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Elastic behaviors of single polymer chains adsorbed on the attractive surface are first investigated using Monte Carlo simulation method based on the bond fluctuation model. We investigate the chain size and shape of adsorbed chains, such as mean-square radius of gyration 〈S2〉, mean-square bond length 〈b2〉, shape factors 〈sfi〉 and 〈δ*〉, and the orientation of chain segments P2〈cos θ〉, to illuminate how the shape of polymer chains changes during the process of tensile elongation. There are some special behaviors of the chain size and shape at the beginning of elongation, especially for strong attraction interaction. For example, mean fraction of adsorbed segments decreases abruptly in the region of small elongation ratio and then decreases slowly with increasing elongation ratio. In fact, the chain size and shape also changes abruptly for small elongation ratio with strong attraction interaction. Some thermodynamics properties are also investigated here. Average Helmholtz free energy increases fast for elongation ratio λ<1.15, especially with strong attraction, and increases slowly for λ>1.15. Similar behaviors are obtained for average energy per bond. Elastic force (f ) and energy contribution to force (fU) are also studied, and we find that elastic force decreases abruptly for λ<1.15, and there is a minimum of elastic force for strong attraction interaction, then increases very slowly with increasing elongation ratio. However, there are different behaviors for weak attraction interaction. For energy contribution to force (fU), there is a maximum value for strong attraction interaction in the region of λ<1.15. Some comparisons with the atomic force microscopy experiments are also made. These investigations may provide some insights into the elastic behaviors of adsorbed polymer chains. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

