(180a) Mechanical Studies of Cellulose Nanocrystals-Polymer Composite Thin Films

Cellulose is one of the most abundant natural materials, typically found in the cell walls of plants. The crystalline form of it can be derived through the hydrolysis of a variety of biopolymers such as wood, cotton, and tunicate in sulfuric acid. This will produce either needle or rod shaped nanocrystals with a range of aspect ratio depending on the raw material. A cellulose single-crystal is very strong; its stiffness is one-seventh that of carbon nanotubes (~1 TPa). The single layer adsorption of cellulose nanocrystals was demonstrated to be fast and produced films of very high loading. This creates a polymer/nanocrystal film with exceptional strength. The low cost of raw material, ease of dispersion, and adsorption quality may allow for more efficient mass production of layer-by-layer composite films using either dipping or spraying methods.

In this study, the polymer/nanocrystal films are made using the layer-by-layer deposition method. Monolayers of chitosan and cellulose nanocrystals were alternatively adsorbed onto Si substrate to produce a thin film. The films' elastic moduli were measured using Brillouin light scattering (for films as thin as 250 nm) in the backscattering geometry. Using this technique, elastic properties of a material are derived from the inelastic scattering of light as it interacts with acoustic phonons. Comparison between the behaviors of surface and bulk acoustic modes allowed us to elucidate the effect of the thickness of individual layers relative to the total film thickness on the overall elastic properties of the composite. The elastic moduli of polymer/nanocrystal composites with crystals of different aspect ratios were measured using Brillouin light scattering. Short cellulose nanocrystals with length of ~200 nm were produced from the hydrolysis of filter paper powder in sulfuric acid. Long crystals with lengths of several microns were produced by the bleaching and hydrolysis of tunicate using sulfuric acid. Layer-by-layer films are fabricated using alternative depositions of chitosan or poly diallyldimethylammonium chloride (PDDA) and cellulose nanocrystals. The moduli of films with long crystals are observed to be dramatically higher than films with short crystals for both chitosan/nanocrystal and PDDA/nanocrystal films. It is also observed that the moduli of chitosan/nanocrystal films are higher than that of PDDA/nanocrystal films.