Studies on Polyelectrolyte Solutions VII. Fast, Heterogeneous, and Slow Diffusion Modes of Poly(diallyl-N,N-dimethylammonium chloride) in Aqueous Alcoholic Salt Solvents

Abstract

Dynamic light scattering measurements were performed on aqueous alcoholic salt solutions of poly(diallyl-N,N-dimethylammonium chloride) (PDADMAC, Mw=2.5×105 g mol−1) in the semidilute regime, where the salts used were NaCl and Na2SO4. The results indicate the existence of three diffusive processes, a fast, heterogeneous, and a slow mode. The fast diffusion mode, Dfast, is interpreted as the coupled motion of a polyion blob with its surrounding small ions. Such a blob contains in average g segments which are connected to give a linear coil where the coil behaves as if it were isolated in a dilute solution. Experimentally, it is found that g increases with increasing salt concentration and increasing alcoholic content. This indicates that the electrostatic segment-segment interactions become more and more screened the poorer the solvent quality is. The slow diffusion mode, Dslow, refers to the motion of large polyion clusters, having an internal ordered structure. These clusters can be as large as 300 nm or more. They are stabilized by intermolecular electrostatic interactions and become destroyed by strong hydrophobic interactions, that is when the degree of polyion dissociation converges versus zero, so that the polyions become neutral. At this limit Dslow disappears and may become equal to Dcop, the cooperative diffusion coefficient of a semidilute solution of neutral polymers. The heterogeneous diffusion mode, Dhetero, is caused by the motion of parts of polyion chains that are not incorporated into the polyion cluster domains. Their total concentration is usually rather small so that an observation is often difficult. At one extreme, such a polyion part can be as small as a blob. Then there is no heterogeneous diffusion mode, that is we have Dhetero=Dfast. At the other extreme, Dhetero may describe the motion of a complete single polyion chain whose dimensions may be as large as a cluster domain so that Dhetero becomes equal Dslow. As a consequence, while Dfast and Dslow are nearly always present, Dhetero may lie too near by Dfast or Dslow, respectively, so that a separation becomes impossible. The simultaneous detection of all three modes is possible only by using the multiple tau technique. With the old correlators and the old conventional analysis methods a decomposition of the diffusion modes is very difficult if not impossible. Mostly one finds only one or two modes, which has lead to the erroneous idea, that there is an extraordinary-ordinary diffusion phase splitting. In reality, however, there is no splitting or phase transition. All three modes are present simultaneously where the only technical problem is their resolution.