Fibril Morphology and Structure

Scanning electron microscopy (SEM) has also been widely used to examine cellulose microfibrils and nanocrystals [21, 22, 24, 26, 32, 34, 41]. The limited resolution of SEM compared to TEM does, however, make detailed analysis of nanosized fibrils and whiskers challenging. In addition, metal coating applied in SEM most likely broadens the nanosized structures. 

Atomic force microscopy (AFM) has the necessary resolution to observe nanosized cellulose fibril structures without the need for metal coatings. AFM has been applied to both MFC [7, 25] and cellulose whiskers [41-44] and to generate surface profiles of films cast from cellulose nanocrystals [44—46]. AFM may, however, overestimate the width of the particles [41, 44] due to the tip-broadening effect (the shape of the tip contributes to the recorded image). One way to overcome this problem is to measure the height of the fibrils, which is not subject to tip-broadening artifacts [43]. 

Solid-state cross polarization magic angle spinning carbon-13 magnetic resonance (CP/MAS C-NMR) spectroscopy has yielded information on the crystallinity, allomorph composition and surface properties of microfibrils [5, 39, 47-49]. Both 

Other methods used to gain information about the structure, size and crystallinity of cellulose microfibrils are X-ray and neutron diffraction analysis [29, 34, 51-54] and various scattering techniques, including small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS) [14, 55, 56]. 

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