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. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The conformation of a polymer chain in solution is intrinsically coupled to the thermodynamic and structural properties of the solvent. Here we study such solvent effects in a system consisting of a flexible interaction-site n-mer chain immersed in a monomeric solvent. Chain conformation is described with a set of intramolecular site-site probability functions. We derive an exact density expansion for these intramolecular probability functions and give a diagrammatic representation of the terms contributing at each order of the expansion. The expansion is tested for a short hard-sphere chain (n=3 or 4) with site diameter σ in a hard-sphere solvent with solvent diameter D. In comparison with Monte Carlo simulation results for 0.2≤D/σ≤100, the expansion (taken to second order) is found to be quantitatively accurate for low to moderate solvent volume fractions for all size ratios. Average chain dimensions are predicted accurately up to liquidlike solvent densities. The hard-sphere chains are compressed with both increasing solvent density and decreasing solvent size. For small solvent (D<σ), depletion effects are found and the chain structure is strongly perturbed even at low solvent volume fractions. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Off-lattice Monte Carlo simulations in the canonical ensemble are used to study polymer-particle interactions in nanocomposite materials. Specifically, nanoscale interactions between long polymer chains (N=550) and strongly adsorbing colloidal particles of comparable size to the polymer coils are quantified and their influence on nanocomposite structure and dynamics investigated. In this work, polymer-particle interactions are computed from the integrated force-distance curve on a pair of particles approaching each other in an isotropic polymer medium. Two distinct contributions to the polymer-particle interaction potential are identified: a damped oscillatory component that is due to chain density fluctuations and a steric repulsive component that arises from polymer confinement between the surfaces of approaching particles. Significantly, in systems where particles are in a dense polymer melt, the latter effect is found to be much stronger than the attractive polymer bridging effect. The polymer-particle interaction potential and the van der Waals potential between particles determine the equilibrium particle structure. Under thermodynamic equilibrium, particle aggregation is observed and there exists a fully developed polymer-particle network at a particle volume fraction of 11.3%. Near-surface polymer chain configurations deduced from our simulations are in good agreement with results from previous simulation studies. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The network structures of polyhedral oligomeric silsesquioxane based nanocomposites are studied by continuous-space Monte Carlo simulations. The nanoporous network contains intercubic pores and mesopores which can be clearly identified in this work. In terms of degree of cross linking and pore size distribution (PSD), effects of linker length, tether rigidity, and number of reactive tethers are examined. It is found that the extent of cross linking as well as the intercubic pore size of the network increases as linker length increases which are consistent with experimental findings. However, the mesopores appear to shift to a smaller radii regime for networks with longer linkers. Networks with rigid tethers contain lots of free linkers, thus, low cross linking density and narrow PSD are observed. On the other hand, reduction of the reactive tethers shows an insignificant effect on the degree of cross linking of the system. The fact that the intercubic pore size increases as the number of reactive tethers decreases causes the nanobuilding blocks to possess larger free volumes and distribute themselves more evenly throughout the system. As a result, it reduces the possibility of forming large mesopores. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A computational strategy to model the amorphous phase of comblike polymers is presented. The strategy, denoted SuSi/CB (CB—configurational bias), combines the strength of an algorithm recently developed to generate reliable microstructures of dense amorphous polymers, which is based on a random search of energy minima, and configurational bias Monte Carlo method. The influence of different parameters used to define the characteristics of SuSi/CB on both the reliability of the generated structures and the computational effort has been examined in detail. Finally, we have modeled and characterized the supramolecular organization of poly(octadecyl acrylate) in the amorphous state. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The effect of confinement, number of branches (functionality), and size of the molecules on various properties as a function of temperature of star-branched polymers confined between two walls was studied using Monte Carlo simulations with the parallel tempering technique. The coil-to-globule transition and the liquidlike to solidlike transition, similar to those observed for linear chains, were characterized in all systems by changes in the heat capacity, internal energy, and radius of gyration. The transitions were also characterized by the most probable isomeric structure at a given temperature. The radius of gyration of the star polymers was smaller than the values of linear chains when the number of arms f increased. For star chains with more than f=5 arms the values of the radius of gyration, and therefore the size of the molecules, were similar for every condition of confinement studied, especially at higher temperatures. As confinement was increased, the difference in the radius of gyration of linear chains and star polymers became even larger. The coil-to-globule transition temperatures shifted to higher temperatures as the size of the chains and the number of arms in a molecule were increased. Effects of confinement were higher on the properties of the system at the smallest separations (less than twice the monomer diameter), where the coil-to-globule transition shifted to lower temperatures. The liquidlike to solidlike transition was present at almost the same temperature for different conditions of confinement, chain size, and number of arms. The behavior of the systems for separations between the walls greater than five bead diameters was similar to the behavior in the unconfined case. Hence, no considerable effect of confinement was found above this separation. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The structure of polymers confined between surfaces is studied using computer simulation and a density functional approach. The simple model system considers the polymer molecule as a pearl necklace of freely jointed hard spheres, having attractions among the beads, confined between attractive surfaces. This approach uses the universality of the free-energy functional to obtain the self-consistent field required in the single chain simulation. The second-order direct correlation function for the uniform bulk fluid required as input has been calculated from the reference interaction site model integral equation theory using mean spherical approximation. The theoretical results are shown to compare well with the Monte Carlo simulation results for varying densities, chain lengths, and with different attractive interaction parameters. The simulation results on the conformational properties give important indications regarding the behavior of chains as they approach the surfaces. