关键词: 棉浆纤维素, 质子型离子液体, 溶解, 流变行为, Cox-Merz定律, 黏弹性

Abstract:

Objective Cellulose is one of the most abundant renewable natural polymers but cannot be effectively dissolved by traditional solvents owing to its highly ordered hydrogen-bond network structure and high crystallinity, which limits the further development and large-scale application of cellulose. Ionic liquids with special structures, due to their strong hydrogen-bond breaking ability, are widely used as a green and efficient solvent for natural polymer dissolution and processing. However, few studies are conducted on protic ionic liquid ([DBNH][Lev]) concerning the dissolution of cellulose and their solution properties. What's more levulinic acid derived from biomass resources endows green properties to [DBNH][Lev].

Method Protic ionic liquid was used as solvent to achieve the efficient dissolution of cotton cellulose under mild conditions. The dissolution mechanism of cellulose in ionic liquid and the steady and dynamic rheological behavior of cellulose solution were systematically studied by using nuclear magnetic resonance and rheological techniques respectively. The influence of factors such as cellulose concentration, shear rate, and temperature on the rheological behavior of cellulose/[DBNH][Lev] solution was thoroughly investigated. The morphology and mechanical properties of generated films from cellulose/PILs solution were studied in view of their potential application.

Results The rheological properties of cellulose are closely related to solvent category, cellulose concentration, cellulose molecular weight and experimental temperature. Firstly, it was identified that [DBNH][Lev] presented satisfactory dissolution ability to cellulose and had good solubility up to 5% to cellulose at 100 ℃. The ketone group in the Lev anion may provide a new hydrogen-bonding acceptor and donor in [DBNH][Lev] due to the keto-enol tautomerism, thus strengthening the interaction via hydrogen bonds between cellulose and [DBNH][Lev]. The steady-state rheological curves of cellulose/[DBNH][Lev] solutions with different mass concentrations at 25 ℃. For all case, a shear-thinning behavior is observed with increases in the shear rate and shear-thinning behavior becomes more remarkable when cellulose increases. Newtonian plateau phenomenon is observed when all samples were sheared at low shear rate. At the same shear rate, the apparent viscosity of cellulose solution gradually decreases with increasing temperature, which is consistent with classical polymer solutions. The power law coefficient n increases with the increasing concentration from 1.01 to 2.53 at 25 ℃. The turnover concentration from dilute to the semi-dilute unentangled regime defined as the overlap concentration (C^*) was 0.83%. The viscosity-temperature dependence of solution was characterized by using the Arrhenius equation, the dissolution activation energy increases when cellulose increases. The cross-over point (gelation point) resulted in a shift to lower frequency when cellulose concentration increases at 25 ℃. It is found that both G' and G″ shift to higher frequency when the temperature decreases because more cellulose chains entangle together in low temperature at C-4 cellulose solution. Finally, it is also found that the generated films have satisfactory mechanical properties, indicating their practical application potential. The generation film at C-5 cellulose solution has the maximum tensile strength of 88.21 MPa and the elongation at breakup to 7.72%.

Conclusion A green and low-cost biomass derived protic ionic liquid was applied to successfully enhance its ability to break cellulose hydrogen bonds and achieve effective dissolution in this research. It has been demonstrated that the keto-enol tautomerism in the levulinic acid anion participates in the hydrogen-bond interaction in the cellulose dissolution process. The trend of shear rate and apparent viscosity of cellulose solutions under different mass concentration conditions is consistent, showing the characteristics of pseudoplastic fluid shear thinning. The apparent viscosity of cellulose is related to cellulose concentration and temperature; The overlap concentration for transition from diluted to semi diluted state is 0.83%, and the empirical Cox-Merz rule is not applicable to cellulose/[DBNH][Lev] solutions due to the apparent viscosity curve cannot overlap well with the complex viscosity curve. Therefore, the obtained results in this research provide a basic insight into the rheological response of cellulose in ionic liquid environment, and provide guidance for the processing of cellulose (such as coating and spinning).

Key words: cotton pulp cellulose, protic ionic liquid, dissolution, rheological behavior, Cox-Merz rule, viscoelasticity