DYE DIFFUSION IN ISOTROPIC AND LIQUID-CRYSTALLINE AQUEOUS (HYDROXYPROPYL)CELLULOSE

Fluorescence photobleaching recovery has been used to measure the self-diffusion of fluorescein dye in dilute and concentrated aqueous (hydroxypropyl)cellulose (HPC). The mobility decreases with the concentration of the semistiff HPC almost exponentially, independently of polymer molecular weight. Arrhenius-type temperature dependence is observed, and there is no dramatic change in the activation energy for dye diffusion as the lyotropic liquid crystalline phase is crossed. The decline in dye mobility with polymer concentration is considerably steeper than in several other systems where small probes or solvents were measured. Measurements by pulsed-gradient spin-echo NMR of N,N-dimethylformamide diffusion in solutions of an even more rigid polymer, poly(gamma-benzyl alpha,L-glutamate), demonstrate that backbone stiffness is not responsible. Interaction between the dye and (hydroxypropyl)cellulose is one possibility. However, at low concentations the microviscosity sensed by the dye is comparable to the viscosity of water, which is inconsistent with strong binding. Neither is there evidence from steady-state or time-resolved fluorescence spectroscopy for strong dye-HPC interactions. Bound water surrounding the somewhat hydrophobic polymer is clearly evident from differential scanning calorimetry. Immobile water should enhance the obstruction created by the polymer, but the measured amount of bound water is insufficient to explain completely the steep decrease of diffusion with added polymer. The possibility of binding too weak to observe by fluorescence spectroscopy is considered, and an effective binding constant between dye and the polymer-water complex is estimated by combining the differential scanning calorimetry and dye mobility data.