bluuebunny/crossref_metadata_2025_split huggingface dataset:
The site of radical attack at the furan ring from MNDO calculations
Authors: Paz José A. , Vega Rebeca , Rieumont Jacques , Montero Luis A. , Alvarez J. Raul 
Published: April 2003

AbstractThe semi‐empirical SCF MNDO method has been used to calculate the radical reactivities for each reaction site in some neutral furan molecules as the average of the HOMO and LUMO probabilities both in 2pz atomic orbitals1SCF: Self‐consistent field; MNDO: modified neglect of differential overlap; HOMO: highest occupied molecular orbital; LUMO: lowest unoccupied molecular orbital.
. The carbon atoms C5 and C2 of of the furan ring are the most favoured sites except for the furan derivatives holding a double bond in the substituent group. Also the SOMO2SOMO: Single occupied molecular orbital.
probabilities of the radicals formed by addition of a hydrogen atom or a vinyl acetate model radical were calculated. Thus, once a radical is added to the C5 position the resulting radical can be partially localized on the carbon C2. Furthermore, the enthalpy of reaction for several radicals was estimated by using the calculated heats of formation of the neutral furan molecules and their radicals. The radical addition to the carbon C 5 resulted in the most exothermic reaction in comparison with other reaction sites of the molecule. However, no correlation was found between the calculated enthalpies and the degradative‐transfer kinetic constants experimentally determined for the radical polymerization of vinyl acetate in the presence of the furan compounds under study.
Kinetic theory of anionic copolymerization with constant monomer ratio, 2 . Molecular weight distribution
Authors: Hu Xiaodong , Yan Deyue , Feng Jing 
Published: April 2003

AbstractThe kinetic differential equations for the anionic copolymerization with constant monomer ratio are treated by Laplace transformation and a graphical technique. A theoretical method is established by which all molecular parameters of the copolymers, such as the molecular distribution (MWD), the average molecular weight and the polydispersity, can be calculated from reaction rate constants, initial conditions and polymerization time. Three‐dimensional plots obtained by numerical computation are presented to illustrate the influence of the reaction conditions on the MWD's of the copolymers.
Thermodynamics of mixing of ethylene oxide oligomers with different end groups in tetrachloromethane: theoretical treatment
Authors: Killmann Erwin , Cordt Frank , Möller Friedrich 
Published: April 2003

AbstractIn a previous paper we reported the enthalpies of mixing of oligomers of ethylene oxide with different end groups in tetrachloromethane. This paper shows the applicability of the modified theoretical treatment, based on a quasichemical equilibrium of the contacting segmental surfaces originated from Huggins, to our experimental results. The experimental data of ethylene oxide oligomers with methoxy end groups can be described with the energy parameter Δεαβ, the equilibrium constant Kαβ and the surface ratio rσ assuming a chemically uniform surface of the oligomer in CCI4. Ethylene oxides with strong polar OH end groups can be treated successfully with an extended version of the theory with two additional parameters K and ΔΔε for systems containing three different kinds of chemical units.
Theoretical study of chain transfer in anionic polymerization. Transfer by the mechanism of side‐group substitution
Authors: Litvinenko Galina I. , Arest‐Yakubovich Alexander A. 
Published: April 2003

AbstractA new theoretical consideration of chain transfer to monomer in the anionic polymerization of hydrocarbon monomers is presented. It is shown that the kinetic scheme used in theoretical studies reported previously contradicts the widespread views on the chemical mechanism of carbanionic reactions. It is suggested that the most probable path of the transfer reaction is the proton abstraction from the side group of the monomer; the terminal double bond of the monomer molecule remains unchanged, and therefore the intermediate species can participate in succeeding reactions as a macromonomer. The molecular characteristics of polymer formed in processes with monomer transfer by side‐group substitution are determined. At high conversion, the polymer formed in such a process is shown to possess a number‐average degree of polymerization, P̄n, approaching the theoretical value for living polymers, and a P̄w exceeding it the more the higher the intensity of transfer. Furthermore, it shows a broad molecular weight distribution and a fairly noticeable degree of branching. These results considerably differ from those previously reported.
Dynamics of multicomponent polymer mixtures: theoretical models
Authors: Benmouna Mustapha , Vilgis Thomas A. , Benoit Henri 
Published: April 2003

