ASSOCIATION DYNAMICS IN SOLUTIONS OF HAIRY-ROD POLYMERS

Photon correlation spectroscopy in the polarized (VV) and depolarized (VH) geometries has been utilized in order to investigate the aggregat ion dynamics in toluene solutions of a hairy-rod polymer, poly(p-pheny lene) with flexible dodecyl side chains. Static and dynamic results fr om the dilute regime suggest that polymer molecules form small aggrega tes, typically trimers, even at very low concentrations. In the semidi lute regime, results at different scattering wavevectors (q) in both t he VV and VH geometry reveal two new relaxation processes (slow and ul tra slow), in addition to the faster cooperative diffusion and reorien tation of the trimers, respectively. They are attributed to the format ion of large anisotropic clusters (of typical size 570 nm), with inher ent crystallization, as revealed by X-ray scattering. After about 2 we eks the clusters settle under the action of gravity and form an opaque sediment. Assessment of the aggregation modes has been carried out by sediment redispersion. The first stages of this process, which exhibi t multiple scattering, reveal a steeper than exponential relaxation, a ssociated with a sedimentation-induced velocity gradient in a transien t pseudonetwork formed by the dispersed clusters. In the single scatte ring limit, reached at later stages, the two relaxation processes are related to reorientation (slow) and number density fluctuation (ultras low) of the anisotropic clusters. On the basis of simple thermodynamic arguments, we argue that rodlike macromolecules exhibit a larger acti vation energy barrier compared to their flexible counterparts, in agre ement with the experimental evidence.