摘要:

AbstractThis article summarizes rheological features of moderately concentrated, diblock copolymer micellar solutions in selective solvents. In solvents of monomeric sizes, spherical micelles (with glassy cores) are arranged on cubic lattices to exhibit elasto‐plastic responses. These lattices are formed due to an osmotic constraint for the conformation of solvated corona blocks of the micelles. In polymeric solvents (homopolymers) chemically identical to the corona blocks, this constraint is screened and the micelles are randomly dispersed in the systems. In this type of blends (micellar dispersions) in short, nonentangling matrices the micelles have no plasticity but exhibit two‐step viscoelastic relaxation: The fast relaxation process corresponds to relaxation of individual corona blocks while the slow process is attributable to diffusion of the micelles. Corresponding to these molecular mechanisms, the fast process exhibits modest damping of the nonlinear relaxation modulus against large step strains while the slow process exhibits much stronger damping. Under steady flow, the micellar dispersion exhibits two‐step shear‐thinning with the molecular origin identical to that for the nonlinear damping. All these rheological features of the micellar lattices and dispersions are strikingly similar, in many aspects, to the features of colloidal crystals and hard‐sphere suspensions of solid

ISSN:0323-7648    

DOI:10.1002/actp.1997.010480701

出版商:Akademie Verlag GmbH    

年代:1997

数据来源: WILEY

摘要:

AbstractA bipolar membrane (BM) is a layered structure composed of one cation and one anion ion‐exchange layers joined together in series. Polymer BMs offer promising applications for many industrial processes (e. g., the use of bipolar electrodialysis for environmentally clean technologies and the treatment of salt‐water effluents) because of their unique electrochemical properties. The most important of these properties is the electric field enhanced (EFE) water dissociation which arises when an electric current is forced through the membrane. This phenomenon occurs at the bipolar junction of the BM, and its coupling with ion transport, though still poorly understood, is the basis of most of the potential applications of BMs. In this review, we will focus on recent work concerning the physical chemistry of BMs and give a general overview of their electrochemical properties, emphasizing both theoretical and experimental aspects. We will model first the electric double layer at the bipolar junction between the two ion‐exchange layers, and describe then the EFE water dissociation occurring in this junction. Later, we will present in detail a complete theoretical model for the coupling of ion transport and water dissociation in BMs, and show that this model is able to explain the experimental trends observed in the electrochemical characterization of polymer BMs by means of membrane potential, current–voltage curve, and impedance measu

ISSN:0323-7648    

DOI:10.1002/actp.1997.010480702

出版商:Akademie Verlag GmbH    

年代:1997

数据来源: WILEY

摘要:

AbstractThe solvatochromic properties of 3‐hexyl‐substituted polythiophenes functionalized with an ester, ether or hydroxy group at the end of the alkyl side chain are reported. An extraordinarily large shift, Δmax= 99 nm, and a dramatic change of the UV/ vis spectral profile are found when the carboxylate is replaced with the hydroxy group by functionalization exchange. The solvatochromic behavior of each polymer is believed to be strictly related to the type of functional group owing to different physical interactions (solvation) between groups different in polarity and solvent molecules. A unique and common chromophoric unit for the final conformation of the 3‐hexyl‐substituted polythiophenes, induced by the solvatochromic effect, is also s

ISSN:0323-7648    

DOI:10.1002/actp.1997.010480703

出版商:Akademie Verlag GmbH    

年代:1997

数据来源: WILEY

摘要:

AbstractThe structure formation of growing spherulites by branching lamellae is simulated by a computer model. In addition to the process of linear growth of lamellae, three different mechanisms are implemented in the simulation program: the fork‐like branching of lamellae at their growth front (“front branching”), the generation of secondary lamellae at the sides of already existing lamellae (“side branching”) and the crystallization of lamellae parallel to existing lamellae (“clustering”). The simulation is performed stepwise and the branching takes place in dependence on branching probability and angle, leading to a multiplication of the lamellae and to disc‐like superstructures. Moreover, the side branching as well as the clustering mechanisms result in a gradient of “density” (crystallized entities per area unit) ins

ISSN:0323-7648    

DOI:10.1002/actp.1997.010480704

出版商:Akademie Verlag GmbH    

年代:1997

数据来源: WILEY

摘要:

AbstractBlock copolymers consisting of polystyrene and a block containing long fluorinated side chains are made by two very effective polymer analogous reactions from commercially available polystyrene‐b‐polybutadiene block copolymers. Since these polymers consist of two mutually immiscible blocks with hydrophobic and very hydrophobic character they form micelles in a wide range of media down to very hydrophobic solvents, e. g. toluene, tetrahydrofurane (THF), perfluorohexane, perfluoro(methylcyclohexane), hexafluorobenzene and 1,1,2‐trichlorotrifluoroethane (Freon 113). First indications that our polymers are useful as effective steric stabilizers in such solvents are given by characterization of some polymer dispersions made in the solvents via precipitation polymerization.In the solid state, mesophase formation typical for block copolymers is observed and characterized by quantitative small angle X‐ray scattering (SAXS). The surface of such films is controlled by the fluorinated block and exhibits an ultra‐low surface energy of the order of γSV= 15 mN/m, below that of polytetrafluoroethylene (PTFE) or polydimethylsiloxanes (PDMS). Unlike PTFE, the blocky nature of the presented polymer enables thermoplastic processing and dissolution in standard solvents, which is of practical relevance. Gas‐permeability measurements on membranes coated with these copolymers exhibit some interesting s

ISSN:0323-7648    

DOI:10.1002/actp.1997.010480705

出版商:Akademie Verlag GmbH    

年代:1997

数据来源: WILEY

摘要:

AbstractSyntheses of aromatic polyketones in the form of particles were performed using a very small monomer concentration (0.05 mol/1) by the precipitation polycondensation method. Some of these particles present highly ordered elongated needle‐like structures (whiskers) particularly for the all‐parapolyketone samples. The use of isophthaloyl instead of terephthaloyl for the same low monomer concentration produces, in addition to the globular particles, bundles of ribbon‐like structures. The average size of the needle‐like particles is 1−5 μm in width and 150−250 μm in length. The factors controlling the morphology texture and size of the whiskers and ribbons are shown to be the polymer structure and the reaction conditions. Formation mechanisms

ISSN:0323-7648    

DOI:10.1002/actp.1997.010480706

出版商:Akademie Verlag GmbH    

年代:1997

数据来源: WILEY

摘要:

AbstractWe have determined by forced Rayleigh scattering the diffusion coefficients of photo‐labeled polystyrene micronetwork spheres (radii ≤ 10 nm) in melts of linear polyvinylmethylether (Mw≤ 40 000 g/mol) at temperatures of 20−80°C. An expected slippage of the spheres through the meshes of the entanglement network appears possible but is still within the experimental uncertainty of our

ISSN:0323-7648    

DOI:10.1002/actp.1997.010480707

出版商:Akademie Verlag GmbH    

年代:1997

数据来源: WILEY