AbstractTheoretical models describing the dynamics of multicomponent polymer mixtures are reviewed. Some detailed derivations are shown to make the basic assumptions clear and to ease the comparison between these models. The effects of random noise and memory functions on the time evolution of dynamical correlation functions are examined. The long‐range hydrodynamic interactions or mode‐coupling effects are also included. A possible extension of these models to the kinetics of spinodal decomposition in solutions of polymer mixtures is briefly discussed.
Multicomponent analysis of poly(propylene) using fractionation, tacticity and molecular weight data
Authors: Cheng H. N. 
Published: April 2003

AbstractHeterogeneous Ziegler‐Natta catalysts are known to contain multiple catalytic sites and produce polymers that contain multiple components. An approach is described in this work that combines fractionation, 13C NMR and molecular weight distribution data to determine the number and the characteristics of the different components found in the polymer. The methodology is applied to published data on poly(propylene). The analyses indicate that poly(propylene) made with heterogeneous Ziegler‐Natta catalysts consists of at least four components. Three of these components obey enantiomorphic‐site models, and the fourth component obeys Bernoullian model.
Anisotropy of local relaxation properties of macromolecules. Polarized luminescence
Authors: Gotlib Yuli Ya. , Neelov Igor M. , Torchinski Isaak A. , Shevelev Vladimir A. 
Published: April 2003

AbstractA tetrahedral lattice model for a polymer chain with three‐unit kinetic elements was used to find the correlation functions which describe the local dynamics of unit vectors oriented in the longitudinal or transverse directions with respect to the chain. These unit vectors can represent the dynamics of the emitting oscillator of the marker. It is shown that the spectra for longitudinal and transverse components of the emitting oscillator differ greatly. It is also shown that at short relaxation times the anisotropy of local relaxation properties of a kinetic chain fragment may be described by an equivalent ellipsoid with an axes ratio of ca. 10.
Anisotropy of local relaxation properties of macromolecules. Spin‐lattice relaxation of 13C nuclei, the nuclear overhauser effect and the estimation of parameters of an equivalent ellipsoid for kinetic segments of polymer chains
Authors: Gotlib Yuli Ya. , Torchinski Isaak A. , Shevelev Vladimir A. 
Published: April 2003

AbstractThe expressions for the functions of spectral density at different orientations of the components of the internuclear vector with respect to the chain backbone, the frequency dependences of the spin‐lattice relaxation time of 13C nuclei (T1C) and the values of the nuclear Overhauser effect (NOE) were obtained for the tetrahedral lattice model of a polymer chain with three‐unit kinetic elements. It was shown that peculiar features of the behavior of T1C and NOE reflect the characteristic properties of the spectra of relaxation (correlation) times for “longitudinal” and “transverse” components of the internuclear vector. It was established that in the range of relatively short times of the relaxation spectrum the dynamics of an anisotropic kinetic segment of the chain may be described with the aid of a simple model of an elongated ellipsoid of rotation with an axial ratio of about 10. It is shown that the equivalent‐ellipsoid model leads to significant differences from a more specific model of chain dynamics when a broad frequency range is considered.
Conformational structure of liquid‐crystalline polyesters with mesogens and spacers in the main chain
Authors: Shilov Sergei , Birshtein Tatiana , Volchek Boris 
Published: April 2003

AbstractThe conformations of thermotropic polyesters with alternating mesogens and poly(methylene) spacers in the main chain were investigated by conformational calculations using the data of linear‐dichroism IR‐spectroscopy, X‐ray analysis and differential scanning calorimetry. The conformations of monomeric units and of the macromolecules as a whole in different phase‐aggregated states were estimated. It is shown that in the liquid‐crystalline (LC) state the polymer chains are strongly extended. The results are compared with the data of various theories of LC ordering.
Melting point depression and phase behavior of poly(ether‐sulfone) and poly(ethylene oxide) blends: Equation‐of‐state theory approach
Authors: Kwon Ick Hwan , Jo Won Ho 
Published: April 2003

