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  • American Institute of Physics (AIP)  (23)
  • 1
    Electronic Resource
    Electronic Resource
    Molecular dynamics analysis of transitions between rotational isomers in polymethylene (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 5348-5354 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Molecular dynamics trajectories have been computed and analyzed for linear chains, with sizes ranging from C10H22 to C100H202, and for cyclic C100H200. All hydrogen atoms are included discretely. All bond lengths, bond angles, and torsion angles are variable. Hazard plots show a tendency, at very short times, for correlations between rotational isomeric transitions at bond i and i±2, in much the same manner as in the Brownian dynamics simulations reported by Helfand and co-workers. This correlation of next nearest neighbor bonds in isolated polyethylene chains is much weaker than the correlation found for next nearest neighbor CH–CH2 bonds in poly(1,4-trans-butadiene) confined to the channel formed by crystalline perhydrotriphenylene [Dodge and Mattice, Macromolecules 24, 2709 (1991)]. Less than half of the rotational isomeric transitions observed in the entire trajectory for C50H102 can be described as strongly coupled next nearest neighbor transitions. If correlated motions are identified with successive transitions, which occur within a time interval of Δt≤1 ps, only 18% of the transitions occur through cooperative motion of bonds i and i±2. An analysis of the entire data set of 2482 rotational isomeric state transitions, observed in a 3.7 ns trajectory for C50H102 at 400 K, was performed using a formalism that treats the transitions at different bonds as being independent. On time scales of 0.1 ns or longer, the analysis based on independent bonds accounts reasonably well for the results from the molecular dynamics simulations. At shorter times the molecular dynamics simulation reveals a higher mobility than implied by the analysis assuming independent bonds, presumably due to the influence of correlations that are important at shorter times.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.461649
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  • 2
    Electronic Resource
    Electronic Resource
    Dynamics of conformational transitions to isomeric states favoring intramolecular excimer formation in aromatic polyesters with methylene or oxyethylene spacers (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 6783-6790 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dynamic rotational isomeric state formalism described in the preceding paper is used to calculate the rate of first passage from non-excimer-forming conformations to excimer-forming conformations in seven aromatic polyesters with different flexible spacers between the aromatic rings. The excimer-forming conformations in these polymers were identified recently by Mendicuti et al. The time dependence of the probability that a flexible spacer will at least once have passed through an excimer-forming conformation can be written as the product of two factors, one of which is time dependent, the other of which is determined by the equilibrium conformational statistics. In the polyesters considered here, the static factor is much more sensitive than the dynamic factor to the identity of the flexible spacers. Consequently, the equilibrium chain statistics provides a good description of the relative excimer population for these polyesters, even at times where the dynamic contribution is significant. The chromophores in these polyesters have such short fluorescence lifetimes that it is only in the most mobile flexible segments that there is an important dynamic contribution to the excimer formation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456297
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  • 3
    Electronic Resource
    Electronic Resource
    Extent of the correlation between the squared radius of gyration and squared end-to-end distance in random flight chains (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 1956-1959 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Manipulation of generator matrix equations permits the evaluation of exact closed-form analytical expressions for 〈r4〉, 〈s4〉, and 〈r2s2〉 for freely jointed chains comprised of n bonds of length l. The squared end-to-end distance and squared radius of gyration for a specified conformation are denoted by r2 and s2, respectively. The expression for 〈r4〉, and the limit for 〈s4〉, agree with results in the literature that were obtained by other methods. The results are used to formulate the three dimensionless ratios, 〈r4〉/〈r2〉2, 〈s4〉/〈s2〉2, and 〈r2s2〉/〈r2〉〈s2〉, that are necessary for evaluation of the correlation coefficient for s2 and r2 in a freely jointed chain. The three dimensionless ratios have asymptotic limits of 5/3, 19/15, and 4/3, respectively. The value of the correlation coefficient decreases from 1 to (5/8)1/2 as n increases from 2 to infinity. In the limit as n goes to infinity, the best linear relationship between s2 and r2 is s2=(〈r2〉+r2)/12. The nature of the deviation of s2 and r2 for specified conformations from the best straight line is documented by an illustrative numerical calculation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456037
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  • 4
    Electronic Resource
    Electronic Resource
