Thermothickening in solutions of telechelic associating polymers and cyclodextrins

Telechelic associating polymers (hydrophilic ethoxylated backbone, hydrophobic n-alkyl end-groups) form viscous solutions in water due to associations between the hydrophobes. The addition of alpha-, beta-, or gamma-cyclodextrin (CD) substantially reduces the solution viscosity because the CD molecules envelop and sequester the hydrophobes in their hydrophobic cavities. The present paper explores the variation in polymer-CD solution viscosity with temperature. We find that, in the case of alpha-CD alone, the solutions show "thermothickening", i.e., the viscosity increases from 25 to ca. 60 degrees C whereupon it reaches a peak value and then drops. In contrast, solutions with beta- and gamma-CD show monotonic drops in viscosity upon heating. At a fixed polymer content, the thermothickening is higher for higher alpha-CD concentrations. We have also studied how surfactants and lipids impact the thermothickening. Addition of single-tailed micelle-forming surfactants causes the viscosity to revert to the more typical decreasing trend with temperature. However, addition of double-tailed lipids to a polymer/alpha-CD solution accentuates the thermothickening behavior. The thermothickening is explained by the propensity of alpha-CDs to unbind from the hydrophobes and form inclusion complexes with the polymer backbone as the temperature is raised.