Search results for " Carl"
showing 10 items of 1737 documents
Anomalous Structure and Scaling of Ring Polymer Brushes
2011
A comparative simulation study of polymer brushes formed by grafting at a planar surface either flexible linear polymers (chain length $N_L$) or (non-catenated) ring polymers (chain length $N_R=2 N_L$) is presented. Two distinct off-lattice models are studied, one by Monte Carlo methods, the other by Molecular Dynamics, using a fast implementation on graphics processing units (GPUs). It is shown that the monomer density profiles $\rho(z)$ in the $z$-direction perpendicular to the surface for rings and linear chains are practically identical, $\rho_R(2 N_L, z)=\rho_L(N_L, z)$. The same applies to the pressure, exerted on a piston at hight z, as well. While the gyration radii components of ri…
Selfdiffusion of polymer chains in solutions and melts
2005
Anomalous diffusion of monomers of polymer chains, as well as motion of these chains as a whole, is discussed with an emphasis on Monte Carlo simulations and simple scaling concepts. While the behavior of isolated chains in good solvents or Theta-solvents without excluded volume interactions is fully accounted for by the Rouse model, the behavior is less clear both for isolated chains in bad solvents and for chains in dense melts. Collapsed chains are shown to diffuse as g3(t) = <([rCM (t) -rCM(0)]2〉 ∝ tξ3 where the (effective?) exponent ξ3 simply seems to be linearly temperature-dependent for temperatures T lower than the Σ-temperature, ξ3 T/Θ. A relaxation time τ oc N3 is found, and scali…
Anomalous Diffusion and Relaxation of Collapsed Polymer Chains
1994
Time-dependent displacement of monomers and the centre-of-gravity motion of a polymer chain at various temperatures below the theta-temperature are studied by Monte Carlo simulation of an off-lattice model. While inner monomers diffuse Rouse-like, [ri(t) − ri(0)]2 t1/2, the centre of mass exhibits pronounced anomalous diffusion, [rc.m.(t) − rc.m.(0)]2 ta, where the exponent a seems to depend on temperature. The resulting anomalous dependence of the relaxation times on chain length is discussed in terms of scaling ideas. A possible relation to a glasslike freezing in of the collapsed globules is pointed out.
Scaling concepts for polymer brushes and their test with computer simulation
2004
After a brief review of the scaling concepts for static and dynamic properties of polymer brushes in good solvents and Theta solvents, the Monte Carlo evidence is discussed. It is shown that under typical conditions the diameter of the last blob is of the order of 10-20% of the brush height, and therefore pronounced deviations from the self-consistent field predictions occur. In bad solvents, lateral microphase separation occurs leading to an irregular pattern of "dimples". Particularly interesting is the response of brushes to shear deformation, and the interaction between two interpenetrating brushes. Recent attempts to understand the resulting shear forces via molecular-dynamics simulati…
Monte carlo simulation of the glass transition of polymer melts
2007
The bond fluctuation model of polymer melts is presented as a reasonable compromise between simulation efficiency and realistic chemical detail. It is shown that inclusion of a potential energy that depends on the length of the effective bonds connecting the effective monomers easily creates a conflict between configurational entropy of dense packing and the energetic tendency of the bonds to stretch. This competition leads to a glass transition of the model, which very well describes many features of real systems.
Unexpectedly normal phase behavior of single homopolymer chains
2006
Employing Monte Carlo simulations, we show that the topology of the phase diagram of a single flexible homopolymer chain changes in dependence on the range of an attractive square well interaction between the monomers. For a range of attraction larger than a critical value, the equilibrium phase diagram of the single polymer chain and the corresponding polymer solution phase diagram exhibit vapor (swollen coil, dilute solution), liquid (collapsed globule, dense solution), and solid phases. Otherwise, the liquid-vapor transition vanishes from the equilibrium phase diagram for both the single chain and the polymer solution. This change in topology of the phase diagram resembles the behavior k…
A Monte Carlo study of diffusion in "living polymers"
1996
We report the first numeric experiments on diffusion in living polymers (polymers that can break and recombine reversibly, and are characterized by an exponential molecular weight distribution). In the simulation we use a modification of the bond fluctuation model which is known to reproduce the correct Rouse dynamics of polymer chains. The diffusion coefficient D reveals a Rouse-type behaviour D ∝ 1/L, where L is the average chain length of the polydisperse system. We also find a D ∝ exp [ − V/2kBT] dependence on the bond energy V, whereas at constant temperature the diffusion coefficient turns out to be inversely proportional, D ∝ ρ−1, to the monomer density of the system ρ in agreement w…
Anomalous scaling of the critical temperature of unmixing with chain length for two-dimensional polymer blends
2003
The thermodynamics, structure and the chain configurations of symmetrical polymer mixtures confined into ultrathin films are studied by Monte Carlo simulations of the bond fluctuation model. It is shown that the Flory-Huggins–type scaling of the critical temperature (Tc ~ N) with chain length N in the bulk is replaced by a weaker increase, Tc ~ N1/2, in an ultrathin film, and this is interpreted in terms of geometric arguments. The pair-correlation function g(r) of monomers from different chains exhibits a pronounced correlation hole, and the density of intermolecular contacts zc decreases with N as zc ~ N−1/2.
Influence of chain stiffness on knottedness in single polymers.
2013
In the present article, we investigate and review the influence of chain stiffness on self-entanglements and knots in a single polymer chain with Monte Carlo simulations spanning good solvent, theta and globular phases. The last-named are of particular importance as a model system for DNA in viral capsids. Intriguingly, the dependence of knot occurrence and complexity with increasing stiffness is non-trivial, but can be understood with a few simple concepts outlined in the present article.
Kinetic Monte Carlo Simulations of Flow-Assisted Polymerization
2012
We performed kinetic Monte Carlo simulations on a model of a polymerization process in the presence of a periodic oscillatory flow to explore the role of mixing in polymerization reactors. Application of an oscillatory flow field helps overcome the diffusive limitations that develop during a polymerization process due to an increase in the molecular weights of polymer chains, thereby giving rise to high rates of polymerization. A systematic increase in the flow strength results in a "dynamic" coil-stretch transition, leading to an elongation of polymer chains. Reactive ends of stretched (polymer) chains react more frequently than the reactive ends of coiled chains, which are screened by oth…