Dynamics of Hairy-Rod Polymers: Semidilute Regime

Abstract

Photon correlation spectroscopy was employed in both the polarized (VV) and depolarized (VH) geometries in order to investigate the dynamics of concentration and orientation fluctuations of model hairy-rod poly(p-phenylenes) in semidilute solution. These materials possess a large inherent optical anisotropy, which allowed the unambiguous detection of orientation relaxation. By probing the dynamics in two solvents of different qualities, chloroform, a good solvent, and toluene, it was possible to distinguish nearly molecular from aggregate processes. Two modes were detected in the VV correlation function: the fast cooperative diffusion, Dc, which increased nearly linearly with concentration (Dc/D0 − 1 ∼ c0.9), and a slow mode, Dslow, which slowed with concentration (Dslow ∼ c-1). The latter process was assigned to the self-diffusion of small aggregates, as confirmed by independent pulsed field gradient NMR measurements. Broad VH correlation functions were obtained at high concentrations and revealed the presence of a dominant mode exhibiting a decay rate, ΓVH, which decreased with increasing q and concentration (ΓVH ∼ c-0.25). This mode was assigned to orientational correlations in space, due to some local ordering of parts of the molecules. These findings cannot be accounted for by the existing theories.