摘要:

This paper reviews the major results concerning the sol‐gel transition of a large variety of systems, including polymeric and colloidal solutions, whose process of gelation has been analyzed in the frame of the percolation model. I first recall the basic ideas of the model and the predictions regarding the main, characteristic and experimentally accessible, parameters. Among them, emphasis is placed on the rheological measurements, but I also discuss the possibilities of other techniques (like dynamic light scattering or electron microscopy) in order, either to investigate the kinetics of network growth, or the three‐dimensional supramolecular structure of fully matured gels.

ISSN:0094-243X    

DOI:10.1063/1.40568

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Addition of gelatine to AOT reverse micelles induces, at the saturation point, the formation of a microstructure in which water spheres are interconnected by gelatin rods. Such structure also allows us to calculate the phase diagram.

ISSN:0094-243X    

DOI:10.1063/1.40569

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The thermally reversible gel phases can be representative, on a visual scale, of the ‘‘soft state’’ of matter. The fibrillar structures found in aqueous biological systems (gels of microtubules, collagen, chitin, actin) can also been observed in organic media. We study the physical gel networks made up with non macromolecular surfactants in organic media. Considering a few examples of binary systems (gelator+solvent), we can survey a large variety of behaviors. The molecular organization within the fibres and the aggregation mechanisms are discussed. Structures appear to be function of the stericity of the gelator, its polarity, its chirality, its chemical functionality and can also be solvent and concentration dependent. Small angle neutron scattering and electron microscopy are some of the complementary techniques used.

ISSN:0094-243X    

DOI:10.1063/1.40570

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Depending on experimental conditions, adsorbed polymer layers on particles can either increase or decrease the stability of a collidal dispersion. The aggregated formed in the latter case are called flocs. Numerous macroscopic techniques are used to clear up the factors that control their formation and it appears that flocculation of suspensions by addition of macromolecules proceeds via various mechanisms form bridging to charge neutralization according to the kind of the polymer‐particle interaction. The organization of the particles inside the aggregates is examined by small angle neutron scattering (SANS). It is mainly governed by the remaining interparticle repulsions. Different structures can be observed when these repulsions decrease: linear necklaces branched clusters, tridimensional aggregates with short range liquid like order and finally fractal objects when the surfaces can be brought to contact.

ISSN:0094-243X    

DOI:10.1063/1.40571

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The percolation phenomenon usually observed at high temperatures occurs at room temperature in the presence of the protein. This process takes place at high water contents at a given cytochromecconcentration or at low water contents increasing the cytochromecconcentration. The increase in the water content in filled micelles induces a phase separation with two optically transparent phases. The upper phase mainly contains isooctane while the lower phase contains all the AOT, water and cytochrome.

ISSN:0094-243X    

DOI:10.1063/1.40572

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Structural changes of AOT reverse micelles are observed in the presence of ribonuclease. A percolation phenomenon takes place at high water contents and for a given temperature. A phase separation is observed with two optically transparent phases. The upper and lower phase both contain isooctane, AOT, enzyme and water. However the lower phase is much more concentrated than the upper one.

ISSN:0094-243X    

DOI:10.1063/1.40573

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Amphilic basic polypeptides Poly(Leu‐Arg‐Arg‐Leu), 2Hcl, Poly(Tyr‐Lys‐Lys‐Tyr), 2Hcl and Ac(Leu‐Lys‐Lys‐Leu)4NHEt, 2TFA, formed by alternating doublets of hydrophobic and hydrophilic aminoacid undergo a coil to &agr; helix transition in the presence of lysolecithin and DMPC vesicles. Circular dichroism and31P NMR strongly suggest the formation of interactions between amino acid residues and phospholipid acyl chains.

ISSN:0094-243X    

DOI:10.1063/1.40574

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The calcium‐induced gelation of low methoxyl pectin has been studied by small‐angle X‐ray and neutron scattering and mechanical spectroscopy. Whilst neutron scattering indicates little polymer conformational change as calcium becomes bounded, the X‐ray data are consistent with junction zones viewed as rod like cylinders whose cross section radius increases from 0.3 to 0.8 nm with the amount of calcium added. Near the sol‐gel transition the results of oscillatory shear measurements agree very well with the prediction of the percolation model. On fully set gels, the mechanical spectrum allow to point out that cross‐links in these gels may be considered as permanent. The magnitude of the equilibrium shear modulus obtained from these dynamic mechanical analysis increases with calcium and pectin concentration and with the ionic strength. Generally solvent expulsion occurs when the above three parameters reach high values.

ISSN:0094-243X    

DOI:10.1063/1.40576

出版商:AIP    

年代:1991

数据来源: AIP