Highly surface-selective nitration of cellulose nanofibers under mildly acidic reaction conditions

Cellulose nitrate (CN) is used in numerous industrial materials, such as propellants, lacquers, and plastics, exploiting its highly flammable, hydrophobic, and plastic characters. The downsizing of cellulose nitrate fibers may enhance their properties. Although a direct nitration of cellulose nanofiber (CNF) is a prospective method for preparing nanosized CN materials, it is difficult because of the susceptibility of CNF to acids. In the previous study, we prepared nitrated cellulose nanofibers (NCNFs) using never-dried CNFs and relatively dilute H2SO4, obtaining a high yield and degree of substitution. In this study, we describe a novel highly surface-selective nitration method using dried CNFs. To prevent the acid hydrolysis of the CNFs, mildly acidic conditions (acetic acid/acetic anhydride/HNO3) were used instead of the conventional mixed-acid systems. Solid- and gel-state NMR studies revealed that the original crystalline structure of the produced NCNF core was retained, even after nitration, whereas the cellulose molecules on the NCNF surface were completely converted to cellulose pernitrates. The NCNFs exhibited morphologies comprising thin nanofiber diameters of approximately 10-50 nm with high specific surface areas of approximately 260 m2 g-1. Thus, unique core-shell NCNFs were prepared, potentially leading to the development of CNF derivatives with novel applications and functions.