Dynamics of Polymeric Liquids in Extensional Flow

Dilute solutions of flexible chains in extensional flow are predicted to undergo a coil-to-stretch transition when the strength of the flow reaches a critical value relative to the inverse of the longest relaxation time, x, of the chains. This criteria is met when [147] 

The flow-strength criteria stated in equation (10.2) has been examined experimentally by Fuller and Leal [72] and Dunlap and Leal [149] using four- and two-roll mills, respectively. These devices allow one to systematically vary the flow type (the relative amount of pure extension to pure rotation). The birefringence was measured for dilute and semidilute polymer solutions as a function of both the magnitude and type of the flow. Simple molecular models of flexible polymer chains suggest that such data, when plotted as a 

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