bluuebunny/crossref_metadata_2025_split huggingface dataset:
Simulation and modelling of transient permeation of organic solvents through polymer films in the case of a concentration dependent diffusion coefficient
Authors: Clément Robert , Nguyen Quang Trong , Grosse Jean Marie , Uchytil Petr 
Published: April 2003

AbstractThe measured transient permeation kinetics of acetic acid from a water‐acetic acid mixture through poly(vinyl alcohol) films could not be accounted for by the Fick law with a constant diffusion coefficient. A new calculation procedure was developed on the basis of simulation results of the Fickian kinetics in which the diffusion coefficient was assumed to increase exponentially with the local permeant concentration. A fast and reliable fitting procedure, which was set up on a Personal Computer, involves an iterative numerical adjustment of (a) the value of the limiting diffusion coefficient D* (i.e., difusion of the permeant in dry polymer) using the onset part of the permeation rate followed by (b) the value at the upstream face of the plasticization term (argument of the exponential function) using the shape of the experimental curve. The values obtained from fitting the model to the transient kinetics showed that the limiting diffusion coefficient increases drastically, but the plasticization term changes little, with increasing temperature.
Narrow molecular weight distribution of condensation polymers achieved by adding monomers in many batches
Authors: Yang Yuliang , Zhang Hongdong , He Junpo 
Published: April 2003

AbstractThe Z transform method has been used to calculate the molecular weight distribution (MWD) of condensation polymers. The MWD obtained by using Z transform is explicitly discrete. The method is illustrated for two cases: (1) further polycondensation of AB prepolymers with certain initial MWD, and (2) polycondensation of AB and Ar (r is the number of A type functional groups) monomers where AB monomers are added in several batches. In the latter case, it is found that the resulting MWD is much narrower than that of one‐batch polycondensation. The trick of producing narrow MWDs of condensation polymers is merely a consequence of keeping AB monomer concentration as low as possible during the reaction in order to suppress the condensation reaction between monomeric AB molecules. The theoretical prediction has been confirmed by Monte Carlo simulation. Therefore, it provides a new possible technique for obtaining narrow MWD polymers through polycondensation reactions.
Self‐assembling of chain molecules in low dimensions: A Monte Carlo study
Authors: Stettin Heiko , Wahab Mirco , Schiller Peter , Mögel Hans‐Jörg 
Published: April 2003

AbstractWe studied the self‐assembling of linear chain molecules in insoluble monolayers due to attractive interactions. We used lattice Monte Carlo simulations in a two‐dimensional system. The molecules consist of segments occupying adjacent lattice sites. The head segments are confined to move along a line whereas the chain segments can arrange in a plane above the heads. Only one interaction parameter is applied. At high densities and small interaction energy the system shows percolation behavior. At moderate and small densities it can be characterized by a monotonous cluster size distribution. Self‐assembling occurs at small densities for strong attractive interactions. The corresponding cluster size distributions indicate preferred cluster sizes which depend upon density and interaction strength. With increasing density the clusters grow. The internal cluster structure depends on the cluster size and the interaction parameter. The clusters tend to minimize their total energy. Molecules at cluster margins contribute less to the cluster energy and are mainly disordered. They cause that the cluster properties strongly depend on the cluster size. Large clusters only have minimum energy if the molecules in the cluster are in stretched‐out conformation. With decreasing interaction strength the clusters get disordered thereby producing less energy‐minimized domain boundaries.
Molecular mechanics analysis of polymer conformation, 4. Local chain motions and molecular relaxations of poly(parabanic acids)
Authors: Konda Masakazu , Kimura Yoshiharu , Kitao Toshio 
Published: April 2003

