Third normal stress difference and component relaxation spectra for bidisperse melts under oscillatory shear

A recently developed rheooptical technique that simultaneously measures infrared dichroism and birefringence is used to study bidisperse, linear polymer melts in an oscillatory shear. The method was used to individually measure the contribution of the high and low molecular weight components to the third normal stress difference. This was accomplished by measuring the infrared dichroism due to deuterium-labeled polymers. Binary blends are prepared from nearly monodisperse pairs of hydrogenated and deuteriated polyisoprenes of molecular weights 53 000 and 370 000 with volume fractions of long polymer, Φ_L, of 0.10, 0.20, 0.30, 0.50, and 0.75. The long-chain relaxation in the blends shows the appearance of a peak for compositions with Φ_L ≤ 0.50 at approximately the same frequency as a similar peak in the pure short polymer. The response of the short-chain relaxation in the blends shows the appearance of a low-frequency shoulder that becomes surprisingly large as the volume fraction of long chains increases. The results are used to test a modified reptation model that includes constraint release and a short-range orientational coupling between chain segments