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We present a computer simulation model of polymer melts representing each chain as one single particle. Besides the position coordinate of each particle, we introduce a parameter nij for each pair of particles i and j within a specified distance from each other. These numbers, called entanglement numbers, describe the deviation of the system of ignored coordinates from its equilibrium state for the given configuration of the centers of mass of the polymers. The deviations of the entanglement numbers from their equilibrium values give rise to transient forces, which, together with the conservative forces derived from the potential of mean force, govern the displacements of the particles. We have applied our model to a melt of C800H1602 chains at 450 K and have found good agreement with experiments and more detailed simulations. Properties addressed in this paper are radial distribution functions, dynamic structure factors, and linear as well as nonlinear rheological properties. [ABSTRACT FROM AUTHOR]

It has been suggested that, due to topological constraints, rings in the melt may assume a more compact shape than Gaussian chains. In this paper, we exploit the availability of narrow fractions of perdeuterated linear and cyclic polydimethylsiloxane (PDMS) and, through the analysis of the small angle neutron scattering (SANS) profiles, demonstrate the difference in scattering properties of linear and cyclic PDMS molecules. As expected for Gaussian chains, for the H/D linear PDMS samples, log-log plots of the scattered intensity versus scattering vector Q display a Q(-2) dependence. However, for H/D cyclic blends, the scaling exponent is higher than 2, as predicted by computer simulations reported in the literature. We show that cyclic molecules in bulk display the characteristic maximum in plots of scattered intensity versus Q(-2) that is expected on the basis of Monte Carlo calculations and from the Casassa equation [E. F. Casassa, J. Polym. Sci. A 3, 605 (1965)]. It is also shown that, for rings, the Debye equation [P. Debye, J. Appl. Phys. 15, 338 (1944)] is no longer appropriate to describe the SANS profiles of H/D cyclic blends, at least up to Mw≈10 000. For these samples, the Casassa form factor gives a better representation of the SANS data and we show that this function which was developed for monodisperse cyclics is still adequate to describe our slightly polydisperse samples. Deviations from all above observations are noted for Mw>11 000 and are attributed to partial contamination of cyclic samples with linear chains. The failure of both the Debye and the Casassa form factors could be due to contamination of the cyclic fractions by linear polymers or to a real conformational change. [ABSTRACT FROM AUTHOR]

In this paper, a method combining the orthogonal array design and the numerical simulation is proposed to optimize the geometry parameters of the melt-blowing slot die. An index, the stagnation temperature, is used to evaluate the performance of the slot die. The stagnation temperature is obtained by simulating the subsonic compressible air jet from the melt-blowing slot die, whereas the optimization is accomplished by the orthogonal array method. Three geometry parameters of the slot die: slot width, nose piece width, and slot angle are investigated. The results show that smaller slot angle and larger slot width will result in a higher stagnation temperature, which is beneficial to the air drawing of the polymer melt and thus to reducing fiber diameter, whereas the effect of nose piece width is insignificant. The optimal geometry parameters of the melt-blowing slot die achieved in this study are: slot width of 1.5 mm, slot angle of 30°, and nose piece width of 2 mm. [ABSTRACT FROM AUTHOR]

The bistability of anti-symmetric thin shallow cylindrical polymer composite shells, made of carbon fiber/epoxy resin, has already been investigated based on the uniform curvature and inextensible deformation assumptions by researchers in detail. In this paper, a non-uniform curvature model that considers the extensible deformations is proposed. Furthermore, a parametric modeling and automatic postprocessing plug-in component for the bistability analysis of polymer composite cylindrical shells is established by means of ABAQUS-software, by which the equilibrium configurations and the load-displacement curves during the snap process can be easily obtained. The presented analytical model is validated by the numerical simulation and literature models, while the factors affecting the bistability of anti-symmetric cylindrical shells are revisited. In addition, the planform effects of anti-symmetric cylindrical shells with rectangular, elliptical and trapezoidal planform are discussed. The results show that the presented analytical model improves the accuracy of the prediction of the principal curvature of second equilibrium configuration and agree well with the numerical results. [ABSTRACT FROM AUTHOR]

