A Green Method for Bacterial Cellulose Electrospinning Using 1-Butyl-3-Methylimidazolium Acetate and γ-Valerolactone

Abstract

Bacterial cellulose (BC) is a highly pure and crystalline cellulose produced via bacterial fermentation. However, due to its chemical structure made of strong hydrogen bonds and its high molecular weight, BC can neither be melted nor dissolved by common solvents. Therefore, processing BC implies the use of very strong, often toxic and dangerous chemicals. In this study, we proved a green method to produce electrospun BC fibers by testing different ionic liquids (ILs), namely, 1-butyl-3-methylimidazolium acetate (BmimAc), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EmimTFSI) and 1-ethyl-3-methylimidazolium dicyanamide (EmimDCA), either individually or as binary mixtures. Moreover, γ-valerolactone (GVL) was tested as a co-solvent derived from renewable sources to replace dimethyl sulfoxide (DMSO), aimed at making the viscosity of the cellulose solutions suitable for electrospinning. A BmimAc and BmimAc/EmimTFSI (1:1 w/w) mixture could dissolve BC up to 3 w%. GVL was successfully applied in combination with BmimAc as an alternative to DMSO. By optimizing the electrospinning parameters, meshes of continuous BC fibers, with average diameters ~0.5 μm, were produced, showing well-defined pore structures and higher water absorption capacity than pristine BC. The results demonstrated that BC could be dissolved and electrospun via a BmimAc/GVL solvent system, obtaining ultrafine fibers with defined morphology, thus suggesting possible greener methods for cellulose processing.

Keywords: cellulose dissolution; fibers; ionic liquids.

Figure 1

(A) Schematic of the electrospinning apparatus used to spin BC solutions, with d = distance from the needle tip to the collector surface. (B) Photograph of the spinning part of the used apparatus.

Figure 2

Characterization of BC as obtained from fermentation: (A) SEM micrograph of the fibrous structure of BC (10 kV, scale bar is 500 nm; magnification 100,000×), and (B) molecular weight distribution of BC; incorporated table showing number-average (Mn), weight-average (Mw), z-average (Mz) and the polydispersity index (Mw/Mn).

Figure 3

Dissolution of BC in different solvent systems at 0.5% w/v concentration and respective optical densities (O.D.): (A) BmimAc, (B) EmimTFSI and (C) BmimAc/EmimTSFI (1:1 w/w). Arrows indicate a magnet, left inside to show transparency.

Figure 4

Morphological analysis of electrospun BC (3.0% w/w) in (A,C,E) BmimAc/GVL and (B,D,F) BmimAc/DMSO. (A,B) SEM micrographs showing the fibers obtained by dissolving BC in (A) BmimAc/GVL and (B) BmimAc/DMSO (15 kV, 4000× magnification, scale bar is 10 µm). (C,D) Bar graphs displaying the void volume distribution into pores discriminated by size ranges for BC dissolved in (C) BmimAc/GVL and (D) BmimAc/DMSO. (E,F) BC fiber diameter distribution into pore size classes, electrospun using (E) BmimAc/GVL and (D) BmimAc/DMSO. Mean ± standard deviation and median are reported for BmimAc/GVL and (D) BmimAc/DMSO; a qualitative trend of the distributions is obtained using the 5th and 4th order polynomial equations, respectively.

Figure 5

FTIR spectra of electrospun BC fibers obtained from BmimAc solutions added with either GVL or DMSO as co-solvents, compared with pristine BC, solvent (BmimAC) and co-solvents alone.

Figure 6

Graph showing the absorption capacity trends during the soaking time of pristine BC and electrospun BC fibers [i.e., 3% (w/w) BC in BmimAc/DMSO], immersed in dd-water and PBS media.