Civil Engineering Reference
In-Depth Information
shear thinning proves to be where the best i ber formation occurs. Unfortunately, the
dependence of shear thinning behavior cannot be completely decoupled from initial
viscosity as the best i ber-forming samples, i.e., those with the most pronounced shear
thinning behavior, are also those with the highest zero shear viscosities. Also it was
found that i bers could be electrospun from solutions with a surface tension greater
than 42 mN m
-1
, regardless of molar fraction of ionic liquid and cosolvent. h is indi-
cates, which would not be surprising, that a certain degree of surface tension is ben-
ei cial for i ber formation. As for shear thinning, the dependence of surface tension
cannot be fully decoupled from viscosity; most of the i ber-forming samples with the
highest surface tensions have relatively high zero shear viscosities. h e study was not
able to make a clear correlation between i ber-forming ability and conductivity of the
solution. h e study showed that solutions of cellulose with conductivity values between
3 and 11 mS cm
-1
could produce i bers. However, the conductivity range studied here
is not exceptionally large. It is likely that a conductivity dependence on i ber formation
would have been seen if a wider range of conductivities had been investigated [78].
Härdelin
et al.
[77] again studied the inl uence of molecular weight and rheologi-
cal behavior on electrospinning cellulose nanoi bers from ionic liquids. h ey found
that depending on treatment conditions, acid depolymerization of cellulose reaches
a leveling-of degree of polymerization. h e cellulose was disintegrated with hydro-
chloric acid with varying times to get cellulose with dif erent degrees of polymeriza-
tion. h e resulted disintegrated cellulose is dissolved in 1-ethyl-3-methylimidazolium
acetate with co-solvent as DMSO in 1:1 ratio to get spinning solution with varying
concentrations from 5 to 15%. h e clear and homogenous solution was then subjected
to electrospinning at a high voltage of 10 to 50 kV and a working distance of 10 cm.
Solution l ow rate was set at 0.5 mL h
-1
and the i bers was collected on a grounded rotat-
ing drum of 10 cm diameter rotating at a speed of 25 rpm wrapped with aluminum foil
and partly submerged in a water bath in order to achieve precipitation of the cellulose
solution into solid polymer i bers. h e co-solvent DMSO is added to lower the viscos-
ity of the solution. h e results showed that the leveling-of degree of polymerization
reached approximately 30 min of acid treatment under the conditions they followed in
the study. Depending on the duration of treatment time, cellulose was depolymerized
by the acid treatment into dif erent molecular weight fractions. Longer acid treatment
time yielded more chain scission of cellulose, which resulted in lower molecular weight
polymer.
All solutions show shear thinning behavior, i.e., the viscosity decreases as the shear
rate increases. h at is an indication of them all being non-Newtonian liquids. h e shear
thinning behavior also increases as the viscosity increases; solutions that have a high vis-
cosity also have a higher degree of shear thinning. Rheological measurements showed
that empirical Cox-Merz rule can be applied to the cellulose solutions prepared in the
study. h e Cox-Merz rule states that the complex viscosity overlaps the steady-state
shear viscosity at the same frequency and shear rate. h e rheological measurements
and the evaluation of electrospinnability of the solutions, with the specii c parameters
used in the study showed that electrospinnable solutions exhibit zero shear viscosi-
ties between 10 to 100 Pas. It was found that polydispersity index values and average
molecular weight for the dif erent celluloses decreases with acid treatment time. Also,
it was found that, as the molecular weight decreases, the onset of shear thinning starts
