Microstructural, thermal, crystallization and water absorption properties of films prepared from never-dried and freeze-dried cellulose nanocrystals

In this paper, we report the microstructural, optical, thermal, crystallization and water absorption properties of films prepared from never-dried (ND) and freeze-dried (FD) cellulose nanocrystals. Morphology of the acid hydrolyzed ND CNCs revealed a needle-like structure, while after freeze-drying, they showed a flake-like morphology. Microstructural analysis of ND CNCs and FD CNCs were further studied via SAXS to probe morphology and interactions. ND CNCs yielded a transparent film with a low surface roughness (14±4 nm), while the FD CNC film evidenced a significant reduction of their transparency due to their higher surface roughness (134±20 nm). Although FTIR and EDX analyses revealed no chemical change occurred during the freeze-drying process, yet a more intense thermal degradation profile was observed for FD CNC film, probably due to the higher oxygen ingress within the gaps created in between the stacked flakes. This, in turn, resulted in a greater loss of crystallinity at a higher temperature (300oC) compared to the ND CNC film. In addition, a rapid decrease in water contact angle of the FD CNC film proved that the microstructural morphology of FD flakes and their orientation within the film had a strong influence in increasing water absorption capacity compared to the ND CNC film.