RHEOLOGY OF SMALL SPHERICAL POLYSTYRENE MICROGELS - A DIRECT PROOF FOR A NEW TRANSPORT MECHANISM IN BULK POLYMERS BESIDES REPTATION

A recently developed synthetic technique, the polymerization in microe mulsions, gives access to a new type of model polymer, spherical micro gels or ''rubbery nanospheres''. These polymer molecules are narrowly distributed and have sizes in the range of 5 nm < R < 50 nm. In this p aper, we report the examination of the viscoelastic behavior of these microgels in the bulk above their glass transition. Opposite to all ex pectations, these microgels show viscous flow up to molecular weights of M(W) = 2 x 10(6) or related hard-sphere radii of R = 9 nm, and the zero-shear viscosity is remarkably lower than the one of linear chains of the same molecular weight. Similarily surprising, it is shown that frequency dependence of the mechanical response looks like the well-k nown Zimm- or reptation-type response, although a Zimm- or reptation-l ike mobility can be strictly excluded by the spherical architecture of the molecules. These results support the existence of a strong, secon dary type of polymer mobility besides reptation which is more cooperat ive in its nature than a single-chain mobility. This mobility should p lay an important role in all bulk polymer systems and might explain th e rather high transport rates and deviations from reptation reported i n many polymer systems with higher complexity.