Rotational isomeric state chains on a high coordination lattice: Dynamic Monte Carlo algorithm details

Computational tool to model the packing of polycyclic chains: Structural analysis of amorphous polythiophene

A very efficient computational procedure, which was previously developed to generate and relax atomistic models of linear and comb-like amorphous polymers, has been adapted to model the amorphous phase of polycyclic systems. The strategy, which is a based in a generation algorithm that eliminates the torsion strain and a simple Monte Carlo Metropolis method to relax the generated structures, has been used to predict the density of amorphous polythiophene by combining NVT and NPT simulations. The theoretical value is in the excellent agreement with the experimental one, the former being overestimated by only 3-5%. Next, the molecular conformation and the packing of the rings were studied in detail. Interestingly, the amorphous phase of polythiophene can be described as a packing of elongated molecular chains more or less aligned in the same direction, in which the thiophene rings close in the space but belonging to different chains tend to adopt approximate parallel or antiparallel displaced pi-stacked arrangements.

Interior segment regrowth configurational-bias algorithm for the efficient sampling and fast relaxation of coarse-grained polyethylene and polyoxyethylene melts on a high coordination lattice

We demonstrate the application of a modified form of the configurational-bias algorithm for the simulation of chain molecules on the second-nearest-neighbor-diamond lattice. Using polyethylene and poly(ethylene-oxide) as model systems we show that the present configurational-bias algorithm can increase the speed of the equilibration by at least a factor of 2-3 or more as compared to the previous method of using a combination of single-bead and pivot moves along with the Metropolis sampling scheme [N. Metropolis, A. W. Rosenbluth, M. N. Rosenbluth, A. H. Teller, and E. Teller, J. Chem. Phys. 21, 1087 (1953)]. The increase in the speed of the equilibration is found to be dependent on the interactions (i.e., the polymer being simulated) and the molecular weight of the chains. In addition, other factors not considered, such as the density, would also have a significant effect. The algorithm is an extension of the conventional configurational-bias method adapted to the regrowth of interior segments of chain molecules. Appropriate biasing probabilities for the trial moves as outlined by Jain and de Pablo for the configurational-bias scheme of chain ends, suitably modified for the interior segments, are utilized [T. S. Jain and J. J. de Pablo, in Simulation Methods for Polymers, edited by M. Kotelyanskii and D. N. Theodorou (Marcel Dekker, New York, 2004), pp. 223-255]. The biasing scheme satisfies the condition of detailed balance and produces efficient sampling with the correct equilibrium probability distribution of states. The method of interior regrowth overcomes the limitations of the original configurational-bias scheme and allows for the simulation of polymers of higher molecular weight linear chains and ring polymers which lack chain ends.

Calculation of pressure using the virtual-volume-variation method and the virial method from chain conformations obtained by Monte Carlo simulations on the second nearest neighbor diamond lattice

For a model system of polyethylene of chain lengths 40 and 100 carbon atoms, we calculated the pressure at different densities and compared them with the experimental values. The simulation was conducted on the second nearest neighbor diamond lattice, and the pressure was calculated using the virtual-volume-variation method after the system was reverse mapped to its fully atomistic form in continuous space and energy minimized. In addition, the pressure was also calculated from the virial route by conducting a short molecular dynamics simulation starting from the energy minimized structure. We show that the pressure obtained from our simulations is quite reasonable in the length of simulation time (in Monte Carlo steps) normally employed in our group. These results provide additional evidence for the equilibration of our model systems, and methodology to calculate the pressure in our lattice models.

Modification of statistical threading in two-component pseudorotaxane melts using the amphiphilic approach and variations in the confinement geometry

Recently we described a coarse-grained model of poly(ethylene oxide) and then employed that model to study the amount of spontaneous threading of cyclic molecules by linear chains in the melt [C. A. Helfer, G. Xu, W. L. Mattice, and C. Pugh, Macromolecules 36, 10071 (2003)]. Since the amount of statistical threading at equilibrium is small, there is interest in identifying physical changes in the system that will increase the threading. We now use that coarse-grained model to investigate the effect on threading of various hypothetical (but feasible) modifications to the two-component system of macrocycles and linear chains in the melt, and different confinement geometries, that can bring about correlations in the arrangement of the rings. Our work follows on the concept of an amphiphilic approach [C. Pugh, J.-Y. Bae, J. R. Scott, and C. L. Wilkins, Macromolecules 30, 8139 (1997)] for increasing the statistical threading in homopolyrotaxane melts. We investigate whether introducing such correlations in the macrocycles can increase the spontaneous threading. This paper shows that some of our modifications can yield more than double the amount of threading seen in purely statistical mixing.