Unusual structure and morphology of cellulose nanocrystals obtained by room temperature acid extraction: effect of cellulose source and pretreatment

Cellulose nanocrystals (CNCs) are attracting increasing interest as renewable nanomaterials for composite fillers and rheological modifiers. Their properties depend on morphology and crystalline structure, governed by cellulose source and hydrolysis conditions. Conventionally, CNCs are extracted by acid hydrolysis at elevated temperatures, favoring rapid depolymerization and limiting alternative reorganization pathways. Herein, sulfuric acid hydrolysis was performed at room temperature as a structure-directing approach to investigate how cellulose source, crystalline polymorphism, and mercerization pretreatment influence CNC formation. Hemp pulp, micronized cellulose powder, and microfibrillated cellulose were studied in native and mercerized forms. Suppressing thermal activation alters the balance between depolymerization, solubilization, and recrystallization, leading to CNCs with distinct crystalline structures and non-classical morphologies. In addition to typical rod-like nanocrystals, hierarchical and flake-like nanoparticles were obtained, including larger particles several hundred nanometers long (average length ≈320 nm) decorated with smaller cellulose II crystallites (≈45 nm length, ≈15 nm width). The results demonstrate that the interplay between cellulose source, pretreatment, and hydrolysis conditions governs CNC morphology and polymorphism. This study provides mechanistic insight into CNC formation under low-temperature acid hydrolysis and demonstrates how controlled access to unconventional CNC architectures and surface features can be exploited for rational design of cellulose-based nanomaterials for advanced applications.