Optical techniques that are sensitive to structural changes induced by a flow field applied to polymers during processing have been used to study the fundamental relationships between applied deformation, mechanical stresses, and flow-induced molecular orientation. But most of the work done so far has used opto-mechanical techniques (i.e. mechanical measurement of stress and deformation, and optical measurement of flow-induced molecular orientation).

This thesis reports the development and application of non-intrusive, opto-electronic techniques for rheo-optical studies on a 300 Pa.s polydimethylsiloxane (PDMS) melt flowing through a 5.00 cm wide converging wedge cell at room temperature. The two techniques used as tools of rheological characterization in the present study are laser doppler anemometry (LDA) to compute strain rate from local velocity measurements, and flow birefringence (double refraction) for measurement of the anisotropic refractive index tensor which, for flexible polymer solutions and melts, provides information the state of stress in the material via the stress-optical law.

Birefringence measurements in extensional flow up to a pressure drop of 689 kPa across the converging wedge cell indicated that stress tensor and polarizability or anisotropic refractive index tensor were linearly related for the polymer over a range of strain rate that extended well into the non-Newtonian region. Along the cell centerline, the extensional flow behaviour of the polymer was studied via birefringence measurements in the linear stress-optical region, and it was found to be extension-thinning in nature. Assuming no boundary layer error, the optical techniques used in the present study provide a valuable test for constitutive relations between stress and deformation in the polymer by comparing predicted orientation angles with experimental measurements in combined shear and extension flows. The two constitutive equations tested were the Power-law model and the Goddard-Miller model (a quasilinear-corotational model). For a two-dimensional converging wedge flow, the orientation angles computed using a two-term version of the Goddard-Miller model with a single Maxwell-type relaxation time constant of 0.009 sec and a zero-shear rate viscosity of 300 Pa.s offered good agreement with experimental measurements at pressure drops of 276 and 483 kPa across the flow cell. Based on this analysis, a stress-optical coefficient of 1.475×10⁻¹⁰ m²/N was obtained for PDMS and compares quite favourably with that reported in literature by Wales [1.35×10⁻¹⁰ m²/N) and Liberman (0.909×10⁻¹⁰m²/N) at the same temperature.