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This work aims at measuring experimentally proton induced secondary electron energy spectra after interaction with gold nano particles (GNPs) and polymer-coated GNPs. Backscattered electron energy spectra were collected over a 0 to 1000 eV energy range using a retarding field analyzer (RFA). This paper presents the spectra obtained for proton beam energies of 0.5 and 2 MeV and diameter 2.5 and 3.8 nm GNPs. The spectra were also measured for 3.8 nm GNPs after 5 and 10 MeV proton irradiations. GNPs were deposited on a 100 nm carbon film. Each experimental spectrum was compared with dedicated simulations based on existing numerical models used in the TRAX and Geant4 Monte Carlo codes. For 100 nm carbon target, good agreement between experimental, TRAX and Geant4 simulation results can be observed. For 3.8 nm GNPs, the TRAX simulations reproduce with good agreement the electron energy spectra produced after 0.5, 2, 5 and 10 MeV proton irradiations, while Geant4 spectra display a lower secondary electron yield at low energy (<600 eV) for all the studied energies. This underestimation can mostly be explained by the 790 eV threshold applied in the condensed history model used by Geant4 which impacts the secondary electron energy distribution. Results obtained for carbon and gold targets highlight the impact of the secondary electron production threshold for proton ionization process considered in condensed history models. The experimental results demonstrate that the single interaction approach used in TRAX is adapted to reproduce secondary electron emission from GNPs. On the other hand, the standard electron generation threshold implement in G4BetheBlochModel and G4BraggModel condensed-history models used in Geant4 is not adapted to reproduce low energy electron emission in gold targets. Finally, the results highlight that the GNP coating leads to a decrease of the electron yield and mostly affects low energy electrons (<500 eV) emitted from GNPs.

Functionalizing nanoparticles with organic ligands, such as oligomers, polymers, DNA, and proteins, is an attractive way to manipulate the interfacial interactions between the nanoparticles and the medium the particles are placed in, and thus control the nanoparticle assembly. In this paper we have conducted a Monte Carlo simulation study on copolymer grafted spherical nanoparticles to show the tremendous potential of using monomer sequence on the copolymers to tune the grafted chain conformation, and thus the effective interactions between copolymer grafted nanoparticles. We have studied AB copolymers with alternating, multiblock, or diblock sequences, where either A monomers or B monomers have monomer-monomer attractive interactions. Our focus has been to show the nontrivial effect of monomer sequence on the conformations of the grafted copolymers at various particle diameters, grafting densities, copolymer chain lengths, and monomer-monomer interactions in an implicit small molecule solvent. We observe that the monomer sequence, particle diameter, and grafting density dictate whether (a) the grafted chains aggregate to bring attractive monomers from multiple grafted chains together (interchain and intrachain monomer aggregation) if the enthalpy gained by doing so offsets the entropic loss caused by stretching of chains, or (b) each grafted chain folds onto itself to bring its attractive monomers together (only intrachain monomer aggregation) if the entropic loss from interchain aggregation cannot be overcome by the enthalpic gain. For six copolymers of chain length N=24 grafted on a spherical particle of diameter D=4, interchain and intrachain monomer aggregation occurs, and the radius of gyration varies nonmonotonically with increasing blockiness of the monomer sequence. At larger particle diameters the grafted chains transition to purely intrachain monomer aggregation. The radius of gyration varies monotonically with monomer sequence for intrachain monomer aggregation because as the sequence becomes blockier (like monomers are grouped together), the copolymer chain has to fold less compactly to maximize the enthalpically favorable contacts while maintaining high conformational entropy. The radius of gyration of alternating and diblock copolymers scales with chain length N through a power law (1/2) = alphaN(nu) with the prefactor alpha and scaling exponent nu, varying with monomer sequence and monomer-monomer attraction strength.

Hypothetical scanning Monte Carlo (HSMC) is a method for calculating the absolute entropy S and free energy F from a given MC trajectory developed recently and applied to liquid argon, TIP3P water, and peptides. In this paper HSMC is extended to random coil polymers by applying it to self-avoiding walks on a square lattice--a simple but difficult model due to strong excluded volume interactions. With HSMC the probability of a given chain is obtained as a product of transition probabilities calculated for each bond by MC simulations and a counting formula. This probability is exact in the sense that it is based on all the interactions of the system and the only approximation is due to finite sampling. The method provides rigorous upper and lower bounds for F, which can be obtained from a very small sample and even from a single chain conformation. HSMC is independent of existing techniques and thus constitutes an independent research tool. The HSMC results are compared to those obtained by other methods, and its application to complex lattice chain models is discussed; we emphasize its ability to treat any type of boundary conditions for which a reference state (with known free energy) might be difficult to define for a thermodynamic integration process. Finally, we stress that the capability of HSMC to extract the absolute entropy from a given sample is important for studying relaxation processes, such as protein folding.

Polymer models describing the statistics of biomolecules under confinement have applications to a wide range of single-molecule experimental techniques and give insight into biologically relevant processes in vivo. In this paper, we determine the transverse position and bending correlation functions for a wormlike chain confined within slits and cylinders (with one and two confined dimensions, respectively) using a mean-field approach that enforces rigid constraints on average. We show the theoretical predictions accurately capture the statistics of a wormlike chain from Monte Carlo simulations in both confining geometries for both weak and strong confinement. We also show that the longitudinal correlation function is accurately computed for a chain confined to a slit and leverages the accuracy of the model to suggest an experimental technique to infer the (often unobservable) transverse statistics from the (directly observable) longitudinal end-to-end distance. [ABSTRACT FROM AUTHOR]

