Counter-ion dynamics in crosslinked poly(styrene sulfonate) systems studied by NMR.

The field dependence of the longitudinal and transverse nuclear magnetic relaxation rates of 23Na+ in aqueous crosslinked Na-poly(styrene sulfonate) (PSS) systems (ion exchange resins) has been obtained as a function of the degree of crosslinking. The relaxation is considerably enhanced relative to solutions of non-crosslinked NaPSS at equal ionizable group concentration. This is due to the dynamic constraints of the polymer chains, which render the averaging of the counter-ion chain interaction less efficient. The field dependence of the relaxation rates in the crosslinked NaPSS systems reveals two processes that are out of the extreme narrowing limit. This is in contrast to the relaxation behavior found in non-crosslinked NaPSS systems. To characterize these processes their correlation times were combined with constants of selfdiffusion to estimate the distances diffused by an ion in order to average the electric field gradient at its nucleus. These two distances are interpreted as characteristic length scales in the network. At all degrees of crosslinking it was found that the smallest of these length scales is roughly equal to the distance between two neighbouring crosslinks. The largest characteristic distance extends over several crosslinks and reflects inhomogeneities in the crosslink concentration. These conclusions were also reached from similar experiments on 7Li+ in LiPSS systems.