bluuebunny/crossref_metadata_2025_split huggingface dataset:
Dipolar concentration correlations in a diblock copolymer melt
Authors: Pakula Tadeusz 
Published: April 2003

AbstractThe conformation of diblock copolymer chains in a computer simulated homogeneous melt is analyzed distinguishing the separation of ends and the end‐to‐end vector orientation of individual chains. It is demonstrated that within the cigar‐like shapes of chains the blocks A and B are spatially separated along the cigar axis constituting concentration dipoles. Consequences of such concentration correlations for the light scattering are discussed. An extra contribution to the depolarized light scattering resulting from the “form birefringence” of individual chains is postulated.
Dynamics of a paraffin crystal
Authors: Liang Guanghe L. , Noid Donald W. , Sumpter Bobby G. , Wunderlich Bernhard 
Published: April 2003

AbstractRecently it has become possible to make a series of simulations on a supercomputer of the molecular motion of a pentacontane (C50H102) crystal containing close to 10000 atoms. Such a crystal is big enough to serve as a model of a macroscopic crystal. It can also serve as a model for polyethylene. The large volume of detailed information on the structure and motion permits a new level of understanding. Information on crystal appearance, pseudo‐symmetric crystal structures, picosecond time‐scale formation of conformational defects and the beginning of the melting process is presented.
A global‐update simulation method for polymer systems
Authors: Heermann Dieter W. , Yixue Li 
Published: April 2003

AbstractWe describe a hybrid Monte Carlo algorithm, combining molecular dynamics with the Monte Carlo method, applied to the simulation of polymer systems. It is shown that observables are independent of the discretization error, and the performance, i.e., autocorrelation times of observables, is analyzed. For a dense system representing a polyethylene melt, we present data for the pair distribution function and the mean square displacement of a chain in the bulk. We also investigate the possibility of using a scaled Hamiltonian in the algorithm.
Gelation as a percolation problem with finite diffusion
Authors: Rosche Matthias , Schulz Michael 
Published: April 2003

AbstractThe formation of polymer networks (gelation process) was studied by a numerical simulation of a lattice spin model with 3 states, which allows the representation of simple diffusion and aggregation processes. Near the percolation threshold, pc, the same critical exponents were detected as in the usual diffusion‐free classical percolation theory. Only non‐universal values (for example pc) show a significant dependence on the ratio between diffusion and reaction constants.
On the sol fraction of Aa1 Bb1−Aa2 Bb2 copolymerisations
Authors: Li Ze‐sheng , Ba Xin‐wu , Sun Chia‐chung , Tang Au‐chin 
Published: April 2003

AbstractBy means of probability theory, the sol fraction of an Aa1 Bb1−Aa2 Bb2 type copolymerisation reaction, in which intramolecular reactions are neglected, is investigated in the post‐gel regime to give the gelation condition explicitly. (Aa1 Bb1 is a monomer with a1 functional groups of type A and b1 functional groups of type B.)
Probabilistic multi‐site modelling of propene polymerization using markov models
Authors: van der Burg Marijke W. , Chadwick John C. , Sudmeijer Olof , Tulleken Herbert J. A. F. 
Published: April 2003

AbstractPossible propagation mechanisms in propene polymerization using heterogeneous catalysts are described by combinations of Bernoulli and first‐ or even second‐order Markov models. Both NMR‐measured pentad fractions and GPC Gel permeation chromatography.
‐measured molecular weight distributions are directly taken into account to fit an appropriate propagation model for a specific catalyst. The analytical data so obtained have been used for an examination of the applicability of multi‐site propagation models and for estimation of the main parameters in each model. The propagation and termination probabilities are estimated, as are “mix” parameters giving the fraction of the polymer population stemming from each kind of propagation mechanism. In this way the subpopulation produced by each site is characterized in terms of tacticity and molecular weight distribution. A three‐site model is required to explain the measured data properly.
Modelling free‐radical copolymerization kinetics—evaluation of the pseudo‐kinetic rate constant method, 1. Molecular weight calculations for linear copolymers
Authors: Xie Tuyu , Hamielec Archie E. 
Published: April 2003