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The interface and surface properties and the wetting behavior of polymer-solvent mixtures are investigated using Monte Carlo simulations and self-consistent field calculations. We carry out Monte Carlo simulations in the framework of a coarse-grained bead-spring model using short chains (oligomers) of NP=5 beads and a monomeric solvent, NS=1. The self-consistent field calculations are based on a simple phenomenological equation of state for compressible binary mixtures and we employ Gaussian chain model. The bulk behavior of the polymer-solvent mixture belongs to type III in the classification of van Konynenburg and Scott [Phil. Trans. R. Soc. London, Ser. A 298, 495 (1980)]. It is characterized by a triple line on which the polymer-liquid coexists with solvent-vapor and a solvent-rich liquid. The solvent is not homogeneously distributed across the dense polymer film but tends to accumulate at the surface and the polymer-vapor interface. This solvent enrichment at the interface and surface becomes more pronounced upon increasing the vapor pressure and alters the surface and interface tensions. This effect gives rise to a nonmonotonic dependence of the contact angle on the vapor pressure and one might observe reentrant wetting. The results of the Monte Carlo simulations and the self-consistent field calculations qualitatively agree. The profiles of drops are investigated by Monte Carlo simulations and a pronounced solvent enrichment is observed at the wedge formed by the substrate and the liquid-vapor interface at the three-phase contact line.© 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present an analysis of the structure of the fluctuation-induced microemulsion phase in a ternary blend of balanced AB diblock copolymers with equal amounts of A and B homopolymers. To this end, graphical analysis methods are employed to characterize two-dimensional configuration snapshots obtained with the recently introduced field-theoretic Monte Carlo method. We find that a microemulsion forms when the mean curvature diameter of the lamellar phase coincides roughly with the periodicity of the lamellar phase. Further, we provide evidence to the effect of a subclassification of the microemulsion into a genuine and a defect-driven region. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present a coarse-grained model for n-alkanes in a supercritical solution, which is exemplified by a mixture of hexadecane and CO2. For pure hexadecane, the Monte Carlo simulations of the coarse-grained model reproduce the experimental phase diagram and the interfacial tension with good accuracy. For the mixture, the phase behavior sensitively depends on the compatibility of the polymer with the solvent. We present a global phase diagram with critical lines, which is in semiquantitative agreement with experiments. In this context we developed two computational schemes: The first adopts Wang-Landau sampling to the off-lattice grand canonical ensemble, the second combines umbrella sampling with an extrapolation scheme to determine the weight function. Additionally, we use Wertheim’s theory (TPT1) to obtain the equation of state for our coarse-grained model of supercritical mixtures and discuss the behavior for longer alkanes. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The effect of solvent quality on the equilibrium structure of a densely branched comb polymer is investigated based on the structure factor analyses by off-lattice Monte Carlo simulations. First, theta temperature (θ∞) must be determined to identify the solvent condition. We locate the characteristic temperature θA(N) at which the second virial coefficient vanishes and the transition temperature θR(N) at which radius of gyration Rg of the chain varies most rapidly with temperature, i.e., d2Rg/dT2|θR=0. N represents the total number of monomers of a comb. As N→∞, θA and θR coincide to a point that is identified as the true theta temperature (θ∞). The structure factors of the main chain, the side chain, and the whole polymer are calculated, respectively. It is found that at T=θ∞, the structural factors S(qRg) for the overall comb polymers match quite well with those of their Gaussian counterparts. When T<θ∞, the overall comb polymer assumes collapsed conformations, similar to a homogeneous sphere. However, the structure factor of the side chain indicates that it always remains in an expanded state regardless of the solvent condition. It is attributed to the strong interactions between side chains. The same effect leads to enhanced rigidity of the main chain in comparison to the linear chain, as clearly observed from the rescaled Kratky plot. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Using Monte Carlo simulations of an off-lattice model, we study the elastic properties of polymer-grafted membranes. Our results are found to be in good agreement with those predicted by the classical path approximation of the self-consistent field theory and scaling theory based on de Gennes’ blob picture. In particular, we found that when the membrane is grafted on both sides by brushes with same molecular weight N and grafting density σ, the excess bending modulus induced by the polymers scales as N3σα where α is consistent with 7/3, as predicted by the self-consistent field theory, and 5/2, as predicted by the scaling theory. When the polymers are grafted to one side of the membrane only, the membrane bends away from the polymers with a spontaneous curvature with a scaling that is consistent with both scaling and self-consistent field theories. When the thickness of the brush exceeds the membrane’s spontaneous radius of curvature, the bending modulus approaches a constant which is of the same order as the bending modulus of the bare membrane. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The effect of solvent quality on dilute and semidilute regimes of polymers in solution is studied by means of Monte Carlo simulations. The equation of state, adsorption near a hard wall, wall–polymer surface tension, and effective depletion potential are all calculated as a function of concentration and solvent quality. We find important differences between polymers in good and θ solvents. In the dilute regime, the physical properties for polymers in a θ solvent closely resemble those of ideal polymers. In the semidilute regime, however, significant differences are found. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The pressure shifts and the electric-field shifts of individual chromophores in an amporphous matrix are—due to strong disorder—subject to broad distributions. By means of single-molecule spectroscopy we measured both the pressure and the electric-field shift of about 800 tetra-tert-butylterrylene molecules in polyisobutylene. We found a significant correlation of 0.52 (Kendall’s correlation coefficient) between the two observables. Analytical calculations and Monte Carlo simulations based on a model by Laird and Skinner predict a nonzero, yet, distinctly smaller correlation. The Monte Carlo simulations showed that the usual assumptions of a spherical shape and isotropic polarizability of the chromophores in glassy systems is an oversimplification of the complex nanoscopic structure and cannot reproduce our experimental results. By taking the molecular anisotropy into account, we obtain agreement of the simulated and the measured correlation between pressure shift and electric-field shift parameter. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present a new approach for simulating the motions of flexible polyelectrolyte chains based on the continuous kink-jump Monte Carlo technique coupled to a lattice field theory based calculation of the Poisson-Boltzmann (PB) electrostatic free energy ''on the fly.'' This approach is compared to the configurational-bias Monte Carlo technique, in which the chains are grown on a lattice and the PB equation is solved for each configuration with a linear scaling multigrid method to obtain the many-body free energy. The two approaches are used to calculate end-to-end distances of charged polymer chains in solutions with varying ionic strengths and give similar numerical results. The configurational-bias Monte Carlo/multigrid PB method is found to be more efficient, while the kink-jump Monte Carlo method shows potential utility for simulating nonequilibrium polyelectrolyte dynamics. [ABSTRACT FROM AUTHOR]

