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

Jingwen Xia,Vladimir Aseyev,Alistair W T King,Ilkka Kilpeläinen

doi:10.1007/s10570-021-04230-w

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 abstractRegular micro-sized particles regenerated from a cellulose-OES mixture of tetrabutylphosphonium acetate:DMSO (70:30 w/w) upon cooling

Highlights

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
Our experiments suggest that organic electrolyte solutions (OESs) containing dimethyl sulfoxide (DMSO) have much higher dissolution capability for microcrystalline cellulose (MCC) than that of OESs containing GVL
There is the proposal that the lower viscosities of the OES solutions lead to faster kinetics of cellulose dissolution, without any increase in ionicity leading contributing to the dissolution capability (Andanson et al 2014)

Summary

Introduction

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. 1–2 wt.% solutions (3 g in a 20 ml vial) in[P4444][OAc]:DMSO (70:30 w/w) were the most successful for regeneration of cellulose, in the form of spherical particles. A high degree of cellulose swelling at elevated temperature (in a good solvent), and possibilities for higher solution saturation concentrations favor regeneration of cellulose in the gel form, upon cooling.

Results

Conclusion