bluuebunny/crossref_metadata_2025_split huggingface dataset:
Perturbed enantiomorphic‐site model
Authors: Cheng H. N. 
Published: April 2003

AbstractThe perturbed enantiomorphic‐site (E) model was developed for the analysis of tacticity of Ziegler‐Natta polymers. Theoretical probability expressions for tacticity diads, triads, tetrads and pentads were derived, and a computer program incorporating perturbed enantiomorphic‐site and Bernoullian models was written to facilitate NMR analysis. Examples are shown of the use of the perturbed models to study the 13C NMR tacticity of poly(propylene) made with homogeneous metallocene catalysts. The perturbed E model is shown to be a suitable theoretical framework for the analysis of many polymers made with these catalysts.
Conformational characteristics of poly(vinylpyrrolidone). solvent‐dependence of the chain dimensions
Authors: Tarazona María Pilar , Saiz Enrique , Gargallo Ligia , Radic Deodato 
Published: April 2003

AbstractConformational energies as function of rotational angles over two consecutive skeletal bonds for both meso and racemic diads of poly(Nvinyl‐2‐pyrrolidone) have been computed. The results of these calculations were used to formulate a statistical model that was then employed to calculate the unperturbed dimensions of this polymer. The conformational energies are sensitive to the Coulombic interactions, which are governed by the dielectric constant, of the solvent, and to the size of the solvent molecules. Consequently, the calculated values of the polymeric chain dimensions are strongly dependent on the nature of the solvent, as it was experimentally found before.
Artificial neural system modeling of Monte Carlo simulations of polymers
Authors: Darsey Jerry A. , Soman Ashish G. , Noid Don W. 
Published: April 2003

AbstractIn this work, a neural network was used to learn features in potential energy surfaces and relate those features to conformational properties of a series of polymers. Specifically, we modeled Monte Carlo simulations of 20 polymers in which we calculated the characteristic ratio and the temperature coefficient of the characteristic ratio for each polymer. We first created 20 rotational potential energy surfaces using MNDO procedures and then used these energy surfaces to produce 10000 chains, each chain 100 bonds long. From these results we calculated the mean‐square end‐to‐end distance, the characteristic ratio and its corresponding temperature coefficient. A neural network was then used to model the results of these Monte Carlo calculations. We found that artificial neural network simulations were highly accurate in predicting the outcome of the Monte Carlo calculations for polymers for which it was not trained. The overall average error for prediction of the characteristic ratio was 4,82%, and the overall average error for prediction of the temperature coefficient was 0,89%.
Configurational‐conformational entropy of 1,4‐polydienes
Authors: Zhou Zhiping , Yan Deyue , Alfonso Giovanni Carlo 
Published: April 2003

AbstractA method based on matrix algebra and on the rotational isomeric state scheme to study the configurational‐ conformational entropy of 1,4‐polydienes with geometrical isomerism has been developed. Bernoullian and first‐and second‐order Markovian statistics for the sequences of cis and trans units along the chains have been considered, and the explicit relation between entropy and mole fraction of cis units has been derived. Calculations performed by using available experimental data for the configurational parameters and the conformational partition functions of 1,4‐polybutadiene and 1,4‐polyisoprene show that entropy is a monotonic function of the geometrical isomer composition. While the entropy of polybutadiene increases with the content of cis units, the reverse is true for polyisoprene.
Monte Carlo simulation of kinetics and chain‐length distribution in radical polymerization
Authors: Lu Jianming , Zhang Hongdong , Yang Yuliang 
Published: April 2003