We present extensive Monte Carlo simulations of tethered chains of length N on adsorbing surfaces, considering the dilute case in good solvents, and analyze our results using scaling arguments. We focus on the mean number M of chain contacts with the adsorbing wall, on the chain’s extension (the radius of gyration) perpendicular and parallel to the adsorbing surface, on the probability distribution of the free end and on the density profile for all monomers. At the critical adsorption strength εc one has Mc∼N[lowercase_phi_synonym], and we find (using the above results) as best candidate [lowercase_phi_synonym] to equal 0.59. However, slight changes in the estimation of εc lead to large deviations in the resulting [lowercase_phi_synonym]; this might be a possible reason for the difference in the [lowercase_phi_synonym] values reported in the literature. We also investigate the dynamical scaling behavior at εc, by focusing on the end-to-end correlation function and on the correlation function of monomers adsorbed at the wall. We find that at εc the dynamic scaling exponent a (which describes the relaxation time of the chain as a function of N) is the same as that of free chains. Furthermore, we find that for tethered chains the modes perpendicular to the surface relax quicker than those parallel to it, which may be seen as a splitting in the relaxation spectrum. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Polarization energies are calculated for a single excess charge on a polyethylene chain in amorphous polyethylene using (i) local segment and nonlocal distributed molecular polarizabilities, (ii) material structures simulated by both general-purpose and specialist Monte Carlo software, and (iii) uniform and Gaussian distributions of charge with different extents of charge delocalization. Local and distributed response lead to results that are essentially the same except that they correspond to different mean polarizabilities. With increasing delocalization of the charge along the chain, the polarization energies shift to higher values and the width of their distribution decreases, the differences being more pronounced for the uniform distribution. The polarization energies for charges delocalized over 10–20 methylene units form a distribution some 1/4 eV wide centered around 1 eV, narrowing significantly for more homogeneous polymer melts. The calculations are relevant to trapping of charge in polyethylene. They also yield the microscopic variation in the potential along the polymer chain caused by the polarization energy difference, and so may provide useful inputs to theories of electronic conduction in polymer materials. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Single-chain simulations of densely branched comb polymers, or “molecular bottle-brushes” with side-chains attached to every (or every second) backbone monomer, were carried out by off-lattice Monte Carlo technique. A coarse-grained model, described by hard spheres connected by harmonic springs, was employed. Backbone lengths of up to 100 units were considered, and compared with the corresponding linear chains. The backbone molecular size was investigated as a function of its length at fixed arm size, and as a function of the arm size at fixed backbone length. The apparent swelling exponents obtained by a power-law fit were found to be larger than those for the corresponding linear polymers, indicative of stiffening of the comb backbone. The probability distribution function for the backbone end-to-end distance was also investigated for different backbone lengths and arm sizes. Analysis of this function yielded the critical exponents, which revealed an increase in the swelling exponent consistent with values found from the molecular size. The apparent persistence length of the backbone was also determined, and was found to increase with increasing branching density. Finally, the static structure factors of the whole bottle-brushes and of their backbones are discussed, which provides another consistent estimate of the swelling exponents. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The influence of monomer structure on the thermodynamic properties of lattice model polymer blends is investigated through Monte Carlo computations. The model of lattice polymers with monomer structure has been used extensively in the context of the lattice cluster theory (LCT), a thermodynamic theory for polymer mixtures in the liquid state. The Monte Carlo computations provide the first unequivocal test of the accuracy of the LCT predictions for binary mixtures of polymers with structured monomers. Four types of monomer structures are analyzed, corresponding to to the monomers of polyethylene, polypropylene, polyethylethylene, and polyisobutylene (PIB). Most computations use chains with M=12 and 24 beads and the total volume fraction of the beads is [lowercase_phi_synonym]=0.6. Both structurally symmetric and asymmetric blends are investigated. For the symmetric case, the predictions of the LCT for the energies of mixing and the liquid–liquid coexistence curves are in qualitative agreement with the Monte Carlo computations, except for the PIB/PIB symmetric blend. For structurally asymmetric blends, the LCT does not capture contributions to the energy of mixing arising solely from structural differences between the components. Computational estimates of the nonideal entropy of mixing indicate that the LCT also underestimates the entropic cost of mixing chains with different structures, thus explaining some discrepancies between the theoretical and the Monte Carlo liquid–liquid coexistence curves. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Single three-dimensional polymers confined to a slab, i.e., to the region between two parallel plane walls, are studied by Monte Carlo simulations. They are described by N-step walks on a simple cubic lattice confined to the region 1≤z≤D. The simulations cover both regions D〈R[sub F] and D>R[sub F] (where R[sub F]∼N[sup ν] is the Flory radius, with ν≈0.587), as well as the cross-over region in between. Chain lengths are up to N=80 000, slab widths up to D=120. In order to test the analysis program and to check for finite size corrections, we actually studied three different models: (a) ordinary random walks (mimicking Θ polymers); (b) self-avoiding walks; and (c) Domb–Joyce walks with the self-repulsion tuned to the point where finite size corrections for free (unrestricted) chains are minimal. For the simulations we employ the pruned-enriched-Rosenbluth method with Markovian anticipation. In addition to the partition sum (which gives us a direct estimate of the forces exerted onto the walls), we measure the density profiles of monomers and of end points transverse to the slab, and the radial extent of the chain parallel to the walls. All scaling laws and some of the universal amplitude ratios are compared to theoretical predictions. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Off-lattice Monte Carlo simulations employing the PIVOT algorithm are used to generate ideal and excluded volume linear polymers in two dimensions. The form factor at small and large wave vectors is calculated from the resulting configurations and compared to the exact equation for ideal chains and to both scaling and renormalization group predictions for excluded volume chains. It is found that using the des Cloizeaux form for the distance distribution function in an analytic calculation of the form factor leads to close agreement with the Monte Carlo data and that simple expressions for both the small and large wave vector expansions reproduce the essential features of the form factor. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

