bluuebunny/crossref_metadata_2025_split huggingface dataset:
Numerical analysis of the rotational relaxation time of spectrin segments and spectrin heterodimer in dilute aqueous solution
Authors: Skjetne Paal , Knudsen Kenneth D. , de La Torre Jose Garcia , Elgsaeter Arnljot 
Published: April 2003

AbstractThe rotational relaxation time for different models of the spectrin αβ‐heterodimer and spectrin subunits were obtained through numerical simulation making use of the so‐called rigid‐body approximation. Based on information on the helix structure of the 106 aminoacids in the repeating motif of spectrin, three different models with varying degree of refinement were set up to represent the spectrin subunits: the two‐bead, the many‐bead and the shell model. For one‐ and two‐subunit spectrin the relaxation time was found to be 20 ± 4 ns and 92 ± 5 ns, respectively. The last result conforms well with available experimental data from transient electric birefringence measurements. For the αβ‐heterodimer two different models were applied: a chain of beads (pearl necklace model) and a more refined so‐called San necklace model. Using the San necklace model, the relaxation time was found to be in the range of 9 to 18 μs (depending on the flexibility of the joint), which is in accordance with what has been obtained from intrinsic viscosity measurements, but considerably higher than experimental data from measurements of transient electric birefringence.
Numerical estimation of unperturbed dimensions of regioirregular poly(3‐ethylthiophene)s
Authors: He Ziru , Yang Xiaozhen , Zhao Delu 
Published: April 2003

AbstractThe unperturbed chain dimensions of poly(3‐ethylthiophene)s have been calculated using the full‐relaxation optimization of conformational energies and the chain statistical mechanics with a rotational isomeric state model of two states. The computation indicates that the conjugated polymer with side groups is more flexible than without, and that the orientation of the ethyl group and the regioirregular sequences perturb the chain conformation to a certain extent. Specially, the calculated chain conformations under θ‐conditions are more rigid than those observed in good solvent. This supports experiments showing that the chain dimensions of conjugated polymers are very sensitive to variations of intermolecular interactions.
Monte Carlo simulation of the chain length distribution in pulsed‐laser polymerization experiments in microemulsion
Authors: Manders Bart G. , van Herk Alex M. , German Anton L. 
Published: April 2003

AbstractThe Monte Carlo method has been used for numerically simulating pulsed‐laser polymerization (PLP) in microemulsion, in order to establish if a shift from inflection point to peak maximum as the best measure of the propagation rate constant, kp, will occur theoretically. Termination is assumed to be instantaneous in the simulations as droplet sizes can be very small in microemulsions. From the results of the simulations it is found that instantaneous termination indeed causes the peak maximum to become the best measure of kp. From these results it can be deduced that in bulk it is not simply the Poisson‐broadening that causes the peak maximum to yield an overestimation of kp. This overestimation is rather caused by the fact that the termination rate is finite leading to an asymmetrical peak in the molecular weight distribution. In combination with broadening this yields the inflection point to be the best measure of kp in the bulk.
Computer simulation of the aggregation of ion pairs in the polymerization of styrene initiated by RCl/SnCl4/NRCl− systems
Authors: Szymanski Ryszard , Matyjaszewski Krzysztof 
Published: April 2003

AbstractComputer simulations show that ion pair aggregation can be responsible for the unusual dependence of the initial rate of polymerization on the concentration of added salt in the cationic polymerization of styrene initiated by RCl/SnCl4/NRCl−. Addition of small amounts of tetraalkylammonium chloride to the system reduces the rate of polymerization due to the decrease of the concentration of propagating free cations. Subsequent salt addition leads to a small rate increase, and then the rate decreases at higher [salt]0/[SnCl4]0 ratio. The simulations show that the rate increase can be ascribed to the formation of aggregates of ion pairs and thus to a higher overall proportion of carbocations resulting in faster polymerization. The decrease of the polymerization rate at higher concentrations of added salt can be explained by the conversion of free SnCl4 to SnCl anions which are weaker Lewis acids. The effect of various equilibrium constants on the total concentration of carbocations as a function of added salt is simulated.
Chain transfer to solvent in two‐state anionic polymerization, 1. Fast exchange between propagating species
Authors: Litvinenko Galina I. , Arest‐Yakubovich Alexander A. 
Published: April 2003

