bluuebunny/crossref_metadata_2025 huggingface dataset:
Liquid‐crystalline ordering in polymer brushes
Authors: Birshtein Tatiana M. , Mercurieva Anna A. , Pryamitsyn Victor A. , Polotzkiy Alexei
Published: April 2003

The mean‐field theory of liquid crystalline (LC) ordering is developed for a polymer brush immersed in a solvent. Additional attraction between neighbouring parallel mesogenic segments of the grafted chains is taken into account. It is shown that LC ordering in this brush is connected with the loss of solubility and occurs as a discrete first‐order phase transition which is similar or even identical to the transition in polymer solution.
The peculiar conformational characteristics of cis‐poly(tert‐butlacetylene)
Authors: Ferro Dino R. , Provasoli Augusto , Costa Giovanna
Published: April 2003

Molecular mechanics was applied to investigate the conformational structure of cis‐poly(tert‐butylacetylene) (c‐PTBA). The program CHAMP, adopting a MM2‐based force field, was used for a thorough search of the minimum‐energy conformers of oligomers, helices and long segments of the c‐PTBA chain. The results show that short oligomers are not good models of the polymer, as their preferred conformations are not allowed inside the chain. Segments of 83 and 85 helices appear as the most probable feature. Junctions between right‐ and left‐handed helices can occur at the cost of 1,7 kcal/mol: such defects may run through the chain, the barrier to their shift being 7,8 kcal/mol. Other conformational defects, having energies in the range of 1–3 kcal/mol above the minimum, form knuckle‐joints in the helix, changing the axial direction by 70–80°. The present calculations suggest a disordered model of c‐PTBA in solution, made of rather stiff helical segments. Such a picture corresponds to a chain less rigid than proposed by previous computations on substituted polyacetylenes, and seems consistent with the observed physical aging of c‐PTBA films. Preliminary packing calculations of right‐ and left‐handed helices lead to density values not much higher than observed, indicating rather low degrees of disorder and free volume in the solid state.
Kinetic model of living radical polymerization
Authors: Yan Deyue , Jiang Hong , Fan Xunpei
Published: April 2003

This work studies the kinetics of living radical polymerization by means of both the nonsteady state approach and the quasi‐stationary state method. Expressions for the numberand weight‐average degress of polymerization and the polydispersity index were derived. Numerical results show that the concentration of residual initiator seriously influences the polydispersity index of the resulting polymer. The calculated outcomes of the non‐steady state approach are evidently different from those of the quasi‐stationary state method when the magnitude of the rate constant of termination is comparable with that of the propagation rate constant, and the difference becomes negligible if the rate constant of the termination (kt) is much larger than that of propagation (kp). The polydispersity index of the resulting polymer increases with decreasing ratios of kt to kp or MO to IO (initial concentrations of monomer and initiator).
Theory and mathematical models of ion retention by watersoluble polymers during membrane filtration
Authors: Spivakov Boris Ya. , Shkinev Valerii M. , Golovanov Vladimir I. , Bayer Ernst , Geckeler Kurt E.
Published: April 2003

Theoretical approaches to the description of ion retention in aqueous solution by soluble polymers during membrane filtration are proposed. Examination of the chemisms responsible for inorganic ion complexation and retention by membrane‐based separations helps to select appropriate hydrophilic polymers and optimum chemical conditions for the process. The ion separation and enrichment data may be formally explained by using mathematical models based on the theory of kinetics of the reactions in partially open systems.
The simulation of molecular weight distributions in polyreaction kinetics by discrete Galerkin methods
Authors: Wulkow Michael
Published: April 2003

This article describes the development of a comprehensive solver for the differential equations arising from the modeling of molecular weight distributions in polyreactions. Based on a series of numerical developments, the software package PREDICI combines new directions in mathematics, chemical computing and implementation. The algorithm is based on a so‐called discrete Galerkin h‐p‐method, which allows the efficient treatment of numerous polymerization reaction types. The abilities of the new concept are demonstrated on challenging examples.
A theoretical investigation of the production of branched copolymers in continuous stirred tank reactors
Authors: Baltsas Apostolis , Achilias Dimitris S. , Kiparissides Costas
Published: April 2003

A comprehensive mathematical model for free‐radical copolymerization reactions has been developed for a homogeneous continuous stirred tank reactor. The present model is based on a fairly general copolymerization scheme accounting for the formation of linear and branched copolymer chains. Both chain transfer to polymer and terminal double bond reactions are considered in order to predict the long chain branching frequency. Changes in molecular weight, composition and degree of branching occurring during the copolymerization reaction are modelled using the method of moments. To break‐down the dependence of the moment equations on higher order moments two different closure methods are considered. The predictive capabilities of the model are examined in relation to the solution copolymerization of methyl methacrylate with vinyl acetate. It is shown that both chain transfer to polymer and terminal double bond reactions significantly contribute to the broadening of the molecular weight and degree of branching distributions. Furthermore, the terminal double bond reaction effects significantly the copolymer number‐average molecular weight and the concentration of terminal double bonds.
Chain collapse by lattice simulation
Authors: Tanaka Genzo , Mattice Wayne L.
Published: April 2003

