Since the introduction of the transmission electron microscope (TEM), researchers have attempted to disintegrate cellulose fibres into single microfibrils for ultrastructural studies. Microfibrils are natural nanofibres with diameters < 100 nm. As an example it appears that already in 1950 the first samples of micro-crystalline and microfibrillated cellulose (MFC) were produced based on ultrasonic treatment and acid hydrolysis of cellulose fibres.
The disintegration of cellulose fibres into their structural components (microfibrils) has also found industrial interest since the beginning of the 80´s. Although microfibrils seem to be the main component of MFC, several studies have shown that fibrillation produces materials which may be inhomogeneous, containing e.g. fibres, fibre fragments, fines and nanofibrils. This fact emphasizes that proper characterisation of a given fibrillated material requires quantification at several scales (Fig.1).
Fig.1 Multiscale assessment of microfibrillated cellulose. Images A)-D) are acquired with a relatively simple scanner technique. The transparencies indicate the degree of fibrillation. A) MFC produced without pre-treatment, 1000 bar. B-D) TEMPO pre-treated samples, homogenized at 200, 600 and 1000 bar, respectively. The arrows in B) indicate poorly-fibrillated fibres. E) and F) are FE-SEM images of the surface structures. Note the nanofibrils exemplified in the high-resolution images.
The degree of fibrillation is commonly given subjectively by visual evaluations of high-resolution microscopy images, applying equipment designed for nano-assessment, e.g. field-emission SEM, AFM and TEM. However, such equipment may limit the field of view considerably, which also introduces a subjective pre-selection of small areas containing nano-structures. Adequate characterisation can additionally include methods for assessing large areas, with a suitable resolution. Keep in mind that one important aspect is not only the quantification of nanofibril morphology, but also the quantification of fibres that are poorly fibrillated (Fig. 1). Methods for assessing relatively large areas and structures at the micrometre scale are thus most valuable for complementing specialized devices for nano-characterisation. Hence, efforts on the development of macro- and micro-characterisation techniques will facilitate the development of objective and rapid methods for assessment of nano-structures. It is thus expected that in a near future such approaches will pave the way for online analysis systems, which is the ultimate goal for effective and automatic quantification of a given nano-structure.
Contact: Gary Chinga-Carrasco.