MACROMOLECULAR DYNAMICS IN EXTENSIONAL FLOWS .2. THE EVOLUTION OF MOLECULAR STRAIN

The opposed jets apparatus has been used to investigate the dynamics o f the coil-stretch transition of polymer solutions in an idealized sta gnation point extensional flow field. Flow simulations generated fluid strain profiles for different geometries. Assuming a molecular uncoil ing model, true birefringence profiles have been converted to molecula r strains for closely monodisperse, high molecular weight aPS under th eta-conditions. This has enabled macromolecular deformation to be foll owed as a function of position and residence time. Non-linear hydrodyn amic friction FENE dumb-bell simulations give qualitative agreement. I nitially, molecular uncoiling is non-affine, consistent with changing hydrodynamic screening on extension. Deformation in a good solvent is more affine. Results are compared with PEO/water to investigate the ef fect of molecular parameters. The effective extensional viscosity has been ascertained by correction for the area of high molecular extensio n. The increase in extensional viscosity due to molecular stretching i s substantial, of the order of the number of equivalent flexible units in the chain. (C) 1997 Elsevier Science Ltd.