We live in an
era of increasing reliance on the very small to satisfy humanity’s endless
needs and desires for new technologies.
Nanotechnology manifests itself in numerous scientific fields, and
polymer chemistry is no exception.
Polymers are generally amorphous, but polymer crystallinity can be
observed if the conditions are right.
Semi-crystalline polymer chains (possesses crystalline and amorphous
phases) such as polyethylene and nylon are often used as barrier films in food,
medicine, and electronics industries.
A barrier is considered highly efficient if small gas molecules are
relegated to permeating through only the amorphous regions of the chains
(crystalline regions are impenetrable). Efficiency can be fine-tuned by varying the
polymer-film processing conditions to suit the desired amount of crystallinity
and chain orientation. Polymer
films can now be made thin enough to effectively confine the crystallization
process to 2D; this leads to surprising results.
Conventionally,
confined polymer chains crystallize into lamellae with thicknesses of ~10-20 nm
with spherelitic morphology.
However, this convention is skirted at the nanoscale, as isotropic
growth is severely hampered to the point of producing lamellar crystal
orientation. This orientation is
usually perpendicular to the layer (edge-on), but parallel orientations have
been reported several times in the literature; mechanisms for orientation
determination remain mysterious for the time being.
Normally,
researchers prepare 2D crystallization of polymers via solution processes such
as spin-coating or Langmuir-Blodgett (LB) techniques, but these are limited by
the solvent requirement and the small quantity of material fabricated. LB techniques enable layered nm
morphologies due to microphase separation of dissimilar block copolymers within
the thin films. Alas, block copolymers
are notoriously difficult to synthesize and align with respect to the direction
of the thin films.
Enter
a new technique known as layer-multiplying extrusion. It uses forced assembly to create alternating layers of two
polymers that number up to the 100,000s.
Almost any melt-processable polymer can be formulated into kilometers of
nanolayered films with thicknesses of ~10 nm. With less material comes an explosion of new previously
unknown properties (“less is more”).
The materials
used in this study are polyethylene oxide (PEO, also known as polyethylene
glycol), which has the following structure:
The other is ethylene-co-acrylic acid (EAA), a
copolymer with much lower crystallinity than PEO:
Films with 33,
257, and 1025 alternating EAA and PEO layers were extruded, with various
thicknesses and composition ratios, including (EAA/PEO vol/vol) 50/50, 70/30,
80/20, and 90/10. The nominal PEO
layer varied from 3.6 µm to 8 nm.
The films were subjected to oxygen permeability tests with respect to to layer thickness. The results are shown below:
The
plots show a significant decrease in O2 permeability. Gas permeability for layered assemblies
is modeled by the following equation.
where
𝜙PEO is the volume fraction of PEO and PPEO and PEAA are the permeabilities of PEO and EAA,
respectively. Upon plugging
determined values of PPEO and
PEAA from literature into
Eq. (1), the result did not agree with the findings reported in the plot above.
Eq. (1) predicts increasing permeability with respect to decreasing PEO
thickness, but the data show the opposite trend. Eq. (1) was then modified to
account for the apparent sensitivity to PPEO
due to the far lesser permeability of PEO; it still did not agree with the
plotted data with the exception of thicker PEO layers as indicated by the
dashed line.
Clearly,
the PEO nanolayers possess some previously unknown crystalline morphology that
bestowed them with staggeringly low permeability. However, differential scanning calorimetry revealed that the
PEO and EAA layers (even the very thin ones) share the same melting enthalpy
and melting temperature as the control films; this means that the changes in
crystalline morphology granting the PEO nanolayers low permeability was not
accompanied by changes in crystallinity nor lamellar thickness.
Upon
examination by AFM, the authors found that the thin 20 nm PEO layers exhibited
single lamellae that extended beyond the field of the AFM image. The single lamellae are said to be very
large single crystals. Reducing
the PEO layer thickness to 8 nm then induces breakage, thereby increasing the
permeability. Fig. 2 below shows
the AFM image of the 20 nm PEO layer, and an accompanying schematic showing a
gas diffusion pathway through the layered assembly.
The lamellar crystalline region is considered impermeable, with the lamellar fold surfaces constituting the permeable amorphous regions. As seen in Fig. 2, the gas pathways depend on the frequency of defects such as lamellar edges. The permeability is now expressed by
where
α is the aspect ratio of the impermeable
platelets (length/width), and 𝜙 is the volume fraction of impermeable platelets; the
platelets are orientated perpendicular to the
flux. For the thinnest PEO layers,
the aspect ratio was as high as 120, which meant the lamellae extended up to 2
µm for the 20 nm thick layers.
Gradually thickening the PEO layer relaxed the restrictions on 3D
growth, which returned the morphology to spherelitic. The results were further confirmed by small-angle
x-ray scattering (SAXS) and wide-angle x-ray scattering (WAXS).
This
work is a major breakthrough in polymeric applications for nanotechnology
because it shows experiment trumping theory, and possibly describes a major
advance for gas-barrier films. Its
importance is amply demonstrated by the 51 citations it has generated since its
publication in 2009. Science
Magazine accepted the paper because of its reliance on well-established
analytical techniques (AFM, differential scanning calorimetry, SAXS, WAXS),
and, more importantly, because of its broad significance in the field of nanoscience.
This
significance is underscored by the novel utilization of a relatively new
technique–coextrusion–on readily available polymers to engineer nanolayered
polymeric formations in sufficient amounts to allow for probing links between
the confined crystalline morphology and the properties exhibited. This opens up new possibilities for
packaging methods, i.e., incorporating polymer nanolayers into common polymeric
films for less cost, thereby reducing the environmental and energy
consequences.
Hi Hellerium,
ReplyDeleteI loved reading this piece! Well written!
Merlen Hogg
silicon
ReplyDeleteThe most common greenhouse plastic is six millimeter four year film.
Metal strips such as those used to mount shelving on walls in the home may be used by the homeowner on a small greenhouse.
See more: Reflectometry