AbstractThe structure of the repeating units of poly(parabanic acid) (PPA), a polyimide‐like glassy heat resistant polymer, was analyzed by MM2 using various model compounds of PPA‐M and PPA‐T consisting of M (1,3‐diazolidine‐2,4,5‐trione‐1,3‐diyl‐1,4‐phenylenemethylene‐1,4‐phenylene) and T (1,3‐diazolidine‐2,4,5‐trione‐1,3‐diyl‐3,3′‐dimethyl‐4,4′‐biphenylylene) units, respectively. Based on the most stable structures obtained, the subunit rotations of the methylene, phenylene, and diazolidine groups were evaluated. The potential mobilities estimated for these subunits can reasonably be correlated with the dynamic mechanical and dielectric properties of PPA. The γ relaxation mode of both PPA‐M and a copolymer PPA‐TM is reasonably explained by the rotation of the methylene group. The major mode of the β relaxations of both PPAs is ascribed to the rotation of the phenylene ring of the M unit, while the β* dielectric relaxation is attributed to the torsional vibration of the parabanic ring involving the adjacent aromatic moieties. These data suggest that PPA is similar to polycarbonate rather than polyimide in terms of the local chain motion. It is shown that the conformational analysis by MM2 is quite informative to understand the structure property relationships of polymers.
Brownian dynamics of grafted polymer chains: time dependent properties
Authors: Neelov Igor M. , Binder Kurt 
Published: April 2003

AbstractResults of computer simulations of polymer layers consisting of chains grafted by one end on an unpenetrable plane are presented. Characteristics of translational and rotational motion of different chain segments and correlation functions of chain radii were calculated both for single layers at different grafting densities s and for two interacting layers at different distances D between parallel grafting planes. Two values of grafting density were used in the latter case. The behavior of different correlation times as function of s and D and the interplay between the interpenetration of the brushes and rotational and translational motion are discussed. Both relaxation functions and mean square displacements are discussed in terms of stretched exponentials, and the behavior of the resulting “Kohlrausch exponents” γ is presented in detail.
Effect of reactor residence time distribution on the size distribution of polymer particles made with heterogeneous Ziegler‐Natta and supported metallocene catalysts. A generic mathematical model
Authors: Soares João B. P. , Hamielec Archie E. 
Published: April 2003

AbstractA generic mathematical model for analyzing the effect of ideal and non‐ideal reactor residence time distributions on the size distribution of polymer particles produced with heterogeneous Ziegler‐Natta and supported metallocene catalysts was developed. It was shown that the residence time distribution in polymerization reactors can have a significant effect on the size distribution of polymer particles and this can lead to imperfect replication of the catalyst particle size distribution.
An investigation of hemibenzopinacolate radicals used in the synthesis of block‐copolymers: ESR measurements and semiempirical calculations
Authors: Hajji Lahoucine , Santos Ramiro Guerrero , Beinert Gérard Joseph , Herz Jean‐Edouard , Bieber André , André Jean‐Jacques 
Published: April 2003

AbstractHemibenzopinacolate radicals may be incorporated in a polymer chain and the latter used as a macromolecular initiator. Kinetics of polymerization are strongly dependent on the substitution of the pinacolate. A large family of substituted (CH3)3SiOC(C6H5)2 radicals has been synthesized and studied by electron spin resonance (ESR) and semiempirical molecular orbital calculations. ESR spectra are well resolved and, except for one derivative, allow an unambiguous determination of the hyperfine coupling splitting (HFS) constants. The ground state properties have been calculated within different approximations, namely MNDO, AM1 and PM3 for the geometry optimization and INDO for spin density. Geometries obtained within AM1 and PM3 approximations are satisfactory. The HFS constants evaluated using McConnell type relations and spin densities are in a quite good agreement with the experimental ones. It turns out that the effect of substitution on kinetics of polymerization cannot be attributed to any drastic change in the charge distribution.
Dynamics and conformational properties of polyampholytes in external electrical fields: Influence of the charge distribution
Authors: Schiessel Helmut , Oshanin Gleb , Blumen Alexander 
Published: April 2003

AbstractIn the present work we study the dynamics and conformational properties of polyampholytes (PAs, polymers containing positive and negative charges) in the presence of external electrical fields. In terms of the Rouse Model of polymer dynamics we obtain for PAs in the so‐called weak coupling limit explicitly the mean square displacement both of the center of mass and of individual beads, and the PAs' end‐to‐end distance. We study both ordered and also random charge distributions along the chains; in the latter case we also consider the role of different correlation lengths.
Monte Carlo simulations of chains confined inside a cube
Authors: Abadie Michel R. L. , Dayantis Jean 
Published: April 2003

