Orientation Dynamics and Correlations in Hairy-Rod Polymers:  Concentrated Regime

We report on the dynamics of orientation fluctuations in substituted poly(p-phenylenes) using dynamic depolarized light scattering. These model hairy-rod polymers, with large inherent optical anisotropy, exhibit enhanced solubility and allow investigation of their dynamics in molecularly dispersed isotropic solutions and concentrations as high as 60 wt %. The high-quality intermediate scattering functions reveal a bimodal relaxation for the orientation fluctuations. Whereas the fast process has an intensity decreasing with the scattering wavevector q following a Debye-like behavior and a rotational relaxation rate with an intercept, the features of the slow mode, exhibiting a broad distribution of relaxation times, are surprisingly opposite: the intensity increases and the rate decreases with q. The concentration dependence of these relaxation modes at different solvents support the notion of unchanged chain conformation and interactions and indicate a slowing down, which is more pronounced for the slow relaxation. With the additional support of dynamic shear rheological measurements, which provide information on the overall rotational motion of these semistiff chains, we attribute the fast process to the collective rotational motion of the Kuhn segments of the persistent poly(p-phenylene) chains and the slow one to the cooperative motion of orientationally correlated pairs of segments.