Monte Carlo simulations have been performed on a self‐avoiding simple cubic lattice chain with the nearest‐neighbor interactions for a range of chain lengths N from 40 to 1000 segments to investigate equilibrium properties of polymer chains from an athermal to a collapsed state. Both the fraction of segments in the clusters and the number of contacts exhibit the three stage process for the chain collapse, consistent with our previous molecular dynamics simulations of a fully atomistic chain. In the collapse region corresponding to the nearest‐neighbor interaction parameter larger than 0.5 for a segment‐solvent pair, polymer chains are quite spherical and both ends lie nearly randomized within the sphere. The peak height of the specific heat is proportional to N(In N)3/11, as predicted by the renormalization group theory.
Theoretical investigation of the molecular structure of aluminium triisopropoxide and its complexes in ring‐opening polymerization
Authors: Parenté Vincent , Brédas Jean‐Luc , Dubois Philippe , Ropson Nathalie , Jérôme Robert
Published: April 2003

We present a theoretical study on aluminium triisopropoxide (Al(OiPr)3) using both empirical (Molecular Mechanics, MM, with Dreiding II force field) and quantum‐chemical (Austin Model 1, AM1, semiempirical Hartree‐Fock) techniques. We determine the most stable geometries for both the tetramer and trimer of aluminium triisopropoxide as well as the thermodynamic characteristics of the equilibrium existing between these two aggregated structures. The theoretical results are compared to experimental data from X‐ray diffraction and 27A1 NMR measurements. For the tetramer, it appears that the optimal equilibrium geometries are in good agreement with the experimental X‐ray diffraction geometry; another geometry is also obtained with both theoretical approaches, which is slightly less stable but of higher symmetry. On the basis of the most stable configurations for the tetramer and trimer aggregates, the variation of free enthalpy (ΔG) between the two aggregated structures has been estimated. The evolution of the theoretical ΔG values indicates a displacement of equilibrium towards the trimer species with temperature, in good agreement with experimental 1H and 27A1 NMR data. Moreover, the AM1 heats of formation show a gain of 33.9 kcal/mol due to the aggregation of four A1(OiPr)3 instead of three, and thus a better stability of the tetramer. The molecular geometries being well described by the theoretical methods used in this study, we also present a model for the ring‐opening polymerization complexes of ε‐caprolactone and lactides.
An ab initio and force field study on the conformation and chain flexibility of the dichlorophosphazene trimer
Authors: Jaeger Raimund , Vancso G. Julius
Published: April 2003

Ab initio molecular orbital calculations have been used to study the conformation, valence electron charge density, and chain flexibility of a dichlorophosphazene trimer (CH3[NP(Cl2)]3CH3). The calculations were carried out at the restricted Hartree‐Fock level with the 6–31 G* basis set. The dichlorophosphazene trimer adopts a planar transcis conformation. The valence electron charge distribution indicates strong charge separations along the backbone of the molecule, and is in agreement with Dewar's island delocalization model for bonding in linear and cyclic phosphazenes. In order to determine the height of the torsional barrier (2,5 kcal/mol), the torsional potential of a central PN bond of the trimer was studied with a rigid rotor scan and geometry optimizations of selected rotamers. The flexibility of the PNP bond angle contributes significantly to the chain flexibility. Based on the results of the ab initio calculations, an empirical force field for the dichlorophosphazene trimer was developed. The energy expression includes bond stretch, angle bend, electrostatic, van der Waals, and torsional potential terms. A relaxed scan with the force field achieves good agreement with the ab initio results for the torsional potential in the vicinity of the stable conformation, and an excellent agreement with the ab initio results on changes in the P2N2P3 bond angle and the N1P2  N2P3 dihedral angle during a full rotation around the N2  P3 bond.
Perturbation expansion of macromolecules with realistic pair interactions in three and four dimensions
Authors: Carl Wilfried
Published: April 2003

We calculated the first‐order perturbation expansion of the mean‐square end‐to‐end distance of polymers in three and four space dimensions. The segments of the chain were assumed to interact via a pair potential with a short‐range repulsive and an additional attractive part. For a purely repulsive potential, the well‐known results of the δ‐function‐pseudopotential approach are recovered, whereas the inclusion of an attractive part leads to a non‐vanishing contribution even for a vanishing binary cluster integral.
Aggregation processes in self‐associating polymer systems: A comparative analysis of theoretical and computer simulation data for micelles in the superstrong segregation regime
Authors: Khalatur Pavel G. , Khokhlov Alexei R. , Nyrkova Irina A. , Semenov Alexander N.
Published: April 2003

