bluuebunny/crossref_metadata_2025_split huggingface dataset:
Strain‐dependent dynamic properties of filled rubber network systems
Authors: Lin Chen‐Ron , Lee Yu‐Der 
Published: April 2003

AbstractA model for strain‐dependent dynamic properties of filler loaded rubber systems has been derived based on the Links‐Nodes‐Blobs (L‐N‐B) model of percolation theory. It is the first time that a L‐N‐B model is applied in the study of dynamic properties of filled rubbers. The density distribution function of the number of singly connected bonds f1a(ϵ) and the apparent yield strain amplitude ϵapp that corresponds to the on‐set point of corruption of the filler network are introduced in the model. Simulation results indicate that both f1a(ϵ) and ϵapp control the break‐down and recombination of the filler network. Two recombination mechanism are adopted in this study. Results of simulations from the extreme ends recombination mechanism match the experimental data better than those from the zero strain recombination mechanism. Also, via the proposed model, the strain‐dependent storage modulus correlates well with the peak loss modulus at a low strain range of around 0.1% to 100%. Moreover, a universal plot of the normalized storage modulus (ZL‐N‐B) as a function of the normalized Log strain amplitude (ϵ0/ϵapp) for different rubber systems is obtained. The loss moduli of systems are also simulated by the L‐N‐B model.
Theory of dilute polyampholyte solutions in external electrical fields
Authors: Schiessel Helmut , Blumen Alexander 
Published: April 2003

AbstractIn this work we display recent findings on the conformations and dynamics of polyampholytes (PAs; polymers with positively and negatively charged monomers) in external electrical fields. We consider the case in which the interactions between the charges are less important (weak coupling limit) and also the case in which they are fundamental (strong coupling case). In the weak coupling limit we present analytical results for Gaussian and also for freely‐jointed chains. Through scaling arguments we discuss the influence of the excluded volume on the PA's configurations. Furthermore we evaluate the dynamics of PAs in the framework of the Rouse and of the Zimm models. In the strong‐coupling regime PAs with vanishing total charge form spherical globules. Using a droplet analogy we examine the response of PAs to external fields and show the onset of an instability reminiscent of a first‐order phase transition.
Computer simulation of network formation via crosslinking copolymerization
Authors: Schröder Uwe Paul , Oppermann Wilhelm 
Published: April 2003

AbstractThe topology of networks formed by radical crosslinking copolymerization is strongly dependent on the type of crosslinker chosen. By a series of computer simulations, the influence of the relative reactivities of crosslinker and monomer on the network properties was investigated. Basic condition of the simulation is the postulation of a diffusion process control of the polymerization reaction. The resulting networks are analyzed with respect to cycle rank, loop formation, dangling ends, sol fraction and crosslinker molecules reacted only by one double bond. The results of the computer simulations indicate that the reactivity of the crosslinker decisively affects the properties of the network. Particularly the cycle rank, which determines the elastic properties of the network, is influenced strongly by the reactivity of the crosslinker. A change of the cycle rank of approximately one decade was observed. The influence of the reactivity was found to be most important in the region where the reactivity of the crosslinker is less than the reactivity of the monomer.
Mean‐square radius of gyration of polysiloxanes
Authors: Zhou Zhiping , Yan Deyue 
Published: April 2003

AbstractThe mean‐square radius of gyration for polysiloxanes has been derived according to the exact definition. Taking account of the examples of symmetrically substituted poly(dimethylsiloxane) and unsymmetrically substituted poly(methylphenylsiloxane), we find that the dependence of 〈S2〉 on the molecular weight follows the general formula 〈S2〉 = aMb with b = 1 ± 0.016, which is analogous to the theoretical outcomes for vinyl or vinylidene polymers even though the skeletal bone of polysiloxanes consists of alternating heteroatoms. A numerical comparison of the rigorous expression of the mean‐square radius of gyration given in this paper with that reported by Flory shows that the difference is obvious for low‐molecular‐weight polymer and it gradually declines with increasing degree of polymerization.
On the use of the gaussian chain as a monte carlo simulation model for the equilibrium properties of polymer solutions
Authors: Jorge Sonia , Freire Juan J. , Rey Antonio 
Published: April 2003