    Dynamics of conformational transitions to isomeric states favoring intramolecular excimer formation in polymeric chains. Application to dimer models of polystyrene (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 6775-6782 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dynamic rotational isomeric states model, which has been recently proposed to investigate the dynamics of local conformational transitions in polymers, is elaborated to formulate the increase in the number of excimer-forming sites through rotational sampling. The transition rate matrix governing the rate of passage between the subsets {φ}a and {φ}b, consisting of excimer-forming and non-excimer-forming conformations, respectively, is modified such that microstates in {φ}b are nonrecurrent, in contrast to the stationary process of conformational transitions in chains in equilibrium. The theory allows for the calculation of the fraction Pt(a) of sites that undergo transition to excimer-forming states at least once prior to time t, as well as mean first passage time from {φ}b to {φ}a, as a function of real chain conformational characteristics and structural properties. Application of the model to the meso and racemic dyads in polystyrene confirms the fact that conformational mobility of the chain plays a major role in intramolecular excimer formation. Comparison with experiments demonstrates that the decay of the monomer fluorescence in styrene dimers is predominantly governed by the process of conformational transitions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456296
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  • 5
    Electronic Resource
    Electronic Resource
    The configurational contribution to the segmental orientation in network chains subject to perturbation by the excluded volume effect (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 1153-1158 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The influence of the excluded volume effect on the configurational contribution to the segmental orientation in network chains is evaluated using rotational isomeric state theory. The chains used are rotational isomeric state models for polyethylene, poly(vinyl bromide), and poly(vinyl chloride), as well as some simplistic models for polyethylene. The long-range interactions may be attractive or repulsive. Chain expansion in response to the excluded volume effect is accompanied by an increase in orientational entropy (or disorder), which is apparent from the decrease in the absolute magnitude of the orientation function. Conversely, any tendency for orientation in a preferred direction is enhanced by chain contraction.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455222
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  • 6
    Electronic Resource
    Electronic Resource
    Simulation of the steric stabilization of polymer colloids by diblock copolymers (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 761-766 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The steric stabilization of polymer colloids by diblock copolymers is simulated on a 22×22×22 cubic lattice with periodic boundary conditions. In the systems described here, the lattice contains 20 chains, the volume fraction of polymer never exceeds 0.0376, and the number of beads in a chain never exceeds 20. The behavior is monitored by evaluation of the dimensionless ratios Mn/M0 and Mw/Mn, where M0 is the mass of an individual chain, and Mn and Mw are the number- and weight-average molecular weights of the particles in the system. By suitable variation in the pairwise interaction energies, one can cause the system to (1) establish a dynamic equilibrium, in which aggregates easily form and dissociate, (2) follow an irreversible path toward the formation of the largest possible aggregate, or (3) reach a long-lived metastable state in which the system prefers smaller aggregates than those that would exist at a true equilibrium. The metastable state arises from the steric stabilization of the dispersed state, as is shown by its characteristic response to the variation in the size of the soluble (stabilizing) block, at constant size of the insoluble block and constant interaction energies. The simulation also documents an interesting influence of the energy of the self-interaction of the insoluble block on the efficiency of the steric stabilization of the metastable state.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460344
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  • 7
    Electronic Resource
    Electronic Resource
    The role of 〈r2μ2〉0 in the response of the mean square dipole moment to the expansion of a family of model chains (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 5512-5517 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The response of the mean square dipole moment 〈μ2〉 to chain expansion has been studied in a family of chains for which 〈r ⋅ μ〉0=0. Here r and μ are the end-to-end and dipole moment vectors, respectively, angle brackets denote the statistical mechanical average, and zero as a subscript denotes the ensemble unperturbed by long range interaction. For some of these chains, 〈μ2〉 increases upon chain expansion, while for other chains 〈μ2〉 decreases when the chain expands. At small expansion, the behavior is described by n0.43 (〈r2μ2〉0/〈r2〉0〈μ2〉0−1), where n denotes the number of bonds.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453635
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  • 8
    Electronic Resource
    Electronic Resource
