bluuebunny/crossref_metadata_2025_split huggingface dataset:
Theory of time‐dependent log‐normal molecular weight distribution of polylefins in heterogeneous Ziegler polymerization
Authors: Keii Tominaga 
Published: April 2003

AbstractAn explanation of the time‐dependent log‐normal molecular weight distribution of polyolefins in heterogeneous Ziegler polymerization is proposed on the basis of the non‐uniformity of surface sites concerning the activation energy of the propagation reaction. It is a theoretical derivation of the Wesslau equation from the usual kinetic model. The theory explains the behavior of the molecular weight distribution, i.e., time and temperature dependences of polydispersity as well as large values of polydispersity in the early stages of polymerization, which have been observed experimentally.
Critical properties for gelation in free‐radical crosslinking copolymerization
Authors: Okay Oğuz , Naghash Hamid J. , Pekcan Önder 
Published: April 2003

AbstractA kinetic model was used to predict the molecular weight developments and the critical properties in free‐radical crosslinking copolymerization. The predictions of the model were compared to the experimental data reported previously. Agreement of the kinetic model with experiments is satisfactory for both low and high crosslinker contents. The model parameters indicate increasing extent of shielding of pendant vinyl groups as the reaction proceeds due to the increasing number of multiple crosslinkages. The calculation results indicate that the real critical exponents can only be observed in the region ε < 10−2–10−3 where experimental studies are very difficult. Outside of this region, the apparent critical exponent γ describing the divergence of the weight‐average molecular weight was found to deviate from the classical value due to the conversion dependent kinetics of free‐radical crosslinking copolymerization.
Molecular modelling of polymers, 1. Prediction of critical values of oligomethylenes
Authors: Tan Ton That Minh , Rode Bernd Michael 
Published: April 2003

AbstractMost procedures for the estimation of critical values of oligomethylenes assume these critical values to depend only on the chain length n and/or the molecular weight M. In this work, we introduce new equations for estimating the critical values by inclusion of quantum chemical data calculated by a semiempirical SCF‐MOSCF‐MO: Self‐consistent field, molecular orbital.
method (AM1). These models give a much superior prediction for the critical values such as the normal boiling point Tb, critical temperature Tc, critical pressure Pc, critical valume Vc, surface tension yb and enthalpy of vaporization ΔHvp of 18 oligomethylenes. The predictions for polyethylene (with infinite chain length) for the values of Tb, Tc, Pc, Vc, ΔHvp and yb are 1029.3 K, 1029.3 K, 1.013 bar, 15723.3 m3/mol, 0 KJ/mol and 0 mN/m, respectively.
The sol fraction and conversions in AaBb type polymerisation
Authors: Xiao Xing‐cai , Huang Xu‐ri , Li Ze‐sheng , Sun Chia‐chung , Tang Au‐chin 
Published: April 2003

AbstractThe sol fraction for the post‐gel regime in AaBb‐type polymerisations is investigated in a way which allows to study the total, the sol, the gel, the tree‐gel and the loop‐gel fractional conversions. Associated with these conversions, corresponding gel points are discussed as the threshold of sol‐gel transition. (AaBb is a monomer with a functional groups of type A and b functional groups of type B.)
On the relation between conformational changes and electronic structure defects in polypyrrole
Authors: Breza Martin , Laurinc Viliam 
Published: April 2003

AbstractUsing the semiempirical quantum‐chemical AM1 method, the dependence of geometry and electronic structure on the torsion angle δ between neighbouring pyrrole rings of quaterpyrrole in various charge and spin states is investigated. The magnitudes of changes in the electronic structure of polypyrrole caused by geometrical changes of the chain may be comparable with those caused by charge and spin fluctuations. A limited amount of negative charge may be spontaneously stabilized on the polypyrrole chain due to chain deformations caused by solid state influences without any charged dopants. Charge fluctuations are most probably concentrated at nitrogen atoms. N—C bonds are less dependent on δ variations as well as on charge and spin perturbations than C—C bonds.
Phenomenological transient shear behavior of liquid crystalline polymers after the start‐up of shear flow
Authors: Kim Kwang Man , Chung In Jae 
Published: April 2003