Abstract: This paper studied through modeling and computer simulation the two-step thermal-induced phase separation phenomenon in a symmetric polymer blend via spinodal decomposition. The two-step phase separation phenomenon involves the two-step process where the initial quench is allowed to phase separate for a certain period of time before the second quench takes place into the unstable region. The second quench occurs at the transition between the early and intermediate stages of spinodal decomposition, which is the time frame when functional polymeric materials with predefined material properties are fabricated. The one-dimensional model consisted of the Cahn–Hilliard theory for spinodal decomposition, and incorporated the Flory–Huggins–deGennes free energy equation, the slow mode mobility theory and reptation model for polymer diffusion. The numerical results replicated frequently reported experimental observations published in the literature. This includes the observation that secondary phase separation occurs only if the second quench is sufficiently deep. Furthermore, the numerical results indicate that a dimensionless diffusion coefficient may be used as a parameter to control the formation and evolution of the phase-separated regions during spinodal decomposition as a means to tailor-make functional polymeric materials with predefined material properties. [Copyright &y& Elsevier]

We focus on highly grafted binary polymer brushes with compatible components in the cases of different chain lengths. Layered structures parallel to the surface that indicating “phase separation” are observed in a series of dissipative particle dynamics simulations. The stretch parameters indicate that the short chains are suppressed in the lower layer of the film, whereas the longer chains are much stretched in the region dominated by the short chains (lower layer) but possess relaxed conformations in the upper layer. By slightly changing the solvent selectivity to prefer the short chains, we find a reversion of the layered structure. Such a sensitive switch of film property implies its potential application as tuning the wettability and adhesion of the surface in industry. [ABSTRACT FROM AUTHOR]

The depletion interaction between proteins caused by addition of either uncharged or partially charged oligomers was studied using the canonical Monte Carlo simulation technique and the integral equation theory. A protein molecule was modeled in two different ways: either as (i) a hard sphere of diameter 30.0 Å with net charge 0, or +5, or (ii) as a hard sphere with discrete charges (depending on the pH of solution) of diameter 45.4 Å. The oligomers were pictured as tangentially jointed, uncharged, or partially charged, hard spheres. The ions of a simple electrolyte present in solution were represented by charged hard spheres distributed in the dielectric continuum. In this study we were particularly interested in changes of the protein-protein pair-distribution function, caused by addition of the oligomer component. In agreement with previous studies we found that addition of a nonadsorbing oligomer reduces the phase stability of solution, which is reflected in the shape of the protein-protein pair-distribution function. The value of this function in protein-protein contact increases with increasing oligomer concentration, and is larger for charged oligomers. The range of the depletion interaction and its strength also depend on the length (number of monomer units) of the oligomer chain. The integral equation theory, based on the Wertheim Ornstein-Zernike approach applied in this study, was found to be in fair agreement with Monte Carlo results only for very short oligomers. The computer simulations for a model mimicking the lysozyme molecule (ii) are in qualitative agreement with small-angle neutron experiments for lysozyme-dextran mixtures. [ABSTRACT FROM AUTHOR]

The authors measure 2H NMR multitime correlation functions to investigate the segmental motion of poly(propylene glycol) containing various amounts of the salt LiClO4. 2H NMR two-time correlation functions indicate that addition of salt affects not only the time scale of the segmental motion, but also the degree of the nonexponential relaxation behavior. To quantify the origin of the nonexponential segmental motion, the authors analyze 2H NMR three-time correlation functions. In general, nonexponential relaxation can result from homogeneous dynamics, i.e., intrinsic nonexponentiality, and from heterogeneous dynamics, i.e., existence of a distribution of correlation times G(ln τ). For the studied high and low salt concentrations, including neat poly(propylene glycol), the analysis shows that both homogeneous and heterogeneous contributions are important. 2H NMR four-time correlation functions allow the authors to measure the lifetime of the dynamical heterogeneities. For the studied salt concentrations, the rate exchange occurs on the same time scale as the segmental motion, indicating short-lived dynamical heterogeneities. To arrive at these results, the authors reconsider the interpretation of 2H NMR three-time correlation functions. Results of analytical calculations and computer simulations show that it is necessary to extend the previous way of analysis so as to include effects due to correlated back-and-forth jumps. [ABSTRACT FROM AUTHOR]

