Diffusion of mesoscopic probes in aqueous polymer solutions measured by fluorescence recovery after photobleaching

Fluorescence recovery after photobleaching (FRAP) has been used to follow the diffusion of mesoscopic probes (1 nm < R < 20 nm) in aqueous poly(ethylene oxide) (PEO) and guar galactomannan solutions. We define "mesoscopic" as the regime for which the size of the diffusing species is of the same order as the screening length ξ in the polymer matrix solution. We show that diffusion depends not only on the dimensionless length scale R/ξ but also on the dimensionless time scale corresponding to the relaxation of the polymer mesh by "constraint release" vs the time for motion of the probe species over the length ξ. Two different FRAP techniques were used: fringe pattern bleaching and recovery (FPBR) and confocal scanning laser microscopy (CSLM). The effect of probe structure on diffusion through polymer matrices was investigated by measurements on probes with differing fractal dimensions (d_f): proteins and polystyrene latex particles behave as rigid spheres (d_f = 3); dextrans are slightly branched polymers with a more expanded conformation (d_f = 2.3); dendrimers fall between these two with a density first decreasing and then increasing with generation. Dendrimers at low generations (G0) and high generations (G9-G10) are compact, while the intermediate generations (G2-G6) are more porous. Probe diffusion was found to be a function of the fractal dimension of the probe: the diffusion of rigid spheres was shown to be more hindered in semidilute and concentrated polymer solutions than dextran molecules with the same hydrodynamic size in free solution. The scaling equation D/D₀ = exp[-β(R/ξ)^δ] fit the experimental results well for mesoscopic, rigid spherical probes. The effects of matrix polymer stiffness and polymer molecular weight were also addressed. At constant screening length ξ (i.e., constant polymer concentration) polymers of different molecular weights are used to demonstrate the region of mesoscopic probe diffusion that is independent of the matrix polymer molecular weight. The dependence of diffusivity on the ratio of the matrix polymer persistence length l_p to the mesh size ξ was shown from measurements using the flexible PEO and more rigid guar as matrix polymers. At equal mesh size, diffusion through the more rigid matrix is hindered relative to that through the more flexible mesh; this effect becomes more pronounced as concentration increases and mesh size decreases.