AbstractA theoretical consideration of molecular weights and molecular weight distribution (MWD) of polymers formed in anionic polymerization proceeding via active centres of two different types under conditions of chain transfer to solvent with a fast exchange between propagating species is presented. Analytical expressions for number‐and weight‐average degrees of polymerization are obtained. Expressions for Pn and Pw are shown to be the same as in a one‐centre process with the apparent intensity of chain transfer proportional to the weight fraction of the polymer formed via “transferring” centres. The polymers formed possess a moderately wide unimodal MWD. The dependence of the polydispersity index on the effective intensity of chain transfer goes through a maximum; for M0/I0 = 103 the maximum value of Pw/Pn is ca. 4,6. The method is suggested for the estimation of the relative reactivity in chain propagation of two active centres from the dependence of molecular weight on initiator mixture composition. The effects of association of active centres on the average molecular weights are analyzed. The case when one of the centres is dormant is also considered.
Chain transfer to solvent in two‐state anionic polymerization, 2. Slow exchange between propagating species
Authors: Litvinenko Galina I. , Arest‐Yakubovich Alexander A. 
Published: April 2003

AbstractThe effect of slow exchange between growing chains on the average degree of polymerization (DP) and the molecular weight distribution (MWD) of polymers formed in a two‐state polymerization accompanied by chain transfer to solvent is considered. For the case when centres of only one type are capable of chain transfer it is shown that the exchange can be regarded as fast or slow, depending on the ratio of characteristic times of exchange (τex) and chain transfer (τtr) for these centres. The conditions for fast exchange, when the MWD of the polymer becomes independent of the rate of exchange, are derived. Under otherwise the same conditions, polymer of the narrowest MWD is formed if the exchange is fast, the number‐average DP being independent of the exchange rate. With increasing ratio τex/τtr, the weight‐average DP of the polymer increases and the MWD changes from unimodal to bimodal. For the case when “non‐transferring” centres are dormant, it was also found that at slow exchange the number‐average DP is greater and the MWD is less broadened than at fast exchange.
Simulations of excluded volume effects due to chain thickness and solvent quality on the chain vector distributions and mean coil dimensions
Authors: Cifra Peter , Bleha Tomáš 
Published: April 2003

AbstractLattice simulations of chains in good, theta and poor solvents were performed with an explicit incorporation of the excluded volume (chain thickness) into the model. The scaling power law for chain dimensions was found to apply in the theta and athermal states and to fail in the region below the theta state where collapsed chains with voids inside a globule occur. The solvent‐induced globule‐to‐coil transition is transparently manifested by changes in the chain vector distribution functions. In the region of small and medium chain elongations, the simulation data can be fitted in the theta solvent by a Gaussian distribution function and in the athermal solvent by a function due to Oono et al. The stored elastic energy of the chains with fixed ends was calculated as a function of the chain‐end displacement for fully flexible molecules in all three types of solvent and for poly(methylene).
Characteristic ratio of the mean‐square end‐to‐end distance of poly(vinylidene fluoride) with microstructural defects
Authors: Zhou Zhiping , Yan Deyue , Alfonso Giovanni C. 
Published: April 2003

AbstractThe segment length distribution of isoregic head‐to‐head and tail‐to‐tail sequences in poly‐(vinylidene fluoride) (PVF2)System. name: poly(1,1‐difluoroethylene).
chains is calculated from 19F NMR literature data. It is found that the average length of inverse segments is very close to one unit; therefore almost all head‐to‐head defects are immediately repaired by an adjacent tail‐to‐tail addition. The role of microstructure on the characteristic ratio of the end‐to‐end distance is then investigated. Results obtained by two different methods indicate that, in agreement with previous Monte Carlo calculations, defects play a minor role for the conformational characteristics of PVF2. However, a slight contraction of about 4% is expected for very defective chains.
The kinetics of free radical polymerizing systems at low conversion, 3. On the variation of the termination rate coefficient with monomer and with temperature
Authors: Russell Gregory T. 
Published: April 2003

