bluuebunny/crossref_metadata_2025_split huggingface dataset:
Energy gap of alternant carbon nanotubes
Authors: Tyutyulkov Nikolai , Dietz Fritz , Baumgarten Martin , Müllen Klaus 
Published: April 2003

AbstractThe energy spectra of two types of non‐helical carbon nanotubes are investigated theoretically. It is shown that either the topological or the correlational contributions determine the width of the band gap. In some cases (for nanotubes with diameter < 1 nm) the electron correlation changes qualitatively the results for the width of the energy gap ΔE: if in the one‐electron approximation the band width ΔE = 0, the contribution of the electron correlation becomes dominant and increases the band gap ΔE > 0. At small diameter of the nanotubes (D > 1 nm) the ground state of the tubes corresponds to semiconductors with moderate or narrow energy gap, while at D > 1 nm ΔE decreases monotonously. The energy spectra of the planar ribbon polymers are compared with those of the iso‐electronic nanotubes.
Spinodal decomposition in asymmetric polymer mixtures
Authors: Zhang Hongdong , Zhang Jianwen , Yang Yuliang 
Published: April 2003

AbstractWe present a preliminary numerical study of spinodal decomposition in an asymmetric polymer mixture, i.e., of polymer with different chain lengths, in three dimensions with full Flory‐Huggins‐de Gennes free energy, numerically integrating the time evolution equations for the conserved order parameter. For the sake of comparison, we also present a numerical study of the symmetric polymer mixture. The results indicate that the scaled structure factor for the asymmetric polymer mixture is much broader than that of a symmetric polymer mixture. It is interesting that the growth exponents are not symmetric around the critical quench, i.e., growth exponents on the two sides of the critical composition are different. In addition to that, the magnitudes of the pair correlation functions of asymmetric mixtures are very small for x larger than the characteristic domain size rg and the oscillations seen in the symmetric mixture are almost absent. We have attributed this finding to the rough interfaces and broader domain size distribution in the phase separated asymmetric polymer mixtures. Therefore, the simulation reveals that the asymmetry plays an important role for the spinodal decomposition dynamics of polymer mixtures.
Some aspects of theory and simulations of polymers in steady flows
Authors: Carl Wilfried 
Published: April 2003

AbstractWe present a study of various properties of bead spring chains in steady flows. The Langevin equation of the normal modes of the chain is solved by Fourier transformation. From the resulting power spectrum, the autocorrelation functions of all configuration‐dependent quantities can be calculated. In equilibrium, the influence of the bead masses on the short‐time dynamics is discussed. The influence of different flow fields (shear, elongational and Kramers potential flow) on the mean‐square chain dimension is calculated. A comparison with results obtained from non‐equilibrium molecular dynamics and Monte Carlo calculations is made. Finally, the influence of shear flow on the configurational and rheological properties of cyclic polymers and on the excluded volume behavior of linear chains is examined.
On the contribution of different reaction routes to polymer formation in reversible polymerization. Oxepane and 1,3‐dioxolane polymerization cases
Authors: Szymanski Ryszard 
Published: April 2003

AbstractThe analysis of the contributions of different propagation routes in reversible polymerization was performed. It was shown by computer simulations that material and kinetic contributions of the given propagation route can differ. An especially large difference between these quantities was found in the simulated polymerization of 1,3‐dioxolane. This allowed to explain the unexpectedly large difference between the values of the apparent rate constant of propagation and the estimated rate constant of the corresponding elementary reaction of propagation in the polymerization of 1,3‐dioxolane.
Calculation of rotational barriers in amorphous polymers
Authors: Tiňo Jozef , Koreň Igor , Mach Pavol , Urban Ján 
Published: April 2003

AbstractThe Monte Carlo method was used for generation of amorphous polyethylene configurations on a diamond lattice. Chain building was performed on the tetrahedral lattice of edges 36, 62 and 43 Å with periodic boundary conditions imposed.32 chains were generated, each with a length of 100 CH2‐groups (resulting density = 0.81 g·cm−3). Small spherical volumes with a radius of 10 Å were chosen at random from the total volume for the calculation of rotational barriers. The rotating bond was chosen to be close to the center of this sphere. We employed the method of molecular mechanics in order to calculate the rotational barriers. The calculation was made for 578 rotating bonds and the obtained distribution of rotational barriers is approximated by the corresponding Γ‐distribution.
Molecular dynamics simulation of the crystal nucleation behavior of a single chain touching a substrate surface
Authors: Yang Xiaozhen , Qian Renyuan 
Published: April 2003