The nonlinear viscoelastic behavior of the Fraenkel-chain model is studied with respect to the constitutive equation of the Rouse model. Distinctly different from the results of the Rouse model, the Fraenkel-chain model gives the following characteristic nonlinear behavior: (a) The two distinct dynamic modes in the relaxation modulus GS(t,λ)—as observed in the linear region reported in Paper I [Y.-H. Lin and A. K. Das, J. Chem. Phys. 126, 074902 (2007), preceding paper]—or in the first normal-stress difference function GΨ1(t,λ) are shown to have different strain dependences: strain hardening for the fast mode and strain softening for the slow mode. (b) The Lodge–Meissner relation GS(t,λ)=GΨ1(t,λ) holds over the whole time range, which has been shown both analytically and by simulation. (c) The second normal-stress difference is nonzero, being positive in the fast-mode region and negative in the slow-mode region. The comparisons between orientation and stress for all tensor components consistently confirm the strong correlation of the slow mode as well as its entropic nature with the segmental-orientation anisotropy as shown in the linear region studied in Paper I. A consequence of this correlation is the applicability of the stress-optical rule in the slow-mode region. This also leads to the expectation that the damping function h(λ)=GS(t,λ)/GS(t,λ→0) and the ratio between the first and second normal-stress differences, N2(t,λ)/N1(t,λ), are described by the orientation tensor which has the same form as that given by Doi and Edwards [J. Chem. Soc. Faraday Trans. 2 74, 1789 (1978); 74, 1802 (1978)] with independent-alignment approximation for an entangled system. The similarity between the slow mode of an entanglement-free Fraenkel-chain system and the terminal mode of an entangled polymer system as observed in the comparison of theory, simulation, and experiment suggests that the close correlation of the entropic nature of the mode with the orientation anisotropy—as of the Fraenkel segment or the primitive step in the Doi–Edwards theory—is a generally valid physical concept in polymer viscoelasticity. [ABSTRACT FROM AUTHOR]

In the first paper of this series, we developed a new one-dimensional Monte Carlo approach for the study of flexible chains that are translocating through a small channel. We also presented a numerical scheme that can be used to obtain exact values for both the escape times and the escape probabilities given an initial pore-polymer configuration. We now present and discuss the fundamental scaling behaviors predicted by this Monte Carlo method. Our most important result is the fact that, in the presence of an external bias E, we observe a change in the scaling law for the translocation time τ as function of the polymer length N: In the general expression τ∼Nβ/E, the exponent changes from β=1 for moderately long chains to β=1+ν or β=2ν for very large values of N (for Rouse and Zimm dynamics, respectively). We also observe an increase in the effective diffusion coefficient due to the presence of entropic pulling on unbiased polymer chains. [ABSTRACT FROM AUTHOR]

The rheological behaviors of polymer brush under oscillatory shear flow were investigated by nonequilibrium Monte Carlo simulation. The grafted chain under oscillatory shear flow exhibited a waggling behavior like a flower, and the segments were found to have different oscillatory phases along the chain contour. Stress tensor was further obtained based on the statistics of sampled configuration distribution functions. The simulation reproduced the abrupt increase of the first normal stress difference N1 with the flow velocity over a critical value, as observed in the experiment of Klein et al. [Nature (London) 352, 143 (1991)]. However, our simulation did not reproduce the brush thickening with shear velocity increased, which was suggested to be responsible for the abrupt increase of N1 in the above-mentioned paper. This simulation demonstrates that the increase of normal stress might be an inherent behavior of polymer brush due to chain deformation under flow. [ABSTRACT FROM AUTHOR]

This paper investigates the conformational and scaling properties of long linear polymer chains. These investigations are done with the aid of Monte Carlo (MC) and molecular dynamics (MD) simulations. Chain lengths that comprise several orders of magnitude to reduce errors of finite size scaling, including the effect of solvent quality, ranging from the athermal limit over the θ-transition to the collapsed state of chains are investigated. Also the effect of polydispersity on linear chains is included which is an important issue in the real fabrication of polymers. A detailed account of the hybrid MD and MC simulation model and the exploited numerical methods is given. Many results of chain properties in the extrapolated limit of infinite chain lengths are documented and universal properties of the chains within their universality class are given. An example of the difference between scaling exponents observed in actual solvents and those observed in the extremes of “good solvents” and “θ-solvents” in simulations is provided by comparing simulation results with experimental data on low density polyethylene. This paper is concluded with an outlook on the extension of this study to branched chain systems of many different branching types. [ABSTRACT FROM AUTHOR]

Background and Aim: One of the best radiotherapy methods that have been suggested for the treatment of cancers such as brain, head and neck, skin, and recently liver is boron neutron capture therapy. Therefore, neutron shielding in the treatment room is important because the absorption of neutrons in the shield may lead to the production of gamma rays, which can have destructive effects. While an increase in density of the elements used in the shield is generally sufficient for gamma radiation shielding, protection against neutrons is more complicated. It is due to variations in neutron interactions with the matter based on their kinetic energy and reaction cross-sections with the shield component atoms. Methods: In this paper, it will be developed a neutron shield that is a proper shield against ionizing radiation, which will be accomplished via the use of experiment-based optimization of materials backed by Monte Carlo simulations. Also, the neutron shielding characteristics of conventional and heavy-weight shields modified with epoxy resin and gadolinium oxide was investigated. The shielding effectiveness against neutrons was initially modelled using the MCNP code. In the end, the comparison of MCNP simulated results and the real experiment was presented. Results: Gadolinium is an efficient additive for weakening low-energy neutrons, but it does not have a good effect in terms of shielding against fast neutrons. Also, Epoxy resin improves the shielding property of the composite against neutron radiation. Conclusion: The results show that the addition of the polymers used in composite shielding and the proposed method for developing the neutron shield can improve the shielding characteristics for neutron beams. [ABSTRACT FROM AUTHOR]

Morphology of symmetric ABCD tetrablock quaterpolymers in melt was studied by the Monte Carlo (MC) simulation, where the volume fractions of the block chains, f, kept the relationships of fA = fD and fB = fC, and the volume fraction of the two mid-blocks φ was defined as φ = fB + fC. Previous self-consistent field theory for ABCD reported morphological change including several structures; however, the scope was limited within a two-dimensional system. To the contrary, in this paper, MC simulations were carried out in three dimensions with changing the φ value finely, which resulted in finding a tetracontinuous structure in the range of 0.625 ⩽ φ ⩽ 0.75. Moreover the tetracontinuous structure has been found to be the gyroid structure, and the mean curvature of the B/C interface is nearly zero. We concluded that the B/C interface must be the Schoen gyroid surface, one of three-dimensional periodic minimal surfaces. The geometrical nature of the A/B interface should be equivalent to that of the C/D interface, and they stand as level surfaces to the Schoen gyroid surface. [ABSTRACT FROM AUTHOR]

