Polyelectrolytes in the presence of multivalent ions: gelation versus segregation

We analyze solutions of strongly charged chains bridged by linkers such as multivalent ions. The gelation induced by the strong short range electrostatic attractions is dramatically suppressed by the long range electrostatic correlations due to the charge along the uncrosslinked monomers and ions. A modified Debye-Huckel approach of crosslinked clusters of charged chains is used to determined the mean field gelation transition self-consistently. Highly dilute polyelectrolyte solutions tend to segregate macroscopically. Semidilute solutions can form gels if the Bjerrum length $l_B$ and the distance between neighboring charged monomers along the chain $b$ are both greater than the ion size $a$

Elasticity near the vulcanization transition

Signatures of the vulcanization transition--amorphous solidification induced by the random crosslinking of macromolecules--include the random localization of a fraction of the particles and the emergence of a nonzero static shear modulus. A semi-microscopic statistical-mechanical theory is presented of the latter signature that accounts for both thermal fluctuations and quenched disorder. It is found (i) that the shear modulus grows continuously from zero at the transition, and does so with the classical exponent, i.e., with the third power of the excess cross-link density and, quite surprisingly, (ii) that near the transition the external stresses do not spoil the spherical symmetry of the localization clouds of the particles.Comment: REVTEX, 5 pages. Minor change

Effect of galactomannans on the viscoelastic behaviour of pectin/calcium networks

An investigation was carried out on the effect of the addition of galactomannans to pectin/calcium networks with different structural and rheological characteristics. For those pectin/calcium gels characterized by an elastic equilibrium modulus, the addition of the galactomannan increased both the storage and loss mod&i, especially at short time scales. This increase was greater than that which could be expected by simple additivity of the viscoelastic properties of each isolated system. The pectin/calcium network remained the continuous gel matrix controlling the viscoelastic behaviour of these systems at low frequency. For pectin/calcium systems close to the sol-gel transition or at low pH, the mixed systems evolved towards the behaviour of Viscoelastic liquids in the presence of increasing concentration of the non-gelling polymer. These overall results suggest that there is no specific interaction and that the changes in the rheological properties of the pectin gels are due to galactomannan microphase separation limited by the entrapment of these macromolecules in the pectin-calcium network