AbstractIn this paper, the Monte Carlo method for numerically simulating the kinetics and chain‐length distribution in radical polymerization is described. Because the Monte Carlo method is not subject to the assumption of steady‐state, it is particularly suitable for studying the kinetic behaviour before the steady‐state has been reached and for systems in which the steady‐state assumption may be violated. Illustrative applications of the algorithm given in this paper not only demonstrate convincingly both the feasibility and usefulness of the algorithm, but also provide some new insight into the illustrative examples. For the case of pseudostationary radical polymerization such as rotating‐sector and pulsed‐laser initiations, we have found that the pseudostationary radical concentration can be reached after two or three initiation periods. However, the number‐average chain‐length x̄n reaches the pseudostationary value much slower than the radical concentration. It is oscillatively reaching the pseudostationary value, and the amplitudes of the oscillations are decreasing with time. We have also found that the chain‐length distribution of the resulting polymer in the case of pseudostationary radical polymerization with termination by combination has stronger periodic modulation. Hence, it should be easier to locate the points of inflection in practice. Therefore, the rate constant of propagation, kp, can be determined precisely for systems which are dominated by a combination‐type of termination.
NMR studies of the effects of isolated variables on polyolefin microstructure and Ziegler‐Natta catalytic sites
Authors: Cheng H. N. 
Published: April 2003

AbstractPolyolefins made with heterogeneous Ziegler‐Natta catalysts contain multiple components as a result of the different catalytic sites present in the catalyst. If a series of similar catalysts are made where only one experimental variable is varied, then a family of similar polymers can be prepared where the polymer microstructure (as studied by NMR) reflects the effects of that variable. An analytical methodology is developed in this work that permits a systematic analysis to be carried out on the NMR data of such polymers. Thus, from the NMR spectra one can deduce the influence of an isolated variable on the polymer components and (indirectly) on the catalytic sites. Illustrated in this work are the effects of Lewis bases on the catalytic sites, catalyst pretreatment with a higher olefin, and catalyst grinding. The approach can be applied to study both homopolymer tacticity and copolymer sequences.
Conformational characteristics of polystyrene
Authors: Rapold Roland F. , Suter Ulrich W. 
Published: April 2003

AbstractConformational energies, computed with a forcefield including coulombic interactions and a simple accounting for the effects of solvent, of meso and racemic 2,4‐diphenylpentane as model substances of polystyrene have been computed as functions of the skeletal torsion angles and the phenyl torsion angles. The relatively high energies of the ḡ conformations rendered these states negligible and no minimum was found in the meso‐tt domain. Three minima for the meso diad (gt, tg, gg) and four minima for the racemo diad (tt, tg, gt, gg) are relevant. A two‐state rotational isomeric state model is applicable with states at φt = 5°C and φg = 110° for both meso and racemo diads. Statistical weight matrices have been derived and values predicted for the characteristic ratio, the fractions of 2,4‐diphenylpentane and 2,4,6‐triphenylheptane at stereo‐chemical equilibrium and the vicinal NMR coupling constants are found to be consistent with experimental results.
Static atomistic modelling of the structure and ring dynamics of bulk amorphous polystyrene
Authors: Rapold Roland F. , Suter Ulrich W. , Theodorou Doros N. 
Published: April 2003

AbstractA detailed static atomistic model of dense, glassy polystyrene is simulated using a well established technique that previously proved successful for simple vinyl polymers. Initial chain conformations that are generated using a Monte Carlo technique including periodic continuation conditions are “relaxed” by potential energy minimization. In total 24 microstructures at densities of 1,07 g/cm3 were obtained with a cube‐edge length of 18,65 Å. Detailed analysis of the minimized structures indicates that intermolecular packing influences create a large variety of chain conformations different from the purely intramolecular ground states. The systems are amorphous, exhibiting random coil behavior. The described structures have been used for a quasistatic simulation of localized motions. These motions include stepwise rotation and oscillation of the phenyl groups. The frequency distribution for the simulated ring motions covers many orders of magnitude. It is very rare that an energy barrier with a reorientation angle indicating a ring “flip” is overcome. Motions with small reorientation of the phenyl rings, and therefore not leading to a ring “flip”, dominate with an average reorientation angle of 16° (±12°). The intermolecular effects of the analyzed processes were found very important and far‐reaching, widely influencing the cooperative motions of molecular groups.
Theory of electrophoretic mobility of polyelectrolyte chains
Authors: Muthukumar Murugappan 
Published: April 2003

