We demonstrate that changing the extrusion speed of thermoplastic elastomer (TPE) fiber can modify its optical transmission by a factor of more than 100. Recently, we created stretchable fiber optic sensors from commercial urethane elastomer fibers that could detect muscle activity when sewn into textiles . By extruding fiber from pellets at slow speeds, optical transmission increased beyond that of the commercial material, potentially leading to sensors that measure deformation on the meter scale instead of cm scale.
Thermoplastic elastomer pellets (Versaflex CL40, PolyOne Corp.) were dehydrated overnight at 60 C, then extruded at 190 C at rates between 0.005 and 0.3 mm/s. The 0.005 mm/s specimen appeared clear upon extrusion while successively faster speeds appeared hazier. Optical transmission was measured with a photodiode and light source. Stress-strain testing showed that the fast-extruded filaments had a greater ultimate tensile strength than filaments extruded at slower speeds.
Waveguiding in a stretchable optical fiber requires a stretchable cladding with lower refractive index than the core. Our previous silicone coating approach worked with the new extruded cores. However, the silicone claddings peeled under extreme strain (>100%). Therefore, we investigated whether solvents could improve cladding adhesion, waveguiding and light-coupling properties of the fibers. Soaking the fibers in NMP (n-methyl-2-pyrrolidinone), then stretching the fibers while the solvent dried, turned out to modify the fibers in a way that solvents alone did not. The clearest, slowest-extruded cores were relatively unchanged by the NMP “soak-and-stretch” treatment, while the faster-extruded, hazier cores developed a porous, textured cladding with a uniform thickness.
The differing optical properties, mechanical properties, and response to solvents are likely caused by a transition from amorphous polymer at low extrusion speeds, to crystalline at high extrusion speeds. The microstructural properties of urethane based TPEs are known to involve pseudo-crosslinks, which are not chemical bonds but rather physical connections between the monomers of the block co-polymer , and which may be disrupted by the soak-and-stretch process. Further investigations with differential scanning calorimetry and FTIR spectroscopy were conducted to identify changes in fiber microstructure at different extrusion speeds.
Varying extrusion speed at constant temperature may tune optical properties along the axis of a fiber, for example creating absorbent regions that are sensitive to length and diameter changes, surrounded by more transmissive segments that carry the signal over long distances. The work also has implications for 3D printing soft optical structures from thermoplastic elastomer.
 C. K. Harnett, H. Zhao, and R. F. Shepherd, Advanced Materials Technologies 2, 9 (2017).
 D. H.W. Feijen, J.L. Muller, J. Julia, D. Salvatella, M. J. Riba, TPE Conference 1, 1. (2001).