CHARGE AND MASS-BALANCE IN POLYELECTROLYTE MULTILAYERS

The ion and polymer content of polyelectrolyte multilayers constructed via layer-by-layer deposition have been directly probed using radioan alytical methods. Multilayers were fabricated using salt-containing or salt-free solutions. Charges on polyions quantitatively balance each other. As-deposited multilayers contain no salt ions within a limit of detection of a few ppm. All excess charge, which is reversed on each deposition step, resides at the surface. Surface charge controls the a mount of polymer deposited and represents, on average, one-half of the charge within a single molecular layer. Internal charge can be regula ted following deposition if one of the polyelectrolytes employed is re dox-active, such as a polyviologen. Under electrochemical reduction, b ulk charge compensation in a polyviologen/poly(styrene sulfonate) mult ilayer is preserved mainly by cation influx. Residual salt ions accumu late as conformational changes occur during repeated electrochemical c ycling. When a thermally labile precursor to poly(p-phenylene vinylene ) is incorporated as polycation, salt cation uptake is observed when p ositive charge is thermally eliminated from the multilayer. Evidence f or disruption of this structure is observed when the charge density on one of the constituents approaches zero. For typical deposition times , usually up to 1 h, polymer deposition is kinetically irreversible, a nd the top layer is not stripped from the surface on exposure to its o ppositely charged counterpart. These results provide further confirmat ion of extensive interpenetration and disorder as well as limited mobi lity within polyelectrolyte multilayers.