Cellulose nanocrystals (CNCs) are routinely produced as aqueous suspensions. These are then typically freeze-dried in order to be added into polymeric composites using melt-blending. However, dispersing freeze-dried CNCs into hydrophobic polymers is a challenge. In this study, our objective was to advance our understanding of the impact of freeze-drying methods on the morphology of dried cellulose nanocrystals (CNCs), and on the tensile properties of the resulting PLA-CNC nanocomposites. CNCs were prepared as aqueous suspensions with 10.7% solids content using a sulfuric acid method, and freeze-dried using a procedure typical to our laboratory. In addition, the CNC aqueous suspension was diluted to 1% and directly freeze-dried or sonicated for 10 or 30 minutes, flash frozen, and freeze-dried. The particle size and morphology of the CNCs before and after freeze drying were determined by microscopy. CNCs were then incorporated into PLA using melt-blending extrusion and injection molding. The PLA-CNC nanocomposites were tested for thermal and mechanical properties. Before freeze-drying, CNCs were nano-scale, while agglomerations were observed after freeze-drying. The agglomerate sizes were reduced with dilution and/or increased sonication time, with fibrillar structures observable after sonication and flash freezing. PLA-CNC composites containing CNCs that were subjected to dilution, sonication for 30 minutes, flash frozen and freeze-dried had higher tensile modulus and strength compared with the other treatments.
Stark, Nicole M.; Wei, Liquing; Sabo, Ronald C.; Reiner, Richard R.; Matuana, Laurent M. 2018. Effect of freeze-drying on the morphology of dried cellulose nanocrystals (CNCs) and tensile properties of poly(lactic) acid-CNC composites. In: Proceedings, ANTEC®2018-society of plastics engineers. 5 p.