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Hindered diffusion of polymers in porous materials
Abstract
Dynamic light scattering (DLS) and forced Rayleigh scattering (FRS) were used to study polymer diffusion in solution in two kinds of porous materials: porous glasses and suspensions and gels formed from fumed silica particles. The diffusants were: dendritic polyamidoamines, linear polystyrenes, and dye-labeled polystyrenes. Polymer diffusion in porous glasses was investigated, by using DLS, as a function of time scale (t), polymer hydrodynamic radius (R$\sb{\rm H}$), and pore radius (R$\sb{\rm P}$). As t increases, the apparent diffusion crosses over from single pore diffusion (in which steric obstruction is weak) to macroscopic diffusion (in which the tortuosity of the pore networks is fully effective). Computer simulated diffusion agreed qualitatively with the crossover observed by DLS. The dependence of hindered diffusion on the size ratio $\lambda\sb{\rm H}$ = R$\sb{\rm H}$/R$\sb{\rm P}$ was studied for dendritic polyamidoamines and linear polystyrenes in porous glasses. For $\lambda\sb{\rm H}$ $\ll$ 1, when hydrodynamic interactions dominate, dendritic polymers diffuse more slowly than linear polymers of comparable $\lambda\sb{\rm H}$. The diffusion results of the dendritic polymer and of the linear flexible polymer agreed quantitatively with the hydrodynamic theories for a hard sphere in a cylindrical pore, and for a random-coil macromolecule in a cylindrical pore, respectively. At large $\lambda\sb{\rm H}$, irregularities in local pore size lead to conformational entropy changes as the macromolecule moves. The experimental data agree qualitatively with the entropy barrier theory. Diffusion of dye-labeled polystyrenes within gels and suspensions formed from fumed silica was studied using FRS. Untreated silica was found to adsorb the labeled polymer, leading to strong hindrance even at very low silica concentration. Thorough quenching of the silica surface by silanization prevented polymer adsorption. The dependence on silica volume fraction of the resulting weakly hindered diffusion in treated silica was found to be consistent with simple theories of steric obstruction.
Subject Area
Polymer chemistry
Recommended Citation
Guo, Yihong, "Hindered diffusion of polymers in porous materials" (1991). Doctoral Dissertations Available from Proquest. AAI9120886.
https://scholarworks.umass.edu/dissertations/AAI9120886