Phase Separation and Stack Alignment in Aqueous Cellulose Nanocrystal Suspension under Weak Magnetic Field
Yimin Mao1,2, Markus Bleuel1,2, Yadong Lyu3
1Department of Materials Science and Engineering , University of Maryland , College Park , Maryland 20742 , United States.
Langmuir : the ACS Journal of Surfaces and Colloids
|June 30, 2018
Summary
Cellulose nanocrystal (CNC) suspensions exhibit spontaneous phase separation. A weak magnetic field aligns CNC stacks in the nematic phase, revealing chiral nematic structures with a distinct helical pitch.
Area of Science:
- Materials Science
- Soft Matter Physics
- Nanotechnology
Background:
- Cellulose nanocrystals (CNCs) are promising nanomaterials with unique self-assembly properties.
- Understanding the isotropic-nematic (I-N) phase transitions is crucial for their application.
- Investigating the structural organization in different phases is key to controlling material properties.
Purpose of the Study:
- To investigate the isotropic-nematic (I-N) transitions in cellulose nanocrystal (CNC) suspensions.
- To characterize the self-assembled structures in both isotropic and nematic phases.
- To explore the effect of a weak magnetic field on CNC orientation and the chiral nature of the nematic phase.
Main Methods:
- Small-angle neutron scattering (SANS) to analyze particle stacking and interparticle distances.
- Atomic force microscopy (AFM) to study particle size distribution and morphology.
- Polarized light microscopy to observe chiral nematic structures and orientation.
- Magnetic field application to induce and study CNC alignment.
Main Results:
- CNC suspension (7.4% mass fraction) spontaneously phase separated into isotropic (6.9%) and nematic (7.9%) layers.
- CNC particles formed stacks (≈37 nm interparticle distance) in both phases.
- Nematic phase contained larger CNC particles; a 0.5 T magnetic field aligned CNC stacks.
- The nematic phase exhibited chirality with a helix pitch of ≈22 μm.
Conclusions:
- CNC suspensions undergo spontaneous phase separation into distinct isotropic and nematic phases.
- Magnetic fields can induce orientation in the nematic phase, highlighting potential for alignment control.
- The chiral nematic phase of CNCs possesses a significant helical structure, influencing self-assembly behavior.
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