AbstractSelf‐avoiding walks (SAWs) and random‐flight walks (RFWs) of various lengths embedded on a simple cubic lattice have been computer generated inside cubes of varying side. If B is the side of the confining cube, we define the reduced cube side size B0 as B0 = (B − 1)/<r2>1/2, where <r2>1/2 is the root‐mean‐square end‐to‐end distance of the non‐confined chains. Dimensionless diagrams are then given of the Monte Carlo estimates for the dimensions, the entropy, and the compressibility parameter PV/(kT) of the confined chains as a function of B0. The comparative behaviour of the confined SAWs and RFWs is established, scaling properties are examined, and the Monte Carlo estimates compared with theory when such theory is available.
Molecular weight distribution in random branching of polymer chains
Authors: Tobita Hidetaka 
Published: April 2003

AbstractAnalytical expressions for the average molecular weights of randomly branched polymer molecules with any primary chain distribution are developed. A full molecular weight distribution (MWD) function is also derived for the case where primary chains conform to the most probable distribution. This MWD function can be separated into the fractional MWDs containing k branch points; therefore, very detailed information on the structure of randomly branched polymers can be obtained. The average molecular weights of the polymer fraction containing k branch points are linear functions of the number of branch points k, and the distribution becomes narrower as k increases. The heterogeneity in the distribution of branch points can make the weight‐average degree of polymerization larger, although it is impossible to form a gel molecule only via branches (T‐shaped junctions) without assistance of crosslinkages (H‐shaped junctions).
Influence of polymolecularity on the second virial coefficient of polymers
Authors: Bareiss Rolf E. , Jung Bernd , Khokhlov Alexei R. 
Published: April 2003

AbstractScaling theory is applied to derive expressions describing the influence of polymolecularity on the second virial coefficient, A2, as obtained from osmotic pressure and light scattering measurements. Numerical values of polymolecularity correction factors are calculated for Schulz‐Zimm and logarithmic normal distributions of the molecular weight, different qualities of the solvent and several ratios of the weight‐average and the number‐average molecular weights M̄w/M̄n. It is found that in the equation \documentclass{article}\pagestyle{empty}\begin{document}$ A_2 = K_{A_2 } \cdot M_{{\rm av}}^{a_{A_2 } } $\end{document} the weight‐average molecular weight is a good approximation for Mav if A2 is measured via light scattering, while the number‐average molecular weight can be inserted for Mav if A2 stems from osmotic pressure measurements.
Nonequilibrium statistical theory of damage and fracture for glassy polymers, 1. The statistical distribution and evolution of microcracks in glassy polymers
Authors: Li Qiang , He Ziru , Song Mingshi , Tang Aoqing 
Published: April 2003

AbstractThe purpose of this paper is to construct a unified theoretical framework to link micro to macro‐mechanical properties of glassy polymers. Starting from a model of microcrack propagation in craze on a mesoscale, the kinetic process of microcrack propagation resulting from fibril breakdown in the crack tip zone is mathematically formulated by a combination of fracture mechanics and fracture kinetics. A microcrack evolution equation involving both the geometric structure parameters of craze and the meso‐mechanical quantities is obtained. After solving this evolution equation, a statistical distribution function of microcrack size which evolves with time and the moment generating function of microcrack size are derived. Any‐order averaged damage functions can be therefore deduced. Specifically, the analytical expressions of the first‐order averaged damage function and its damage rate are presented, which correspond to a similar definition of damage mechanics.
Effect of the topology on the gaussian elasticity of a network. The example of a fractal topology
Authors: Bastide Jacques , Boué François 
Published: April 2003

