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Polymer extrusion system

Produces consistent, long, ribbon-like, nanomaterials that can be stacked for increased stress resistance

SEM images of ribbon-like PMMA fibers fabricated using a 7-chevron device at different sheath-to-core flow-rate ratios: (A) 100:1 (B) 500:1 (C) 2500:1.

Microfibers and nanofibers are being developed for use in high-strength materials, optical devices, and molecular electronics. Nanofibers are of particular interest due to their potential uses in high-strength applications that can benefit both commercial and defense sectors. Such fibers are usually fabricated by gel spinning, which typically produces fibers tens of microns in diameter. Smaller fibers are then produced by pulling, which also increases the molecular alignment. Fibers below 10 microns are rarely produced by this method, and the shape can only be round. These artificially made fibers are then aligned into high-strength composites used in products like bulletproof vests.

Navy researchers had envisioned that proper stacking of non-round fibers, such as ribbon-like fibers, could accommodate a higher-impact load by transferring stress more efficiently, but methods to fabricate these have been lacking. Techniques like crystallization, electrospinning, molding and templating, and extrusion have all been used to fabricate fibers – each with its own drawbacks. Extrusion is the most common technique for producing microfibers and nanofibers, but the shape of the fibers is usually dictated by the round geometry of the nozzle used to introduce the materials. In recent years, electrospinning has been adopted for producing nanofibers, but all the fibers are likewise round and the degree to which the size of the fibers can be controlled is limited.

The best way to overcome these shortcomings is by eliminating the physical nozzle altogether. To that point, the Navy research team developed a simple sheath flow microfluidic device as a tool for fabricating polymethylmethacrylate (PMMA) fibers. Unlike other microfluidic sheath flow devices that employ multiple sheath inlets to focus the sample stream and are complicated to fabricate, the Navy approach uses simple integrated features on the walls of the fluidic channel to sheath a core stream. Hydrodynamic forces control the shape and size of the PMMA fibers such that round or flat PMMA microfibers and nanofibers are produced. PMMA was chosen for its availability and wide use in composite fabrication. This technique should be applicable to most polymers for which suitable strong and weak solvents are available.

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