Cellulose dissolution: insights on the contributions of solvent-induced decrystallization and chain disentanglement

The dissolution of cellulose is a critical step for the efficient utilization of this renewable resource as a starting material for the synthesis of high value-added functional polymers and chemicals and also for biofuel production. The recalcitrance of semicrystalline cellulose microfibrils presents a major barrier to cellulose dissolution. Despite research efforts, important aspects of cellulose dissolution such as solvent-induced decrystallization and chain disentanglement are not well-understood. Here we address these fundamental issues with the practical goal of gaining insights into the swelling and dissolution of cellulose that cannot be obtained from macroscopic experimental data. To this end, we have used a newly-developed phenomenological model that captures the phenomena governing the dissolution of semicrystalline polymers as well as the thermodynamics and kinetics of dissolution. This model fits well experimental data for swelling and dissolution of cotton fibers in the ionic liquid [bmim]Cl, and allows the quantification of two important aspects, i.e., solvent effectiveness in cellulose (1) decrystallization and (2) chain disentanglement, the balance of which controls the mechanism and kinetics of cellulose dissolution. The activation parameters of cellulose decrystallization, estimated using the obtained decrystallization constant values, reveal that the decrystallization of cellulose in [bmim]Cl is associated with positive enthalpy and entropy and it is also very sensitive to temperature. When the solvent effectiveness in the disruption of cellulose crystals is relatively lower than its ability to disentangle the chains, the kinetics of dissolution are controlled by decrystallization. Furthermore, conditions that facilitate cellulose chain disentanglement, in addition to increasing the rate of dissolution, can result in faster decrystallization. The solvent effectiveness in chain disentanglement is the only factor that determines the decrease of the cellulose fiber radius. In cases where the fiber dissolution rate is lower than the decrystallization rate, the dissolution of cellulose is mostly controlled by the solvent ability to disentangle the chains. The insights obtained from this study improve the understanding of cellulose–solvent interactions underlying decrystallization and disentanglement and their contributions in controlling the kinetics of cellulose swelling and dissolution.