The design of functional materials capable of sequestering CO2 from the atmosphere or capturing that emitted from human activities is one of the major challenges in materials science today. Porous organic polymers have already proven efficient for the selective adsorption of CO2. Various combinations of building units and functionalities can create different topologies of porous organic systems. We have investigated the effect of two trigonal connectors (triphenylamine and triphenylpyridine) and three nitrogen–nitrogen linkages (azo, azoxy and azodioxy) on the geometrical and adsorption properties of porous organic polymers. The computational chemistry methods (calculation of binding energies, electrostatic potential maps and grand-canonical Monte Carlo simulations) were used to evaluate the effects of various structural patterns and four different layer stacking modes on the CO2 adsorption. In the experimental part of the work, we synthesized and characterized (by IR and 13C CP/MAS NMR spectroscopy, powder X-ray diffraction, elemental analysis, thermogravimetric analysis and nitrogen adsorption–desorption measurements) a new azo-linked pyridine-based polymer which exhibits high surface area of 606 m2 g−1 and the CO2 uptake of 32 mg g−1, which matches nicely with the results of the GCMC study. The computational results indicated that azoxy and azodioxy linkages strongly promote CO2 adsorption and this can be tentatively predicted from the calculated electrostatic potential values. Although the correlation between the calculated electrostatic potential values and CO2 uptake is not always straightforward, it can provide a simple model for evaluating the CO2 adsorption properties of porous organic polymers prior to synthesis. [ABSTRACT FROM AUTHOR]

We investigated the role of particle potential symmetry on self-assembly by Monte Carlo simulation with a particular view toward synthetically creating structures of prescribed form and function. First, we established a general tendency for the rotational potential symmetries of the particles to be locally preserved upon self-assembly. Specifically, we found that a dipolar particle potential, having a continuous rotational symmetry about the dipolar axis, gives rise to chain formation, while particles with multipolar potentials (e.g., square quadrupole) having discrete rotational symmetries lead to the self-assembly of "random surface" polymers preserving the rotational symmetries of the particles within these sheet structures. Surprisingly, these changes in self-assembly geometry with the particle potential symmetry are also accompanied by significant changes in the thermodynamic character and in the kinetics of the self-assembly process. Linear chain growth involves a continuous chain growth process in which the chains break and reform readily, while the growth of the two-dimensional polymers only occurs after an "initiation" or "nucleation" time that fluctuates from run to run. We show that the introduction of artificial seeds provides an effective method for controlling the structure and growth kinetics of sheet-like polymers. The significance of these distinct modes of polymerization on the functional character of self-assembly growth is illustrated by constructing an artificial centrosome structure derived from particles having continuous and discrete rotational potential symmetries. [ABSTRACT FROM AUTHOR]

Stochastic Brownian dynamics is an extremely powerful way to simulate the polymer dynamics in solutions and melts. Mathematically, these models are described by stochastic differential equations. The most challenging problems are the Monte Carlo algorithm, which simulates the motion of a large number of model particles and hence requires an enormous amount of computer time. It is therefore necessary to develop an efficient numerical method in operational emergency response applications. In this paper, we give an improved multilevel Monte Carlo (improved-MLMC) method based on equilibrium control variables at each level to calculate the propagation of polymers. The improved-MLMC method can be shown to result in asymptotically optimal random errors and reduce total cost when compared to the standard Monte Carlo and MLMC methods. Finally, the effect of the Wi number (dimensionless parameter) on the total cost of the presented MLMC method is also discussed in detail. [ABSTRACT FROM AUTHOR]

In efforts to produce polymeric materials with tailored physical properties, significant interest has grown around the ability to control the spatial organization of nanoparticles in polymer nanocomposites. One way to achieve controlled particle arrangement is by grafting the nanoparticle surface with polymers that are compatible with the matrix, thus manipulating the interfacial interactions between the nanoparticles and the polymer matrix. Previous work has shown that the molecular weight of the grafted polymer, both at high grafting density and low grafting density, plays a key role in dictating the effective inter-particle interactions in a polymer matrix. At high grafting density nanoparticles disperse (aggregate) if the graft molecular weight is higher (lower) than the matrix molecular weight. At low grafting density the longer grafts can better shield the nanoparticle surface from direct particle-particle contacts than the shorter grafts and lead to the dispersion of the grafted particles in the matrix. Despite the importance of graft molecular weight, and evidence of non-trivial effects of polydispersity of chains grafted on flat surfaces, most theoretical work on polymer grafted nanoparticles has only focused on monodisperse grafted chains. In this paper, we focus on how bidispersity in grafted chain lengths affects the grafted chain conformations and inter-particle interactions in an implicit solvent and in a dense homopolymer polymer matrix. We first present the effects of bidispersity on grafted chain conformations in a single polymer grafted particle using purely Monte Carlo (MC) simulations. This is followed by calculations of the potential of mean force (PMF) between two grafted particles in a polymer matrix using a self-consistent Polymer Reference Interaction Site Model theory-Monte Carlo simulation approach. Monte Carlo simulations of a single polymer grafted particle in an implicit solvent show that in the bidisperse polymer grafted particles with an equal number of short and long grafts at low to medium grafting density, the short grafts are in a more coiled up conformation (lower radius of gyration) than their monodisperse counterparts to provide a larger free volume to the longer grafts so they can gain conformational entropy. The longer grafts do not show much difference in conformation from their monodisperse counterparts at low grafting density, but at medium grafting density the longer grafts exhibit less stretched conformations (lower radius of gyration) as compared to their monodisperse counterparts. In the presence of an explicit homopolymer matrix, the longer grafts are more compressed by the matrix homopolymer chains than the short grafts. We observe that the potential of mean force between bidisperse grafted particles has features of the PMF of monodisperse grafted particles with short grafts and monodisperse grafted particles with long grafts. The value of the PMF at contact is governed by the short grafts and values at large inter-particle distances are governed by the longer grafts. Further comparison of the PMF for bidisperse and monodisperse polymer grafted particles in a homopolymer matrix at varying parameters shows that the effects of matrix chain length, matrix packing fraction, grafting density, and particle curvature on the PMF between bidisperse polymer grafted particles are similar to those seen between monodisperse polymer grafted particles. [ABSTRACT FROM AUTHOR]

