bluuebunny/crossref_metadata_2025_split huggingface dataset:
Linear chain surfactants at a planar interface: a comparative Monte Carlo study of several lattice models
Authors: Haas Frank M. , Lai Pik‐Yin , Binder Kurt 
Published: April 2003

AbstractLinear chain surfactants in a densely packed arrangement (such as alkane chains in lipid monolayers in the “uniform tilt” structures) are described by a crude coarse‐grained model where the endgroups grafted on the interface form a regular lattice and the chains are described by the bond fluctuation model with chains containing N = 4 effective monomers only. Square‐well interactions between the monomers are studied for both the attractive and repulsive case for three choices of the interaction range. None of these models exhibits a structure with uniform tilt. For attractive interactions the last bond has a strong tendency to fold back thus leading to a very high density close to the interface. Only when an intrachain‐potential favoring stiff chain configurations also is included one can obtain configurations with uniform tilt order. Although related models (with much longer chain lengths and lower grafting densities) are very useful for the study of polymer brushes, the present case of very short chains in a high‐density state clearly is plagued by various lattice artefacts and it is concluded that for modelling linear chain surfactants one should use an off‐lattice model even on a coarse‐grained level.
Isomerism and symmetry of stacked polymeric phthalocyaninnes and related polymers
Authors: Knothe Gerhard 
Published: April 2003

AbstractThe number of isomers ir,t for stacked polymeric phthalocyanines in which each phthalocyanine ring is tetrasubstituted or octasubstituted with two different substituents (each benzene moiety substituted with both substitutents) is determined to be

where m is the number of substituted phthalonitrile molecules incorporated into the polymer and x is the number of phthalocyanine macrocycles (degree of polymerization; m = 4x). Specifically, for a stacked polymeric phthalocyanine with x = 2; there exist 298 isomers. The stacked polymeric phthalocyanines with substituted phthalocyanine rings possess different symmetry groups (D2h, D2d, C4, C2V or CS E as sole symmetry operation). The results are valid for stacking of other macrocycles similar to phthalocyanine such as prophyrins.
Polyelectrolyte/Ionomer behavior in polymer gel collapse
Authors: Khokhlov Alexei R. , Kramarenko Elena Yu. 
Published: April 2003

AbstractThe theory of collapse of weakly charged polyelectrolyte gels is generalized by taking into account the possibility of counter ion trapping with the formation of ion pairs, which becomes progressively important as the gel shrinks and the dielectric constant of the gel medium decreases. A phenomenon well known in the theory of ionomers, namely the aggregation of ion pairs due to dipole‐dipole interactions with the formation of multiplets, is also taken into account. These multiplets act as additional physical cross‐links. It is shown that accounting for the two effects mentioned above generally leads to an increase of the region of stability of the collapsed phase and to an increase of the jump in volume at the transition point. The most important, qualitatively new effect is the possibility of existence of a new supercollapsed state of a polymer gel which is very close to the densely packed dry gel. The reason for the thermodynamic stability of the supercollapsed state is a loop of positive feedback: the decrease of the volume of the gel leads to a decrease of the dielectric constant and hence to progressive formation of ion pairs, thus the concentration of mobile counter ions and the corresponding osmotic pressure decrease, the gel shrinks further etc. It is possible to realize the phase transitions between all three states of a polymer gel: swollen, ordinary collapsed and supercollapsed.
Viscosity of weakly charged polyelectrolyte solutions: The screening of hydrodynamic interactions
Authors: Borsali Redouane , Vilgis Thomas A. , Benmouna Mustapha 
Published: April 2003