In this work we investigate how a pattern imposed in a copolymer film at a certain distance from the surface propagates through the film onto an adsorbing heterogeneous surface. We bias the copolymer film to adopt a specified target pattern and then use simulation to design a surface pattern that helps the adsorbed film to maintain that target pattern. We examine the effect of varying the copolymer chain length, the size of the target pattern, and the distance from the surface where the target pattern is applied, z′, on the extent of pattern transfer. For each chain length, target pattern, and z′ we compare the energy of the system when a pattern is applied in the bulk to the energy when no pattern is applied in order to understand why a certain pattern size is transferred to the surface with higher fidelity than the others. At constant chain length, pattern transfer is best when the pattern size brings the energy of the system close to the energy when no pattern is applied. At constant pattern size, pattern transfer is best in the systems with longer chains. This is because longer chains are more likely to adsorb as brushes and loops which then helps transfer the pattern through the adsorbed film down to the surface. [ABSTRACT FROM AUTHOR]

Large-scale computer simulations show that two orthogonal external fields can control the orientation of lamellar microdomains in diblock copolymers in three dimensions and lead to an enhanced long-range ordering. [ABSTRACT FROM AUTHOR]

There is a rapidly growing interest in performing computer simulations in a closed space, avoiding periodic boundary conditions. To extend the range of potential systems to include also macromolecules, we describe an algorithm for computer simulations of polymer chain molecules on S3, a hypersphere in four dimensions. In particular, we show how to generate initial conformations with a bond angle distribution given by the persistence length of the chain and how to calculate the bending forces for a molecule moving on S3. Furthermore, we discuss how to describe the shape of a macromolecule on S3, by deriving the radius of gyration tensor in this non-Euclidean space. The results from both Monte Carlo and Brownian dynamics simulations in the infinite dilution limit show that the results on S3 and in R3 coincide, both with respect to the size and shape as well as for the diffusion coefficient. All data on S3 can also be described by master curves by suitable scaling by the corresponding values in R3. We thus show how to extend the use of spherical boundary conditions, which are most effective for calculating electrostatic forces, to polymer chain molecules, making it possible to perform simulations on S3 also for polyelectrolyte systems. [ABSTRACT FROM AUTHOR]

Fluorescence lifetime measurements in a polymer chain are modeled using a memory function expansion, computer simulations, and simple scaling arguments. Unless the quenching rate is localized and infinitely fast, the fluorescence lifetime is generally not equivalent to the first passage time. The fluorescence lifetime distribution is decomposed into memory functions that can be measured separately in single-molecule experiments. The leading order of the expansion gives the Wilemski–Fixman (WF) approximation, and the convergence of higher order terms determines its validity. Simulations of the fluorescence quenching on a Rouse chain verify the accuracy of the WF approximation at small contact radii, short contour lengths, and small quenching rates. Detailed investigation of the average fluorescence lifetime reveals two competing mechanisms: the independent motion of end-to-end vector, which dominates at small contact radius, and the slowest relaxation of polymer, which dominates at large contact radius. The Wilemski–Fixman rate is used in combination with scaling arguments to predict the dependence of fluorescence lifetime on the contour length. Our predictions for the scaling of the average lifetime with the contour length are in good agreement with both simulations and recent experiments by Eaton and his group [L. J. Lapidus, W. A. Eaton, and J. Hofrichter, Proc. Natl. Acad. Sci. U.S.A. 97, 7220 (2000)]. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The mean first passage time for the close approach of the ends of a polymer is investigated by computer simulation of the Brownian dynamics of a Rouse model. There are N bonds, of length b, and the contact distance is a. The results for N=50, 75, and 100 are compared with theoretical predictions by Wilemski and Fixman (WF), and by Szabo, Schulten, and Schulten (SSS). Theory differs from simulation by a factor of 1.3 to 3 when a is comparable to b but improves considerably when a is reduced. Closest agreement is obtained at a/b=0.1 for WF with a δ function sink, where theory is within the statistical error (≊5%) of the simulation for each case. [ABSTRACT FROM AUTHOR]