AbstractBy considering fractal topologies of polymer networks, we show that one can obtain much smaller values of the elastic modulus than predicted by classical models for the same density of crosslinks. In a regular fractal such as the Sierpinski gasket, one can replace the segments joining the modes by Gaussian chains. This allows one to draw some micronetworks of rank k = 1, 2 etc., with no dangling ends except for the few external points (3 for any rank). Within the frame of the phantom network model, we calculate the elastic modulus of the micronetwork. It varies as pk−1 for p < 1, which leads, even for rather small orders k such as 2, 3, to values lower than the classical James and Guth modulus, which actually corresponds to the connectivity of a tree. Thus there is an influence of the topology. Fluctuations of the positions of crosslinks and of the distance between two crosslinks are estimated and are found to correspond to a lower deformation of the chains. Whether these topologies are more than an example and could explain the anomalies observed experimentally is a postponed discussion.
Liquid‐crystalline ordering in polymer brushes
Authors: Birshtein Tatiana M. , Mercurieva Anna A. , Pryamitsyn Victor A. , Polotzkiy Alexei 
Published: April 2003

AbstractThe mean‐field theory of liquid crystalline (LC) ordering is developed for a polymer brush immersed in a solvent. Additional attraction between neighbouring parallel mesogenic segments of the grafted chains is taken into account. It is shown that LC ordering in this brush is connected with the loss of solubility and occurs as a discrete first‐order phase transition which is similar or even identical to the transition in polymer solution.
The peculiar conformational characteristics of cis‐poly(tert‐butlacetylene)
Authors: Ferro Dino R. , Provasoli Augusto , Costa Giovanna 
Published: April 2003

AbstractMolecular mechanics was applied to investigate the conformational structure of cis‐poly(tert‐butylacetylene) (c‐PTBA). The program CHAMP, adopting a MM2‐based force field, was used for a thorough search of the minimum‐energy conformers of oligomers, helices and long segments of the c‐PTBA chain. The results show that short oligomers are not good models of the polymer, as their preferred conformations are not allowed inside the chain. Segments of 83 and 85 helices appear as the most probable feature. Junctions between right‐ and left‐handed helices can occur at the cost of 1,7 kcal/mol: such defects may run through the chain, the barrier to their shift being 7,8 kcal/mol. Other conformational defects, having energies in the range of 1–3 kcal/mol above the minimum, form knuckle‐joints in the helix, changing the axial direction by 70–80°. The present calculations suggest a disordered model of c‐PTBA in solution, made of rather stiff helical segments. Such a picture corresponds to a chain less rigid than proposed by previous computations on substituted polyacetylenes, and seems consistent with the observed physical aging of c‐PTBA films. Preliminary packing calculations of right‐ and left‐handed helices lead to density values not much higher than observed, indicating rather low degrees of disorder and free volume in the solid state.
Kinetic model of living radical polymerization
Authors: Yan Deyue , Jiang Hong , Fan Xunpei 
Published: April 2003

AbstractThis work studies the kinetics of living radical polymerization by means of both the nonsteady state approach and the quasi‐stationary state method. Expressions for the numberand weight‐average degress of polymerization and the polydispersity index were derived. Numerical results show that the concentration of residual initiator seriously influences the polydispersity index of the resulting polymer. The calculated outcomes of the non‐steady state approach are evidently different from those of the quasi‐stationary state method when the magnitude of the rate constant of termination is comparable with that of the propagation rate constant, and the difference becomes negligible if the rate constant of the termination (kt) is much larger than that of propagation (kp). The polydispersity index of the resulting polymer increases with decreasing ratios of kt to kp or MO to IO (initial concentrations of monomer and initiator).
Theory and mathematical models of ion retention by watersoluble polymers during membrane filtration
Authors: Spivakov Boris Ya. , Shkinev Valerii M. , Golovanov Vladimir I. , Bayer Ernst , Geckeler Kurt E. 
Published: April 2003

AbstractTheoretical approaches to the description of ion retention in aqueous solution by soluble polymers during membrane filtration are proposed. Examination of the chemisms responsible for inorganic ion complexation and retention by membrane‐based separations helps to select appropriate hydrophilic polymers and optimum chemical conditions for the process. The ion separation and enrichment data may be formally explained by using mathematical models based on the theory of kinetics of the reactions in partially open systems.
The simulation of molecular weight distributions in polyreaction kinetics by discrete Galerkin methods
Authors: Wulkow Michael 
Published: April 2003

