bluuebunny/crossref_metadata_2025_split huggingface dataset:
Real‐time AFM study of lamellar growth of semi‐crystalline polymers
Authors: Luo Yan‐Hong , Jiang Yong , Jin Xi‐Gao , Li Lin , Chan Chi‐Ming 
Published: April 2003

AbstractAtomic force microscopy (AFM) has been used to study the lamellar development during the crystallization and melting processes of poly(bisphenol A‐co‐alkyl ether) (BA‐Cn) films. High‐resolution and real‐time AFM phase imaging enables us to observe the detailed growth process of the lamellae. At the early stage of the lamellar growth, embryos appeared firstly and some disappeared on the film surface after a period of time. The stable embryo developed into a single lamella. Then the lamella developed into a lamellar sheaf through branching and splaying. Our results revealed that the branches of the lamellae were formed by induced nucleation and it was also dependent on the crystallization temperature. Real‐time AFM study of the melting, recrystallization and remelting processes of lamellae indicated that the thermal stability of different segments of a single lamella is different and that the thermal stability of the different lamellae is also different even if they develop at the same annealing temperature. The orientation and the development of the lamellae at the characteristic eyes and boundaries of the spherulites are observed in details.
Production of nanocomposites of PET‐montmorillonite clay by an extrusion process
Authors: Sanchez‐Solis A. , Garcia‐Rejon A. , Manero O. 
Published: April 2003

AbstractPolyethylene terephthalate (PET) was mixed by extrusion with clay (montmorillonite) to produce nanocomposites. Additives (pentaerythrytol and maleic anhydride) were added to induce compatibility between the particles and the matrix. Thermal, mechanical and Theological properties of the nanocomposites were evaluated. The nanocomposites with additives presented improved mechanical strength. In shear, the viscoelastic properties decrease with rising particle concentration and additives content. The resulting new materiales present processing advantages and potential improvements in performance.
Amphiphilic and mesomorphic fullerene‐based dendrimers
Authors: Guillon Daniel , Nierengarten Jean‐François , Gallani Jean‐Louis , Eckert Jean‐François , Rio Yannick , del Pilar Carreon Maria , Dardel Blaise , Deschenaux Robert 
Published: April 2003

AbstractGrowing attention is currently devoted to large dendritic structures for applications in nanotechnology and materials science. In this respect, the incorporation of such compounds into thin ordered films appears to be an important issue. One of the most widely pursued approaches to structurally ordered dendrimer assemblies has been the preparation of Langmuir films at the air‐water interface. We report on the case of a diblock globular fullerene‐based dendrimer and show that peripheral substitution of the dendrimer with hydrophobic chains on one hemisphere and hydrophilic groups on the other provides the required hydrophobic/hydrophilic balance allowing the formation of stable Langmuir films. A second approach has been to consider the case of fullerene containing dendrimers terminated by mesogenic groups such as cyanobiphenyl subunits. Whatever the generation is, up to the fourth one, all these compounds exhibit a well‐defined liquid crystalline smectic A phase. The molecular organisation within the smectic layers is found to be monolayered or bilayered depending on the generation. For the smallest dendrimers, the organisation is mainly governed by the size of the fullerene moiety, whereas for the higher ones, it is governed by the interactions between the terminal mesogenic groups. These two approaches appear particularly interesting for functional groups such as fullerenes, which are not well adapted to be organised in nanoscale architectures. The present study shows that fullerenes can indeed be introduced into different types of ordered structure when they have been chemically adequately modified.
Analysis of polyamides by size exclusion chromatography and laser light scattering
Authors: Chen Jinglu , Radke Wolfgang , Pasch Harald 
Published: April 2003

AbstractThe molar mass analysis of polyamides is complicated due to the fact that only a limited range of solvents can be used and association and aggregation phenomena have to be screened by adding electrolytes to the mobile phase. Optimum SEC behaviour is obtained when hexafluoroisopropanol + 0.05 mol/L potassium trifluoroacetate is used as the mobile phase. The calibration of the SEC system can be conducted in different ways. While a calibration with narrow disperse polymethyl methacrylate standards does not yield accurate molar mass information, the quantification can be done using an “artificial” calibration curve. This calibration curve is obtained by correcting the PMMA calibration curve with polyamide molar mass data from light scattering. The resulting molar mass distributions for different types of polyamides are compared with molar masses that are determined by size exclusion chromatography with a light scattering detector and an excellent correlation is obtained.
Triblock copolymers of lactide with poly(ethylene glycol) and influence of stereochemical composition
Authors: Onyari John M. , Huang Samuel J. 
Published: April 2003

