Moisture Absorption and Absorption Kinetics in Polyelectrolyte Films:  Influence of Film Thickness

Abstract

Specular X-ray reflectivity (XR) and quartz crystal microbalance (QCM) measurements were used to determine the absorption of water into thin poly(4-ammonium styrenesulfonic acid) films from saturated vapor at 25 degrees C. The effect of film thickness on the absorption kinetics and overall absorption was investigated in the range of thickness from (3 to 200) nm. The equilibrium swelling of all the films irrespective of film thickness was (0.57+/-0.03) volume fraction. Although the equilibrium absorption is independent ofthickness, the absorption rate substantially decreases for film thickness < 100 nm. For the thinnest film (3 nm), there is a 5 orders of magnitude decrease in the diffusion coefficient for water.

Solvent retention in thin polymer films studied by gas chromatography

“…Segregation of solvent at the polymer/substrate interface of supported polymer films has already been observed in absorption experiments in which the films are put in the presence of the solvent vapor [21][22][23][24][25]42]. It was attributed to the preferential interaction of the solvent with the substrate [25].…”

“…The absorption rate of water vapor in thin films of poly(vinyl pyrrolidone) [25] and in the polyelectrolyte poly(4-ammonium styrene sulfonic acid) [23] was also found to be independent of film thickness. Note, however, that we work in a rather different regime of solvent concentrations.…”

“…To analyze their data, Vogt et al [23,25] assumed that the absorption rate was purely determined by the diffusion of the absorbed molecules, after the diffusion models developed for bulk and bulky films [55]. In this case, an absorption rate independent of film thickness means that D/ h 2 (where D is the diffusion coefficient of the absorbed molecules in the film) is independent of film thickness and, therefore, that D varies as h 2 [23,25].…”

“…In this case, an absorption rate independent of film thickness means that D/ h 2 (where D is the diffusion coefficient of the absorbed molecules in the film) is independent of film thickness and, therefore, that D varies as h 2 [23,25]. If we make the same assumption, our observation also imply that the diffusion coefficient of toluene in the films varies as h 2 and, therefore, decreases by several orders of magnitude as the film thickness decreases down to ∼10 nm.…”

“…Some of the results mentioned above suggest that the solvent could be segregated at the polymer/substrate interface [19,20]. Water absorbed in films of different polymers has also been shown to be located at the polymer/substrate interface [21][22][23][24][25]. Another relevant question is the dependence of the amount of retained solvent with the film thickness.…”