Hydrophobically Modified Polymers. Rheology and Molecular Associations

Abstract: Structure- property relationships and thickening mechanism in aqueous solution of two hydrophobically modified polymers (HM-P) with different architecture have been investigated. The two polymers, hydrophobically modified ethyl hydroxyethyl cellulose (HM-EHEC) and hydrophobically modified polyethylene glycol (HM-PEG), are commonly used as associative thickeners in the paint industry. The strength of the hydrophobic associations in an aqueous solution of a HM-P is very much influenced by the structure of the HM-P. Longer hydrophobic groups give stronger association and longer relaxation times resulting in a more elastic consistency to the solution. The thickening mechanisms of HM-EHEC and HM-PEG were studied by addition of cyclodextrin (CD). In an aqueous solution a CD molecule can form a complex with a hydrophobic molecule or part of a molecule provided that the hydrophobic group fits into the cavity of the cyclodextrin molecule. CD binds primarily to hydrophobic side- or end-groups of the polymer and not to hydrophobic segments of the polymer backbone. The addition of CD provides unique information about the thickening mechanism that can not be achieved by the addition of other substances that disconnect all types of hydrophobic associations, for example surfactants at high concentration. A remarkable observation is that in a HM-PEG solution of an intermediate concentration it is enough to terminate only a small fraction of the total amount of associative linkages to reduce the viscosity almost to the same level as for a solution of an unmodified PEG.