On the basis of an extended Flory–Huggins expression for the mixing free energy of the long-chain binary polymer blend, we showed that liquid–liquid demixing can be driven solely by the propensities of crystallization differing with respect to components. Demixing prior to crystallization on cooling was demonstrated by dynamic Monte Carlo simulations of the lattice model for a symmetric polymer blend where only one component is crystallizable. We discussed the implications of this principle to several experimental observations including the (in)compatibility of polymer chains with different stereochemical compositions, the memory effect of polymer crystallization, and the spinodal decomposition at an early stage of polymer crystallization. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The equilibrium thermodynamics of confined linear homopolymers between two impenetrable walls was investigated by means of Monte Carlo simulations in the canonical ensemble with the parallel tempering algorithm. The influence of confinement and chain size on the potential energy, heat capacity, radius of gyration, and the end-to-end distance was investigated as a function of temperature. A bead–spring model was used to simulate the chains. Two conformational changes were observed regardless of the differences in confinement of chain size: The coil-to-globule transition that resembles the gas to liquid transition and the liquidlike to solidlike transition. An additional transition between solid states was also observed for the smallest chain size studied (16 beads). Results indicate a shift of the coil-to-globule transition temperature to lower values as the slit width approaches the two-dimensional case (wall separation equal to bead diameter), and to higher temperature regions as the chain length increases. For separations greater than five bead diameters, the thermodynamic behavior was similar to the behavior in unconfined space. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The physical processes accompanying an illumination of azobenzene chromophores embedded in a polymer matrix with spatially modulated and linearly polarized light are studied experimentally and simulated using the Monte Carlo (MC) method. The MC simulations base on a simple three-channel kinetic model of trans-cis type photoisomerization cycles in azobenezene molecules. The method focus on studying the kinetics of diffraction grating recording and erasing under various conditions of illumination, light polarization and recording in the presence or absence of a background light. By modeling the “macroscopic” light diffraction efficiency one gets insight into the role played by microscopic processes (angular hole burning, angular redistribution, rotational diffusion) up to the high-temperature limit, characterized by a high mobility of polymer chains. The obtained qualitative agreement between experimental data and the modeling confirms that the main processes underlying grating recording and erasing are well identified. The experimentally observed two-exponential functional dependence of diffraction efficiency η as function of time is well explained by the MC modeling. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The structure of telechelic polymers is investigated using off-lattice Monte Carlo simulations and the polymer reference interaction site model (PRISM) integral equation theory. The polymer molecules are modeled as tangent-sphere freely-jointed chains where all beads interact via a hard sphere potential and end beads interact via an additional short-ranged attractive potential. The static properties, i.e., conformational properties, end-bead aggregation, intermolecular pair correlations, and partial static structure factors are investigated as a function of density and temperature. For a given density, as the temperature is lowered, the chain ends aggregate to form multiplets. For a given temperature, this tendency is greater at higher densities. Predictions of the PRISM theory for the pair correlation functions and partial static structure factors are compared to the simulation results. Three different closure approximations, the reference-Molecular mean spherical approximation (R-MMSA), the reference-molecular Percus–Yevick closure with the high temperature approximation (R-MPY/HTA), and the linearized form of the R-MPY/HTA, are tested. At high temperatures, the linearized R-MPY/HTA and R-MMSA closures are in good agreement with the simulations, but the R-MPY/HTA closure underestimates the correlation between monomers. At low temperatures, when multiplets are formed, none of the closures are accurate for the pair correlation functions or the partial static structure factors. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Using the configurational-bias Monte Carlo method combined with the parallel tempering technique, solvent driven changes in polymer conformations in a supercritical solvent are systematically investigated using simple molecular models. The solvent is modelled as a square-well fluid, and the polymer is made up of a flexible chain of tangentially touching hard spheres and/or square-well spheres. The mean square end-to-end distance and radius of gyration are computed for various ratios of square-well and hard-sphere segments and a range of thermodynamic conditions. The simulation results show that conformation behavior of amphiphilic chains is rather complex and it is not a simple combination of the conformation behavior of the attractive and repulsive chains. The main finding is that the conformations of amphiphilic chains is determined primarily by attractive intermolecular interactions between the polymer segments and molecules of the solvent. [ABSTRACT FROM AUTHOR]

