Humidity Swelling Hysteresis In Electrostatically Crosslinked Polyelectrolyte Multilayer Films

Polyelectrolyte multilayers (PEMs) are thin polymer films created via the alternating adsorption of positively and negatively charged polyelectrolytes from aqueous solutions.  Although PEMs are generally insoluble due to internal electrostatic crosslinks formed during the deposition process, the films remain moisture-responsive and hence are good candidates as humidity sensors and stimuli-responsive coatings.  In this talk, we demonstrate that PEMs assembled from poly(allylamine hydrochloride) and poly(acrylic acid) demonstrate humidity swelling/deswelling hysteresis, which ultimately can be attributed to the internal crosslink structure of the PEM.  The hysteresis phenomenon is remarkably persistent, with a characteristic relaxation time on the order of hours to days.  At even longer times, films retain a swelling difference of a few percent, demonstrating that a PEM can maintain a “memory” of its drying history.  These results suggest that careful control over PEM drying and storage may be warranted, as such treatments may affect subsequently measured film properties.