bluuebunny/crossref_metadata_2025_split huggingface dataset:
Reptation theory: geometrical and topological aspects
Authors: Kholodenko Arkady L. 
Published: April 2003

AbstractThis paper discusses topological and geometrical aspects of reptation theory which are common to all versions of reptation theory. These are: the postulated existence of the tube, the functional relationship between the tube diameteraand the polymer/monomer densityp, the crossover from the Rouse to reptation regime. Statistical mechanics of the geometrically confined polymer chain is reanalyzed by careful separation of the diffusive motion of the chain into the longitudinal and transversal parts. Connection between old results and the new formalism is established. It is shown that the longitudinal motion resembles that known for directed polymers. This provides a source of the effective rigidification of the reptating chain's backbone thus facilitating the viscosity exponent to be larger than 3. The transversal motion is also reanalyzed. It is shown that the diffusion on the Bethe lattice used before to describe the transversal (planar) motion (conformational statistics) of the trapped chain is actually the diffusion on the universal covering of the corresponding Riemannian surface. This fact allows to reanalyze the tube stability using topological arguments. Detailed numerical comparison of the obtained new theoretical results with available experimental and Monte Carlo data is provided. Very good agreement between theory and experiment is found. It is also shown that the emerging physical picture of the tube destruction is isomorphic to that which was developed earlier with the help of the quantum Hall effect analogy (J. Phys. I4, 843 (1994)). Remarkable connections between the reptation theory and the theory of quantum chaotic/mesoscopic systems are established thus making the reptation theory part of the more general theory of quantum chaotic systems.
Polymer coupling and theory of on‐random crosslinking: an analytical solution
Authors: Zhu Shiping , Ma Min 
Published: April 2003

AbstractGel formation is an important feature in free‐radical polymer coupling. Due to the different possible combination reactivities of each polymer backbone radical, polymer chains are crosslinked in a non‐random manner. Equations of the moments have been derived to predict the pregel molecular weight development and the crosslink density at gel point. This work provides an analytical solution for the differential equations. The model agrees with the Flory‐Stockmayer gelation theory under the condition of random crosslinking. The magnitude of deviations from the classical theory for non‐random crosslinking depends on the product of the radical termination reactivity ratios (r1r2), the ratio of the rate constants of backbone radical generation (k), the ratio of the weight‐average chain lengths of primary polymers (y), and the polymer weight fractions (w2).
Extended kinetic theory of polymeric fluids
Authors: Grmela Miroslav , Dlugogorski Bogdan Z. , Lebon Georgy 
Published: April 2003

AbstractThe setting of classical configuration space kinetic theory of polymeric fluids is extended by adopting the field of internal momentum into the set of independent state variables. The internal momentum predicated by the extended theory compares well with the internal momentum seen in molecular simulations. The extended theory is also shown to provide an intrinscially consistent formulation of the time evolution of macromolecules with internal friction.
Correction of scaling exponents for self‐avoiding walks in two dimensions
Authors: Dayantis Jean , Palierne Jean‐François 
Published: April 2003

AbstractThe first and second correction‐to‐scaling exponents for two‐dimensional self‐avoiding walks have been estimated using exact enumeration data up to twenty‐two steps, and Monte Carlo simulation data from twenty‐three up to two hundred steps. It was found that Δ1, the first correction‐to‐scaling exponent, is not the same when deduced from root‐mean‐square end‐to‐end distance and root‐mean‐square radius of gyration data. In the former case it was found that Δ1 = 0.84 ± 0.10, and in the latter case it was found that Δ1 = 1.13 ± 0.03. These results are discussed with reference to previous estimates. The reliability of our Monte Carlo data has been checked making two different tests, including the use of a different random number generator, and it is concluded that apparently no artefact can account for the above different values found for Δ1.
Polydisperse polymer liquid crystals near the anisotropic‐isotropic transition
Authors: Brostow Witold , Walasek Janusz 
Published: April 2003