Single domain proteins are thought to be tightly packed. The introduction of voids by mutations is often regarded as destabilizing. In this study we show that packing density for single domain proteins decreases with chain length. We find that the radius of gyration provides a poor description of protein packing but the alpha contact number we introduce here characterize proteins well. We further demonstrate that protein-like scaling relationship between packing density and chain length is observed in off-lattice self-avoiding walks. A key problem in studying compact chain polymers is the attrition problem: It is difficult to generate independent samples of compact long self-avoiding walks. We develop an algorithm based on the framework of sequential Monte Carlo and succeed in generating populations of compact long chain off-lattice polymers up to length N= 2000. Results based on analysis of these chain polymers suggest that maintaining high packing density is only characteristic of short chain proteins. We found that the scaling behavior of packing density with chain length of proteins is a generic feature of random polymers satisfying loose constraint in compactness. We conclude that proteins are not optimized by evolution to eliminate packing voids. [ABSTRACT FROM AUTHOR]

The second virial coefficient, A[SUB2], is evaluated between pairs of short chain molecules by direct simulations using a parallel tempering Monte Carlo method where the centers of mass of the two molecules are coupled by a harmonic spring. Three off-lattice polymer models are considered, one with rigid bonds and two with flexible bonds, represented by the finitely extensible nonlinear elastic potential with different stiffness. All the models considered account for excluded volume interactions via the Lennard-Jones potential. In order to obtain the second virial coefficient we calculate the effective intermolecular interaction between the two polymer chains. As expected this intermolecular interaction is found to be strongly dependent upon chain length and temperature. For all three models the θ temperature (θ[SUBn]), defined as the temperature at which the second virial coefficient vanishes for chains of finite length, varies as θ[SUBn]-θ[SUB∞]&vprop;n[SUP-1/2], where n is the number of bonds in the polymer chains and θ[SUB∞] is the θ point for an infinitely long chain. Introducing flexibility into the model has two effects upon θ[SUBn]; the θ temperature is reduced with increasing flexibility, and the n dependence of θ[SUBn] is suppressed. For a particular choice of spring constant an n-independent θ temperature is found. We also compare our results with those obtained from experimental studies of polystyrene in decalin and cyclohexane, and for poly(methyl methacrylate) in a water and tert-butyl alcohol mixture, and show that all the data can be collapsed onto a single universal curve without any adjustable parameters. We are thus able to relate both A[SUB2] and the excluded volume parameter v, to the chain interaction parameter z, in a way relating not only the data for different molecular weights and temperatures, but also for different polymers in different solvents. [ABSTRACT FROM AUTHOR]

A density of states Monte Carlo formalism is used to calculate the interfacial tension of a simple fluid (3D Ising model) and of free-standing polymer films on a lattice. Good agreement is found between the results of these calculations and literature values. The proposed approach is efficient and provides the surface tension over a wide range of temperature from a single simulation. It has the additional advantage of being equally applicable to systems interacting through continuous potential energy functions, discontinuous functions, and systems on a lattice. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A Monte Carlo (MC) study is made of the mean-square radius of gyration 〈S²〉 and the gyration-radius expansion factor α[sub S] for the freely rotating chain of bond angle 109° and with the Lennard-Jones (LJ) 6-12 intramolecular potentials between beads in a cutoff version for the number n of bonds in the chain ranging from 10 to 1500 at the reduced temperature ranging from 3.6 to 8.0, which is defined as the absolute temperature multiplied by the Boltzmann constant and divided by the depth of the well of the LJ potential. It is shown that the ratio 〈S²〉/n approaches asymptotically a constant independent of n for very large n at the value 3.72±0.05 of the reduced temperature, which value is equal to the reduced Θ temperature Θ[sup *] of the MC model system, and that possible effects of chain ends on 〈S²&Rang; and therefore on α[sub S] are negligibly small. Taking the values of 〈S²〉 at Θ[sup *] as the unperturbed ones, α[sup 2, sub S] is evaluated from those at various reduced temperatures higher than Θ[sup *]. It is then found that the behavior of α[sup 2, sub S] may be well explained in the quasi-two-parameter scheme or is in good agreement with that of real experimental data. Further, the binary cluster integral for a bead in the chain is found to be much smaller in magnitude than that for a single isolated bead at reduced temperatures higher than Θ[sup *], the result being consistent with a previous finding. [ABSTRACT FROM AUTHOR]

