DIFFUSION OF WEAKLY CONFINED STAR AND LINEAR-POLYMERS AND STRONGLY CONFINED LINEAR-POLYMERS IN A POROUS MATERIAL

We report the direct measurement, by dynamic light scattering, of the macroscopic diffusion coefficient D∞ (at low q) of linear, four-arm star and eight-arm star polyisoprene in silica controlled pore glass, for values of the confinement parameter λH = RH/Rp < 0.1, where RH is the hydrodynamic radius of the polymer and Rp is the radius of the pores in the glass. The reduced diffusion in the pores is found to depend on the molecular architecture of the polymer. For a given λH, the branched polymers diffuse slower than the linear polymer; it is also found that eight-arm stars diffuse more slowly than fourarm stars of the same hydrodynamic radius. The results are compared to hydrodynamic theories for hard spheres in isolated cylindrical pores. Our results indicate that the hydrodynamic radius of a polymer derived from its diffusion coefficient in dilute unbounded solution does not uniquely describe the hydrodynamic effects of a constraining wall on its diffusion behavior. We also report measurements of the macroscopic diffusion D∞ of strongly confined linear polystyrene chains with λH values up to 0.74. Over the range of confinement investigated, an asymptotic region described by a power law relationship between D∞ and molecular weight is not observed. Our data suggest that the presence of two length scales in the porous material could play an important role in the diffusion of strongly confined chains, in accordance with the theoretical model of Muthukumar and Baumgartner. © 1990, American Chemical Society. All rights reserved.