摘要:

AbstractThe 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 deformat

ISSN:1022-1344    

DOI:10.1002/mats.1992.040010401

出版商:Hüthig&Wepf Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractThe 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 seco

ISSN:1022-1344    

DOI:10.1002/mats.1992.040010402

出版商:Hüthig&Wepf Verlag    

年代:1992

数据来源: WILEY

摘要:

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 χ2function between the measured values,G′(ω) andG″(ω), and those obtained by the simulated spectrum. The number of relaxation modes is smoothly increased until no improvement on the minimum value of χ2is 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 ofG′ andG″ 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

ISSN:1022-1344    

DOI:10.1002/mats.1992.040010403

出版商:Hüthig&Wepf Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractIn a previous paper we reported the enthalpies of mixing of oligomers of ethylene oxide with different end groups in tetrachloromethane. This paper shows the applicability of the modified theoretical treatment, based on a quasichemical equilibrium of the contacting segmental surfaces originated from Huggins, to our experimental results. The experimental data of ethylene oxide oligomers with methoxy end groups can be described with the energy parameter Δεαβ, the equilibrium constantKαβand the surface ratiorσassuming a chemically uniform surface of the oligomer in CCI4. Ethylene oxides with strong polar OH end groups can be treated successfully with an extended version of the theory with two additional parametersKand ΔΔε for systems containing three different kinds of chemi

ISSN:1022-1344    

DOI:10.1002/mats.1992.040010404

出版商:Hüthig&Wepf Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractGel points in random polymerizations of the general type ΣiRA f ai+ ΣjRB f bjin which A‐groups react with A‐ and B‐groups, and B‐groups react only with A‐groups are considered. (The symbols Σiand Σisignify that the A‐ and B‐bearing reactants RA f aiand RB f bjcan be mixtures of monomers of different functionalities, denoted generally asfaiandfbj.) 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 aspaas a function ofpb. 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 f ai+ ΣjRB f bjpolymerization, and, in the limit of A‐groups only reacting with A‐groups

ISSN:1022-1344    

DOI:10.1002/mats.1992.040010405

出版商:Hüthig&Wepf Verlag    

年代:1992

数据来源: WILEY

摘要:

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 MN

ISSN:1022-1344    

DOI:10.1002/mats.1992.040010406

出版商:Hüthig&Wepf Verlag    

年代:1992

数据来源: WILEY

ISSN:1022-1344    

DOI:10.1002/mats.1992.040010407

出版商:Hüthig&Wepf Verlag    

年代:1992

数据来源: WILEY

ISSN:1022-1344    

DOI:10.1002/mats.1992.040010408

出版商:Hüthig&Wepf Verlag    

年代:1992

数据来源: WILEY