Renewable Hybrid Biopolymer/Polyaniline Composites for Humidity Sensing

The current study was focused on electrically conductive polyaniline (PANI)-based polymer composites and their films, which contain poly(vinyl alcohol) (PVA). The goal of this research was to evaluate PANI composites that contain biopolymer platforms (chitosan or cellulose micro-powder) as materials for humidity sensing, which were compared against composites that contain carbon nanofibers in lieu of the biopolymer. Ternary film materials were prepared by a two-step method: (1) in situ polymerization of PANI onto the biopolymer or carbon support to yield a binary composite, and (2) physical blending of the resulting binary composite with PVA to yield a ternary composite in a film form. The following mechanical properties were estimated: (a) elastic (Young's) modulus and stiffness, (b) ductility and toughness, and (c) yield and tensile strength. The Young's modulus improved as the chitosan fraction increased, which indicates that chitosan can be used as a mechanical reinforcement additive for PANI-based composites. The electrical properties were characterized by plots of electrical conductivity versus relative humidity, whereas mechanical properties were evaluated using tensile testing. Overall, chitosan-based films showed good mechanical properties, moderate electrical conductivity, and favorable high humidity response and moisture uptake. Cellulose- and carbon-based composites were generally inferior to chitosan for most properties, except for higher conductivity. Due to the high brittleness of carbon-based composites, the practical utility of carbon nanofibers for humidity sensing is limited in the case of pristine carbon materials. This work reveals that ternary PANI-based composites are potential candidates for humidity sensor materials, where moderate PANI-to-chitosan/cellulose/carbon ratios enable good moisture uptake and electromechanical properties of composites. PANI/biopolymer composites are a promising class of sustainable materials due to the ability to tailor their structure and properties, in contrast to conventional ceramic humidity sensor materials.