FLUORESCENCE ANISOTROPY MEASUREMENTS ON A POLYMER MELT AS A FUNCTION OF APPLIED SHEAR-STRESS

Polybutadiene tagged with anthracene was synthesized and used as a fluorescent molecular probe to study shear-induced orientation in a matrix polymer melt. With the tagged polybutadiene doped into a polybutadiene matrix at 0.1 % concentration, steady-state fluorescence anisotropy measurements were carried out under zero shear and under finite shear conditions using an optically instrumented cone and plate rheometer. Measurements were made over a shear rate range for which the specimen displayed non-Newtonian behavior, 2.64 X 10(-3) to 5.3 s-1. Anisotropy was observed to decrease with increasing applied shear stress. The magnitude of the effect is small and is attributed to shear-induced orientation of the probe molecule which is engaged in the entanglement network of the host polymer. Diluting the entanglement network using plasticizer produced a smaller effect. For polybutadiene plasticized with 50 % cetane, we observed that anisotropy was independent of the applied shear stress, indicating that the probe molecule was not participating in the orientation of the matrix entanglement network. A relationship between anisotropy, chromophore relaxation time, and orientation factors was derived and used to deduce an orientation distribution of fluorescent absorption dipoles and to illustrate the difference between shear and extension stress observations. Extension experiments, carried out using a cross-linked polybutadiene specimen, showed that anisotropy increased as a function of applied extensional stress.