The classical bond-fluctuation model (BFM) is an efficient lattice Monte Carlo algorithm for coarse-grained polymer chains where each monomer occupies exclusively a certain number of lattice sites. In this paper we propose a generalization of the BFM where we relax this constraint and allow the overlap of monomers subject to a finite energy penalty [variant_greek_epsilon]. This is done to vary systematically the dimensionless compressibility g of the solution in order to investigate the influence of density fluctuations in dense polymer melts on various static properties at constant overall monomer density. The compressibility is obtained directly from the low-wave vector limit of the static structure factor. We consider, e.g., the intrachain bond-bond correlation function P(s) of two bonds separated by s monomers along the chain. It is shown that the excluded volume interactions are never fully screened for very long chains. If distances smaller than the thermal blob size are probed (s≤g) the chains are swollen according to the classical Fixman expansion where, e.g., P(s)∼g-1s-1/2. More importantly, the polymers behave on larger distances (s>g) like swollen chains of incompressible blobs with P(s)∼g0s-3/2. [ABSTRACT FROM AUTHOR]

In this paper, we construct an effective model for the dynamics of an excluded-volume chain under confinement by extending the formalism of Rouse modes. We make specific predictions about the behavior of the modes for a single polymer confined to a tube. The results are tested against Monte Carlo simulations using the bond-fluctuation algorithm which uses a lattice representation of the polymer chain with excluded-volume effects. [ABSTRACT FROM AUTHOR]

Monte Carlo simulations of a model fluid in which the particles interact via a continuous potential that has a logarithmic divergence at a pair separation of σ, which we introduced in J. G. Powles et al., Proc. R. Soc. London, Ser. A 455, 3725 (1999), have been carried out. The potential has the form, [lowercase_phi_synonym](r)=-ε ln(f(r)), where ε sets the energy scale and f(r)=1-(σ/r)m. The value of m chosen was 12 but the qualitative trends depend only weakly on the value of m, providing it is greater than 3. The potential is entirely repulsive and has a logarithmic divergence as ∼-ln(r/σ-1) in the r→σ limit. Predictions of the previous paper that the internal energy can be computed at all temperatures using the standard statistical mechanics formula for continuous potentials are verified here. The pressure can be calculated using the usual virial expression for continuous potentials, although there are practical limitations in resolving the increasingly important contribution from the r→σ limit at reduced temperatures greater than ∼5. The mean square force ≤F2≥ and infinite frequency shear G∞ and bulk K∞ moduli are only finite for T*=kBT/ε<1. The logarithmic fluid’s physical properties become increasingly more like that of the hard sphere fluid with increasing temperature, showing a sharp transition in the behavior of the mean square force and infinite frequency elastic constants at T*=1. The logarithmic fluid is shown to exhibit a solid-fluid phase transition. [ABSTRACT FROM AUTHOR]

Proteins contain many voids, which are unfilled spaces enclosed in the interior. A few of them have shapes compatible to ligands and substrates and are important for protein functions. An important general question is how the need for maintaining functional voids is influenced by, and affects other aspects of proteins structures and properties (e.g., protein folding stability, kinetic accessibility, and evolution selection pressure). In this paper, we examine in detail the effects of maintaining voids of different shapes and sizes using two-dimensional lattice models. We study the propensity for conformations to form a void of specific shape, which is related to the entropic cost of void maintenance. We also study the location that voids of a specific shape and size tend to form, and the influence of compactness on the formation of such voids. As enumeration is infeasible for long chain polymer, a key development in this work is the design of a novel sequential Monte Carlo strategy for generating large number of sample conformations under very constraining restrictions. Our method is validated by comparing results obtained from sampling and from enumeration for short polymer chains. We succeeded in accurate estimation of entropic cost of void maintenance, with and without an increasing number of restrictive conditions, such as loops forming the wall of void with fixed length, with additionally fixed starting position in the sequence. Additionally, we have identified the key structural properties of voids that are important in determining the entropic cost of void formation. We have further developed a parametric model to predict quantitatively void entropy. Our model is highly effective, and these results indicate that voids representing functional sites can be used as an improved model for studying the evolution of protein functions and how protein function relates to protein stability. [ABSTRACT FROM AUTHOR]

A dynamic Monte Carlo simulation of the collapse transition of polymer chains is presented. The chains are represented as self-avoiding walks on the simple cubic lattice with a nearest-neighbor contact potential to model the effect of solvent quality. The knot state of the chains is determined using the knot group procedure presented in the accompanying paper. The equilibrium knot spectrum and the equilibrium rms radius of gyration as functions of the chain length and the contact potential are reported. The collapse transition was studied following quenches from good-to poor-solvent conditions. Our results confirm the prediction that the newly formed globule is not yet at equilibrium, since it has not yet achieved its equilibrium knot spectrum. For our model system, the relaxation of the knot spectrum is about an order of magnitude slower than that of the radius of gyration. The collapse transition is also studied for a model in which both ends of the chain remain in good-solvent conditions. Over the time scale of these simulations, knot formation is frustrated in this inhomogeneous model, verifying that the mechanism of knotting is the tunneling of chain ends in and out of the globule. [ABSTRACT FROM AUTHOR]

This paper reports the first computational estimation of the comb polymers’ third virial coefficients. The number of the chains in the comb polymers range from 5 to 11. An algorithm that counts the contributing terms of the third virial coefficients in an accelerated manner is presented along with its efficiency dependence on the polymers’ size. In addition, the second virial coefficients are estimated for the comb polymers and compared to previously reported results. [ABSTRACT FROM AUTHOR]