AbstractPolymerization of L‐lactide (LLA) and poly(ethylene glycol) (PEG) at 120°C using stannous 2‐ethyl hexanoate results in ABA triblock copolymers in good yields. The glass transition temperature of the copolymers shows strong dependence on the composition. One‐ and two‐phase triblock copolymers can be prepared by controlling the molecular weight of the PEG soft block, as well as the initial LLA/PEG charge ratio. Incorporation of racemic lactide during polymerization drastically affects the crystallinity of the block copolymers obtained. 1H and 13C (solid state) NMR are powerful tools for characterization of the polymer materials.
13C NMR studies of olefin copolymer microstructure: a tool for understanding active center behavior in ansa‐zirconocene‐based catalysis
Authors: Sacchi Maria Carmela , Forlini Fabrizio , Losio Simona , Tritto Incoronata , Locatelli Paolo 
Published: April 2003

AbstractIn the present paper the effects of the solvent, a Lewis base and of non‐polymerizable olefin on copolymerization of propene with higher α‐olefins performed with isospecific ansa‐zirconocene catalysts are investigated. A wide range of solvents with increasing polarity, donor character and steric hindrance has been tested, and their effects on copolymer yield, composition and microstructure thoroughly analyzed. Our results demonstrate that the solvent can have a significant influence on the copolymer composition, even though the solvent polarity is not the relevant factor. Polymerizations carried out in coordinating solvents, e.g. aromatic solvents, lead to the formation of products with considerably decreased higher α‐olefin content. The reduced incorporation of the higher α‐olefin is explained in terms of competition between the nucleophilic medium and the olefin monomer for coordination to the active polymerisation site. The dependence of polymer microstructure on the solvent nucleophilicity is further elucidated by copolymerization studies performed in the presence of diisopropyldimethoxysilane as a Lewis base. The addition of the non‐polymerizable olefin, 2‐methyl‐1‐pentene, which does not significantly affect comonomer composition, seems to slightly modify comonomer distribution. The sum of the results obtained give us valuable information regarding the mechanism of polymerisation at the active centers.
Monte Carlo simulation of tetrahedral chains, 5. The pivot‐algorithm for non‐athermal linear and star‐branched polymers
Authors: Zifferer Gerhard 
Published: April 2003

AbstractBy use of the pivot algorithm, star‐branched chains with F = 3–12 arms of length n, nF = 480, and linear chains (F = 2) are generated on a tetrahedral lattice. In order to simulate different qualities of the solvent, specific short‐range interactions are taken into account. Whereas in athermal systems a new configuration — which is obtained by rotating that part of an arm which contains the chain end around a randomly selected bond by ± 120° — is accepted if it is self‐avoiding, for non‐athermal systems the Metropolis‐Rosenbluth criterion with respect to energy must be satisfied in addition. Calculating the energy of the configuration, nearest‐neighbour interactions (each contact contributes an energy Φ·kT, Φ < 0 characterizing endothermal solutions and Φ > 0 exothermal ones) are considered only; no energy is introduced to distinguish between trans and gauche bonds. A rather quick response of chain properties to the variation of thermodynamic conditions is demonstrated. The average acceptance fraction f̄ has its maximum value for athermal conditions, slightly decreases for exothermal conditions and strongly decreases with increasingly negative Φ‐values. However, for Φ = −0,5 — representing a solution near theta‐conditions — still 25% of attempted moves are accepted for all F‐values examined. The dependence of global properties (characterized by the mean‐square radius of gyration) on Φ and F is in full accordance with most Monte Carlo results.
Simulation of pseudostationary radical polymerization, 1. Chain‐length distributions for arbitrary initiation profiles
Authors: Olaj Oskar Friedrich , Zifferer Gerhard 
Published: April 2003

