bluuebunny/crossref_metadata_2025 huggingface dataset:
Water‐soluble polymeric materials with the ability to bind metal ions
Authors: Rivas Bernabé L. , Pereira Eduardo
Published: April 2003

Water‐soluble polymers containing carboxylic acid, phosphonic acid, and sulfonic acid groups were investigated as polychelatogens by means of the liquid phase polymer‐based retention technique, LPR, under different experimental conditions. The retention profiles were determined and the maximum retention capacity (MRC) of these polymers was determined at different pH and polymer‐metal ion ratio. The metal ions investigated were: Ag(I), Cu(II), Co(II), Ni(II), and Cr(III). An important effect of the pH and the ligand type was observed on the metal ion retention. The MRC values of the divalent metal ions were very similar and higher than that for the trivalent cation, indicating that the polymer‐metal ion interaction was basically of an electrostatic type.
Electron spin resonance studies of reversible addition‐fragmentation transfer polymerisation
Authors: Calitz F.M. , Tonge M.P. , Sanderson R.D.
Published: April 2003

The polymerisation of styrene in the presence of cumyl dithiobenzoate (RAFT agent) was studied. Electron spin resonance spectroscopy was used to determine the time‐dependence of the concentrations of the intermediate radical species, as a function of temperature and relative initial amounts of reactants. The intermediate radical concentration was found to be much lower than predicted by the studies of Barner‐Kowollik et al.[Barner‐Kowollik, C.; Quinn, J. F.; Morsley, D. R.; Davis, T. P. J. Polym. Sci. Part A: Polym. Chem. 2001, 39, 1353], but consistent with that observed by Hawthorne et al.[Hawthorne, D. G.; Moad, G.; Rizzardo, E.; Thang, S. H. Macromolecules 1999, 32, 5457] and Kwak et al.[Kwak, Y.; Goto, A.; Tsujii, Y.; Murata, Y.; Komatsu, K.; Fukuda, T. Macromolecules 2002, 35, 3026] Several observations that are not consistent with the current RAFT model were made. Intermediate radical signals were observed long after a significant amount of initiator remained and an extra signal appeared in the ESR spectrum at longer reaction times.
RAFT polymerisation in water‐borne organic dispersions
Authors: Tonge M.P. , McLeary J.B. , Vosloo J.J. , Sanderson R.D.
Published: April 2003

Two different approaches to achieving controlled reversible addition‐fragmentation chain transfer polymerisation in water‐borne dispersions have been attempted, both giving reasonable control over molar mass evolution. Molar mass increased linearly with conversion, and the colloidal instability that has sometimes been observed in similar systems (e.g. [de Brouwer, H.; Monteiro, M. J.; Tsavalas, J. G.; Schork, F. J. Macromolecules 2000, 33, 9239]) was avoided. The similarity of results for the two quite different approaches can largely be reconciled utilising the theory of Luo et al.[Luo, Y.; Tsavalas, J.; Schork, F. J. Macromolecules 2001, 34, 5501]

The average number of radicals per particle, in terms of both propagating and RAFT intermediate radicals were compared, using two current models in the literature for the ratio of propagating to intermediate species. [Barner‐Kowollik, C.; Quinn, J. F.; Morsley, D. R.; Davis, T. P. J. Polym. Sci. Part A: Polym. Chem. 2001, 39, 1353; Monteiro, M. J.; de Brouwer, H. Macromolecules 2001, 34, 349; Kwak, Y.; Goto, A.; Tsujii, Y.; Murata, Y.; Komatsu, K.; Fukuda, T. Macromolecules 2002, 35, 3026] Possible implications for water‐borne dispersions under such conditions are that the maximum value of the average number of propagating radicals per particle may be significantly lower than 0.5, depending on the dominant mechanism for radical loss.
Propene copolymers: an overview of recent research
Authors: van Reenen Albert J.
Published: April 2003

In recent years, some of the focus in our laboratories has been on the use of metallocene catalysts, mostly in conjunction with methylalumoxane as cocatalyst, in the polymerisation of, inter alia, propylene. This paper reviews the main areas of research either already published or about to be published. In particular, the area of propylene copolymers is addressed. The copolymers of propylene with the linear α‐olefins C4 ‐ C10 and C12, C14 and C18; the branched olefins 4‐methyl‐1‐pentene, 4,4 dimethyl‐1‐pentene, 3‐methyl‐1‐butene and 3‐methyl‐1‐hexene; and other olefins (3‐(1‐adamantyl)propene and 1,5 hexadiene) are briefly discussed and relevant results highlighted. The effect of comonomer on the reaction mechanism with both syndiospecific and isospecific metallocene catalysts is also discussed.
Theory of static scattering from weakly charged copolymers in solution
Authors: Benmouna Mustapha , Vilgis Thomas A. , François Jeanne
Published: April 2003

The static scattering properties of weakly charged copolymers in solution are investigated. The cases of symmetrical and nonsymmetrical diblocks are considered and various structure factors calculated. The electrostatic interaction is introduced via a combination of the random‐phase approximation and the Debye‐Hückel approximation. Several cases of charge distributions are considered. In particular, diblock copolymers made of neutral and charged monomers and oppositely charged monomers are considered. All the cases deal with systems in the homogeneous disordered phase where no mesophase structures or micelle formations can be observed.
Statics and dynamics of single chains in polymer blends and copolymer melts — results beyond the random phase approximation
Authors: Vilgis Thomas A. , Benmouna Mustapha
Published: April 2003

