Cooperative interactions of unlike macromolecules 2:: NMR and theoretical study of electrostatic binding of sodium poly(styrenesulfonate)s to copolymers with variably distributed cationic groups

Cooperative electrostatic binding of sodium poly(styrenesulfonate)s (PSS) of varying chain length to poly[diallyl(dimethyl)ammonium chloride-co-acrylamide] (DADMAC-AA) copolymers with varying charge density were investigated using H-1, Na-23, and Cl-35 NMR spectra and relaxations and H-1 pulsed-gradient stimulated echo (PGSE) self-diffusion experiments. First, DADMAC-AA copolymers were shown to exhibit increasing chain rigidity and tighter counterion binding both with increasing charge density and progressive dilution by D2O. These effects can be alleviated by increasing the ion strength of the solution and are thus mainly due to the polyelectrolyte effect. Second, p-toluenesulfonate as a model of a lone PSS unit was shown to bind to DADMAC-AA to a less-than-statistical degree ct indicating thus a positive single-group increment of binding Gibbs energy for PSS, DeltaG((1)) > 0. Third, PSS of various chain lengths were found to bind to DADMAC-AA to a larger-than-statistical (mostly almost quantitative) alpha, exhibiting thus an intramolecular cooperativity of the second (or true) kind. In the cases of lower cooperativity (short PSS and DADMAC-AA with low charge density), alpha approximate to 0.6 and is lowered to 0.35 when increasing the ion strength to 1 hi NaCl. For large cooperativity (longer PSS and high-charge-density DADMAC-AA), alpha is somewhat decreased (to alpha approximate to 0.9) at low ion strength but increases to almost 1.0 in 1 M NaCl. Mobility studies of the AA sequences in the products obtained at low ion strength, using transverse relaxations and PGSE measurements, indicate the existence of uncoupled regions (rather than single groups), hence non-equilibrium coupling due to the parking problem. As increased ion screening by an added electrolyte as well as prolonged storage of the sample increases alpha to almost 1.0, this behavior can be diagnosed as a pseudo-irreversible or kinetic effect.