Several thermoplastic polymers are suitable and commercially available [39 ,40 (link)] for a PL-POF core. We have chosen the cycloolefin (COP) polymer, which was previously identified as the best host material for a specific photoluminescent dye, Lumogen Red 305 (LR305), as the formation of dye aggregates is less pronounced in this material [15 (link)]. COP granulates (Zeonor 1020R, refractive index 1.53) were purchased from Zeon Europe GmbH, Düsseldorf, Germany. The photoluminescent dye, LR305 (BASF, Basel, Switzerland), was added to the COP core at a concentration of 0.01 wt.%. The terpolymer of tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride polymer (THVP), has become a common choice for the sheath material of melt-processed POFs [12 (link),15 (link),29 (link),31 (link)]. THVP is characterized by good transparency, low RI, good melt-spinnability, and semi-crystallinity [41 (link)]. THVP granulates (THVP 2030GZ, RI = 1.35) were purchased from 3M GmbH, Rüschlikon, Switzerland. Acetone and methoxyperfluorobutane were purchased from Sigma Aldrich, Buchs, Switzerland, in order to remove the sheath for subsequent studies of the optical and structural properties of the core material. All materials were used as provided without purification.
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