New co-drawing strategies for the fabrication of glass/polymer fibers

. Among the different fundamental aspects that govern the design and development of elongated multimaterial structures via the preform-to-fiber technique, material association methodologies hold a crucial role. They greatly impact the number, complexity and possible combinations of functions that can be integrated within single fibers, thus defining their applicability. In this work, co-drawing strategies to produce monofilament microfibers from unique glass-polymer associations are discussed.


Introduction
In recent years, multimaterial fibers have enabled the development of a large number of novel photonic devices with tremendous innovative potential in numerous domains, such as biophotonic, environmental science, optoelectronics, remote sensing, medicine and so on [1,2].The elaboration of such small-scale objects relies on the ability to combine through the thermal drawing process several materials (glasses, metals, polymers, semiconductors, etc.) and, by extension, several functionalities within the same elongated structure [3,4].From a fabrication standpoint, new materials associations are always sought to produce ever more original fiber architectures, thus solving modern technological challenges [5,6].Glass/polymer combinations offer for instance great opportunities for the design and fabrication of functional optical fibers.This claim is illustrated in this work in which a new co-drawing strategy of glasses with thermoplastics based on the molten core technique is presented.Following, development of a quasi-exposed core fiber using the proposed method is described.

Applying the molten-core technique to glass/polymer associations
The molten core method (MCM) appears as one of the main attractive solutions for the design of intricate hybrid elongated structures among the available multimaterial fiber preparation strategies [7].The MCM relies on the following principle: a core material is inserted within an amorphous cladding to form a macroscopic preform and the assembly is subsequently stretched into thin continuous fibers using conventional fiber-drawing equipment, with the particularity that during the stretching procedure, the core material is in a molten state, while the amorphous cladding material is only softened.In other words, the cladding acts as a support to restrict the flow of the liquid core material.In the early development of the molten core technique [8], the method referred to thermal elongation procedures during which chemical interactions within the core materials mix or between the molten core and the cladding would occur.This process would allow the production novel materials that could not be synthesized otherwise or could not be integrated into fibers through the conventional drawing process.The technique now designates, in a broader way, thermal elongation procedures during which a core material is drawn while being in the molten state [9].To this date, the MCM has mainly been exploited for the in-fiber incorporation of inorganic, non-conventionally stretchable materials (glass compositions with high rareearth concentrations, semi-conductors, metals, etc.), but the integration of polymers into glass-based elongated structures using the same methodology has not been studied.It is proposed here to assess for the first time the fiber drawing ability of selected thermoplastics using the molten core method [10].First, the conditions in which the MCM can be applied to integrate polymeric materials within larger glass frames are discussed, and general thermal parameters are established.Following, experimental investigations are carried out and several fibered structures produced from unprecedented glass/polymer associations are examined, as shown on Fig. 1.It is demonstrated that amorphous and semicrystalline polymers can be co-drawn with low glass transition temperature oxide glasses while being in a low viscosity regime.This result constitutes an important breakthrough as glass/polymer associations are almost exclusively limited to the combination of chalcogenide glasses and high-performance amorphous thermoplastics.

Evolutive glass/polymer fibers
Pushing further, we then demonstrate the development of evolutive glass fibers integrating thin polymer structures [11].It is found that large surface area polymer films with micrometric thickness can be successfully embedded within larger oxide-glass structures through the thermal drawing process and without compromising the organic material physicochemical integrity.This enables the production of elongated geometries in which specific domains are delimited by a polymer inclusion, which greatly simplifies post-drawing functionalization procedures.A functional fiber architecture which integrates a waveguiding core is produced using the methodology.The fiber profile consists in a regular cylindrical core/cladding structure, except that the central chalcogenide core is wrapped within a thin polyether sulfone (PES) film with sub-micron thickness and embedded in a tellurite clad.Selective etching of the cladding is performed to produce quasi-exposed-core fibers.Fig. 2. 3D diagram and pictures of the proposed quasi-exposed core fiber architecture.The transmission spectra of the waveguide in air (red line) and in deionized water (blue symbols) is also presented.
The purpose of the polymer is then twofold: (i) it preserves the chalcogenide glass core from the hydrochloric acid treatment carried out to remove the oxide glass cladding, and (ii) it provides mechanical protection for the etched portion of the fiber.The resulting fiber device is then tested in a proof-of-principle sensing experiment based on evanescent wave spectroscopy of liquids, as depicted on Fig. 2. Numerical investigations of modal properties are also carried out to further optimize the proposed fiber geometry and improve its sensing performances through interactions between the evanescent field and an analyte.We believe that small footprint optical detection systems could benefit from the use of such unique and evolutive photonic objects.This research was funded by from French program "Investments for the Future" operated by the National Research Agency (ISITE-BFC, 4DMeta project, contract ANR-15-IDEX-03; and EIPHI Graduate School, SMILE project, contract ANR-17-EURE-0002).The authors also acknowledge support from the French ANR (through the TRAFIC project, contract ANR-18-CE08-0016-03), from CNRS Institute of Physics (Emergence 2017, 3D-printed optical fibers), and from European Regional Development Fund.This work benefited from the facilities of the SMARTLIGHT platform funded by the French ANR (EQUIPEX+ contract "ANR-21-ESRE-0040") and the Région Bourgogne Franche-Comté.

Fig. 1 .
Fig.1.3D diagram describing the molten core method applied to glass-polymer associations and pictures of fibers obtained using the proposed methodology. /doi.org/10.1051/epjconf/202328705040