Structural Changes in Polymer Gels Probed by Fluorescence Correlation Spectroscopy

Introduction Previous studies, e.g., by NMR,1,2 fluorescence pho­ tobleaching,3 and dynamic light scattering (DLS),4,5 have shown that the diffusion of noninteracting probe particles in polymer solutions and gels primarily de­ pends on the polymer concentration and the size of the probe. In interpreting these results, it is widely assumed that polymer gels behave like semidilute polymer solu­ tions, while the structural differences due to the pres­ ence of permanent cross-links are ignored. These dif­ ferences have been revealed and characterized by scat­ tering experiments, such as small-angle neutron scat­ tering (SANS) and light scattering, which show signifi­ cant structural rearrangement of the polymer chains upon gelation.6 This observation is corroborated by results from elasticity measurements, indicating that gelation is accompanied by an increase in the elastic modulus of the samples.7 However, the effect of crosslinking on the diffusion of small particles in a gel has yet to be fully elucidated and understood.4,7 In this paper, we demonstrate how fluorescence correlation spectroscopy (FCS) can provide quantitative measurements on the diffusion of particles in polymer systems. We apply FCS to measure the diffusion time of fluorescent TAMRA molecules in poly(vinyl alcohol) (PVA) solutions and gels prepared at various polymer concentrations and cross-link densities. The measure­ ments indicate that the diffusion of probe particles is affected not only by the polymer concentration but also by the cross-link density of the gel. Remarkably, we find a simple linear relation between the diffusion times and the elastic moduli of the same gels.