We present new results on the behaviour of single chains in homo‐and copolymer blends which are beyond the rendom phase approximation (RPA). The radius of gyration of a chain in the blend was found to depend strongly on the interactions as well as the effective diffusion constant of a single chain in the blend or copolymer melt. Note that these results cannot be obtained in the original formulation of the RPA. They are based on the effective Hamiltonian of the chain in the medium. In the equation found for the self diffusion constant of a single chain, Ds, the second term is directly the value of the effective potential at zero wave vector. Thus the measurement of Ds would detect the screened potential at q = 0. Experimental verification of these results is in progress.
Wetting transitions of asymmetric binary polymer mixtures
Authors: Chen Zheng Yu , Noolandi Jaan
Published: April 2003

We examine the effect on surface‐wetting phase transitions in polymer mixtures when the degrees of polymerization of the two components are different. It has been demonstrated by Schmidt and Binder (J. Phys. (Paris) 46, 1631 (1985)) that in a symmetric polymer mixture a second‐order wetting transition occurs if the wall‐polymer interaction f(ϕ0) has a negative curvature f″(ϕ0), where ϕ0 is the surface volume fraction of the polymer component preferred by the wall. We found that in an asymmetric mixture this is not necessarily the case.
A kinetic tree‐like model of non‐linear alternating copolymerization of monomers with non‐equal reactivities of functional groups and substitution effect
Authors: Galina Henryk , Kaczmarski Krzysztof , Para Beata , Sanecka Bożena
Published: April 2003

A kinetic model is presented that can be used to predict moments of size distribution of polymer species in the system where a tri‐functional monomer with groups of different reactivity (such as glycerol) reacts irreversibly with a bi‐functional monomer with no cycle formation. A Smoluchowski‐like equation is used to follow the time evolution of the size distribution of polymer molecules up to the gel point. The results are confronted with calculations made by using the statistical cascade theory. The two methods of calculations give slightly different results. The reasons for the differences are discussed.
Chain linear dimensions in the surface‐enriched layer of polymer mixtures
Authors: Wang Jian‐Sheng , Binder Kurt
Published: April 2003

We calculate the mean‐square end‐to‐end distances and mean‐square gyration radii using the bond fluctuation model for a binary polymer blend in the presence of a wall by Monte Carlo simulation. In the bulk, the size of the minority, low‐concentration polymer species is compressed compared to the majority one. In the vicinity of the wall, where the minority polymer concentration is enriched due to attraction from the wall, the dimensions of the two types of polymers are approximately equal and are essentially the same as in an athermal polymer melt. Thus, the geometric constraint is more important to the structure of the polymers than the polymer‐polymer and polymer‐wall interactions.
Collapse of polyelectrolyte networks induced by their interaction with an oppositely charged surfactant. Theory
Authors: Khokhlov Alexei R. , Kramarenko Elena Yu. , Makhaeva Elena E. , Starodubtzev Sergei G.
Published: April 2003

A very simple theory of swelling and collapse of weakly charged polyelectrolyte networks in the solution of an oppositely charged surfactant has been developed. The following contributions to the free energy were taken into account: free energy of volume interaction and of elastic deformation of the network chains, free energy connected with micelle formation and free energy of translational motion of all mobile ions in the system (translational entropy). Both the cases of a solution of charged surfactant and that of a mixed solution of charged and neutral surfactant components have been taken into account. It has been shown that the behaviour of the network depends on the total surfactant concentration in the system and corresponds to one of the three following regimes: At low concentration, micelles inside the network are not formed and the behaviour of the polymer network is similar to that of a network swelling in the solution of a lowmolecular‐weight salt (regime 1). In the second regime, surfactant concentration inside the network exceeds the critical micelle concentration and micelles are formed; in this regime the network collapses because surfactant molecules, aggregated in micelles, cease to create “exerting” osmotic pressure in the network sample. In the third regime, at very high surfactant concentration, formation of additional micelles inside the network ceases, and the network dimensions coincide with those of the corresponding neutral network.
Kinetics of polymer analogous reactions
Authors: Szymanski Ryszard
Published: April 2003

The kinetics of reactions of polymer units depending on the closest vicinity is analyzed. The derived equation can be applied to transformations of homopolymers into copolymers. It directly allows (by fitting of the calculated kinetic curve to experimental data) to determine the corresponding rate constants, provided the analyzed reaction is zero or pseudo‐zero order with respect to the transformation reagent (and first order with respect to the polymer units). The Proposed procedure of numerical calculations allows to adopt the derived equation to other systems. It is shown in the Appendix that the distribution of blocks in the resulting copolymer is not Bernoullian, contrary to previous claims or assumptions.
Kinetics theory of anionic copolymerization with constant monomer ratio, 1. Sequence distribution
Authors: Hu Xiaodong , Yan Deyue
Published: April 2003

The chain microstructure of copolymers generated via anionic copolymerization with constant monomer ratio is studied theoretically by using the linear operator technique to solve the kinetic differential equations. The microstructural parameters for copolymers, such as the functions of sequence distributions, average length of the sequence and fractions of dyads and triads, are derived and further connected with initial conditions and reaction time so that one can predict the chain microstructure for copolymers from the polymerization conditions.
Fractal dimensions of polymeric phthalocynines
Authors: Knothe Gerhard
Published: April 2003