We present an extension of our previous theory describing aggregation processes in self‐associating polymer systems, i.e., in copolymers with strongly attracting groups. In particular, the formation and properties of micelles are studied in detail for the superstrong segregation regime. We have explicitly taken into account the effect of the radius of attraction, rc, and the structural defects of the micellar core on the micellar shape and stability. It has been shown that for a given aggregation number, Q, the shape of the micellar core is determined by a balance between the interfacial free energy and the energy of structural defects. As the effective energy of attraction increases, the most stable shape of the core can change from spherical to disk‐like (oblate ellipsoid). A transition between spherical and disk‐like core geometries takes place at nearly equal interfacial and defect free energies. In the case of long‐range attraction (i.e., for large rc), we have shown that the surface term must dominate, thus stabilizing a spherical shape of the core. Alternatively, for small, rc, the non‐spherical (disk‐like) geometry of the micellar core becomes most stable. The critical radius of transition, r, theoretically predicted, is in a reasonable agreement with computer simulations presented in our previous paper. If rc is further decreased and the effective energy of attraction is fixed, then the large aggregate can disintegrate and form smaller disk‐like splinters. We have located this equilibrium transition point. The critical value r thus obtained is in very good agreement with molecular dynamics calculations performed by us. At last, the theory predicts the formation of strongly elongated (stripe‐like) micelles in concentrated solutions. Such structures were also observed in our computer experiments.
Quantum chemical studies of propene, ethylene, acetylene and dihydrogen reactivity in the insertion reaction into the titanium‐alkyl bond
Authors: Zakharov Ivan I. , Zakharov Vladimir A. , Zhidomirov Georgii M.
Published: April 2003

Using the ab initio method of SCF MO LCAO

SCF MO LCAO: Self‐consistent field molecular orbital linear combination of atomic orbitals.

in a valency‐splitted basis of the Gaussian functions we have studied the addition of various monomers (C3H8, C2H4, C2H2) and dihydrogen to the titanium‐alkyl bond in the complex H2TiCH3. The structure of transition states in the insertion reaction, heats of π‐complex formation and activation energies for the insertion of the coordinated monomers have been calculated. The calculation results show that the reactivity decreases in the order C2H2 > C2H4 > C3H8 > H2. According to the results obtained, the energy of the π*‐antibonding orbital of monomers can serve as an index of relative reactivity in the insertion reaction into the metal‐alkyl bond.
The incorporation of side reactions into pseudostationary polymerization, 1. The closed solution of a kinetic scheme comprising chain‐transfer to monomer or solvent
Authors: Zifferer Gerhard , Olaj Oskar Friedrich
Published: April 2003

A procedure is developed that allows the calculation of chain length distributions of polymers prepared by periodic modulation of the initiation process, considering termination by disproportionation and chain transfer. For the case of a (pseudostationary) laserpulse initiated polymerization process a closed solution can be derived for the chain length distribution (cld) of dead polymer and its Oth, 1st and 2nd moments. By analysis of the detectability of the “extra‐peaks” appearing in the cld (which represent the key for the determination of propagation constants) as a function of the parameters chosen the optimum conditions can be predicted for experiments.
The incorporation of side reactions into pseudostationary polymerization, 2. The limiting case of a small amount of chain transfer
Authors: Zifferer Gerhard , Olaj Oskar Friedrich
Published: April 2003

Approximate closed expressions for the chain length distribution and the moments of dead polymer prepared by periodic modulation of the initiation process are derived for the limit of a small amount of chain transfer (considering termination by disproportionation). The expressions obtained are given in terms of chain length distribution and moments of a reference system without chain transfer. The range of validity of these approximations is examined for the special case of initiation by laser‐pulses. In addition, the first six moments of the chain‐length distribution of laser‐pulse initiated polymers (in the absence of chain transfer) are given.
Copolymers with controlled distribution of comonomers along the chain, 1. Structure, thermodynamics and dynamic properties of gradient copolymers. Computer simulation
Authors: Pakula Tadeusz , Matyjaszewski Krzysztof
Published: April 2003

Properties of copolymer systems with various composition distributions of comonomers A and B along the chain are analyzed by means of computer simulation using the Cooperative Motion Algorithm. Extreme cases of random and block copolymers, as well as the intermediate cases of gradient copolymers with various composition gradients along the chain are considered. Thermodynamic, static and dynamic properties of systems are predicted in a broad temperature range including both the homogeneous and the microphase separation regime. A possibility to control the microphase separation temperature and consequently a possibility to control a shift of the properties along the temperature scale by changing the monomer distribution within copolymer chains is demonstrated.
Random fluorinated copolymers of tetrafluoroethylene: a study of the inclusion/exclusion of defects from the crystal by conformational and packing energy calculations
Authors: Napolitano Roberto , Pucciariello Rachele
Published: April 2003

A conformational analysis has been performed on the isolated chains of copolymers of tetrafluoroethylene with hexafluoropropene, chlorotrifluoroethylene or perfluoromethyl vinyl ether, in comparison with polytetrafluoroethylene. The lowest energy conformations in accordance with the chain repeating distance of polytetrafluoroethylene in the low temperature crystal phase have been used in packing energy calculations. The results of both conformational and packing energy calculations suggest that the Cl group is easily tolerated in the crystal phase. The —CF3 group could also be tolerated in the crystal phase but at the cost of conformational and crystal lattice deformations. On the contrary, it can be concluded that the —OCF3 group is excluded from the crystal phase because its presence should require high energy values and wide deformations in the unit cell parameters.
On the evaluation of the anisotropic‐isotropic transition volume fraction for a semirigid polymer
Authors: Figari Giuseppe , Costa Camilla
Published: April 2003

