Structure and mobility of cyclohexane as a solvent for Trans-Polyisoprene

Solutions of {\it trans}$-$polyisoprene in cyclohexane are investigated in atomistic detail at different compositions at two different temperatures. We investigate the influence of polymer concentration on the dynamics of the solvent molecules and the structure of the solvation shell. The double bonds along the polymer backbone are preferentially approached by the solvent molecules. The mobility of cyclohexane molecules decreases with increasing polymer concentration at ambient conditions. The reorientation of molecules becomes more anisotropic with concentration as the polymer hinders the reorientation of the molecular plane. At elevated temperatures the influence of the polymer is weaker and the reorientation of the solvent is more isotropic. Additionally, a fast and efficient way to set up atomistic simulations is shown in detail in which the initial simulations increase in length and in the simulation time-step. The excess energy from initial overlaps is removed by resetting the velocities at regular intervals.Comment: 6 pages, 3 figure

Properties of Poly (isoprene) - Model Building in the Melt and in Solution

Properties of 1,4-\textit{trans} poly (isoprene) at ambient conditions are determined by simulations on two length scales based on two different models: a full-atomistic and a mesoscopic one. The models are linked via a mapping scheme such that one mesoscopic bead represents one chemical repeat unit. Melts as well as solutions of several chain lengths were investigated and mapped individually to the meso-scale. The resulting models are compared to each other. The meso-scale models could be simulated over a large variety of chain lengths and time-scales relevant for experimental comparison. Concerning static properties, we determined the persistence length of our systems and the scaling behavior of the radius of gyration. The latter was compared to experiments and the agreement is satisfactory. Furthermore, we find deviations from Rouse dynamics at all chain lengths at ambient conditions.Comment: 11 pictures 7 figure

Density of states of a binary Lennard-Jones Glass

We calculate the density of states of a binary Lennard-Jones glass using a recently proposed Monte Carlo algorithm. Unlike traditional molecular simulation approaches, the algorithm samples distinct configurations according to self-consistent estimates of the density of states, thereby giving rise to uniform internal-energy histograms. The method is applied to simulate the equilibrium, low-temperature thermodynamic properties of a widely studied glass former consisting of a binary mixture of Lennard-Jones particles. We show how a density-of-states algorithm can be combined with particle identity swaps and configurational bias techniques to study that system. Results are presented for the energy and entropy below the mode coupling temperature.Comment: 6 pages, 3 figures, accepted by J Chem Phy

Constant Pressure Hybrid Molecular Dynamics-Monte Carlo Simulations

New hybrid Molecular Dynamics-Monte Carlo methods are proposed to increase the efficiency of constant-pressure simulations. Two variations of the isobaric Molecular Dynamics component of the algorithms are considered. In the first, we use the extended-ensemble method of Andersen [H. C. Andersen J. Chem. Phys. {\bf 72},2384 (1980)]. In the second, we arrive at a new constant-pressure Monte Carlo technique based on the reversible generalization of the weak-coupling barostat [H. J. C. Berendsen et. al J. Chem. Phys. {\bf 81}, 3684(1984)]. This latter technique turns out to be highly effective in equilibrating and maintaining a target pressure. It is superior to the extended-ensemble method, which in turn is superior to simple volume-rescaling algorithms. The efficiency of the proposed methods is demonstrated by studying two systems. The first is a simple Lennard-Jones fluid. The second is a mixture of polyethylene chains of 200 monomers.Comment: 10 pages, 4 figure

Local Reorientation Dynamics of Semiflexible Polymers in the Melt

The reorientation dynamics of local tangent vectors of chains in isotropic amorphous melts containing semiflexible model polymers was studied by molecular dynamics simulations. The reorientation is strongly influenced both by the local chain stiffness and by the overall chain length. It takes place by two different subsequent processes: A short-time non-exponential decay and a long-time exponential reorientation arising from the relaxation of medium-size chain segments. Both processes depend on stiffness and chain length. The strong influence of the chain length on the chain dynamics is in marked contrast to its negligible effect on the static structure of the melt. The local structure shows only a small dependence on the stiffness, and is independent of chain length. Calculated correlation functions related to double-quantum NMR experiments are in qualitative agreement with experiments on entangled melts. A plateau is observed in the dependence of segment reorientation on the mean-squared displacement of the corresponding chain segments. This plateau confirms, on one hand, the existence of reptation dynamics. On the other hand, it shows how the reptation picture has to be adapted if, instead of fully flexible chains, semirigid chains are considered.Comment: 29 pages, several figures, accepted by Macromolecule

Local chain ordering in amorphous polymer melts: Influence of chain stiffness

Molecular dynamics simulation of a generic polymer model is applied to study melts of polymers with different types of intrinsic stiffness. Important static observables of the single chain such as gyration radius or persistence length are determined. Additionally we investigate the overall static melt structure including pair correlation function, structure function and orientational correlation function.Comment: 13 pages, 15 figures, PCCP accepte

Orientation Correlation in Simplified Models of Polymer Melts

We investigate mutual local chain order in systems of fully flexible polymer melts in a simple generic bead-spring model. The excluded-volume interaction together with the connectivity leads to local ordering effects which are independent of chain length between 25 and 700 monomers, i.e. in the Rouse as well as in the reptation regime. These ordering phenomena extend to a distance of about 3 to 4 monomer sizes and decay to zero afterwards.Comment: 5 pages, 3 figure

Density of States Monte Carlo Method for Simulation of Fluids

A Monte Carlo method based on a density-of-states sampling is proposed for study of arbitrary statistical mechanical ensembles in a continuum. A random walk in the two-dimensional space of particle number and energy is used to estimate the density of states of the system; this density of states is continuously updated as the random walk visits individual states. The validity and usefulness of the method are demonstrated by applying it to the simulation of a Lennard-Jones fluid. Results for its thermodynamic properties, including the vapor-liquid phase coexistence curve, are shown to be in good agreement with high-accuracy literature data.Comment: 5 pages, 3 figures, accepted by J Chem Phy

Local Structure and Dynamics of Trans-polyisoprene oligomers

Mono- and poly-disperse melts of oligomers (average length 10 monomers) of trans-1,4-polyisoprene are simulated in full atomistic detail. The force-field is developed by means of a mixture of ab initio quantum-chemistry and an automatic generation of empirical parameters. Comparisons to NMR and scattering experiments validate the model. The local reorientation dynamics shows that for C$-$H vectors there is a two-stage process consisting of an initial decay and a late-stage decorrelation originating from overall reorientation. The atomistic model can be successfully mapped onto a simple model including only beads for the monomers with bond springs and bond angle potentials. End-bridging Monte Carlo as an equilibration stage and molecular dynamics as the subsequent simulation method together prove to be a useful method for polymer simulations.Comment: 25 pages, 15 figures, accepted by Macromolecule