AbstractThe mode‐mode coupling approximation (MMCA) is used to describe the behavior of the reduced viscosity of weakly charged polyelectrolyte solutions in the presence of screening of hydrodynamic interactions. This effect is introduced through a dynamic correlation length ζH in the Oseen tensor description. The results show that as ζH goes from zero (Rouse model) to infinity (Zimm model), the general qualitative behavior of ηr as function of the polyelectrolyte concentration Cp does not change but its value decreases significantly with a slight shift in the peak position toward higher Cp values.
Lattice model of heterogeneous polymer chains in the liquid‐crystalline state, 1 statistical properties
Authors: Medvedev Gregory A. , Gotlib Yuli Y. 
Published: April 2003

AbstractA heterogeneous lattice model for main‐chain liquid‐crystalline polymers is proposed. Phase transition parameters and statistical properties of the chain in the ordered phase are treated in the framework of a mean‐field approximation. The order parameters for the mesogen and the spacer bonds, the chain shape anisotropy and the fraction of stretched isomers are obtained. Dependence of the Curie point on the spacer length and flexibility is considered. The “odd‐even” effect is shown to increase drastically with the spacer flexibility. A strong dependence of the conformation changes in the ordered state on the spacer properties is obtained.
Monte Carlo simulation of tetrahedral chains, 7 the shape of linear and star‐branched polymers near to theta‐conditions
Authors: Zifferer Gerhard 
Published: April 2003

AbstractBy use of the pivot algorithm, star‐branched chains with F = 4, 8 and 12 arms of length n and linear chains (F = 2) are generated on a tetrahedral lattice (120 ≤ nF ≤ 3 840). By taking into account nearest neighbour interactions (each contact contributes an energy ϕ kT to the total energy of the configuration) a variation of the thermodynamic quality of the solvent is simulated by a variation of the energy parameter ϕ near the value of ϕθ = ‐0,475, characteristic of theta‐conditions. For theta‐conditions various quantities characteristic of the instantaneous shape of polymers exhibit similar values as found for nonreversal random walks; furthermore, while linear theta‐chains are slightly less asymmetric than athermal ones, the opposite behaviour is found for star‐branched polymers. Clearly, for all thermodynamic conditions the asymmetry of configurations decreases with increasing number of arms but remains appreciable even for F = 12.
Effect of polyfunctional chain transfer agents on molecular weight distribution in free‐radical polymerization, 2. The case C ≤ 1
Authors: Yuan Cui Ming , Farina Mario 
Published: April 2003

AbstractIn this part of the series, the influence of polyfunctional chain transfer agents with transfer constant C ≤ 1 on the molecular weight distribution was studied. The analytical expressions for the number‐ and weight‐average degree of polymerization, and dispersion index were derived by kinetic and statistical methods. The expression for the molecular weight distribution can only be obtained by statistical methods. Some numerical examples on the dependence of distribution parameters as a function of the functionality of transfer agents f and transfer constants are illustrated. A critical value of the chain transfer constant was found to exist, which permits the synthesis of linear (for f = 2) or branched polymers (f > 2) with DPw/DPn approximately equal to 2 during the entire course of the polymerization.
Band structure of quasi‐1‐dimensional polycondensed π‐systems, 3 energy spectra of anti‐aromatic polymers
Authors: Dietz Fritz , Müllen Klaus , Baumgarten Martin , Tyutyulkov Nikolai 
Published: April 2003

AbstractThe band structure of a class of quasi 1‐dimensional (1‐D) polymers with anti‐aromatic structural units — anti‐aromatic polymers (AAP) — has been investigated theoretically. The energy gap (EG) of the AAP is significantly smaller compared to the EG of polymers with aromatic building blocks having the same number of π‐centers in the elementary units. The influence of the electron correlation in the calculation of the EG is investigated. As with the anti‐aromatic molecules the Jahn‐Teller distortion influences the geometric configuration of the AAP.
Neural networks and graph theory as computational tools for predicting polymer properties
Authors: Sumpter Bobby G. , Noid Donald W. 
Published: April 2003

AbstractA new computational methodology is presented for making rapid and accurate predictions of chemical, physical and mechanical properties of polymers based on their molecular structure. The method uses a set of topological indices derived from graph theory to numerically describe the structure of a monomeric repeating unit for a given polymer (structural descriptors) and relates these indices to a set of polymer properties by utilizing an artificial neural network. The neural network is able to efficiently formulate all of the correlations (i.e., between structural descriptor‐property, property‐property, structural descriptor‐structural descriptor: both linear and nonlinear dependencies) necessary to make accurate predictions. Results have been obtained for up to 9 properties of 357 different polymers with an average prediction error of < 3% and a maximum error of 12%, demonstrating superiority over other quantitative structure/property relationships for polymers.
Approximate quantum‐mechanical hamiltonians for slow internal rotations of macromolecules
Authors: Alvarez‐Estrada Ramón F. 
Published: April 2003

AbstractSlow internal rotations of three‐dimensional macromolecules are studied based upon quantum mechanics, the Born‐Oppenheimer approximation and related assumptions, and a variational inequality. Difficulties arising from vibrational and electronic energies are pointed out and bypassed consistently. Approximate quantum hamiltonians for internal rotations, in terms of dynamical variables studied recently, are obtained.
The study of network formation as a cluster‐cluster diffusion‐limited aggregation process: Modelling of the curing of an epoxy‐resin using the Monte Carlo method
Authors: Netz Paulo A. , Samios Dimitrios 
Published: April 2003

AbstractThe study of network formation during the curing of an epoxy‐resin with anhydride and tertiary amine has been carried out by the Monte Carlo method. Both dynamical and structural properties are studied. The model resembles the well known cluster‐cluster aggregation: the reaction is modelled as a “geometrical” phenomenon and neglects attractive and repulsive energies except for an excluded volume condition. A reaction occurs always when two active sites are in the vicinity to each other, meaning that the “chemical” processes are taken to be much faster than diffusion (as in diffusion‐limited aggregation). Results are presented of two‐dimensional square lattice simulations with periodic boundary conditions and are discussed with respect to the proposed mechanism of the curing reaction and to the assumptions of the diffusion‐limited nature of the processes. The scope and limitations of such two‐dimensional simulations are discussed.
Conformational and packing energy calculations on ethylene copolymers
Authors: Napolitano Roberto , Pucciariello Rachele , Villani Vincenzo 
Published: April 2003

AbstractA conformational analysis was performed on the isolated chains of ethylene‐propene and ethylene‐1‐butene copolymers. The lowest energy conformations in accordance with the chain repeating distance of polyethylene were used in packing energy calculations. The results of our calculations suggest that both methyl and ethyl groups are tolerated in the crystalline conformation of a single polyethylene chain, but only the methyl group is acceptable in the crystalline state if the packing energies and the lattice deformations are taken into account.
Microphase separation in poor‐solvent polyelectrolyte solutions: Phase diagram
Authors: Dormidontova Elena E. , Erukhimovich Igor Ya. , Khokhlov Alexei R. 
Published: April 2003

AbstractMicrophase separation in solutions of weakly charged polyelectrolytes in poor solvents is studied in the weak segregation limit within the framework of the mean field approximation using a method first developed by Leibler. As a result a complete phase diagram of the solution near the critical point is obtained. The regions of the stability of the disordered, homogeneous phase and of body‐centered cubic (bcc), triangular and lamellar microdomain structures, as well as the phase separation regions are determined. The most striking difference in comparison with the corresponding diagram for block‐copolymer melts is the existence of broad phase separation regions even for monodisperse systems. As the quality of solvent becomes poorer, the triangular microdomain structure remains the most stable among microdomain phases of other symmetry.
A model ladder polymer physisorbed on a Lennard‐Jones surface
Authors: Hentschke Reinhard 
Published: April 2003

