Gels of hydrophobically modified hydroxyethyl cellulose cross-linked by amylose. Competition with cyclodextrin
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Previous work has shown that amylose (AM) can cross-link hydrophobically modified polymers by inclusion complexation, whereby thermoreversible cold-setting gels are formed. In this work, the competition between AM and cyclodextrin (CD) for the formation of inclusion complexes with hydrophobically modified hydroxyethyl cellulose (HMHEC) is investigated. A detailed study of viscosity, NMR self-diffusion, and chemical shifts of the two-component mixture, CD and HMHEC, was performed. The results imply that 2:1 (CD:polymer hydrophobe) complexes may be formed. The three-component mixtures, HMHEC/AM/CD, were investigated by rheology, NMR self-diffusion, and intensities of the NMR resonance peaks. The CD molecules competed efficiently with the AM molecules, as seen by a decreased storage modulus, an increased self-diffusion of AM and HMHEC, and increased NMR intensities of the HMHEC hydrophobes, as the concentration of CD increased in the solution. A high concentration of CD is needed in the mixtures to inhibit all interactions between HMHEC and AM, and it was shown that there still is an effect of AM at excess CD concentration in the mixtures.