Ionic Conductivity Enhancement of Polyelectrolyte Multilayers by Variation of Charge Balance

Schlicke J, Hoffmann K, Lorenz M, Schönhoff M, and Cramer C

Abstract

Three different approaches to enhance the dc conductivity of poly(diallyldimethylammonium)/poly(acrylic acid) (PDADMA/PAA)n polyelectrolyte multilayers (PEMs) are investigated. The first two approaches concern the effect of varied preparation conditions. First, an increase of the pH value of preparation modifies the charge degree of PAA and results in dc conductivities increased by more than 1 order of magnitude. In a second approach, PEMs are prepared in the presence of 0.1 M LiCl solutions. The increase of the conductivities at a higher charge degree of PAA is further amplified as compared to the preparation from water: The dc conductivity increases from (8 ± 2) nS·cm-1 at pH 3 to (6 ± 2) μS·cm-1 for pH 8.5. This strong effect is attributed to enhanced ion mobilities caused by increased hydration, and is discussed with regard to film composition and structure. The third approach involves successive postpreparational treatments of (PDADMA/PAA)n PEMs prepared at pH 4. Such multilayers are immersed into 0.1 M LiCl solutions of stepwise increasing pH to modify the charge balance. The structural changes induced by postpreparational treatment are elucidated by in situ studies utilizing attenuated total reflection infrared spectroscopy (ATR-IR). Upon pH treatment the PAA dissociation is enhanced only in the presence of salt, and the conductivities increase exponentially with pH. Two mechanisms of charge balancing are discussed to explain the adjustment of the films induced by postpreparational treatment: (i) intrinsic charge adjustment by partial desorption of PAA and the relocation of PDADMA and (ii) extrinsic charge compensation by Li+ ion incorporation. Though extrinsic compensation by Li+ ions may be involved, the dominant influence on film conductivity isnot an enhanced charge carrier density but rather a highly ionic polymer matrix environment, which promotes hydration and thus exponentially enhances charge carrier mobility.