Biaxial deformation of a polymer under shear: NMR test of the Doi-Edwards model with convected constraint release

H-2 NMR quadrupole interaction spectroscopy has been used to measure the deformation of a 670 kD poly(dimethylsiloxane) melt under shear in a Couette cell. The signals were acquired from a per deuterated benzene probe molecule which provides a motionally averaged sampling of the entire segmental ensemble. We have measured the dependence on shear rate of the S-XX (velocity), S-YY (velocity gradient), S-ZZ (vorticity), and S-XY (shear) elements of the segmental alignment tensor, as well as the angular dependence of the deuterium quadrupole splitting at fixed shear rate. We show that the data agree quite well with the Doi-Edwards theory but significantly better when convected constraint release effects are included, These fits return a value for the tube disengagement time of 100 ms.