Formation of hairy cellulose nanocrystals by cryogrinding

From a green chemistry perspective, cryogrinding of cellulose fibers conducted under mild conditions is introduced as a rapid, facile, and scalable methodology for the mechanochemical synthesis of cellulose nanocrystals. Traditionally, they are synthesized in a solution using acid/oxidation-based methods in a multi-stage process. This work presents a method for the production of hairy carboxylated nanocrystalline cellulose (HNC) through a mechanochemical process from wood kraft pulp, using two different procedures: (1) dry-cryogrinding (HNC-1 and HNC-2), and (2) cryogrinding with mono-chloroacetate (MCA) under alkaline conditions (HNC-10 and HNC-40). Solid carbon-13 NMR and FTIR spectroscopy proved the presence of carboxyl groups on the nanoparticles. XRD shows that HNC-1 and HNC-2 corresponded to the type I allomorph of cellulose while HNC-10 and HNC-40 corresponded to cellulose II. The crystallinity of nanocelluloses decreases with increasing milling time. HNC nanoparticles produced by cryogrinding are electrosterically stable due to the presence of both protruding chains (“hairs”) and anionic charges, as confirmed by DLS. All HNC particles produced by cryogrinding are rod-shaped with a similar diameter, but HNC-1 and HNC-2 particles are about three times longer than HNC-10 and HNC-40. The unique molecular and crystal structure of the produced HNCs may be important for subsequent chemical modification and developing new applications.