Large scale molecular dynamics and grand canonical Monte Carlo simulation techniques are used to study the behavior of the interdiffusion of a solvent into an entangled polymer matrix as the state of the polymer changes from a melt to a glass. The weight gain by the polymer increases with time t as t1/2 in agreement with Fickian diffusion for all cases studied, although the diffusivity is found to be strongly concentration dependent especially as one approaches the glass transition temperature of the polymer. The diffusivity as a function of solvent concentration determined using the one-dimensional Fick’s model of the diffusion equation is compared to the diffusivity calculated using the Darken equation from simulations of equilibrated solvent-polymer solutions. The diffusivity calculated using these two different approaches are in good agreement. The behavior of the diffusivity strongly depends on the state of the polymer and is related to the shape of the solvent concentration profile.© 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

This paper develops reliability-based design provisions for eccentrically loaded rectangular concrete short columns reinforced with fiber-reinforced polymer (FRP) bars. First, closed-form solutions for the axial force-moment strength interaction diagram were presented. Subsequently, the statistical parameters of the resistance model were estimated based on available experimental data and Monte Carlo simulations. The first-order second-moment (FOSM) method was then applied to calibrate strength reduction factors to meet the uniform target reliability level βT = 4.0. To verify the results obtained, a comparison between reliability indexes obtained from Monte Carlo simulations and those from the FOSM was conducted. Finally, sensitivity analyses were performed to evaluate the influence of various design parameters on the reliability index, indicating that the reliability of FRP-reinforced concrete columns is dependent on the axial reinforcement stiffness Efρf. As a result, this study recommended strength reduction factors of 0.60 for the columns with Efρf ≤ 2 GPa (290 ksi), 0.65 for those with Efρf > 4 GPa (580 ksi), and a linear variation between the two. [ABSTRACT FROM AUTHOR]

In this paper we present a comprehensive lattice Monte Carlo study of long chain conformations at solid–polymer melt interfaces. Segmental scale interfacial features, like the bond orientational distribution were found to be independent of surface–segment energetics, and statistically identical with Helfand’s predictions for the full-occupancy, infinite chain length limit. Conformational statistics of chains longer than 5–6 statistical segments followed the predictions of the Scheutjens–Fleer theory and the same power laws as a single ideal chain at the critical value of the surface–segment adsorption free-energy. Our simulations tested the predictions of random walk next to a ‘‘reflective’’ surface statistics for the spatial variations of chain dimensions and chain center of mass density. It was found that these statistics furnish the correct long chain limit, independently of surface–segment energetics. The random walk next to a ‘‘reflective’’ boundary predictions for the ‘‘adsorbed’’ amount and the distributions of tail, loop, and train number, as well as tail and loop size were in quantitative agreement with the simulation data. The correspondence between random walks and real (or simulated) chains required the knowledge of a single, microscopic parameter, the number of chemical segments per statistical segment, a∞. This quantity was very close to the average length of adsorbed sequences (trains), in the long chain limit. Our simulations tested thoroughly and established firmly the validity of ‘‘reflective’’ boundary statistics in the melt. The inevitability of these statistics has broad implications on ‘‘desorption’’ kinetics, chain mobility, and chain relaxation, which are currently under study. [ABSTRACT FROM AUTHOR]

Many variants of RNA, DNA, and even proteins can be considered semiflexible polymers, where bending stiffness, as a type of energetic penalty, competes with attractive van der Waals forces in structure formation processes. Here, we systematically investigate the effect of the bending stiffness on ground-state conformations of a generic coarse-grained model for semiflexible polymers. This model possesses multiple transition barriers. Therefore, we employ advanced generalized-ensemble Monte Carlo methods to search for the lowest-energy conformations. As the formation of distinct versatile ground-state conformations, including compact globules, rod-like bundles, and toroids, strongly depends on the strength of the bending restraint, we also performed a detailed analysis of contact and distance maps. [ABSTRACT FROM AUTHOR]

In this paper we continue our recent analysis [K. Haydukivska et al., J. Mol. Liq., 2021, 328, 115456] of complex molecules with two branching points at both ends of the linear backbone with f1 and f2 side arms starting from them, known as the pom-pom polymers. Here, we analyze the asymmetric case, f1 ≠ f2, by applying both the analytical approach, based on the direct polymer renormalization, and computer simulations using both dissipative particle dynamics and Monte Carlo methods. We study the role played by the molecular asymmetry of average polymer conformations, considering the infinite dilution regime and good solvent conditions. The quantitative estimates are reported for the set of universal size and shape characteristics of such molecules and for their individual branches, all the functions of f1 and f2. In particular, we evaluate the size ratio of the gyration radii of symmetric and asymmetric pom-pom topologies with the same molecular weight and quantitatively reveal an increase of the effective size of a molecule caused by its asymmetry. We also introduce and analyse the asymmetry factor and estimate the shift of the center of mass caused by the presence of side stars, which can serve as another characteristic of the asymmetry of pom-pom structure. [ABSTRACT FROM AUTHOR]

Cover Picture: Non-Shock Ignition Probability of Octahydro-1,3,5,7-Tetranitro-Tetrazocine-Based Polymer Bonded Explosives Based on Microcrack Stochastic Distribution (Prop., Explos., Pyrotech. Keywords: Non-shock ignition probability; Microcrack stochastic distribution; Steven test; Viscoelastic cracking constitutive model; Monte Carlo method EN Non-shock ignition probability Microcrack stochastic distribution Steven test Viscoelastic cracking constitutive model Monte Carlo method 525 525 1 04/10/20 20200401 NES 200401 The cover picture is based on the Full Paper Non-Shock Ignition Probability of Octahydro-1,3,5,7-Tetranitro-Tetrazocine-Based Polymer Bonded Explosives Based on Microcrack Stochastic Distribution by Rui Liu et al., https://doi.org/10.1002/prep.201900313. Non-shock ignition probability, Microcrack stochastic distribution, Steven test, Viscoelastic cracking constitutive model, Monte Carlo method. [Extracted from the article]

