Civil Engineering Reference
In-Depth Information
nanoi bers [69, 73]. Cellulose is successfully dissolved in lithium chloride (LiCl)/N,N-
dimethylacetamide (DMAc) [74, 75]. Aiming at replacing the noxious solvents com-
monly employed, ionic-liquid-based solvents have been recently explored as novel
nonvolatile and nonl ammable media for the electrospinning of polymers. Swatloski
et al.
[76] were the i rst to report the ef ective dissolution of cellulose in ionic liquids.
Room temperature ionic liquids such as 1-ethyl-3-methylimidazolium acetate along
with dimethyl sulfoxide [77-79], 1-decyl-3-methylimidazolium chloride ionic liquids
[79] , 1-butyl-3-methylimidazolium bis(tril uoromethylsulfonyl)imide (BMITFSI)
[80] , N-methylmorpholine-N-oxide (NMMO), etc. [81] , are reported to prepare cel-
lulose solution and are electrospun to nanosized cellulose i bers at room tempera-
ture. Among these, ionic liquids have received the epithet of “green solvents” and can
potentially be used as safe solvent due to their low melting temperature, high thermal
stability, chemical stability, wide liquid phase range, hydrophobicity in some cases,
nonl amability, very low or negligible vapor pressure, low toxicity and simple recy-
cling process [82, 83]. In addition, they were shown to be non-derivatizing solvents
for cellulose (with no covalent interactions involved), while it was demonstrated that
solvation predominantly occurs by hydrogen bonding interactions between the sugar
building blocks and the ionic liquid anion [84]. As a result, the dissolution of cellulose
in ionic liquids has provided a new platform for cellulose processing. Moreover, their
physicochemical properties can be tuned by using dif erent combinations of cations
and anions. Room temperature ionic liquids are primarily molten salts composed of
bulky and asymmetric organic anions and cations having low melting point and exist
as liquids at or below room temperature [85]. However, pure cellulose solution in ionic
liquid is not easily electrospun due to its high viscosity and low volatility. Even the
room temperature ionic liquids (RTILs) are touted as environmentally-friendly sol-
vents; ionic liquids do not evaporate completely between the electrospinning spin-
neret and the collector due to their low volatility, a coagulation bath is necessary to
form the i bers. To improve the spinnability of cellulose solution in RTILs, several
methods such as addition of co-solvent [86-89] or mixture with artii cial polymers
were reported [90-95]. Multicomponent systems, however, contain both a nonvolatile
salt and a volatile component. In all of these cases, completely removing the solvent
from the cellulose during the electrospinning process has required particular inge-
nuity in developing the spinning system. Researchers have added coagulant baths
to the spinning line to remove components that do not volatilize in the spinning air
gap and have incorporated temperature control at the spinneret, the collector, or the
coagulation bath. h e concentration of cellulose in solutions successfully electrospun
into i bers depends on both the cellulose molecular weight (expressed as degree of
polymerization, DP) and the solvent composition. h e source of the cellulose, from
wood pulps, cotton linters, or cotton i bers, does not appear to have a signii cant ef ect.
Owing to the mechanical strength of cellulose due to the presence of strong hydrogen
bonds, before electrospun, cellulose has to be pretreated to weaken the bonds between
the chains [75] . h e pretreatment process proposed by Li
et al.
[96] included the fol-
lowing processes: (i) the immersion of natural cellulose in water for overnight and the
subsequent removal of the water, (ii) the washing of the swollen cellulose with metha-
nol twice consecutively, and (iii) exchanges with DMAc.
