Re-examination of dynamics of polyeletrolytes in salt-free dilute solutions by designing and using a novel neutral-charged-neutral reversible polymer

By using the Staudinger ligation to attach 4-{5'-[1''-(dimethylamino)- ethylideneamino]pentyl}- 1 -methyl-2-(diphenylphosphino)terephthalate to poly(p-azidomethylstyrene)-co-polystyrene (PAMS-co-PS), we successfully prepared a novel polymer that can undergo a neutral-charged-neutral transition in DMF with 0.5% H₂O when the solution is alternatively bubbled with CO ₂ and N₂. Such a reversible change is confirmed by its sharp conductivity variation. Armed with this polymer, we re-examined dynamics of salt-free polyelectrolyte dilute solutions by using laser light scattering (LLS). As expected, there exists only one diffusive relaxation mode in the neutral state. The bubbling of CO₂ decreases the scattering intensity and splits this initial diffusive relaxation mode into a fast and a slow diffusive mode, and their scattering intensity contributions are independent of the scattering angle, indicating that the slow mode is not related to some scattering objects larger than the LLS observation length (∼35nm < 1/q < ∼190nm). The bubbling ofN₂ gradually diminishes the slow mode and returns both the scattering intensity and solution dynamics back to their initial neutral state. The change from dilute to semidilute solution slows the fast relaxation down and suppresses the slow mode. On the other hand, the addition of LiBr (50 mM) can completely suppress electrostatic interaction.