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Characterization of Concentration-Dependent Gelation Behavior of Aqueous TEMPO-Cellulose Nanocrystal Dispersions Using Dynamic Light Scattering Yaxin Zhou, Shuji Fujisawa, Tsuguyuki Saito, and Akira Isogai Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.8b01689 • Publication Date (Web): 17 Dec 2018 Downloaded from http://pubs.acs.org on December 18, 2018
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Biomacromolecules
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Characterization of Concentration-Dependent Gelation
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Behavior of Aqueous TEMPO-Cellulose Nanocrystal
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Dispersions Using Dynamic Light Scattering
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Yaxin Zhou,† Shuji Fujisawa,† Tsuguyuki Saito,† and Akira Isogai*,†
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†
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University of Tokyo, Tokyo 113-8657, Japan
Department of Biomaterials Science, Graduate School of Agricultural and Life Sciences, The
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ABSTRACT: Cellulose nanofibrils (CNFs) and cellulose nanocrystals (CNCs) with high and
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low aspect ratios, respectively, were prepared from wood cellulose by catalytic oxidation with
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2,2,6,6-tetramethylpiperidine-1-oxyl
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Cavitation-induced force was used to prepare TEMPO-CNCs from aqueous suspensions of
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TEMPO-oxidized celluloses. Aqueous dispersions of the TEMPO-CNF and TEMPO-CNCs
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with different solid concentrations were prepared by dilution or condensation. Dynamic light
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scattering (DLS) was used to determine the solid concentrations at the transition points from
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the dilute to semi-dilute regions and from the semi-dilute to dense gel regions in the aqueous
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TEMPO-CNF and TEMPO-CNC dispersions. All the DLS data corresponded well to the fitting
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curves of the normalized time-correlation functions. The solid concentration at the gelation
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point increased from 0.40% w/v for the TEMPO-CNF dispersions to 1.71 % w/v for the
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TEMPO-CNC dispersions, and the aspect ratio decreased from 134 to 57, respectively. The
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solid concentrations of the TEMPO-CNF and TEMPO-CNC dispersions at the gelation point
(TEMPO)
and
subsequent
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sonication
in
water.
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calculated using effective-medium theory based on their aspect ratios corresponded well with
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those experimentally determined by DLS.
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KEYWORDS: Cellulose nanocrystal, TEMPO-CNC, Dynamic light scattering, Gelation point,
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Effective-medium theory
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INTRODUCTION
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Nanocelluloses are prepared from plant cellulose fibers and bacterial, tunicate, and algal
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celluloses by mechanical disintegration in water with or without chemical or enzymatic
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pretreatment. Cellulose molecules in terrestrial and vascular plant cell walls form crystalline
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cellulose microfibrils with small widths of ~3 nm and lengths over several microns as the
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smallest elements.1,2 Nanocelluloses with widths of 3–100 nm are prepared from plant
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cellulose fibers with widths of 20–40 µm and lengths of 1–3 mm by downsizing, with their
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nanostructures originating from the cellulose microfibrils. Plant nanocelluloses are roughly
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categorized as either cellulose nanofibrils (CNFs) or cellulose nanocrystals (CNCs) depending
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on their lengths; CNFs have lengths >300 nm and CNCs have lengths 2 nm were different between SBKP-120 and
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MCC-60, each sample had a highly positive and constant V value at the distances >2 nm in
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pure water. Moreover, both SBKP-120 and MCC-60 had positive (or repulsive) potential
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energies at the distances >0.07–0.09 nm and >0.24–0.27 nm for the crossed and parallel
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orientations, respectively. These results indicate that electrostatic repulsions owing to the
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anionic sodium carboxylate groups densely present on TEMPO-CNC surfaces always work
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between the TEMPO-CNC elements in water.
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It has been reported that TEMPO-CNFs form small clusters in aqueous dispersions.32 In each
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cluster, some TEMPO-CNF elements have nematic-ordered and self-assembled structures
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owing to close packing. These structures result in the formation of birefringence for the
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aqueous dispersions and nano-layered and plywood-like dense structures in cast-dried
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TEMPO-CNF films. The nematic-ordered structures are formed for the TEMPO-CNF/water
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dispersions
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surface-charged TEMPO-CNF elements.32 In the case of TEMPO-CNCs, therefore, the
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concentration-dependent aggregation may occur between the clusters (not nano-elements) in
because
electrostatic
repulsions
efficiently
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work
between
the
highly
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Biomacromolecules
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water, different from the conventional CNCs or other nanocelluloses.15,17,43,44 This is probably
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the reason why the gelation points of TEMPO-CNCs obtained in Figure 6 and Table 2 are
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primarily governed by their morphologies and concentrations in water rather than their surface
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charges. Further studies are, however, needed to clarify the detailed gelation or
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concentration-dependent aggregation mechanism of TEMPO-CNCs in water.
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Interaction potential energy V (kT)
449 10.0
a
5.0 2.5 0.0 -2.5 -5.0
0
5
456 457 458 459 460 461 462
SBKP-120 in pure-water MCC-60 in pure water
7.5
10
15
20
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Distance from solid surface (nm) Interaction potential energy V (kT)
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200
b
SBKP-120 in pure water MCC-60 in pure water
150 100 50 0 -50 -100
0
5
10
15
20
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Distance from solid surface (nm)
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Figure 7. Interaction potential energy V between two cylindrical TEMPO-CNCs in parallel (a)
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and crossed (b) orientations, calculated using the DLVO theory,45 and sizes and carboxylate
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Biomacromolecules
contents (Table 1 in the main manuscript) of SBKP-120 and MCC-60.
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CONCLUSION
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The solid concentrations of aqueous dispersions of TEMPO-CNCs with lengths