Cellulose nanocrystals (CNCs), isolated e.g. by sulfuric acid hydrolysis from cellulosic fibers, form colloidally stable suspensions of cellulose in their native crystalline form (cellulose I). While the application of electric or magnetic field was known to induce an alignment of the CNCs respectively parallel or perpendicular to their longer axis, the precise mechanism of this orientation coupling remained poorly documented. In this invited talk, we report with two recent experiments how we addressed qualitatively and quantitatively the origin of electric and magnetic alignment of individual CNCs, exploiting the static and dynamic alignment of dilute suspensions under external fields, and monitoring their orientation from the induced birefringence. Electric field alignment was performed on surfactant-stabilized cotton-sourced CNCs in apolar solvent. While static field alignment with increasing field strength allowed to measure the Kerr constant, accounting for both permanent and induced electric dipoles, the use of fast relaxation in dynamic field reversal configuration allowed for discriminating the two contributions, with a dominant permanent dipole as high as 4000 Debye. Such giant dipole in native CNCs could be explained by the build-up of the polarity in the parallel chain conformation of Cellulose I. Diamagnetic properties of cellulose Iβ were experimentally investigated from the magnetic alignment of dilute aqueous CNC suspension. Micron-long tunicate cellulose crystals were aligned using strong magnetic source (up to 17.5 T) to reach nearly 90% alignment. The induced birefringence was then analyzed using three orientation-coupling scenarios (two uniaxial models and one biaxial), while both intrinsic and shape birefringence of CNCs in water were accounted for. Finally, a thorough statistical analysis of the CNC shape distribution allowed for an estimation of the CNC individual volume and therefore of the diamagnetic anisotropy (Δχ = −0.95×10−6) of cellulose Iβ, slightly higher than the one we deduced from Pascal additivity law.

Electric and magnetic properties of cellulose nanocrystals in suspension revealed by field-induced birefringence

255th National Meeting (2018)

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