AbstractThis paper shows that variations with temperature and with monomer of overall termination rate coefficients, denoted 〈kt〉, are not what they seem. This is important because such variations, as determined experimentally, are usually taken as making direct statements about the actual termination reaction. For example, it has always been assumed that the activation energy of 〈kt〉, signified Ea (〈kt〉), reflects the activation energy of the rate determining step behind the termination reaction. However it is shown that purely as a consequence of termination rate coefficients being chain length dependent, this is not so. In fact, simply given chain length dependent termination, Ea (〈kt〉) will be a function of the means of initiation, of the initiator itself, of the initiator concentration [I], and of whether [I] is held constant or varied with temperature. These results not only suggest interesting experiments for microscopic probing of the termination process, but they also help explain the wide variety of experimental Ea(〈kt〉) found in the literature. Much literature data concerning 〈kt〉 is also made sense of in the second half of this paper. Here variations of 〈kt〉 with monomer are examined, and it is shown that in order to explain these, it is likely that variations of rates of propagation and of transfer are often of major importance. Also considered are the effects on 〈kt〉 of diffusion coefficient variations, these of course being directly related to the rate coefficients of the actual termination process. Interesting results, ones relevant to experimental data interpretation, again emerge. For all these calculations a termination model appropriate for very low conversion bulk and solution polymerizations is used, and steady state 〈kt〉 computed. However the principles that emerge are general, and their relevance is contingent only upon termination being chain length dependent. It is therefore to be envisaged that these principles will assist in explaining far more actual polymerization data than has been looked at here.
The extension of the “restricted” penultimate scheme of binary copolymerization to ternary systems
Authors: Olaj Oskar Friedrich , Schnöll‐Bitai Irene 
Published: April 2003

AbstractThe so‐called “restricted” penultimate scheme of binary copolymerization — first presented by Fukuda et al. — is extended to ternary systems without introducing extra parameters not inherent in the three binary systems which form the ternary systems. The crucial assumption is that the restricted penultimate effect is only dependent (1) on the type of monomer forming the terminal radical and (2) the monomer replacing this type of monomer in the penultimate unit, but is independent of the monomer to be added. Accordingly, the scheme is fully described by quantities taken from the binary copolymerizations forming the ternary system. This general principle is still valid when the number of different monomers in the copolymerization system is even increased beyond three. Comparison between predicted values of the phenomenological propagation constant k̄p with the few experimental data available for terpolymerization systems indicates a satisfactory agreement.
Fourier‐transformation of model chain pair configurations and their analysis in fourier‐space
Authors: Zifferer Gerhard , Olaj Oskar Friedrich 
Published: April 2003

AbstractLight scattering experiments may be used to determine a lot of quantities characteristic of a polymer. Numeric analysis of calculated scattering intensities of ensembles of polymer model chains in dilute solution (taking into account intramolecular as well as intermolecular interferences) according to the familiar methods and comparison of the results to the respective quantities evaluated directly in real space allows a check of the liability of data extracted from light scattering experiments. It turned out that the mean‐square radius of gyration as well as the excluded volume (which may be regarded as the second moment of the pair distribution function) are nearly perfectly reproduced. Higher moments, however, are subject to some error which is due to the characteristic changes in size, shape and orientation of polymers caused by intermolecular interaction which are not accounted for in the theories forming the basis of the analysis of light scattering data.
Theory of the orientational ordering for dimers with conformational flexibility
Authors: Mercurieva Anna , Birshtein Tatiana 
Published: April 2003

