Phase-separation of cellulose from ionic liquid upon cooling: preparation of microsized particles

Abstract

Cellulose is an historical polymer, for which its processing possibilities have been limited by the absence of a melting point and insolubility in all non-derivatizing molecular solvents. More recently, ionic liquids (ILs) have been used for cellulose dissolution and regeneration, for example, in the development of textile fiber spinning processes. In some cases, organic electrolyte solutions (OESs), that are binary mixtures of an ionic liquid and a polar aprotic co-solvent, can show even better technical dissolution capacities for cellulose than the pure ILs. Herein we use OESs consisting of two tetraalkylphosphonium acetate ILs and dimethyl sulfoxide or γ-valerolactone, as co-solvents. Cellulose can be first dissolved in these OESs at 120 °C and then regenerated, upon cooling, leading to micro and macro phase-separation. This phenomenon much resembles the upper-critical solution temperature (UCST) type thermodynamic transition. This observed UCST-like behavior of these systems allows for the controlled regeneration of cellulose into colloidal dispersions of spherical microscale particles (spherulites), with highly ordered shape and size. While this phenomenon has been reported for other IL and NMMO-based systems, the mechanisms and phase-behavior have not been well defined. The particles are obtained below the phase-separation temperature as a result of controlled multi-molecular association. The regeneration process is a consequence of multi-parameter interdependence, where the polymer characteristics, OES composition, temperature, cooling rate and time all play their roles. The influence of the experimental conditions, cellulose concentration and the effect of time on regeneration of cellulose in the form of preferential gel or particles is discussed. Graphical abstract Regular micro-sized particles regenerated from a cellulose-OES mixture of tetrabutylphosphonium acetate:DMSO (70:30 w/w) upon cooling

Thermo‐Reversible Cellulose Micro Phase‐Separation in Mixtures of Methyltributylphosphonium Acetate and γ‐Valerolactone or DMSO

“…In more dilute MCC solutions (2‐6 wt %, in [P 4441 ][OAc]:DMSO 80 : 20 w/w), roughly spherical micro‐scale structures are formed and can be observed via simple optical microscopy (Figure 10), upon cooling from the solution below the SGTT. These kinds of spherulites have been observed previously, for systems comprised of cellulose in: 1) N ‐methylmorpholine‐ N ‐oxide ( N MMO) hydrates, [37] 2) the ionic liquid 1‐allyl‐3‐methylimidazolium chloride ([amim]Cl), [38] 3) from aqueous solution by crystallization of cellooligomers, after enzymatic polymerization of β ‐cellobiosyl fluoride, [39] and 4) by regeneration from tetrabutylphosphonium acetate ([P 4444 ][OAc]) electrolyte solutions [40] . This last article focuses on a similar electrolyte system to the present article, which is more robust than the current system for regeneration of uniform particles.…”

“…Gel‐formation seems to occur initially, as temperature decreases, with clouding only occurring at even lower temperatures (in lower mass transfer regimes), and after some time. This may be one reason why this system is not as robust as those previously studied by Clough [17] and Xia [40] . This points the way towards careful selection of IL and co‐solvent, to develop more robust systems.…”

“…This may be one reason why this system is not as robust as those previously studied by Clough [17] and Xia. [40] This points the way towards careful selection of IL and co‐solvent, to develop more robust systems. While the rheology and UV measurements were performed using different molecular weight high‐purity celluloses (PHK pulp and MCC), the temperature onset of gelation is expected to be the same but the effect of gelation will more noticable for the higher molecular weight PHK pulp.…”

“…These kinds of spherulites have been observed previously, for systems comprised of cellulose in: 1) N-methylmorpholine-N-oxide (NMMO) hydrates, [37] 2) the ionic liquid 1allyl-3-methylimidazolium chloride ([amim]Cl), [38] 3) from aqueous solution by crystallization of cellooligomers, after enzymatic polymerization of β-cellobiosyl fluoride, [39] and 4) by regeneration from tetrabutylphosphonium acetate ([P 4444 ][OAc]) electrolyte solutions. [40] This last article focuses on a similar electrolyte system to the present article, which is more robust than the current system for regeneration of uniform particles. Typically, in polymer solutions with an LCST or UCST, as the solvent becomes poor the polymer moves from a random coil to a collapsed globule conformation and, without stabilisation, the globules aggregate into larger particles.…”

The dialkylcarbonate route to ionic liquids: purer, safer, greener?

“…cellulose concentration, solvent composition, temperature and cooling rate, etc. 102,103 Guglielmero et al synthesized and studied the thermal properties of several dicarboxylate ionic liquids. Although the majority of them were made from halide-based ionic liquid precursors, with an expensive anion exchange step (based on silver salt, or ion exchange resin), those with a butylmethylimidazolium cation were reached thanks to the DMC route.…”