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  • American Institute of Physics (AIP)  (23)
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  • 11
    Digitale Medien
    Digitale Medien
    Monte Carlo simulation of the adsorption of diblock copolymers from a nonselective solvent. II. Structure of adsorbed layer (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 7508-7514 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The adsorption of a diblock copolymer from a nonselective solvent is studied by using a Monte Carlo method. Simulation results for the structure of the adsorbed layer, segment density profiles, and the conformational properties are presented for adsorption energy per segment varying from −0.25 to −1.25, in units of kT, and for different volume fractions and different block sizes. It is found that the bound fraction for the anchoring block A increases as the surface affinity increases, decreases as the length of the anchoring block increases, and decreases as the volume fraction of copolymer increases. The probability of end adsorption decreases, while the probability for flattened chains increases as the surface affinity increases. The probability of adsorption for the middle beads is larger than that for the end beads. The segment density profiles show the maximum of the adsorption for the anchoring block is at the first layer near the surface. The surface coverage for block A at the first layer increases as the concentration of copolymer increases and as the surface affinity increases. The height and position of the adsorption peak for the nonanchoring blocks are determined by the length of the nonanchoring block and the surface affinity. The data of the end-to-end distance of adsorbed chains show that the nonanchoring block is stretched.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.464691
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  • 12
    Digitale Medien
    Digitale Medien
    Molecular dynamics of paraffins in the n-alkane/urea clathrate (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 9138-9143 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Simulations of the molecular dynamics of n-CNH2N+2 in the n-alkane/urea clathrate have been performed for N=13,23,33, for a period of 1 ns. Parallel simulations were performed starting from conformations in which all CH2–CH2 bonds were in the trans conformation, and starting from a conformation in which one of the terminal CH2–CH2 bonds was in a gauche conformation. These simulations suggest less than 3% of the terminal CH2–CH2 bonds would be in a gauche state at equilibrium. The gauche content is much smaller at the CH2–CH2 bonds further removed from the ends of the chain. No significant differences are seen in the simulations performed with n-alkanes with N=13,23,33. The mobility of the C–H bond vectors about the axis defined by the channel is greater at the ends than in the middle of the chain. The angular distribution is nearly randomized in 1 ns for the C–H vector in a methyl group, but a longer time scale would be required for randomization of this distribution for the C–H vector bonded to the central methylene group in C33H68.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.462223
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  • 13
    Digitale Medien
    Digitale Medien
    Segmental distribution functions for a micelle comprised of small symmetric diblock copolymers (short chain amphiphiles) (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 5341-5347 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The micelle formed by short chain amphiphiles (small diblock copolymers) in a selective solvent has been simulated on a cubic lattice. The system contains twenty chains, each with ten beads of A and ten beads of B, at a volume fraction of 0.0376. These chains form a single micelle when the pairwise interaction of nonbonded beads in the insoluble block contribute an energy of −0.5 kT, all other interaction energies being zero. The micelle consists of an internal core that is a close packed region occupied almost exclusively by the insoluble blocks, and a corona, the outer portion of which consists primarily of solvent and the soluble blocks. However, the interface between these two regions is not sharp, as judged by distribution functions for the two types of beads, and for the junctions between the blocks. Letting 〈r 2j〉 denote the mean square radius of gyration of the junctions, the close packed core of insoluble beads extends from the center of mass no farther than ∼0.5 〈r 2j〉. The free ends of the insoluble block have a mean square radius of gyration of 0.88 〈r 2j〉, which places them, on average, farther from the center of mass than a randomly chosen bead in the insoluble block. The results are not compatible with models for a diblock copolymer micelle that assume a sharp interface, nor with models that assume the free ends of the insoluble block seek out the center of mass, or are randomly distributed throughout the core.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.461648
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  • 14
    Digitale Medien
    Digitale Medien
    Creation, migration, and termination of conformational defects in poly(1,4-trans-butadiene) in form II and in the inclusion complex in perhydrotriphenylene (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 3279-3284 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The histories of the conformational defects created by transitions between anticlinal± states, either singly or correlated as next-nearest neighbors, have been studied using molecular dynamics trajectories computed for a chain of poly(1,4-trans-butadiene) in form II and in the inclusion complex with perhydrotriphenylene. The most likely fate of a newly created conformational defect is immediate reversion to the preferred conformation. The remaining defects can undergo diffusion. The average contour lengths for the paths covered in the diffusion are on the order of ten CH–CH2 bonds for a long poly(1,4-trans-butadiene) in the inclusion complex in perhydrotriphenylene. The diffusion is terminated either by loss of the defect out the end of the chain, or by the annihilation of two defects that encounter one another. The average time between hops is much shorter for the chain in the inclusion complex than for the chain in form II. The time-averaged number of defects in methyl terminated decamers is virtually identical in the two systems. For much longer chains, the poly(1,4-trans-butadiene) in the inclusion complex should contain more defects than the chain in form II.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.461973
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  • 15
    Digitale Medien
    Digitale Medien
    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
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: 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.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.461649
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  • 16
    Digitale Medien
    Digitale Medien
    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
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: 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.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.456037
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  • 17
    Digitale Medien
    Digitale Medien
    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
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: 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.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.460344
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  • 18
    Digitale Medien
    Digitale Medien
    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
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: 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.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.456296
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  • 19
    Digitale Medien
    Digitale Medien
    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
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: 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.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.456297
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  • 20
    Digitale Medien
    Digitale Medien
    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
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: 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.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.455222
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