| The extensional degradation of polymer solutions in a fast transient flow is studied. Extensional flows are prevalent in many polymer processing operations, such as film stretching, fiber spinning, and blow molding, but their effect on polymer degradation is not well understood. Degradation in this type of flow field is much more extensive than in a pure shear flow due to the lack of vorticity. This research extends solution degradation studies to the semidilute and concentrated regimes, where entanglements are expected to affect chain scission. The effect of scission on samples with different polydispersities and the effect of solvent viscosity on chain scission is also studied. Polystyrene solutions are degraded in an opposed pistons, multipass extensional flow apparatus. This device cycles the solution across a sharp contraction, exposing the coils to a high extension rate in a very short time period. The centerline strain rate is varied over a two decade range ({dollar}5times 10sp3{dollar} to {dollar}5rmtimes 10sp5 ssp{lcub}-1{rcub}{dollar}) by changing the piston speed and the contraction ratio.; Degradation is quantified by changes in the molecular weight distribution (MWD). Chains are broken near the center, a phenomenon explained by the mechanism of coil unraveling in a strong flow. For a polydisperse sample, the high molecular weight species are preferentially ruptured. There is a critical molecular weight (M{dollar}sb{lcub}rm c{rcub}{dollar}) above which all chains are broken, but low molecular weight chains are not fractured. Multiple passes through the high extension rate region increase chain scission, although degradation is the greatest in the first pass through the high extension rate region. For a monodisperse sample, experiments in different concentration regimes point to a dual mechanism of chain scission. For isolated chains, intramolecular entanglements are responsible for scission. For interacting chains, both intra- and intermolecular entanglements result in bond rupture.; The flow field is modeled using POLYFLOW to determine the magnitude of important variables associated with the chain scission process. The centerline extension rate, energy dissipation rate, residence time in the high extension rate zone, the fluid strain, and the flow character are calculated and mapped onto the flow field. |
