Diffusion of Macromolecules in Polymer Solutions and Gels: A Laser Scanning Confocal Microscopy Study

Abstract

Laser scanning confocal microscopy combined with fluorescence recovery after photobleaching is an effective tool to measure the diffusion coefficients of macromolecules in cross-linked hydrogels and polymer solutions. In this study, the effects of enzyme treatment on the diffusion of macromolecules (FITC-dextran) in guar solutions and titanium-guar hydrogels are examined. Enzyme treatment with β-mannanase, a polymer backbone cleaving enzyme, quickly increases the diffusion coefficient of the probe molecules in both solutions and hydrogels to that in water. Enzyme treatment of guar solutions and hydrogels with α-galactosidase, a side chain cleaving enzyme, displays a unique behavior due to changes in the fine structure of guar. The removal of galactose branches from the mannan backbone of guar creates additional hyperentanglements (i.e., cross-links), which reduce the water holding capacity of guar and induce syneresis. If the depth at which the diffusion coefficient is measured remains constant, a minimum is observed in the diffusion coefficient as α-galactosidase enzyme treatment time increases. At the site of measurement, the sample changes from a homogeneous guar system to a phase-separated polymer-rich hydrogel and finally to a dilute polymer phase as the polymer-rich hydrogel phase precipitates below the site of measurement. The diffusion coefficient in the dilute polymer phase increases to that in water, while the diffusion coefficient in the hydrogel phase continues to decrease to a value of approximately 6 × 10-8 cm2/s.