AbstractA procedure is developed which allows to treat arbitrary periodic initiation profiles (asymmetric and symmetric triangle profiles, sinusoidal profiles, Gaussian profiles etc.) in pseudostationary radical polymerization. Using an iterative method these profiles are transformed into the (likewise periodic) radical profiles and into the chain‐length distributions of the resulting polymer in case of termination by disproportionation. These distributions are analysed for the position of their inflection points which may be used for experimental determination of the elementary rate constant of chain propagation kp. It turned out that for all profiles that have at least one discontinuity (e.g. asymmetric triangle profiles) the position of the point of inflection is a correct measure of kp for a conveniently wide range of experimental parameters. In case of profiles without discontinuity (symmetric triangle profiles, sinusoidal and Gaussian profiles) the position of the inflection point is shifted to lower values which means that the kp values determined on this basis will be a little too small. In most cases, however, the error introduced by this fact will not exceed the overall error of the experiment so that in practice the method of determining kp in pseudostationary polymerization is not restricted to those profiles which exhibit discontinuities.
The kth radius of gyration for a polycondensation reaction with identical structural units AaBb
Authors: Li Ze‐sheng , Ba Xin‐wu , Sun Chia‐chung , Tang Au‐chin 
Published: April 2003

AbstractFor a polycondensation reaction with identical structural units AaBb (i.e. each unit has a functional groups of type A and b of type B) the kth radius of gyration \documentclass{article}\pagestyle{empty}\begin{document}$ \begin{array}{*{20}c} {\left\langle {R^2 } \right\rangle _k = \sum\limits_n {n^k \cdot \left\langle {R_n^2 } \right\rangle P_n } } & {({\rm sum over all }n{\rm - mers; see Eqs}{\rm . (11) and (17))}{\rm }} \\ \end{array} $\end{document} is investigated. By means of a differential method, a recursion formula that holds true for both the pre‐gel and the post‐gel state is obtained for the evaluation of the kth radius.
Application of the Marquardt‐Levenberg procedure to the determination of discrete relaxation spectra
Authors: Carrot Christian , Guillet Jacques , May Jean‐François , Puaux Jean‐Pierre 
Published: April 2003

AbstractThe application of the Marquardt‐Levenberg procedure for the determination of discrete relaxation spectra from dynamic mechanical experiments on molten polymers is reviewed. It allows the calculation of a minimum number of relaxation modes, relaxation times and contribution of each time to the modulus in the terminal zone. All parameters are kept freely adjustable. The Marquardt‐Levenberg procedure enables the minimisation of the χ2 function between the measured values, G′(ω) and G″(ω), and those obtained by the simulated spectrum. The number of relaxation modes is smoothly increased until no improvement on the minimum value of χ2 is observed. The criterium to stop the iterative procedure can be the appearance of physically improper values (negative values), but it is shown that it is better to stop the increase of the number of modes before the problem becomes ill‐posed. This can be done by a comparison between the uncertainties on the calculated parameters and the values of these parameters. The uncertainties can be calculated by the Marquardt‐Levenberg algorithm at each step of the calculation. The method has been tested on its ability to recover a given spectrum from model data and on a set of real experimental measurements on a monodisperse polystyrene. The calculated spectra enable calculation of the various rheological parameters. The agreement between the calculated values of G′ and G″ obtained from the recovered spectra, and the experimental ones is very good and the relative errors are very low (less than 4%). Furthermore, the calculated rheological parameters are found to be very close to those that can be evaluated from the experimental curves.
Generalized statistical gelation theory
Authors: Rolfes Heidi , Stepto Robert F. T. 
Published: April 2003

AbstractGel points in random polymerizations of the general type ΣiRA + ΣjRB in which A‐groups react with A‐ and B‐groups, and B‐groups react only with A‐groups are considered. (The symbols Σi and Σi signify that the A‐ and B‐bearing reactants RA and RB can be mixtures of monomers of different functionalities, denoted generally as fai and fbj.) The usual case of A‐groups reacting only with B‐groups is a special case of the present theory. The effects of chemical kinetics, the competitive reaction of A‐ and B‐groups, are separated from the generalized statistical condition for gelation. The former are used to define reaction curves and the latter, gelation curves. Both types of curve are represented as pa as a function of pb. For a given polymerization, gelation occurs when the reaction curve and the gelation curve intersect. When A‐groups react only with B‐groups, the gel points are those for the usual type of ΣiRA + ΣjRB polymerization, and, in the limit of A‐groups only reacting with A‐groups, the gel points are those for ΣiRA self polymerizations.
Quantum chemical approach to the mechanism of reactions in CH3Li/n CH2CHCN systems
Authors: Yakimansky Alexander V. , Erussalimsky Boris L. 
Published: April 2003