AbstractTransient stresses of liquid crystalline polymers appear as damped oscillatory patterns after the start‐up of shear flow. They are examined by using the constitutive equations which were modified to include the idea of an initial domain state in the Larson‐Doi polydomain equation set. In order to be consistent with the phenomenological transient shear stress after the start‐up in the region of low shear rates, the Hinch‐Leal closure approximations were adopted for the domain‐averaged (mesoscopic) fourth‐order tensors of the director field. It is predicted as an important result that the amplitude of the first overshoot in the transient stress increases with an increase in the shear rate imposed at the start‐up, which was found to be largely in agreement with phenomenology. In addition, the effects of parameters such as molecular concentration, initial domain size, and intensities of viscous and elastic contributions on the transient stress under the start‐up process were also evaluated.
Thermodynamic treatment of ligand‐receptor interactions
Authors: Di Noto Vito , Via Lisa Dalla , Toninello Antonio , Vidali Maurizio 
Published: April 2003

AbstractThis work reports the development of a physico‐chemical model for ligand‐receptor interactions. It considers receptors having groups of different binding sites in equilibrium and far from equilibrium where each of these may have multiple occupancies. The obtained results under appropriate simplifications give the well known Scatchard, Hill and Adair relations and provide a very interesting tool for studying receptor‐ligand phenomena and for determining the related parameters and interaction energies.
Molecular simulation of the comparative flexibility of bridging linkages in poly(aryl ether sulfone)s and poly(aryl ether ketone)s from a study of isolated oligomers
Authors: Hamerton Ian , Heald C. Richard , Howlin Brendan J. 
Published: April 2003

AbstractAtomistic molecular modelling of the bridging linkages in poly(aryl ether sulfone)s and poly(aryl ether ketone)s have been studied in vacuo. The results of these simulations indicate that models that have bond lengths and angles that agree with X‐ray data are produced from the energy minimised structures and additionally that conformational analysis of these linkages produces low energy conformers that are also accessible in the X‐ray data. The results of these simulations provide confidence in the modelling techniques used and indicate the potential for these models in further simulation of the bulk amorphous state.
Quantum‐Monte‐Carlo simulations on linear chains
Authors: Kreitmeier Stefan N. , Noid Donald W. , Sumpter Bobby G. 
Published: April 2003

AbstractThe diffusion Quantum‐Monte‐Carlo method is applied to a set of coupled oscillators as a first approach to a polyethylene chain. The obtained energy eigenvalues are in good agreement with normal mode analysis. For chains shorter than 50 atoms a distinction between center oscillators and end oscillators is possible. For longer chains non‐negligible deviations are discussed.
Molecular reinforcement: A force field evaluation
Authors: Wendling Jürgen , Wendorff Joachim H. 
Published: April 2003

AbstractComputer simulations on the basis of a force field approach are used to test the concept of molecular reinforcement and to get an insight into the parameters controlling the reinforcement effect. The finding is that molecular reinforcement may be as effective as fiber reinforcement. The molecular interactions between the matrix and the rigid molecules replace the adhesion between fiber and matrix and the axial ratio of the molecules plays the same role as the axial ratio of the fiber in the case of fiber reinforcement.
Molecular modelling of polymers, 3. Prediction of glass transition temperatures of poly(acrylic acid), poly(methacrylic acid) and polyacrylamide derivatives
Authors: Tan Ton That Minh , Rode Bernd Michael 
Published: April 2003

