Thermal Unlocking of Cellulose Mercerization: High-Temperature Low-Concentration Phase Transition and the Twist−Stabilize−Lock Mechanism

Abstract Solid-state polymorphic transition from cellulose I to II (mercerization) generally requires high alkali concentrations (>12−15 wt %) to overcome hydrogen-bonding networks and topological barriers. In this study, we elucidated the phase behavior of cellulose in a high-temperature, low-concentration regime using in situ Fourier-transform infrared spectroscopy in D2O and non-negative matrix factorization (NMF). Phase mapping demonstrated that the alkali threshold for mercerization decreased to approximately 5 wt % above 60 °C. NMF analysis established kinetic markers that distinguished alkali-induced structural evolution from overlapping spectral responses. The results suggest that elevated temperature promotes transient local loosening of cohesive contacts, accompanied by reorganization of structured water and increased conformational freedom. Spectroscopic tracking of backbone geometry revealed a Twist−Stabilize−Lock mechanism initiated by reduction of the glycosidic bond angle. This thermally assisted unlocking process facilitates alkali-assisted structural rearrangement under dilute conditions, extending mercerization into a low-alkali regime distinct from dissolution−regeneration pathways. These findings provide a mechanistic basis for reducing alkali consumption and designing thermally assisted cellulose processing and surface-modification strategies.