Charge and Mass Balance in Polyelectrolyte Multilayers

Abstract

The ion and polymer content of polyelectrolyte multilayers constructed via layer-by-layer deposition have been directly probed using radioanalytical 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 amount of polymer deposited and represents, on average, one-half of the charge within a single molecular layer. Internal charge can be regulated following deposition if one of the polyelectrolytes employed is redox-active, such as a polyviologen. Under electrochemical reduction, bulk charge compensation in a polyviologen/poly(styrene sulfonate) multilayer is preserved mainly by cation influx. Residual salt ions accumulate as conformational changes occur during repeated electrochemical cycling. When a thermally labile precursor to poly(p-phenylene vinylene) is incorporated as polycation, salt cation uptake is observed when positive charge is thermally eliminated from the multilayer. Evidence for 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, and the top layer is not stripped from the surface on exposure to its oppositely charged counterpart. These results provide further confirmation of extensive interpenetration and disorder as well as limited mobility within polyelectrolyte multilayers.

Time‐Controlled Desorption of Ultrathin Polymer Films Triggered by Enzymatic Degradation

“…As suggested in previous reports, the charges that exist inside the LbL assemblies are compensated by electrostatic interactions. [8] Accordingly, the enzyme adsorbed on the surface seemed to hydrolyze the DNA in a stepwise fashion, thus progressing to the inner layer. Small amounts of the assembly as well as the enzyme remained even after sufficient reaction time, possibly because of difficulties associated with desorption (Figure 2 b).…”

“…This result suggests that the present methodology enables three-dimensional regulation of the surfaces of the nanostructure of ultrathin polymer films. Notably, the enzyme incorporated inside the film composed of (PDDA-DNA) 8 -PDDA-DNase I-PDDA-(PDDA-DNA) 8 and prepared by stepwise assembly did not degrade the DNA at concentrations of both cations of 5 mm, indicating that the ions did not diffuse into the film to allow enzymatic activity.…”

Deposition Mechanisms in Layer-by-Layer or Step-by-Step Deposition Methods: From Elastic and Impermeable Films to Soft Membranes with Ion Exchange Properties

“…Usually the linearly growing films display "internal charge compensation," this means that, with exception to the film/substrate and film/solution interface, the amount of charges provided by the polycation exactly matches the amount of charges provided by the polyanion, meaning that the bulk of the film does not need to incorporate counter ions to ensure its electrical neutrality [57]. These films are nevertheless hydrated [58,59] and display some porosity at the nanometer scale.…”

Ion Transfer in Layer-by-Layer Films

“…Many factors influence the formation process of polyelectrolyte LbL films, including polymer type, molecular weight, concentration, deposition time, salt type and concentration, pH, and solvent composition. [2][3][4][5] Among them, salt concentration has shown to have predominant effect on the resultant film structure. Polyelectrolytes are the most thoroughly investigated class of substances in LbL studies; thus, the preparation parameters are well correlated with the film structures made of polyelectrolytes.…”