AbstractThe establishment of the relations between the glass transition temperatures of polymers, such as poly(acrylic acid), poly(methacrylic acid), polyacrylamide and their derivatives, and the electronic structure of their monomers calculated by various methods such as the quantum chemical methods AM1 and PM3, and the Gasteiger‐Hückel method has been investigated. Partial charges of some important atoms in the monomer together with the degree of substitution and chain length of the hydrocarbon group of ester or amide functions of the monomer were used as descriptors. The results indicate that the quantum chemical methods PM3 and especially AM1 lead to models of superior predictability compared to the one obtained by the Gasteiger‐Hückel method.
Bivariate chain length and long chain branching distribution for copolymerization of olefins and polyolefin chains containing terminal double‐bonds
Authors: Soares João B. P. , Hamielec Archie E. 
Published: April 2003

AbstractPolyolefins containing long chain branches can be synthesized using certain metallocene catalysts such as Dow Chemical's constrained geometry catalyst. These polyolefins combine the excellent mechanical properties of polymers with narrow molecular weight distribution with the easy processability of polymers containing long chain branches. A mathematical model for the chain length distribution for these novel polyolefins was derived from basic principles and an analytical solution for the chain length distributions of the populations containing different number of long chain branches per polymer molecule was obtained. The analytical solution agrees with the direct solution of the population balances and with a Monte‐Carlo simulation model. It is also shown that this solution applies for copolymers using pseudo‐kinetic rate constants and Stockmayer's bivariate distribution.
Physically meaningful discrete relaxation spectrum: Rheological behaviour of monodisperse polymer melts
Authors: Smail Rachid , Carrot Christian , Guillet Jacques 
Published: April 2003

AbstractA discrete relaxation spectrum using a small number of modes is derived to describe the relaxation behaviour of monodisperse polystyrene melts. Attention is focused on keeping the physical meaning of the relaxation times. Therefore, molecular dynamics theories are used in the terminal, plateau and transition zones and in the case of entangled melts. The spectrum is divided into three parts corresponding to three assumed different relaxation mechanisms: Rouse mechanism at short times, modified Rouse for tube equilibration and reptation with tube fluctuation in the long time range. This description only requires the knowledge of a limited number of parameters, namely the molecular weight of the polymer under investigation, the scaling law for Newtonian viscosity, the plateau modulus and the molecular weight between entanglements. The method is sucessfully applied to entangled monodisperse polystyrene melts and enables the recovery of final spectra containing only a few relaxation modes.
Computer simulation of DNA orientation and deformation in a shear flow field
Authors: Ødegaard‐Jensen Arvid , Elvingson Christer , Håkansson Christer 
Published: April 2003

AbstractThe structure and orientation of semiflexible chain molecules in a shear flow field were studied by Brownian dynamics simulation. Molecules in the size range 200 nm to 1 μm were modeled as chains of spherical subunits with parameters chosen to mimic the size and persistence length of B‐DNA. The analysis of the steady‐state orientation showed a rather broad and asymmetric distribution. The simulations also showed that the orientation of the largest main axis of the moment of inertia tensor is significantly higher compared to the orientation obtained from averaging over the individual bonds in the molecules, the latter procedure being the relevant case when comparing with, e.g., linear dichroism experiments.
On the equation of state for thermal polymer solutions and melts with attractive interaction
Authors: Wolfgardt Matthias , Binder Kurt 
Published: April 2003

AbstractWe perform Monte Carlo simulations of a lattice model for polymer melts, i. e., the bond fluctuation model in three dimensions. By using an energy parameter that prefers relatively long bonds, the model exhibits a glass transition at low temperatures, in close qualitative similarity to experiment. We modify this model by adding an attractive interaction of variable strength. We demonstrate that a small interaction strength has only a very small effect on the static properties of the melt. For a fixed strength of the potential, the chemical potential is measured by a modified particle‐insertion method over a large range of temperatures and densities. The osmotic pressure is obtained by thermodynamic integration. In contrast to the original version our extended model exhibits a positive thermal expansion.
Aggregation processes in self‐associating polymer systems: Computer simulation study of micelles in the superstrong segregation regime
Authors: Khalatur Pavel G. , Khokhlov Alexei R. , Nyrkova Irina A. , Semenov Alexander N. 
Published: April 2003

