Concentration driven cocrystallisation and percolation in all-cellulose nanocomposites

All-cellulose nanocomposites reinforced by cellulose nanocrystals (CNC) were produced using a solvent consisting of 1-butyl-3-methylimidazolium chloride and dimethyl sulfoxide. Microcrystalline cellulose (MCC) was pre-dissolved at high temperature in the solvent. Freeze-dried CNC were then added to the slurry at room temperature, thereby avoiding complete CNC dissolution. Solid all-cellulose composite films were obtained by film casting, solvent exchange and drying. The MCC to CNC ratio was kept constant while the solvent content was incremented. The short-range and long-range cellulose-cellulose interactions in the solid materials were respectively assessed by Fourier-transform infrared spectroscopy and X-ray diffraction. The CNC used in this work contained both cellulose I and cellulose II. The cellulose concentration in the mixture drastically changed the overall crystallinity as well as the cellulose I to cellulose II ratio in the ACC. Cellulose II was formed by recrystallisation of the dissolved fractions. These fractions include the pre-dissolved MCC and the cellulose II portion of the CNC. Cocrystallisation with the cellulose I CNC acting as a template was also evidenced. This phenomenon was controlled by the initial solvent content. The correlation between the hygromechanical properties and the nanostructure features of the ACC was investigated by humidity-controlled dynamic mechanical analysis (RH-DMA). The introduction of the cocrystallisation and percolation concepts provided a thorough explanation for the humidity dependency of the storage modulus.