One-pot synthesis of 2-bromopropionyl esterified cellulose nanofibrils as hydrophobic coating and film

Hydrophobic 2-bromopropionyl esterified cellulose nanofibrils (Br-CNFs) have been facilely produced via one-pot esterification of cellulose with 2-bromopropionyl bromide (BPB) then in situ disintegrated by ultrasonication in the same reaction media. Br-CNFs optimally produced by this robust esterification-ultrasonication approach, i.e., 5 : 1 BPB to anhydroglucose (AGU) molar ratio, 23 °C, 6 h and ultrasonication (50% amplitude, 30 min), were 4.6 nm thick, 29.3 nm wide, and 1 μm long in 71% yield and 48% crystallinity. Successful 2-bromopropionyl esterification of cellulose was confirmed by FTIR and (1)H NMR. The degree of substitution (DS) of surface hydroxyl to 2-bromopropanoate was determined to be between 0.53 (DS(s)) based on XRD and Br-CNF dimensions and 0.56 (DS(NMR)) from solution-state (1)H NMR. Br-CNF dispersions in DMF exhibited Newtonian behaviors at concentrations below and shear thinning behaviors above 0.5%, enabling homogeneous deposition at dilute concentrations up to 0.01% into a few nm ultra-thin layers as well as blade coating of gel into ca. 100 μm thick film, all similarly hydrophobic with surface water contact angles (WCAs) in the range of 70–75°. The ultra-high modulus and strength film from gel coating further showed the potential for dual high-strength and hydrophobic applications of Br-CNFs.