As a result of their structure, polymeric phthalocyanines have four fractal dimensions for every size/shape/dilation combination. The construction of polymeric phthalocyanines resembles known fractals. A structure model of the polymer is extended. For infinitely large polymeric phthalocyanines, the same limiting value for the fractal dimensions (D = 2) was found to be valid for all structures.
Shear of a planar polyelectrolyte brush
Authors: Birshtein Tatyana M. , Zhulina Ekatarina B.
Published: April 2003

The behavior of a planar polyelectrolyte brush subjected to normal and tangential external forces is considered. A new “polyelectrolyte effect” is predicted: shear of a free polyelectrolyte brush leads to a decrease in brush thickness contrary to the case of a free neutral brush. Such behavior is equivalent to that of a neutral brush subjected to an external normal stretching force. In the case of a polyelectrolyte brush this force is created by the osmotic pressure of mobile counterions neutralizing grafted chain charges. Addition of salt diminishes the polyelectrolyte effect and changes the sign of correlation between brush thickness and tangential deformation.
Liquid‐crystalline ordering in two‐dimensional systems with discrete symmetry
Authors: Mercurieva Anna A. , Birshtein Tatyana M.
Published: April 2003

The mean‐field theories of liquid‐crystalline (nematic) ordering developed for three‐dimensional systems are applied to describe two‐dimensional systems of both geometrically anisotropic and anisotropically interacting particles. Systems with discrete symmetry (lattice models) for which long‐range order is possible are considered on the base of the Landau free‐energy expansion. It is shown that the Hamiltonian describing the energy of intermolecular interactions may be written in a common form for lyotropic and thermotropic systems. The mean‐field theory gives a continuous phase transition (second‐order phase transition) for a square lattice, whereas for a triangular lattice it gives a phase transition with latent heat (first‐order phase transition) like for three‐dimensional systems. These results are compared with results of the exact theories (two‐dimensional Ising and Potts models). It is concluded that for realistic two‐dimensional models the orientational in‐plane ordering is not sharper than a second‐order phase transition.
Multiblock copolymers as polymeric surfactants: are “pancakes” better than “dumbbells”?
Authors: Noolandi Jaan
Published: April 2003

It is suggested that multiblock copolymers which form a pancake at the interface between two incompatible homopolymers would be more efficient as compatibilizers than di‐ or triblock copolymers. Less of the multiblock copolymers would be lost into the bulk phases as micelles or mesophases than di‐ or triblock copolymers if some degree of irregularity is included in the multiblock architecture, either in block length or chemical sequence distribution, without compromising the thermodynamic driving forces which localize the copolymers at the interface. This molecular design strategy is aimed at reducing the ability of the surfactant to form a separate phase, thereby increasing the interfacial activity in a nonequilibrium system with a large interfacial area. The multiblock loops which would preferentially orient into the most compatible homopolymer phases would have to exceed a certain critical size in order to avoid the entropic penalty of having an extremely flat pancake, and to provide good adhesion between the bulk phases.
Conformational analysis and electronic structure calculations of S‐1,2‐bis(2‐dibutenoyl)glycerol and S‐1,2‐bis(2,4‐hexadienoyl)glycerol, two analogues of 1,2‐bis(2,4‐octadecadienoyl)‐sn‐glycero‐3‐phosphorylcholine
Authors: Faupel Peter , Buß Volker
Published: April 2003

As a model for 1,2‐bis(2,4‐octadecadienoyl)‐sn‐glycero‐3‐phosphorylcholine, a doubly unsaturated membrane‐forming lipid molecule, force‐field (MMP2) calculations were performed on S‐1,2‐bis(2‐dibutenoyl)glycerol, and CNDO/S‐calculations2

CNDO: Complete neglect of differential overlap, a semi‐empirical quantum‐mechanical method.

on the derived minimum‐energy conformations of S‐1,2‐bis(2,4‐hexadienoyl)glycerol. The energy hypersurface especially with respect to the dihedral angles along the C(1)‐C(2) and the two CO ester bonds was explored and the rotational strength as a function of these angles was calculated. The two gauche‐forms were found to be most stable, with a slight preference for the g−‐form. The experimental circular dichroism data obtained for 1,2‐bis(2,4‐hexadienoyl)‐sn‐glycero‐3‐phosphorylcholine, the corresponding phosphorylcholine, indicate a dynamic equilibrium between two forms of opposite chirality possibly involving the g+‐and the g−‐forms.
Intramolecular phase separation of a copolymer chain with mobile primary structure
Authors: Dormidontova Elena E. , Grosberg Alexander Yu. , Khokhlov Alexei R.
Published: April 2003

The phenomenon of intramolecular phase separation of a single copolymer chain with mobile primary structure (i.e., a polymer chain with reversibly adsorbable ligands) in dilute solution is investigated on the basis of a Flory‐type interpolation theory of the coil‐globule phase transition. It is shown that under certain critical conditions the stability of the homogeneous primary structure is violated and intramolecular phase separation takes place. Phase diagrams for the cases of rigid and flexible chains are calculated. The main characteristics of a polymer chain in the two‐phase state (such as fraction of monomeric units, swelling coefficient and fraction of units with ligands for coexisting intramolecular phases) are investigated.
Molecular arrangements and conformations of liquid unbranched alkanes in narrow slits
Authors: Vacatello Michele , Auriemma Finizia
Published: April 2003

