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

Artur Hedlund,Hans Theliander,Tobias Köhnke,Ulf Olsson,Joel Hagman

doi:10.1007/s10570-018-2168-6

Abstract

Coagulation of cellulose solutions is a process whereby many useful materials with variable microstructures and properties can be produced. This study investigates the complexity of the phase separation that generates the structural heterogeneity of such materials. The ionic liquid, 1-ethyl-3-methylimidazolium acetate ([C2mim][OAc]), and a co-solvent, dimethylsulfoxide (DMSO), are used to dissolve microcrystalline cellulose in concentrations from 5 to 25 wt%. The solutions are coagulated in water or 2-propanol (2PrOH). The coagulated material is then washed and solvent exchanged (water → 2PrOH → butanone → cyclohexane) in order to preserve the generated microstructures upon subsequent drying before analysis. Sweep electron microscopy images of 50 k magnification reveal open-pore fibrillar structures. The crystalline constituents of those fibrils are estimated using wide-angle X-ray spectroscopy and specific surface area data. It is found that the crystalline order or crystallite size is reduced by an increase in cellulose concentration, by the use of the co-solvent DMSO, or by the use of 2PrOH instead of water as the coagulant. Because previous theories cannot explain these trends, an alternative explanation is presented here focused on solid–liquid versus liquid–liquid phase separations.Graphical abstract

Highlights

Cellulose is the major biopolymer that provides mechanical reinforcement of the cell walls in landliving plants
In most native supramolecular structures of cellulose, the polymers are hierarchically organized in crystalline nanofibrils, which are bundled into larger fibrils that are wound at an angle around the plant cell wall
For 2PrOH, our analysis suggests that * 3 flat ‘‘crystallites,’’ two (110) planes thick each, could be stacked to produce the thickness of such ‘‘nano ribbons.’’ This implies that the smallest fibril dimension would be slightly smaller if the fibrils are ribbons, but the general picture remains largely unaffected by the shape of the cross section

Summary

Introduction

Cellulose is the major biopolymer that provides mechanical reinforcement of the cell walls in landliving plants. At 0.2 Mton/a of fiber, lyocell remains a niche-product relative to the 5 Mton/a of viscose (The_Fiber_Year_GmbH 2017). This is probably due to issues with process safety and related costs. In order to expand the use of cellulosic materials, new sustainable and resourceefficient cellulose dissolution-regeneration processes are required together with the ability to control material properties. To succeed in this endeavor, the effects of process conditions on the properties of the precipitated cellulosic material from new solvents must be better understood

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Conclusion