A numerical model based on the method polynomial regression is developed to simulate the mechanical behavior of reinforced concrete beams strengthened with a carbon-fiber-reinforced polymer and subjected to four-point bending. The results obtained are in good agreement with data of laboratory tests. [ABSTRACT FROM AUTHOR]

Simulating complex fluids or in general complex molecular systems requires approaches covering decades of time and length scales. This usually cannot be achieved within one simulation model. Over the years many different methods and models have been developed ranging from rather generic models, representing most efficiently the universal statistical mechanical properties of e.g. polymers, to all atom models and even quantum mechanical treatments. While these allow for scientifically very important studies in their own right, only a combination and close link between models of different levels allows for a truly quantitative description of materials and processes. In the present contribution we discuss an adaptive resolution approach where different levels of detail are treated within one simulation and the molecules are free to diffuse between different regions in space, where the molecules interact with different interaction potentials. [ABSTRACT FROM AUTHOR]

This review discusses several computational methods used on different length and time scales for the simulation of material behavior. First, the importance of physical modeling and its relation to computer simulation on multiscales is discussed. Then, computational methods used on different scales are shortly reviewed, before we focus on the molecular dynamics (MD) method. Here we survey in a tutorial-like fashion some key issues including several MD optimization techniques. Thereafter, computational examples for the capabilities of numerical simulations in materials research are discussed. We focus on recent results of shock wave simulations of a solid which are based on two different modeling approaches and we discuss their respective assets and drawbacks with a view to their application on multiscales. Then, the prospects of computer simulations on the molecular length scale using coarse-grained MD methods are covered by means of examples pertaining to complex topological polymer structures including star-polymers, biomacromolecules such as polyelectrolytes and polymers with intrinsic stiffness. This review ends by highlighting new emerging interdisciplinary applications of computational methods in the field of medical engineering where the application of concepts of polymer physics and of shock waves to biological systems holds a lot of promise for improving medical applications such as extracorporeal shock wave lithotripsy or tumor treatment. [ABSTRACT FROM AUTHOR]

Neutron scattering is a unique probe of the structural and dynamic properties of polymeric materials. Two properties of the neutron particle have played a major role in the development of the technique: (i) length and timescales accessed by neutron scattering simultaneously match those of typical molecular processes, and (ii) the capability to manipulate the contrast by deuteration. Starting from what could now be regarded as rather simple experiments on common polymeric systems, the technique has been developed into a tool used to elucidate structure and dynamics of increasingly complex, multi-component materials. Exploiting the high penetration power of the neutron particle, in situ measurements can be performed, so that structural changes can be followed mimicking processing conditions. With continuous improvements in neutron flux and instrumentation, the range of systems that can be investigated is continuously expanding, as well as the accessible spatial and time range instrumentally accessible. Recent years have seen not only advances in synthetic chemistry, but also in molecular modelling and neutron scattering has afforded a way to compare experimental data and computer simulations. With plans being made for next generation neutron sources, e.g. the second target station at ISIS and SNS in the USA, the future is looking very bright for neutron scattering. [ABSTRACT FROM AUTHOR]

Experiment and computer simulation are two complementary tools to understand the dynamics and behavior of biopolymers in solution. One particular area of interest is the ensemble of conformations populated by a particular molecule in solution. For example, what fraction of a protein sample exists in its folded conformation? How often does a particular peptide form an alpha helix versus a beta hairpin? To address these questions, it is important to determine the sensitivity of a particular experiment to changes in the distribution of molecular conformations. Consequently, a general analytic formalism is proposed to determine the sensitivity of a spectroscopic observable to the underlying distribution of conformations. A particular strength of the approach is that it provides an expression for a weighted average across conformational substates that is independent of the averaging function used. The formalism is described and applied to experimental and simulated nuclear Overhauser enhancement (NOE) and 3J-coupling data on peptides in solution. [ABSTRACT FROM AUTHOR]