A RHEOOPTICAL STUDY OF POLYDIMETHYLSILOXANE MELT

Results from a rheo-optical study of polydimethylsiloxane (PDMS) melt flowing through a converging wedge cell at room temperature for pressu re drops of 276, 483, and 689 kPa are described. Birefringence measure ments in extensional flow confirmed that stress and polarizability ten sors were linearly related over a range of strain rate that extended w ell into the non-Newtonian region. Local velocity measurements were ma de using Laser Doppler Anemometry (LDA) in order to compute the rate-o f-strain tensor. Along the cell centerline, extensional flow behavior of the polymer was studied via birefringence measurements in the linea r stress-optical region. Also, constitutive equations were tested in c ombined shear and extension flow by comparing their predicted orientat ion angles with experimental values. For this study, the two constitut ive equations tested were the Power-law model and the Goddard-Miller m odel (a quasilinear corotational model). The orientation angles comput ed using a two-term version of the Goddard-Miller model with a single Maxwell-type relaxation time constant of 0.009 s and a zero-shear visc osity of 300 Pa s agreed quite well with experimental results.