Unexpected Thickness-Driven Volume Expansion in Tunicate Cellulose Nanocrystal-Based Multilayer Films

Abstract

Cellulose nanocrystals are rod-like nanoparticles obtained from different cellulose sources (plants, bacteria, algae, or tunicates) and represent a class of renewable, biodegradable, and mechanically robust nanomaterials with great potential for the design of advanced functional materials. In this study, we report on the preparation of multilayer composites based on tunicate cellulose nanocrystals and poly(vinyl amine) using the layer-by-layer assembly technique. The assembly of these nanostructured films was characterized by ellipsometry. Surprisingly, films composed of tunicin nanocrystals and poly(vinyl amine) showed a highly unusual film growth behavior by dipping, spray- and spin-assisted assembly, and grazing incidence spraying methods, which contrasted with previous reports. When the film thickness reached approximately 50–60 nm, the deposition of additional layers induced a sharp increase in thickness that nearly doubled the volume of the film by expansion in the direction normal to the surface. This behavior was also observed by atomic force microscopy, ruling out any optical artifact due to ellipsometry. Ellipsometry measurements revealed that this thickness jump (i) was independent of the assembly conditions, (ii) was associated with an increase in film porosity, and (iii) clearly correlated with the aspect ratio and rigidity of the tunicate cellulose nanocrystals. This unexpected phenomenon was not observed for films assembled from either shorter cellulose nanocrystals extracted from cotton or wood, or from longer and more flexible cellulose nanofibrils in combination with poly(vinyl amine). Such a pronounced structural transition has never been previously observed, and its origin remains unexplained despite the experiments performed to date.