It has been found that a method allowing the calculation of an anisotropic‐isotropic transition volume fraction for a given semirigid polymer can be derived from the equations recently proposed by Blonski and coworkers to evaluate the equilibrium compositions of isotropic and anisotropic phases coexisting within ternary systems semirigid polymer/flexible polymer/solvent. By using the term anisotropic‐isotropic transition volume fraction of a semirigid polymer we will refer to its smallest volume fraction giving an anisotropic character to liquid ternary phases also including a flexible polymer and a solvent. Its evaluation simplifies the construction of the corresponding ternary diagram and can also be seen as a fast and quantitative way of estimating the influence exerted by the semirigid polymer, through its intrinsic anisotropy, on ternary phases.
Local chain dynamics of a model polycarbonate near glass transition temperature: A molecular dynamics simulation
Authors: Fan Cun Feng , Çagin Tahir , Shi Wen , Smith Kennith A.
Published: April 2003

Constant pressure constant temperature molecular dynamics method is employed to investigate the atomistic scale dynamics of a model Bisphenol A polycarbonate in the vicinity of its glass transition temperature. First, the glass transition temperature and the thermal expansion coefficients of the polymer are predicted by performing simulations at different temperatures. To explore the significance of different modes of motion, various types of time correlation functions are utilized in analyzing the trajectories. In these nanosecond scale simulations, the motion of the chain segments is found to be highly localized with little reorientation of the vectors representing these segments. Detailed analysis of trajectories and the correlation functions of the backbone dihedrals and side methyl groups indicates that they exhibit numerous conformational transitions. The activation energies of the conformational transitions obtained from the simulation are generally larger than the potential barriers for the rotations of these dihedrals, however, both show the same trend. We also have estimated the phenylene ring flip activation energy as 12.6 kcal/mol and the flip frequency as 0.77 MHz at 300 K. These values fall either fall within the range determined by various NMR spectroscopy experiments or slightly out of the range. The study shows that the conformational transitions between the adjacent dihedrals are strongly correlated. Three basic cooperative modes are identified from the simulation. They are: a positive synchronous rotation of two phenylene rings, a negative synchronous rotation of two phenylene rings, and a carbonate group rotation. Above the glass transition temperature, the large scale cooperative motions become much more significant.
Flow effects in the polymer cyclization reaction
Authors: Kolb Alexander , Marques Carlos M. , Fredrickson Glenn H.
Published: April 2003

We investigate the effect of shear and uniaxial extensional flows on the rate of the cyclization reaction in polymers. We find that for such an intramolecular diffusion‐controlled reaction the flow always reduces the reaction rate, in contrast to the previously studied case of intermolecular reactions, where linear flows enhance the reaction rate.
Macromolecular reaction and interdiffusion in a compatible polymer blend
Authors: Yashin Victor , Kudryavtsev Yaroslav , Govorun Elena , Litmanovich Arkady
Published: April 2003

A theory describing slow macromolecular reaction and interdiffuion in a compatible polymer blend is suggested based on the linear non‐equilibrium thermodynamic principles. A simple model system is considered. In a blend consisting initially of homopolymers A and B, the transformation A → B proceeds with the B units accelerating the reaction. A system of diffusive‐reaction equations for relevant macroscopic variables is derived. The randomness of the reacting chains' structure gives rise to a new interdiffusion mode in addition to the reacting polymer‐homopolymer B interdiffusion. Numerical calculations reveal that the diffusive intermixing of reacting chains of different composition may significantly affect the reaction rate and the local compositional heterogeneity as well. It is possible to discriminate the fast‐ and the slow‐mode theories of interdiffusion using reaction kinetics data. Under certain conditions, the reaction may proceed in a non‐trivial autowave‐like regime.
Molecular modeling of adsorption and ordering at solid interfaces
Authors: Hentschke Reinhard
Published: April 2003

Interfaces between liquids and solid surfaces are of considerable scientific as well as technological interest, in particular in the context of the adsorption and organization of molecular films. In recent years the direct observation of the molecular structure and often even the dynamics of ordered monolayers at such hidden interfaces has been made possible by the rapid development in scanning probe microscopy. Nevertheless, there is still a lack of understanding with respect to the formation and organization of such films and their interaction with the experimental apparatus. Here computer modeling plays an increasing role as both the complexity of the interfaces and the available computer power increase. This article addresses the application of phenomenological molecular modeling to physisorption at solid surfaces with a special emphasis on the liquid‐solid interface. The paper presents an overview over different modeling approaches and illustrates their application in a series of examples ranging from the simulation of adsorption isotherms of small molecules to the prediction of the structure of physisorbed layers for larger molecules.
Molecular force field study concerning the host properties of carbosilane dendrimers
Authors: Lach Christian , Brizzolara Davide , Frey Holger
Published: April 2003