AbstractAn explicit formula is derived for the electrophoretic mobility of a linear polyelectrolyte chain in terms of the molecular weight, Debye length and solvent quality. The nature of the crossover between different regimes as the salt concentration and molecular weight are varied is presented. The effect of chain statistics intermediate between the Gaussian and rodlike limits on the electrophoretic mobility is analysed for the first time.
Lattice model of heterogeneous polymer chains in the liquid‐crystalline state, 2 local relaxation properties
Authors: Gotlib Yuli Y. , Medvedev Gregory A. 
Published: April 2003

AbstractUsing a lattice description, the short‐ and medium‐scale dynamics of the heterogeneous model of main‐chain liquid‐crystalline polymers have been considered. Chain mobility is assumed to occur by means of three‐ and four‐bond kinetic‐unit jumps. Mobility and force coefficient tensors, and linear relaxation equations for the average projections of chain bonds have been obtained. Both acoustic and optical branches of the relaxation spectrum are shown to be split into longitudinal and transverse components. A pronounced dependence of relaxation spectrum shape and splitting magnitude on the spacer length has been found.
Monte carlo simulation of polymers in steady potential flows
Authors: Carl Wilfried , De Pablo Juan J. 
Published: April 2003

AbstractThis work presents a study of the configurational properties of bead‐spring chains in steady‐state potential flows. These properties are obtained from off‐lattice Monte Carlo simulations. The results of our simulations compare favourably with theoretical results for phantom chains. We investigate the effects of excluded volume and hydrodynamic interaction on the configurations of the chains, and compare our results to the predictions of scaling arguments. The scaling law observed here for the latter case deviates from theoretical predictions but is in agreement with findings reported from molecular dynamics simulations.
Effect of polyfunctional chain transfer agents on molecular weight distribution in free‐radical polymerization, 1. The ideal case: C equals one
Authors: Yuan Cui Ming , Silvestro Giuseppe Di , Farina Mario 
Published: April 2003

AbstractThe theoretical approach to free‐radical polymerization in the presence of polyfunctional transfer agents can be derived in a simple way if the effects of primary chains and termination reactions are neglected. Under these conditions, and for values of the chain transfer constant C = 1, the analytical expressions for the number‐ and weight‐average degree of polymerization, dispersion index and weight distribution function were derived through the elementary probability theory and generating functions. Numerical examples are reported on the relationship between distribution parameters, functionality and monomer conversion.
Influence of reactor operational policies on distributions of chemical composition, molar mass, sequence length and sequence frequency of multicomponent polymers obtained by free‐radical polymerization
Authors: Engelmann Uwe , Schmidt‐Naake Gudrun 
Published: April 2003

AbstractRemarkable shifts in chemical composition, molar mass, sequence length and sequence frequency will occur in the course of free‐radical multicomponent polymerizations, if the reactivities of the comonomers are different and a simple batch reactor is used. It is shown for the terpolymerization system of methyl methacrylate/styrene/maleic anhydride that a semi‐batch reactor with appropriate regimes is suitable to obtain products with improved chemical, molecular and sequential homogeneity. However, if polymers with defined inhomogeneities like bimodal molar mass or chemical composition distributions are desired, these are also obtainable by use of appropriate operational policies, which will be illustrated for the homopolymerization of methyl methacrylate and the binary copolymerization of methyl methacrylate/maleic anhydride. Concerning the instantaneous mean chemical composition of polymer molecules, which cannot be measured directly, a new procedure is presented to determine this quantity experimentally.
Ab initio quantum‐mechanical calculations of the propagation enthalpies for the radical and anionic polymerizations of formaldehyde and methanimine
Authors: Sana Michel , Leroy Georges , Dewispelaere Jean‐Pierre , Wilante Claude , Penelle Jacques 
Published: April 2003