AbstractWe have explored the performance of a simulation model for Gaussian chains at different concentrations in a good solvent. The Gaussian statistics for the distances between contiguous beads in the model is directly implemented in the individual moves of a Monte Carlo algorithm. When the results of conformational properties for the Gaussian model are compared with those provided by a freely jointed model in the same conditions, significant differences arise at finite concentrations. The modeled Gaussian chain yields incorrect results for the quadratic average dimensions 〈R2〉 and 〈S2〉 at high concentrations, but correctly reproduces the results for the scaled end‐to‐end distance distribution function at any concentration, showing the effects of the screening of excluded volume when concentration increases. The reason for the wrong behavior of the simulated Gaussian model comes from a strong distortion of the “bond distance” distribution as a result of the concentration increase. We conclude that this model can only be safely applied to infinitely dilute solutions.
Computer simulation of irreversible gelation of polymers with stickers
Authors: Khalatur Pavel G. , Men'shikova Larisa V. , Khokhlov Alexei R. 
Published: April 2003

AbstractThe “bond fluctuation model” is used for Monte Carlo simulations of irreversible aggregation in solutions of associating macromolecules with regularly spaced stickers. The irreversible aggregation process follows the kinetically‐limited‐aggregation model first proposed by Eden. The fractal structures produced in the course of the aggregation are analyzed depending on the number of chains involved in the final cluster, n, chain length, N, and the number of stickers per chain, ns. It is shown that the chains with ns ≥ 2 form aggregates crosslinking the chains in a network‐like structure. The mesh size of this network mainly depends on the chain length between two stickers; there is also a weaker dependence on the number of associating groups per chain, ns. The chains connecting two aggregates turn out to be strongly extended. It is shown that the aggregates have a rather broad size distribution and that there is always a significant fraction of free single stickers. The inter‐ and intrachain screening effects control the local morphology of the aggregates.
Shape asymmetry of star‐branched random walks and nonreversal random walks
Authors: Zifferer Gerhard 
Published: April 2003

AbstractStar‐branched random walks with 3, 4, 6, 8 and 12 arms (the total chain‐length ranging from N = 49 to 1925) have been produced and analysed with respect to their instantaneous shape. The short‐chain behaviour of nonreversal random walk stars (NRRWs) embedded in various lattices is compared to that of star‐branched freely jointed (off‐lattice) chains (RWs). While for all types of NRRW‐stars examined as well as for RW‐stars with bonds of constant length shape‐asymmetry increases with increasing chain‐lengths, the opposite behaviour is found for RW‐stars with Gaussian‐distributed bond‐lengths. The amount of short‐chain effects is strongly dependent on the number of arms and on the lattice type used. In the limit of infinitely large molecules, however, quantities characteristic of the shape converge to common values for all types of RWs and NRRWs examined.
Absolute propagation rate coefficients in radical polymerization from gel permeation chromatography of polymers produced by intermittent initiation
Authors: Hungenberg Klaus‐Dieter , Knoll Konrad , Wulkow Michael 
Published: April 2003

AbstractThe pulsed laser polymerization technique is now a well accepted method to determine propagation rate coefficients for radical polymerization from molar mass distributions resulting from intermittent initiation. A simplified apparatus for the periodic photoinitiation is used which is much less expensive than the laser equipment. The usefulness of the simplified equipment was proved by the determination of kp for styrene at technically relevant temperatures up to 130°C for the first time. Furthermore, careful inspection of the molar mass distribution (mmd) reveals that depending on the reaction conditions, inflection points (Li) can not only be found at integer multiples of kp • to • [M] but also at 0.5i • kp • to • [M], i = 1, 2, 3, … . A rule to find the inflection points leading to correct values for kp is proposed. It is shown that the shape of the mmd inter alia depends on the amount of primary radical termination compared to the termination reaction between growing chains. With dominant primary termination, the maxima of the distribution will give the correct kp, whereas in the absence of primary termination the inflection points should be used. Experimental conditions like initiator concentration, light intensity etc. may influence the position of the Li at least to some extent, and so may give a small but principal error or uncertainty in kp. A new mathematical method for the time‐dependent simulation of the resulting mmd is presented which allows the calculations being performed on a PC within an acceptable time.
The model theory of viscoelastic relaxation properties of bulk cross‐linked polymers. Interchain friction effects
Authors: Gotlib Yuli Ya. , Gurtovenko Andrew A. 
Published: April 2003

