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Polymer induced liquid crystal phase behavior of cellulose nanocrystal dispersions - PubMed

  • ️Sat Jan 01 2022

. 2022 Oct 6;4(22):4863-4870.

doi: 10.1039/d2na00303a. eCollection 2022 Nov 8.

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Polymer induced liquid crystal phase behavior of cellulose nanocrystal dispersions

Qiyao Sun et al. Nanoscale Adv. 2022.

Abstract

Cellulose nanocrystals (CNCs) are a promising bio-based material that has attracted significant attention in the fabrication of functional hybrid materials. The rod-like shape and negative surface charge of CNCs enable their rich colloidal behavior, such as a liquid crystalline phase and hydrogel formation that can be mediated by different additives. This study investigates the effect of depletion-induced attraction in the presence of non-absorbing polyethylene glycol (PEG) of different molecular weights in CNC aqueous dispersions, where the polymer molecules deplete the space around particles, apply osmotic pressure and drive the phase transition. Polarized light microscopy (PLM), rheology, small angle X-ray scattering (SAXS) and atomic force microscopy (AFM) are used to characterize the phase behavior over a time period of one month. In our results, pure CNC dispersion shows three typical liquid crystal shear rheology regimes and cholesteric self-assembly behavior. Tactoid nucleation, growth and coalescence are observed microscopically, and eventually the dispersion presents macroscopic phase separation. PEG with lower molecular weight induces weak attractive depletion forces. Tactoid growth is limited, and the whole system turns into a fully nematic phase macroscopically. With PEG of higher molecular weight, attractive depletion force becomes predominant, thus CNC self-assembly is inhibited and nematic hydrogel formation is triggered. Overall, we demonstrate that depletion induced attraction forces by the addition of PEG enable precise tuning of CNC self-assembly and phase behavior with controllable mechanical strength and optical activity. These findings deepen our fundamental understanding of cellulose nanocrystals and advance their application in colloidal systems and nanomaterials.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1
Fig. 1. Schematic for nematic and cholesteric liquid crystal phase formation.
Fig. 2
Fig. 2. Macroscopic images, development of tactoid morphology (tactoid droplet volume vs. aspect ratio) over time, and microscopic observation of pure 3.5 wt% CNCs (A), 3.5 wt% CNCs with 4, 6 and 8 wt% 20 kDa PEG (B) and 3.5 wt% CNCs with 4, 6 and 8 wt% 200 kDa PEG (C).
Fig. 3
Fig. 3. Time-series Cox–Merz rule rheological analysis: (A) valid regime of complex viscosity (η*, solid) and steady shear viscosity (η, empty) versus angular frequency (ω) and shear rate () (detail inertia and minimum torque boundary calculation in the ESI†); (B) dynamic storage (G′, full) and loss modulus (G′′, empty) versus angular frequency (ω); (C) steady shear viscosity (η) versus shear rate () of pure 3.5% CNCs at 0 week, 1 week and 4 weeks.
Fig. 4
Fig. 4. Time-series Cox–Merz rule rheological analysis: 3.5 wt% CNCs with 4% 20 kDa PEG, 6 wt% 20 kDa PEG and 8% 20 kDa PEG at 0 week (A), 1 week (B) and 4 weeks (C); 4% 200 kDa PEG, 6 wt% 200 kDa PEG and 8 wt% 200 kDa PEG at 0 week (D), 1 week (E) and 4 weeks (F).
Fig. 5
Fig. 5. Time-series Lorentz-corrected scattering intensities as a function of scattering vector q of pure 3.5 wt% CNCs (A), 3.5 wt% CNCs with 4 wt% 20 kDa PEG (B), 3.5 wt% CNCs with 4 wt% 200 kDa PEG (C) at 0 week, 1 week and 4 weeks.
Fig. 6
Fig. 6. AFM images and orientation distribution of contour segments within selected areas of pure 3.5 wt% CNCs (A), 3.5 wt% CNCs with 4 wt% 20 kDa PEG (B), 3.5 wt% CNCs with 4 wt% 200 kDa PEG (C) at 1 week. Scale bar: 500 nm.

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