| Abstract: | Based on a linear stress-optical relation, off-center and centerline velocities along with the orientation angle measurements (via the birefringence method) can be used to test constitutive equations for polymeric melts without the need to measure viscosity hood stresses directly. For polydimethylsiloxane (Dow Corning type 200 grade 300 Pa · s) melt in a 60° wedge flow cell (inward flow) at a pressure drop of 70 psi, the orientation angles computed by the Power-Law model and Goddard-Miller model with two terms in the Taylor Series and a single relaxation time of 0.005 s were compared with the experimentally measured values. It was found that the Goddard-Miller model agreed quite well with the measured data. The stress-optical coefficient (C) was evaluated from the velocity, birefringence, and orientation data. It was calculated to be 1.44 × 10?10 m2/N for combined shear and extension using the two-term Goddard-Miller Model. For centerline flow (purely extensional), the zero-extension-rate elongational viscosity (η 0) was calculated to be 930 Pa · s, slightly greater than three times the zero-shear-rate viscosity (η 0). The cell can be adapted for transient flow experiments. |