Understanding and controlling vesicle shapes is fundamental challenge in biophysics and materials design. In this paper, we employ the Monte Carlo method to investigate the shape of soft vesicle induced by semiflexible polymer outside in two dimensions. The effect of bending stiffness κ of polymer and the strength ε V P of attractive interaction between vesicle and polymer on the shape of vesicle is discussed in detail in the present paper. It is found that the shape of vesicle is influenced by κ and ε V P . Typical shape of vesicles is observed, such as circular, cigar-like, double vesicle, and racquet-like. To engineer vesicle shape transformations is helpful for exploiting the richness of vesicle geometries for desired applications. [ABSTRACT FROM AUTHOR]

Representing polymers by random walks on a lattice is a fruitful approach largely exploited to study configurational statistics of polymer chains and to develop efficient Monte Carlo algorithms. Nevertheless, the stretching and the folding/unfolding of polymer chains within the Gibbs (isotensional) and the Helmholtz (isometric) ensembles of the statistical mechanics have not been yet thoroughly analysed by means of the lattice methodology. This topic, motivated by the recent introduction of several single-molecule force spectroscopy techniques, is investigated in the present paper. In particular, we analyse the force-extension curves under the Gibbs and Helmholtz conditions and we give a proof of the ensembles equivalence in the thermodynamic limit for polymers represented by a standard random walk on a lattice. Then, we generalize these concepts for lattice polymers that can undergo conformational transitions or, equivalently, for chains composed of bistable or two-state elements (that can be either folded or unfolded). In this case, the isotensional condition leads to a plateau-like force-extension response, whereas the isometric condition causes a sawtooth-like force-extension curve, as predicted by numerous experiments. The equivalence of the ensembles is finally proved also for lattice polymer systems exhibiting conformational transitions. [ABSTRACT FROM AUTHOR]

This paper reveals an interface transition characteristics in LDPE/Al2O3 nanocomposites. A nonlinear variation of dielectric permittivity in nanocomposites is closely related to the interface characteristics, such as the effective thickness, the volume fraction, and the relative permittivity. The interface with a low relative permittivity is defined as a phase between nanoparticle and polymer matrix. After that, the effective permittivities of the nanocomposites are calculated by the three-phase models. The linear model which considers the interface volume by Monte Carlo calculation indicated a significant increase of interface volume fraction at low loadings, which decreased the permittivity. However, it failed to fit the permittivity at high loadings. The interface power law (IPL) model considering the interface overlap and the shape and orientation of particle is difficult to fit the measured data, particularly at low loadings. It seems that neither of the models can well fit the measured data. However, through comparison of the two models, the permittivity behaviors in low and high loading regions can be understood. The improved model with power law relation is likely useful for explaining the reduced permittivity at low loadings, while a revised IPL model could locally agree with the increased permittivity at high loadings. In light of the comparisons, it can be inferred that the reduction of εr at low loadings is ascribed to the large interface volume (low permittivity), and the enhancement of εr at high loadings is caused by the nanoparticles. A transition of interface characteristics is summarized, which is responsible for the change of dielectric properties. [ABSTRACT FROM AUTHOR]

Due to the difficulty of accurately characterizing properties such as the molecular weight (Mn) and grafting density (σ) of gradient brushes (GBs), these properties are traditionally assumed to be uniform in space to simplify analysis. Applying a stochastic reaction model (SRM) developed for heterogeneous polymerizations, we explored surface-initiated polymerizations (SIPs) with initiator gradients in lattice Monte Carlo simulations to examine this assumption. An initial exploration of SIPs with 'homogeneously' distributed initiators revealed that increasing σ slows down the polymerization process, resulting in polymers with lower molecular weight and larger dispersity (Đ) for a given reaction time. In SIPs with an initiator gradient, we observed that the properties of the polymers are position-dependent, with lower Mn and larger Đ in regions of higher σ, indicating the non-uniform properties of polymers in GBs. The results reveal a significant deviation in the scaling behavior of brush height with σ compared to experimental data and theoretical predictions, and this deviation is attributed to the non-uniform Mn and Đ. [ABSTRACT FROM AUTHOR]

To better characterize the properties of surface-initiated polymers, simultaneous bulk- and surface-initiated polymerizations are usually carried out by assuming that the properties of the surface-initiated polymers resemble those of the bulk-initiated polymers. Through a Monte Carlo simulation using a heterogeneous stochastic reaction model, it was discovered that the bulk-initiated polymers exhibit a higher molecular weight and a lower dispersity than the corresponding surface-initiated polymers, which indicates that the equivalent assumption is invalid. Furthermore, the molecular weight distributions of the two types of polymers are also different, suggesting different polymerization mechanisms. The results can be simply explained by the heterogeneous distributions of reactants in the system. This study is helpful to better understand surface-initiated polymerization. [ABSTRACT FROM AUTHOR]

We present a new concept of fluorescence suppression in Raman microscopy based on micro-spatially offset Raman spectroscopy which is applicable to thin stratified turbid (diffusely scattering) matrices permitting the retrieval of the Raman signals of sublayers below intensely fluorescing turbid over-layers. The method is demonstrated to yield good quality Raman spectra with dramatically suppressed fluorescence backgrounds enabling the retrieval of Raman sublayer signals even in situations where conventional Raman microscopy spectra are fully overwhelmed by intense fluorescence. The concept performance was studied theoretically using Monte Carlo simulations indicating the potential of up to an order or two of magnitude suppression of overlayer fluorescence backgrounds relative to the Raman sublayer signals. The technique applicability was conceptually demonstrated on layered samples involving paints, polymers and stones yielding fluorescence suppression factors between 12 to above 430. The technique has potential applications in a number of analytical areas including cultural heritage, archaeology, polymers, food, pharmaceutical, biological, biomedical, forensics and catalytic sciences and quality control in manufacture. [ABSTRACT FROM AUTHOR]