AbstractIn the self‐consistent field approximation a theory of the orientational ordering is developed for the melt of dimers containing two mesogenic fragments capable of conformational reorganization. Discrete conformations of the dimers (rotational isomers) are characterized by the values of valence angles. Both three‐dimensional and two‐dimensional melts are considered. It is shown that the presence of bent isomers in the melt reduces the temperature of the phase transition to the anisotropic phase. For a three‐dimensional system with first‐order phase transition to the ordered state, the jump of the order parameter at the transition point for dimers with conformational flexibility appears to be greater than for rigid dimers. For dimers with two isomers (a linear and a bent one) the order parameter and the statistical weight of the linear isomer at the transition point depend nonmonotonously on the statistical weight of this isomer in the isotropic phase. The effect of “supercooling” of the isotropic phase for linear conformations of flexible dimers is discussed.
Distributions of the unperturbed dimensions of Markov‐chain copolymers
Authors: Jung Bernd 
Published: April 2003

AbstractThis investigation shows that Markov‐chain copolymers can be regarded as random copolymers the segment lengths of which depend on the copolymerization parameters. It was possible to derive simple analytical formulae for the mean‐square end‐to‐end distance (〈r2〉), the Kuhn length, and the distribution of r2 under theta‐conditions. The results of these equations are in excellent agreement with data from simulations. It is shown that 〈r2〉 as well as the non‐uniformity of r2 increase strongly with increasing probabilities of homopropagation, i.e., with increasing mean homosequence lengths. Furthermore it is demonstrated by simulation that even chains of identical length and composition show a distribution of r2 because of different arrangements of the sequences inside the chains. For chains or chain segments shorter than the average homosequences, a double‐peak distribution of r2 is found. The equations derived in this paper can be applied to real copolymers as well as to chains the curvature of which is altered locally by the association of ligands.
Monte Carlo simulation of freely jointed chains with variable bond lengths
Authors: Reiter Johannes , Duering Edgardo R. 
Published: April 2003

AbstractMonte Carlo simulation of freely jointed off‐lattice chains with variable bond length is usually done with local random displacements of beads and with reptation moves (displacements of a bead along a chain). In dense systems, the acceptance ratio of reptations decreases strongly with density. We discuss versions of reptation moves, which are effective in dense systems. The idea, which comes from lattice systems, is to use a pseudovacancy (walker), which has the same size as a bead of a chain. The walker is attached to a neighbor chain and then another bead of that chain is cleaved. This is equivalent to a reptation move and a nonlocal displacement of the walker and since no free volume is needed, the move can be used with advantage in dense systems. A related technique are cooperative motions, which were introduced by T. Pakula for lattice models, where several chains change their conformation concomitantly. Such cooperative loops are implemented in the Monte Carlo algorithm by creating a temporary walker by cleaving a bead from a chain, moving it with reptations and finally annihilating the walker by attaching it to the same chain it was cleaved from. These moves and the condition of detailed balance are discussed in detail. As an example, we study the integrated autocorrelation time τint for the radius of gyration for a two‐dimensional system. For reduced densities larger than 0,4, we find that with standard reptations and local bead displacements τint increases strongly with density. If reptations with either a permanent or a temporary walker are used in addition to local moves, the integrated autocorrelation time changes only very little with density and very dense systems can still be simulated efficiently.
Modelling the free‐radical polymerization of methyl methacrylate over the complete range of conversion
Authors: Panke Dietrich 
Published: April 2003

AbstractA model is proposed for the radical‐induced polymerization of methyl methacrylate which combines concepts given in the literature. Consecutive steps for diffusion and reaction are used for both the propagation and the termination process, and the termination at higher degrees of conversion is dominated by reaction diffusion as described by Buback. The diffusional contributions to the processes are semi‐empirically based on the free‐volume theory, following a suggestion made by Marten and Hamielec, and the diffusion of the macroradicals is assumed to depend on the instantaneous molar mass (as representative for their chain length) and on the cumulative molar mass (representing the matrix of the dead polymer). With one fixed and four adjustable parameters, the model successfully not only describes the bulk polymerization but also takes account of the presence of solvent, chain transfer agent and pre‐polymer.
A new kinetic model for the accumulation of poly(3‐hydroxybutyrate) in alcaligenes eutrophus, 1. Granule growth
Authors: Kurja Jenci , Zirkzee Hennie F. , De Koning Gertjan M. , Maxwell Ian A. 
Published: April 2003