AbstractThe electronic and geometric structures of the CH3Li/acrylonitrile (AN) and CH3Li/2AN systems were investigated by the semiempirical quantum‐chemical MNDO1MNDO: Modified neglect of differential overlap.
method. For the CH3Li/AN system a fundamental difference was found between the predictions of the most favourable reaction path (initiation or termination of polymerization) based on the comparison of two intermediate complexes (IC) on the one hand and of two transition states (TS) on the other hand. The lower energy of the TS of initiation reaction as compared to the energy of the TS of termination reaction correlates with the experimentally observed predominance of the initiation reaction. In contrast to this, a comparison of the two IC energies shows no preference of one of the reactions. In the case of the CH3Li/2AN system, a comparison of different IC by their energies shows qualitative agreement with experimental data. The results of the MNDO calculations were verified by ab‐initio STO‐3G and 3‐21G calculations. The well‐known enhancement of the termination reactions of anionic active sites (AS) of AN in the stage of trimer AS was interpreted on the basis of geometric and electronic structures of the Li(CH2)5CN model active sites investigated by the MNDO method.
Evidence for the time‐temperature superposition principle from Monte‐Carlo simulations of the glass transition in two‐dimensional polymer melts
Authors: Wittmann Hans‐Peter , Kremer Kurt , Binder Kurt 
Published: April 2003

AbstractThe bond fluctuation model on a square lattice with a bond‐length dependent potential exhibits in simulations of slow cooling a kinetic glass transition where the system falls out of equilibrium. Extending previous work, the relaxation functions of gyration radius and end‐to‐end distance, and the bond autocorrelation function of the polymers are presented and related to the time‐dependent displacements of inner monomeric units and center of gravity of the whole chains, respectively. Over a wide temperature range the data can be collapsed on master curves satisfying the time‐temperature superposition principle for Rouse dynamics.
Angular correlations within coiled polymer molecules
Authors: Bruns Wolfgang 
Published: April 2003

AbstractMonte Carlo techniques were used to investigate angular correlations between the principal axes of inertia and the end‐to‐end vector as well as the first two normal coordinates. On the average, the first normal coordinate is nearly parallel to the longest principal axis. The parallelism is less distinct in the case of the end‐to‐end vector and the longest principal axis. The assumption made by some authors that the second normal coordinate is coplanar with the two longest principal axes could not be confirmed.
The configurational free energy of a polymer chain
Authors: Ganazzoli Fabio , Allegra Giuseppe , Colombo Emanuele , De Vitis Mario 
Published: April 2003

AbstractThe configurational, or elastic, free energy Ael of a polymer chain is discussed in terms of the Fourier configurational approach. The importance of accounting for all degrees of freedom of the chain is shown in comparison with affine mean‐field theories and with scaling theories of chain expansion and contraction. In case of strong contraction the chain does show neither affinity nor self‐similarity, and we get Ael ∼ N1/3, N being the number of chain bonds. Conversely, in case of good‐solvent expansion we find Ael ∼ N. The same result holds in the vicinity of the Θ‐temperature, where Ael is also proportional to [(T − Θ)/T]2.
Existence of nonlinear resonances in polyethylene
Authors: Noid Donald W. , Sumpter Bobby G. 
Published: April 2003

AbstractIn this paper, we demonstrate that the internal dynamics and vibrational spectra of polyethylene are influenced by Fermi and other nonlinear resonances. In previous work, the first semiclassical theory and quantum numbers (in parabolic coordinates) appropriate for this type of resonance were developed. We find from detailed molecular dynamics simulations that the same types of quantum numbers are applicable for polyethylene stretch‐bend mode interactions. In addition, a one‐to‐one resonance is found to exist from one local CH stretch to another CH stretch along the polymer chain.
Theory of nematic networks with finite extensibility
Authors: Rusakov Victor V. , Shliomis Mark I. 
Published: April 2003

