Swelling Dynamics of Multilayer Films of Weak Polyelectrolytes

The swelling behavior of thin polyelectrolyte multilayers assembled from poly(acrylic acid) and poly(allylamine hydrochloride) under various pH conditions was characterized in real time by in situ single wavelength ellipsometry. Both the rates and extent of swelling were found to be independent of the solution environment, yet they depended strongly on the pH conditions under which the layers were fabricated. The transport mechanism could be determined by analyzing the rate of swelling and was also observed to vary significantly with the initial assembly conditions of the multilayer films. The swelling behavior was non-Fickian for films constructed with a deposition pH of 3.5, 5.0, and 6.5 for both polyelectrolytes. The time scales to reach maximum swelling depended strongly on pH assembly conditions and varied over at least 3 orders of magnitude from seconds to tens of minutes. Swelling of the films was also observed to depend strongly on the ambient humidity to which the films were exposed before measurement; for example, when equilibrated at 45% relative humidity prior to swelling, films assembled at pH = 3.5 required ∼1800 s to reach equilibrium, while the same films reached maximum swelling in ∼3 s when pre-exposed to an environment of 20% relative humidity.