Fluorescence Studies of Hydrophobically Modified Hydroxyethylcellulose (HMHEC) and Pyrene-Labeled HMHEC

We report pyrene fluorescence probe experiments which examine the association in aqueous solution of a hydrophobically modified hydroxyethylcellulose (HMHEC) of M w ≈ 300 000 containing on average 1 C 16 H 33 group/143 anhydroglucose units as well as the transformation of this polymer into a pyrene-labeled derivative HMHEC-Py-1 containing 1 pyrene/200 glucose units. Fluorescence studies of mixtures of HMHEC-Py-1 + HMHEC in water demonstrate that both pyrene−pyrene and pyrene−C 16 associations occur, and we are able to infer that even in the most dilute solutions that we can examine these polymers exist as small aggregates of several polymer molecules. HMHEC-Py-1 exhibits excimer emission at concentrations as low as 0.01 g/L. As the polymer concentration is increased, one observes a sharp increase in the reduced viscosity at 1.5−2.0 g/L. Over the same range of concentrations HMHEC-Py-1 exhibits an increase in I E / I M values, indicating that the viscosity increase is due to a substantial increase in polymer association. HMHEC solubilizes pyrene as a fluorescent probe. The limiting solubility, approximately 1 pyrene/35 C 16 alkyl groups, and the partition coefficient (5 × 10 4 ) are much smaller than those found for other more flexible associating polymers, yet the I 1 / I 3 value is consistent with solubilization of pyrene in domains large enough to minimize exposure of the probe to water. We infer that HMHEC in water forms hydrophobic domains with a large size polydispersity. The mean aggregate size is small, but hydrophobic probes such as pyrene and bis(1-pyrenylmethyl) ether are solubilized preferentially in the largest hydrophobic domains.