We present results on the profiles of the first- and second-rank bond-order parameters, 〈P[sub 1](cos θ)〉 and 〈P[sub 2](cos θ)〉, of the grafted polymer melts simulated in atomistic detail in Part I of this work, with the end-bridging Monte Carlo (EBMC) algorithm. The 〈P[sub 1](cos θ)〉 and 〈P[sub 2](cos θ)〉 profiles are discussed in terms of bond distance from the grafting surface and bond coordinate along chain contour. A methodology is also presented for calculating the deuterium (²H) nuclear magnetic resonance (NMR) spectrum of the simulated grafted melts by analyzing the atomistic configurations accumulated in the course of the EBMC simulations. The calculation of the deuterium NMR spectrum is found to be very sensitive to the details of the profile of the second-rank bond-order parameter. For the systems simulated in the present work (melts of linear C[sub 78] and C[sub 156] polyethylene melts at grafting densities σ ranging from 1.31 to 2.62 nm[sup -2]), calculated ²H NMR splittings are on the order of a few kilohertz, in agreement with experimentally measured values through deuterium NMR spectroscopy. [ABSTRACT FROM AUTHOR]

We study the dynamics of polymers in a random disordered medium of fixed obstacles using kinetic Monte Carlo methods. The polymers can have monomers which have attractive (A-type), repulsive (R-type) or neutral (H-type) interactions with the fixed obstacles that comprise the disordered medium. Several classes of homopolymers and heteropolymers with diverse sequences have been studied. Our most noteworthy result is that, above a threshold temperature, polymer bearing monomers that are attracted to the disordered medium translocate faster through the medium than those bearing neutral or repulsive monomers. We discuss how a delicate balance between energetic and entropic factors leads to this counterintuitive outcome. By examining heteropolymers with different sequences, we also find that the dependence of mobility on average composition is stronger than that on higher order correlations characterizing the sequence distribution. Connections between our results and experiments with synthetic and biological systems are noted. [ABSTRACT FROM AUTHOR]

We report on a careful analysis of finite size effects on pressure measurements in Monte Carlo computer simulations of isotropic athermal solutions of flexible polymer chains by means of the repulsive wall method. We find finite size corrections to the pressure due to surface effects. These corrections are inversely proportional to the thickness of the simulation box, both if we keep the average density (in the canonical ensemble) or the chemical potential (in the grand canonical ensemble) constant in course of the preparation of the starting conformation. We propose a modification of the repulsive wall method which allows avoidance of these finite size effects and to estimate the pressure for an infinite system when running simulations in a finite box. [ABSTRACT FROM AUTHOR]

An efficient algorithm for counting contributing terms in the calculation of second and third virial coefficients of the lattice polymer model was proposed. The algorithm was applied to linear and three-arm star polymers. The algorithm's efficiency was demonstrated, and the obtained results were compared to both experimental and computational results already reported. To the authors' best knowledge, the estimation of the third virial coefficient of the three-arm star polymer is the first reported. [ABSTRACT FROM AUTHOR]

An off-lattice bead-spring model of a polymer solution in a container with impenetrable walls is used to study the depletion interaction of a colloid particle with the planar wall by means of a Monte Carlo simulation. As expected, this interaction is found to depend essentially on the ratio p = R/R[sub g] of the particle radius R to the mean radius of gyration R[sub g] of the polymer chains in the case of dilute and semidilute solutions. For large particle to polymer size ratio ρ > 1 this effective force is attractive and decreases steadily with growing distance D of the colloid from the wall. It is found to scale linearly with p in agreement with recent theoretical predictions. In the opposite case of ρ < 1 the depletion force is found to change nonmonotonically with D and go through a maximum at a particular distance D[sub max]≤R[sub g]. In both cases, however, local variations of the polymer density profile, which we detect at higher polymer concentrations, are found to influence the depletion force and even to change it locally from attraction to repulsion. The monomer density distribution far away from/or around the colloid in the vicinity of the wall is also investigated and related to the observed behavior of the depletion force. [ABSTRACT FROM AUTHOR]

Investigates the effect of topological deformation on the mobility and diffusivity of a polymer chain in a good solvent by off-lattice dynamic Monte Carlo simulations. Expression of the topological deformation of the polymer through the knotted structure; Finding that the deformation yields an extra contribution to the resistance associated with a linear chain. [ABSTRACT FROM AUTHOR]

Using Monte Carlo simulations of a coarse grained lattice model, we study conformations and concentration fluctuations of polymers confined into thin films. We vary the chain length from N = 32 to 512. As we reduce the film thickness to the unperturbed radius of gyration R[sub g], the chain extension parallel to the surface increase only by about 5% for the largest chain length. Confinement has, however, a much more pronounced effect on intra- and intermolecular correlations: the surfaces make the chain fold back into the volume it occupies and squeeze out neighboring chains. Decreasing the film thickness, we reduce the interdigitation of chains. This gives rise to deviations from the ideal Gaussian chain structure for wave vectors qR[sub g]> 1, which are qualitatively similar to corrections observed in semidilute (bulk) solutions. Fitting the structure factor with a Debye function in this Kratky regime, we systematically overestimate the chain extension by up to 15%. Additional (minor) deviations from the incompressible random phase approximation at large wave vectors are found. As we reduce the film thickness the correlation function of the centers of mass develops a very strong correlation hole and resembles in thin films that of a two-dimensional fluid of soft particles. [ABSTRACT FROM AUTHOR]