The effect of structural variations at the core of carbosilane dendrimers has been studied using Molecular Dynamics. Several derivatives of the parent dendrimer have been modeled and characterized with respect to the dimensions of the inner cavities which lie on the order of 5–15 Å. The “denseness” of the outer shell has been studied by calculating solvent accessible surfaces and excluded volumes as a function of the radius of the probe sphere. The higher generation dendrimers were found to possess dense outer shells with holes of the order of 2–3 Å. Dendron separation especially with respect to clefts in the molecule is discussed. It was found that in low generation carbosilane dendrimers the dendrons are clearly separated while at higher generations a substantial amount of interpenetration occurs. In addition, results are presented showing that carbosilane dendrimer surfaces are fractal only over a narrow range of length scales. In this range, the fractal dimensions of the surfaces are of the order of 2.1.
Calculation of vapor pressures not requiring the derivatives of the energy of mixing
Authors: Horst Raland
Published: April 2003

A method is presented for the calculation of vapor pressures exclusively on the basis of the energy of mixing, the knowledge of chemical potentials is not required. The only condition used for the calculation is the minimum of the energy of mixing of the overall system in equilibrium. The gas phase is treated as an ideal gas, for the liquid phase no specific thermodynamic description is assumed. The method is demonstrated for a mixture of two solvents and one polymer. The system water/poly(ethylene oxide), the thermodynamics of which are described by an equation that can only be solved numerically thus impeding the calculation of chemical potentials, serves as an example. Interaction parameters are determined by fitting calculated vapor pressures to literature data.
Polymer‐polymer interaction parameters by inverse gas chromatography: A novel molecular interpretation of nonrandom partitioning of solvent probes in polymer blends
Authors: Tan Zhanjie , Vancso G. Julius
Published: April 2003

Non‐random partitioning of molecular probes in a polymeric mixture was examined with Kirkwood‐Buff‐Zimm (KBZ) cluster integrals. Equations were derived to give relationships of thermodynamic quantities that can be obtained by using inverse gas chromatography (IGC). The mer‐mer correlation structure factor S*(AB) (0) in a binary mixture of polymers A and B in terms of the spatial distribution of a probe, S, around a molecule A and a molecule B is discussed. The derivation allows a direct assessment of the correlation structure factor obtained either from scattering experiments or from IGC measurements. The common polymer‐polymer interaction parameter from the two types of experiments is discussed with respect to criticism concerning IGC. The detailed molecular description of the probe behavior in a polymeric mixture at infinite dilution of the probe given in this work can be used in IGC studies of the microstructure of polymer mixtures or other mixtures of liquids. In order to illustrate our approach, the structure factor and the interaction parameter of the blends of polystyrene (A) and poly(butyl methacrylate) (B) were evaluated using IGC results of DiPaola‐Baranyi and Degre.
Probing chain ends in adsorbed polymer layers
Authors: Johner Albert , Joanny Jean‐François
Published: April 2003

We study theoretically the pull‐out of polymer chains from an adsorbed polymer layer by sticking of the chain ends on an opposing surface using scaling arguments and a mean field theory. When only one chain is pulled out from the layer, we extend previous results obtained for a single adsorbed chain and calculate the force necessary to extract the chain from the layer. We then discuss end adsorption from an adsorbed layer of polymers bearing specific end groups onto a second surface. Two bridging regimes are predicted: a diffuse layer regime at weak separations (or/and weak interaction) and a large separation strong interaction regime where the bridges stretch into a brush like structure. Bridging fractions and force profiles are displayed that could be compared to atomic force microscope or surface force apparatus experiments.
On the determination and ramifications of chaos in many‐body systems: A model study of polyethylene
Authors: Newman David E. , Watts Christopher , Sumpter Bobby G. , Noid Donald W.
Published: April 2003

Computational methods for elucidating information related to the dynamical behavior of multidimensional systems are presented and applied in the area of macromolecular dynamics. Results indicate that the dimensionality of chaotic dynamics for a model of polyethylene can only be characterized for a very narrow range of extremely low temperature (0–2 K). These results provide evidence that suggests the dynamics of this system are completely chaotic at temperatures as low as 10 K. The preponderance of increasing density of low frequency floppy vibrational modes in large molecular systems provides a facile mechanism for the onset of global chaos. Ramifications of such global chaos to the accurate modeling and simulation of macromolecular dynamics is discussed.
Conformational transitions of end‐adsorbed triblock copolymers in a nonselective solvent
Authors: Haliloglu Turkan , Mattice Wayne L.
Published: April 2003