AbstractMelting‐point depression in miscible polymer blends is interpreted with Flory‐Prigogine's equation‐of‐state theory (FP theory) and Sanchez‐Lacombe's lattice fluid theory (LF theory). The equations for equilibrium melting point depression in polymer mixtures are proposed from both the FP and LF theories. For miscible poly(ether‐sulfone) (PES)/poly(ethylene oxide) (PEO) blends, the proposed equations are tested. The interaction parameters, X12 in FP theory and ζ12 in LF theory, can be determined with these equations. The theoretically predicted equilibrium melting‐point depression is subdivided into three terms, namely, the equation‐of‐state, the entropy and the contact interaction terms. When the estimated interaction parameters are converted to the heat of mixing by use of both theories, the composition dependence of the heat of mixing can be properly predicted. Using the interaction parameters obtained from the melting‐point depression in PES/PEO blends, the spinodal curves are simulated from both the FP and LF theories.
Computer simulations of atomic force microscopy: crystalline polymers and the effects of surface contaminants
Authors: Sumpter Bobby G. , Getino Coral , Noid Donald W. , Wunderlich Bernhard 
Published: April 2003

AbstractThe atomistic dynamics of the interaction of an atomic force microscopic (AFM) probe with a crystalline polyethylene surface was examined by using the molecular dynamics method. The results show that the internal dynamics of the polymer crystal is such that rapid relaxation occurs, providing for a large amount of structural reversibility and making it possible to perform nondestructive AFM experiments. However, surface and/or AFM tip defects or contaminants (such as those which can be induced by polar molecules adsorbed on the surface), can result in significant perturbations in the AFM images produced, causing large and sharp structures to appear on the surface topology. A rationale of the mechanisms responsible for the image distortions is presented, and a relationship to defects observed in AFM and STM experiments is given.
A Monte‐Carlo study of ring formation during step‐growth polymerization in three dimensions
Authors: Fawcett Allan H. , Mee Richard A. W. , McBride Frederick V. 
Published: April 2003

AbstractThe formation of ring and chain molecules during an irreversible step‐growth polymerization has been modelled on a three‐dimensional twenty six choice cubic lattice, and examined by the Monte‐Carlo method. Movements were not allowed in the system in this preliminary study. The limiting value of the extent of reaction was found to be p = 0,9837 (±0,0003), and the number fraction of molecules found to be rings was 0,300 (±0,045), but the weight fraction was much smaller, 0,056 (±0,005).
Diffusion controlled copolymerization kinetics
Authors: Maxwell Ian A. , Russell Gregory T. 
Published: April 2003

AbstractThe Smoluchowski equation is used to derive equations for predicting the values of diffusion controlled propagation and termination rate coefficients in copolymerization. None of the resulting expressions are any more complicated than the analogous homopolymerization results. Diffusion controlled propagation rate coefficients in copolymerization are predicated to depend only very weakly on chain end type, and thus the propagation kinetics of high conversion copolymerizations are expected to be considerably simpler than those of low conversions. In considering termination, expressions for the rate coefficient for termination by ‘reaction diffusion’ are first of all derived. It is possible that under some conditions (for example, high conversions) these expressions describe the overall rate of termination, but in general, overall rates of termination will be determined by rates of center of mass diffusion, which are chain length dependent. For this reason the reaction diffusion results are incorporated into a more general treatment of termination in which chain end motion is both by translational and reaction diffusion; this treatment is envisaged as applying from intermediate conversions onward. Considerable effort is subsequently invested in developing simple descriptions of copolymerization in which rate coefficients (and in particular those for termination) are allowed to be chain length dependent. An important feature of this paper is that for every parameter usilized, a means of associating to it a value appropriate for a real system is discussed; in most cases this association is readily carried out.
Kinetic model for a polymerization process where the rate constants of both propagation and termination by monomer depend on chain‐length
Authors: Feng Jing , Yan Deyue 
Published: April 2003