AbstractThe moment equations for binary copolymerization in the context of the terminal model have been solved numerically for a batch reactor operating over a wide range of conditions. Calculated number‐ and weight‐average molecular weights were compared with those found using pseudo‐kinetic rate constants with the method of moments and with the instantaneous property method for homopolymerization. With the pseudo‐kinetic rate constant method under polymerization conditions where number‐average molecular weights (M̄n) are below about 103 the error in calculating M̄n exceeds 5%. The error increases rapidly with decrease in molecular weight for M̄n < 103. M̄n measured experimentally for polymer chains (homo‐ and copolymers) have error limits of greater than ±5% at the 95% confidence level. Therefore, for all practical purposes, the pseudo‐kinetic rate constant method is valid for M̄n greater than 103. Errors in calculating weight‐average molecular weights (M̄w) or higher averages are always smaller than those for M̄n when applying the pseudo‐kinetic rate constant method. The assumptions involved in molecular weight modelling using the pseudo‐kinetic rate constant approach are thus proven to be valid, and therefore it is recommended that the pseudo‐kinetic rate constant method be employed with the instantaneous property method to calculate the full molecular weight distribution and averages for linear chains synthesized by multicomponent chain growth polymerization.
Solvent effects on free‐radical polymerization, 4. Modelling of hompolymerization
Authors: Czerwinski Wojciech Krzysztof 
Published: April 2003

AbstractA new so‐called reactant‐solvent complex model is proposed to describe the effect of solvent on chain propagation in homopolymerization. It takes into account complex formation of both monomer and radical with solvent by equilibria. Evaluation methods presented permit to estimate the complex equilibrium constant K which is assumed to be nearly the same for both monomer and radical complexation and the relative reactivity ratio r11, for complexed monomer. Measured reaction rates as function of monomer concentration are needed for calculations.
Calculation of phase equilibria in random copolymer systems
Authors: Wohlfarth Christian 
Published: April 2003

AbstractSynthesis and application of copolymers are not seldom connected with different phase equilibria. Their precise knowledge is of importance for industrial processing as well as it is a profound basis for a better understanding of the nature and thermodynamics of such systems. As a common situation today, enough experimental information is seldom available in the necessary or desired amount, and a lot of model calculation is, therefore, more or less unavoidable to cover the desired ranges of application. Different equations‐of‐state as well as lattice models are discussed with respect to their applicability for calculating liquid‐liquid and gas‐liquid phase equilibria in copolymer solutions and blends. Examples for high‐pressure phase equilibria in monomer/copolymer mixtures, liquid‐liquid demixing in copolymer blends and for the isotropicnematic phase equilibrium in systems with rigid rod‐like copolymers characterized by distributions of rigid and flexible chain parts are given. The effects of copolymer polydispersity are included by means of continuous thermodynamics. Literature references for original sources, earlier reviews and further applications round up this paper.
A simple estimate of the polymer theta‐point on different lattices
Authors: Barat Krishna 
Published: April 2003

AbstractWe check here numerically and establish the validity of a simple estimate for the polymer theta‐point on various pure and diluted lattices. Our study on two‐dimensional lattices supports a linear relationship of the θ‐point value with the average number Zeff of non‐bonded (nearest) neighbours on a random self‐avoiding walk (SAW). This Zeff in turn can be estimated from the connectivity constant μ of SAWs of the corresponding lattice.
A model for the creep of oriented high‐modulus fibers
Authors: Schuppert Andreas A. 
Published: April 2003

AbstractRecently it has been shown that the creep of oriented high‐modulus fibers satisfies a logarithmic time law over very long time intervals. Moreover, the modulus of these fibers increases due to the creep. This was interpreted by the annihilation of conformation defects during creep. In structures with strong intermolecular interactions as in aramides, however, the direct annihilation of defects is difficult to explain. Moreover a direct annihilation leads to a false time law if no further assumption about the properties of the defects are made. In this paper a model for the transport of conformation defects in oriented polymer chains imbedded into an oriented fibrillary structure with strong intermolecular interactions is presented. This modified transport model leads directly to the experimentally observed time law without any further assumptions about the defect properties.
Fast force field expressions for computer simulations
Authors: Jung Bernd 
Published: April 2003