Conformational dynamics of triblock copolymers end‐adsorbed onto a surface from a nonselective solvent have been studied by a Monte Carlo lattice simulation technique. The triblock copolymers are A B A with NA = 10 and NB = 5, 10, 20 and 40, at surface interaction parameters ε = −0.5 and −1.0. A is the adsorbing block and B is the nonadsorbing block. The number of chains in the periodic box is varied over the range 100 ≤ n ≤ 500. The triblock copolymer can exist in three states: loop (L), tail (T) and free (F) chain. Fractions of those conformations at different system parameters (n, NB and ε) and the associated lifetimes are calculated from the simulations. A kinetic scheme is constructed and the corresponding transition rate matrix is used in the master formalism equation, the solution of which yields the dynamic modes of the system. A correlation function is defined to produce a single overall view of the rate of transitions between different conformational states. The degree of surface interaction has strong influence on the rate of transitions between the states, in particular, longer average lifetimes and a wider distribution of lifetimes of the loop conformation contribute to the slower decay of the correlation curves. At high surface adsorption, larger NB decrease the rate of all transitions at all values of n. A weak dependence on n is observed for all sizes of NB at both surface energies.
Study of the conformational disorder in polyesters giving liquid‐crystalline phases
Authors: Pirozzi Beniamino , Napolitano Roberto , Tuzi Angela , Centore Roberto
Published: April 2003

Conformational and packing energy analyses have been performed by molecular mechanics calculations on the polyester derived from the condensation of 2‐(4‐hydroxyphenyl)‐6‐hydroxybenzoxazole with terephthalic acid, which presents a high degree of crystallinity in spite of the head‐to‐tail constitutional disorder. The line repetition groups in accordance with the experimental value of the c axis have been taken into account. Possible conformations of chains having constitutional and conformational disorder in the crystalline phase have been proposed. Packing energy calculations performed on pairs of chains give the close packing distances and indicate the best symmetry elements. The results are in agreement with the unit cell obtained by X‐ray data, the thermodynamical properties of the polymer and the existence of liquid‐crystal phases.
Recent advances in polymer molecular dynamics simulation and data analysis
Authors: Tuzun Robert E. , Noid Donald W. , Sumpter Bobby G. , Wozny Christopher E.
Published: April 2003

Significant advances in molecular simulation methodology over the past decade have greatly reduced the traditional size‐timescale bottleneck in molecular dynamics calculations. The development of the geometric statement function method allows for systems up to several hundred thousand atoms to be simulated for up to several nanoseconds in reasonable times on standard workstations. For constant energy simulations, the use of symplectic integrators ensures accurate dynamics, even at long simulation times, without velocity or other artificial rescaling schemes. Finally, new methods of frequency estimation allow for accurate vibrational mode frequency calculations even in the presence of chaotic motion on time scales twenty times shorter than the standard fast Fourier transform, with an additional improvement in the sensitivity of the results when initial dynamics conditions are carefully chosen.
Local relaxation of polymers in dense media: Cooperative kinematics theory and applications
Authors: Bahar Ivet
Published: April 2003

Cooperative kinematics (CK) theory and its recent applications are presented. CK theory has been developed as an efficient approach for predicting the mechanism of segmental relaxation processes in bulk polymers. The theory aims at determining the most probable changes in atomic coordinates, occurring collectively in response to a given, external or localized, structural perturbation. The basic postulate is the minimization of the energy change involved in the overall conformational motion, which naturally yields the optimal pathway of cooperative relaxation. Attention has been confined here to the collective motions accompanying the rotational transitions of backbone bonds in polyethylene (PE) and polybutadiene (PB). The strong dependence of the mechanism of motions on the geometry of the repeat unit and on chain connectivity is emphasized. The differences in the types of correlated transitions operating in different structures, the effective conformational energy changes triggered by bond rotational jumps, and the correlation lengths for particular bond isomerizations are analyzed. The reorientations of CH bond vectors in cis‐ and trans‐PB are also examined to explain the shorter correlation time of cis units, compared to trans, detected by NMR. A good agreement between various CK predictions and results from molecular dynamics (MD) simulations is obtained. The fact that CK calculations are at least two orders of magnitude faster than MD simulations invites attention to the utility of the CK method as an efficient tool for elucidating the pathway of motion in complex systems.
Atomistic simulations of phenoxy ring flips in the glassy region of a semicrystalline polyimide
Authors: Natarajan Upendra , Mattice Wayne L.
Published: April 2003

A quasi‐static atomistic simulation methodology is used to study the 180° (π) flip motion of phenoxy rings in the amorphous glass of a semicrystalline polyimide. The π‐flip is carried out for the rings in various local microenvironments in the polymer. Results show that the activation energy distribution for the flip covers a broad range of values. The energetics, short and long scale matrix movements, and the interrelation between cooperativity and polymer structure are also studied. The results are in qualitative agreement with previous simulation and experimental work on other glassy polymers such as bisphenol‐A polycarbonate and polystyrene. The nature of the movement of the amorphous matrix, in response to the ring flip, is far reaching with greater atomic displacements closer to the phenoxy ring. Intermolecular van der Waals and electrostatic interactions contribute significantly to the energy changes associated with the ring flip. Cooperativity is observed for torsion angles along the repeat unit containing the flipped ring. The mean value of the activation energy compares reasonably well to the experimental value for the flip that has been observed previously through solid‐state NMR experiments.
Liquid‐crystalline ordering in solutions of polyelectrolytes
Authors: Nyrkova Irina A. , Shusharina Nadezhda P. , Khokhlov Alexei R.
Published: April 2003