AbstractPolydisperse systems of polymer liquid crystal (PLC) macromolecules consisting of LC and flexible sequences are considered, particularly at the phase transition LC‐i from an anisotropic to the isotropic phase. The Flory and Matheson lattice model is used for formulation of the partition function; the orientation distribution function for LC orienting potential interactions introduced by Flory and Ronca is included. Using a Landau approach to calculating the Helmholtz function of the system as a function of the orientation parameter s, we obtain the formulae for parameters of the model in the vicinity of the LC‐i transition. The critical values at the transition of s, of the strength of orienting interactions, the system temperature, and the anisotropy of LC sequences are calculated and discussed as functions of the polydispersity parameter d. Regions of d are found where polydispersity has a weak influence on the LC‐i equilibrium parameters. There is also a region of d values in which the influence of polydispersity is quite pronounced.
Modeling of the transition temperature for the helical denaturation of α‐keratin intermediate filaments
Authors: Knopp Birgitta , Jung Bernd , Wortmann Franz‐Josef 
Published: April 2003

AbstractSimulations of the stability of the secondary and tertiary structure of the α‐keratin intermediate filament (IF) monomeric unit of wool are reported. Based on the assumed secondary structure three segments of the primary structure were selected: 1A, L12, and a part of 2B. Starting with an ideal α‐helical conformation for each IF‐segment, molecular dynamics simulations were carried out on the atomistic level at various temperatures in vaccum using the CFF91 force field. In either simulation the expected destabilization of the helical structure with increasing simulation temperature was observed. By use of different procedures of analysis, transition temperatures for the α‐helical denaturation were determined that are significantly higher for the supposedly α‐helical segments 1A and 2B than for the linker segment L12. The different stabilities of segments 1A and L12 were further verified through simulations in water environment that show the linker segment to be non‐helical at room temperature. The lower transition temperature of segment L12 confirms the expectation that its amino acid sequence leads to increased conformational flexibility. The mobility of the water molecules surrounding the IF‐segment is found to be significantly decreased by protein/water interactions.
Semiflexible chain molecules with nonuniform curvature, 2. . Brownian dynamics simulations of electric dichroism orientation dynamics
Authors: Håkansson Christer , Elvingson Christer 
Published: April 2003

AbstractThe hydrodynamically induced orientation dynamics of charged polymer molecules during electric dichroism has been studied by Brownian dynamics simulations. The effect of a locally changed flexibility or bending angle has been of special interest. The results show that simultaneously changing the bending angles and the corresponding force constants so as to keep the persistence length constant does not produce any large changes in the orientation dynamics or molecular shape although, quantitatively, a larger number of kinks in the chain will give a smaller steady‐state orientation. Releasing the constraint of a fixed persistence length when randomly increasing or decreasing the flexibility at different positions in the molecule drastically changes the dynamics, and the corresponding results are discussed and also compared with the orientation obtained when introducing the inhomogeneities in a cooperative manner.
The swelling of nematic elastomers by nematogenic solvents
Authors: Wang Xin‐Jiu , Warner Mark 
Published: April 2003

AbstractThe swelling of a nematogenic elastomer by an isotropic solvent was earlier predicted (Macromolecules 25, 445 (1992)) to be more complex than that of a classical elastomer. We extend here to the swelling by solvents having a nematic‐isotropic transition in the pure phase in the region of that of the pure elastomer. The phase diagrams are qualitatively different from the first case, but still with the novel features of a triple point and a gel‐gel biphasic gap reported in the initial study. Where the cross‐coupling is stronger than the self‐couplings we have an azeotropic point.
Simulation of pseudostationary radical polymerization, 3. Periodic modulation of the termination process for termination by disproportionation
Authors: Zifferer Gerhard , Olaj Oskar Friedrich 
Published: April 2003

AbstractA procedure is developed that allows the calculation of chain‐length distributions of polymers prepared by periodic modulation of the termination rate constant, considering termination by disproportionation. Applying some (hypothetical) field which suppresses — or at least diminishes — the termination rate constant periodically, results in step‐function‐like chain‐length distributions. In principle, the same information may be obtained as in the case of periodical variation of the rate of initiation.
Molecular weight distribution of a living polymer generated from a bifunctional initiator with non‐equal initiation rate constants
Authors: Dong Hai , Yan Deyue 
Published: April 2003

