1. Enhancing the Gas Barrier Properties of Polylactic Acid by Means of Electrospun Ultrathin
Zein Fibers
Maria A. Busolo1,2
, Sergio Torres-Giner1
and Jose M. Lagaron1
1
Novel Materials and Nanotechnology Lab., IATA-CSIC (Valencia, Spain),
email: lagaron@iata.csic.es
2
Nanobiomatters, S.L. (Valencia, Spain)
Abstract
Ultrathin fibers of 100-500 nm in diameter were obtained
by electrospinning from an alcoholic solution of zein, a
prolamine from maize. The ultrathin zein fibers generated
were incorporated as a reinforcement to a poly(lactic
acid)/polyethylene glycol (PLA/PEG) matrix by a
proprietary method to generate a novel multilayer
structure consisting of a zein fiber layer laminated in a
sandwich to two PLA/PEG layers.
As the zein nanofibers retained the ultrathin structure
inside the resulting PLA/PEG matrix, the overall
composite remained transparent and colorless. The
incorporation of the zein did not significantly affect the
thermal behaviour or the water permeability of the original
PLA/PEG matrix, but surprisingly it did reduce the
oxygen permeability of the matrix by ca. 70%. This
technology could thus be potentially applied to obtain
highly transparent PLA films with enhanced barrier
properties of interest in for instance packaging
applications.
Introduction
The electrospinning technique was developed and
patented at the beginning of the 20th century, but in recent
years it has been revisited by the growing interest in
nanotechnology. It is a simple, versatile and efficient
method to produce high-performance polymeric fibers
with diameters ranging from the micro to the nanoscale1
.
The electrospinning technique requires a voltage
power supply, a syringe containing the polymer solution
with a needle, and a grounded collector screen. The
spinning process is achieved by application of a high
voltage electric field that induces charges on the solution
polymer surface. The hemispherical droplet at the tip of
the needle stretches and a conical jet projection of the
newly created fibers emerges. As the solvent evaporates
before the jet reaches the collector, solid fibers are
collected in the form of non-woven mats. Some
parameters rule the process, and consequently, the final
properties of the fibers. Electrospinning parameters
(needle-to-collector distance, flow rate, voltage); polymer
solution characteristics (concentration, solvent nature,
viscosity) as well as room conditions (temperature,
relative humidity) define the morphology of the products,
fibers or beads, and their dimensions1,2
.
Because of their very large area to volume ratio,
superior mechanical performance, and the versatility in the
design, electrospun ultrathin fibers are being considered in
different areas such as filtration, nanotube reinforcements,
sensors, cosmetics, catalysis, protective clothing,
biomedicine, space applications, semiconductors and
micro- or nano-electronics3-6
.
Zein is a natural, non-water soluble prolamine
obtained from maize that has been used to improve barrier
properties of bags and packagings due to its low
permeability to oxygen and carbon dioxide under dry and
medium relative humidity conditions. Electrospinning
procedures to obtain zein nanofibers from acidic and
alcoholic solutions have recently been reported2,7,8
. Zein
conventional fibers have been incorporated to fabrics for
work clothes reducing their permeability to liquids and
gases, and increasing their mechanical resistance, wear
and washing durability9
.
Polylactic acid (PLA) is a linear biodegradable and
biocompatible polyester produced from renewable
resources with interesting physical properties, which make
it an attractive alternative for polymers derived from fossil
origin. In recent years, PLA has been used in many
differents applications: sutures, implants and
biocompounds for drug delivery systems (biomedicine);
bottles and packagings for food and beverages, disposable
fabrics, personal care products, etc. However, some
properties such as thougness, HDT and gas barrier are not
suficiently enough for its use in various applications10
.
The water permeability of PLA is much lower than that of
proteins and polysacharides but it is still higher than the
permeability of conventional polyolefins and polyethylene
terephthalate (PET)11
. By adding nanoaditives to PLA,
nanocomposites with enhanced mechanical properties,
thermal resistence, barrier properties, and dimensional
stability can be obtained, even at low filler
concentrations12
. Although many studies have reported
interesting blends of PLA with other polymers of
ANTEC 2009 / 2763

2. enhanced performance13,14
, the latest incorporation of
nanosized fibers and layered silicates, as reinforcing
materials to PLA, can potentially result in advanced
materials of wider applications15-17
. The addition of
polyethylene glycol (PEG), as a plastizacing agent, to
PLA has also been reported to lead to increased flexibility
and ductibility18
.
This paper presents the optical, barrier and thermal
properties of a novel PLA multilayer composite material
prepared by incorporation of electrospun zein fiber mats
between two PLA/PEG layers with potential interest in
packaging applications.
Materials
A poly(lactic) acid (PLA) extrusion grade with an
average molecular weigh of 150.000 g/mol was supplied
by Natureworks (USA). Zein powder from maize was
purchased from Sigma Aldrich Chemical and was used
without further purification. Polyethyleneglycol, 850-950
g/mol (Fluka Chemika) was used as both zein
compatibilizer and plastizicer for the PLA matrix. Ethanol
and trichloromethane 99%, were supplied by Panreac
Synthesis.
