NMR as a Probe into the Local and Semilocal Order in Cooperative Interactions of Polyelectrolytes

Polyelectrolytes such as poly(N-diallyldimethylammonium chloride) (PDADMAC) exhibit some local and semilocal order both in their conformation dynamics (dynamic correlations) and the average density of their counterion cloud (the radial density distribution). Both aspects of order are subject to cooperative fluctuations as well as mutual reinforcement. They can be observed in particular in dilute solutions, i.e. at concentrations of the charged groups less than 5 mmol/L. At such dilution, ¹H NMR is the method of choice for the study of conformation dynamics. In addition to the recently published analysis of transverse relaxation and saturation transfer experiments, relaxations of double-quantum coherence, pseudo-solid echo and solid echo responses and exchange 2D spectra are reported here. For the radial distribution of counterions with spin-3/2 nuclei (⁷Li, ²³Na, ³⁵Cl, ⁸¹Br), the relaxation model proposed by Halle, Wennerstrom and Picullel is applied and the use of either combined longitudinal and transverse relaxation or DQC ( T ₂ ³ ) relaxation is shown. The results show that the counterion condensation, i.e. relative ordering of the counterion cloud, decreases in less densely charged polyions and vanishes for the charge separation about 2.5 nm.