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.