Complexation of Poly(vinyl alcohol)-Congo Red Aqueous Solutions. 1. Viscosity Behavior and Gelation Mechanism

Abstract

The complexation of poly (vinyl alcohol) (PVA) with congo red (CR) ions in aqueous solutions was investigated as functions of PVA (Cpva) and congo red concentrations (Ccr) and temperature. The PVA-CR complexation resulted in a gelation around pH 7 when both Cpva and Ccr were high enough. However, in the case of intermediate PVA concentrations in monomer unit, i.e., 0.6 mol/L, a sol-gel-solgel-type transition was observed with increasing CR concentration. The origin of this "re-entrant"-type phase transition was studied by estimation of the enthalpy of cross-link formation and by capillary viscometry. The gelation was classified into two steps: (a) weak cross-link and (b) strong cross-link formations. These mechanisms were characterized by the value of the enthalpy of cross-link, AH. The former is in the order of -5 kcal/mol and the latter -20 kcal/mol. This viscosity behaviors in a dilute polymer concentration regime were also classified into several regions: When both CCr and Cpva were low, the complex solution showed a typical viscosity behavior for a polyelectrolyte solution. With increasing Ccr and Cpva, there appeared a region where a time-dependent viscosity thickening took place. With further increase in Cpva, the viscosity recovered a behavior similar to that for a PVA solution without CR. The connection between these rheological properties and sol-gel transition is discussed by taking into account the ionic screening effect and complexation equilibria.

On the nature of crosslinks in thermoreversible gels

“…From their presented dynamic mechanical Figure 10. Schematic representation of the weak (a) and strong (b) crosslink structures in poly(vinyl alcohol) -Congo Red complexes, according to Shibayama [50] and (c) of an octafunctional crosslink, according to Te Nijenhuis [4,18]. measurements the present author was able to calculate the functionality of the crosslinks to be 7.6±0.4, which is in agreement with the thermodynamic results [4,18].…”

“…In this way complexation might lead to so-called weak and strong crosslinks, as shown in Figs.10a and 10b [50], where in both cases the crosslink functionality is equal to 4, i.e. the number of polymer chains leaving the crosslink.…”

Universality and Specificity of Polymer Gels Viewed by Scattering Methods

“…21a. 67 As shown here, the sol-gel transition has a re-entrant feature with respect to C CR in the concentration regime of 0:45 C PVA 0:75 M. 67 This is due to ionization of PVA chains to form a diol structure, followed by formation of di-diol structure as discussed elsewhere. 74 As a mater of fact, Figure 21b shows a series of photographs of PVA/CR showing a sol-gel-sol transition.…”

Supramolecular Aggregates of Dendritic Multishell Architectures as Universal Nanocarriers

“…This observation may be a result of the fact that these molecules are preferentially located in the interfacial region and behave as "cosurfactants" that reduce the interfacial tension and thus prevent the formation of larger aggregates. In contrast, linear guest molecules might act as noncovalent linkers between the assemblies constituents owing to their extended geometry (congo red, [19] b-carotene). All these supramolecular aggregates showed high stabilities, were detectable in solution for more than a year, and survived filtration procedures as well as size exclusion chromatography.…”