AbstractThe ladder or band polymer considered here differs from “ordinary” main chain polymers through its strongly elongated cross section as opposed to the usually assumed cylindrical cross section of the latter. This cross sectional shape anisotropy and the attendant anisotropy of the bending stiffness give rise to a competition between the polymer's adsorption energy and its undulation entropy. Following the self‐consistent field approach the present work studies the effects due to this competition in terms of the orientation order parameter of the ladder plane and the normal distribution of the polymer with respect to a smooth Lennard‐Jones surface. In particular, we show that it is possible to induce abrupt orientation changes of the ladder‐plane by only slight variations of the torsion and bending persistence lengths of the polymer or the polymer‐surface interaction.
Construction of approximate entropic forces for finitely extensible nonlinear elastic (FENE) polymers
Authors: Slater Gary W. , Hubert Sylvain J. , Nixon Grant I. 
Published: April 2003

AbstractWhen the stress applied to a Rouse‐like polymer chain is large enough, one must use anharmonic entropic spring forces in order to keep the chain contour length from increasing to unphysical values. Although one can derive “exact” equations relating the spring extension to the entropic force produced by a finitely extensible non‐linear elastic (FENE) random‐walk polymer, such expressions are usually of little interest because their complexity would entail large evaluation times in numerical studies by computer. Moreover, these expressions can rarely be used directly in analytical studies. In this article, we describe a systematic method to construct analytically simple yet numerically accurate expressions to relate the entropic force to the extension of an entropic spring for a random‐walk polymer chain in arbitrary dimension d ≥ 2. These expressions are modified Pade approximants which yield the correct asymptotic behaviours in both the small and large extension limits. It is shown that the well‐known Warner empirical approximation is but a limiting case (for infinite dimensions).
Dynamics of star branched polymers in a matrix of linear chains — a Monte Carlo study
Authors: Sikorski Andrzej , Kolinski Andrzej , Skolnick Jeffrey 
Published: April 2003

AbstractA simple cubic lattice model of the melt of 3‐arm star‐branched polymers of various length dissolved in a matrix of long linear chains (n1 = 800 beads) is studied using a dynamic Monte Carlo method. The total polymer volume fraction is equal to 0,5, while the volume fraction of the star polymers is about ten times smaller. The static and dynamic properties of these systems are compared with the corresponding model systems of isolated star‐branched polymers and with the melt of linear chains. It has been found that the number of dynamic entanglements for the star polymers with arm length up to 400 segments is too small for the onset of the arm retraction mechanism of polymer relaxation. In this regime dynamics of star‐branched polymers is close to the dynamics of linear polymers at corresponding concentration and with equivalent chain length. The entanglement length for star polymers appears to be somewhat larger compared with linear chains.
Statistical thermodynamics in the framework of the lattice fluid model, 3. Binary mixture of polydisperse polymers of special distribution with strong interactions
Authors: An Lijia , Jiang Bingzheng , Hu Yuxin , Jiang Zhenhua , Tang Xinyi 
Published: April 2003

AbstractFor a binary mixture of polydisperse polymers with strong interactions, the free energy, the equation of state, the chemical potentials and the spinodal are formulated on the basis of the lattice fluid model. Further, the spinodal curves for the system with strong interactions are simulated by using mixtures of special‐distribution polymers, i.e., Schulz distribution and uniform distribution. From comparison with the experimental results, it can be seen that the shift of the spinodal curves with the polydispersity and the strong‐interaction energy parameter is qualitatively identical with the conclusions of Nishi and Kwei, and Goh, Paul and Barlow.
Increase of the average reactivity of active species due to a shift to the more reactive species in ionic propagation accompanied by termination
Authors: Penczek Stanislaw , Vairon Jean‐Pierre , Szymanski Ryszard 
Published: April 2003

