Cooperative Ordering and Kinetics of Cellulose Nanocrystal Alignment in a Magnetic Field

Abstract

Cellulose nanocrystals (CNCs) are emerging nanomaterials which form chiral nematic liquid crystals above a critical concentration (C*) and additionally orient within electromagnetic fields. The control over CNC alignment is significant for material processing and end-use; to date, magnetic alignment has only been demonstrated using strong fields over extended or arbitrary timescales. This work investigates the effects of comparatively weak magnetic fields (0 – 1.2 T) and CNC concentration (1.65 – 8.25 wt%) on the kinetics and degree of CNC ordering using small angle x-ray scattering. Interparticle spacing, correlation length, and orientation order parameters (η and S) increased with time and field strength following a sigmoidal profile. In a 1.2 T magnetic field for CNC suspensions above C*, partial alignment occurred in under 2 min followed by slower cooperative ordering to achieve near-perfect alignment in under 200 min (S = –0.499 where S = –0.5 indicates perfect anti-alignment). At 0.56 T, near-perfect alignment was also achieved, yet the ordering was 36% slower. Outside of a magnetic field, the order parameter plateaued at 52% alignment (S = –0.26) after 5 hours, showcasing the drastic effects of relatively weak magnetic fields on CNC alignment. For suspensions below C*, no magnetic alignment was detected.