Fluorescence Polarization Measurements of the Local Viscosity of Hydroxypropyl Guar in Solution

The local viscosity experienced by the chain backbone of fluorescein-labeled hydroxypropylguar (HPG) in aqueous solution has been studied using time-resolved and steady-state fluorescence polarization spectroscopy. These measurements have been undertaken over a range of HPG concentrations and in a series of glycerol/water solutions of varying Newtonian viscosity. The results indicate that despite the bulk viscosities varying over several orders of magnitude as a function of HPG concentration, the rotational dynamics of the fluorophore attached to the backbone are only measurably affected at concentrations of ∼10 wt %. Samples of the HPG at concentrations as high as 10 wt % therefore contain regions with viscosity similar to the aqueous phase as sensed by the probe molecule. Intermolecular interaction between the chains introduces a slow rotational mode at concentrations approaching 10 wt %. The rotational correlation time of the probe attached to the HPG backbone shows a square root dependence on the solvent viscosity in the glycerol/water mixtures. This may be attributed to the restricted motion of the probe molecule around the central axis in two dimensions.