A rheological investigation of the complex formation between hydrophobically modified ethyl (hydroxy ethyl) cellulose and cyclodextrin

Abstract

The thickening effect of a hydrophobically modified polymer in an aqueous solution is dependent on intermolecular hydrophobic associations, and if the polymer concentration is significantly above the overlap concentration also on chain entanglements. In this investigation we have added different cyclodextrins (CD) in order to decouple hydrophobic polymer–polymer associations via inclusion complex formation with the polymer hydrophobic tails. Both size and hydrophobicity of the cavity of the CD-molecules were found to have an effect on the process. In addition, the influence of chemical structure of the polymer hydrophobic tails was investigated. Either a linear C 14-chain or a more bulky nonylphenol group was used. The viscosity as a function of CD-concentration first decreased strongly, and then attained a constant value. At excess CD the viscosity became virtually the same as in a solution of the unmodified parent polymer, provided that complex formation was not sterically hindered. This suggests that all hydrophobic links, originating from the hydrophobic modification process, which influence the rheology could be deactivated. On the other hand, with combinations where the complex formation was hindered to a certain degree the initial decrease was less accentuated, and also, the viscosity leveled out at a significantly higher value. In an attempt to rationalize the data a simple model based on the assumption that each complex formed deactivates one rheologically active link was used. In combination with the Langmuir adsorption model the number of complexes as a function of CD concentration could be obtained. This model also gave a value of the complex formation constant. Furthermore, in solutions where all hydrophobic links could be deactivated the results from the model suggested that all polymer hydrophobic tails were originally active in forming the network.