AbstractA new kinetic model is proposed for the accumulation of poly(3‐hydroxybutyrate) (PHB) in Alcaligenes eutrophus. The model developed is closely related to the descriptive model of Harkins for the emulsion polymerization process. The PHB accumulation is discussed in terms of polymerization events such as initiation, propagation and termination, taking into account the heterogeneous nature of the system. Experimental results on the PHB accumulation in Alcaligenes eutrophus can be described in both a quantitative and qualitative manner by the model. Further, it is shown that the decrease in the rate of polymerization during the PHB accumulation can be explained by coalescence of the granules. The decrease of the molecular weight of the polymer formed during the accumulation process can be fully explained in terms of the kinetic mechanisms of progapation and chain transfer, and the effect of granule size on these.
Bifurcational analysis of the isotropic‐nematic phase transition of rigid rod polymers subjected to biaxial stretching flow
Authors: Rey Alejandro D. 
Published: April 2003

AbstractA bifurcational analysis is performed on Doi's equation of nematodynamics that describes the non‐equlibrium isotropic‐nematic phase transition of rigid rod polymers in the presence of steady biaxial stretching flow. The symmetry of the flow and of the governing order parameter equations are shown to be the source of a rich bifurcation, symmetry breaking, and multistability behavior involving two physically equivalent biaxial nematic phases, one uniaxial nematic phase and one uniaxial paranematic phase. According to the relative intensity of the nematic ordering field and stretching rate, the uniaxial isotropic‐biaxial nematic transition may be continuous (2nd order), discontinuous (1st order), or it may exhibit a tricritical non‐equilibrium phase transition point. The solutions to the Doi equations of nematodynamics are found to be consistent with those of Khokhlov and Semenov [Macromolecules15, 1272 (1982)], which are based on a version of the Onsager theory of isotropic‐nematic phase transitions. The present simulations provide a useful guide for orientation control in biaxial stretching flows.
Strain effects on the microphase separation in interpenetrating polymer networks and cross‐linked polymer mixtures
Authors: Stepanow Semjon , Frisch Harry L. 
Published: April 2003

AbstractWe study the microphase separation in cross‐linked polymer mixtures and interpenetrating polymer networks by taking into account the coupling between the fluctuations of the order parameter and the strain tensor. We predict that thermal fluctuations of the order parameter decrease the compression and shear moduli and that compression of the sample decreases the critical temperature of microphase separation. We also compute the scattering function in the case of shear and uniaxial deformations.
Molecular dynamics treatment of torsional interactions accompanied by dissociation
Authors: Tuzun Robert E. , Noid Donald W. , Sumpter Bobby G. 
Published: April 2003

AbstractWhen three atoms in a bend sequence move into a linear configuration, any torsional angle involving those three atoms becomes undefined and its Cartesian derivatives singular. In molecular dynamics simulations, this is most likely to occur during and after a bond‐breaking process, leading to spuriously large torsional forces and energies which invalidate the simulation. By including appropriate bend angle switching functions in torsional interaction terms, one may approach the linear bend limit in a well‐defined manner and make torsional forces vanish around this limit, as expected on physical grounds. Although this expense can be greatly reduced by observing that any internal force must not move the center of mass of the particles involved. Several approximations explored in this paper can reduce this effort even further.
On the contribution of covalent and ionic species to the chain growth. Monte Carlo simulations and theoretical considerations
Authors: Szymanski Ryszard 
Published: April 2003

AbstractContributions of the covalent and ionic species to the chain building in cationic reversible polymerizations are analyzed by means of Monte Carlo simulations and by solving the formulated set of equations. The validity of the equation for the contribution of covalent propagation, derived for reversible polymerization with simplifying assumptions, is discussed. It is shown that repeating units incorporated into the chain by covalent propagations are distributed uniformly only when direct monomer insertion into covalent bonds is not possible, or when intramolecular interconversion of covalent and ionic species is very fast in comparison with propagation.