AbstractWe present the results of extensive molecular dynamics and Monte Carlo studies of the self‐organization in the solution of short polymer chains with strongly attracting head groups at their end. The formation of micelles (multiplets) is studied in detail. Both two dimensional (2d) and three‐dimensional (3d) systems are considered. The off‐lattice and lattice models under study incorporate physical factors which control micelle structure and growth in the so‐called superstrong segregation regime. These factors include (i) conformational effects associated with short‐range excluded‐volume interaction between the tails of flexible‐chain molecules and (ii) very strong attraction of head groups. Our computer simulations of 3d micelles, constructed a priori from chains with strong attraction of head groups (with the characteristic energy ≈ 10 kBT), show that size and shape of the micellar core depends crucially on the radius rc of the interaction of head‐groups. If the value of rc is comparable with chain length, then micelles of nearly spherical shape emerges. The decrease of rc can induce a sharp polymorphic transition from the micellar core which is spheric in shape to a disk‐like (bilayer‐shaped) aggregate. Such molecular organization differs from the commonly held notion of a radially symmetric micellar core. On the other hand, these findings fall into line with a recent theory of the super strong segregation regime. When the starting configuration is a random one (i.e., no micelles were a priori formed) the type of final microstructures, emerging as a result of micellization in the superstrong segregation regime, also depends essentially on the radius of head‐head attraction. In the case of three‐dimensional systems and/or short range attractive potentials we always obtain many small spherically shaped aggregates which, once formed at initial stages of micellization, remain stable for all time scales. Such a behavior is due to both the strong head‐head attraction and the screening (repulsive) action of micellar shells creating insurmountable potential barriers. As a result, we deal with kinetically “frozen‐in” microstructures which are not reversible and cannot exchange molecules with one another. In dense systems, we observe the formation of a (quasi) periodic pattern of alternating microdomains.
Mean square radius of gyration and hydrodynamic radius of jointed star (dumbbell) and H‐comb polymers
Authors: Radke Wolfgang , Müller Axel H. E. 
Published: April 2003

AbstractEquations for the mean square radius of gyration and the hydrodynamic radius for jointed stars (dumbbells) and H‐combs are derived, based on random flight statistics for each subchain. Comparision with literature data on computer simulations and experimental data for H‐combs show good agreement for the g‐value of the mean square radius of gyration even in good solvents. This suggests that for the mean square radius of gyration the relative dimension of a H‐comb relative to the linear molecule of the same degree of polymerization is not altered significantly by long range interactions, as in the case of star polymers. For the hydrodynamic radius the situation is different. Fair agreement is found when comparing our results with viscosity measurements under θ conditions, while in good solvents the viscosity of the branched molecule is more reduced than predicted by our results.
Efficient computation of potential energy first and second derivatives for molecular dynamics, normal coordinate analysis, and molecular mechanics calculations
Authors: Tuzun Robert E. , Noid Donald W. , Sumpter Bobby G. 
Published: April 2003

AbstractBy using two‐ and three‐body internal coordinates and their derivatives as intermediates, it is possible to enormously simplify formulas for three‐ and four‐body internal coordinates and their derivatives. Using this approach, simple formulas are presented for stretch (two‐body), two types of bend (three‐body), and wag and two types of torsion (four‐body) internal coordinates and their first and second derivatives. The formulas are eminently suitable for economizing molecular dynamics and molecular mechanics calculations and normal coordinate analysis of complicated potential energy surfaces. Efficient methods for computing derivatives of entire potential energy terms, and in particular cross terms with switching functions, are presented.
Calculation of phase diagrams not requiring the derivatives of the Gibbs energy for multinary mixtures
Authors: Horst Roland 
Published: April 2003

