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Light scattering from flexible polymer solutions in elongational flow
Abstract
The non-Newtonian rheology of dilute polymer solutions can be traced to the flow-induced rearrangements in the conformation of flexible, isolated macromolecules. Such rearrangements are most pronounced in strong elongational flows; when such a flow is imposed on a random coil polymer for a sufficient time, the chain can become highly distorted and the fluid properties highly non-Newtonian. A direct determination of the magnitude of flow-induced polymer stretching will lead to a better understanding of the physical mechanisms responsible for the unique rheology of polymer solutions. In this study, light scattering methods are employed for the first time to probe the deformation of flexible polymer coils in dilute solutions undergoing uniaxial elongational flows. Such flows are created at the stagnation point region of an opposed jets flow device. This device is positioned so that its stagnation point can be superimposed on the scattering volume of a specially constructed variable angle light scattering instrument. Flow birefringence is also monitored. After suitable analysis, the two optical measurements provide the radius of gyration parallel to the stretching direction and the degree of segmental orientation; both quantities are averages over a 100 $\mu$m size region. Data are collected from the good solvent pair polystyrene/toluene as a function of elongation rate and polymer molecular weight. The scattering results indicate limited stretching, even at elongation rates greatly exceeding the reciprocal of the Zimm relaxation time. No evidence is found for complete chain unravelling. The birefringence does saturate at large elongation rates, indicating that segmental orientation and global deformation are not necessarily achieved simultaneously. It is suggested that measurements performed at the stagnation point actually reflect a larger spatial region of limited average residence time and that this time is sufficient for segmental orientation but not for chain unravelling. In addition to flexible polymer solutions, birefringence is also used to monitor the behavior of stiff polymers in the same flow. Distinctions between flexible and stiff chains are readily discerned in the spatial variation of the birefringence intensity.
Subject Area
Polymer chemistry|Chemical engineering
Recommended Citation
Menasveta, Malika Jean, "Light scattering from flexible polymer solutions in elongational flow" (1992). Doctoral Dissertations Available from Proquest. AAI9219468.
https://scholarworks.umass.edu/dissertations/AAI9219468