Monte Carlo method has been used to determine the efficiency for proton production and to study the energy and angular distributions of the generated protons. The ENDF library of cross sections is used to simulate the interactions between the neutrons and the atoms in a polyethylene (PE) layer, while the ranges of protons with different energies in PE are determined using the Stopping and Range of Ions in Matter (SRIM) computer code. The efficiency of proton production increases with the PE layer thickness. However the proton escaping from a certain polyethylene volume is highly dependent on the neutron energy and target thickness, except for a very thin PE layer. The energy and angular distributions of protons are also estimated in the present paper, showing that, for the range of energy and thickness considered, the proton flux escaping is dependent on the PE layer thickness, with the presence of an optimal thickness for a fixed primary neutron energy. [ABSTRACT FROM AUTHOR]

Stretching of semicrystalline polymer materials is fundamentally important in their mechanical performance and industrial processing. By means of dynamic Monte Carlo simulations, we compared the parallel stretching processes between the initially bulk amorphous and semicrystalline polymers at various temperatures. In the early stage of stretching, semicrystalline polymers perform local and global melting-recrystallization behaviors at low and high temperatures, while the memory effects occur upon global melting-recrystallization at middle temperatures. However, the final crystallinities, crystalline bond orientations, chain-folding probabilities, residual stresses, and crystallite morphologies at high enough strains appear as the same at each temperature, irrelevant to the initially amorphous and semicrystalline polymers, indicating that the common post-growth melting-reorganization processes determine the final products. In addition, both final products harvest the highest crystallinities in the middle temperature region because the postgrowth stage yields the vast nuclei followed with less extent of crystal growth in the low temperature region and few nuclei followed with large extent of crystal growth in the high temperature region. Our observations imply that a large enough strain can effectively remove the thermal history of polymers, similar to the thermal treatment at a high enough temperature; therefore, the fracture strength of semicrystalline polymers depends upon their final structures in stretching, not related to their nascent semicrystalline structures. [ABSTRACT FROM AUTHOR]

Concrete is a common material used as a shielding against ionizing radiation due to the relatively low cost and the ability to meet the structural function. State of the art in concrete shielding is that while in case of gamma radiation an increase in density by a change of aggregate is usually efficient enough, protection against neutrons is more complex. It is due to the differences in interactions of neutrons with the matter, depending on their kinetic energy and reaction cross-sections with the component atoms of the cement paste and the aggregate. Last progress in concrete evolution due to use of polymer additives (e.g. superplasticizers) together with reactive additions (e.g. silica fume) allows for a new look at the concrete design for radiation shielding purposes. One of the main advantages of concrete is its composite-type and there is a potential for the optimization of its constituents as well as mixture proportions. The paper presents the preliminary results of the project NGS-Concrete – new-generation shielding concrete against ionizing radiation. The aim of the project is to design the composition of concrete against ionizing radiation, achieved by the use of experiment based multi-criteria optimization of materials supported by the Monte Carlo simulations. The purpose of presented studies was to evaluate neutron shielding properties of ordinary and heavy-weight magnetite concrete modified with epoxy resin and gadolinium oxide. At first the shielding efficiency against neutrons from LWR neutron flux source and Pu-Be was simulated in MCNP code. At the end the comparison of MCNP simulated results and real experiment was presented. It was proved that both methods of modification can improve neutron shielding properties concrete but gadolinium oxide is an efficient additive only for low energy neutron attenuation. [ABSTRACT FROM AUTHOR]

The process during which a polymer translocates through a nanopore depends on many physical parameters and fundamental mechanisms. We propose a new one-dimensional lattice Monte Carlo algorithm that integrates various effects such as the entropic forces acting on the subchains that are outside the channel, the external forces that are pulling the polymer through the pore, and the frictional effects that involve the chain and its environment. Our novel approach allows us to study the polymer as a single Brownian particle diffusing while subjected to a position-dependent force that includes both the external driving forces and the internal entropic bias. Frictional effects outside and inside the pore are also considered. This Monte Carlo method is much more efficient than other simulation methods, and it can be used to obtain scaling laws for various polymer translocation regimes. In this first part, we derive the model and describe a subtle numerical approach that gives exact results for both the escape probability and the mean translocation time (and higher moments of its distribution). The scaling laws obtained from this model will be presented and discussed in the second part of this series. [ABSTRACT FROM AUTHOR]

We simulated the translocation process of a polymer chain from a source container to a drain container through a short nanochannel. We utilized the bond fluctuation model coupled with Monte Carlo dynamics in our simulations. The calculation results show that the excluded volume effect significantly affects the polymer’s translocation time τ. This time depends nonmonotonically on the polymer length N. For a fixed nanochannel length, τ decreases when the polymer length increases. τ, however, increases when the polymer length exceeds a certain threshold. This observation differs from those predicated for a Gaussian chain. In this paper, we will further present our findings to explain this phenomenon. The knowledge we gain from this research can enhance the understanding of complex transport processes in many biological systems. [ABSTRACT FROM AUTHOR]

Bounded potentials are good models to represent the effective two-body interaction in some colloidal systems, such as the dilute solutions of polymer chains in good solvents. The simplest bounded potential is that of penetrable spheres, which takes a positive finite value if the two spheres are overlapped, being 0 otherwise. Even in the one-dimensional case, the penetrable-rod model is far from trivial, since interactions are not restricted to nearest neighbors and so its exact solution is not known. In this paper the structural properties of one-dimensional penetrable rods are studied. We first derive the exact correlation functions of the penetrable-rod fluids to second order in density at any temperature, as well as in the high-temperature and zero-temperature limits at any density. It is seen that, in contrast to what is generally believed, the Percus-Yevick equation does not yield the exact cavity function in the hard-rod limit. Next, two simple analytic theories are constructed: a high-temperature approximation based on the exact asymptotic behavior in the limit T→∞ and a low-temperature approximation inspired by the exact result in the opposite limit T→0. Finally, we perform Monte Carlo simulations for a wide range of temperatures and densities to assess the validity of both theories. It is found that they complement each other quite well, exhibiting a good agreement with the simulation data within their respective domains of applicability and becoming practically equivalent on the borderline of those domains. A comparison with numerical solutions of the Percus-Yevick and the hypernetted-chain approximations is also carried out. Finally, a perspective on the extension of our two heuristic theories to the more realistic three-dimensional case is provided. [ABSTRACT FROM AUTHOR]

