Polyelectrolyte Uptake by PEMs at High Salt

Upon a jump in salt concentration, a polyelectrolyte multilayer (PEM) constructed by the layer-by-layer process will swell, and in consequence, uptake from solution a large additional mass of the capping polyelectrolyte. Here, swelling and uptake are monitored in time by the quartz crystal microbalance with dissipation (QCM-D) method as a function of elevated salt concentration (0.75M$<$[NaCl]$<$2.5 M) during the uptake of poly(styrene sulfonate) (PSS, MW$\sim $70,000 g/mol) by poly(diallyldimethylammonium chloride)/PSS PEMs made at [NaCl]=0.5M. For [NaCl] less than $\sim $1 M, PSS adds only at/near the PEM surface, while for higher [NaCl], PSS fully permeates the PEM, contributing a PSS mass approaching, even exceeding, that already present; higher salt concentration leads to faster and greater PSS uptake. Above [NaCl]=1.0 M, uptake is diffusive, characterized by surprisingly large and sharply [NaCl]-dependent diffusion coefficients ($\sim $10$^{-14} - 10^{-12}$ cm$^{2}$/s). This uptake process opens a general opportunity for facile bulk and surface modifications of PEMs.