Molecular weight dependence of component dynamics in bidisperse melt rheology

Simultaneous measurement of infrared dichroism and birefringence is used to study component relaxation in bimodal molecular weight distribution melts. Nearly monodisperse poly(ethylenepropylene) samples of molecular weights 53, 125 and 370K, all above the critical molecular weight for entanglement, were used. Results for the step-strain relaxation of each component and of the total sample are presented and discussed for binary blends of 10, 20, 30, 50 and 75 vol.% of the higher molecular weigtht species for three sets of blends: 53K/125K, 53K/370K, and 125K/370K. For each sample, the relaxation dynamics of the blend and of each component depend upon the two polymer relaxation times and the blend composition. Effects of intermolecular orientational coupling interactions were observed, and a coupling strength of 0.45 was measured. The results of these experiments are compared to a reptation-based constraint release model, and a qualitative agreement is found. Graphs. 30 ref.--AA