AbstractThis article discusses the viscoelastic relaxation properties of bulk cross‐linked polymers. The simple dynamic model of a “mesh‐like” polymer network is considered for the description of the behavior of bulk cross‐linked polymers at stretching. This dynamic network model directly takes into account the following features of bulk polymers: (1) the existence of a fixed average volume of the polymer network, (2) the interconnection between the gradient of the effective viscous medium and the macroscopic deformation of the bulk polymer sample, and (3) the presence of mutual interchain friction. The low‐frequency network dynamics determined by the motions with scales greater than the distance between the network junctions is mostly studied. The simplified “coarse‐grained” cubic network model is used. The total stress which arises in bulk cross‐linked polymers and the viscoelastic functions (the dynamic modulus and viscosity) are calculated. The influence of the interchain friction and of the fixed average volume of the network on the total stress and on the dynamic modulus and viscosity is discussed. The results obtained are compared with the results of the theoretical approaches developed earlier.
Non‐uniqueness in determination of terminal and penultimate model reactivity ratios in the styrene‐methyl methacrylate free‐radical copolymerization system
Authors: Kaim Andrzej , Oracz Paweł 
Published: April 2003

AbstractThe terminal and penultimate model reactivity ratios in the free‐radical copolymerization of styrene and methyl methacrylate in bulk at 40°C were calculated by means of the simplex and scanning methods. Calculations showed that for the terminal model r1 and r2 vary in comparatively narrow ranges of 0.548–0.552 and 0.480–0.483, respectively. For the penultimate model, the most accurate reactivity ratios calculated by the simplex method, which were r11 = 0.727, r22 = 0.490, r21 = 2.890, r12 = 4.583, are surrounded with sets of reactivity ratio values of equal accuracy. The ranges of variation were found to be 0.711–0.746, 0.487–0.492, 2.810–2.970 and 4.213–5.049, respectively. Numerical values of the penultimate r‐parameters calculated with the simplex method depend, due to the structure of the multidimensional space (r11, r22, r21, r12, σ), on the initial guess for the r‐parameters. Use of the covariance matrix for the estimation of the indetermination ranges is discussed.
The chemical composition component of the distribution of chain length and long chain branching for copolymerization of olefins and polyolefin chains containing terminal double‐bonds
Authors: Soares João B. P. , Hamielec Archie E. 
Published: April 2003

AbstractAn analytical solution for the distribution of molecular weight, long chain branching and chemical composition of polyolefins produced under steady‐state conditions with the mechanism of terminal branching is derived. The model predictions agree well with Monte‐Carlo simulation results. This detailed description of the microstructure of polymer chain populations with different levels of long chain branching can be useful in establishing structure‐property relations for these polymers.
Mean‐square radius of gyration of polymer chains
Authors: Zhou Zhiping , Yan Deyue 
Published: April 2003

AbstractThe calculations of the mean‐square radius of gyration for more than thirty sorts of polymer chains are reviewed on the basis of a unified approach. A general expression of the mean‐square radius of gyration was developed for polymer chains with side groups and/or heteroatoms. It consists of two parts. The first part is the mean‐square radius of gyration of a model chain, in which every side group, R, was considered to be located in the centroid of the substituent flanking the related skeletal atom, and the second one is the total contribution of the square radius of gyration of every substituent around its centroid. Numerical calculations showed that the logarithmic relationship between the mean‐square radius of gyration and the degree of polymerization becomes linear when x is greater than 100, and the dependence of the mean‐square radius of gyration on the molecular weight can be expressed by the general formula 〈S2〉 = aMb, which was supported by a number of experimental measurements. A comparison of our expression for the mean‐square radius of gyration with that reported by Flory was made. The difference is obvious in the range of lower molecular weight, and gradually declines with increasing degree of polymerization.
The “fast” and “slow” mode theories of interdiffusion in polymer mixtures: Resolution of a controversy
Authors: Akcasu A. Ziya 
Published: April 2003

