C. Bruel, J.R. Tavares, P.J. Carreau, M.-C. Heuzey
Keywords: cellulose nanocrystals, thin films, self-assembly, solvent casting, colloidal stability
Summary:Cellulose nanocrystals (CNC) are biosourced and biodegradable rod-like nanomaterials with high expectations regarding their potential as fillers for nanocomposite applications. Despite CNC’s mechanical and chiral properties, CNC-based composites have been somewhat disappointing due to the difficulty encountered when dispersing these predominantly polar particles in non-polar matrices. This illustrates the need for protocols that improve dispersion and allow the transfer of CNC’s useful properties to less polar media. Understanding of the mechanisms underlying the formation of CNC chiral self-assemblies is a key factor for developing novel applications in composite reinforcement, optical sensors, optical encryption, and chiral templating. The process for CNC production, which has been scaled up to 1 ton/day, involves a sulfuric acid hydrolysis that partially esterifies the hydroxyl groups of cellulose, leading to the introduction of sulfate half-ester groups on the surface. Particles are then neutralized through the addition of sodium hydroxide and spray-dried. By introducing ~3.4 –OSO3Na groups per 100 anhydroglucose rings, the process yields CNC that are negatively charged upon redispersion, provided that the solvent has a dielectric constant, ε, sufficiently high to trigger the dissociation of the O(δ-)-Na(δ+) bond. Dispersion is expected to induce significant dissociation and electrostatic stabilization for solvents with ε > ~11 at room temperature. In water (ε = 80.1 at 20 °C), our CNC have a zeta potential of -48 mV at a pH of ~6.8 and form chiral nematic self-assemblies from a given concentration threshold. These structures are preserved upon drying, thus yielding iridescent thin films. Addition of a water-soluble polymer such as poly(ethylene glycol) (PEG) interferes only slightly with the self-ordering process: the optical properties of these water-cast polymer nanocomposites are preserved down to a CNC weight fraction of 60-70 wt% while 2-D percolation networks of CNC form down to 0.25 wt% in nanoparticles. This water-based process is, however, unfit for non-polar polymers, which include poly(lactic acid) (PLA). In recent years, PLA/CNC nanocomposites – a fully biosourced material- have been a hot topic in the literature and most of the solvent-based protocols employed N,N-dimethyl-formamide (DMF). DMF, itself a high dielectric constant solvent (ε = 38.3 at 20 °C), is expected to be a good media to trigger self-organization of the CNC. A comparative rheological study of PEG and PLA/CNC nanocomposites, however, showed that CNC disperse better and form stronger percolation networks in PEG than in PLA. The state of the CNC dispersion in water and DMF was proposed as a potential explanation for this difference. Indeed, while CNC form stable colloidal suspensions in water, they sediment within 40 h in DMF. Here, we performed a comparative study of solvent-cast CNC thin films. Using water, DMF, and binary mixtures thereof as solvents, our results highlight the increasing competition occurring between sedimentation and evaporation driven self-ordering as the volume fraction of DMF in the initial media increases. The loss in iridescent properties is monitored through optical microscopy at the top and bottom layers of the thin films. On-going work focuses on identifying additives susceptible to preserve CNC’s self-ordering potential upon DMF casting.