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Polymer Engineering Laboratory, Université de Lyon / French National Centre for Scientific Research (CNRS), France
Keywords: polymer materials, multilayer structures, micro- and nanolayer coextrusion
Many living things in nature have microscopic multilayer structures resembling a “layer cake,” including butterfly wings (Figure 1), shells and trees. Inspired by nature, people have designed many high-performance multilayer materials, such as barrier packaging films, body armor, glass-fiber-reinforced plastics, armored-vehicle shells and sound-absorbing tiles. Polymer-based multilayer composite films or sheets account for a substantial share of practical applications in water treatment, optical devices, barrier packaging, electromagnetic shielding, vibration damping and noise reduction. Materials scientists are therefore interested in how to manufacture such multilayer polymer composites in large quantities.
Figure 1: The multilayer structure of a butterfly wing.
Traditional methods for making multilayer polymer materials generally rely on molecular assembly, such as layer-by-layer assembly (LbL). Such methods, however, are usually confined to the laboratory, with low production efficiency that does not meet industrial requirements. To address this, the US company Dow first proposed the concept of “multilayer coextrusion” in the 1970s for the large-scale preparation of polymer-based multilayer films [1]. Figure 2 shows a typical multilayer coextrusion system. It mainly consists of two extruders, a connector or feedblock, layer-multiplier units, and a take-off and cooling assembly [2]. During operation, two polymers (A and B) are separately melt-extruded from the two extruders and combined in the connector into a single two-layer melt stream. This passes through a series of layer-multiplier units, whose splitting and stacking action repeatedly doubles the number of melt layers. Finally, the take-off and cooling assembly cools and sets the material, producing a sheet or film with alternating A/B layers.
Figure 2: Schematic of micro- and nanolayer polymer coextrusion.
The operating principle of a layer-multiplier unit is illustrated in the dashed box in Figure 2. As the polymer melt enters the unit, it is first divided into two streams perpendicular to the flow direction. One stream flows upward and spreads horizontally, becoming thinner and wider, while the other flows downward and also spreads horizontally. The two streams are then recombined at the outlet. This process doubles the number of layers while leaving the overall thickness unchanged. Using n layer-multiplier units produces a composite with 2n+1 layers, considerably reducing the production steps compared with laboratory-based LbL methods. Since the overall material thickness remains unchanged during stacking, each individual layer becomes thinner as the number of layers increases. With several layer-multiplier units, individual layers can be reduced to the nanoscale. For example, the micrograph on the right of Figure 2 shows a polycarbonate (PC)/polymethyl methacrylate (PMMA) composite film with 4096 layers, prepared using 11 layer-multiplier units. Each layer is approximately 30 nm thick. Unlike molecular-assembly-based LbL methods, multilayer coextrusion creates the micro- and nanostructures of alternating multilayer polymer films through a top-down approach. It is also often called “forced assembly” in polymer processing. Coextrusion can now continuously manufacture large quantities of multilayer polymer composites with nanoscale layered structures. The resulting films can have thousands or tens of thousands of layers, with individual-layer thickness controlled at the micro- or nanoscale. Minimum layer thicknesses can reach 10 nm, even comparable to the radius of gyration of a polymer chain.
Micro- and nanolayer coextrusion can rapidly and efficiently prepare multilayer polymer composite films with controllable micro- and nanostructures. It has therefore continued to attract materials scientists' attention in recent years. The resulting polymer-based multilayer composites exhibit excellent mechanical, optical, barrier and electrical properties because of pronounced interface and spatial-confinement effects, and have found widespread applications in daily life [3–6].
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References:[1] Alfey, T.; Schrenk, W. J. Multipolymer systems, Science 1980, 208 (4446): 813-818.
[2] Lu, B.; Lamnawar, K.; Maazouz, A.; Sudre, G. Critical role of interfacial diffusion and diffuse interphases formed in multi-micro-/nanolayered polymer films based on poly(vinylidene fluoride) and poly(methyl methacrylate), ACS Appl. Mater. Interfaces 2018, DOI: 10.1021/acsami.8b09064.
[3] Baer, E.; Hiltner, A.; Keith, H. Hierarchical structure in polymeric materials, Science 1987, 235(4792): 1015-1022.
[4] Weber, M. F.; Stover, C. A.; Gilbert, L. R.; Nevitt, T. J.; Ouderkirk, A. J. Giant birefringent optics in multilayer polymer mirrors, Science 2000, 287(5462): 2451-2456.
[5] Wang, H.; Keum, J. K.; Hiltner, A.; Baer, E.; Freeman, B.; Rozanski, A.; Galeski, A. Confined crystallization of polyethylene oxide in nanolayer assemblies, Science 2009, 323(5915): 757-760.
[6] Baer, E.; Zhu, L. 50th Anniversary perspective: dielectric phenomena in polymers and multilayered dielectric films, Macromolecules 2017, 50(6), 2239-2256.