AbstractThe gas phase enthalpies of formation for oligomeric radicals and anions H(CH2NH)n* and H(CH2O)n* were theoretically determined by ab initio quantum‐mechanical calculations with n in the range 1 to 6. From these results, the reaction enthalpies for each of the first five propagation steps of the polymerization were estimated for methanimine (H2C = NH) and formaldehyde (H2C = O). At the same step of oligomerization, enthalpies associated with anionic polymerizations are always more negative than enthalpies corresponding to radical polymerizations, but the difference between them decreases with increasing n. Both Delta;H (propagation) vs. n curves tend rapidly, particularly for radical polymerizations, towards an asymptotic value independent of the mode of polymerization and equal to ‐ 12 kcal/mol for formaldehyde and ‐ 14 kcal/mol for methanimine. Experimental data for the gas phase polymerization of formaldehyde are in good agreement with our theoretical value. These results demonstrate that heats of polymerization can be reasonably estimated by intensive calculation methods if a careful choice of the reaction mimicking the propagation step is done.
Statics and dynamics of heterogeneous polymer networks
Authors: Vilgis Thomas A. , Heinrich Gert 
Published: April 2003

AbstractThis paper discusses the effect of heterogeneities in crosslink density on static properties, such as the mechanical stress‐strain relation (free energy) and scattering effects as occur in small‐angle neutron experiments. The second part of the paper is devoted to the segmental dynamics and the long‐time relaxation of polymer networks. It is shown that the heterogeneities slow down the relaxation at long times.
Molecular arrangements of liquids of chain molecules near solid surfaces in the presence of a diluent
Authors: Vacatello Michele 
Published: April 2003

AbstractThe molecular arrangements near solid surfaces and in narrow slits of liquid tridecane, containing 5 wt.‐% of a diluent having the size and the interactions of a methylene group, have been studied by atomistic computer simulations using the Monte Carlo technique. The presence of the diluent does not modify the tendency to form segmental layers near the solid surfaces. However, the diluent is preferentially adsorbed at these surfaces, replacing part of the chain ends. As a result, the ordered structures with nearly two‐dimensional chains found for the pure liquid in analogous conditions are partly destroyed.
Semiclassical theory of group vibrations for linear polymer chains
Authors: Wozny Christopher E. , Sumpter Bobby G. , Noid Donald W. 
Published: April 2003

AbstractThe semiclassical spectral method (SSM), which employs molecular dynamics and spectral estimators to determine transition frequencies between quantum energy levels in molecular and atomic systems, is extended to include calculations for linear polymer chains. Average differences between harmonic and transition frequencies for the symmetric and asymmetric stretch modes of polyethylene are reported to be around 50 cm−1 with smaller average frequency shifts for the other group vibration dispersion curves for a certain potential surface describing the macromolecule.
Sorption of gases in glassy polymers: A probabilistic model
Authors: Favre Eric , Schaetzel Pierre , Nguyen Quang Trong , Néel Jean 
Published: April 2003

AbstractThe description of the sorption mechanism of gases in glassy in glassy polymers is of importance for membrane‐based gas separation processes as well as for packaging applications. Dual mode theory, postulating a Henry and Langmuir process to occur and leading to an equation with three adjustable parameters, is usually proposed in order to describe the isotherms. A simple mechanistic model, based on an unequal affinity between the gas molecules and the matrix regions occupied either by polymer or a previously sorbed gas molecule, is presented in this paper; in contrast to dual mode theory, this approach does not postulate the existence of microvoids in the glassy matrix. Integration leads to an equation with two adjustable parameters, which is shown to describe precisely already reported isotherms.
Thermodynamic modelling of polymer solutions with a modified Staverman equation
Authors: Pfennig Andreas 
Published: April 2003

AbstractA model describing the thermodynamic behaviour of polymer solutions is derived which explicitly accounts for the flexibility of the polymer chains. Based on computer simulations on various lattices it is shown that the flexibility of a polymer chain can be modelled by distinguishing different polymer conformations. Here each conformation is characterized by its corresponding number of external contact sites. The equilibrium between the different conformations is then solved for any polymer concentration and any combination of interaction energies utilizing a modified Staverman equation. The model predictions are in good agreement with the results of the computer simulations which were performed using the simple‐sampling and the slithering‐snake algorithm. Since the knowledge of the distribution of the conformations of a single polymer chain on an empty lattice is a prerequisite to perform the model calculations, Poisson distribution functions are fitted to the results of the corresponding computer simulations. The generalization of these distribution functions not only facilitates the use of the new model but also allows to model polymers of varying chain stiffness.
Application of numerical methods for the calculation of size distribution functions of polymers produced by ionic polymerizations
Authors: Ermakov Igor , De Clercq Ronny , Goethals Eric 
Published: April 2003