The isotropic‐nematic phase transition in solutions of stiff‐chain polyelectrolytes is considered on the basis of the Onsager approach for the limiting cases of charged rods and persistent macrolecules. We plotted a full family of phase diagrams for orientational transitions in solutions of polyelectrolytes with any charge density as a function of the low‐molecular‐weight salt concentration. It is shown that charging of macromolecules can lead not only to the renormalization of the chain diameter (discussed by Onsager) or to the twisting effect (predicted by Odijk et al.), but in some cases also to a variety of other regimes, including transitions with very narrow phase separation regions, or with a second orientational transition to a highly oriented nematic phase upon the increase of polymer concentration.
Polymer‐mode‐coupling theory of the slow dynamics of entangled macromolecular fluids
Authors: Schweizer Kenneth S. , Fuchs Matthias , Szamel G. , Guenza Marina , Tang Hai
Published: April 2003

A microscopic statistical dynamical theory of the slow dynamics of entangled macromolecular fluids has been formulated at the level of effective generalized Langevin equations‐of‐motion of a tagged polymer. A novel macromolecular version of mode‐coupling theory is employed to approximately capture the cooperative motions of entangled polymers induced by the long range, self‐similiar interchain correlations. Polymer integral equation methods are used to determine the required equilibrium structural input. Entanglements arise due to time and space correlations of the excluded volume forces exerted by the surrounding matrix on a tagged macromolecule. A spatially resolved description of entanglement constraint amplitudes relates the fluctuating forces to fluid structure. Constraint relaxation proceeds via three parallel processes: probe center‐of‐mass translation and shape fluctuations, and collective matrix relaxation. Asymptotic scaling law predictions for the molecular weight and concentration dependences of transport coefficients and relaxation times of chain polymer solutions and melts are in qualitative agreement with the phenomenological reptation theory. Predictions for finite frequency properties such as anomalous diffusion, and shear stress and dielectric relaxation, are derived. Enhanced, power law dissipation for properties controlled by conformational relaxation is predicted, with the corresponding frequency scaling exponents in good agreement with experiments but differing from reptation behavior. For experimentally accessible chain lengths strong finite size corrections for the transport coefficients arise due to entanglement constraint porosity and constraint release. Successful quantitative applications to many experimental data sets suggest the theory provides a unified microscopic understanding of the non‐asymptotic scaling laws observed for the viscosity, dielectric relaxation time, and solution self and tracer diffusion constants. Generalization to fractal macromolecular architectures allows semi‐quantitative treatment of ring and spherical microgel melts, and tracer diffusion in gels. A theory for the influence of concentration fluctuations in entangled polymer blends and diblock copolymers has also been developed. Self‐diffusion in blends is quantitatively suppressed due to dynamical constraints associated with domain formation. Much stronger suppression of diffusion and chain relaxation is predicted near and well below the order‐disorder transition of diblock copolymer melts due to microdomain formation. New dynamical scaling laws are predicted, and quantitative agreement of the theory with recent measurements on polyolefin diblocks is demonstrated. Limitations of the theory, open problems, and possible future directions are discussed.
A monte carlo lattice study of living polymers in a confined geometry
Authors: Rouault Yannick , Milchev Andrey
Published: April 2003

We investigate the changes in the average chain length of a solution of semi‐flexible living polymers between two hard repulsive walls as the width of the slit, D, is varied. Two different Monte Carlo models, that of the ‘slithering snake’ and of the ‘independent monomer states’ are employed in order to simulate a polydisperse system of chain molecules confined in a gap which is either closed (with fixed total density), or open and in contact with an external reservoir. It appears that the mean chain length L in a state of equilibrium polymerization depends essentially on the geometry constraints for sufficiently small D. We find that in the case of an open slit the mean length L(D) decreases with D → 0 for flexible chains whereas it grows if the chains are sufficiently stiff. As the width of a closed gap D is decreased, in a three‐dimensional gap L(D) gradually decreases for absolutely flexible chains whereas for semi‐rigid chains it goes through a minimum at D = 2 and then grows again for D = 1. In two dimensions, in a closed strip the average chain length L(D) for both flexible and rigid macromolecules goes through a sharp minimum and then grows steeply in compliance with a predicted divergence for semi‐rigid polymers as D → 0. We attribute the observed discrepancies of our numeric experiments with some recent analytic predictions to the ordering effect of container walls on the polymer solution when chain stiffness and excluded volume interactions are taken into account.
Nonlinear least squares fitting applied to copolymerization modeling
Authors: Van Herk Alex M. , Dröge Thomas
Published: April 2003

