Macromolecular dynamics in extensional flows: 1. Birefringence and viscometry

The opposed jets apparatus is used to investigate the dynamics of the coil-stretch transition of polymer solutions in an idealized stagnation point extensional flow-field. A linear CCD detector allows optical retardation profiles to be recorded as the strain rate is varied. A numerical transformation enables true birefringence profiles to be produced, which enable the assessment of localized molecular orientation and stretching around the stagnation point. High molecular weight, closely monodisperse aPS and PEO are studied in θ and good solvents. Flow modification effects are apparent to extremely low concentration (≈c*/100). Under θ conditions, the width of the transition is consistent with the residual polydispersity. Simultaneous measurements of flow resistance enable the determination of the effective extensional viscosity. The increase in extensional viscosity due to molecular stretching is found to be of the order of the number of equivalent flexible units in the chain, after correction is made for the area of high molecular extension.