Complexation between hydrophobically modified polymers and amylose. Gelation and competition by surfactants
Sammanfattning: Aqueous mixtures of hydrophobically modified polymers (HMP) with amylose have been investigated by rheology and 1H NMR. Amylose can cross-link HMP chains by forming single-helical inclusion complexes with the hydrophobic side chains of the HMP, leading to thermoreversible, elastic, and thixotropic gels at ambient temperatures. The complexation ability, the strength of the association, and the viscoelastic behaviour of the gels vary with the concentrations of amylose and HMP. The rheology of the gels can be further controlled by the length of the alkyl chains of the HMP and by the addition of short hydrophilic ”spacers” between the alkyl chains and the HMP backbone. An increase in the alkyl chain length gives larger increase in the gel strength than a similar increase in spacer length. Linear surfactants also form inclusion complexes with amylose. Two types of structures, loose fibrous networks and unilamellar vesicles, were observed by cryogenic transmission electron microscopy of dilute aqueous mixtures of amylose and sodium dodecyl sulfate. Both types of structures are believed to consist of inclusion complexes where SDS-molecules are enclosed in amylose helices. Surfactants can compete with the amylose-HMP complexation. The strength of the competition varies with the length and bulkiness of the hydrophobic ”tail” of the surfactant. In mixtures of surfactant, amylose, and HMP, the HMP hydrophobes are engaged in two types of complexes, i.e., mixed micelles with added surfactant and inclusion complexes with amylose, over a large concentration range of the added surfactant. A range of different surfactants, when added to amylose-free HMP solutions, were found to give rise to the well-established viscosity maximum with increasing surfactant concentration, due to a mixed micellization between surfactant molecules and the hydrophobic side chains of the HMP. The surfactants affect both the lifetime and the structure of the HMP – surfactant network. The viscosity maximum is the result of two opposing effects of increasing surfactant concentration: an increase in the lifetime of mixed micellar cross-links and a decrease in the number of cross-links. The difference in viscosifying ability between different surfactant is due to differences in the lifetimes of the mixed micellar cross-links.
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