The choice of the statistical method to determine the reactivity ratios in copolymerization modeling is shown to be very important. Problems in literature, as well as possible pitfalls when using available statistical programs that are in itself correct are pointed out. These problems mainly involve (knowledge of) the error structure, as the error structure determines the weighting scheme of the data points in fitting procedures. A simple, robust, statistically correct non‐linear least squares (NLLS) method is reintroduced which is based on the visualization of the sum of squares of residuals in the so‐called sum of squares space (SSS). The advantages of this method include the fact that the method is easy to understand and can be implemented in simple computer programs, as well as the fact that the method allows important aspects of the error structure to be incorporated. Furthermore, in the SSS the joint confidence interval (JCI) with exact shape can be constructed. The exact shape of the JCI is not always ellipsoidal and following a normal distribution, depending on the linearity of the fitted equations. This can sometimes lead to wrong conclusions.
Iatrogenic complications and risks of nerve conduction studies and needle electromyography
Authors: Al‐Shekhlee Amer , Shapiro Barbara E. , Preston David C.
Published: April 2003

Electrodiagnostic procedures are routinely performed in patients with a variety of neuromuscular disorders. These studies are generally well tolerated and rarely thought to be associated with any significant side effects. However, needle electromyography is an invasive procedure and under certain situations has the potential to be associated with iatrogenic complications, including bleeding, infection, nerve injury, pneumothorax, and other local trauma. Similar complications are possible if needles are used for either stimulating or recording. In addition, like all other electrical devices and monitoring equipment connected to patients, electrodiagnostic testing carries the risk of stray leakage currents that under certain circumstances can result in electrical injury, especially in patients in the intensive care setting. Similarly, certain precautions are required during nerve conduction studies (NCS) in patients with pacemakers and other similar cardiac devices. In this review, we address the known and theoretical complications of NCS and needle electrode examination, and the possible methods to avoid such hazards. Muscle Nerve 27: 517–526, 2003
Gentamicin fails to increase dystrophin expression in dystrophin‐deficient muscle
Authors: Dunant Patrick , Walter Maggie C. , Karpati George , Lochmüller Hanns
Published: April 2003

A recent report that aminoglycoside antibiotics restored the expression of functional dystrophin to skeletal muscles of mdx mice, a model of Duchenne muscular dystrophy (DMD), raised hopes that DMD may be treatable by a conventional drug. Subsequently, several human trials were initiated for evaluating gentamicin therapy in selected DMD patients. An increase of dystrophin expression was not detected in one human trial that was fully reported. Here, we report that we were unable to replicate previously published beneficial results by gentamicin treatment in the mdx mouse. Therefore, we believe that additional animal experimentation is required to further evaluate the possibility of in vivo aminoglycoside therapy of DMD. Muscle Nerve 27: 624–627, 2003
Clinical and genetic analysis of CMT1B in a Nigerian family
Authors: Kakar R. , Ma W. , Dutra A. , Seltzer W.K. , Grewal R.P.
Published: April 2003

We report a Nigerian family with a late‐onset autosomal dominant neuropathy consistent with Charcot‐Marie‐Tooth disease. Electrophysiological examination of the index patient confirmed a severe demyelinating neuropathy with secondary axonal features. Sequence analysis of the myelin protein zero (MPZ) gene identified a C‐to‐G transversion at nucleotide position 234, resulting in a serine‐to‐tryptophan mutation in codon 78 (S78W) of the translated protein. The presence of this novel missense mutation suggests a diagnosis of Charcot‐Marie‐Tooth disease type 1B. Our study confirms the worldwide distribution of this disorder and extends the genetic spectrum of mutations in the MPZ gene. Muscle Nerve 27: 628–630, 2003
Dendritic cells in hyperplastic thymuses from patients with myasthenia gravis
Authors: Nagane Yuriko , Utsugisawa Kimiaki , Obara Daiji , Yamagata Munehisa , Tohgi Hideo
Published: April 2003

To investigate the role of dendritic cells (DCs) in the hyperplastic myasthenia gravis (MG) thymus, we studied the frequency and distribution of three mature DC phenotypes (CD83+CD11c+, CD86+CD11c+, and HLA‐DR+CD11c+) in samples from patients with MG whose symptoms dramatically improved following thymectomy and in non‐MG control thymuses. In hyperplastic MG thymuses, mature DCs were much more numerous in nonmedullary areas, such as the subcapsular/outer cortex; around the germinal centers; and in extralobular connective tissue, particularly around blood vessels. Mature DCs strongly coexpressed CD44 and appeared to be components of a CD44–highly positive (CD44high) cell population migrating from the vascular system. Furthermore, in the hyperplastic MG thymus, the expression of secondary lymphoid‐tissue chemokine (SLC) markedly increased especially around extralobular blood vessels, where the CD44high cell population accumulated. These findings suggest that DCs may migrate into the hyperplastic thymus from the vascular system via mechanisms that involve CD44 and SLC. DCs may present self‐antigens, thereby promoting the priming and/or boosting of potentially autoreactive T cells against the acetylcholine receptor. Muscle Nerve 27: 582–589, 2003