Controlling Thermoplastic Elastomer Optical Properties by Mechanical Processing
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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2019.19
Controlling Thermoplastic Elastomer Optical Properties by Mechanical Processing Margaret Campbell1, Paramjot Singh2, Kunal Kate2, Cindy K. Harnett2 1
University of Kentucky College of Engineering, 351 Administration Dr, Lexington, KY 40506, U.S.A.
2
University of Louisville J.B. Speed School of Engineering, 2210 S Brook St, Louisville, KY 40208, U.S.A.
ABSTRACT We demonstrate that the extrusion speed of thermoplastic urethane elastomer can modify its optical transmission by a factor of more than 100. Varying extrusion speed at constant temperature may tune optical properties along the axis of a filament, for example creating absorbent regions that are sensitive to length and diameter changes, surrounded by more transmissive segments that carry the sensor signal over long distances. Such waveguiding in a stretchable optical fiber requires a stretchable cladding with lower refractive index than the core. In experiments toward a rugged, stretchable fiber cladding, we investigated whether solvents could modify the outer structure of the filaments. Soaking the filaments in NMP (nmethyl-2-pyrrolidone), then stretching the filaments while the solvent dried, turned out to modify the filaments in a way that solvents alone did not, creating porosity and reducing the appearance of optical clarity.
INTRODUCTION Thermoplastic polyurethane (TPU) is an amorphous thermoplastic which softens around its glass transition temperature, Tg. Heating and extrusion transforms TPU polymer pellets into fibers, and since TPU has good optical transmission, the fibers can act as optical waveguides for visible light. In this work we investigated the effect of extrusion speed on optical clarity, and also investigated how the surface of TPU filaments could be modified by a combination of solvent exposure and mechanical stretching. The motivation for modifying the surface is to control waveguiding and scattering. For longdistance waveguiding a low refractive index, smooth coating is desired, while for scattering light into or out of the waveguide core, a rough surface helps. A rough surface can also provide greater bonding area for a cladding that will experience strong forces during stretching. Optical scattering from a rough surface isn’t optimal for long-distance
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(>1 km) communication fibers, but for ( 0.2 mm/s. We observed that filaments extruded at 0.2 mm/s and 0.3 mm/s developed the deeply-textured surface of Figure 1 after soak-andstretch processing, while those extruded at the slower speeds (0.01 mm/s, 0.0075 and 0.005 mm/s) did not.
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