Molecular orientation and dynamics of flexible polymers in strongly deforming flow fields

Kilfoil, Maria (2001) Molecular orientation and dynamics of flexible polymers in strongly deforming flow fields. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

A method of spatially resolved magnetic resonance spectroscopy has been developed to allow studies of order and dynamics in complex fluids having transverse relaxation times on the order of tens of milliseconds, studies which were otherwise not possible using existing techniques. The model of Doi and Edwards is a microscopic description for stress transmission in concentrated polymer solutions and melts under deformation. Central to the Doi-Edwards model is the dependence of the stress on bond orientational order of the chain segments. Different elements of the segmental alignment tensor for a polymer melt under strong shearing flow are measured here using localized deuterium NMR spectroscopy on a 61 OK molecular weight poly (dimethyl siloxane) melt in a concentric cylinder Couette rheometric cell. This approach provides a new means of testing the Doi-Edwards model and its refinements, in the important regime far from equilibrium where the entangled polymers exhibit nonlinear viscoelastic behaviour. -- The same rheo-NMR methodology is also used to test predictions of the model of Leslie and Ericksen which describes director dynamics in semi-flexible rod-like polymers subjected to viscous stresses. Director dynamics are studied in a lyotropic liquid crystal polymer PBLG (300K) in a highly ordered, nematic phase in a planar extensional flow around a stagnation point. In addition, bulk 2H NMR studies are carried out on PBLG under shear, in concentric cylinder Couette and cone and plate rheometric cells. Magnetic alignment (equivalent to the dynamic Freedericksz transition) is investigated in all three cells following deformation. Values are obtained for the Leslie viscosity coefficients a2 and a3, scaled by the diamagnetic susceptibility. Possible development of mesoscale structure under shear is discussed.

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