic-nematic transition: ccβcε(u+Uβc)=6. cc is the critical concentration and βc=1/kBTc is the inverse critical temperature. At the transition we find that the order parameter has a universal value of 0.25. We present universal plots for the order parameter and free energy of the system as a function of polymer concentration and temperature.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Macromolecules 24 (1991), S. 3459-3461 
    ISSN: 1520-5835
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part B: Polymer Physics 28 (1990), S. 2585-2606 
    ISSN: 0887-6266
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: A comparison between the entirely statistical and the combined kinetic and statistical model for amine-epoxy networks, both studied previously in the literature is made. The chemistry studied involves the generation of a secondary site which reacts in the network through chainwise addition rather than the stepwise addition mode of the primary reaction. It is shown that the combined model is exact and the naïve statistical models are approximate. The assumptions underlying the two models are delineated. It is shown, that under certain circumstances, the predictions of the two models can differ by several hundred percent. Further, the combined model is extended to predict the postgel properties of the network.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
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