AbstractA kinetic model with non‐constant propagation and termination rate constants is given for a sort of anionic polymerization. The expression of molecular weight distribution and other molecular parameters are derived by a non‐steady state procedure. The effect of the reaction conditions on the molecular parameters are illustrated by numerical examples. The theoretical curves of molecular weight distribution with two or three peaks are similar to those of polymers generated in the anionic polymerization of polar monomers in nonpolar solvents.
Kinetic modelling in living polymerizations: The effect of modifiers on the degree of polymerization in batch and continuous stirred‐tank reactors
Authors: Puskas Judit E. 
Published: April 2003

AbstractEquations for the calculation of the degree of polymerization in 'modified' living polymerizations (living polymerizations in the presence of deliberately added terminating and transfer agents) are developed from a mechanistic/kinetic model. It is shown that a modifier, acting by the mechanism discussed, would cause more chain transfer in a continuous stirred tank reactor (CSTR) than in a batch. This is found to be the case in the living polymerization of 1,3‐butadiene initiated by butyllithium in nonpolar solvents, such as cyclohexane, when 1,2‐butadiene was used as a modifier. Under similar conditions no chain transfer was detected in batch polymerizations, while ≈ 20% transfer was found in a CSTR. The equations developed can be used to calculate the ratio of the rates of propagation and termination, kp/kt. CSTR data yielded kp/kt ≈ 480 at 90°C; re‐evaluation of batch data published earlier by Adams et al. for the same system in hexane solvent yields kp/kt ≈ 1250 and 670 at 50° C and 70°C, respectively.
Collapse of a polymer gel induced by complex formation with linear polymers
Authors: Khokhlov Alexei R. , Kramarenko Elena Yu. 
Published: April 2003

AbstractThe thermodynamic theory of swelling and collapse of networks in the solution of linear polymers capable to form complexes with the network chains has been developed. The interaction of components is taken into account in the framework of the Flory‐Huggins approximation. It was shown that complex formation in most of the cases leads to the contraction of the network, which can be either continuous or discrete. The influence of the solvent quality for the network and for the complex on the behavior of the network has been studied. If the solvent is poor for complexes the network reswells at high concentration of linear chains in the outer solution. This reswelling is realized as a steep conformation transition.
Stress tensor in model polymer systems with periodic boundaries
Authors: Theodorou Doros N. , Boone Travis D. , Dodd Lawrence R. , Mansfield Kevin F. 
Published: April 2003

AbstractThe calculation of the stress tensor from molecular simulations of atomistic model polymer systems employing periodic boundary conditions is discussed. Starting from the dynamical equations governing the motion of sites, correct double summation forms of the atomic and the molecular virial equations are derived, which are valid for flexible, infinitely stiff and rigid chain models even in the presence of interactions between different images of the same parent macromolecule. A new expression for the true instantaneous stress (flux of momentum through the faces of the simulation box) is derived and shown to exhibit large fluctuations when applied in molecular dynamics simulations. A new equation for the thermodynamic stress, cast exclusively in terms of intermolecular forces on interaction sites, is also derived. Application to Monte Carlo simulations shows that the molecular virial expression exhibits the smallest fluctuations among all stress expressions discussed, and thus allows computation of the thermodynamic stress with least uncertainty. A scheme is developed for the calculation of surface tension from intermolecular forces only.
Statistical thermodynamics of polydisperse polymer blends with strong interaction
Authors: An Lijia , Ma Rongtang , Kou Xichun , Tang Xinyi , Jiang Bingzheng 
Published: April 2003

AbstractA statistical thermodynamics theory of polydisperse polymer mixtures with strong interaction between dissimilar components based on a lattice fluid model is formulated. Expressions for the free energy, equation of state, phase stability and spinodal for a polydisperse, binary polymer mixture with strong interaction are derived.
Monte carlo study of the dynamics of star‐branched polymers
Authors: Sikorski Andrzej 
Published: April 2003

AbstractA uniform star‐branched polymer model with f = 3 arms based on a simple cubic lattice was studied by means of the dynamic Monte Carlo method. The model chain is athermal with excluded‐volume interactions and it is flexible. A new type of local micromodification was introduced to make the branching point movable. Static properties of the star polymer are in accordance with other theoretical predictions and experimental evidence. Scaling of the self diffusion constant and the terminal relaxation times is close to those of the Rouse theory and to simulation results of linear chains.
Kinetics of critical fluctuations in ternary polymer solutions
Authors: Benmouna Mustapha , Fischer Erhard W. , Seils Joachim , Vilgis Thomas A. 
Published: April 2003