AbstractThe theory of interdiffusion of a pair of components in multicomponent polymer mixtures is reviewed from a statistical point of view, and the foundation of the “fast” and “slow” mode theories, as well as the more recent “ANK” theory of interdiffusion is critically examined. The ANK theory reproduces the results of the slow and fast mode theories as the two limits when the vacancy concentration is varied from zero to a large value, and shows that the interdiffusion coefficient in a binary compressible mixture at finite vacancy concentrations can not in general be expressed only in terms of the tracer diffusion coefficients of the components, but it involves in addition the cooperative diffusion coefficient which characterizes the relaxation of total density fluctuations. The predictions of the ANK expression for the molecular dependence of the kinetic factor is compared with recent scattering experiments.
The role of the orientation tensor in the rheology of flexible polymers
Authors: Wagner Manfred H. , Geiger Kalman 
Published: April 2003

AbstractThe stress‐strain relations for transversely isotropic deformations of linear and branched polymer melts as well as of (crosslinked) rubbers are discussed in terms of the orientation tensor. It is shown that orientation and network strand extension are decoupled, and that the relative tube diameter and its inverse, the molecular stress function f, can be extracted directly from experimental data, if the effect of network orientation is accounted for by the order parameter. The tension of the average network strand increases with increasing deformation. This is caused by an increasing restriction of lateral movement of polymer chains due to deformation. At small strains, f2 is found to be linear in the average stretch for melts as well as for rubbers, which corresponds to a constant volume of the tube. At large strains, melts show maximum molecular tension, depending on the degree of longchain branching, while rubbers show maximum extensibility.
Application of molecular mechanics to refine and understand the crystal structure of polythiophene and its oligomers
Authors: Ferro Dino R. , Porzio William , Destri Silvia , Ragazzi Massimo , Brückner Sergio 
Published: April 2003

AbstractA molecular mechanics computational procedure, previously used for the refinement and the analysis of several crystalline polymers, was applied to investigate the crystal structures of the tetramer (T4) and hexamer (T6) of thiophene, as well as the crystal structure of polythiophene (PT). Simultaneous minimization of intra‐ and intermolecular energies of the T4 and T6 structures, obtained by Rietveld analysis of powder X‐ray diffraction profiles, leads to molecular conformations showing smaller deviations from the ring co‐planarity than the original models. For both oligomers the calculations confirm that the molecular centre of inversion is not a crystallographic centre of symmetry, as also revealed by X‐ray diffraction of the T6 single crystal. This surprising effect appears to arise from intermolecular interactions between the terminal residues, hence is not relevant with respect to the PT polymer structure. The small energy cost for constraining the molecules at the crystallographic centre of symmetry is in agreement with experimental findings that reveal the existence of polymorphs for both T4 and T6. The calculations on the T6 single crystal were used to upgrade the MM2‐like force field, which was then used to determine the minimum‐energy model of the monoclinic crystal structure of polythiophene.
Atomistic computer simulation of mechanical properties of composites
Authors: Steinfath Matthias , Bruns Wolfgang 
Published: April 2003

AbstractAn atomistic computer simulation is performed in order to investigate structure and mechanical behaviour of materials composed of fibres embedded in a polymer matrix. A graphite‐poly(propylene) system is used as a model, the energy of which has been minimized. Unlike the models of other authors the bond angles of the polymer chains are assumed as flexible. The properties found were compared with those of the bulk polymer. The results show that the presence of the graphite surface affects the structure of the polymer as well as the values of the elastic constants within a layer of 7 Å thickness. In this layer the density has a maximum and the chain segments are oriented parallel by the graphite plane. A long range order, however, cannot be observed.
Styrene‐methyl methacrylate copolymer microstructure according to terminal and penultimate model for free‐radical copolymerization
Authors: Kaim Andrzej 
Published: April 2003