AbstractMolecular dynamics simulation has been used in exploring the crystal nucleation behaviour of a single chain touching a substrate surface. It shows that a polyethylene chain (980 CH2) changed its overall shape from an isotropic coil to an oriented one in the case of touching a substrate surface of amorphous carbons at 300 K. Most repeats of the chain were aligned and ordered in a zigzag package. Surprisingly, the direction of the package is not parallel to the plane of the substrate, but almost perpendicular to it. This is in accordance with experimental observations.
On the chirality of paper
Authors: ten Bosch Alexandra 
Published: April 2003

AbstractOn a macroscopic level, a paper sheet has a fiber structure similar to the chains of a molecular chiral membrane. The continuum model of membranes based on chiral elasticity can be applied to calculate the spontaneous curl observed in paper sheets. Twist of the paper sheet is shown to occur in the presence of a tilt relative to the sheet geometry of the constituent cellulose fibers. The dependence of the twist angle on the width of the sheet, on external mechanical fields and on the elastic constants of the paper is given.
Apparent glassy dynamics of an adsorbed polymer chain in the bond‐fluctuation model
Authors: Lai Pik‐Yin 
Published: April 2003

AbstractThe dynamics of a single homopolymer chain strongly adsorbed on a flat surface is investigated by Monte Carlo simulation using the bond‐fluctuation model. Previously observed anomalous glassy dynamics at low temperatures [Phys. Rev. E 49, 5420 (1994)] is revisited in detail. We also study the dynamics of a polymer chain confined in a narrow slit of width of a monomer and a modified bond‐fluctuation model in three dimensions to include more relevant bond vectors. We show that the previously observed glassy dynamics is due to an artifact of the bond‐fluctuation model in three dimensions in that it does not cross‐over to the two dimensional bond‐fluctuation model with the correct Rouse dynamics. With the proper inclusion of the extra bond vectors P(2,2,0) and P(3,2,0), no glassy behavior appears for the strongly adsorbed chain.
A theoretical approach to the thermochemistry of radical and anionic polymerizations of various unsaturated monomers
Authors: Leroy Georges , Dewispelaere Jean‐Pierre , Benkadour Hanan , Wilante Claude 
Published: April 2003

AbstractIn this work, we have calculated the thermodynamic parameters of the first steps of the free radical and anionic polymerizations of various unsaturated monomers, using ab initio methods of quantum chemistry. The enthalpies and entropies of polymerization were estimated assuming that they correspond to those of the model reaction A  B(p) + HABAH(p′) → HABABAH(p′) where p and p′ stand for the physical state of the considered species. The enthalpies of polymerization were rationalized using the equation ΔH = −ΔΣ NABEAB + SE(A  B) + SE(HABAH) − SE(HABABAH) where NAB is the number of A  B bonds, EAB the corresponding bond energy, − ΔΣ NABEAB the variation of the sum of the bond energy terms, and SE(X) the thermodynamic stabilization energy of compound X. The preferential mode of polymerization of each monomer was derived from the enthalpies of the initiation and initial propagation steps of the two types of polymerization. Thus, we were able to make some comments concerning the feasibility of the polymerization of the monomers under consideration.
Theoretical considerations on the directionality in polymerization mechanism and polymer type
Authors: Weegels Peter Louis 
Published: April 2003

AbstractThe directionality (Dn) of a polymerization mechanism can be defined as the average orientation of the reaction or the degree of randomness of head‐to‐tail, head‐to‐head, tail‐to‐tail or tail‐to‐head reactions within the polymer at the n‐th monomer or reaction (n ≫ 2). Directionality of a polymer type can be defined as the average orientation inside a polymer. If the directionality is random, Dn = 0.5. For full directionality, Dn = 1. It is demonstrated that these situations correspond to step and chain polymerization, respectively. Directionality can be as important for the functional properties of polymers as length and composition distributions and the functionality of the original monomers.
The molecular structure and polarizabilities of poly(aryl ether ketone)s
Authors: Shibata Mitsuhiro , Yosomiya Ryutoku , Ke Yang‐Chuan , Zhen Yu‐Bin , Wu Zhong‐Wen 
Published: April 2003