Procedures
PLA-PEG films preparation
The PLA/PEG films were prepared by solution
casting. A solution containing 4.75 wt.-% of PLA and 0.25
wt.-% of PEG in chloroform was prepared by stirring on a
hot plate at 40ºC until complete pellet dissolution. The
resulting solution was then cast onto Petri dishes (15 cm
diameter), and 100 microns thick films were obtained after
solvent evaporation at ambient conditions. The obtained
PLA/PEG film was vacuum dry overnight at 70ºC, to
remove residual solvent.
Electrospinning process
Electrospinning was performed using a plastic syringe
containing the protein solutions which were disposed in
horizontal lying on a digitally controlled syringe pump
while the needle was directly in vertical towards the
collector. Further details of the basic electrospinning setup
can be found elsewhere2
.
Zein ultrathin fibers were obtained from
electrospinning of 33wt.-% of zein in 80% ethanol/water
solutions prepared under magnetic stirring at room
temperature. The governing parameters were fixed at 14
kV of power voltage, 15 cm of tip-to-collector distance
and 0.33 ml/h of volumetric flow-rate. Environmental
conditions were maintained stable at 24ºC and 60 %RH by
having the equipment enclosed in a specific chamber with
temperature and humidity control.
Multilayer Assemble
By using a proprietary method (patent pending) of
application, the protein fiber mats were laminated to two
PLA/PEG layers to obtain a multilayer system where an
ultrathin layer of zein fibers was trapped between two
symmetrical PLA/PEG layers.
Optical microscopy
The morphology of the films was investigated by
optical microscopy. Observations were performed in
1x1cm film samples, using a Nikkon Eclipse 90i
microscope with imaging software NIS Element BR 2.30.
Scanning electronic microscopy (SEM)
SEM imaging of the cryofractured composite was
measured using a Hitachi S-4100 equipment operating at 8
kV.
Thermal properties
The thermal behaviour of PLA/PEG-zein composites
was analyzed by differential scanning calorimetry (DSC)
using a Perkin Elmer DSC 7. Samples of approximately 5
mg were heated in aluminium pans from 40ºC to 170ºC at
a rate of 10ºC/min. Similar thermal runs were performed
in empty pans in order to get a base line.
Water vapour permeability (WVP)
The water vapour permeability of the films was
determined according to the ASTM method E96, using
aluminium cells with internal and external diameters of 3.5
and 6 cm, respectively. Each sample film was sealed to a
permeation cell containing liquid water, and then the
permeation cells were left under controlled environmental
conditions (24ºC, 40% relative humidity) and weighed
regularly until steady-state was reached. The water vapour
transmission rate was easily determined from the slope of
the cell weight loss vs. time plot following equation 1.
WVP = ( G * L ) / ( A * ∆p ) (1)
where G is the slope of weigh loss vs. time straight
line, L is the film thickness, and ∆p is the vapour partial
pressure differential across the film. All tests were
conducted in duplicate, and aluminium films were used as
blanks to evaluate water vapour loss through the sealings.
Oxygen permeability
The oxygen transmission rate was measured using an
Oxtran 100 equipment (Mocon, Minneapolis, USA), at 24
ºC and 80% relative humidity. The samples were
previously purged with nitrogen during 20h before being
exposed to the oxygen flow. The oxygen permeability was
calculated from the oxygen transmission rate at
equilibrium conditions by:
PO2 = (Q * L ) / ∆p (2)
2764 / ANTEC 2009

3. where Q is the oxygen transmission rate, L is the sample
thickness, and ∆p is partial pressure gradient across the
sample.
Results and discussion
Morphology of the zein fibers
SEM micrographs show that zein electrospun fibers
form a non woven ultrathin structure while they are
generated (Figure 1). The attained fiber diameters range
between 100 to 500 nm. However, the fiber length can
reach up to several centimeters.
Composite characterization
Figure 2 shows an optical micrograph of one of the
multilayer systems prepared. From this a regular
disposition of the ultrathin zein fibers is observed from
within the PLA/PEG matrix. By cryofracture of the
assemble, the fibers layer can be hardly distinguished
within the multilayer system, indicating that a strong
adhesion between the zein fiber mats and the PLA/PEG
layers takes place and that the zein layer is very small, i.e.
few microns, in comparison to the overall thickness.
Simple optical pictures indicate that the final
composite system is completely transparent and colorless
similarly as the neat PLA/PEG film (Figure 3). It should
be borne in mind that transparency is a key property in
many applications but especially in food packaging. The
results indicate that the multilayer system can be made in a
way in which no apparent losses in optical properties are
generated.
Thermal properties of PLA/PEG-zein composites
Figure 4 shows the DSC thermograms of the
PLA/PEG film and of the PLA/PEG-zein composite
material developed. From the DSC experiments the heat of
fusion (∆Hm), melting point (Tm) and cold crystallization
temperature (Tc) of the studied materials was determined
and is summarized in Table 1. The results indicate that the
protein fibers do not influence the thermal behavior of the
PLA/PEG matrix since they are not randomly mixed with
the polymer but forming a separate intermediate layer.
