20. - 22. 10. 2009, Roznov pod Radhostem, Czech Republic, EU
EFFECT OF SODIUM CHLORIDE ON THE NEEDLE ELECTROSPINNING
OF POLY (VINYL ALCOHOL)
Anh Tuan DAO, Oldrich JIRSAK
Technical University of Liberec, Studentska 2, 461 17 Liberec, Czech Republic,
e-mail: [email protected]
Abstract
Poly (vinyl alcohol) (PVA) is one of the most popular polymers used as a raw material for nanofibres
production via electrospinning. It is commonly processed from water solutions. Water insoluble PVA
nanofibres are obtained using a crosslinking treatment.
PVA nanofibres find many end uses including medical applications because of its non-
toxicity. PVA
electrospinning process and its throughput can be influenced by various additives. Influence of NaCl on PVA
electrospinning is studied in the paper and the results are discussed as a contribution to the understanding
of electrospinning mechanism.
In this work, water solutions of PVA with concentration of 12 weights per cent were studied. The solutions
contained 0, 0.5 and 1.0 weight per cent of NaCl. Some characteristics of those solutions were measured
such as surface tension, electric conductivity, and rheological properties to determine the effects of salt on
the material properties. While the effects of salt to surface tension and selected rheological properties are
not significant, the conductivity of solutions increases strongly with increasing concentration of salt. Beside
these, the LCR Meter and special voltage meter were used to determine electrical properties of those
solutions, as well as the changing behavior of those properties under the voltage. Electrical properties of
samples such as capacitance, inductance, and resistance were measured. The time dependence of
conductivity of solutions under DC voltage was also measured. In addition, high speed camera was used to
see the effects of salt concentration in needle electrospinning process. The results show that NaCl strongly
affects the needle electrospinning process as well as properties of produced nanofibres.
Keywords: electrospinning, poly (vinyl alcohol), sodium chloride, jets length, nanofibers.
1.
INTRODUCTION
Poly (vinyl alcohol) (PVA) is a water-soluble synthetic polymer. PVA has excellent film forming, emulsifying,
and adhesive properties. PVA was discovered in 1915 by F Klatte. The stoichiometric saponification of poly
(vinyl acetate) with caustic soda to yield PVA was first described in 1924 by W O Hermann and W Haehnel.
Since its discovery, PVA has found many uses and new ones are still being added. Recently, PVA became
one of the most popular materials in electrospinning. From many works, it has been seen that, it is very easy
when PVA was used as material in electrospinning, and because of its non-toxicity and it can be dissolved
easily in water so PVA can be used as a unique material, or PVA can be used in combination with ather
polymers which are not electrospinnable themselves, but they have special functions for end use purposes
[2, 3].
20. - 22. 10. 2009, Roznov pod Radhostem, Czech Republic, EU
Electrospinning is a method of producing nanofibres by using an electrostatic repulsive force and an electric
field between two electrodes, by applying high voltage to a polymer solution or a melt, so that it can produce
a web of nanofibres [1, 2 and 3]. Electrical properties of a polymer solution or a melt affect the eletrospinning
process and properties of the product [4, 5]. Sodium chloride was used in this work to change electrical
properties of PVA. The results of the effects of changing in electrical properties to electrospinning process as
a contribution to understanding of electrospinning mechanism.
2.
EXPERIMENTAL
It has been known from many experiments that electrospinníng process and spinnabilty of PVA strongly
depend on polymer molecular weight, concentration and content of additives such as salts. In this work,
atactic PVA with degree of saponification 88 percent was used. The additive sodium chloride (NaCl) with
concentration of 0%, 0.5% and 1% was used to change properties of PVA solutions. The symbols of those
solutions are showed in table 1.
Table 1: PVA solutions studied
Symbol
Molecular
Hydrolysis Grade
Concentration of
Concentration of
weight (Mw)
(%)
polymer (%)
salt (%)
PVA12-0
80000
88
12
0
PVA12-0.5
80000
88
12
0.5
PVA12-1
80000
88
12
1
Experiment 1
Following properties of PVA solutions were measured|: surface tension, electric conductivity in alternating
current, rheological behavior and electrical properties. The results are showed in the table 2 and following
figures.