AbstractThis work deals with the kinetics of the living polymerization initiated by a bifunctional initiator with different rate constants of the first and the second step of initiation. The expressions of the molecular weight distribution (MWD) function, the number‐ and weight‐average degrees of polymerization and the polydispersity index were derived rigorously. Numerical results show that the nonequal initiation rate constants often lead to a bimodal MWD except the ratio of ki to ki′ being very large (>500, for instance) or relatively small (<1, for example). The MWD of the resultant polymer is rather narrow even in the presence of double peaks. With decreasing initiator concentration, the MWD at full conversion becomes narrower, and the shoulder peak of the MWD curve declines.
Simultaneous determination of free‐radical propagation and transfer rates from a novel type of PLP‐SEC experiment
Authors: Buback Michael , Laemmel Ralf A. 
Published: April 2003

AbstractA novel type of pulsed laser polymerization (PLP)‐size exclusion chromatography (SEC) experiment is presented in which laser pulse sequences are applied at considerable dark time intervals between each pulse package. The pulse sequences give rise to the characteristically structured molecular weight distribution (MWD) from which kp may be derived. Polymer produced during the extended dark time intervals determines the MWD at high molecular weights and allows for the measurement of chain transfer rates. The method by which propagation and transfer rates are accessible from one experiment is illustrated by simulations using the program package PREDICI.
Application of a tie molecule model to the postyielding deformation of crystalline polymers
Authors: Takayanagi Motowo , Nitta Koh‐hei 
Published: April 2003

AbstractOur past paper reported that the postyielding deformation of crystalline polymers such as polyethylene (PE), poly(oxymethylene) (POM), poly(propylene) (PP) and nylon 6 (Ny 6) was expressed by master curves with a characteristic constant for each polymer when normalized true stress and true strain are plotted in both logarithmic graphs and suitably shifted. For a molecular understanding of the postyielding process, we present a tie molecule model which assumes that the tie molecules are pulled out from the lamellar fragment at a constant number of tie molecules. The limit in applicability of the model is in the final stage of fiber formation. Fundamental equations of the model are solved to give a critical point at which all the molecular parameters can be uniquely determined from the characteristic constant for each polymer. At the critical point, the tie molecule length monotonously increases at a fixed number of tie molecules. The limit of the tie molecule length increases in the order of PE, POM, PP and Ny 6. By using an empirical relationship between the area fraction of tie molecules and the degree of crystallinity of these polymers, it is found that the order of area fraction of the tie molecules in Ny 6, PP, POM and PE is entirely reversed to the order of weight fraction of the tie molecules of the above polymers.
Spatial extension of the bipolaronic defect in highly conducting polymers based on five‐membered aromatic rings
Authors: Alemán Carlos 
Published: April 2003

AbstractThe molecular geometries of neutral and dicationic oligomers comprising 7, 10, 13 and 16 rings of thiophene, furan and pyrrole were determined using quantum mechanical calculations. The usual benzenoid structure was characterized for neutral oligomers, whereas a quinoid structural defect which extends over six rings was found for dications. Analysis of the transition region, where the quinoid structure changes to the benzenoid one, revealed structural differences between thiophene, furan and pyrrole derivatives.
Strain‐dependent dynamic properties of filled rubber network systems, 2: The physical meaning of parameters in the L‐N‐B model and their applicability
Authors: Lin Chen‐Ron , Lee Yu‐Der 
Published: April 2003