AbstractThe application of numerical methods for the calculation of the molecular weight distribution of living ionic polymerization, carried out in the presence of monofunctional and polyfunctional transfer agents, is described. The methods used include the numerical solution of a system of differential equations by the Runge‐Kutta‐Merson procedure and the statistical Monte‐Carlo approach. When a monofunctional transfer agent is used, the Runge‐Kutta‐Merson procedure is quite useful for the calculation of the molecular weight distribution for various polymerization systems. When a polymerization is carried out in the presence of a polyfunctional transfer agent, the mechanism includes the coupling of polymer chains. Due to the complexity of the system, the Runge‐Kutta‐Merson procedure is hardly applicable and problems of this type should be solved by a Monte Carlo simulation procedure. Once a computer program has been written, both methods allow the chemist to calculate the molecular weight distribution of a polymer as a function of the different kinetic parameters of the polymerization.
Polydomains in liquid‐crystal elastomers
Authors: ten Bosch Alexandra , Varichon Lionel 
Published: April 2003

AbstractIn liquid‐crystal elastomers, the simultaneous presence of rubber elasticity due to the crosslinked backbone chains and of optical birefringence due to the mesogens in the side chains lead to exceptional physical properties. An elastic deformation of the network influences the order of the mesogens and, therefore, the optical properties. A theory based on a Landau‐de Gennes expansion of the free energy is proposed. In the opaque polydomain phase, the local orientation is given by a compromise between the external mechanical field and a local anchoring interaction. As the field is increased, it becomes energetically favorable for the mesogens to align parallel to the mechanical field, and a transition to a transparent monodomain structure occurs. Results for the average orientation, the stress and the chain conformation are given.
Stability analysis and scattering properties of charged crosslinked blends in solution
Authors: Benmouna Mustapha , Vilgis Thomas A. , Benhamou Mabrouk , Babaoui M. , Daoud Mohamed 
Published: April 2003

AbstractThe phase behavior and scattering properties of crosslinked charged polymer blends in solution are examined. De Gennes' analogy between the effects of crosslinks in a network and electrostatic charges in a dielectric medium on the scattering properties and the phase behavior of crosslinked polymer blends is used. This analogy is extended to include the effects of screening in order to improve its agreement with the experimental data in the small q range. The excluded volume interactions are explicitely introduced through the blob model. The enhancement of compatibility of the mixture and the increase of its rigidity constant are evaluated in certain cases. The limit of neutral polymers is discussed because it corresponds to the case of charged mixtures in the presence of excess salt. The kinetics of the microphase separation transition is briefly discussed. A dynamical model including the long range hydrodynamic interaction is presented. The relaxation frequency of the stable modes and the growth rate of unstable ones are also considered.
On the plateau modulus mixing rule of miscible blends
Authors: Lomellini Paolo 
Published: April 2003

AbstractBased on Graessley and Edwards scaling concepts, a new mixing rule for the entanglement density of miscible blends has been proposed. The arguments here used apply when the interactions in the blend do not affect too much the entanglement probability. The rule was compared with others (linear and geometric mixing rules) based on the entanglement idea of fixed interchain contact points. The predicted plateau moduli were successfully compared with experimental data of polystyrene‐poly(dimethylphenylene oxide) blends.
Are the continuum and the lattice representation of freely jointed chains equivalent?
Authors: Glatting Gerhard , Winkler Roland G. , Reineker Peter 
Published: April 2003

AbstractThe partition function and force elongation relation of freely jointed chains of rigid rod segments are calculated for arbitrary dimensions in the continuum and on a lattice. The qualitative differences between these models are discussed for polymers embedded in spaces of various dimensions. These investigations show that chains of rigid rod segments in higher dimensions are equivalent to chains of segments with internal degree of freedom in lower dimensions. Networks composed of these chains show a similar behaviour as uniaxial stretching experiments of rubber‐like materials.