Diffusion in Ionic Liquid–Cellulose Solutions during Coagulation in Water: Mass Transport and Coagulation Rate Measurements

Abstract

This article describes central features of the mass transport during the coagulation in water of cellulose–1-ethyl-3-methyl­imidazoium acetate ([C2mim]­[OAc]) solutions, namely, that the diffusivities are mainly affected by the relative concentrations of water and [C2mim]­[OAc], that the concentration of cellulose does not affect diffusivities and coagulation rates, that the diffusivities of low-M w compounds are similar to those in aqueous [C2mim]­[OAc] solutions without macromolecules, that the polymer concentration is diluted by the large influx of coagulant causing a positive net mass gain, NMG, from diffusive fluxes, and that such NMG, although observed only as a function in time, is also a function in space that has local peaks significantly higher than the mean NMG value. The conclusion from the first three findings was that the diffusion advances through a liquid phase which possesses a continuous pore network and most of the volume. The precipitated cellulose is concentrated into fibrils whose inhibitive effect on the diffusion of small molecules through the surrounding phase is marginal. This key understanding about mass transport during coagulation also simplifies numerical modeling significantly.

Water-induced crystallization and nano-scale spinodal decomposition of cellulose in NMMO and ionic liquid dope

“…A critical review of the literature data is available in recent report by Hedlund et al (2017). The reported value of diffusion coefficient of water and NMMO in NMMO/cellulose dope measured by visual inspection of diffusion front (Biganska2005), after correction of equation as pointed out by Hedlund et al (multiplication by factor of 4), was 4.4×10 -9 m 2 /s, which is higher than the selfdiffusion coefficient of water at ambient conditions (2.57×10 -9 m 2 /s).…”

Microstructures of cellulose coagulated in water and alcohols from 1-ethyl-3-methylimidazolium acetate: contrasting coagulation mechanisms

“…Based on comparisons of the SEM images of different samples, it is apparent that the material that was coagulated from the 5 wt% cellulose solution is denser than expected. A previous study (Hedlund et al 2017) found that the dry content of the cellulose structure before solvent exchange and drying correlates with each solution's cellulose concentration. This indicates that the 5 wt% cellulose material underwent a large degree of volume collapse during solvent exchange and drying, which disqualifies it from the general comparison of pore structures observed using SEM.…”

“…Coagulation was performed by covering a steel rod (diameter of 4 mm) with a 0.35 mm thick solution layer and immersing the rod in water or 2PrOH for a sufficient amount of time to coagulate the full layer thickness, cf. (Hedlund et al 2017). The resulting cellulose tube (* 0.3 g wet) was pulled off the rod and stored in a small flask with about 4 ml of the coagulant used.…”

Air gap spinning of a cellulose solution in [DBNH][OAc] ionic liquid with a novel vertically arranged spinning bath to simulate a closed loop operation in the Ioncell® process

“…Those changes in the mechanical properties might be related to changes in the kinetics of the coagulation and regeneration steps, which rates would be directly affected by the amount of IL in the bath. 3,7,12 Crystalline structure of the spun fibers Background corrected intensity profiles from 0, 15, 30, 45, and 60 IL concentration at DR 3 are shown in Figure S6 in the ESI. No shift was observed for the peak position by different IL concentrations, indicating the identical cellulose II crystal structure for all the IL concentrations in the spin bath.…”

“…The cellulose filaments regeneration kinetic is controlled by the relative diffusive fluxes of the solvent (IL) from the solution into the bath and the nonsolvent (H 2 O) from the bath into the gradually forming filament 3 . 7 The diffusive fluxes of the solvent and nonsolvent are proportional to the concentration gradients between the incipient filament and the bath. In addition, the diffusivities of the cation and anion in a H 2 O/IL mixture may change with the water mole fraction in the coagulation bath 8 .…”