Realistic, atomistic models of liquid tridecane in broad slits (>3 nm) and in narrow slits of thickness 1,2 nm and 1,0 nm have been obtained using the Monte Carlo technique. The setup of the models is such that the molecules in the slits are in equilibrium with the bulk liquid. The surfaces of the plates are modelled as two‐dimensional arrays of hexagonally packed units having the same size and interaction parameters of a methylene group. The regions adjacent to the plates in slits with thickness > 3 nm are characterized by a well defined tendency to form partially ordered layer structures, while molecules at a distance from the plates larger than 1,5 nm are unperturbed. The simultaneous presence of two plates increases the tendency to form layer structures when their distance is 1,2 nm, while this tendency is almost totally destroyed when the slit is squeezed down to a thickness of 1,0 nm. This is also associated with a 10% decrease of the density in the latter slit.
Diffraction modelling of poly(vinyl chloride): An approach to the crystallinity enigma
Authors: Hobson Rachel J. , Windle Alan H.
Published: April 2003

This paper demonstrates that changes which are seen to occur in experimental X‐ray diffraction patterns of poly(vinyl chloride) (PVC) with varying sample tacticity are indicative of a crystal structure in which both isotactic and syndiotactic sequences are incorporated within the same lattice. The isotactic sequences are in a conformation first suggested by Juijn et al. which enables them to emulate the shape of the syndiotactic chain. We present a short summary of molecular and diffraction modelling results which show how the intensities of the 200 and 110 peaks change with crystallite tacticity. The results provide a rational explanation for the long standing enigma that the observed crystallinity of atactic PVC is too high to be accounted for in terms of crystallization of syndiotactic sequences only.
Thermodynamics of swelling of latex particles with two monomers: A sensitivity analysis
Authors: Maxwell Ian A. , Noel Lilian F. J. , Schoonbrood Harold A. S. , German Anton L.
Published: April 2003

A sensitivity analysis is performed to determine at what conditions the simplified model for swelling of latex particles by two monomers or two solvents is valid. This model proposes that, inter alia, the fractions of two monomers in the latex particles and in the monomer droplets are equal. The model is a simplified version of that proposed by Ugelstad et al. and results from the use of three assumptions, namely (1) the molar volumes of the monomers are equal, (2) the interaction parameters between the two monomers are equal and (3) the interaction parameters between each of the monomers and the polymer are equal. It is shown by simulation that these assumptions are of algebraic necessity only. The simple model is always valid when utilizing realistic values for all the appropriate parameters for partially water soluble monomers that swell the polymer.
Free‐radical multicomponent polymerization reactors and chemical composition distribution
Authors: Engelmann Uwe , Schmidt‐Naake Gudrun
Published: April 2003

Formulae to calculate the statistically caused instantaneous copolymer composition distribution as well as the chemical distribution of accumulated macromolecules, which is due to polymerization statistics and shifts in mean polymer composition during the reaction process, are derived on the basis of a universal model for free‐radical solution polymerization with any number of monomers proceeding in a batch, semi‐batch or continuous, ideally mixed vessel. The influence of the reactor type on chemical composition distributions is investigated for a copolymerization of different reactive components (methyl methacrylate/styrene/maleic anhydride), a system with nearly equal reactive monomers (methyl methacrylate/styrene), and the ternary polymerization of methyl methacrylate/styrene/maleic anhydride. Though products of constant mean composition are obtainable in a semi‐batch or steady‐state continuous reactors, considerable statistical dispersion cannot be removed in any case.
Short‐chain behaviour of star‐branched nonreversal random walks embedded in a tetrahedral lattice
Authors: Zifferer Gerhard
Published: April 2003

Analytical results are presented for the mean‐square dimensions of star‐branched nonreversal random walk polymers on a tetrahedral lattice considering short‐chain effects and the influence of the core (centre) of the star. They are checked against numerical results obtained from highly precise Monte Carlo calculations. Short‐chain effects are studied in detail. In addition, the deviation from proportionality between mean‐square dimensions and the total chain‐length is evaluated and compared to that for self‐avoiding chains and stars near theta‐conditions, the total chain‐lengths ranging from 125 to 3845 segments.
Static and dynamic molecular mechanics modelling and X‐ray scattering calculations for a cyclic siloxane macromolecule
Authors: Pachter Ruth , Bunning Timothy J. , Crane Robert L. , Adams W. Wade , Socci Edward P. , Farmer Barry L.
Published: April 2003

A liquid crystalline system consisting of cyclic penta(methylsiloxane) cores with biphenylyl 4‐propoxybenzoate mesogens is studied with respect to its molecular conformation and the intermolecular ordering of pendant groups by employing molecular mechaincs calculations, while a molecular dynamics simulation indicates the degree of conformational flexibility. The calculated X‐ray scattering patterns for these structures provide insight into the origin of the experimentally observed results. A good agreement is found between the calculated and experimental reflections data, supporting the ‘cylinder’ global topology as the basis for forming a ‘columnar’ structure. Further, a comparison of the calculated and experimental meriodional sections shows a strong similarity of both the meridional intensities and d‐spacings.
Application of conservation law to NMR measurements of poly(propylene) stereoregularity
Authors: van der Burg Marijke W. , Chadwick John C. , Sudmeijer Olof , Tulleken Herbert J. A. F.
Published: April 2003

