Scission of flexible polymers in contraction flow: Predicting the effects of multiple passages.

When injected through a contraction, high molecular weight polymer solutions exhibit a sharp increase in apparent viscosity, which originates from stretching polymer chains above a critical extension rate. This chain stretching can also induce polymer scission, which then decreases the extensional viscosity. In practice, the two phenomena are difficult to separate. Moreover, these phenomena are often observed in situations where flow instabilities appear. In order to disentangle the two effects, we have measured the pressure-flow rate relation for polymer solutions passing through a hyperbolic contraction. The ratio of the pressure drop to that of the (Newtonian) solvent exhibits a maximum due to the competition between polymer extension and scission. We find a geometry-dependent relation between the flow rates at which the maximum occurs for successive passages in a given contraction, which appears to be independent of molecular weight, concentration, solvent quality, and viscosity and can be used to predict the scission under successive passes. [ABSTRACT FROM AUTHOR]