The recent development of X-ray Photoelectron Spectroscopy (XPS) instrumentation with spatial resolution down to several microns has advanced the capability of elemental and chemical state imaging. XPS imaging analysis has been applied in understanding the delamination problems of siloxane coatings on polymethyl-methacrylate (PMMA) polymer. It was found that delamination occurred by interfacial failure, and the coating suffered complete delamination from a PMMA substrate. This example offered an opportunity for the investigation of X-ray damage on polymers encountered in XPS imaging analysis. This paper also demonstrated how to construct a constrained peak model with the aid of chemical knowledge and supporting evidence of the sample. Monte Carlo error analysis was used to determine the validity of the peak fit models used. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

[Display omitted] • Different folding types such as two-state folding, folding through intermediates, as well as metastability, are presented with an effective order parameter for polymers. • Gyration tensor-based analysis does not need any reference state before the simulation as that is the need for the other order parameters which are chosen in other studies in the same content. • Our paper presents the use of very useful parameters to detect conformational phase transitions in polymer chains. In the earlier works related to this aspect presents another order parameter that needs a reference conformation. Multicanonical simulations of hydrophobic-polar heteropolymers were performed with a simple and effective coarse-grained off-lattice model to identify the folding pathways and the topology of the energy landscape with gyration tensor-based parameters. We examined six exemplified sequences of heteropolymers and illustrated three of them which present definitely different folding types such as two-state folding, folding through intermediates, as well as metastability. The major advantage of this study is our chosen "order" parameter, namely gyration tensor-based analysis, does not require any reference state before the simulation as that is the need for the other order parameters which are chosen in other studies in the same context. Our paper presents the use of very useful parameters to detect conformational phase transitions in polymer chains. Another order parameter that requires a reference conformation is presented in earlier works related to this aspect. This extends the simulation into two steps. With our analysis, there is no need for a global minimum conformation so-called reference state and our simulation will be completed in one step. Furthermore, there is no relative evaluation according to the reference state. [ABSTRACT FROM AUTHOR]

Studies the phase behavior of single homopolymers in a simple hydrophobic/hydrophilic off-lattice model with sequence independent local interactions. Monte Carlo methods used in the study; Dynamics of the algorithms; Measurement of specific heat and bond-bond correlation function.

Polymer nanocomposites consist of nanoscale additives embedded in a polymer matrix and are widely used in the automobiles, optics and microelectronics industries. Since the composition and the morphology of the polymer nanocomposite impact its macroscopic properties, significant efforts have been made to understand how parameters, such as polymer and nanoparticle chemistries, molecular weight of the matrix polymers and nanoparticle size, help tune the morphology. Theory and simulations have proven to be useful tools in this field due to their ability to link molecular level interactions, the morphology and the macroscopic properties. Due to the computational intensity of molecular simulations of a dense polymer matrix, there has been a strong effort on the theoretical front to develop methodologies that map out equilibrium structure and phase behaviour of polymer nanocomposites over a large parameter space. In this paper, we review the details of the self-consistent polymer reference interaction site model (PRISM)–Monte Carlo (MC) simulation method which integrates theory and simulation to study phase behaviour in polymer nanocomposites. We discuss two specific cases of polymer nanocomposites containing polymer-grafted nanoparticles with chemical and physical heterogeneity in grafts in which this self-consistent PRISM–MC approach has been used to study effective inter-filler interactions and phase behaviour. [ABSTRACT FROM AUTHOR]

The polymer chain growth in brushes takes place in a non-uniform environment which is one of the reasons of higher dispersity. Some active chain ends grow in monomer-reach regions while others in polymer-rich regions where the monomer concentration is lower. This effect depends on grafting density but also on the polymerization and diffusion rates. If chains grow fast, they adopt non-equilibrium conformations which change the polymer density profile and the growth conditions. In this paper we study this effect and chain relaxation using Monte Carlo simulation method. The growth of polymer brushes by living polymerization at various conditions was simulated until some average molecular weight was attained. Then, the polymerization was stopped and the changes of polymer parameters in time were monitored. The chain end-to-end distance increases and consequently the brush thickness increases in time. Polymer concentration decreases, especially near the grafted surface. The effect is stronger for higher polymerization rate and for higher grafting density. For low polymerization rates and lower grafting density, chain relaxation takes place during polymerization and the brush structure essentially does not change in time whenthe polymerization is finished. Such conditions assure also lower dispersity. Image 1 • After fast polymerization, a polymer brush is in a compact dense state. • Such a brush relaxes to an equilibrium state in which the chains are more extended. • Relaxation time depends mainly on average DP n , not on the individual chain length. • During slow polymerization, brushes are close to an equilibrium state. [ABSTRACT FROM AUTHOR]

Abstract: Presented in this paper is a simulation of the interaction of neutrons emitted from a cylindrical Am–Be source with a CR-39 detector. A Fortran90 program, called Neutron.f90, has been written to determine the specifications of secondary particles (alpha particle or proton) and the kinematics of the interactions. A Monte Carlo method was used to simulate which interactions occurred with the constituents of the CR-39 detector, H, C and O atoms. Due to the detector composition, elastic and nonelastic scattering of neutrons is possible. In this paper, nonelastic scattering includes: inelastic scattering; the emission of an alpha particle (n,α) reaction and the emission of a proton (n,p) reaction. Cross sections of other reactions are very small and negligible in comparison with the above mentioned. The energy deposited per unit mass per one neutron was calculated. In addition the contribution to the energy deposited by the alpha particles and protons were determined separately. The program Neutron.f90 computes the coordinates of points in which interactions occur i.e. where secondary particles appear. The energy and angular distributions of protons are also represented in this paper. Another Fortran90 program, called Track_Visibility.f90, was written to calculate the number of visible proton tracks and to calculate the energy deposited per one neutron per one visible track. [ABSTRACT FROM AUTHOR]