AbstractA method for the calculation of phase diagrams (tie lines and binodal, spinodal, critical points and their stability) based exclusively on the Gibbs energy of mixing with no need of its derivatives with respect to the composition variables is extended to multinary mixtures for any number of components. The mathematical description of the (K‐1)‐dimensional phase diagram of a K‐component mixture is presented. The method is demonstrated for a quinternary blend of five polymers exhibiting a closed miscibility gap; all binary, ternary and quaternary subsystems are completely miscible. The phase separation in the quinternary system is caused by very favorable interactions in the ternary subsytem 1/2/3 and the binary subsystem 4/5.
Phenomenological theory of textured mesophase polymers in weak flows
Authors: Rey Alejandro D. 
Published: April 2003

AbstractA macroscopic viscoelastic model for incompressible, isothermal, homogeneous lyotropic mesophases exhibiting the nematic polydomain texture is presented. Under equilibrium static conditions the model describes a three dimensional tessellation, where each region or nematic domain has a characteristic size and orientation, and where the polydomain texture has a random orientation. Close form expressions that define the characteristic texture size and the number of randomly oriented domains are given. When subjecting the model lyotropic liquid crystalline polymer displaying the polydomain texture to a steady rectilinear shear flow, the predicted characteristic texture size decreases with increasing shear rates. The power law scaling relations of texture size with shear rate are in excellent agreement with the experimental measurements. The steady shear flow orientation predictions, characterized by a positive shear dependent alignment angle and a low orientation, are in agreement with experimental data.
Configurational‐conformational statistics of stereoirregular polystyrene
Authors: Zhou Zhiping , Yan Deyue 
Published: April 2003

AbstractThe dependence of the configurational‐conformational characteristics, such as the mean‐square end‐to‐end distance, the mean‐square radius of gyration, and the temperature coefficient of the stereoirregular polystyrene chains on the fraction of meso dyads, Pm was investigated, using the periodic condition method. The calculation shows that polystyrene has the most compact chain when Pm = 0.7, and the temperature coefficient of the characteristic ratio of the mean‐square end‐to‐end distance reaches the minimum, −0.89 × 10−3 K−1, at Pm = 0.5. The theoretical result of the mean‐square radius of gyration for atactic polystyrene, 〈S2〉1/2 = 0.2245 M0.5106 shows good agreement with the experimental measurement in both bulk amorphous state and θ‐solvent.
Comparison of two force fields in MD‐simulations of α‐helical structures in keratins
Authors: Knopp Birgitta , Jung Bernd , Wortmann Franz‐Josef 
Published: April 2003

AbstractMolecular dynamic (MD) simulations based on two different force fields, CVFF and CFF91, were carried out in order to check their feasibility for the structural investigation of the wool intermediate filament (IF) monomeric unit. Selecting an ideal α‐helix as start conformation, all MD‐simulations with CVFF in vaccum show the α‐helix to be unstable. Independently of the amino acid sequence of the α‐helix, a new helical structure with a larger diameter arises during the MD‐simulation, due to a shift of the intrahelical hydrogen bonds. However in simulations with surrounding water the α‐helix remains stable throughout the simulations with the CVFF force field. In contrast to this, MD‐simulations in vaccume based on the CFF91 force field are able to determine different stabilities for the α‐helical start conformation of various IF‐segments, that agree well with secondary structure predictions. The simulation results obtained with CFF91 in vacuum can like wise be verified using an explicit water environment. We found that higher partial charges attributed to the atoms of the amide groups that form the intrahelical hydrogen bonds are the reason for the superiority of the CFF91 force field.
Computer simulation of DNA gel electrophoresis: influence of solid friction on linear and circular chains
Authors: Widgren Stefan , Elvingson Christer 
Published: April 2003

AbstractThe effect of solid friction between DNA molecules and gel fibres during gel electrophoresis has been studied using Brownian dynamics simulations. To investigate the effect of the topology of the polyelectrolyte chains, we have studied both linear and circular molecules. The results show that the migration properties of linear molecules seem to be almost uneffected by solid friction. There is, however, a significant effect on the mobility of circular molecules of the same size. Increasing the electric field strength increases the mobility of the linear molecules while for the circular chains, the mobility goes through a maximum when friction is included.