Formation of cellulose microspheres and nanocrystals from mildly carboxylated fibers

The ultrastructure of mildly carboxylated swollen tracheids from which cellulose microspheres and cellulose nanocrystals (CNC) were formed was investigated. A mild etherification of spruce kraft pulp was performed to introduce 1.3 and 2.5 mmol/g carboxyl groups on cellulose chains. 1.3 mmol/g carboxymethyl fibers (CMF) were dissolved partially in alkaline water to form balloons and collars on the tracheid. Primary wall, expanded S1, swollen S2, wrinkled S3, spiral bands of S1, parallel microfibrils of S2 and their transverse splitting were observed on swollen fibers. It was concluded that balloons, collars and wrinkled S3 were formed due to different cellulose microfibril features in different layers of the tracheid cell wall. Microspheres with a size up to about 0.6 µm were observed by field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). It is shown that they originated from transverse splitting of S2 microfibrils occurring at locations where more hemicelluloses and amorphous cellulose were present. The microspheres contain bundles of well-known CNCs. After homogenization and sonication of an aqueous dispersion of 2.5 mmol/g CMF, acoustic attenuation spectroscopy showed the presence of nanoparticles with a size distribution of 18–208 nm. Similar sizes of nanorods were observed by TEM.Graphical abstract