AbstractMicrostructure of styrene‐methyl methacrylate copolymers was determined according to the “bootstrap model” description using the terminal and the penultimate model of copolymerization. The implication of non‐uniqueness in determining the penultimate model reactivity ratios from the copolymer microstructure is discussed. The styrene centred triad sequences in the copolymer obtained in the free‐radical copolymerization of styrene and methyl methacrylate in bulk at 40°C were calculated using various sets of terminal and penultimate model reactivity ratios with very similar standard deviations. Calculations showed that for both models, the terminal model and the penultimate model, different reactivity ratios with very similar standard deviations are equally well suited for the quantitative description of the triad sequence distribution in the copolymer. The terminal and penultimate models produce different styrene centred triad sequence distributions in the copolymer, the difference depending on the penultimate model reactivity ratios used for calculation and on the type of the styrene centred triad sequence. The best equivalence of the two kinetic models was found when the initial r‐parameters guess used for determining the penultimate reactivity ratios by the simplex method were r11 = r21 = r1 and r22 = r12 = r2. The results suggest that the penultimate effect on the styrene‐methyl methacrylate free‐radical copolymerization system is limited.
Fractal models of the shrinkage compensation mechanism for unsaturated polyesters
Authors: Merle Gérard , Vincent Nicole 
Published: April 2003

AbstractThe shrinkage compensation mechanism in sheet molding compounds is governed by the morphology resulting from phase separation and microvoids formation. The observed microvoid morphology is quantified by using fractal geometry, in 2D and 3D spaces. The correlation between the morphologies of the matrix with phase separation and those of the microvoids is demonstrated by simulation. The pressure effect is studied. The fact that the shrinkage compensation has a pressure dependent optimum is explained by a model for the compensated volume. Thus, the “fractal pillar effect” is a model that represents properly speaking the compromise between pressure and volume creation shown by such a smart material.
Simulations of rodlike molecules represented by anisotropic Lennard‐Jones potentials
Authors: Tanaka Genzo , Mattice Wayne L. 
Published: April 2003

AbstractIn the interests of carrying out computer simulations efficiently for systems of rodlike molecules, the model proposed by Berne and coworkers, in which each rod is represented by an anisotropic Lennard‐Jones potential, was extensively compared with a site‐site model. Using this potential, Monte Carlo simulations were carried out for rodlike molecules with aspect ratio (Ra) ranging from 3 to 50. Short rods such as Ra < 10 exhibit an isotropic phase for essentially the entire range of volume fraction ϕ, while long ones (Ra > 20) show an isotropic phase at only very low ϕ < 0.07. The attractive part of the potential makes rodlike molecules associate as well as align even at low ϕ. At high ϕ around or above the transition, the repulsive potential dominates and the attractive potential becomes secondary. Based on simulation results, some implications for designing molecular composites are offered.
Kinetics of spinodal decomposition in mixtures of low molecular weight liquid crystals and flexible polymers
Authors: Lin Zhiqun , Zhang Hongdong , Yang Yuliang 
Published: April 2003

AbstractIn this paper, the time‐dependent Ginzburg‐Landau model for mixtures containing nematogens has been applied to mixtures of low molecular weight liquid crystals and flexible polymers. Dynamic equations for the time evolution of concentration and orientation fluctuations and the structure factors for these fluctuations are given. It is shown that the coupling between concentration and orientation fluctuations is absent in the isotropic spinodal region, thus the evolution of the structure factors for the concentration fluctuations falls into the Cahn‐Hilliard classic category and it exhibits no maximum in the structure factors of orientation fluctuations. We should emphasize that, in the anisotropic spinodal region, both concentration and orientation structure factors possess a maximum but not coincide with each other and both are shifting to smaller wave numbers according to the scaling relation, qmax ˜ τ−α, as time increases. The value of a closely correlates to the interfacial free‐energy parameters.