AbstractThe molecular structure and polarizabilities of poly(aryl ether ketone)s were investigated with quantum chemical calculations, based on the fully optimized structures of model compounds (S5: ArCOArOArOArCOAr and BS6: ArOArCOArArCOArOAr) for their chain segments. The results showed that the average dihedral angle between their aromatic rings was in the range of 36°−37° relative to the molecular zig‐zag plane. Especially, when a biphenyl group was introduced into the molecular chain of this polymer, its two aromatic rings were distorted by 48.0° to each other. Also, the calculation results of refractive indices for pure crystals obtained by using the structural parameters were in good accordance with those reported in the literature.
Monte Carlo simulation studies of the interfaces between polymeric and other solids as models for fiber‐matrix interactions in advanced composite materials
Authors: Baschnagel Jörg , Binder Kurt 
Published: April 2003

AbstractAs a coarse‐grained model for dense amorphous polymer systems interacting with solid walls (i.e., the fiber surface in a composite), the bond fluctuation model of flexible polymer chains confined between two repulsive surfaces is studied by extensive Monte Carlo simulations. Choosing a potential for the length of an effective bond that favors rather long bonds, the full temperature region from ordinary polymer melts down to the glass transition is accessible. It is shown that in the supercooled state near the glass transition an “interphase” forms near the walls, where the structure of the melt is influenced by the surface. This “interphase” already shows up in static properties, but also has an effect on monomer mobilities and the corresponding relaxation behavior of the polymer matrix. The thickness of the interphase is extracted from monomer density oscillations near the walls and is found to be strongly temperature dependent. It is ultimately larger than the gyration radius of the polymer chains. Effects of shear deformation on this model are simulated by choosing asymmetric jump rates near the moving wall (large jump rate in the direction of motion, and a small rate against it). It is studied how this dynamic perturbation propagates into the bulk of the polymer matrix.
Orientational ordering of a polymer chain in a strong dipole field. Tetrahedral lattice model
Authors: Gotlib Yulii Ya. , Lyulin Sergey V. 
Published: April 2003

AbstractOrientational and conformational properties have been investigated of a rotational isomeric model of a polar polymer chain on a tetrahedral lattice in a strong electric or mechanical field of dipole symmetry. Two types of dipole moment distribution along the chain are discussed: (A) constant signs of longitudinal components of the dipole moments, and (B) alternating signs of the longitudinal components of the dipole moments. The second case represents polymer chains such as \documentclass{article}\pagestyle{empty}\begin{document}$ \rlap{--} ({\rm CH}_{\rm 2} \hbox{---} {\rm CR}_{\rm 2} {\rm \rlap{--} )}_n $\end{document}>/UEQN> when the dipole moments are oriented along the bisector of the CR2‐angle, i.e., normal to the extended trans‐chain conformation. It is shown that only a discrete most probable orientation of the lattice relative to the field should be considered, namely that coinciding with one of the symmetry axes of the lattices. The average dipole order parameter and dichroic functions (quadrupole order parameter) are calculated in a strong external dipole field for unit vectors with different orientation relative to the chain backbone. The quadrupole order parameter for different unit vectors is obtained also as a function of chain elongation. The polarizability induced by an additional weak dipole field is calculated as a function of the magnitude of the strong external dipole field. For the model considered here the order parameters are more strongly influenced by the external field than those for the freely jointed chain (FJC) model having the same distribution of dipole moments along the chain. The orientational ordering of the chain in a dipole field is higher than in a quadrupole field of the same magnitude.
The structure and phase transitions in polymer blends, diblock copolymers and liquid crystalline polymers: The Landau‐Ginzburg approach
Authors: Hołyst Robert , Vilgis Thomas A. 
Published: April 2003