The properties of simplified lattice models of confined branched polymers were studied by the Monte Carlo method. Model chains were located between two parallel impenetrable surfaces. The distance between surfaces, as well as the chain length, were varied. The model chains consisted of f=3 branches of equal length (star-branched polymers) and were based on a simple cubic lattice. The model was athermal and the excluded volume was introduced. Monte Carlo sampling algorithm using local chain micromodifications was applied. Both static and dynamic properties of the system were investigated. The differences between star-branched chains and linear ones which were trapped between walls were discussed. The behavior of static properties made it possible to construct a universal curve presenting the effect of surface-to-surface distance on chain dimensions. The possible explanation of the shape of this curve was given. The changes of dynamic properties were discussed in the context of possible chain’s mechanism of motion. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Three-dimensional dynamic Monte Carlo simulations of polymer translocation through a cylindrical hole in a planar slab under the influence of an external driving force are performed. The driving force is intended to emulate the effect of a static electric field applied in an electrolytic solution containing charged monomer particles, as is relevant to the translocation of certain biopolymers through protein channel pores embedded in cell membranes. The time evolution of the probability distribution of the translocation coordinate (the number of monomers that have passed through the pore) is extracted from three-dimensional (3-D) simulations over a range of polymer chain lengths. These distributions are compared to the predictions of a 1-D Smoluchowski equation model of the translocation coordinate dynamics. Good agreement is found, with the effective diffusion constant for the 1-D Smoluchowski model being nearly independent of chain length. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Examines the Monte Carlo simulation of polymer blends in the absence of interactions. Difference of the microphases length with the mean radius of gyration; Calculation of the structure function and different interaction parameters; Comparison of the results to theoretical models.

Reports on a Monte Carlo and numerical self-consistent field study of end-tethered polymers in good solvent. Calculation of properties of teh system as functions of the degree of polymerization and surface density of the chains; Lower limit on the range of surface coverage.

Examines the crossover region from the swollen region to the semidilute regime using polymer chains of degree up to N=2000 via Monte Carlo simulation. Analysis of chain end-to-end distance as a function of the number of chain links expressed in terms of their connectivity; Use of scattering functions to study structure of polymer chains.

Discusses the observation of the molten globule state in a Monte Carlo simulation of the coil-to-globule transition of a homopolymer chain. Implication of the existence of the special state on the process of the melting of the globule; Resemblance of the sate to the molten globule state of a protein chain.

Presents a Monte Carlo scheme for the efficient simulation of multi-polymer systems. Insertion of chains into the system using a biased growth technique; Success rates of the growth process; Testing of the method using self-avoiding walks on a cubic lattice.

Examines the conformational properties of linear polymer brushes. Usage of the Monte Carlo method and molecular dynamics simulations in conformational property description; Focus on the isotropic-nematic phase transitions from collapsed to strongly stretched regimes of the brushes; Impact of orientational interactions on the isotropic temperature.

Investigates the Monte Carlo simulation of Lennard-Jones chains grafted polymers in supercritical solvent. Enhancement of solvent density relative to bulk density; Effects of solvent bulk density on solvents; Similarities between interaction force and surface point apparatus on surfaces with adsorbed copolymers.

Examines a cubic lattice model of star-branched polymers using Monte Carlo simulations. Analysis of a shape of branched macromolecules by means of the gyration tensor principal moments and asphericity factor; Changes of the parameters during the transition from a random coil to a collapsed globule; Formation of an isotropic globule in lower temperatures.

Examines the influence of long-range chain connectivity on the thermodynamic properties of athermal compressible single component and binary polymer mixtures. Influence of the excluded volume of the chain molecule; Context of intramolecular self-contacts; Discussion of results for athermal pure components and mixtures.

A model for a multichain polymer system in three-dimensional continuous space is studied by link cell Monte Carlo methods, using systems up to chain length N=64 and up to 16 384 monomers. The chains consist of beads with a hard core connected by rather stiff harmonic bonds, with a repulsive Lennard-Jones-type interaction between beads chosen such that chains cannot cross each other during their random motions. On RISC workstations the model performs only about a factor of 4 slower than the bond fluctuation lattice model, the qualitative behavior of the time-dependent mean-square displacements and relaxation functions being rather similar to the latter. For the model without obstacles, it is shown that the present continuum model can be approximately mapped on the lattice bond fluctuation model by a suitable rescaling of chain length and volume fraction. But the distinctive advantage of the present model is that it can be applied easily to random media (described by randomly placed rigid obstacles), without the severe ergodicity problems (locked-in configurations) arising in lattice models for such systems. It is shown that static properties of the chains stay nearly unaffected by the obstacles, while the chain motions are considerably slowed down in this frozen environment. [ABSTRACT FROM AUTHOR]