AbstractIn Part 1 of this series, the strain‐dependent dynamic properties of filler loaded rubber systems were modelled with a newly derived L‐N‐B (links‐nodes‐blobs) model. In this paper, the physical meaning and significance of parameters of the L‐N‐B model are described and compared with experimental results. Furthermore, with the combination of a van der Waals force model, these parameters can be described in terms of basic physical material properties of filler and rubber matrix. The temperature dependence of dynamic properties of filled rubbers is successfully described in terms of the series combination of the angular deformation element \documentclass{article}\pagestyle{empty}\begin{document}$\frac{{2d\bar G}}{{mL_1 a^2 }}$\end{document} in series with the tension element \documentclass{article}\pagestyle{empty}\begin{document}$\frac{{dQ}}{{L_1 a^2 }}$\end{document}. Moreover, the normalized storage modulus can be expressed as a master curve \documentclass{article}\pagestyle{empty}\begin{document}$Z_{L - N - B} = {\rm Ei}\left[ {\frac{{ - a_2 mQ\varepsilon _b }}{{2d\bar G}} \bullet L_1 } \right]/{\rm Ei}\left[ {\frac{{ - a_2 mQ\varepsilon _b }}{{2d\bar G}}} \right]$\end{document}. The parameter \documentclass{article}\pagestyle{empty}\begin{document}$\frac{{ - a_2 mQ\varepsilon _b }}{{2d\bar G}}$\end{document} is almost a constant for different carbon black filled systems. The distributions of blob and link in a L‐N‐B model are similar for carbon black filled rubber systems. The applicability and the limitation of the L‐N‐B model are also discussed and illustrated with experimental results. Beside carbon black filled rubber systems, silica filled rubber and filler‐in‐oil systems are discussed.
Intramolecularly ordered globules and the collapse of short chains
Authors: Ganazzoli Fabio 
Published: April 2003

AbstractWe investigate the poor‐solvent collapse of short chains having different stiffness through self‐consistent minimization of the intramolecular free energy under the constraint of fixed segment lengths between adjacent beads. At first the chains form the Random Gaussian Globule, where the beads are distributed at random at the same average distance from the centre of mass, while the segments show very little correlation. At a larger attractive potential, this collapsed globule undergoes a transition to one or more ordered compact states, depending on the chain stiffness. Under very strong contraction, all chains are described as a Compact Ordered Globule: the beads are again at a constant average distance from the centre of mass, while the segments jump back and forth at the globule's wall with a very large correlation. At intermediate contraction, the thinner and stiffer chains form the Oscillating Ordered Globule wherein the beads are alternatively distributed on two concentric on two concentric shells. In this case, we also find metastable states with nonsymmetrical conformations of the chain with respect to its ends. We also briefly discuss the thermodynamics of the coil‐globule and globule‐globule transitions, showing that in long polymer chains these ordered conformations cannot involve the whole chain. However, we suggest that they might still be found as local globules that form for kinetic reasons.
Stereoselectivity of an early propagation step in the anionic polymerization of methyl methacrylate – ab initio SCF, MNDO and AM1 study
Authors: Dybal Jiří , Kříž Jaroslav 
Published: April 2003

AbstractOn the basis of previous experimental and theoretical studies of low‐molecular models of living poly(methyl methacrylate) and their coordination interactions, molecular states preceding and involving the propagation step of the anionic polymerization of methyl methacrylate (MMA) were investigated. Previously published structures of the intracoordinated form of the model dimer dimethyl 2‐lithio‐2,4,4‐trimethylglutarate (DMMALi) and its free anion (DMMA) optimized using ab initio SCF 3‐21G were reinspected and a new, similar but more stable, geometry was found. AM1 and ab initio 3‐21G calculations of the addition of MMA to the anion derived from methyl isobutyrate were used to explore the applicability of semiempirical methods to the present problem. Intermediate complexes and transition states of the iso‐ and syndiotactic addition of MMA to DMMA and DMMALi were simulated using MNDO and AM1. The activation energy of MMA addition to DMMA is predicted to be substantially lower for the syndiotactic transition state. In contrast, only isotactic MMA addition is possible to the energetically favored intra‐coordinated form of DMMALi (c‐DMMALi). Syndiotactic addition to the uncoordinated form of DMMALi is apparently possible according to MNDO. Considering the earlier finding that c‐DMMALi is favored by dilution and lower temperature in solvating media such as tetrahydrofuran, the differences in stereoregularity of the anionic polymerization of MMA caused by concentration, temperature or solvent polarity can be plausibly explained.
Statistical branching of heterochains
Authors: Tobita Hidetaka 
Published: April 2003

