Small-angle Neutron Scattering Study on Cellulose Nanocrystal Solution: Phase Behavior and Magnetic Alignment

Cellulose nanocrystals (CNC) were prepared using a sulfuric acid hydrolysis method. CNC dispersions were characterized using small-angle neutron scattering (SANS) technique at both single particle and concentrated suspension levels. The former revealed a parallelepiped particle shape with a length of $\sim$150 nm, and the cross-sectional dimensions of $\sim$3$\times$20 nm. The CNC dispersion showed lyotropic liquid crystal behavior which could be qualitatively described by Onsager's model for rod-like particle solution. Between CNC concentrations (mass fraction) of $\sim$6\% to $\sim$8\%, the homogenous solution spontaneously phase separated into a dense phase having birefringence, and an optically isotropic phase. The birefringent phase showed chiral nematic characteristics under polarized microscope, with chiral pitch distance on the micron scale; with inter-particle distances of $\sim$40 nm, as revealed by SANS. The CNC stacking can be quantitatively examined using a 1D para-crystal model. Under weak magnetic field (0.4 T), the chiral nematic stack in the birefringent phase can be re-oriented with the pitch direction aligned with the magnetic field. The isotropic phase cannot be aligned under weak magnetic field.