AbstractThis article describes the development of a comprehensive solver for the differential equations arising from the modeling of molecular weight distributions in polyreactions. Based on a series of numerical developments, the software package PREDICI combines new directions in mathematics, chemical computing and implementation. The algorithm is based on a so‐called discrete Galerkin h‐p‐method, which allows the efficient treatment of numerous polymerization reaction types. The abilities of the new concept are demonstrated on challenging examples.
A theoretical investigation of the production of branched copolymers in continuous stirred tank reactors
Authors: Baltsas Apostolis , Achilias Dimitris S. , Kiparissides Costas 
Published: April 2003

AbstractA comprehensive mathematical model for free‐radical copolymerization reactions has been developed for a homogeneous continuous stirred tank reactor. The present model is based on a fairly general copolymerization scheme accounting for the formation of linear and branched copolymer chains. Both chain transfer to polymer and terminal double bond reactions are considered in order to predict the long chain branching frequency. Changes in molecular weight, composition and degree of branching occurring during the copolymerization reaction are modelled using the method of moments. To break‐down the dependence of the moment equations on higher order moments two different closure methods are considered. The predictive capabilities of the model are examined in relation to the solution copolymerization of methyl methacrylate with vinyl acetate. It is shown that both chain transfer to polymer and terminal double bond reactions significantly contribute to the broadening of the molecular weight and degree of branching distributions. Furthermore, the terminal double bond reaction effects significantly the copolymer number‐average molecular weight and the concentration of terminal double bonds.
Chain collapse by lattice simulation
Authors: Tanaka Genzo , Mattice Wayne L. 
Published: April 2003

AbstractMonte Carlo simulations have been performed on a self‐avoiding simple cubic lattice chain with the nearest‐neighbor interactions for a range of chain lengths N from 40 to 1000 segments to investigate equilibrium properties of polymer chains from an athermal to a collapsed state. Both the fraction of segments in the clusters and the number of contacts exhibit the three stage process for the chain collapse, consistent with our previous molecular dynamics simulations of a fully atomistic chain. In the collapse region corresponding to the nearest‐neighbor interaction parameter larger than 0.5 for a segment‐solvent pair, polymer chains are quite spherical and both ends lie nearly randomized within the sphere. The peak height of the specific heat is proportional to N(In N)3/11, as predicted by the renormalization group theory.
Theoretical investigation of the molecular structure of aluminium triisopropoxide and its complexes in ring‐opening polymerization
Authors: Parenté Vincent , Brédas Jean‐Luc , Dubois Philippe , Ropson Nathalie , Jérôme Robert 
Published: April 2003

AbstractWe present a theoretical study on aluminium triisopropoxide (Al(OiPr)3) using both empirical (Molecular Mechanics, MM, with Dreiding II force field) and quantum‐chemical (Austin Model 1, AM1, semiempirical Hartree‐Fock) techniques. We determine the most stable geometries for both the tetramer and trimer of aluminium triisopropoxide as well as the thermodynamic characteristics of the equilibrium existing between these two aggregated structures. The theoretical results are compared to experimental data from X‐ray diffraction and 27A1 NMR measurements. For the tetramer, it appears that the optimal equilibrium geometries are in good agreement with the experimental X‐ray diffraction geometry; another geometry is also obtained with both theoretical approaches, which is slightly less stable but of higher symmetry. On the basis of the most stable configurations for the tetramer and trimer aggregates, the variation of free enthalpy (ΔG) between the two aggregated structures has been estimated. The evolution of the theoretical ΔG values indicates a displacement of equilibrium towards the trimer species with temperature, in good agreement with experimental 1H and 27A1 NMR data. Moreover, the AM1 heats of formation show a gain of 33.9 kcal/mol due to the aggregation of four A1(OiPr)3 instead of three, and thus a better stability of the tetramer. The molecular geometries being well described by the theoretical methods used in this study, we also present a model for the ring‐opening polymerization complexes of ε‐caprolactone and lactides.