Civil Engineering Reference
In-Depth Information
at higher shear rates. h is ef ect on the shear thinning behavior of the solution is attrib-
uted to the long polymer chains present in the solution. h e existence of a well-dei ned
zero shear viscosity indicates that the cellulose in our solutions is genuinely dissolved
on the molecular level, forming l exible overlapping coils in the solvent, and that chain
interactions are dominated by topological constraints (entanglements). h e rheologi-
cal studies showed that cellulose molecules in the solution are quite l exible, forming
random coils in the solvent system, and the intramolecular hydrogen bonds in cellulose
are largely broken by the interactions with the solvent. It was observed that, the mor-
phology of the i bers greatly demands the solution viscosity, which can be controlled by
polymer concentration and/or molecular weight. As the viscosity of the spinning solu-
tion increases the average i ber diameter increased and appeared as thick i bers made
up of bundles of several individual thin i bers, while the viscosity is below a certain
value but still above the critical viscosity (the viscosity at which the i ber can be formed)
form the i bers with bead morphology or even the drops or clumps of the polymer. h e
solutions exhibiting too low viscosity (below the critical viscosity), either caused by
low cellulose concentration or long acid treatment time, appear to electrospray instead
of electrospinning, hence no i bers are formed. h e solutions with too high viscosity
become dii cult to handle and are thus non-spinnable.
Freire
et al.
[79] prepared nanosized and biodegradable cellulose i bers by electros-
pinning at room temperature using a nonvolatile pure ionic liquid or a binary mixture
of two selected ionic liquids. h e cellulose used has molecular weight (Mw) of 53000.
From the large array of possible ionic liquids, 1-ethyl-3-methylimidazolium acetate,
([C
2
mim][CH
3
CO
2
]), was selected as the main solvent as a result of its desirable prop-
erties, namely low toxicity, low viscosity, low melting temperature, low corrosiveness,
favorable biodegradability and high cellulose dissolution capacity [84]. h e increased
basicity of the acetate anion makes it more ei cient at disrupting the intra- and inter-
molecular hydrogen bonding network in cellulose compared to chloride-based ionic
liquids [84]. Furthermore, the low viscosity and melting temperature of 1-ethyl-3-me-
thylimidazolium acetate facilitate the dissolution, handling, and electrospinning of
cellulose at temperatures near to room temperature. To further enhance the solvent
thermophysical properties, in particular aiming at a reduction of the surface ten-
sion, a second and surface active ionic liquid 1-decyl-3-methylimidazolium chloride,
([C
10
mim]Cl) was used as an additive (in a mole fraction ratio of 0.10 with respect to
1-ethyl-3-methylimidazolium acetate), and the electrospinning of cellulose was further
performed using the binary mixture of ionic liquids. h e electrospinning of 8 wt% cel-
lulose in 1-ethyl-3-methylimidazolium acetate medium showed to produce electros-
pun i bers with average diameters within (470 ± 110) nm, while cellulose i bers from
the binary mixture of ionic liquids presented average diameters within (120 ± 55) nm.
Compared to the i bers obtained from the pure ionic liquid, smaller, and more homoge-
neous i bers were obtained with binary mixture. h erefore, the improved solvent prop-
erties contributed to a stabilization of the i bers morphology, as well as to a decrease
in the i bers diameter, resulting in ultra-thin regenerated cellulose. Surface tensions of
chloride-based ionic liquids previously used in the electrospinning of cellulose range
between 48.2 mN m
-1
to 61.9 mN m
-1
at temperatures close to room temperature [88,
89], and thus, the reduction of the i bers size obtained in this work is also a result
of the surface tension decrease in the overall polymeric solution. h e comparison of