A new method is presented for data reconciliation in the 13C NMR determination of the pentad distribution in regioregular poly(propylene). Deviations of experimentally measured data from mathematically derived conservation laws are corrected by a maximum‐likelihood method. In this method some assumptions are made about the structure of the erros to obtain better estimates of the correct data. The validity of this procedure was demonstrated with Monte Carlo simulation, after which the method has been applied to real 13C NMR data to show what corrections may be obtained. Conservation laws for hexads and heptads are derived as well, for possible future use.
Modelling free‐radical copolymerization kinetics—evaluation of the pseudo‐kinetic rate constant method, 2. Molecular weight calculations for copolymers with long chain branching
Authors: Xie Tuyu Y. , Hamielec Archie E.
Published: April 2003

The full moment equations and equations using pseudo‐kinetic rate constants for binary copolymerization with chain transfer to polymer in the context of the terminal model have been developed and solved numerically for a batch reactor operating over a wide range of conditions. Calculated number‐ and weight‐average molecular weights (M̄n and M̄w) were compared with those found using the pseudo‐kinetic rate constant method (PKRCM). The results show that the weight‐average molecular weights calculated using PKRCM are in agreement with those found using the method of full moments for binary copolymerization when polymeric radical fractions φ1˙ and φ2˙ of type 1 and 2 (radical centers are on monomer types 1 and 2 for a binary copolymerization) are calculated accounting for chain transfer to small molecules and polymer reactions in addition to propagation reactions. Errors in calculating M̄w using PKRCM are not always negligible when polymer radical fractions are calculated neglecting chain transfer to small molecules and polymer. In this case, the relative error in M̄w by PKRCM increases with increase in monomer conversion, extent of copolymer compositional drift and chain transfer to polymer rates. The errors in calculating M̄w, however, vanish over the entire monomer conversion range for all polymerization conditions when chain transfer reactions are properly taken into account. It is theoretically proven that the pseudo‐kinetic rate constant for chain transfer to polymer is valid for copolymerizations. One can therefore conclude that the pseudo‐kinetic rate constant method is a valid method for molecular weight modelling for binary and multicomponent polymerizations.
Isomerism and symmetry of bridged polymeric phthalocyanines
Authors: Knothe Gerhard
Published: April 2003

Bridged polymeric phthalocyanines exist in different isomeric forms of varying symmetry. The number of isomers and the symmetry depends on the structure, i. e. the size and shape, of the polymeric phthalocyanine. Equations are derived for calculating the number of isomers for bridged polymeric phthalocyanines, and symmetry point groups for various shapes (linear, square, rectangular) are discussed.
Modelling of crosslinking and cyclization in free‐radical copolymerization of vinyl/divinyl monomers
Authors: Zhu S. , Hamielec A. E. , Pelton R. H.
Published: April 2003

The Tobita‐Hamielec kinetic model of the crosslink density distribution is further elaborated and extended to incorporate divinyl loop formation. Divinyl loop formation, primary and secondary cyclization, and their effects on the elastically effective crosslink density distribution are extensively examined as a function of cyclization parameters and double bond reactivities. Under conditions of equal double bond rectivity and negligible cyclization, primary chains born at different conversions have the same crosslink density. The variance of the crosslink density distribution is mainly due to unequal reactivities of double bonds and secondary cyclization. Divinyl loop formation and primary cyclization contribute neither to gelation nor to network elasticity and have an insignificant effect on the variance of the crosslink density distribution.
Monte Carlo simulation of tetrahedral chains, 6. Linear and star‐branched polymers near to theta conditions
Authors: Zifferer Gerhard
Published: April 2003

By use of the pivot algorithm, star‐branched chains with F = 4, 8 and 12 arms of length n and linear chains (F = 2) are generated on a tetrahedral lattice (120 ⩽ nF ⩽ 3 840). By taking into account non‐bonded nearest‐neighbour interactions (each contact contributes an energy ϕ · kT to the total energy of the configuration) a variation of the thermodynamic quality of the solvent is simulated by a variation of the energy parameter ϕ in the range −0,425 to −0,525. The energy parameter ϕ⊙ = −0,475, characteristic of theta conditions, was evaluated by use of an intramolecular criterion (proportionality between mean‐square dimensions and total chain‐length) and confirmed by a crossover scaling analysis. Theta dimensions are found to be larger than those of nonreversal random walks, the deviation growing with increasing number of arms.
Molecular dynamics simulation of semirigid polymers
Authors: Fusco Roberto , Longo Luca , Caccianotti Luciano , Aratari Carola , Allegra Giuseppe
Published: April 2003

