Names: Lindsey Vanderlyn and Lamiya Zavari
Date Due: 09/19/2011
Testing salt induced spider-net linkage in electrospun nanowebs of poly(vinyl alcohol) and
poly(ethylene-co-vinyl alcohol)
Applications for nonwoven membranes, which can be spun out of synthetic and natural
polymers, are extremely varied. They include filtration, insulation, bone and tissue scaffolding,
and wound dressing. Electrospun membranes are suited to these applications because of their
high surface to volume ratios and their interconnecting porous networks; however, they can often
be very fragile (Bhardwaj & Kundu, 2010; Yang, Li, & Nie, 2007). Electrospinning is a process
for creating non-woven membranes (Wong, Hutter, Zinke-Allman, & Wang, 2009). A polymer
solution, such as poly(vinyl alcohol) (PVA) is placed in a capillary needle. When a voltage is
induced between the needle and a collector plate, a surface charge is induced in the solution
causing a spherical drop on the end of the needle to deform into a conical shape, called a “Taylor
Cone”. After a critical voltage is reached, the electrostatic repulsion force of the surface charges
overcomes the surface tension of the solution and the charged fluid is released from the tip of the
Taylor Cone. High surface charge densities enhance a whipping mode, where bending of the jet
creates highly stretched fiber as well as rapid evaporation of the solvent. The fibers are
deposited on the collector in the form of non-woven mat. Individual fiber diameters are usually
between 100 nanometers and 1 micrometer (Bhardwaj & Kundu, 2010; Yang et al., 2007).
PVA is a semi-crystalline, hydrophilic polymer with fairly good chemical and thermal
stability. It is water soluble with a high permeability rate as well as being very biocompatible
(Koski, Yim, & Shivkumar, 2003). PVA also has good chemical resistance and excellent
mechanical properties (Nirmala et al., 2011) and is synthesized from poly(vinyl acetate) through
hydrolysis and can be broken into two groups, partially hydrolyzed and fully hydrolyzed. PVA
is chemically stable at normal temperatures and its physical and chemical properties have led it
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Names: Lindsey Vanderlyn and Lamiya Zavari
Date Due: 09/19/2011
to be used in film, contact lenses, and biomedical applications (Zhang, Yuan, Wu, Han, & Sheng,
2004).
Poly(ethylene-co-vinyl alcohol) (EVOH) is hydrolyzed from poly(ethylene-co-vinyl
actetate), and is most commonly available in 55-70 mol% vinyl alcohol content (Kenawy et al.,
2003; Marias et al., 2006). The gas barrier properties exhibited by EVOH and its resistance to
chemicals make it a useful material in food packaging, especially as an oxygen barrier (Marias et
al., 2006). When intended to be used with liquid or high humidity, EVOH needs to be laminated
with a hydrophobic material such as polyethylene to preserve its ability as an oxygen barrier,
which is greatly compromised by the absorption of water (Milosavljevic & Thomas, 2006).
In a study done by Kenawy et al. (2003), EVOH polymers were electrospun in solutions
with a solvent of 70/30% isopropanol/water, with compositions of 2.5-20% w/v. The ideal time
for mixing these solutions was found to be about 2-3 hours, and the ideal temperature about 80°
C. The solutions were cooled to room temperature, approximately 22-24° C, before
electrospinning. The polymer began to precipitate after a few hours, so solutions were
electrospun within 2-3 hours after mixing them. If not, a precipitated mixture could be
solubilized again at 50°C for 10 minutes. The electrospinning set-up consisted of a blunt-end
needle and a syringe, which was filled with solution and placed about 20 cm from the target
surface. The flow rate of the solution was 10-18 mL/h. For 62 mol% vinyl alcohol, spinning
started at 5% w/v solution, and the most uniform fibers were obtained at 10% w/v (Kenawy et
al., 2003). We plan to use these parameters when spinning our EVOH and alter them to fit our
specific procedures.
Non-woven membranes can be affected by many different factors including
concentration, conductivity, and viscosity of the solution, diameter of the syringe needle,
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Names: Lindsey Vanderlyn and Lamiya Zavari
Date Due: 09/19/2011
distance between the needle and the collection plate, and electric voltage (Yang et al., 2007).
Changing the viscosity of the solution has been found to affect the length and diameter of the
fibers, by changing how quickly the solution is released from the Taylor Cone (Huang, Zhang,
Kotaki, & Ramakrishna, 2003). High molecular weight polymer solutions are usually used
because they provide an optimum viscosity for electrospinning (Bhardwaj & Kundu, 2010).