AbstractIt is the aim of this work to develop analytical force field expressions that can be rapidly evaluated by computers. The new expressions approximate the energy hypersurfaces as described by the usual force fields. The energies and the derivatives of the energy expressions, i. e. the forces acting on the atoms, in most cases can be very quickly calculated as functions of one squared distance of two atoms per interaction, avoiding slow operations like cosine, square root etc. Formulae and algorithms are given to calculate the parameters needed from those of the AMBER force field.
Simulation model for network formation in free‐radical crosslinking copolymerization: Pregelation period
Authors: Tobita Hidetaka 
Published: April 2003

AbstractA new simulation model for network formation in free‐radical copolymerization of vinyl and divinyl monomers is proposed. This model is based on the crosslinking density distribution of the primary polymer molecules that results from a kinetically controlled network formation. The crosslinking density distribution provides information on how each chain is connected to other chains and therefore, a detailed analysis of the kinetics of network formation becomes possible by application of Monte Carlo simulations. In this method, not only averages but also various distributions, such as molecular weight distribution and distribution of crosslinked units as well as of unreacted pendant double bonds among various polymer molecules, can be calculated. The present theory is a direct solution for the Bethe lattice formed under nonequilibrium conditions, and therefore, it can be used to examine the applicability of the earlier theories of network formation to kinetically controlled systems. The present method is quite general and can be applied to various complex reactions systems that involve crosslinking, branching, cryclization and degradation in a nonequilibrium system.
Quantum chemical interpretation of the stereospecific effects characteristic of the polymerization in acrylonitrile‐RLi and methyl acrylate‐RLi systems
Authors: Yakimansky Alexander V. , Erussalimsky Boris L. 
Published: April 2003

AbstractWith the aim to find the causes of the different stereochemical behavior characteristic of the anionic acrylonitrile (AN) and methyl acrylate (MA) growing chains, the following model systems were investigated using the MNDO1Modified neglect of diatomic overlap.
(method: 1) compounds XCH2CH2CH(X)Li simulating racemic (r) and meso (m) diads of AN (X = CN) and MA (X = COOCH3) at the active end of the growing chain; (2) intermediate complexes of a MA r‐diad with a MA molecule in the mutual orientations which should lead to the formation of a rr‐ or rm‐triad; (3) compounds XCH2(CH2CH(X))2Li simulating mm‐, mr‐, rm‐, or rr‐triads of MA and their complexes with H2O and ethylenediamine (EDA). It was found that reaction of a MA r‐diad with a MA molecule should preferably lead to the formation of a rm‐triad in the absence of EDA and to the formation of a rr‐triad in the presence of EDA. For AN none of the possible transformations of active diads into triads was found to be preferred. These results are in qualitative agreement with the experimental data.
Quantum semiempirical studies on the formation of aromatic polyamides, 1. Effect of structural parameters of diamines on the polyamidation reaction
Authors: Lozano Angel E. , de Abajo Javier , De la Campa José G. 
Published: April 2003

AbstractA semiempirical quantum‐mechanical study of structural and electronic parameters of aromatic diamines was performed using the method AM1. The diamines have been considered as condensation monomers, and the reactivities of both amino groups against acyl chlorides have been computed. The study shows that the amidation reaction is controlled mainly by the global charge on the amino group and, in a much smaller extent, by the frontier orbitals. This behaviour is consistent with the large energy gap between the HOMO1HOMO: highest occupied molecular orbital.
of the diamine and the LUMO2LUMO: lowest unoccupied molecular orbital.
of the acyl chloride. It has been found also that the electronic nature of the substituents, affecting the charge of the amino groups, greatly affects the carbons directly joined to these groups. This permits to correlate the reactivities with the calculated charges on these carbons:
Chain collapse of star polymers
Authors: Allegra Giuseppe , Vitis Mario De , Ganazzoli Fabio 
Published: April 2003

