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.4

Recent Advances in Chiral Nematic Structure and Iridescent Color of Cellulose Nanocrystal Films

“…With well-sealed liquid crystalline samples, the disclination lines often disappear with time, giving mono-domains with uniform director orientation. Magnetic fields may be used to generate well-oriented uniform suspensions and films [24,25,26]. …”

Benchmarking Cellulose Nanocrystals: From the Laboratory to Industrial Production

“…23 The literature commonly presents CNCs as reinforcing agents in nanocomposites, rheological modifiers/stabilizers, and as additives in biomedical devices; these and many other applications are described in a number of comprehensive review papers. 9,18,[23][24][25][26][27][28][29] In addition to their high aspect ratio and large surface area, CNCs have unique physical properties, such as their ability to self-assemble into chiral nematic liquid crystalline phases, [30][31][32][33] align in magnetic [34][35][36][37][38][39] and electric fields, [40][41][42] and exhibit piezoelectric reponsivity. 43 Our work on CNCs has primarily focused on developing hybrid nanomaterials such as films, aerogels, and liquid formulated products.…”

“…[51][52][53][54][55] Although we have targeted advanced applications such as industrial coatings, 56,57 tissue scaffolds, 6,48,50 energy storage 2,58 water purification 59 and food and cosmetics, [44][45][46][47] we remain committed to thorough characterization of CNC particles and interfaces. This has included characterization of chemical, physical, mechanical and selfassembly properties, 34,52,[60][61][62] as well as the development of new methods to predict and assess CNC dispersion. 63,64 Although CNCs have been extracted from a wide variety of natural cellulose sources using numerous methods, 65 CNCs obtained via sulfuric acid hydrolysis from cotton or wood have been the primary focus of both academia and industry.…”

“…119 The formation of these structures in all of the CNCs investigated is an advantage for future applications. However, despite the many academic demonstrations and control of CNC selfassembly in magnetic [34][35][36][37][38][39] and electric fields, [40][41][42] exploitation of this behaviour in a commercially relevant and scalable processes remains to be seen. The most significant impact of Soxhlet extraction observed in this work was in the crystal structure of FPL CNCs.…”

Controlling the Photonic Properties of Cholesteric Cellulose Nanocrystal Films with Magnets

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a variety of parameters. [28,30,31] While the use of external fields, such as electric [32][33][34] and magnetic, [29,[35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50] have been shown to provide a powerful control over the CNC orientation in suspensions, they have been so far considered impractical as a tool to tailor the photonic pro perties of the films. Therefore, the manipulation of the orientation of the helix m has so far not been addressed, or was at best limited to favor its vertical orientation uniformly across the sample.

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