Abstract:

Objective Cellulose fibers at high moisture content result in excessive free water content inside the pores of the fibers due to excessive swelling, which leads to increased hydrolysis of reactive dyes and limits the development of dyeing processes with high color fixation rates. Therefore, this study aims to reveal the dynamic swelling mechanism of cellulose at different moisture contents (40%-60%) on an atomic scale using molecular dynamics (MD) simulations, which provides a reference to promote the model construction of cellulose fiber pores and provides theoretical support for the control of fiber moisture content in the process of reactive dye dyeing of cellulose fibers.
Method Equilibrium molecular dynamics simulations were applied to simulate the swelling process of cellulose fiber at different moisture contents of 40%-60%. A cellulose pore model with a crystalline-amorphous-crystalline (CC/AC/CC) sandwich structure was constructed using the CHARMM36 force field and the simulation results were validated by the proportion of bound water measured by TD-NMR (T2 relaxation analysis). The system was simulated in NPT setup. Key metrics include free volume fraction analysis, density profiles, hydrogen bonding and diffusion coefficients. TD-NMR quantification of strongly bound water (0.07%) was performed to verify the accuracy of the simulation. Adsorption site competition was analyzed by radial distribution function and hydroxyl coordination number (O2, O3, O4, O6).
Results An important mechanism of cellulose hygroscopic swelling is revealed from an atomic perspective by integrating TD-NMR and MD simulations. Water molecules preferentially accumulated in the crystalline-amorphous (CC/AC) interface pores, forming a distinct longitudinal density gradient and uniform penetration in the AC region. The maximum increase in free volume fraction and a decrease in cellulose density to 1.081 g/cm³ were observed at moisture contents of 40%-50%, and the increment slowed down as moisture content further increased, suggesting that this moisture content interval is an isolated gap evolving into a connected three-dimensional network. TD-NMR and simulations consistently confirumed that the stable proportion of strongly bound water within the cellulose pores is 0.07%, which confirms the reliability of this model of cellulose pores. At high moisture contents, enhanced pore connectivity improves mass transfer efficiency, while the reduction of localized water clusters drives nonlinear diffusion kinetics with reduced diffusion coefficients. Competition for adsorption sites is related to the moisture level: at low moisture content, O6/O4 hydroxyl sites dominate, while swelling-induced structural relaxation shifts the adsorption predominance to the low-affinity O2/O3 sites.
Conclusion A cellulose pore model was constructed by molecular dynamics simulations and validated it by TD-NMR experiments to reveal the dynamic mechanism of cellulose hygroscopic swelling.The 40%-50% moisture content interval is the critical humidity threshold for cellulose pores to transition from isolated pore structure to three-dimensional network pores, and the nonlinear diffusion kinetics of water molecules is affected by local aggregation effects, thus controlling the fiber with liquid carrying rate at 40% can reduce the hydrolysis of reactive dyes. Competitive adsorption mechanisms (O6/O4→O2/O3) exist for hydration sites on cellulose. The practical significance includes: providing a reference for the construction of a multi-scale model of cellulose fiber pores, reducing the hydrolysis of dyes by controlling the liquid-carrying rate of fabrics which provides a theoretical basis and ideas for optimizing the dyeing process of reactive dyes. Future work is necessary to investigate how the pores of cellulose fibers swell at moisture contents below 40%.

Key words: cellulose fiber, fiber channel, molecular dynamics simulation, hygroscopic swelling, moisture content, time-domain nuclear magnetic resonance, cotton fabric