Development of a vapor phase polymerization method using a wet-on-wet process to coat polypyrrole on never-dried nanocellulose crystals for fabrication of compression strain sensor

Vapor-phase polymerization (VPP) of polypyrrole (PPy) is a versatile and simple form of surface restraint polymerization onto a variety of substrates; only the polymerization initiator (an oxidant) is adsorbed onto the surface. However, traditional VPP features substrate (film, foam and fabric) impregnation/coating with an oxidant, followed by short drying to evaporate excess solvent, and then pyrrole polymerization onto the oxidant coated surface with controlled relative humidity. When seeking to coat PPy on water dispersed nano-materials such as never-dried nanocellulose and chitosan nanoparticles, in-situ chemical oxidative polymerization is most often used. However, bulk polymerization and impurity formation hinder application of the technique to wet substrates. Here, we develop an aqueous wet-on-wet vapor-phase polymerization (WOW-VPP) technique to apply restraint PPy coatings onto wet substrates. Never-dried cellulose nanocrystals (CNCs) served as the model material; we formed PPy-nanocoated hybrids (CNC-PPy), which were then transformed into a lightweight aerogel serving as a compression strain sensor. The sensor evidenced good performance under repeated compression cycles as well as resistivity changes under various applied loads. For proof-of-concept, we used WOW-VPP to coat other wet substrates including polyvinyl alcohol (PVA). The WOW-VPP approach to fabrication of PPy nanohybrids is not associated with bulk polymerization, and is performed at room temperature ambient pressure, renewable and sustainable. Furthermore, the process is solution-processed, simple, and amenable to scale-up.