摘要:

AbstractThe Ahmad‐Stepto (A‐S) theory of gelation in RA2+ RBfpolymerisations has been improved and developed to include general polymerisations of A‐ and B‐bearing reactants, namely,\documentclass{article}\pagestyle{empty}\begin{document}$$ {\sum\limits_{\rm i}}{\rm RA}{_{{\rm fai}}} + {\sum\limits_{{\rm i}}}{\rm RB}{_{{\rm fbi}}} $$\end{document}polymerisations and\documentclass{article}\pagestyle{empty}\begin{document}$$ {\sum\limits_{\rm i}}{\rm RA}{_{{\rm fai}}} $$\end{document}self‐polymerisations. It is found that the values of the ring‐forming parameter needed to fit experimental data on polyester‐forming RA2+ RB3polymerisations are much nearer to those expected from solution properties than the values required by A‐S theory. The present theory is proposed as a useful basis for the prediction of unknown gel points from reactant structures and reac

ISSN:0258-0322    

DOI:10.1002/masy.19930760103

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractThere have now been a number of experimental studies on the preparation and elastomeric properties of random bimodal networks of polydimethylsiloxane. The mole per cent of the short chains and their molecular masses covered a wide range, thus resulting in various polydisperse chain‐length distributions. The networks were studied with regard to their stress‐strain isotherms in elongation, and values of their moduli in the large‐deformation (phantom) limit were found to depend on the chain‐length distribution. This Important result is in disagreement with phantom network theory, which assumes the elastic modulus is dependent only on the mean value of chain lengths such that the cycle rank of the network is preserved. The effective functionality of the long chains was found to depend on the number of short chains present. Better agreement with experiment was obtained when account was taken of the connectivity of the very short chains, in what is essentially a bimodal distribution of both network chain lengths and cross‐link functionalities. Relevant here is the fact that as the degree of chemical cross‐linking Increases, the shear modulus G moves away from the affine limit, toward the phantom limit. This increase toward phantom behavior is presumably due to the fact that the mutual interspersion of chains is less when the chains are shorter, even in the small‐

ISSN:0258-0322    

DOI:10.1002/masy.19930760104

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractThe network formation in the radical polymerization of multivinyl monomers, especially including diallyl esters and dimethacrylates, is dealt by focusing our attention on the mechanistic discussion on deviation from ideal network formation. Thus, in the bulk polymerization of diallyl phthalate, the actual gel point was obtained to be 6.9 times higher compared with the theoretical one. In common multivinyl polymerization systems, the discrepancy was more than 10 times and sometimes, more than 102. Moreover, the extent of deviation was enhanced with increasing primary chain length, the content of pendant vinyl groups in the prepolymer, and dilution. In order to interpret reasonably the greatly delayed gelation different structural factors were considered. The primary factor concerns the significance of the thermodynamic excluded volume effect on the intermolecular crosslinking reaction between growing radical and prepolymer, especially at high molecular weight. Beyond the theoretical gel point, a secondary factor is related to the intramolecular crosslinking which becomes progressively important with conversion. The latter leads to the restriction of segmental motion of prepolymer and, moreover, imposes the steric hindrance, inducing the significance of the reduced reactivity of prepolymer as a tertiary factor. Solvent effect was observed as much delayed gelation in a good solvent as opposed to Walling's results, although this is expected by considering the significance of excluded volume effect.