The chain rigidity of poly(p‐hydroxybenzoate) was estimated through the theoretical evaluation of its persistence length (Lp). A non‐Brownian molecular dynamics (MD) simulation of an isolated chain with 20 monomeric units was performed. The sampled conformational population was analyzed and the orientational correlation function between monomeric units along the chain was calculated. An algorithm based on the worm‐like chain model was applied to evaluate the persistence length. The results were compared with those obtained from equilibrium models like the freely‐rotating‐chain and the rotational‐matrix method with fluctuations. Equilibrium models give different results depending on the degree of accuracy used in describing the monomeric unit. The inclusion of thermal fluctuations is crucial to obtain realistic results. These coincide with those given by MD simulation when only nearest‐neighbour orientational correlations are taken into account: inclusion of higher‐order correlation terms leads to lower values of the persistence length. The origin of this discrepancy was investigated. The MD simulation results are characterized by an overrepresentation of conformations with a short end‐to‐end distance resulting from an anomalous energy concentration in the first bending mode of the chain. In analogy with previous simulation results from systems characterized by a week coupling amoung their degrees of freedom, failure in the energy equipartition is proposed as a likely explanation of the anomalous dynamical behaviour.
Monte‐Carlo simulation of copolymer mass spectra
Authors: Montaudo Maurizio S.
Published: April 2003

A computer program, MACO5, was developed which employs the Monte‐Carlo approach to simulate the time‐variation of the mass spectral peak intensities of a copolymer sample during the synthesis of an A/B copolyester via reactive blending of a mixture of homopolyesters A and B or when a copolymer undergoes partial degradation. The program was first used to simulate literature data consisting of a series of mass spectra of a butadiene/styrene copolymer subjected to partial ozonolysis. It was also used to investigate the role of terminal groups in reactive blending processes. Two models were developed: the first one describes inner‐group ester‐ester exchange reactions, the second describes the process in which and active terminal groups attacks an ester group. The predictions of the two models are compared with recent experimental mass spectral data concerning a poly(ethylene adipate‐co‐ethylene terephalate) copolyester.
Modelling free‐radical copolymerization kinetics, 3. Molecular weight calculations for copolymers with crosslinking
Authors: Xie Tuyu , Hamielec Archie E.
Published: April 2003

A comprehensive model for molecular weight calculations of free‐radical crosslinking copolymerizations was developed using the pseudo‐kinetic rate constants and the method of moments. The moments of copolymer chain distributions are defined in such a way so that the molecular weight averages of crosslinking copolymers can be calculated using the moments. The present model is based on a general crosslinking copolymerization scheme, accounting for chain transfer to small molecules and polymer, bimolecular termination, and crosslinking reactions. The influence of crosslinking reactions on molecular weight development is discussed. The effects of the reactivity of pendant double bonds on the moments development were further demonstrated using model simulations. The simulations results suggest that the higher‐order molecular weight averages are very sensitive to the reactivity of pendant double bonds. It was found that chain transfer to polymer affects the gelation point significantly. The radical fractions must be calculated accounting for chain transfer reactions in addition to propagations in order to properly evaluate pseudo‐kinetic rate constants. The present model was used to predict kinetic behavior and molecular weight development of styrene/m‐divinylbenzene and styrene/ethylene dimethacrylate free‐radical crosslinking copolymerizations in benzene solution at 60°C. It was found that the present model is in excellent agreement with the experimental data published in the literature. Model predictions and experimental data show that the reactivity of pendant double bonds is much lower than that of vinyl and divinyl monomers. The simulation results suggest that the assumption of the same reactivity of functional groups is likely not valid for many free‐radical crosslinking copolymerizations. The present model based on a kinetics approach can be used to predict molecular weight development for vinyl/divinyl free‐radical crosslinking copolymerizations and to estimate kinetic parameters in the pre‐gelation period.
Dependence of flow behaviour of nematics in shear on the form of the mean‐field potential
Authors: Minale Mario , Apuzzo Giuseppe , Greco Francesco , Marrucci Giuseppe
Published: April 2003

The dynamic transition of nematics in (slow) shear from flow‐aligning to tumbling behaviour is here predicted by using the rigid rod‐like molecular model together with an extension of the Maier‐Saupe mean‐field potential. Such an extension, already available in the literature with reference to equilibrium data, includes interactions of rank higher than the basic quadrupolar one considered by Maier and Saupe. The theoretical predictions are then compared with the few existing sets of dynamic data. Although an acceptable qualitative agreement is found, the theory generally overpredicts the value of the relevant ratio α3/α2 of Leslie coefficients.
Monte Carlo studies of the conformational statistics of polymers. Polyethylene
Authors: Raucci Rosa , Vacatello Michele
Published: April 2003

The conformational distribution of unperturbed polyethylene (PE) has been studied utilizing the Monte Carlo approach. The results are in excellent agreement with experiments on the average dimensions of PE chains, as well as with the molecular scattering function obtained in mixtures of deuterated and hydrogenous PE at 400 K. Maps of the free energy as a function of two consecutive torsional angles confirm that an approximate description of the conformational distribution of PE can be given in terms of three rotational isomeric (RIS) states. The location of the states and the corresponding energy and entropy parameters, which can be separately evaluated from the Monte Carlo results, are compared with literature results obtained by internal energy calculations for butane and pentane.
Linear chain surfactants at a planar interface: a comparative Monte Carlo study of several lattice models
Authors: Haas Frank M. , Lai Pik‐Yin , Binder Kurt
Published: April 2003

Linear chain surfactants in a densely packed arrangement (such as alkane chains in lipid monolayers in the “uniform tilt” structures) are described by a crude coarse‐grained model where the endgroups grafted on the interface form a regular lattice and the chains are described by the bond fluctuation model with chains containing N = 4 effective monomers only. Square‐well interactions between the monomers are studied for both the attractive and repulsive case for three choices of the interaction range. None of these models exhibits a structure with uniform tilt. For attractive interactions the last bond has a strong tendency to fold back thus leading to a very high density close to the interface. Only when an intrachain‐potential favoring stiff chain configurations also is included one can obtain configurations with uniform tilt order. Although related models (with much longer chain lengths and lower grafting densities) are very useful for the study of polymer brushes, the present case of very short chains in a high‐density state clearly is plagued by various lattice artefacts and it is concluded that for modelling linear chain surfactants one should use an off‐lattice model even on a coarse‐grained level.
Isomerism and symmetry of stacked polymeric phthalocyaninnes and related polymers
Authors: Knothe Gerhard
Published: April 2003

The number of isomers ir,t for stacked polymeric phthalocyanines in which each phthalocyanine ring is tetrasubstituted or octasubstituted with two different substituents (each benzene moiety substituted with both substitutents) is determined to be

where m is the number of substituted phthalonitrile molecules incorporated into the polymer and x is the number of phthalocyanine macrocycles (degree of polymerization; m = 4x). Specifically, for a stacked polymeric phthalocyanine with x = 2; there exist 298 isomers. The stacked polymeric phthalocyanines with substituted phthalocyanine rings possess different symmetry groups (D2h, D2d, C4, C2V or CS E as sole symmetry operation). The results are valid for stacking of other macrocycles similar to phthalocyanine such as prophyrins.
Polyelectrolyte/Ionomer behavior in polymer gel collapse
Authors: Khokhlov Alexei R. , Kramarenko Elena Yu.
Published: April 2003

The theory of collapse of weakly charged polyelectrolyte gels is generalized by taking into account the possibility of counter ion trapping with the formation of ion pairs, which becomes progressively important as the gel shrinks and the dielectric constant of the gel medium decreases. A phenomenon well known in the theory of ionomers, namely the aggregation of ion pairs due to dipole‐dipole interactions with the formation of multiplets, is also taken into account. These multiplets act as additional physical cross‐links. It is shown that accounting for the two effects mentioned above generally leads to an increase of the region of stability of the collapsed phase and to an increase of the jump in volume at the transition point. The most important, qualitatively new effect is the possibility of existence of a new supercollapsed state of a polymer gel which is very close to the densely packed dry gel. The reason for the thermodynamic stability of the supercollapsed state is a loop of positive feedback: the decrease of the volume of the gel leads to a decrease of the dielectric constant and hence to progressive formation of ion pairs, thus the concentration of mobile counter ions and the corresponding osmotic pressure decrease, the gel shrinks further etc. It is possible to realize the phase transitions between all three states of a polymer gel: swollen, ordinary collapsed and supercollapsed.
Viscosity of weakly charged polyelectrolyte solutions: The screening of hydrodynamic interactions
Authors: Borsali Redouane , Vilgis Thomas A. , Benmouna Mustapha
Published: April 2003

The mode‐mode coupling approximation (MMCA) is used to describe the behavior of the reduced viscosity of weakly charged polyelectrolyte solutions in the presence of screening of hydrodynamic interactions. This effect is introduced through a dynamic correlation length ζH in the Oseen tensor description. The results show that as ζH goes from zero (Rouse model) to infinity (Zimm model), the general qualitative behavior of ηr as function of the polyelectrolyte concentration Cp does not change but its value decreases significantly with a slight shift in the peak position toward higher Cp values.
Lattice model of heterogeneous polymer chains in the liquid‐crystalline state, 1 statistical properties
Authors: Medvedev Gregory A. , Gotlib Yuli Y.
Published: April 2003

A heterogeneous lattice model for main‐chain liquid‐crystalline polymers is proposed. Phase transition parameters and statistical properties of the chain in the ordered phase are treated in the framework of a mean‐field approximation. The order parameters for the mesogen and the spacer bonds, the chain shape anisotropy and the fraction of stretched isomers are obtained. Dependence of the Curie point on the spacer length and flexibility is considered. The “odd‐even” effect is shown to increase drastically with the spacer flexibility. A strong dependence of the conformation changes in the ordered state on the spacer properties is obtained.
Monte Carlo simulation of tetrahedral chains, 7 the shape of linear and star‐branched polymers near to theta‐conditions
Authors: Zifferer Gerhard
Published: April 2003

By use of the pivot algorithm, star‐branched chains with F = 4, 8 and 12 arms of length n and linear chains (F = 2) are generated on a tetrahedral lattice (120 ≤ nF ≤ 3 840). By taking into account nearest neighbour interactions (each contact contributes an energy ϕ kT to the total energy of the configuration) a variation of the thermodynamic quality of the solvent is simulated by a variation of the energy parameter ϕ near the value of ϕθ = ‐0,475, characteristic of theta‐conditions. For theta‐conditions various quantities characteristic of the instantaneous shape of polymers exhibit similar values as found for nonreversal random walks; furthermore, while linear theta‐chains are slightly less asymmetric than athermal ones, the opposite behaviour is found for star‐branched polymers. Clearly, for all thermodynamic conditions the asymmetry of configurations decreases with increasing number of arms but remains appreciable even for F = 12.
Effect of polyfunctional chain transfer agents on molecular weight distribution in free‐radical polymerization, 2. The case C ≤ 1
Authors: Yuan Cui Ming , Farina Mario
Published: April 2003