AbstractThe time evolution of the static structure factor following a sudden temperature jump ΔT is investigated in the case of a symmetric mixture of two polymers and a solvent in the optical theta conditions. The two reverse processes corresponding to ΔT > 0 and ΔT < 0 are considered, and the kinetics of approach towards the final states are investigated both in the Rouse limit and in the presence of hydrodynamic interactions.
A new application of statistical moment theory to evaluate the swelling behavior of thermo‐responsive gels
Authors: Yoshida Masaru , Asano Masaharu , Omichi Hideki , Miyajima Masaharu , Takahashi Yasuo , Yamanaka Hidetoshi , Yonezawa Noriyuki , Katakai Ryoichi 
Published: April 2003

AbstractThe loosely cross‐linked poly(acryloyl‐L‐proline methyl ester) gel, which is known as one of the typical thermo‐responsive gels, shows a volume transition around approximately 14°C in pure water. This volume transition is shifted to higher temperature by treatment in aqueous sodium dodecyl sulfate (SDS). The analysis of the swelling‐temperature curve (frequency distribution) was performed according to a moment analysis, in which the area under the swelling‐temperature curve (AUC), mean swelling transition temperature (MSTT) and variance of swelling transition temperature (VSTT) were calculated. It is shown that the apparent volume transition temperature can be estimated from MSTT and that VSTT can be used to characterize the shape of the corresponding curves.
Penultimate model description of the propagation kinetics for the free radical copolymerization of styrene and methyl methacrylate
Authors: Schweer Johannes 
Published: April 2003

AbstractRecently experimental data on the free radical copolymerization of styrene and methyl methacrylate in bulk were shown to obey penultimate model kinetics. These data were now reevaluated with respect to copolymerization parameters by means of a simultaneous fit of the penultimate model both to the copolymer composition and to the overall propagation rate coefficient, and in dependence on the monomer feed composition. Different simultaneous fit procedures were used and compared. The following parameters are finally suggested:r11= 0,498;r21= 0,589;r22= 0,463;r12= 0,547;s1= 0,478 ands2= 0,256.
On microphase segregation of interpenetrating polymer networks
Authors: Frisch Harry L. , Grosberg Alexander Yu. 
Published: April 2003

AbstractThe topological entanglements between subchains of two interpenetrating polymer networks are described in the simplest approximation supposing that the primitive path of each subchain is influenced due to the shift of one network relatively to the other. The entanglement contribution to the free energy of the networks is shown to behave as 1/q2 for the state with deviation from uniform densities with the wave vector of order q. This contribution is shown to cause the microphase type of segregation.
Computer simulations of molecular motion in liquid crystals by the method of Brownian dynamics
Authors: Darinsky A. , Lyulin A. , Neelov I. 
Published: April 2003

AbstractComputer simulations of the molecular motion of polymer chains in the presence of a strong nematic field were carried out by the method of Brownian dynamics. Two models were studied: the first model (linear liquid crystal) is a freely jointed chain with rigid bonds, the second model (comb‐like liquid crystal) is a chain with fixed bond angles and rigid side groups. The influence of ordering on chain conformations, orientational and translational mobility and spectra of relaxation times was investigated.
Monte Carlo studies of static properties of interacting lattice polymers with the cooperative‐motion algorithm
Authors: Gauger Andreas , Weyersberg Andrea , Pakula Tadeusz 
Published: April 2003

AbstractIn this paper we review some applications of the cooperative‐motion simulation algorithm to dense polymer melts. The basic idea behind the algorithm and its implementation on a computer are discussed. Furthermore we show how to include intra‐and intermolecular interactions in the simulation. Exemplary results are presented concerning on one hand the static properties of athermal and semiflexible lattice polymers and on the other hand the phase behaviour of polymer mixtures and diblock copolymers.