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.     

Twiston vs. random fluctuation as the source of the rapid motion of polyethylene in the inclusion complex with perhydrotriphenylene

Recently molecular dynamics simulations were performed for polyethylene in the inclusion complex with perhydrotriphenylene. The system contained ninety molecules of perhydrotriphenylene, arranged in six stacks of fifteen molecules each, and one molecule of n-tetracontane, C₄₀H₈₂. The internal CH₂-CH₂ bonds in n-tetracontane have a very strong preference for the trans state. Nevertheless, the chain exhibits a high degree of internal flexibility. This motion produces a characteristic pattern in δ|ψ N + i ψi| vs. N, where ψ_i describes the instantaneous angle of a C-H bond vector at carbon atom i about the axis defined by the channel, and δ denotes the fluctuation. The pattern expected for δ|ψ N + i ψi| vs. N is derived for the case where the rapid internal motion is produced by a twiston. It is compared with the results from the simulation and from the expectation for the case where the rapid motion arises from uncorrelated internal fluctuations within the trans state at each CH₂-CH₂ bond.     

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.     

Configurations conducive to the formation of intramolecular excimers in poly(N‐vinyl carbazole) and its copolymers

The ratios of the intensity of excimer and monomer emissions, denoted I_E/I_M, in poly(N‐vinyl carbazole) and copolymers of N‐vinyl carbazole and methyl methacrylate were measured with steady‐state fluorescence. Measurements were performed in dilute solutions of several fluid solvents at 25 °C and in a solid matrix of poly(methyl methacrylate) at room temperature. The values of I_E/I_M depended on the nature of the solvent, the emission wavelength, and the copolymer composition. Molecular dynamics simulations were performed for diastereoisomers of 2,4‐di(N‐carbazolyl)pentane and for isotactic and syndiotactic trichromophoric copolymer fragments to assist in the identification of the thermally accessible conformations capable of forming intramolecular excimers and the configurational relationship of the carbazole units in these complexes. Nearest neighbor carbazole groups made the dominant contribution to the excimers. Excimers were more likely in isotactic sequences than in syndiotactic sequences, as was also the case for the low‐energy excimer arising from the complete overlap of two carbazole units. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1272–1281, 2001     

An atom-based explanation for the absence of excimer emission in the fluorescence of dilute solutions of poly(pentafluorostyrene)

The steady-state fluorescence emission spectrum of poly(pentafluorostyrene) in dilute fluid solution shows no excimer emission. An atomic level modeling study explains why this polymer cannot form an excimer. Repulsive Coulombic interactions prohibit the attainment of the extensive overlap of the two rings in the classic face-to-face sandwich conformation of a singlet excimer.     

Topological effects on static and dynamic properties in an amorphous nanofiber composed of cyclic polymers

The static and dynamic properties of an amorphous polymer nanofiber are investigated via Monte Carlo simulation. Each nanofiber is composed of rings represented by 50 beads. The results are compared with recent simulation, by a similar method, of a nanofiber composed of linear chains with the same number of beads. The coarse‐grained model is one that permits reverse‐mapping of the macromolecules to C₁₀₀H₂₀₂ or C₁₀₀H₂₀₀. The radial density profiles can be fitted to a hyperbolic tangent function. The orientation of the chains is similar in the two fibers. The cyclic chains move slower than the linear chains, both on the scale of individual beads and on the scale of entire molecules. Both fibers experience an increase in mobility in the surface region, with this increase being larger in the fibers composed of linear chains.     

Stability of pesticides on solid-phase extraction disks after incubation at various temperatures and for various time intervals: interlaboratory study

An interlaboratory study was conducted at 8 locations to assess the stability of pesticides on solid-phase extraction (SPE) disks after incubation at various temperatures and for various time intervals. Deionized water fortified with selected pesticides was extracted by using 2 types of SPE filtration disks (Empore C18 and Speedisk C18XF), and after extraction, the disks were incubated at 3 temperatures (25, 40, and 55 degrees C) and for 2 time intervals (4 and 14 days). Deionized water was fortified with atrazine, carbofuran, and chlorpyrifos by all participating laboratories. In addition, some of the laboratories included 2 of the following pesticides: metolachlor, metribuzin, simazine, chlorothalonil, and malathion. Concurrently, fortified water samples were extracted with the incubated samples by using each disk type at 4 and 14 days. Pesticides had equivalent or greater stability on > or = 1 of the C18 disk types, compared with storage in water. The lowest recoveries of carbofuran (6%) and chlorpyrifos (7%) were obtained at 55 degrees C after storage for 14 days in incubated water. At 55 degrees C after 14 days, the lowest recovery for atrazine was 65% obtained by using Empore disks. Pesticide-specific losses occurred on the C18 disks in this study, underlining the importance of temperature and time interval when water is extracted at remote field locations and the SPE disks containing the extracted pesticides are transported or shipped to a laboratory for elution and analysis.