Although a higher voltage can generate finer nanofibers and decrease the diameter of beads, it
does not reduce the number of beads formed. Increasing the voltage has actually been found to
cause more beaded and uneven fibers (Huang et al., 2003). Increased beading can also be caused
by having too high of a flow rate of solution from the syringe (Bhardwaj & Kundu, 2010).
Another way to change the membranes is by adding a filler, such as a salt, to the polymer
solution. Fillers, including both organic and inorganic substances, have been found to greatly
reduce or eliminate the presence of beads in the nanowebs (Huang et al., 2003).
It has been found that by adding a filler, such as a salt, into the polymer solution, the
mechanical properties can be improved. With an addition of 1 wt% salt to a PLDA polymer the
fibers were thinner and completely free of beads (Huang et al., 2003). In a study performed by
Wu, Fan, Qin, and Zhang (2007) it was found that the addition of TiO2 to PVA resulted in a
membrane with better thermal radiative properties, because the average fiber diameter had
decreased. The addition of a salt, such as NaCl or KBr, in Nylon 6 (N6) or PVA, has been found
to create spider-net links between main fibers, linking them together (Barakat, Kanjwal, Sheikh,
& Kim, 2009). The spider net links have been found to alter mechanical strength, surface
morphology, thermal properties, wettability, ultraviolet light blocking abilities, and antimicrobial
properties. In fact, with the addition of TiO2 to N6, it was found that the maximum thermal
degradation was shifted 2.6 ˚C higher, mechanical strength was increased, strain was reduced,
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Names: Lindsey Vanderlyn and Lamiya Zavari
Date Due: 09/19/2011
hydrophilicity was increased, and fouling was prevented (Pant et al., 2010). The increased
strength and other changes caused by the addition of salts could be very useful in applications
such as protective clothing
In order to create this linkage, the optimum amount of salt is about 1.5 wt%. The spidernet link formations are believed to be created by free ions, resulting from the dissociation of the
ionic salts, such as that caused by placing them in water (Huang et al., 2003). It has also been
found that most of the salt in the solution was found in the spider-net links. It was discovered in a
recent study, that higher concentrations of salt such as 5 wt% or 10 wt% caused little to no
spider-net link formation and unequal distribution of the salt throughout the web (Pant et al.,
2010). In a study conducted by Barakat et al. (2009), it was determined that when the salts were
mixed with the polymer solutions for longer amounts of time, the resulting spider-net links were
more present and developed. The best length of stirring was determined to be about 24 hours,
and these spider-link webs were shown to have greater structural stability and resistance to
strain. However, if the solution was not mixed for long enough, the spider-net link formation was
inconsistent. It was proposed, in this experiment, that the formation of webs would be similar in
all polymer solutions that have a polar base. Our experiment will be similar to this one; however,
instead of testing the effects of salts on N6 and PVA, we will be testing them on PVA and
EVOH, both of which have water, a polar liquid, as a solvent. Our goal is to determine if, like
hypothesized by Barakat et al. (2009), the web formations are created in other solutions with
polar solvents.
To test the effect of adding a salt to the polymer solution, we plan to test mechanical
properties. Mechanical properties such as strain softening behavior, extensibility, maximum
stress, energy loss, and deformation characteristics can be measured. Stress is the force per unit
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Names: Lindsey Vanderlyn and Lamiya Zavari
Date Due: 09/19/2011
area applied to the material and strain is how much the material changes its shape; in this case, it
would be how much the nanofiber bends from the straight position. Extensibility of a fiber is the
strain at which that fiber snaps, and maximum or peak stress is the highest stress value reached
before the fiber snaps. Deformation measures how closely a fiber returns to its original shape
after stretching (Carlisle, Coulais, & Guthold, 2010). Surface morphology can also be observed
using scanning electron microscopy (SEM), with which the presence of spider-net links can be
determined (Barakat et al., 2009).
In review, our project will be to test spider-net link formation in two different polymers,
PVA and EVOH, both of which are biocompatible. The addition of spider-net links has been
found to increase the mechanical strength of the electrospun mat, making it more attractive for
applications such as protective clothing (Barakat et al., 2009; Pant et al., 2010). We will attempt
to induce spider-net links by adding 1.5 wt% salt to the polymers, and we will be careful to avoid
using more than this amount so that we do not have uneven spider-net link formation. The
addition of the salts should also help to reduce beading (Barakat et al., 2009; Huang et al., 2003).
Our project will differ from other research that has been done, because we plan on using other
salts than those previously tested, and we will be testing the formation of these spider-net links
on EVOH.
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Names: Lindsey Vanderlyn and Lamiya Zavari
Date Due: 09/19/2011
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