AbstractThe polymer systems are discussed in the framework of the Landau‐Ginzburg model. The model is derived from the mesoscopic Edwards Hamiltonian via the conditional partition function. We discuss flexible, semiflexible and rigid polymers. The following systems are studied: polymer blends, flexible diblock and multi‐block copolymer melts, random copolymer melts, ring polymers, rigid‐flexible diblock copolymer melts, mixtures of copolymers and homopolymers and mixtures of liquid crystalline polymers. Three methods are used to study the systems: mean‐field model, self consistent one‐loop approximation and self consistent field theory. The following problems are studied and discussed: the phase diagrams, scattering intensities and correlation functions, single chain statistics and behavior of single chains close to critical points, fluctuations induced shift of phase boundaries. In particular we shall discuss shrinking of the polymer chains close to the critical point in polymer blends, size of the Ginzburg region in polymer blends and shift of the critical temperature. In the rigid‐flexible diblock copolymers we shall discuss the density nematic order parameter correlation function. The correlation functions in this system are found to oscillate with the characteristic period equal to the length of the rigid part of the diblock copolymer. The density and nematic order parameter measured along the given direction are anticorrelated. In the flexible diblock copolymer system we shall discuss various phases including the double diamond and gyroid structures. The single chain statistics in the disordered phase of a flexible diblock copolymer system is shown to deviate from the Gaussian statistics due to fluctuations. In the one loop approximation one shows that the diblock copolymer chain is stretched in the point where two incompatible blocks meet but also that each block shrinks close to the microphase separation transition. The stretching outweights shrinking and the net result is the increase of the radius of gyration above the Gaussian value. Certain properties of homopolymer/copolymer systems are discussed. Diblock copolymers solubilize two incompatible homopolymers by forming a monolayer interface between them. The interface has a positive saddle splay modulus which means that the interfaces in the disordered phase should be characterized by a negative Gaussian curvature. We also show that in such a mixture the Lifshitz tricritical point is encountered. The properties of this unusual point are presented. The Lifshitz, equimaxima and disorder lines are shown to provide a useful tool for studying local ordering in polymer mixtures. In the liquid crystalline mixtures the isotropic nematic phase transition is discussed. We concentrate on static, equilibrium properties of the polymer systems.
On phase equilibria, interfacial tension and phase growth in ternary polymer blends
Authors: He David Qiwei , Kwak Soonjong , Nauman E. Bruce 
Published: April 2003

AbstractA gradient squared free energy functional of the Landau‐Ginzburg type is combined with Flory‐Huggins theory to calculate minimum domain sizes, concentration profiles and interfacial tensions in ternary polymer blends. The dynamic equations governing spinodal decomposition are linearized to show that the minimum size for growth is identical to the thermodynamic minimum on phase volume. It is shown that unseparated, third components are enriched at the interface, reduce interfacial tension, increase stability and increase the minimum domain sizes. Enrichment of the third component at the interface causes concentrations at the major components to lie outside their binodal limits at a distance from the interface. Although the effects are most pronounced when the third component is a compatibilizer, the general phenomena remain true even when the third component is relatively incompatible. Generalizations to blends of N components are presented, and a robust method for calculating multicomponent phase diagrams is described.
Influence of polymolecularity on the second virial coefficient of polymers, 2
Authors: Jung Bernd , Bareiss Rolf E. 
Published: April 2003

AbstractIt is shown that the recently developed scaling theory for the second virial coefficient, A2, of dilute solutions of polymers with non‐uniform molecular weight distributions in a good solvent is able to explain in principle all seemingly contradictory observations reported in the literature, in contrast to all other theories proposed so far.
Modeling of molecular weight distribution in pulsed laser free‐radical homopolymerizations
Authors: Buback Michael , Busch Markus , Lämmel Ralf A. 
Published: April 2003