However, it is remarkable to observe that the Tm of the
PLA/PEG blend decreases compared to the neat PLA due
to molecular interactions with the plasticizer13
.
Barrier properties
The water vapour permeability results of the films are
gathered in Table 2. From this, it can be seen that the zein
fibers do not significantly alter the water barrier of the
PLA/PEG blend, even when zein films are about 10 fold
more permeable to water than the PLA/PEG blend19,20
.
Thus, while zein is a relatively hydrophobic protein it does
still become strongly plasticized by water21
. The reason
why water permeability does not go up in the system could
be related to the fact that PLA does not uptake water to a
significant extent. The hydrophobicity of the matrix may
thus be constraining the fibers in the multilayer system as
previously observed in other systems, such as cellulose
fibers in PE, leading to very little plasticization or
swelling of the zein layer. Another important contributing
reason is the very small layer of the zein fibers compared
to the overall thickness of the PLA/PEG film. The PLA is
the higher barrier in the multilayer structure and is also the
dominant phase determining the final barrier.
Table 2 also indicates that the PLA films are actually
41% less permeable to water than the actual PLA/PEG
blend. So, the addition of PEG to the PLA polymer does
bring a negative effect on the water permeability of the
blend due to the higher free volume attained and the
corresponding changes in diffusion and also in solubility
of water in this blend. In this context, it is also worth
mentioning that the water vapour of the composite is not
better than its oil-based counterpart PET, which has a
WVP of 3x10-15
(Kg m / s m2
Pa)20
. As a result the
resulting composite does not outperform the polyester in
terms of water barrier.
However, the oxygen barrier results are very
significant (see Table 3). In this table zein fibers were able
to reduce the oxygen permeability of the PLA/PEG matrix
drastically (71%). This entails that the PLA/PEG-zein
composite is 24% more barrier than the neat PLA, is
equivalent to polypropylene (PP)22
and is 10 fold more
permeable than PET19
. The reason for the oxygen barrier
improvement is related to the fact that dry zein is a very
high barrier polymer to oxygen and therefore imposes a
stronger blocking effect in a multilayer system16
.
Application of a simple series modeling is supporting the
fact that if the permeability difference between the layers
is very high, a thinner layer with higher barrier will impact
more strongly the overall barrier of the multilayer system
as observed with the oxygen barrier.
Conclusions
Electrospun zein nanofibers of 100-500 nm in
diameter were obtained from alcoholic zein solutions. A
new methodology alternative to melt mixing or solvent
casting allowed the incorporation of electrospun ultrathin
fiber in a PLA/PEG blend with no alteration of their
structure and original morphology. The resulting
composite film was transparent and colorless.
Thermal analysis showed that melting and
crystallization behavior of the PLA/PEG matrix did not
change due to the incorporation of the ultrathin fibers. In
the same way, water vapor permeability of PLA/PEG
matrix did not significantly increase due to the
ANTEC 2009 / 2765

4. incorporation of the ultrathin fibers. However, the oxygen
permeability was seen to be reduced by 71% compared to
the pure matrix
Acknowledgements
The authors would like to acknowledge the Spanish MEC
project MAT2006-10261-C03 for financial support; and
Dr. Eugenia Nuñez for helpful comments.
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2766 / ANTEC 2009

5. Tables
Table 1. Thermal properties of PLA/PEG, PLA/PEG-
zein
Sample Tc (ºC) St. dev. Tm (ºC) St. dev. ∆∆∆∆Hm (J/g)
PLA/PEG 78.3 0.6 134.4 0.4 9.0
PLA/PEG-
zein
78.9 1.4 134.4 1.1 9.9
Table 2. Water vapor permeability of PLA/PEG,
PAL/PEG-zein, PLA and zein
Sample WVP (Kg m/s m2
Pa)
PLA/PEG (3.96 ± 1.13) x 10-14
PLA/PEG-zein (3.88 ± 0.17) x 10-14
PLA19 2.30 x 10-14
Zein20 3.36 x10-13
Table 3. Oxygen permeability of PLA/PEG,
PAL/PEG-zein (24ºC, 80% RH) and other polymers
Sample PO2 (m3
m/m2
sPa)
PLA/PEG (5.85 ± 0.25) x 10-18
PLA/PEG + zein nanofibers (1.68 ± 1.50) x 10-18
PET19
4.26 x 10-19
PP22
8.6 x 10-18
PLA19
2.21 x 10-18
Figures
Figure 1. Zein fiber mats by SEM (scale marker is 5 µm)
(a) (b)
Figure 2. PLA/PEG-zein composite micrographs: (a) top
view by optical microscopy (scale marker is 100 µm); (b)
cryofractured side view by SEM (scale marker is 10 µm).
(a) (b)
Figure 3. Images of nanocomposite films: (a)PLA/PEG;
(b) PLA/PEG-zein ultrathin fibers
Figure 4. DSC heating thermograms of the PLA/PEG
blend (top) and of the PLA/PEG-zein composite (bottom)
Keywords: Electrospinning, zein, poly(lactic acid), gas
barrier properties.
ANTEC 2009 / 2767