Plant-Inspired Polyaleuritate–Nanocellulose Composite Photonic Films

Abstract

Plant epidermis is a complex composite material composed by the cuticle and the epidermal cells. In order to prevent dehydration the cuticle is a water barrier composed of an outer layer (proper cuticle) connected to the cell wall of the epidermal cells via a complex matrix often referred to as cutinised cell wall, that acts as compatibilizer for the water repellent cutin and the hydrophilic polysaccharides in the cell walls. Here, biomimetic plant epidermis-inspired films with selective reflection properties were prepared by formation of an aliphatic polyester coating on chiral nematic cellulose nanocrystal (CNC) films. Aleuritic acid, a polyhydroxylated fatty acid, was sprayed on CNC films and polymerized by hot-pressing. The micromorphology of the resultant samples was characterized by scanning electron microscopy (SEM). Polarised optical microscopy confirmed the CNCs helicoidal organization in the films, responsible for the reflection of circularly polarised light, before and after the hot-pressing. The chemical analysis by attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) confirmed the polymerization of aleuritic acid into polyaleuritate with differences between filter paper and woodpulp substrates that were ascribed to water elimination during polycondensation. The characterization of the mechanical (Young’s modulus and hardness from nanoindentation tests) and hydrodynamic (water uptake and water vapor transmission rate) properties indicated that this process enhances the robustness and waterproof behaviour of CNC films. These properties were comparable to those of commercial and biodegradable materials commonly used in packaging such as polyesters and cellulose derivatives, thus making these natural composite ideal for optically responsive packaging applications.J.A.H.-G. acknowledges the funding by the Spanish “Ministerio de Ciencia, Innovación y Universidades”, project numbers RTI2018-096896-J-I00 and RYC2018-025079-I