In this part of the series, the influence of polyfunctional chain transfer agents with transfer constant C ≤ 1 on the molecular weight distribution was studied. The analytical expressions for the number‐ and weight‐average degree of polymerization, and dispersion index were derived by kinetic and statistical methods. The expression for the molecular weight distribution can only be obtained by statistical methods. Some numerical examples on the dependence of distribution parameters as a function of the functionality of transfer agents f and transfer constants are illustrated. A critical value of the chain transfer constant was found to exist, which permits the synthesis of linear (for f = 2) or branched polymers (f > 2) with DPw/DPn approximately equal to 2 during the entire course of the polymerization.
Band structure of quasi‐1‐dimensional polycondensed π‐systems, 3 energy spectra of anti‐aromatic polymers
Authors: Dietz Fritz , Müllen Klaus , Baumgarten Martin , Tyutyulkov Nikolai
Published: April 2003

The band structure of a class of quasi 1‐dimensional (1‐D) polymers with anti‐aromatic structural units — anti‐aromatic polymers (AAP) — has been investigated theoretically. The energy gap (EG) of the AAP is significantly smaller compared to the EG of polymers with aromatic building blocks having the same number of π‐centers in the elementary units. The influence of the electron correlation in the calculation of the EG is investigated. As with the anti‐aromatic molecules the Jahn‐Teller distortion influences the geometric configuration of the AAP.
Neural networks and graph theory as computational tools for predicting polymer properties
Authors: Sumpter Bobby G. , Noid Donald W.
Published: April 2003

A new computational methodology is presented for making rapid and accurate predictions of chemical, physical and mechanical properties of polymers based on their molecular structure. The method uses a set of topological indices derived from graph theory to numerically describe the structure of a monomeric repeating unit for a given polymer (structural descriptors) and relates these indices to a set of polymer properties by utilizing an artificial neural network. The neural network is able to efficiently formulate all of the correlations (i.e., between structural descriptor‐property, property‐property, structural descriptor‐structural descriptor: both linear and nonlinear dependencies) necessary to make accurate predictions. Results have been obtained for up to 9 properties of 357 different polymers with an average prediction error of < 3% and a maximum error of 12%, demonstrating superiority over other quantitative structure/property relationships for polymers.
Approximate quantum‐mechanical hamiltonians for slow internal rotations of macromolecules
Authors: Alvarez‐Estrada Ramón F.
Published: April 2003

Slow internal rotations of three‐dimensional macromolecules are studied based upon quantum mechanics, the Born‐Oppenheimer approximation and related assumptions, and a variational inequality. Difficulties arising from vibrational and electronic energies are pointed out and bypassed consistently. Approximate quantum hamiltonians for internal rotations, in terms of dynamical variables studied recently, are obtained.
The study of network formation as a cluster‐cluster diffusion‐limited aggregation process: Modelling of the curing of an epoxy‐resin using the Monte Carlo method
Authors: Netz Paulo A. , Samios Dimitrios
Published: April 2003

The study of network formation during the curing of an epoxy‐resin with anhydride and tertiary amine has been carried out by the Monte Carlo method. Both dynamical and structural properties are studied. The model resembles the well known cluster‐cluster aggregation: the reaction is modelled as a “geometrical” phenomenon and neglects attractive and repulsive energies except for an excluded volume condition. A reaction occurs always when two active sites are in the vicinity to each other, meaning that the “chemical” processes are taken to be much faster than diffusion (as in diffusion‐limited aggregation). Results are presented of two‐dimensional square lattice simulations with periodic boundary conditions and are discussed with respect to the proposed mechanism of the curing reaction and to the assumptions of the diffusion‐limited nature of the processes. The scope and limitations of such two‐dimensional simulations are discussed.
Conformational and packing energy calculations on ethylene copolymers
Authors: Napolitano Roberto , Pucciariello Rachele , Villani Vincenzo
Published: April 2003

A conformational analysis was performed on the isolated chains of ethylene‐propene and ethylene‐1‐butene copolymers. The lowest energy conformations in accordance with the chain repeating distance of polyethylene were used in packing energy calculations. The results of our calculations suggest that both methyl and ethyl groups are tolerated in the crystalline conformation of a single polyethylene chain, but only the methyl group is acceptable in the crystalline state if the packing energies and the lattice deformations are taken into account.
Microphase separation in poor‐solvent polyelectrolyte solutions: Phase diagram
Authors: Dormidontova Elena E. , Erukhimovich Igor Ya. , Khokhlov Alexei R.
Published: April 2003

Microphase separation in solutions of weakly charged polyelectrolytes in poor solvents is studied in the weak segregation limit within the framework of the mean field approximation using a method first developed by Leibler. As a result a complete phase diagram of the solution near the critical point is obtained. The regions of the stability of the disordered, homogeneous phase and of body‐centered cubic (bcc), triangular and lamellar microdomain structures, as well as the phase separation regions are determined. The most striking difference in comparison with the corresponding diagram for block‐copolymer melts is the existence of broad phase separation regions even for monodisperse systems. As the quality of solvent becomes poorer, the triangular microdomain structure remains the most stable among microdomain phases of other symmetry.