    Monte Carlo simulation on separation and co-crystallization of a mixture of short polyethylene chains in a thin film (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 117 (2002), S. 3440-3447 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Thin films of a mixture of two kinds of short polyethylene chains, n-tricontane and n-tetracontane, are simulated both at high and low temperatures by a dynamic Monte Carlo method on a high coordination lattice, which bridges the coarse-grained and the fully atomistic simulations. Films are obtained from an equilibrated model for the melt by increasing one of the three periodic boundaries to a very large value. The melting temperatures (Tm) of the two pure components in the simulation are obtained from a sharp transition of various parameters, such as the probability of trans conformation of C–C bonds, orientation order parameters, energies, etc. At high temperature, 420 K (above Tm of the two pure components), the chains have a trend to separate with each other. The shorter chains are enriched on the free surface due to their larger fraction of chain ends. At low temperature (below Tm of the two pure components), two situations exist, which depend on the prior history of the mixture. If the quench takes place from a homogeneous mixture the chains crystallize together with almost the same density profile except for a large fluctuation in the bulk region of the thin film. On the other hand, if the quench takes place from the equilibrated thin film at 420 K, the shorter chains remain enriched on the surface, and the segregation of the shorter chain is enhanced. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1492281
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  • 9
    Electronic Resource
    Electronic Resource
    Role of the attractive portion of the Lennard-Jones potential in the homogeneity of melts of isotactic and syndiotactic polypropylene (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 8221-8225 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Recently Clancy et al. reported a Monte Carlo simulation that reproduces the tendency, observed by Mülhaupt and co-workers, for demixing of melts of isotactic polypropylene (iPP) and syndiotactic polypropylene (sPP). The short-range intermolecular interactions in the coarse-grained chains in the simulation are controlled by a rotational isomeric state model for PP that was described by Suter et al. The intermolecular interactions of the beads are controlled by a discretized representation of a continuous Lennard-Jones (LJ) potential that describes the interaction of two molecules of propane. The work reported here reveals that the outcome is sensitive to the truncation of this LJ potential. A weak tendency for demixing of the 50:50 iPP:sPP melt is seen if the LJ potential is truncated so that only its repulsive part is used in the simulation. Inhomogeneity is enhanced if the truncation of the LJ potential is shifted to larger distances, so that the attractive branch of the LJ potential is incorporated in the simulation. This observation in the simulation leads to conclusive identification of the mechanism responsible for demixing of the 50:50 melt. It also implies that the same mechanism may affect the miscibility of melts of other vinyl polymers in which the chains differ in stereochemical composition. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1409361
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  • 10
    Electronic Resource
    Electronic Resource
    Monte Carlo simulation of the crystallization and annealing of a freestanding thin film of n-tetracontane (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 2277-2283 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A freestanding thin film of n-tetracontane chains is simulated by a Monte Carlo (MC) method on a high coordination lattice. The coarse-grained chains, represented by 20 beads each, can be reverse-mapped into the fully atomistic description, C40H82. The Hamiltonian includes a short-range interaction based on a rotational isomeric state model and a long-range interaction obtained from a Lennard-Jones potential energy function. When the melt is instantaneously quenched from 473 to 298 K, crystallization initiates in the surface region and propagates into the interior of the film, as was found in a prior molecular dynamics simulation of a united atom model of polyethylene [M. Ito, M. Matsumoto, and M. Doi, Fluid Phase Equilibria, 144, 395 (1998)]. Several repetitions of the MC simulation, starting from different configurations of the melt at 473 K, reveal that two distinctly different structures can be obtained. Usually the independently initiated crystals at the two surfaces of the thin film produce a disordered grain boundary when they impinge on one another as a consequence of propagation into the interior of the film. This grain boundary was also observed by Ito et al. However, if the MC simulation is repeated many times, there are a few instances in which the independently initiated crystals happen to have a similar orientation, and then crystallization propagates completely through the thin film without producing a grain boundary in the interior. A well-defined melting phenomenon is observed at about 390 K when the film without the grain boundary is heated. Annealing at 380 K of the film with the grain boundary causes growth of one crystal at the expense of the other. This growth eventually leads to a completely crystalline film, with elimination of the grain boundary. Therefore, the stable structure of the thin film is the one that is completely crystalline, with no grain boundary in the interior, even though rapid quenching is more likely to lead to a structure with a grain boundary. The MC simulation can anneal the imperfect structure into the more perfect one. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1431587
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