ISSN:0258-0322    

DOI:10.1002/masy.19930760105

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractA multi‐purpose computer program has been developed for describing copolymerization and network formation in step reactions. It operates at two hierarchical levels: a generation level and an application level. At the generation level, the class of problems is defined and prepared, and this information is used to generate automatically a problem‐specific program based on generalized concepts of the branching theory. At the application level, the problem‐specific program can be used for model calculations with all possible variations in recipes and kinetic parameters within the boundaries of the specific class of processes conc

ISSN:0258-0322    

DOI:10.1002/masy.19930760106

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractSome of the existing definitions of the term ‘gel’ are discussed and shortcomings are identified. A purely phenomenological definition is proposed: a gel is a soft, solid or solid‐like material consisting of two or more components one of which is a liquid, present in substantial quantity. ‘Solid‐like gels’ are further defined in terms of the dynamic mechanical

ISSN:0258-0322    

DOI:10.1002/masy.19930760107

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractThis paper consists of two parts. In the first part, the strain dependence of entanglements or constraints acting along the contour of a Gaussian chain is derived. The functional form of the elastic free energy resulting from constraints along the chain contour is identical to that obtained in the Flory constrained junction model. In the second part, the effective number of constraints acting along the contour of a chain is discussed. The choice of this number results from postulates imposed by various authors and cannot be derived by rigorous theory. The paper compares the predictions based on these assumptions with experimental data from model networks.

ISSN:0258-0322    

DOI:10.1002/masy.19930760108

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractCrosslinked polyelectrolytes exhibit extreme swelling in aqueous systems; fluid uptake usually amounts to 100 to 1000 times of their dry volume. The mechanical and stress‐optical properties of such highly swollen gels show some peculiarities because the network chains are stretched far beyond the Gaussian limit. The shear modulus increases markedly at high degrees of swelling, and the stress‐optical coefficient decreases.A quantitative treatment has to be based on non‐Gaussian chain statistics. In order to provide experimental data, stress‐optical and modulus measurements were carried out on hydrogels made from partially hydrolyzed poly‐(acrylamide) and other polyelectrolytes. The degree of swelling was adjusted by using aqueous NaCl‐solutions of different concentration as the swelling medium. Using a description based on the inverse Langevin function, the course of the shear modulus on the degree of swelling can be fitted to the experimental data. The optical modulus, n/(λ2‐λ−1), is predicted with somewhat less agreement. However, a different set of parameters has to be used for the mechanical and optical properties. This shows that the treatment is not

ISSN:0258-0322    

DOI:10.1002/masy.19930760109

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractThe viscoelastic behaviour of arterial elastin in aqueous ethylene glycol has been studied to compare the effects of two solvents of differing polarity. The behaviour of elastin in low concentrations of ethylene glycol was similar to that of elastin that had been indirectly dehydrated using polyethylene glycol as an osmotic agent. That is, at low concentrations the dominant effect of ethylene glycol was to reduce the activity of the water molecules. However, at high concentrations, ethylene glycol appeared to interact directly with the elastin, altering both the swelling and the viscoelastic behaviour of the network.

ISSN:0258-0322    

DOI:10.1002/masy.19930760110

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractAmylose gel is obtained by cooling amylose aqueous solution at a certain cooling rate. In order to clarify the gelation process of amylose in the mixed solvent of water and DMSO, the characteristics of turbidity and dynamic viscoelasticity of the solution as a function of temperature have been studied by optical and rheological methods, respectively. Accordingly, cloud temperature (Tc) at which the aggregation of amylose occurs, and gelation temperature (Tgel) at which the elasticity began to appear were obtained. Tc and Tgel were strongly dependent on cooling rate, and these values shifted to higher temperatures with decreasing cooling rate. However, there was difference between Tc and Tgel, and Tc was higher than Tgel, indicating the gelation process occurs in two stages.

ISSN:0258-0322    

DOI:10.1002/masy.19930760111

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractThe variability of rheological properties in cohesive sediment suspensions are studied and related to the flocculation process. It was found that the steady state shear stress increases exponentially with increasing sediment concentration and increases logarithmically by increasing the salinity of the suspension. The salinity dependence of the shear stress was correlated with changes in the floc‐size. These results reflect the dominant influence of fluid‐sediment interactions on the flocculation behavior of the suspension. An analytic model predicting shear stress as a function of the physical and electrochemical properties of the fine sediment suspension is develo

ISSN:0258-0322    

DOI:10.1002/masy.19930760112

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY