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Abstract:

Scission of polymers results in large viscosity losses, which is major concern in different industrial applications. In this work we study the relevance of the two characteristic numbers: Weissenberg and Deborah number for polymer scission. Weissenberg number defines scission process to be a critical stress criterion (instantaneous process) and gives the actual force required to break the C-C bond. While a Deborah number, defines the scission process to be an energy activated process; when a force is applied to stretch the polymer the energy barrier required decreases (gradual process). To understand the most relevant criterion for scission, semi-dilute unentangled polymers solutions are injected through specially- designed microfluidic hyperbolic contraction geometries with Hencky strains ranging from 1.4 to 4 units. The ratio of the pressure drop of the polymer solution to that of the (Newtonian) solvent has a maximum when plotted as function of flow rate. This maximum results from the competition between polymer extension and scission. For a given Hencky strain, the maximum in pressure drop ratio curve occurs for a fixed Weissenberg number. With increase in Hencky strain the maximum in pressure drop ratio curve occurs for smaller Weissenberg number. At the maximum in pressure drop ratio, the Deborah number is exponentially related to the strain rate. From the equivalence with the TABS model, rate of scission has a similar exponential dependence with the force required to break the polymer chain, we suggest scission process to be an energy activated mechanism. Furthermore, scission is also studied from the double passages. A scission model is proposed to predict the quantity of scission in the first passage by characterizing the pressure drop ratio of the second passage. This scission model consists of scission exponent b, which defines the broadness of transition from no scission to scission dominated regime and is a function of the Hencky strain.