Integrated Study of Water Sorption/Desorption Behavior of Weak Polyelectrolyte Layer-by-Layer Films

Abstract

We present an integrated study of the water sorption/desorption behavior of layer-by-layer (LbL) assembled films made of two oppositely charged weak polyelectrolytes, poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA). The water sorption and desorption of PAH/PAA LbL films are investigated using quartz crystal microbalance with dissipation (QCM-D) monitoring, which allows for the simultaneous determination of the swelling/deswelling ratio and mechanical properties of the LbL films as well as the real-time monitoring of the sorption/desorption dynamics as a function of relative humidity (RH). It is shown that PAH/PAA LbL films that exhibit significant swelling/deswelling hysteresis during water sorption and desorption have higher shear moduli than those that exhibit small hysteresis. The diffusion mechanisms during sorption and desorption, studied as a function of relative humidity (RH), also show hysteresis, which correlates with the humidity-induced swelling/deswelling hysteresis of the films. During sorption, all LbL films initially exhibit Super Case II diffusion at low RH. At high RH, the water sorption mechanism gradually transforms to anomalous diffusion in (PAH/PAA) LbL films with large hysteresis, whereas it changes to Fickian diffusion in (PAH/PAA) films with small hysteresis. Fickian diffusion indicates that the latter films become significantly plasticized at high RH, and thus, the chain mobility is significantly enhanced. During desorption, the transport mechanisms of all of the films are Super Case II diffusion at high RH (>80%). The mechanism gradually changes to anomalous diffusion in the films with large hysteresis at low RH (20–60%), whereas the transport mechanism in the films with small hysteresis changes rapidly to anomalous diffusion at relatively high RH (60–80%). Our results indicate that humidity-induced swelling/deswelling hysteretic behaviors of PAH/PAA LbL films can be attributed to impeded chain relaxation that hampers the response of the LbL films to changes in the activity of water in the gas phase. Our study provides an integrated perspective on the water sorption/desorption properties of weak polyelectrolyte LbL films, which will be useful in developing water-sensitive devices such as humidity sensors, actuators, and gas barriers using weak polyelectrolyte layer-by-layer films.