Table 2: Properties of samples
Sample
Surface tension
Conductivity
Capacitance
(mN/m)
(mS/cm)
(µF)
PVA12-0
44.9
0.616
19
PVA12-0.5
45.9
7.6
27.9
PVA12-1
45.9
13.8
27
In the rheological experiments, the dependence of solutions viscosities on shear rate was determined. These
results are shown in Fig. 1.
20. - 22. 10. 2009, Roznov pod Radhostem, Czech Republic, EU
Viscosity [Pas]
0.8
PVA12-0
PVA12-0.5
PVA12-1
0.7
0.6
0.5
1
10
100
Shear rate [1/s]
Fig. 1: Dependence of viscosities on shear rate.
Experiment 2:
An electrical circuit was set up to determine the dependence of resistance of polymer solution under effects
of direct voltage. The diagram of the circuit was showed in Fig. 2.
Fig 2: Diagram of the circuit
It has been known that, under DC voltage, ions inside polymer solutions move toward to electrodes,
therefore, amount of ions decrease, resistance or resistivity of polymer solutions increase, caused change in
current of the circuit.
From this diagram in Fig. 2 we have
R1 =
(U 0 − U 2 )
R2
U2
U0: Voltage of power source (0.7 V).
U2: Voltage of constant resistance R2 (V).
R2: Constant resistance (100000 Ω)
(1)
20. - 22. 10. 2009, Roznov pod Radhostem, Czech Republic, EU
Another concept is electrical resistivity. That is a measure of how strongly a material opposes the flow of
electric current. To determine specific resistance, we can use formula (2):
ρ=R
A
L
(2)
ρ: Specific resistance (Ωm)
R: Resistance (Ω)
2
A: Area of the cell (m )
L: Distance between plates (m)
To be more advantage in comparison, another concept was used. That is the relative resistivity ρr. Relative
resistivity was calculated by the ratio of resistivity at the time t (ρt) to started resistivity (ρ0).
ρr =
ρt
ρ0
(3)
Relative Resistivity
We have the dependence of relative resistivity on time flow in Fig. 3.
7
PVA12-0
PVA12-0.5
PVA12-1
5
3
1
0
10
20
30
Time [s]
Fig 3: Dependence of relative resistivity on time
Experiment 3
During electrospinning, high rate camera was used to record the process. The length of jets during was
obtained from video records. Here are some pictures captured the jet length of the polymer solutions during
process:
20. - 22. 10. 2009, Roznov pod Radhostem, Czech Republic, EU
Fig 4: Jet length of samples (a: PVA12-0; b: PVA12-0.5; c: PVA12-1)
The lengths of jets are shown in Table 3.
Properties of products
Scanning electro microscope was used to take pictures of nano-fibers. Then LUCIA software was used to
determine diameter of fibers and make the distribution of fiber diameter.
(a)
(b)
(c)
Fig 5: SEM pictures of products (a: PVA12-0; b: PVA12-0.5; c: PVA12-1)
20. - 22. 10. 2009, Roznov pod Radhostem, Czech Republic, EU
PVA12-0
PVA12-0.5
30
20
Fraction [%]
Fraction [%]
25
15
10
5
20
10
0
0
140 160 180 200 220 240 260 280 300 320
110 150 190 230 270 310 350 390 430
Fiber Diameter [nm]
Fiber diameter [nm]
(a)
(b)
PVA12-1
Fraction [%]
20
15
10
5
0
120 160 200 240 280 320 360 400 440 480
Fiber diameter [nm]
(c)
Fig 6: Distributions of fiber diameters
3.
