OLIGOMERS AS MOLECULAR PROBES OF ORIENTATIONAL COUPLING INTERACTIONS IN POLYMER MELTS AND NETWORKS

Simultaneous measurement of infrared dichroism and birefringence was used to study orientational coupling effects between a series of monodisperse polybutadiene oligomers and the surrounding matrix, which consisted of either polybutadiene polymer melt or polybutadiene network. The oligomers had molecular weights ranging from well below the critical molecular weight for entanglement M(e), to several times M(e). For the oligomer/network system, the magnitude of the coupling coefficient was found to remain constant at about unity over the range of network crosslink densities. oligomer molecular weights and degrees of swelling examined, thereby confirming the D n.m.r. experiments performed on the same samples. For the oligomer/polymer system, orientational coupling effects were examined as a function of oligomer molecular weight and temperature. It was found that unentangled oligomers experienced almost complete coupling, while fully entangled melts were subject to a much lower coupling. In addition, it was found that the magnitude of the coupling is independent of temperature, indicating that these orientation correlations are entropic in nature, arising from the packing entropy of chain segments. An orientational dependent lattice model, proposed originally by DiMarzio, was found suitable for predicting experimental results.