AbstractThe self‐consistent approach to free‐energy optimization is applied to regular star polymers in bad solvents (T < Θ); the Gaussian approximation and the Zimm‐Kilb description of Fourier modes are both warranted under these conditions leading to chain collapse. As currently done in this approach, the interatomic‐contact free energy is taken to be a single‐valued function of the mean‐square radius of gyration 〈S2〉, whence the universal plots of the mean‐square distances between two atoms and between the atoms and the center of mass are obtained as a function of the contraction ratio α = 〈S2〉/〈S2〉0 (the zero subscript denotes the unperturbed state at T = Θ) and of the degree of branching. The density as a function of the distance from the center of mass as well as the equilibrium structure factor are likewise evaluated. The decrease of αs with an increase of the undercooling (Θ − T) at a fixed molecular weight is sharper with a smaller number of arms, being sharpest with the linear chain. The relative fluctuation of the radius of gyration, expressed as (〈S4〉 − 〈S2〉2)/〈S2〉2, increases at first with the undercooling, showing a maximum close to the transition temperature which is more pronounced the smaller is the degree of branching, then decreases sharply at strong contration. The average location of the free ends of the collapsed molecule is close to, or even outside, the surface of the resulting globule, their mean‐square distance from the center of mass being 2 〈S2〉. Conversely, the mean‐square distance of the branch point from the center of mass if 2〈S2〉/f, f being the number of arms.
Theoretical study of chain transfer in anionic polymerization, 2. Copolymerization accompanied by chain transfer to solvent
Authors: Litvinenko Galina I. , Arest‐Yakubovich Alexander A. 
Published: April 2003

AbstractThe main factors determining molar mass characteristics of copolymers formed in nonterminating copolymerization under the conditions of chain transfer to solvent are studied theoretically. The dependences of the mean polymerization degrees on conversion and monomer feed composition for various values of reactivity ratios are obtained. The results obtained greately differ from those for homopolymerization. This is explained by the contribution of cross‐propagation reactions. In particular, it is shown that at ≪ and ≪ the azeotropic copolymerization proceeds like the living one even if homopolymerization of each monomer is accompained by extensive chain transfer to solvent.
A mean field theory for the swelling of a gaussian polyion
Authors: Bratko Dušan , Dawson Kenneth A. 
Published: April 2003

AbstractThe effect of electrostatic forces among the ionized units on the structure of a polyion is studied using a simplified mean‐field model that represents the polymer as a swollen Gaussian coil. Screened Coulombic interactions are approximated by the pairwise additive Yukawa potential. The swelling parameter is estimated in a variational procedure analogous to Flory's theory for neutral polymers. The analytic solution to the variational problem is given. The method is shown to provide a fair description of the polyion configuration at lower ionization or in the presence of moderate concentrations of simple salt in the solution.
Computer simulations of two‐dimensional trifunctional bimodal networks
Authors: Sakrak Gokhan , Bahar Ivet , Erman Burak 
Published: April 2003

AbstractTransformation of mean chain dimensions in a bimodal Gaussian network is analyzed by generating a two‐dimensional trifunctional network using Monte Carlo technique and deforming it, subject to the balance of mean forces at each junction. Instantaneous fluctuations in chain dimensions are not considered in the simulations. The problem is, therefore, equivalent to the analysis of a two‐dimensional deterministic system of two sets, short and long, of linear springs, joined at trifunctional junctions. The molecular deformation of network chains, the distribution of orientation of chain vectors and the reduced stresses are studied as a function of extension ratio and network topology.
Preliminary potential energy calculations of cellulose iα crystal structure
Authors: Aabloo Alvo , French Alfred D. 
Published: April 2003

AbstractPacking energy calculations were used to evaluate various models of Iα cellulose, based on unit cell dimensions proposed by Sugiyama et al. Both a rigid‐ring method, PLMR, and a full‐optimization molecular mechanics technique, MM3, were used. The model found to be best with both methods was packed “up” (the z coordinate of O5 is greater than that of C5); O6 atoms were in tg positions, forming sheets of hydrogen bonded chains. With the PLMR program, the energy of the best model was almost 3 kcal/mol lower than the second best model. The MM3 studies also showed substantially higher energies for the alternative models. Also, some alternative PLMR models had substantially higher atomic movement during MM3 optimization.
Polymer deformation in various flows
Authors: Carl Wilfried , Bruns Wolfgang 
Published: April 2003