AbstractA general theory for the statistical branching of heterochains is proposed on the basis of the random sampling technique. Consider a polymer mixture that consists of N types of chains whose weight fractions are wi (i = 1, 2, …, N), and number‐ and weight‐average chain lengths are P̄np,i and P̄wp,i, respectively. Suppose the transition probability that a branch point on a chain of type i is connected to a chain end of a type j chain is given by pij. When the branching density of chains of type i is ρi, the weight‐average chain length is given by \documentclass{article}\pagestyle{empty}\begin{document}$\bar P_w = W\sum\nolimits_{m = 0}^\infty T ^m \sum\nolimits_{n = 0}^\infty {SU} ^n 1$\end{document}, where S is the diagonal matrix whose elements are \documentclass{article}\pagestyle{empty}\begin{document}$S_{ii} = \bar P_{wp,i}$\end{document}, 1 is the column vector whose elements are all unity, U is the transition matrix whose elements are given by \documentclass{article}\pagestyle{empty}\begin{document}$u_{ij} = \rho _i p_{ij} P_{np,j} ,T$\end{document} is another transition matrix whose elements are given by tij = (wj/wi)Uji, and W is the row vector whose elements are wi. Simpler expressions of P̄w are presented for binary systems. In addition to the multicomponent systems, the present equation could also be used such as for free‐radical polymerization with long‐chain branching, by considering primary chains formed at different times as different types of polymer chains. For the prediction of the full molecular weight distribution, a Monte Carlo simulation method is used to illustrate the resulting distribution profiles.
Geometric properties of micelles formed by triblock copolymers and solubilizates in dilute solutions
Authors: Xing Li , Mattice Wayne L. 
Published: April 2003

AbstractIn this work we study the geometric properties of the triblock copolymer micelles with solubilized low‐mass molecules in a selective solvent using the Monte Carlo technique on a cubic lattice. The triblock copolymers are of the ABA type, with the two insoluble blocks at the ends. The size of the micelles is characterized by the squared radius of gyration of the micellar core, Rg2, while the shape is treated by the asphericity b and the acylindricity c, which are defined in terms of the principal moments of the radius of gyration tensor. The parameters varied are the amount of solubilizate molecules, the polymer concentration, the interaction parameters between A and B, A and solvent, solute and solvent, solute and B block, and the A and B block length. The micelle size, characterized by Rg2, grows with increasing concentration of the solubilizates and/or the polymer, and stronger interactions between the incompatible species. The A block length is found not to modify Rg2 monotonously, while an increase in B block length results in a decrease in Rg2 at high concentrations. As the size expands, the micellar shape becomes less spherical but retains its cylindricity. In addition to an increase in the averaged Rg2, the distribution of Rg2 becomes broader and the system less homogeneous.
Atomistic monte carlo simulations of polymethylene fluids
Authors: Vacatello Michele 
Published: April 2003

AbstractAtomistic Monte Carlo simulations of polymethylene fluids in various environments have been of great importance for improving our understanding of the molecular arrangements and conformations in systems of chain molecules. This paper describes models, calculations and principal results of a number of simulations performed in the last two decades for liquid alkanes in bulk and in the proximity of solid surfaces, and for systems of long chain amphiphilic molecules in monolayers, double layers, micelles and droplets.
Chain‐length dependent termination in pulsed‐laser polymerization, 1. Interaction with propagation
Authors: Olaj Oskar Friedrich , Kornherr Andreas , Zifferer Gerhard 
Published: April 2003

AbstractUsing a new simulation procedure in which each individual propagation step is subjected to a Poisson process it was proved that in case of chain‐length dependent termination the apparent rate of propagation no longer coincides with the true one. This is caused by the polydispersity of the chain‐length distribution of the growing chains: shorter chains are removed preferentially. This effect is comparatively small although significant. The consequences for the determination of the rate constant of chain propagation kp are nearly negligible.
A fast way of solving Jonah and coworkers' equations to evaluate anisotropic‐isotropic transition temperatures of semirigid polymers
Authors: Figari Giuseppe , Costa Camilla 
Published: April 2003

AbstractA fast and simple method has been found to solve the set of equations recently proposed by Jonah, Brostow and Hess [Macromolecules 26, 76 (1993)], allowing the evaluation of the anisotropic‐isotropic transition temperature for a semirigid liquid polymer. Starting with given values of η (average length of the rigid sequences coexisting with flexible sequences inside the semiflexible polymer chains) and θ (degree of rigidity of the semirigid polymer), a reduced transition temperature τ is readily obtained, together with the transition values of the anisotropic order parameters s and γ.