Different orientation patterns of cellulose nanocrystal films prepared from aqueous suspensions by shearing under evaporation

Abstract

Oriented films were prepared from aqueous suspensions of cellulose nanocrystal (CNC; microfibril fragment of sulfuric acid-hydrolyzed cotton) by a shearing method. Rotating glass vials each containing a 3–4 wt% CNC/water suspension under evaporation resulted in formation of translucent films of CNC per se. Structural characterization of the dry films was carried out by use of X-ray diffractometry and optical and scanning electron microscopy. The orientation pattern of CNCs in the films was much affected by pH condition of the starting suspensions; that is, the longitudinal axes of CNCs aligned preferentially perpendicular to the shear direction (SD) in the acidic condition of pH = 2.0, while an ordinary orientation of CNCs aligning parallel to SD was observed in the neutral condition of pH = 6.7 (adjusted with NaOH addition to the acidic suspension, however). To interpret the two distinct orientation patterns, first, it was inspected whether a mesomorphic ordered phase arrived or not in the two sheared and dried suspensions, different from each other in the counterions of surface-sulfated CNCs. As to the orientation development from the suspension of pH = 2, it was particularly assumed that the arising nematic planar domains would have been rolled up into a transversely extended body with the director perpendicular to SD. For the two film preparations, the orientation parameter of the longitudinal axis of CNC was quantified by WAXD intensity measurements, and the data were compared with those for other CNC-oriented materials such as CNC/polymer composites synthesized by immobilizing CNC suspensions via magnetic field application.