AbstractUsing a Langevin‐like approach, the deformation of a polymer, modelled as a bead‐spring chain, is calculated in simple shear, elongational and Kramers potential flow. Analytic expressions for the mean‐square end‐to‐end distance, radius of gyration, segment‐segment distance, static structure factor up to O(q4) and the intrinsic elongational viscosity are given. Near equilibrium, preaveraged hydrodynamic interaction is taken into account.
Polymer chains confined into tubes with attractive walls: A Monte Carlo simulation
Authors: Milchev Andrey , Paul Wolfgang , Binder Kurt 
Published: April 2003

AbstractA bead‐spring off‐lattice model of a polymer chain with repulsive interactions among repeating units confined into straight tubes of various cross sections, DT2, is studied by Monte Carlo simulation. We are also varying the chain length from N = 16 to 128 and the strength of a short‐range attractive interaction between the repeating units and the walls of the tube. Longitudinal and perpendicular static linear dimensions of the chains are analyzed, as well as the density profile of repeating units across the tube. These data are interpreted in terms of scaling concepts describing the crossover between three‐dimensional and quasi‐one‐dimensional chain conformations and the adsorption transition of chains at flat infinite walls, respectively. We also study the time‐dependent mean‐square displacements of repeating units and obtain various relaxation times. It is shown that both relaxation times scaling proportional to N2 and to N3 play a role in the reptative motion of the chain in the tubes.
Conformational properties of weakly charged polyelectrolytes from Monte Carlo simulations
Authors: Seidel Christian , Schlacken Hartmut , Müller Ingolf 
Published: April 2003

AbstractWe present results of off‐lattice Monte Carlo simulations of weakly charged chains where the charged beads interact with a screened Coulomb potential of Debye‐Hückel type. The behaviour of the mean square end‐to‐end distance and mean‐square radius of gyration versus chain length is discussed for various solution conditions characterized by the parameter ζ = 2λD/b, with λD being the Debye screening length and b the contour distance between neighbouring charged beads. We also evaluate the probability‐density distribution for the end‐to‐end distance R. From the angular correlation function 〈cos γk〉 we estimate the persistence length as well as the chain length dependence of 〈R2〉 and 〈RG2〉. With increasing ζ it is found to come close to that which is directly obtained from 〈R2(N)〈 and 〈RG2(N)〉. Furthermore, the nonexponential decay of correlations indicates our charged chains to be not wormlike.
Statistical thermodynamics of single‐chain single crystals
Authors: Chen Erqiang , Bu Haishan , Hu Xiulan 
Published: April 2003

AbstractA simple statistical mechanical theory is developed assuming a chain‐folded crystal. Taking account of the side‐surface energy of the single‐chain single crystal, the partition function and the free energy are calculated. Also, the expressions for equilibrium thickness le and melting temperature Tm, e of the single‐chain single crystal are obtained, and they are similar to that given by a thermodynamic analysis of the single‐chain single crystal. It is shown that le depends on temperature and on the molar mass of the single‐chain single crystal, while Tm, e depends on the molar mass. Furthermore, it is found that the side‐surface and the fold‐surface free energies γ and γe can be explained on a molecular basis.
Gel properties in free radical crosslinking copolymerization: A kinetic approach
Authors: Okay Oǧuz 
Published: April 2003

AbstractA kinetic model is presented for prediction of the gel properties in free‐radical monovinyldivinyl monomer copolymerization. It is an extension of the kinetic model proposed recently for the sol species. The model involves the moment equations of the primary chains in the gel, and predicts the gel growth and structure as a function of the reaction time. Formulae for the chain length averages of the primary chains in the sol and in the gel were derived. The equations for calculating the number‐ and weight‐average primary chain lengths are identical to the equations derived by statistical methods.