SEM imaging of chiral nematic films cast from cellulose nanocrystal suspensions

• Published in: Cellulose (London) Vol. 19; no. 5; pp. 1599 - 1605
• Main Authors: Majoinen, Johanna, Kontturi, Eero, Ikkala, Olli, Gray, Derek G.
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer Netherlands 01.10.2012; Springer Nature B.V
• Subjects: Bioorganic Chemistry; Cellulose; Ceramics; Chemistry; Chemistry and Materials Science; Composites; Evaporation; Fracture surfaces; Glass; image analysis; ionic strength; Morphology; Nanocrystals; Nanoparticles; Natural Materials; Optical properties; Organic Chemistry; Original Paper; Physical Chemistry; Polymer Sciences; Porosity; Self-assembly; Sustainable Development; Chiral nematic films; Left-handed helicoidal structures; Electron microscope images; Film cross-sections; Fracture surface; Cellulose nanocrystals
• DOI: 10.1007/s10570-012-9733-1
• ISSN: 0969-0239

The chiral nematic self-assembly of aqueous suspensions of cellulose nanocrystals is partially preserved on evaporation of water, but the ordering of the rod-like nanoparticles may become distorted by changes in volume, ionic strength and surface and convective forces during evaporation, thus affecting the morphology and optical properties of the dried film. Proposed applications for these solids with chiral nematic order require confirmation of their structure. A SEM examination of the fracture surface of a slowly-dried film showed a surprisingly regular fan-like pattern which is shown to be characteristic of cross-sections of the left-handed helicoidal arrangement of nanocrystals, where the helicoidal axis was almost perpendicular to the film surfaces. Superimposed on this pattern was what appeared to be a regular porosity, which is postulated to result from pull-out of the nanocrystals oriented orthogonal to the fracture surface.