Rheological characteristics of novel cellulose/superbase-derived ionic liquid solutions and the coagulation process towards regenerated cellulose films

Abstract

The preparation of regenerated cellulose (RC) materials with high performance via dissolution processing is dependent on the properties of the cellulose solution and the conditions of the coagulation bath. Herein, a novel sustainable superbase-derived ionic liquid (SIL, [DBUH][CH₃CH₂OCH₂COO]) with excellent solubility (14.83 wt% at 80 °C) for dissolving wood pulp (DWP; DP = 526) is demonstrated. The rheological characteristics of DWP/[DBUH][CH₃CH₂OCH₂COO] solutions and the regeneration process of cellulose were first systematically investigated. The results indicated that the DWP concentration significantly affected the apparent viscosity (η), structural viscosity index (Δη), overlap concentration (c*), entanglement concentration (c_e), storage modulus (G′), and loss modulus (G′′). Notably, a 12 wt% DWP solution possessed favourable temperature-sensitive properties with an activation energy (E_η) of 55.428 kJ mol⁻¹ and a gel–sol transition temperature of 59.5 °C, presenting superior viscoelasticity to that of imidazolium-based ILs/cellulose in fibre spinning. Moreover, these properties of DWP solutions offered essential advantages for the subsequent preparation of RC films. Comparing the properties of RC films prepared in various coagulation baths, cellulose films regenerated from ethanol exhibited high transparency (up to 90% at 800 nm) and outstanding mechanical properties (tensile strength: 120 MPa; elongation at break: 12%). In this regard, it was inferred that the diffusion rates between [DBUH][CH₃CH₂OCH₂COO] and ethanol contributed to the construction of RC films with high crystallinity and a dense morphology. Hence, [DBUH][CH₃CH₂OCH₂COO] as a solvent and ethanol as a coagulation bath can be a promising processing platform for manufacturing high-performance cellulose materials.