AbstractA statistical molecular model of polymer networks, which takes into account inter‐molecular orientational interaction and constancy of the chain contour length, is proposed. A simple formula for the network free energy valid for any deformations is obtained. A system of equations which describes orientational‐elastic interactions in polymer networks is derived. The influence of deformation and degree of cross linking on the conditions of network phase transition into the orientationally ordered state of the nematic type is studied. The deformation curves considerably deviate from those predicted by the classical elasticity theory. A comparison of the proposed theory with experimental data for natural rubbers is carried out.
Conformation of 4‐ethoxy‐4′‐biphenylyl cyanide in the isotropic and anisotropic liquid‐crystalline state — a rotational isomeric state simulation of 2H NMR spectra
Authors: Abe Akihiro , Kimura Noritaka , Nakamura Mamoru 
Published: April 2003

AbstractThe conformation of the ethoxy tail flanking the cyanobiphenyl core has been studied in the liquid‐crystalline phase as well as in the isotropic solution. In solution, the vicinal coupling constant 3JCH for the moiety CphOCH (Cph: phenyl carbon) provided the information regarding the rotation around the OC bond. Phenetole was mostly used as a model compound. The conventional rotational isomeric state (RIS) analysis yielded the energy difference between the gauche and trans state to be of the order of 1,6 to 1,4 kcal/mol in CDCl3, C6D6 and dimethyl‐d6 sulfoxide. In this treatment, the rotational state around the neighboring bond (CphO) was taken to occur at 0 or π. The deuterated compound, 4‐ethoxy‐4′‐biphenylyl‐d7 cyanide, was prepared and studied in the liquid‐crystalline state by 2H NMR. The orientational order parameters of the mesogenic core were determined by the analysis of dipolar and quadrupolar splitting data due to aromatic deuterons. The quadrupolar splitting data were also obtained for the deuterated ethoxy tail. The conformation of the tail was elucidated according to the RIS simulation scheme previously proposed. The fraction of the trans conformation ft was estimated to be 0,79 around the nematic‐isotropic transition temperature (TNI = 91,9°C). It was suggested that the conformation of the ethoxy tail remains nearly unaffected by the phase transition in the immediate vicinity of TNI. The fraction ft increases moderately toward an asymptotic value as temperature decreases.
Statistics of stiff polymer chains near an adsorbing surface
Authors: Khokhlov Alexei R. , Ternovsky Felix F. , Zheligovskaya Ekaterina A. 
Published: April 2003

AbstractThe exact solution of the problem of adsorption of a long ideal polymer chain with variable degree of stiffness on a plane surface is presented. It is shown that the adsorption of stiff polymer chains is a second‐order phase transition; in the adsorbed state “train” (i.e. adsorbed) sections are relatively longer and loop sections relatively shorter than for flexible chains. This effect is very pronounced: already for moderately stiff chains the number of Kuhn segment lengths in one “train” section at the temperature T = Tcr/2 (Tcr is the critical temperature for adsorption transition) can reach several thousands, and deviation from the surface occurs only in the form of small “hairpins”. The maximum length of the chain, which at the given conditions would flatten completely on the surface, is estimated.
Viscosity from ternary mixtures of two weakly charged polyelectrolytes in solution
Authors: Borsali Redouane , Rinaudo Marguerite 
Published: April 2003

AbstractThe mode‐mode coupling approximation (MMCA) is applied to describe the behavior of the reduced viscosity ηr from ternary mixtures of two weakly charged polyelectrolytes in solution. Similarly (ε = +1) as well as oppositely (ε = −1) weakly charged polyions are discussed in this work. The variation of ηr with the total polyelectrolyte concentration CT exhibits a peak. The position and the height of this peak show for a given concentration of added salt CS a sensitive variation with: the charge parameters (degrees of ionization) f1 and f2 of both polyelectrolytes, the proportion of polyelectrolyte 1 within the mixture (x = C1/CT) and the Flory interaction parameter χF between monomeric units of different species. This calculation is presented within the framework of the Rouse model where no hydrodynamic interactions are taken into account.