AbstractThe presented simulations demonstrate that in polymerizations proceeding on two kinds of species, differing in reactivity and being in equilibrium, the expected decrease of the rate of polymerization due to termination may happen to be compensated by the relative increase of concentration of the more reactive species. This takes place, for instance, in the polymerization proceeding simultaneously on ions and ion pairs if ions are more reactive. Because of termination the total concentration of ionic species during the course of polymerization decreases while the proportion of ions increases due to increasing dilution. The maximum compensation is observed when simultaneously k(ions)/k(ion pairs) → and Kd/[I]0 → 0, where k are the propagation rate constants, Kd is the equilibrium constant of dissociation and [I]0 is the starting concentration of initiator. Then, the degree of compensation (the ratio of the rate with compensation to the rate without termination) is becoming equal to ([P*]/[P*]0)1/2, where [P*] is the actual, total concentration of the growing species and [P*]0 is the initial total concentration (before any termination has taken place).
Structure, energy spectra and magnetic properties of poly(m‐aniline)s
Authors: Madjarova Galia , Baumgarten Martin , Müllen Klaus , Tyutyulkov Nikolai 
Published: April 2003

AbstractThe energy spectra and magnetic properties of a large class of one‐dimensional poly(m‐aniline)s (PMA) and further model polymers are investigated theoretically. The band structure of those PMA's which are aza‐analogs of the alternant non‐classical (non‐Kekulé) hydrocarbons (polymers) is characterized by a wide gap in which there is a narrow half‐filled band (HFB). The different contributions to the effective spin exchange between the unpaired electrons in the HFB: Coulomb (Hund), kinetic and indirect exchange interactions are calculated. While it has been shown earlier that PMA exhibits a net spin exchange of ferromagnetic nature, this approach enables a detailed understanding of the influence of substituents on the nitrogen centers and changes in the aromatic bridging unit. The ferromagnetic nature of the spin coupling in the singly bridged PMA models therebye prevails independent of the structural changes. The HFB width of those PMA's which are derivatives (aza‐analogs) of ladder‐type non‐alternant hydrocarbons is very large and the Wannier functions are delocalized. For this group of polymers the delocalized, non‐magnetic state is favored and, therefore, they may be good candidates for testing electrical conduction.
Edge profiles at lamellar phase/melt interfaces
Authors: Hołyst Robert 
Published: April 2003

AbstractWe study in the framework of the continuum theory of dislocations the structure of the interface between an AB diblock copolymer lamellar film deposited on a solid substrate and an A‐homopolymer melt. The dislocation inside the lamellar phase induces steps at the interface. The shape of the profile at the edge of a step (edge profile) depends on the distance of the dislocation from the interface. The profile and the equilibrium location of the dislocations are both studied as a function of the film thickness, D. For large D, the dislocation is stabilized at a finite distance, heq, from the interface, due to the small surface tension and large surface bending elastic constant, Ks. For zero surface tension, heq ≈ Ks/(2K), where K is the bulk bending elastic constant. For small D, heq is mainly determined by the proximity of the solid substrate. The edge profile along the interface is a monotonic function of the distance along the interface for large D of the film and becomes nonmonotonic for small D. Also the dislocation energy strongly depends on D for small D. The theory is discussed in connection to recent experimental studies of diblock copolymer films deposited on a solid substrate.
A comparative study of the master equation approach and the discrete galerkin method for the simulation of free radical polymerization
Authors: Seeßelberg Markus , Thorn Matthias 
Published: April 2003

AbstractThe present article deals with the mathematical treatment of free radical polymerization reactions. As a typical example the synthesis of poly(methyl methacrylate) under realistic experimental conditions is investigated. Since the mathematical treatment of the kinetic rate equations raises severe numerical problems, alternative approaches are required. In this paper two of these methods, i.e. the discrete Galerkin method and the master equation approach, are compared. The discrete Galerkin method circumvents difficulties encountered by the direct integration of the kinetic rate equations but requires much a priori knowledge of the chemical reaction system. Within the framework of the master equation approach the polymerization reaction is regarded as a stochastic process. For the simulation of this stochastic process a modified algorithm is presented. The example of the polymerization of methyl methacrylate shows that the master equation approach is an efficient tool in the simulation of free radical polymerization reactions.