AbstractThe use of pulsed lasers to induce radical polymerization with a subsequent analysis of the polymer molecular weight distribution (MWD) through size exclusion chromatography (PLP‐SEC method) revolutionized the determination of propagation rate coefficients kp. In this paper the program package PREDICI is applied to the simulation of PLP‐SEC orginated MWDs. The comparison with SPEEDUP‐simulations from Hutchinson et al. Shows that both programs are equally well suited for the calculation of the MWD of PLP samples. The emphasis is on the study of the effects that the experimental parameters, i.e. the primary radical concentration in a specific experiment, the gradients of this radical concentration, and above all the instrumental broadening of SEC analysis, have on the measured MWD, and therewith on the kp determination via PLP experiments. By detailed consideration of a styrene bulk polymerization experiment at 70°C and ambient pressure, it is shown that the PLP‐SEC technique at primary radical concentrations between those given by the limiting cases of the low and high termination rate limits and with simulatioenous high quality of the SEC (low broadening) is very well suited for the kp determination. As the MWD of samples from PLP is influenced in a rather complex fashion by the kinetics of the polymerizing system, by the choice of the experimental parameters determining laser‐induced primary free radical concentration, and by the performance of the SEC device, it appears highly recommendable to accompany PLP studies by simulation.
Computer simulation of solutions of telechelic polymers with associating end‐groups
Authors: Khalatur Pavel G. , Khokhlov Alexei R. 
Published: April 2003

AbstractWe present the results of numerical Monte Carlo simulations of solutions of telechelic chains with strongly attracting end‐groups. Formation of micelles (aggregates), their structure and structural characteristics of the system as a whole are studied in detail. The features revealed in computer experiments are qualitatively similar to the recent theoretical predictions. In particular, we show that micelles formed by telechelic chains attract each other even if the solvent is good for the soluble blocks forming micellar shells. As a result, A “micellar gel” structure with a number of chain “bridges” connecting micelles is formed. The bridging chains turn out to be significantly stretched. Furthermore, we observe a pronounced maximum in the wave‐vector dependence of the static structure factor for the associating end‐units which is a manifestation of a quasiperiodic pattern of alternating microdomains consisting of dense micellar cores and the swollen soluble chain blocks.
Dielectric relaxation of polymer networks built from macromolecules with dipole moment directed along the end‐to‐end chain vector
Authors: Gotlib Yuli Ya. , Gurtovenko Andrei A. 
Published: April 2003

AbstractThe dielectric relaxation properties are considered for polymer networks built from polar macromolecules with the dipole moment directed along the end‐to‐end chain vector. The viscoeleastic cubic model of a regular network is used. The fixed average volume of a polymer network is ensured by the effective internal pressure. The dynamic models of polymer networks with external and interchain friction are studied. Two cases are considered: (1) polar chains cross‐linked in a network at their ends, and (2) a densely cross‐linked network with many network junctions per polar chain. The expressions for the autocorrelation functions of the total dipole moment of a network, which determine the dielectric susceptibility, are calculated. The relaxation spectrum of the autocorrelation function consists of two regions: the high‐frequency relaxation spectrum of a chain fragment between two neighbouring junctions (intrachain relaxation spectrum) and the lowfrequency interchain relaxation spectrum. The interchain relaxation spectrum is determined by cooperative motions of chains which form a network. The characteristic time of this spectrum for networks of type (1) is the relaxation time of a chain between junctions τmin. For networks of type (2) a second time scale τ1 exists, which corresponds to motions inside the volume occupied by a single long polar chain included in a network. It leads to different time behaviour of the autocorrelation functions for both network models. The existence of only interchain friction in the network model leads to a cut‐off of the relaxation spectrum at the time τmax depending on the volume of viscous interchain interactions.
Ellipsoidal potential parameterization for bisphenol‐A polycarbonate
Authors: Zimmer Klaus M. , Linke Andreas , Heermann Dieter W. , Batoulis Jannis , Bürger Thorsten 
Published: April 2003

AbstractWe present a coarse‐grained parameterization for the intra‐ and intermolecular potential for bisphenol‐A polycarbonate (BPA‐PC). The parameterization is based on the ellipsoidal‐shaped monomer unit model applied to BPA‐PC. Because of the symmetries of the model a Lennard‐Jones type or even a mere repulsive potential with the usual parameters suffices to model the non‐bonded interactions. We determined the potential parameters from ab‐initio quantum calculations for the BPA‐PC monomer. Results of molecular dynamics simulations using our model compare favourably with experimental data.