DISCUSSION
The numbers in Table 2 and Figure 1 show that NaCl influences electrical propeties of solution considerably,
whereas changes of surface tension and viscosity are not significant. The time dependence of relative
resistivity is shown in Figure 2. The graphs show that under direct voltage (0.7V), relative resistivity of
PVA12-0 increase almost two times faster than PVA12-0.5, and PVA12-1. Conductivity of PVA decreases
due to orientation of macromolecules in direct electric field, whereas conductivity linked with small ionic
charge cariers is less time dependent.
Experiment 3 shows the jet length during electrospinning process. The lengths of jets were measured by
using LUCIA software. The results of jet length measurement are in Table 3.
Table 3: Length of jets and average diameter of nanofibers.
Solutions
Jet length (mm)
Average fibre diameter (nm)
PVA12-0
PVA12-0.5
PVA12-1
5
4.2
2.9
205
237
253
It has been known that the jet forming is the first stage of electrospinning. Jet stands for equilibrium state of a
tug of war between electrostatic and capillary forces. The capillary forces cause jet particles to flock together
20. - 22. 10. 2009, Roznov pod Radhostem, Czech Republic, EU
to minimize jet surface area and surface energy, resulting from short distance inter-molecular interactions at
quantum level. Electrostatic forces make the charged jet body disintegrate due to long range repulsive
Coulombic forces between ions of the same signs. When electric forces are stronger than capillary forces, jet
will be split [1]. As the results from figure 3, the relative resistivity of PVA12-0 grows quicker than PVA12-0.5
and PVA12-1. It means electric density of PVA12-0 grows up slower than PVA12-0.5 and PVA12-1. In other
words, it can be understood that the jet from PVA12-0 solution need longer time to be split, or jet length is
longer. Comparison between PVA12-0.5 and PVA12-1 in the figure 3 shows that the change of relative
resistivity in time of both solutions is not significantly different. But the result in table 2 shows that
conductivity of PVA12-1 is much higher than PVA12-0.5. It means PVA12-1 take shorter time to get suitable
electric density for splitting of jet.
From figure 5 and figure 6 we can see that the average fiber diameter and fiber diameter distribution spun
from the three solutions are a little different. PVA12-1 has broader fiber diameter distribution and greater
average fiber diameter. It has been known that, after splitting, one jet will be separated into many smaller
parts. Sizes of these parts are different. In the same conditions, with shorter length of jet, it also means the
diameter of jet is bigger, the range of size of those smaller parts is larger, and the average sizes of them are
also bigger. This causes the fibers diameter of PVA12-1 is highest, and also has broader fiber distribution.
Another effect is that, with higher amount of ions, the electric force is higher. It causes more intensive
elongation process and smaller fiber diameters. So even these three solutions have big difference in amount
of ions, they still have a little difference in average diameter.
4.
CONCLUSIONS
The additive sodium chloride NaCl almost does not affect to surface tension and rheological properties of
PVA. But it affects strongly electrical properties of PVA.
After putting NaCl in to PVA solution, conductivity of solution increased significantly, the relative resistivity
under DC voltage is reduced; both of them cause the length of jet during electrospinning process become
shorter with increasing concentration of NaCl.
The diameter of nanofibers increases slightly with concentration of NaCl. The concentration of NaCl also
influences the distribution of fiber diameter. The higher concentration of NaCl, the larger is distribution of
fiber diameters.
REFERENCES
[1]
D.Lukas, A. Sarkar, L. Martinova, K. Vodsed’alkova, D. Lubasova et al. Physical principles of
elctrospinning, Textile Progress Vol. 41, No.2, 2009
[2]
S. Ramakrishna, K. Fujihara, W. Teo, T. Lim, and Z. Ma, An introduction to electrospinning and
nanofibres, World Scientific Publishing Co., Singapor, 2005.
[3]
Anthony L.Andrady, Science and Technology of Polymer Nanofibers; John Wiley & Sons, Inc.,
Hoboken, New Jersey, 2008
[4]
Chan Kim, Sang-Hee Park, Wan-Jin Lee, Kap-Seung Yang; Electrochimica Acta 50 (2004) 877–881.
[5]
Chan Kim; Journal of Power Sources 142 (2005) 382–388